INVESTIGATION of DRY HORIZONTAL STIRRED MILLING

APPLICATIONS FOR CEMENT GRINDING CIRCUITS

KURU YATAY KARIŞTIRMALI DEĞİRMEN

TEKNOLOJİSİNİN ÇİMENTO ENDÜSTRİSİNDE KULLANIM OLANAKLARININ

ARAŞTIRILMASI

OKAY ALTUN

Prof. Dr. A. HAKAN BENZER

Supervisor

Submitted to Institute of Graduate Studies in Science

of Hacettepe University as a partial fulfillment

to the requirements for the award of the degree of

DOCTOR OF PHILOSOPHY

in

MINING ENGINEERING

2013

This study named “Investigation of Dry Horizontal Stirred Milling Applications for Cement Grinding Circuits” by OKAY ALTUN has been accepted as a thesis for the degree of DOCTOR OF PHILOSOPHY in MINING ENGINEERING by the below mentioned examining committee members.

Head

Prof. Dr. Ali İhsan AROL

Supervisor

Prof. Dr. A. Hakan BENZER

Member

Prof. Dr. Ş. Levent ERGÜN

Member

Prof. Dr. Zafir EKMEKÇİ

Member

Assist. Prof. Dr. İlkay Bengü ÇELİK

This thesis has been approved as a thesis for the degree of DOCTOR OF PHILOSOPHY in MINING ENGINEERING by the Board of Directors of the Institute of Graduate Studies in Science of Hacettepe University.

Prof. Dr. Fatma SEVİN DÜZ

Director of the Institute of

Graduate Studies in Science

In loving memory of my beloved father

İsmail Oktay ALTUN (1953-…)

ETHICS

In this thesis study, prepared in accordance with the spelling rules of Institute of Graduate Studies in Science of Hacettepe University,

I declare that

all the information and documents have been obtained in the base of the academic rules

all audio-visual and written information and results have been presented according to the rules of scientific standards

in case of using other works, related studies have been cited in accordance with the scientific standards

all cited studies have been fully referenced

I did not do any distortion in the data set

And any part of this thesis has not been presented as another thesis study at this or any other university

26 / 06 / 2013

Okay ALTUN

i

ABSTRACT

INVESTIGATION of DRY HORIZONTAL STIRRED MILLING

APPLICATIONS FOR CEMENT GRINDING CIRCUITS

OKAY ALTUN

Doctor of Philosophy, Mining Engineering Department

Supervisor: Prof. Dr. A. HAKAN BENZER

June 2013, 170 pages

Within the thesis study, possible applications of dry horizontal stirred mill were

investigated for cement industry where fine grinding is prominent with regards to

improved product quality. The main objectives of this work were to determine

grinding problems, to study the effects of design and operating variables on

grinding performance, to optimize the milling operation, to carry out modelling

studies and then use these models in order to simulate different cement grinding

circuit configurations where stirred mills were employed.

In this context, dry horizontal stirred mill was developed with the partnership of

Netzsch-Feinmahltechnik and commissioned at a cement plant. Different flow

streams of the grinding circuit were sampled and then subjected to grinding

operation under different operating conditions. Within the scope of the

experimental studies, the influences of the parameters affecting the specific

energy consumption of the mill such as stirrer speed, media filling, media size,

feed rate were investigated. In addition to those, the influences of material and

design variables i.e., feed size distribution, mill chamber geometry and stirrer type

were assessed. Throughout the studies, experimental studies were performed with

two chamber designs having 23 L and 42 L volume and three different stirrer

types.

ii

As a result of the experimental studies it was understood that, material

transportation was a parameter influencing the grinding performance directly and

in case of adjusting the grinding chemical type and dosage, besides the air flow

from the feed inlet properly, 18% improvement in product quality was achievable.

Stirrer speed affected the product fineness however below 4.34 m/s and above 6.5

m/s the effectiveness of the parameter decreased. Media loading of the mill was

needed to be maximized. The studies indicated that, in case the rest of the

operating parameters were constant, 50% increase in Blaine value could be

observed when media loading was changed from 30% to 60%. In this study, the

advantages of using fine media were reported. With this mill configuration and

material, it was determined that up to 27% energy saving was achievable with the

use of 4 mm media size when compared to 6 and 8 mm. The experimental studies

with different mill chamber geometries and stirrer types showed that 42 L mill had

better grinding performance (compared to 23 L) besides disc type stirrer provided

more energy efficient grinding operation.

Following the performance evaluation studies, the benefits of employing dry

horizontal stirred mill on cement grinding circuits were discussed with the aid of

simulation studies. In this context, simulation scenarios for open and closed circuit

grinding operations were prepared. The studies showed that energy saving up to

35% was achievable when dry horizontal stirred mill was used with open circuited

ball mill and 16% decrease in specific energy consumption could be observed with

the closed circuit configuration. In addition to energy saving, the overall circuit

production could be increased by %100.

Keywords: Dry stirred mill, stirred mill, cement grinding, fine grinding.

iii

ÖZET

KURU YATAY KARIŞTIRMALI DEĞİRMEN TEKNOLOJİSİNİN

ÇİMENTO ENDÜSTRİSİNDE KULLANIM OLANAKLARININ

ARAŞTIRILMASI

OKAY ALTUN

Doktora, Maden Mühendisliği Bölümü

Danışman: Prof. Dr. A. HAKAN BENZER

Haziran 2013, 170 sayfa

Bu tez çalışması kapsamında, kuru yatay karıştırmalı değirmen teknolojisinin ince

öğütmenin önem arz ettiği çimento endüstrisinde kullanım olanakları araştırılmıştır.

Çalışmanın temel amaçları, değirmende oluşabilecek öğütme sorunlarını tespit

etmek, tasarım ve işletme değişkenlerinin öğütme performansına olan etkilerini

incelemek, değirmen için en uygun işletme koşullarını belirlemek, modelleme

çalışmalarını yürütmek ve sonrasında bu modelleri farklı çimento öğütme devresi

alternatiflerinin simülasyonu sırasında kullanmak olarak sıralanabilir.

Bu amaçla Netzsch-Feinmahltechnik firması ile birlikte yatay karıştırmalı değirmen

geliştirilmiş ve çimento öğütme devresinde kurulmuştur. Halihazırda çalışan

çimento öğütme devresinde farklı akış kollarında örnekleme çalışmaları

yürütülmüş ve bu malzemeler farklı işletme koşullarında öğütme işlemine maruz

bırakılmıştır. Deneysel çalışmalar kapsamında, değirmende özgül enerji tüketimini

etkileyen karıştırıcı hızı, bilya doluluğu, bilya boyu, besleme hızı değişkenlerinin

etkileri incelenmiştir. Bunlara ek olarak malzeme ve tasarım değişkenleri olan

malzeme besleme boyu, değirmen haznesi ve karıştırıcı tipi etkileri de denemeler

kapsamında değerlendirilmiştir. Çalışmalar süresince 23 Litre ve 42 Litre hacim

iv

değerine sahip öğütme haznelerinde ve üç farklı karıştırıcı tipinde deneysel

çalışmalar gerçekleştirilmiştir.

Yürütülen çalışmalar neticesinde malzeme taşınmasının, kuru öğütmenin

verimliliğini etkileyen önemli bir değişken olduğu ve kimyasal tipi ve dozajının

ayrıca değirmen besleme bölümünden verilen havanın uygun oranlarda

ayarlanması durumunda ürün kalitesinde %18 değerine varan iyileştirme olduğu

belirlenmiştir. Karıştırıcı hızının değirmenden alınan ürünün inceliğini etkilediği

ancak 4.34 m/s değerinin altında ve 6.5 m/s değerinin üzerinde etkisini yitirdiği

tespit edilmiştir. Değirmende bilya doluluğunun olabilecek en yüksek seviyede

tutulması gerektiği belirlenmiştir. Diğer işletme koşulları sabit kalmak koşuluyla,

bilya doluluğunun %30 değerinden %60’a çıkarılması ile Blaine değerinde %50

oranında artış olduğu tespit edilmiştir. Çalışmalar neticesinde değirmende ince

bilya kullanımının avantajlı olduğu ve mevcut tasarım için kullanılabilecek en ince

bilya boyunun 4 mm olduğu belirlenmiştir. Ayrıca iri bilyaya kıyasla besleme

boyuna bağlı olarak %27’ye varan enerji tasarrufu sağladığı da gözlemlenmiştir.

Değirmen tasarım değişkenleri olan öğütme haznesi ve karıştırıcı tipinin de

öğütme performansını etkilediği, daha büyük hacim değerine sahip haznenin

öğütme performansının daha iyi olduğu (42 Litre) ve disk tipi karıştırıcının da diğer

tasarımlara oranla daha enerji verimli olduğu anlaşılmıştır.

Performans çalışmalarını takiben yürütülen simülasyon çalışmaları ile çimento

öğütme devresinde farklı noktalarda kullanılan kuru yatay karıştırmalı değirmenin

devreye sağladığı katkılar tartışılmıştır. Çalışmalar, açık devre çimento

öğütmelerinde karıştırmalı değirmenlerin son öğütmede kullanılması durumunda

%35, kapalı öğütme devrelerinde ise %16’ya varan enerji tasarrufu sağladığı

belirlenmiştir. Enerji tasarrufuna ek olarak devre üretim kapasitelerinde %100’e

varan artış tespit edilmiştir.

Anahtar kelimeler: Kuru karıştırmalı değirmen, karıştırmalı değirmen, çimento

öğütme, ince öğütme

v

ACKNOWLEDGEMENTS

I owe a debt of sincere thanks to;

The Head of Hacettepe University Mine Engineering Department who helped me

to benefit from the department’s resources,

Prof. Dr. A. Hakan BENZER, my distinguished thesis supervisor, who put forward

his both financial and moral support during thesis preparation phase and stated his

opinions during the process of adding value to this specific topic, Prof. Dr. Ş.

Levent ERGÜN and Prof. Dr. Zafir EKMEKÇİ, who are the thesis monitoring

committee members,

Udo ENDERLE, the Managing Director of Netzsch Feinmahltechnik, who provided

equipment and technical support, Levent ONAT, the previous Quality Manager of

Set Italcementi, who provided support in laboratory studies, Murat AKAY, the

quality chief of SET Ankara Cement Plant and Kerim DOĞAN, the plant production

manager of SET Ankara Cement Plant, Alper TOPRAK, Konuray DEMİR, Dr.

Namık AYDOĞAN, Dr. Hakan DÜNDAR and A. SARGIN, my valuable colleagues

who helped me in experimental studies and helped this thesis to develop,

Assoc. Prof. Dr. Abdullah OBUT and Assoc. Prof. Dr. Aubrey MAINZA, for their

valuable contributions during thesis writing process,

To F. Çiğdem ALTUN, my precious mother, who never avoided her financial and

moral support during my tiring studying period and made endless contributions to

my current success, Okan ALTUN my one and only brother who had stood by me

with his character-wise stance, Deniz ALTUN, my wife, who have been playing a

supportive role on my decisions with the love, respect and her objective attitude

since she walked into my life, and her precious family,

Emre YILMAZKAYA, Ediz KANBİR and Yelda YAMATMA, my distinguished

friends who always stood by me with their friendship, Dr. Serkan DİKMEN,

Neslihan TOK, department technicians Mustafa YILMAZ and Işın ASLIYÜKSEK,

department secretary Sıddık YILMAZOĞLU and Birgül ATAY,

I would like also to thank Hacettepe University Research Foundation Unit (Project

No: 013 T06 604 006) for their financial support.

vi

TABLE of CONTENTS

ETHICS ................................................................................................................... ii

ABSTRACT .............................................................................................................. i

ÖZET ...................................................................................................................... iii

ACKNOWLEDGEMENTS........................................................................................ v

TABLE of CONTENTS ........................................................................................... vi

1. INTRODUCTION ............................................................................................. 1

2. STIRRED MEDIA MILL ................................................................................... 8

2.1. Vertical Stirred Media Mills ........................................................................ 13

2.2. Horizontal Stirred Media Mills .................................................................... 18

2.3. Comparison between Vertical and Horizontal Stirred Mills ........................ 22

2.4. Operating Parameters and Their Influences on Grinding Performance of

Stirred Mill ......................................................................................................... 26

2.4.1. Stirrer Speed ....................................................................................... 26

2.4.2. Media Size and Density ....................................................................... 28

2.4.3. Media Filling ........................................................................................ 30

2.4.4. Feed Rate ........................................................................................... 31

2.4.5. Mill Geometry ...................................................................................... 31

2.4.6. Rheology of the material (Grinding aids) ............................................. 32

2.5. Motion of Suspension and Single Grinding Media ..................................... 33

2.5.1. Motion of Suspension .......................................................................... 33

2.5.2. Motion of Single Grinding Media ......................................................... 35

2.6. Modelling Studies ...................................................................................... 37

2.6.1. DEM Models ........................................................................................ 37

2.6.2. PEPT Technique ................................................................................. 38

vii

2.6.3. Stressing Models ................................................................................. 39

2.7. Scale-up of Stirred Mills ............................................................................. 44

3. EXPERIMENTAL STUDIES & INITIAL TESTWORKS .................................. 46

3.1. Description of the Experimental Apparatus ................................................ 46

3.1.1. Internal Structure of Dry Horizontal Stirred Mill ................................... 48

3.1.2. Power Draw Measurements ................................................................ 50

3.2. Sampling & Material Characterization Studies ........................................... 52

3.3. The Observations during Initial Test Works ............................................... 57

3.3.1. Grinding Problems with 23 L Mill ......................................................... 57

3.3.2. Grinding Problems with 42 L Chamber ................................................ 59

3.4. Reproducibility of the Grinding Results ...................................................... 62

4. INFLUENCES of OPERATING and DESIGN PARAMETERS on GRINDING

PERFORMANCE ................................................................................................. 64

4.1. The Effects of Grinding Chemicals ............................................................ 64

4.1.1. The tests with EPCT-04 (Glycol-based chemical) ............................... 65

4.1.2. The tests with EPCT-02 (Triethanolamine (TEA)-based chemical) ..... 67

4.1.3. The tests with EPCT-01 (Triisopropanolamine (TIPA)-based chemical)

...................................................................................................................... 69

4.1.4. Comparison of the Chemical Performances ........................................ 71

4.2. The Effects of Air Flow Rate ...................................................................... 73

4.3. The Effects of Stirrer Speed ....................................................................... 74

4.4. The Effects of Feed Rate ........................................................................... 79

4.5. The Effects of Media Filling ........................................................................ 81

4.6. The Effects of Ball Size .............................................................................. 84

4.6.1. 4-6 mm Comparison ............................................................................ 84

4.6.2. 4-6-8 mm Comparison ......................................................................... 85

4.6.3. 4-3 mm Comparison ............................................................................ 86

viii

4.7. The Effects of Feed Size Distribution ......................................................... 88

4.7.1. Grinding Tests Performed with Final Product Stream ......................... 89

4.7.2. Grinding Tests Performed with Separator Reject Stream .................... 93

4.7.3. Grinding Tests Performed with Mill Filter Return Stream .................... 97

4.7.4. Performance Evaluation of Stirred Mill at Different Feed Fineness ... 101

4.8. The Effects of Mill Geometry .................................................................... 104

4.9. The Effects of Stirrer Type ....................................................................... 108

5. MODELLING of DRY HORIZONTAL STIRRED MILL ................................. 112

6. SIMULATION STUDIES .............................................................................. 121

6.1. Selected Circuit Configurations for Simulation Studies ............................ 121

6.2. Mass Balance and Model Fitting of the Circuits ....................................... 125

6.3. Simulation Scenarios Prepared for Dry Stirred Mill Operation ................. 129

6.3.1. The Use of Dry Horizontal Stirred Mill in Finish Grinding .................. 129

6.3.2. The Use of Dry Horizontal Stirred Mill on Filter Return Stream ......... 133

7. RESULTS and DISCUSSIONS ................................................................... 136

8. CONCLUSIONS .......................................................................................... 143

REFERENCES ................................................................................................... 144

APPENDICES .................................................................................................... 152

CURRICULUM VITAE ........................................................................................ 169

1

1. INTRODUCTION

The purpose of this thesis study is to provide information on the design and

application of a new dry stirred fine grinding mill. The application of this mill was

focussed on the cement industry but can be extended to other commodities.

Cement production is an intensive energy use process. Nowadays the cement

manufacturers are pushing the operational limits of their existing cement grinding

circuits as the demand for high quality product and high production capacities

have steadily been increasing. These circumstances cause already inefficient

grinding circuits to be operated even more inefficiently that is thought to be

overcome by introducing innovative solutions.

Increased demand on efficient grinding technologies had pushed machine

manufacturers to search for new milling mechanisms. Until now wide varieties of

mill mechanisms have been developed to improve energy efficiency of

comminution operations. Napier-Munn et al. [1] classified the mills aiming to

process different types of materials according to their operating size ranges and

energy consumption values as given in Figure 1.1. As can be seen from Figure

1.1, many mills are available from coarse to fine end and the selection totally

depends on the application.

Figure 1.1. Energy figures of different mills [1]

Among the technologies given in Figure 1.1 , stirred media mills have been used in

fine and ultra-fine grinding applications. They are among the proven technologies

used in PGM, gold, copper-zinc processing plants for wet grinding purpose. In

2

particular, it had been reported that the use of IsaMill® in McArthur River and

George Fisher ores, where grind size P80 of 7 µm was required, improved flotation

recoveries for both zinc and lead ores by 5-10% [2]. Successive wet operations

raised the question as to whether it is applicable in dry milling which is expected to

be prominent in the future due to the environmental reasons (CO2 emissions,

water recovery and water efficiency). As mentioned previously cement industry is

in need of energy efficient equipment to reduce CO2 emissions and to decrease

energy consumptions. Therefore the use of stirred mill in cement grinding area can

be considered as one of the step changes. In this study, possible applications of

stirred mill technology on cement grinding circuits were investigated. For this

purpose a prototype dry stirred mill was developed and several experimental

studies were performed.

Dry stirred media mill technology can be employed where fine grinding is required.

It is possible to develop various circuit alternatives improving energy efficiency of

the circuit without deteriorating cement quality. Moreover, the quality deficiencies

coming from compression machines used in finish grinding (VRM or HPGR) could

be regulated by the use of stirred mills. In the following figures simplified flow

sheets of possible circuit configurations are illustrated.

In Figure 1.2, stirred milling application on mill filter return stream is illustrated. In

some of the cases the fineness of mill filter return stream does not meet the final

product specifications therefore it is fed to the separator feed stream again. It is

thought that the use of stirred mill on this stream would increase throughput rate of

the circuit up to a certain extent.

Figure 1.2. Filter return stream application of dry horizontal stirred mill

3

The use of a stirred mill on the separator reject stream may be another alternative

(Figure 1.3). In general, because of high circulating loads of the grinding circuits,

by-pass of the air separators may reach up to 30%. That means considerable

amount of unclassified material is recirculated back into the mill that overloads the

circuit. It is thought that stirred milling on this stream would grind that size of

material down to a finer size range thus improving the energy efficiency of the

operation.

Figure 1.3. Separator reject stream application of dry horizontal stirred mill

Stirred milling technology may also be applied on final product stream. In Figure

1.4 and Figure 1.5 possible circuit configurations are illustrated. In these

configurations, the grinding circuits would be pushed to produce coarser product

then stirred mill technology would perform further grinding to obtain the target size

resulting in increased capacity. It is thought that open circuit configuration (Figure

1.5) would benefit more from stirred milling compared to closed circuit. Because

such a circuit design would provide more flexible operation in terms of controlling

the product size and increasing throughput rate.

4

Figure 1.4. Final product stream application of dry horizontal stirred mill for closed circuit cement grinding

Figure 1.5. Final product stream application of dry horizontal stirred mill for open circuit cement grinding

Cement manufacturing process was used as a starting point of using dry

horizontal stirred mills. After successful application of the technology, the

experience would be transferred to the mineral grinding applications in circuits

where dry grinding is required. As the ultimate aim is to grind an ore in an efficient

way, in the near future it is highly possible to design flow sheets comprising only

HPGR, air classifier and dry stirred mill (Figure 1.6, Figure 1.7) [3] or VRM

followed by dry stirred milling (Figure 1.8).

5

Figure 1.6. Grinding circuit with closed circuited HPGR (VSK separator) and dry stirred mill

Figure 1.7. Grinding circuit with closed circuited HPGR (with two-stage air classifier) and dry stirred mill

6

Figure 1.8. Grinding circuit with VRM and dry stirred mill

As mentioned above, there would be wide range of applications for dry horizontal

stirred mill if it is operated efficiently and provide benefits to the grinding circuit in

terms of energy saving and production rate increase. Within the context of the

study several experimental studies were performed with a prototype dry horizontal

stirred mill developed by Netzsch-Feinmahltechnik GmbH. The mill was

commissioned at a cement plant where sampling campaigns were carried out at

different streams of the circuit, final product, separator reject, and mill filter return.

As a result, various feed size distributions were tested and the performance of the

mill was assessed. The sampling studies around stirred mill was performed when

the mill reached to a steady state condition which was understood by observing

power draw and production rate of the mill. Provided that both of the parameters

were steady, then the samples were collected and subjected to characterization

works. In terms of characterization, size distributions, surface area and strength

properties was determined.

Throughout comprehensive test studies, understanding the behaviour of milling

operations was aimed initially. In this context, grinding problems and solutions, the

influences of operating parameters, i.e., stirrer speed, media filling, material

rheology, air flow rate inside the mill, media size, feed size distribution, mill

geometry and stirrer type were investigated and compared with the related

literature. After all, the relationships between the energy utilization and size

reduction values were developed that was used to assess the mill performance.

The performance data was then used to optimize the milling conditions.

7

Within the context of the thesis, modelling of dry horizontal stirred mill was

accomplished with the perfect mixing approach. These model structures were

used to simulate various circuit configurations that stirred mills were employed. In

this context, the applications on finish grinding for open and closed cement

grinding circuits and mill filter return stream were investigated. The results were

promising that lead to save energy as well as the increase in production rate of the

overall grinding circuits. Finally, a scaling-up methodology was developed for dry

horizontal stirred mill irrespective of mill geometry.

8

2. STIRRED MEDIA MILL

Stirred media milling is not a new concept; it is a well-known process since 1928.

In 1928, Klein and Szegvari proposed a mill model operating in wet mode with

agitators on the shaft and using spherical grinding media inside. The first industrial

scale operation of stirred mills found in 1948 in pigment grinding for paint and

lacquer industries [4]. Since that time, many developments have taken place and

now many types are available for industrial use. In Table 2.1 the applications of

stirred media mills, where reducing feed size of 100 µm down to 1-10 µm is

required, are presented [4; 5].

Table 2.1. Stirred media milling applications

Industry Product

Paint + lacquer

primer coating

Lacquer

dispersion paints

Ink

printing inks

textile inks

activating pigment crudes

Food industry

cocoa nibs

milk chocolate

peanuts

Coal, energy

coal-oil mixture

coal-water slurries

gas turbines (micronized)

desulfurization

Minerals

limestone

filler industry

paper coatings

flue gas

kaoline

gypsum

aluminium oxide

precious metals liberation

9

Several types of stirred media mills have been developed so far for commercial

use. One of the earliest machines was a shot mill in which media size was in

range between 0.5 mm and 2 mm, and was fluidized by the disc type attritors

mounted on the shaft. Besides, a product separation zone was mounted inside of

the mill to prevent the discharge of media and unground material [6].

In the late 30’s, the USBM developed an attritor mill aiming to clean the surface of

minerals to improve their flotation efficiency (Figure 2.1). The attritor was then

used for grinding purpose. The USBM attritor mill had water jacket surrounding mill

shell and cage type rotor that turned in a stator. This machine was then modified

and patented by Union Process and named as Szegvari attritor system (Figure

2.2). Szegvari mill was also in a water jacketed container where the media was

fluidized by the attritors. The use of the water jacket was important especially

when processing heat sensitive materials such as metal oxide coatings.

Figure 2.1. USBM attritor grinding compartment [6]

10

Figure 2.2. Union process [6]

Stehr [4] illustrated modern stirred media mills of continuously operated as shown

in Figure 2.3. Those mills can be operated both in vertical and horizontal

configurations and the selection of the configuration depends on the process

variables such as viscosity, feed rate, density as well as the distribution of the

media along the mill chamber et cetera. The distribution of media directly affects

the performance of grinding operation therefore grinding chamber with equally

distributed media is the most preferable one.

Figure 2.3. Stirred media mill types [4]

In stirred mills, basically two types of agitators are available which are; perforated

disc and pinned agitator. The main difference between them is the method of

transferring the energy. The perforated discs transfer the energy to the

11

suspension–media mixture via adhesion forces while the pinned agitators transfer

the energy via force displacement. Because of its characteristics, the pinned type

agitator found its application in the processing of higher density media and high

viscosity material [4].

Innovation of the stirred media mills brought higher energy efficiencies, due to their

operational characteristics and particle breakage mechanism (mainly attrition), in

fine and ultrafine grinding applications when compared to conventional techniques

(ball mill). The energy efficiency of the stirred media milling is illustrated

graphically (Figure 2.4). The data gathered from vibratory ball mills, tumbling mills

and stirred media mills (in chalcopyrite grinding) indicate that at the same product

fineness, stirred media milling requires less amount of energy compared to the

vibrating ball mill and conventional ball mill in both wet and dry operation. Energy

efficient grinding operations at fine product sizes indicate these mills are well-

adapted to fine grinding applications [6].

Figure 2.4. Comparison of different grinding systems [6]

Energy efficient operation of stirred mills has been reported by different

researchers [7; 8; 9; 10; 11]. Figure 2.5 illustrates that the energy consumption of

the ball mill increases significantly below 75 µm product size and below 30 µm the

trend gets steeper compared to the stirred mills. Therefore, the stirred media mills

have found applications in regrinding, fine grinding and very fine grinding

operations. In addition to energy saving operations, some other properties of

stirred mills make them advantageous over ball mills. These are; lower capital

cost, lower installation cost, less floor space, fewer moving parts, less noise,

higher level of controllability, lower maintenance cost and greater operational

safety [12].

12

Figure 2.5. The energy comparison between ball mill and stirred mill [8]

Based on the features mentioned above, stirred media mill has proven its worth

and found worldwide applications in mineral industry. There are many types of

stirred mills available developed by different manufacturers which include;

Sala Agitated Mill or SAMTM (Grinding Division of Metso Group, UK)

MaxxMill® (Maschinenfabrik Gustav Eirich, Germany)

Vertimill®, SMD® (Metso)

Tower mill (Nippon Eirich , Japan)

IsaMillTM (Netzsch-Feinmahltechnik GmbH, Germany)

Referring to the list given above, the stirred media mills can be classified into two

[13] as shown in Figure 2.6.

Figure 2.6. The classification of stirred media mills [13]

13

2.1. Vertical Stirred Media Mills

In this section, the structures and operating principles of vertical stirred media mills

are explained briefly.

The history of vertical stirred media mills goes back to 1940’s when a prototype

mill was introduced into the Japanese mineral industry with financial support of

Asahi Glass Corp. Preliminary studies reported that less specific energy was

utilized when compared to the ball milling technology. Ultimately, a test mill was

manufactured and became a Tower Mill owned by Japan Tower Mill Company in

1965 [14]. Nowadays the Nippon Eirich has got the license for Tower Mill

production.

Tower Mills agitate the grinding media filled in mill shell with screw equipped with

double helical flights (Figure 2.7). The material is ground in the mill shell under the

influence of the impact and abrasion mechanisms. This technology has gained

experiences both in wet and dry grinding applications. They have been used for

wet grinding purpose in lead-zinc circuits and gold ore, where grind size of 25 µm

is required, to have an increased liberation degree. Moreover, cement grinding

circuits producing ultra-fine products preferred Tower Mill technology as well.

Figure 2.7. Vertical stirred media mill [14]

14

Eirich [15] reported that 242 mills had been sold in 15 different countries by the

end of 1998. The largest Tower Mill was manufactured for limestone grinding and

had a motor power of 1,120 kW and a capacity of 65 tph (Table 2.2).

Table 2.2. The specifications and the applications of Tower mill [15]

Type Power (kW)

Total Height (m)

Mill Diameter (m) Applications

ETM-20 22 5.6 0.7 Zinc ore/Slaked lime residue

ETM-50 55 6.9 1 Zinc oxide/Slaked lime

ETM-300 280 10.8 1.8 Limestone/Coaks/Zircon Sand

ETM-500 370 12.5 2.4 Limestone/Lead Zinc conc.

ETM-1500 1120 14.9 3.3 Limestone

SALA Agitated Mill (SAM) from Allis Mineral Systems GmbH (Figure 2.8) was

developed in 1986 to provide energy efficient grinding operation while producing

ultrafine particles. It was used both in wet and dry grinding applications. The mill

was limited with a maximum feed size of 0.5 mm so it was particularly preferred in

dry secondary grinding of high-strength cements, pigments and fillers down to P80

of 5 µm [16]. The SALA mill with motor power ranging from 7.5 to 200 kW and 50

tph maximum throughput was available for industrial use [16].

15

Figure 2.8. SALA agitated mill [16]

Metso Company [17] manufactured two stirred media mill models named as,

Vertimill® and SMD® (Figure 2.9). In the market Vertimills® with motor capacities

15 to 2240 kW and throughput rates up to 500 tph can be found and they are

capable of grinding material top size of 6 mm down to 20 µm [18].

Figure 2.9. The internal structure of Vertimill® (left) and SMD® [17]

16

Vertimills® (Figure 2.10) may be filled with different types of media, i.e., steel,

ceramic or natural pebbles, stirred by overhung double helix screw. The media is

risen within the screw flights and falls downward in the space between the flight

tips and the inside diameters of the mill body. The feed material enters into mill

body via a feed chute, which is placed on the top of the grinding chamber, and

initially subjected to a pre-classification by an uprising velocity provided by an

external recycle pump. In pre-classification process, the fine particles are removed

from inside the mill to prevent overgrinding, while the coarse particles are drawn

into the media and reground. The ground pulp overflows from the mill body and

moves into a splitter box. The splitter box is equipped with a dart valve and control

devices which splits the pulp into two streams; a process stream and a recycle

stream. The recycle stream is controlled to produce an optimum uprising velocity

in the mill body for the specific grinding application. Vertimills® can be operated

both in open and closed circuit configurations [19].

Figure 2.10. Vertimill® layout [19]

Metso company licensed the SMD® technology from ECC International and has

been in use since 1998 for submicron material production (<15 µm). SMD®

technology is able to process feed size up to 100 µm with slurry solid

17

concentration of 20-60% w/w [20]. The maximum installed power reported by the

manufacturers is 1,100 kW and depending on the ore type and the target fineness

specific energy consumptions range between 5-100 kWh/t. SMD® mills consist of

an octagonal body which supports the suspended internal multi-armed impeller.

Feed slurry enters through a port on the top of the detritor. The impellers

thoroughly mix the feed slurry with the media. A predominantly axial flow regime

throughout the grinding charge provides intense interparticle abrasion. This action

utilizes the applied energy and maximizes grinding efficiency. The axial flow within

the charge constantly circulates the particles across the media retention screens.

The milled product discharges through these screens that are located around the

top half of the unit. The launder collects the product as it flows through the screen.

The number of exit screens depends on the grinding requirements and the

required feed flow rate [20].

The MaxxMill® (Figure 2.11) manufactured by Maschinenfabrik Gustav Eirich

GmbH found applications in dry and wet fine grinding. The main internal

components of the mill are; rotating cylindrical grinding pan (1), one or two

eccentric agitators (2) and a static deflector of the material being ground

associated with the feeding tube (3). MaxxMills® may be operated at media fillings

up to 80% with media composition of 2-10 mm [21].

Figure 2.11. Schematic representation of MaxxMill® [21]

Feed material mixed with carrier fluid enters the machine through the pipe. The

fine material is sucked from the upper layer through the product outlet. The

coarser material of classifier together with fresh feed is fed to the mill from the

bottom of the chamber. Usually, the media inside the mill does not leave the

18

chamber, however, a sieve at the top is needed, in particular when processing

high viscous slurries, in order to prevent discharging of the media. MaxxMills® with

capacities ranging from 100 kg/h to 20 tph can be found depending both on the

material properties and the target fineness [3; 22; 23].

2.2. Horizontal Stirred Media Mills

In this section, operating principles and industrial applications of DRAIS mill and

IsaMill® are explained.

DRAIS (Direct Perl Horizontal Stirred Mill) mill was invented by Durr in 1976 [4].

Figure 2.12 illustrates auxiliary components and chamber of DRAIS mill [24].

Figure 2.12. DRAIS mill in wet grinding application [24]

In a DRAIS mill, the agitator having circumferential speed of 4 to 20 m/s is located

at the centre of the grinding chamber where media filling may reach up to 90% of

the effective volume. The water and solid components are fed separately then

conveyed via a screw feeder into the mill chamber. The pulp passing from the

inside of the mill is subjected to the grinding action and the product discharges

from the product outlet. Media types having various densities and diameters are

available for grinding purposes in a DRAIS mill (Table 2.3).

19

Table 2.3. The types of media

Density (g/cm3) Diameter (mm)

Sand 2.8 0.4-0.8

Hard glass 2.9 0.3-3.5

Zirconium oxide 5.4 0.5-3.5

Steel shot 7.6 0.2-1.5

Chrome steel 7.85 1-12

The largest DRAIS mill commissioned had 1000 L volume with motor power of 320

kW and throughput rate of 10 tph. They were employed for limestone, titanium

dioxide, barium sulfate and magnetic oxides grinding applications [4].

The IsaMill® was developed early in 1990s in a partnership between Netzsch-

Feinmahltechnik GmbH and McArthur River mine while aiming to find a solution for

an efficient fine grinding technique that would upgrade the concentrate and make

zinc deposit economical to mine. In order to upgrade the concentrate P80 of 7 µm

was targeted, where conventional grinding machines utilizes high specific energy

[25]. Therefore IsaMill® project was initiated. The chronology of the mill

development is given in Table 2.4.

Table 2.4. The chronology of IsaMill® development

Jan 1992 LME 100 model, powered by a 55 kW motor in pilot plant at Mt Isa

Nov 1992 LME 500 model, powered by 205 kW, then 250 kW at Hilton Concentrator, Mt Isa

Nov 1993 ISA 1500 model powered by a 900 kW motor installed in Lead/Zinc concentrator

Dec 1994 IsaMill M3000 powered by 1.1 mW motor installed in Lead/Zinc concentrator

Dec 2003 IsaMill M10000 powered by 2.6 MW motor installed at Anglo Platinum

IsaMill® is a high speed horizontal mill having high power intensity. The layout of

IsaMill® is illustrated in Figure 2.13. The grinding shell, which can easily be moved

out for maintenance purpose, includes grinding discs mounted on the shaft.

20

Grinding discs agitate the media and slurry mixture continuously. The product

separator lying at the end of the mill has two major effects on grinding which are;

keeping the media inside and producing a narrow sized product due to its

classification effect.

Figure 2.13. IsaMill® layout

The minimum media size to be used in grinding may be down to 0.25 mm for steel

and 0.3-0.42 for ceramic type (Table 2.5). The media filling may reach up to 85%

and stirred at tip speed ranging from 6-22 m/s. These characteristics of IsaMill®

increase the probability of media to particle collision at fine end and enable to

process fine materials with higher throughputs which is essential for the

economics of mineral industry [26].

Table 2.5. Media size for different materials

Zirconium Silicate Media (mm) Steel Media (mm)

0.3-0.42 0.25

0.6-0.8 0.45

0.8-1 1

2

The comparisons regarding the design and operating conditions between the

conventional mills and IsaMill® are given in Table 2.6 and Table 2.7.

21

Table 2.6. Power intensities of different grinding devices [26]

Mill Diameter

(m)

Mill Length

(m)

Installed Power

(kW)

Mill Volume

(m3)

Power Intensity

(kW/m3)

Autogenous Mill 10 4.5 6400 353 18

Ball Mill 5 6.4 2600 126 21

Regrind Ball Mill 3.2 4.8 740 39 19

Tower Mill 2.5 2.5 520 12 42

IsaMill 1.3 3 1120 3 280

Table 2.7. Media size, power intensity and number of grinding balls for different mills

Media Size

(mm) No. Balls /

m3 Surface

Area

Ball Mill 20 95500 120

Tower Mill 12 440000 200

IsaMill 1 1150000000 3600

The agitator design affects the grinding performance of IsaMill® significantly. Up to

date, many types of agitators have been developed. The peg type agitator was

designed by Netzsch Feinmahltechnik and specifically used for grinding and

dispersing of carbon black heat set inks (Figure 2.14).

Figure 2.14. The John mill with short pegs [27]

A John mill [27] is another type of horizontal stirred mills which uses peg type

agitators. In contrast to other stirred mill geometries the grinding tank of John Mill

22

has counter pegs attached to it. As the media is accelerated by the pegs on the

shaft, it streams past the counter peg on the chamber wall creating extremely high

shearing and impact forces to tear the agglomerates apart. As the media streams

collide immediately after the peg, impact grinding occurs for particle size reduction.

This type is a proper selection for the operations with very high viscous pulps and

high solid rates. It can be operated up to 90% solid for industrial paint applications

[27].

In addition to peg type agitator, molinex type was also introduced into stirred

milling operations (Figure 2.15). Molinex discs are cast with half-moon shaped

cavity. The action of the discs is to accelerate the media centripetally from the

inner edge of the discs and centrifugally from the outer diameter of the disc. This

creates shearing and impact forces between the disc and the media next to the

disc. Molinex systems’ design is well adapted for ceramic applications or

processes requiring minimum contamination. In a typical grinding operation, solid

concentration ranges from 5 to 85% [27].

Figure 2.15. Molinex type agitator [27]

2.3. Comparison between Vertical and Horizontal Stirred Mills

As can be understood from Section 2.1 and Section 2.2, the main difference

between the vertical and horizontal structures comes from the operational

variations. Horizontal stirred mills can be operated at higher agitator speeds (6-22

m/s) and at lower media size (0.25 mm). These characteristics of horizontal mills

make them more energy efficient over the vertical configuration [26; 28]. In

addition to energy efficiency, the scaling-up of the mills is also an important

parameter for selecting the suitable configuration. The vertical stirred mills have

23

scale-up problems because of the start-up torque [26]. Manufacturers pointed out

that mechanical design of vertical mill was dominated by start-up torque on the

bottom stirrer after a shut-down. This dominated the design of stirrer and shaft. On

the other hand, in horizontal mills, many stirrers are available to stir the settled

load thus scale-up procedure was easier compared to vertical mills. Another

difference between the horizontal and the vertical configuration is the shape of the

product size distributions. It has been shown that the Tower Mill® produces

product with wider size distribution compared to IsaMill® at the same target size.

Because IsaMill® grinds coarser particles selectively and produces minimum

amount of fines resulting in producing product with narrow size distribution. This

behaviour of the mill provides better liberation of the valuable minerals and

maximizes the flotation recovery since the flotation feed is more uniform [26].

In the literature, the grinding performances of vertical and horizontal configurations

were compared. In one of the case studies, grinding performance of 4 L IsaMill®

and 40 L Tower Mill® were evaluated. IsaMill® was operated with 3.5 mm ceramic

media and the Tower Mill® was operated with 12 mm steel media. Each of the test

works was performed with feed material having 50% solid content [28]. Figure 2.16

illustrates the relationship between the specific energy consumption and P80 sizes

obtained from both mills. As can be seen from Figure 2.16, IsaMill® was able to

grind feed material down to 13 µm where the minimum product size that Tower

Mill® produced was 31 µm. Besides, IsaMill® utilized less energy in contrast to

Tower Milling® in particular below 65 µm range.

24

Figure 2.16. IsaMill® vs Tower Mill® in magnetite grinding [28]

The grinding performances of IsaMill® and Tower Mill® were also compared in lead

and zinc regrinding circuits. The regrind circuit shown in Figure 2.17 was part of

bulk lead/zinc retreatment circuit where the Tower Mill® was operated and there

were some over-grinding problems for galena-bearing minerals. Because of the

over-grinding problems, the circuit was modified and the IsaMill® followed by

Jameson Cell was commissioned (Figure 2.18). With the new configuration, the

overall lead recovery was increased by 5% [26].

Figure 2.17. Tower Mill® circuit configuration [26]

25

Figure 2.18. Modified lead regrind circuit [26]

In another case study, some laboratory tests were performed for a lead/zinc mine

demanding an increase in the overall throughput of the circuit (Figure 2.19) by

replacing the Tower Mill® technology with IsaMill® [29]. Preliminary laboratory tests

with 20 L IsaMill® implied that IsaMill® utilized less energy compared to the Tower

Milling (Figure 2.20). As can be seen from Figure 2.20, IsaMill® was able to

produce P80 of 45 µm to 50 µm, where the Tower Mill® produced P80 of 100 µm at

the same energy level.

Figure 2.19. Lead/zinc mine grinding section [29]

26

Figure 2.20. Energy-size reduction comparison between Tower Mill® and IsaMill®

[29]

2.4. Operating Parameters and Their Influences on Grinding Performance of Stirred Mill

Grinding in a stirred media mill is influenced by many parameters;

Stirrer type, speed

Media Size and density

Media filling

Throughput rate

Mill geometry

These parameters mainly affect the specific energy consumption of the mill that

ultimately influences the grinding performance [4; 11; 30; 31; 32].

In addition to design and operating conditions of the mill, slurry rheology should

also be taken into consideration while evaluating milling performance. In wet or dry

grinding processes, the slurry rheology can be modified by adding grinding

chemicals (grinding aid, dispersant) that affect the grinding operation directly.

2.4.1. Stirrer Speed

Stirrer speed mainly increases the probability of media to particle collision by

creating high energy intensity environment. It has been proved by many studies

that increasing stirrer speed increases the energy consumption and produces finer

product [3; 8; 32; 33; 34; 35; 36].

Sadler III et al. [36] in their study performed several grinding tests with batch

operated attrition mill at different stirrer speeds and measured the loss in weight of

27

(-400+350) µm fraction at different time intervals (Figure 2.21). They reported that

the higher the stirrer speed, the higher the loss in weight of fraction.

Figure 2.21. Effect of stirrer speed [36]

Fadhel and Frances [34] performed test works with horizontal Drais-Perl Mill and

plotted the grinding results at different stirrer speeds as a function of time of

grinding. As seen from Figure 2.22, increasing stirrer speed decreases the d50 of

the product.

Figure 2.22. Effect of stirrer speed [34]

Jankovic [8] from his studies on vertical stirred mills concluded that stirrer speed

and product size were inversely proportional to each other (Figure 2.23a) and also

showed that the use of smaller size media reduced the effect of stirrer speed

(Figure 2.23b).

28

Figure 2.23. Effect of stirrer speed (a) 1.7-1.2 mm media (b) 0.85-0.6 mm media [8]

2.4.2. Media Size and Density

Proper media size selection improves the efficiency of grinding process. In a

stirred mill operation, the use of smaller size media has two major effects on

grinding process which are; decreases the energy consumption due to the fluidity

of the bulk media and produces finer product [32; 37]. However, there is a lower

limit of media size that it becomes too small to grind the particles effectively.

Mankosa et al. [38] performed several test studies to investigate the influences of

media size on coal grinding. The results showed that the product size distribution

became finer and less energy was utilized when media size got finer. Fadhel and

Frances [34], Jankovic [8], Kwade et al. [39], Mende et al. [40], Persson and

Forssberg [41], Schollbach [42], Wang and Forssberg [43] also reported similar

results in their studies.

Figure 2.24. Effect of media size [38]

29

The grinding performance of media depends both on feed size and target size of

the product. Fadhel and Frances [34] in batch grinding tests proved that smaller

size media should have been preferred as the feed size to the mill got finer (Figure

2.25). They applied short and long grinding times and observed the changes in d50

at varying media size. Figure 2.25a illustrates test results obtained at short

grinding times. In this range (2.2-1.8) mm media size was found more effective

when compared to the other media size classes. This observation is the opposite

of what was seen with the finest media size (Figure 2.25b, 0.56-0.4 mm). It is

understood from the test works that there is an optimum ratio of Dball /Dparticle and

this has been a subject of many researchers.

Figure 2.25. Effect of media size (a) for short grinding times, (b) for longer grinding times [34].

Mankosa et al. [38] studied on determining the optimum ratio of Dball /Dparticle by

investigating the breakage rate of the grinding action (Figure 2.26). The studies

showed that breakage rate increased with increasing Dball/Dparticle ratio up to a point

that (20:1) media became too small to nip the particles. Zheng et al. [32] in their

study determined the optimum ratio of Dball/Dparticle as 12:1 to obtain efficient

grinding performance.

30

Figure 2.26. Effect of media size on breakage rate [38]

In stirred milling, major part of the energy is utilized in stirring and lifting the ball

charge. Therefore the change in media density influences the energy consumption

directly. In general, the use of denser media results in consuming more energy

compared to the lighter one [44]. Grinding media used for stirred milling can be

made up of various materials such as steel, slag, glass, ceramic etc. Zheng et al.

[32] investigated the operational differences between the steel and glass balls and

concluded that the use of steel balls resulted in obtaining better grinding

performance but consumed almost double of the energy utilized by the glass

beads. Mankosa et al. [38] also compared the performances of steel and glass

balls and found that the steel balls produced finer material. The benefits of using

higher density media were also reported by Farber et al. [37].

2.4.3. Media Filling

Media filling is a parameter affecting ball to material amount ratio, thus the product

fineness as well. Besides, the main effect of media filling is observed on the power

draw and the specific energy consumption. Generally, the mills are recommended

to be operated at maximum media fillings due to the improved grinding

performances. In order to prove the benefits of higher media fillings, Sadler III et

al. [36] changed the mill load gradually and measured the loss in each size fraction

for each case. They concluded that better grinding performances were achieved

when the mill was operated at higher media fillings (Figure 2.27). Similar results

were also reported by Persson and Forssberg [41] and Sivamohan and Vachot

[45].

31

Figure 2.27. Effect of mill charge on grinding performance [36]

2.4.4. Feed Rate

In any kind of grinding operation, specific energy is one of the most important

parameters related to the product fineness. The specific energy is a function of

feed rate, thus there is a direct correlation between the feed rate and the product

fineness as well. At the same media filling, it is expected that, decreasing the feed

rate increases the surface area of the product. Wang et al. [3] in their dry MaxxMill

studies (Figure 2.28) and Pilevneli et al. [35] in cement grinding tests investigated

the effects of feed rate on grinding performance and they concluded that feed rate

was inversely proportional to the product size.

Figure 2.28. Effect of feed rate on surface area of the product [3]

2.4.5. Mill Geometry

In a grinding process, the environment where grinding action takes place

influences the performance directly. In the literature some research studies

investigating the effects of mill geometry on stirred media mill performance are

reported.

32

Stender et al. [46] performed grinding tests at different mill geometries and media

sizes. The studies showed that the improved grinding performance was achieved

with the smallest grinding chamber (0.73 L) filled with fine media (355 µm). Zheng

et al. [32] in their study investigated the effects mill geometry on both torque (N.m)

of the stirrer and the product fineness (Figure 2.29). They performed various test

studies with different stirrer diameters (D) and mill diameters (T). It was revealed

that the diameter of stirrer affected the torque directly (Figure 2.29a). It was also

found that, reducing D/T ratio of mill resulted in obtaining coarser product (Figure

2.29b).

Figure 2.29. Effect of mill geometry on (a) the torque (b) the product fineness [32]

2.4.6. Rheology of the material (Grinding aids)

Grinding operation produces newly formed surfaces which are charged electrically.

Because of the electrical charges, the particles tend to agglomerate or coat on the

grinding media that lowers the efficiency of grinding process. Nowadays the

grinding chemicals have been widely used in various wet and dry grinding

applications to prevent agglomeration and coating on the media. Main function of

them is to neutralize the surface charges of particles [47; 48]. It was indicated by

some of the studies that the use of chemicals increased the overall throughput of

the process for a given product size by improving efficiency of the grinding and

classification operation [47].

Since the stirred media mills are used in ultrafine grinding applications, grinding

chemicals or surface modifiers are needed to carry out efficient grinding process.

In the literature, many studies explaining the effects of chemical on stirred milling

are reported. Kapur et al. [49] investigated the effects of chemicals on stirred

a

b

33

milling operation and concluded that the viscosity of the slurry was reduced

significantly with the use of chemicals. Zheng et al. [50] tested various types of

chemicals in limestone grinding with a stirred media mill. They concluded at the

same operating conditions that the surface area of the product could be doubled

with the use of proper chemical type. Choi et al. [51] in their stirred mill test works

showed that it was possible to reduce the utilized energy by 37% with the use of

grinding chemicals.

The operating and design parameters presented above mainly affect the specific

energy consumption of the grinding operation by influencing the motion of media

and suspension mixture inside the mill. In order to reveal the flow field

characteristics of the media and suspension, many studies have been performed

so far. In the following sections the theory behind the motion of media and

suspension in a stirred mill is presented.

2.5. Motion of Suspension and Single Grinding Media

2.5.1. Motion of Suspension

Blecher et al. [52] investigated the flow fields of fluid around an agitator under

laminar flow conditions (10≤Re≤2000). Figure 2.30 illustrates the results of the

calculations done in the non-dimensional r-z plane. The axis on the left lies to the

agitator shaft where the right axis is limited by the grinding wall. The bottom and

top boundaries of the figure represent the middle of the agitator disc and between

two discs respectively. As illustrated in Figure 2.30, with increasing radial distance

to the shaft, a velocity profile develops with decreasing values in z-direction. The

highest velocity gradients are found at the disc surface. When the profile on r-

direction is investigated, it is seen that, maximum speed occur at the outer radius

of the disc.

34

Figure 2.30. Flow field of fluid at Re=2000 [52]

In Figure 2.31, the streamlines investigating the motion of fluid is presented. It is

indicated that the fluid, which is under the influence of disc movement, is initially

directed towards the chamber wall then is diverted to the direction of the top

symmetry and finally flows back to the starting point. During the circulation of the

fluid, high gradient zones are observed at the disc surface, the chamber wall and

the gap between disc and wall.

Figure 2.31. Streamlines of the stirred fluid [52]

Theuerkauf and Schwedes [53] also investigated the flow field of fluid. In contrast

to studies conducted by Blecher et al. [52], the analysis were performed under

turbulent flow conditions. In Figure 2.32, the circumferential velocity gradients

calculated between two stirrer discs are illustrated. The solid line represents the

velocity of fluid. As seen from Figure 2.32, the velocity increases linearly until

R/Rtip=1. Then, a sudden decrease is observed and finally increases slightly again

35

at the zone of chamber wall due to the recirculating fluid between the wall and

stirrer.

Figure 2.32. Circumferential fluid velocity distributions at Re=16000 [53]

In Figure 2.33, radial and axial flow fields are given. The results imply that there

are mainly two high energy zones which are; around disc surface and at the

chamber wall. The studies also indicate that, within these zones, the velocity of

fluid does not exceed 24% of the stirrer speed.

Figure 2.33. Radial-axial fluid velocity distribution at Re=16000 [53]

2.5.2. Motion of Single Grinding Media

In Figure 2.34, the study performed by Blecher et al. [52] investigating the motion

of different size of single media (Rball/Rattritor diameter) is illustrated. The studies

showed that the small size beads (Rb/Rd=1/240) followed a trajectory with high

energy zones in contrast to larger size beads. In Figure 2.35, the particle

trajectories as a function of Re number are illustrated. It is concluded that a

grinding media passes through the high energy zones (chamber wall and disc

surface) when Re number is between 800 and 2000.

36

Figure 2.34. Motion of a single media [52]

Figure 2.35. Particle trajectory as a function of Re [52]

Theuerkauf and Schwedes [53] also investigated motion of a single grinding media

(Figure 2.36). As seen from the figure, velocity of a grinding media increases with

increasing R/Rtip and reaches to its maximum value at R/Rtip =1 then it decreases

towards the chamber wall.

Figure 2.36. Circumferential grinding bead velocity distributions [53]

37

Eskin et al. [54] studied on analysing motion of a grinding media having different

densities with the aid of numerical calculations. It was concluded that the denser

media accumulated near the chamber wall while the lighter ones accumulated at

the centre of the chamber.

The motion analysis of media and suspension mixture emphasize the importance

of high energy zones. It is thought that better grinding performances are achieved

when most of the media passes through these zones. Blecher et al. [52] defined a

parameter called “motion index” that includes Reynold’s number, the size of media

and stirrer, the density of media and fluid parameters. They reported that for small

values of motion index, improved grinding performance was achieved owing to

having homogenously distributed media along the mill chamber. Conversely, it

was indicated that higher motion index values resulted in obtaining deteriorated

grinding performance.

2.6. Modelling Studies

For a better understanding of a grinding process it is useful to develop models

based on the flow characteristics of media and fluid. These models may then be

used to improve efficiency of grinding process. For this purpose, stressing models,

DEM (Discrete element method), CFD (Computational fluid dynamics) and PEPT

(Positron emission particle tracking) techniques have been developed to date.

2.6.1. DEM Models

Jayasundara et al. [55] had performed research studies on DEM modelling of

stirred mills. They characterized the particle flow in terms of microdynamic

variables (porosity, collision energy and collision frequency) and presented the

results as shown in Figure 2.37. Figure 2.37 indicates that at lower media fillings

and stirrer speeds, the particles are accumulated at the bottom of the chamber

therefore inefficient grinding environment is provided. On the other hand,

increasing either solid concentration or tip speed agitates the particles more

vigorously thus increases the probability of collision. Jayasundara et al. [55] also

examined the flow pattern of the particles and found that the particles near the

discs moved much faster than those around the chamber wall.

38

Figure 2.37. Axial view of mill chamber for different filling (Ø) and stirrer speed (s) a) Ø=40% s=300rpm b) Ø=60% s=300rpm c) Ø=60% s=800rpm [55]

Yang et al. [56] investigated the influences of operational conditions on the flow of

grinding media with the aid of DEM technique. The results showed that;

The lower media fillings resulted in accumulation of the media at the bottom

of the chamber.

Increasing the media filling increased flow velocity.

There was an optimum stirrer speed as further increase did not change the

flow velocity and energy dissipation.

Both tip speed and media filling had influence on power draw of the mill.

2.6.2. PEPT Technique

Positron Emission Particle Tracking (PEPT) method was developed to trace the

motion of a media in an IsaMill [57]. In this technique, glass or ceramic tracers are

made radioactive through bombardment in a cyclotron (Figure 2.38). Depending

on the activity of the tracer, stream lines are recorded every second and tracer

position is determined using triangulation routine [57].

PEPT technique was developed as an alternative method to DEM and in the

literature, results from comparison made between CFD, DEM and PEPT outputs

can be found [58].

39

Figure 2.38. PEPT camera [57]

2.6.3. Stressing Models

In stirred media mill, a grinding operation takes place between two grinding media

where the particles are stressed. However, the particles are stressed when they

are captured by the media and not carried by the fluid. Below, three cases of

particle stressing are listed;

Single particle stressing.

More than one particle is captured and all of them are stressed. In this case

the largest particle is subjected to the maximum stress.

A particle bed is captured and stressed.

Kwade [59] stated that the number of particles captured between two media was

function of solid concentration and particle size (x) and determined by ratio of

diameter of active volume between two grinding media (dact) (Figure 2.39) and

average distance between two particles in suspension.

Figure 2.39. Active volume [59]

40

In stressing models of stirred mill, two parameters come forward which are; the

intensity of stress and the number of stress events. In the next section

mathematical expressions of these parameters are presented.

2.6.3.1. Estimation of Stress Intensity

Kwade and Stender [60] reported that the stress intensity parameter was

influenced by the motion of grinding media which directly influences the grinding

performance of the mill. As explained in motion analysis section, grinding action

mainly takes place around stirrers and in the zone of chamber wall due to the

centrifugal action created by stirrer. As a result of this movement, media gain

kinetic energy and the theory relates stress intensity with the kinetic energy.

Kwade et al. [39] calculated stress intensity parameter as function of

circumferential speed of the discs, size and density of the media (Equation 2.1).

𝑆𝐼 = 𝐷𝑏3 ∗ (𝜌𝑏 − 𝜌) ∗ 𝑣𝑑

2 (2.1)

Where;

Db (m) : Size of the media

Vd (m/s) : Circumferential speed of the discs

ρb (t/m3) : Density of the media

ρ : Density of the material

In order to reveal the effects of stress intensity on grinding performance, Kwade et

al. [39] performed several studies with limestone at different stress intensity (SI)

and specific energy inputs. In Figure 2.40, change in median particle size as a

function of different operating conditions is presented. As seen from the figure, no

matter how much energy is applied, the trend of the curve does not change. The

median size decreases with increasing SI and reaches to a minimum point at a

certain level. Beyond this point the material starts to get coarser due to high

energy loses, therefore it is at the optimum value of stress intensity that efficient

operation is performed [46; 59; 61; 62; 63].

41

Figure 2.40. Product fineness as a function of stress intensity and specific energy [39]

2.6.3.2. Estimation of Number of Stress Events

Kwade [62] estimated number of stress events (Equation 2.2) as a function of

number of media contacts (Nc), the probability that a particle is caught and

stressed (Ps) and the number of product particles inside the mill (Np). Then the

mathematical expressions of each parameter were revealed.

𝑆𝑁 =𝑁𝑐𝑃𝑠

𝑁𝑝 (2.2)

The number of media contacts (Equation 2.3) is proportional to the number of

revolutions of the stirrer and number of grinding media in the chamber (NGM).

𝑁𝑐 ∝ 𝑛𝑡𝑁𝐺𝑀 ∝ 𝑛𝑡𝑉𝐺𝐶∅𝐺𝑀(1−𝜀)

𝜋

6𝑑𝐺𝑀3 (2.3)

Where;

n (s-1) : the number of revolutions of the stirrer per unit time

t (s) : the milling time

42

VGC (m3) : the volume of the grinding chamber

ØGM : the filling ratio of the grinding media

Ε : the porosity of the bulk of grinding media

dGM (m) : the diameter of the grinding media.

The probability of a particle to be caught and sufficiently stressed by grinding

media (Ps) depends both on material and the type of grinding process. In case of

grinding crystalline materials [62] the probability is proportional to the active

volume between two grinding media which is affected by the diameters of them

(Equation 2.4).

𝑃𝑠 ∝ 𝑑𝐺𝑀 (2.4)

As given in Equation 2.5, the number of product particles inside the mill (Np) is

proportional to the overall volume of them that is expressed as;

𝑁𝑃 ∝ 𝑉𝑃 = 𝑉𝐺𝐶(1 − ∅𝐺𝑀(1 − 𝜀))𝑐𝑉 (2.5)

When the equations of each parameter are put into Equation 2.2, Equation 2.6 is

obtained;

𝑆𝑁 ∝ 𝑛. 𝑡.∅𝐺𝑀(1−𝜀)

(1−∅𝐺𝑀(1−𝜀))𝑐𝑉

𝑥3

𝑑𝐺𝑀2 (2.6)

Where;

x : mean product size

43

In stirred milling, energy consumption of grinding operation correlates with stress

intensity and the number of stress events parameters (Equation 2.7). Kwade and

Stender [60] stated that constant grinding result, which is beneficial for scaling-up

of the mill, was achievable if the two of these parameters were kept constant.

Relations between these parameters are illustrated in Figure 2.41 and Figure 2.42.

𝐸 ∝ 𝑆𝐼. 𝑆𝑁 (2.7)

Figure 2.41 shows stress intensity-dependent change in energy utilization. The

points of signature plots produced at different stress intensity and energy levels for

a given median particle size (2 µm) shows that energy consumption decreases

until a certain value of stress intensity then it starts to increase [62]. The minimum

value of the curve is called as the optimum energy requirement for a specified

product size. Figure 2.42 illustrates the correlation between stress intensity and

stress number parameters. It indicates that these two parameters are inversely

proportional to each other. At very small stress intensities the trend tends to go

infinity indicating no evident comminution takes place.

Figure 2.41. Relation between stress intensity and specific energy [62]

44

Figure 2.42. Relation between stress intensity and stress number [62]

2.7. Scale-up of Stirred Mills

Scale-up of stirred mills is basically done with the data produced from grinding

tests performed with a small scale mill. As a result of the tests, calculated net

specific energy consumption is plotted against product size thus the material is

characterized in terms of energy-size reduction relationship. In the literature some

studies have been performed to compare the grinding results obtained from small

scale mill and the industrial one [64]. In Figure 2.43, grinding results of 4 L and

4000 L mill are given. It is understood that energy and size reduction relation of

the two mills are in the same trend. Consequently, scale-up of the mill is done from

small scale without any problem.

Figure 2.43. Energy consumption and P80 relation [64]

45

Curry et al. [65] also indicated that, scale-up procedure was applied successfully

from 4 L to 1000 L mill. Karbstein et al. [66] conducted several test works at

different mill chamber volumes and found that the grinding performances of

different mills were similar to each other (Figure 2.44). However the study reported

that, mill chamber volume should have been at least 1 L in order to produce

consistent results.

Figure 2.44. Grinding performance of different mill volumes [66]

46

3. EXPERIMENTAL STUDIES & INITIAL TESTWORKS

3.1. Description of the Experimental Apparatus

The experimental apparatus was the dry horizontal stirred mill. The dry horizontal

stirred mill used in this study was developed in a partnership with Netzsch-

Feinmahltechnik GmbH. Netzsch-Feinmahltechnik is the manufacturer of the

IsaMill® for wet grinding that had been applied in the fine grinding of platinum,

copper, zinc ores et cetera. Netzsch-Feinmahltechnik GmbH manufactures both

vertical and horizontal stirred mills, such as IsaMill®, however in this study

horizontal configuration was preferred due to its supposed operational benefits

explained in Section 2.3.

The structure of the dry machine is similar to the IsaMill®. Photographs of the key

components of the dry horizontal stirred mill used in the experiments are given in

Figure 3.1. A schematic of the equipment is provided in Figure 3.2. This is a novel

comminution device designed to produce fine particles from the feed in a dry

grinding environment. The key components for the operation of the mill are; control

panel, mill chamber, stirrer type and product separator. In this study, two types of

mill chambers having different volumes were used (Table 3.1). As given in Table

3.1, only the 23 L mill was built with a water jacket.

Figure 3.1. 23 L (on the left) and 42 L Mill (1-feed hopper, 2-control panel, 3-grinding chamber, 4-product outlet)

1 2

3

4

47

Figure 3.2. Simplified drawing of the dry stirred mill

Table 3.1. Technical specifications of the two chambers

42 L Mill 23 L Mill

D (cm) 29.7 23.7

L (cm) 74 75

Di (cm) 26.4 20.4

V (L) 42 23

Water Jacket X

Where;

D : Diameter of the mill chamber

L : Length of the mill chamber

Di : Effective Diameter of the mill chamber

V : Volume of the mill

Operating parameters of the mill, e.g. rotor speed and feed rate, are adjusted via

frequency converter mounted on the control panel. In operation, the material is fed

into the mill chamber via the feed hopper. Then, it is subjected to the grinding

action in grinding chamber. Finally, the product comes out from the product outlet

(Figure 3.1). In order to improve material transportation, air can be introduced from

the feed inlet when necessary (Figure 3.2). The benefits of using air will be

explained in details in Section 3.3. The equipment specifications, which include

motor power, allowable stirrer speed, maximum feed rate and air flow rate, of the

mill are given in Table 3.2.

Feed

Product Air

Stirrer Shaft

48

Table 3.2. Equipment specifications of the dry stirred mill for both chambers

Motor Power (kW) 18

Maximum Feed Rate (kg/h) 400

Stirrer Speed (m/s) 1.08 - 9.76

Maximum Air Flow rate (L/h) 1000

3.1.1. Internal Structure of Dry Horizontal Stirred Mill

A stirred mill, internally, is composed of a shaft on which the stirrers are mounted

and a product separator that lies at the end of the mill in order to keep the media

inside and discharge only the ground material. In this study, three types of stirrers

were used. Photographs of the stirrer types and product separator used in this

study are given in Figure 3.3 up to Figure 3.7.

Figure 3.3. The stirrer types 1-wing, 2-disc, 3-cross

Figure 3.4. Stirred mill with wing type stirrer

1

Discharge end

2

Wing Type Stirrer

3

49

Figure 3.5. Stirred mill with disc type stirrer

Figure 3.6. Stirred mill with cross type stirrer

Figure 3.7. Product separator

Discharge end

Disc Type Stirrer

Cross Type Stirrer

50

3.1.2. Power Draw Measurements

The literature reports that, power drawn by the stirred mill is influenced by media

density, media filling, media size, stirrer type, stirrer speed, grinding chemical

amount, pulp density and volume of the mill chamber parameters [67; 68; 69; 70].

In this section, power draw of the dry stirred mill was measured under different

conditions to investigate the effects of operating parameters.

The measured power draw of the mill is defined as gross power and it comprises

no load power (without media and material) together with the power consumed to

lift the media, material mixture. No load power is function of internal components’

specifications (shaft, stirrer type etc.). Therefore, a change in mill design affects

the no-load power directly. In order to have robust scaling-up operation, this no

load power is subtracted from gross power then a relationship between the net

specific energy consumption and size reduction (Equation 3.1, Equation 3.2) can

be evaluated. Such a finding is then used to calculate what size of mill motor is

required for a given size reduction at different throughputs.

Net Power Consumption (kW) = Gross Power-No Load Power (3.1)

Net Specific Energy Consumption (kWh/t) = Net Power/Throughput Rate (3.2)

The no load power measured at different type of stirrers are illustrated in Figure

3.8. No significant differences were observed between the stirrers tested at low

stirrer speeds. However above the speed of 6 m/s, disc type stirrer seems

advantageous as it draws slightly less power compared to the other types.

Therefore, the rest of the power measurements were performed with disc type

stirrer and the results are shown graphically in Figure 3.9 and Figure 3.10.

51

Figure 3.8. The influences of stirrer type on no load power draw

Figure 3.9 illustrates power draw measurements at different media fillings. The

following observations can be drawn:

Stirrer speed is directly proportional to power draw

As the media size decreases, less power is drawn by the mill for all

volumetric filling tested.

Figure 3.9. The influence of media size on power draw

52

When the measurements given in Figure 3.9 are classified for each media size,

Figure 3.10 is obtained. As seen from the figure, both media filling and stirrer

speed parameters are directly proportional to the power draw of the mill.

Figure 3.10. The influences of stirrer speed and media filling on power draw

3.2. Sampling & Material Characterization Studies

In order to carry out grinding tests with dry stirred mill, considerable amount of

sample was needed that could be collected directly from a cement grinding circuit.

In this context, a closed circuit grinding system was chosen (Figure 3.11) and

samples were collected with the aid of apparatus (Figure 3.12) prepared for

separator reject, final product and mill filter return streams. In this way, the mill

performance at varying feed size distribution was investigated as well. Sampling

studies were performed during CEM I 42.5R type cement and around 5 tons of

material was collected from each of the streams.

53

Figure 3.11. The flow sheet of the sampled cement grinding circuit

Figure 3.12. View of the sampler tool mounted on each stream

The typical size distributions and mean sizes of each stream are given in Figure

3.13 and Table 3.3 respectively. Additionally, the chemical compositions are given

in Table 3.4.

Figure 3.13. Typical size distributions of the sampled streams

54

Table 3.3. Mean size of each stream

d50 (µm)

Final Product 15

Mill Filter Return 25

Separator Reject 66

Table 3.4. The chemical composition of the sampled streams

Final Product Filter Return Separator Reject

CaO % 63.24 63.16 63.85

SiO2 % 19.65 20.10 20.61

Al2O3 % 4.91 5.70 5.88

Fe2O3 % 3.35 3.39 3.48

MgO % 2.00 2.31 2.27

SO3 % 3.04 2.21 1.62

K2O % 0.74 0.85 0.69

Na2O % 0.49 0.38 0.27

LOI % 2.62 1.90 1.29

Dry stirred mill test works commenced with the adjustment of operating conditions

evaluating the stirred mill performance and followed by sampling campaign around

the mill. Prior to sampling studies, steady state conditions of the mill were provided

that is, minimum fluctuations were observed in power draw and product tonnage.

Typical trends of the mill parameters until reaching the equilibrium stage is

illustrated in Figure 3.14.

Figure 3.14. The trends of the mill parameters until steady state condition

55

During stable state condition, the feed and product streams were sampled then the

mill was crash stopped and the chamber was removed to weigh the material inside

of the mill. This measurement was used to calculate the material load parameter

(Equation 3.3). This parameter explains how much of the interstitial volume in the

charge is filled with material. When this volume is totally filled with material, the

value equals to 100%.

100*)m/kg(VolumeBulk*4.0*%LoadBall*)m(VolumeMill

)kg(MilltheInsideAmountMaterial=LoadMaterial

33 (3.3)

In this study, the material load varied between 100-105% at maximum media filling

(60%) and at maximum feed rate (400 kg/h). Therefore the values over 105% can

be stated as the overloaded milling conditions.

Collected samples were then subjected to characterization studies. In this context,

the size distribution and Blaine surface area of the samples were determined.

Laser sizing measurements were accomplished with Sympatek (Figure 3.15) at

Hacettepe University Mining Engineering Department and Blaine tests were

performed with Atom Teknik device (Figure 3.16) at SET Italcementi Ankara

Cement Plant. Moreover, in some of the studies cement properties (Section 4.1),

i.e., strength and water demand, were determined to evaluate the influences of

grinding on cement properties. These tests were performed at SET Italcementi

Ankara Cement Plant. Following the experimental studies, the size distributions

obtained from each test were used both to calculate the reduction ratio (F50/P50)

and the shape or the slope of the distribution. The shape of the size distribution

directly affects the downstream processes or material properties, i.e., cement

strength, thus it is an important parameter that should be evaluated. In determining

the slope of the size distribution curve, n parameter in RRBS equation [73], was

calculated to find out whether the operating conditions had some effects on the

slope of the size distribution curve. This parameter was estimated by applying

non-linear regression technique where the equation was fitted with the minimum

change in size distribution. In order to show the agreement between the RRBS

equation and the size distributions, the curves given in Figure 3.13 was fitted and

56

the measured and calculated values were graphed for each stream (Figure 3.17).

The R2 of the fitting calculations was 0.998 for each of the cases thus it can be

concluded as RRBS equation can be used in assessing the width of the size

distribution curves.

Figure 3.15. Sympatek laser sizer

Figure 3.16. Atom teknik Blaine measurement device

Figure 3.17. The agreement between the measured and calculated size

distributions

57

3.3. The Observations during Initial Test Works

Since the dry horizontal stirred mill was newly introduced equipment into the

cement grinding area, some grinding problems occurred while conducting initial

test works. The studies performed to sort these problems out provided better

understanding of milling operation and in this section the

solutions/recommendations are explained plainly. As indicated in previous

sections, two different chamber designs were available within the thesis therefore

the solutions were developed for each of the designs. The adjusted milling

conditions throughout the studies are given in Table 3.5.

Table 3.5. The milling conditions of the initial test studies

Media Type Steel

Stirrer Type Disc

Media Size (mm) 4

Stirrer Speed (m/s) 4.34

Media Filling (%) 60

Feed Rate (kg/h) 250

3.3.1. Grinding Problems with 23 L Mill

As indicated in Section 3.1, the 23 L mill was manufactured with the water jacket

surrounding the chamber. Within the study, several test works proved that water

circulation from inside the water jacket improved the efficiency of grinding

operation. In this section, two of the case studies are presented which are;

grinding with d50 of 15 µm and d50 of 25 µm. In the first case study the final product

stream with d50 of 15 µm was tried to be ground through the mill at specified

milling conditions (Table 3.5). Initially, the water was not circulated inside the

jacket to observe what kind of operational difficulties could be encountered.

At start-up, the temperature of the mill chamber was around 25°C. After 15

minutes of grinding time it went up to 78°C. In the meantime, the amount of the

discharged material, in other words production rate of the mill, decreased

considerably although the feed rate of the mill was constant. It was certain that the

rise in temperature caused accumulation of the material inside the chamber and

consequently the production rate of the mill decreased evidently. In the end, power

draw of the mill exceeded 17 kW and got close to its installed power thus the mill

58

stopped itself due to the safety regulations. The change in the temperature and

power draw of the mill are illustrated graphically in Figure 3.18.

Figure 3.18. Graphical representation of the change in operational power and

chamber temperature of 23 L mill

After all, the mill chamber was removed to observe the mill inside, i.e., material

accumulation and coatings on the media, stirrers and chamber (Figure 3.19). As

can be seen from the figure, the material coated on the mill chamber wall and

internal parts reduced efficiency of the grinding operation. Such an inefficient

grinding environment made the transportation of the material through the mill

difficult and finally stopped the grinding operation. All of these symptoms implied

that temperature was an issue needed to be solved as it directly affected material

transportation along the mill.

Figure 3.19. Coatings on the internal parts and mill chamber

Due to the adverse effect of temperature on milling performance, another study

focussed on using water circulation in order to keep the chamber temperature at

constant level that would improve the process and provide a sustainable grinding

operation. In the case of circulating water, the maximum temperature measured on

59

the chamber was 46°C after 26 minutes of grinding time and the grinding operation

was performed without deteriorating the production rate of the mill that is, no

material accumulation inside the chamber was observed.

In the second case, the filter return stream with d50 of 25 µm was tried to be

ground. The similar problems to that of d50 of 15 µm were observed and then

solved in the same manner. All of the studies concluded that heating of the mill

chamber affected the material transportation adversely as it reduced the grinding

efficiency and the use of water jacketed chamber could be one of the solutions

ahead.

3.3.2. Grinding Problems with 42 L Chamber

The previous studies showed that the heating of the mill chamber caused serious

operational problems that should have been overcome to obtain a sustainable

grinding operation. As given previously, the 42 L mill chamber was manufactured

without water jacket. Therefore during the initial test studies, mill chamber

temperature increased from 25°C to 72°C that affected material transportation

along the mill adversely. Consequently, the production rate of the mill decreased in

spite of being operated at the same feed rate. Ultimately, the material accumulated

inside the mill chamber stopped the grinding operation. It was obvious that besides

the water jacketed chamber, another solution was necessary in order to improve

transportation behaviour of the bulk material.

In case of wet milling, water as a fluid carries the ground particles through the

discharge end thus the accumulation of the material is prevented. From this point

of view, either fluidizing the bulk material or the use of fluid was thought to be

beneficial for dry milling. In this context, the effects of grinding chemicals and air

supply were investigated. The grinding chemicals have been used in grinding

operations for many years. In brief, they neutralize the surfaces of the particles

thus the bulk material is fluidized. Their beneficial usage has been proved by many

studies in particular in dry grinding area (Section 2.4.6). Within the scope of the

thesis, the influences of grinding chemicals on grinding performance of the mill

were investigated. The preliminary test studies with chemicals on material with d50

of 25 µm and d50 of 15 µm showed that despite of increased temperature, the

power draw and the production rate of the mill were steady and the temperature

60

reached up to a constant level after 30 minutes of grinding time. The graphical

representation of the results are presented in Figure 3.20.

Figure 3.20. Graphical representation of the change in operational power and

chamber temperature of 42 L mill

For a better understanding of the influences of chemicals, the view of discharge

end section before and after the grinding chemicals are added is illustrated in

Figure 3.21 and Figure 3.22 respectively. As can be seen from the figures, the use

of chemicals reduced the accumulations at discharge end as well as the coatings

on the mill internal parts and on the media. These studies conclude that, it is

beneficial to use chemicals in order to provide a sustainable grinding operation.

Figure 3.21. Mill discharge section before using chemicals

61

Figure 3.22. The effect of chemicals on discharge end section, internal parts and

media

Throughout the initial test studies, there were some milling conditions that the use

of grinding chemicals alone did not provide any benefits regarding improvement in

transportation (Table 3.6) thus grinding operation could not be performed. The use

of fluid, in this case air, inside the mill was another solution developed to improve

transportation when the mill was operated with fine size media (3 mm). For this

purpose, air nozzle was placed to the feed inlet with the maximum flow rate of

1000 L/h. In case of supplying air from the feed inlet, the drawbacks arisen from

the use of fine media were overcome.

Table 3.6. The milling conditions of the test with 3 mm media

Media Type steel

Stirrer Type disc

Grinding Chemical (g/t) 500-1000

Feed Size, F50 (µm) 15.25

Stirrer Speed (m/s) 4.34

Media Filling (%) 60

Media Size (mm) 3

The benefits of using air were also observed when grinding relatively coarser

materials on which the use of grinding chemicals were not effective. The size

distributions and the mean sizes of the coarse materials are presented in Figure

3.23 and Table 3.7 respectively. The problem with the grinding of coarser material

was its interaction with grinding chemicals since no improved effects of chemicals

were observed therefore grinding operation could not be performed. In this case

air supply from the feed inlet improved the process and made it possible to grind

coarse particles as well.

62

Figure 3.23. The feed size distributions of coarse grinding tests

Table 3.7. Mean sizes of the size distributions illustrated in Figure 3.23

d50 (µm)

Slag -1 mm 519

Slag -500 µm 342

Slag -300 µm 194

Separator Reject -300 µm 47

These studies concluded that the cooling of the mill chamber, the use of grinding

chemicals and air were all effective on material transportation along the mill

chamber.

3.4. Reproducibility of the Grinding Results

For any kind of equipment, obtaining reproducible results has crucial importance in

order to carry out the assessments confidently. In this context, grinding

performance of the dry stirred mill was questioned as to whether the same

operating conditions lead obtaining similar grinding results. During the evaluations,

the product size distribution and energy consumption parameters were taken into

consideration. The milling conditions that the assessments were performed are

given in Table 3.8.

63

Table 3.8. Milling conditions of the reproducibility test studies

Mill Volume (L) 42

Media Type Steel

Stirrer Type Disc

Media Size (mm) 4

Media Filling (%) 60

Stirrer Speed (m/s) 5.71

Feed Rate (kg/h) 320

Within the study, totally 3 grinding tests were carried out and the changes in size

distributions and energy consumptions were observed. The results given in Table

3.9 and Figure 3.24 indicate that the mill processing same feed material

characteristics, i.e., size distribution, chemical composition, produces similar

grinding results.

Table 3.9. Energy consumption of each test and mean sizes of products

Power (kW) Specific Energy Cons. (kWh/t) P50 (µm)

Test 1 5.6 17.5 11.89

Test 2 5.6 17.5 11.79

Test 3 5.66 17.7 12.07

Figure 3.24. The size distribution curves of each test

64

4. INFLUENCES of OPERATING and DESIGN PARAMETERS on GRINDING PERFORMANCE

In comminution power draw and energy utilizations are key features that needs to

be evaluated for a given device under different operating conditions. The other

important aspect that determines if the equipment can be successful in operation

is throughput. The objectives of this study that was carried out on the novel

horizontal stirred mill operation were to;

Evaluate the performance of the device in terms of energy utilisation.

Evaluate the influence of key operating and design variables such as, grinding

chemicals, air supply, stirrer speed, media filling, media size, throughput rate,

feed size, stirrer type and mill geometry, on the energy and grind size. Grinding

chemical and air supply mainly affect transportation through the mill and their

optimization is of critical importance (Section 3.3). Therefore, these parameters

were optimized initially and then the rest of the studies (stirrer speed, media

filling etc.) were carried out at optimized conditions. The test works presented

in this section were performed under the conditions given in Table 4.1.

Determine the optimum operating conditions of the device that minimum

energy is consumed for a given grind size.

Table 4.1. The milling conditions adjusted for performance tests

Mill Volume (L) 42

Media Type Steel

Stirrer Type Disc

4.1. The Effects of Grinding Chemicals

Referring to Section 2.4.6, the grinding chemicals neutralize the surface charges

of the particles thus the agglomerations and the coatings on the mill internal parts

are prevented and bulk material becomes fluidized. Ultimately an efficient milling

environment is obtained and grinding performance is improved.

The importance of using chemicals has been revealed in previous section. In

general, these chemicals provide efficient grinding environment. However, their

optimum use is of critical importance since some of the case studies have showed

that excessive use of them could influence the strength performance adversely as

65

a result of the set retarding-effect [71]. It is reported that this behaviour is observed

with some type of cements. Moreover, chemicals bring extra operating cost to the

operation therefore, their dosage and type are needed to be adjusted for the

material processed. In addition to their significant contribution on transportation of

the bulk material, the chemicals may also have an improved effect on strength

development of cement. Up to 10% development in ultimate strength of the

cement is achievable with the use of special chemicals [72].

Within the scope of this study, determining the most suited chemical for dry stirred

milling application was aimed. Three different chemicals varying in chemical

compositions were available and the test studies were carried out at different

chemical dosages to find out the optimum usage that benefitted most to cement

properties and milling performance. These chemicals were manufactured by

Chryso Company coded as EPCT-01, EPCT-02 and EPCT-04 which were

Triisopropanolamine (TIPA)-based, Triethanolamine (TEA)-based and Glycol-

based chemicals respectively. Following the grinding tests, performance of each

chemical was evaluated by considering their contribution to size reduction,

material amount inside the mill, surface area development, strength development

of cement as well as the water demand of cement mortar. The economic facts

should also have been considered while evaluating the performances of the

chemicals however, since they were produced for a special aim, their market

values were not estimated. The milling conditions adjusted during the chemical

tests are presented in Table 4.2.

Table 4.2. The milling conditions adjusted for chemical test

Media Size (mm) 4

Media Filling (%) 60

4.1.1. The tests with EPCT-04 (Glycol-based chemical)

The grinding tests with EPCT-04 were performed at three different chemical

dosages. The experimental conditions and the obtained results are given in Table

4.3. Figure 4.1 and Table 4.4 show the particle size distributions and the changes

in cement properties respectively.

66

Table 4.3. The milling conditions and the obtained results for EPCT-04 tests

The experimental conditions Test 1 Test 2 Test 3

Chemical Dosage (g/t) 1200 1000 700

Stirrer Speed (m/s) 5.42

Mill Power (kW) 5.16 5.25 5.43

Feed Material Temp (°C) 53

Cement Temp (°C) 94

Feed Rate (kg/h) 303

Feed Size F50 (µm) 14.11

Blaine of feed (cm2/g) 3277

n (RRBS slope of feed) 1.00

The experimental results Test 1 Test 2 Test 3

Specific Energy (kWh/t) 17.04 17.32 17.92

Product Size, P50 (µm) 11.33 11.46 11.34

Blaine (cm2/g) 4224 4351 4263

n (RRBS Slope) 1.01 1.01 1.00

Figure 4.1. The size distributions obtained from the EPCT-04 tests

67

Table 4.4. Cement properties of EPCT-04 tests

2 Days Strength

(MPa)

7 Days Strength

(MPa)

28 Days Strength

(MPa) Water Demand %

Feed 29.6 40.8 49.6 28.2

700 g/t 31.0 40.8 52.3 29.2

1000 g/t 31.1 43.1 54.0 29.4

1200 g/t 30.0 41.7 53.1 29.2

The test results obtained from EPCT 04 were evaluated with regarding to changes

in the shape of size distribution curves and size reduction, energy consumptions

and cement properties. Changing the chemical dosage had no influence both on

the shape of product size distributions (no selective grinding of coarser particles

was observed) and size reduction performance of the mill. The main influence of

the chemical was observed on power draw or energy consumption of the mill.

From dosage of 700 g/t to 1000 g/t, energy saving around 3.3% was obtained.

From cement properties point of view, the strength of the cement mortar increased

as a result of the grinding operation. The highest increase was observed at the

dosage of 1000 g/t where the strength was improved by 8.87% with respect to

feed material. However, a further increase in chemical amount (1200 g/t) had an

adverse effect on strength (28 days) as it decreased by 1.7%. Finally, the water

demand of cement mortar increased as a function of grinding action but chemical

dosage had no influence on it.

4.1.2. The tests with EPCT-02 (Triethanolamine (TEA)-based chemical)

Grinding test studies with EPCT-02 were performed at two different dosages

(Table 4.5). The size distribution curves and cement properties of each test are

given in Figure 4.2 and Table 4.6 respectively.

68

Table 4.5. The milling conditions and the obtained results for EPCT-02 tests

The experimental conditions Test 4 Test 5

Chemical Dosage (g/t) 1000 700

Stirrer Speed (m/s) 5.42

Mill Power (kW) 5.9 5.9

Feed Material Temp (°C) 53

Product Temp (°C) 93

Feed Rate (kg/h) 295

Feed Size F50 (µm) 14.11

Blaine of feed (cm2/g) 3277

n (RRBS slope of feed) 1.00

The experimental results Test 4 Test 5

Specific Energy (kWh/t) 19.96 19.75

Product Size, P50 (µm) 11.29 11.08

Blaine (cm2/g) 4451 4213

n (RRBS Slope) 0.98 0.98

Figure 4.2. The size distributions obtained from the EPCT-02 tests

Table 4.6. Cement properties of EPCT-02 tests

2 Days Strength

(MPa)

7 Days Strength

(MPa)

28 Days Strength

(MPa) Water Demand %

Feed 29.6 40.8 49.6 28.2

1000 g/t 33.4 44.8 52.4 28.4

700 g/t 32.4 43.6 52.1 28.4

69

The test results showed that, the feed and product size distribution were parallel to

each other. That is, n parameter was constant for each case. With regards to

cement properties, the strength of the ground material increased by 5.6% (1000

g/t) with respect to the feed material. Changing the chemical dosage did not affect

the size reduction performance of the mill as well as the strength development of

the cement mortar. Besides, water demand of the ground products was affected

slightly.

4.1.3. The tests with EPCT-01 (Triisopropanolamine (TIPA)-based chemical)

The grinding tests with EPCT-01 were performed at three different dosages. The

experimental conditions and the obtained results are given in Table 4.7. Figure 4.3

illustrates the feed and product size distributions and Table 4.8 presents the

strength developments of cement mortar.

Table 4.7. The milling conditions and the obtained results for EPCT-01 tests

The experimental conditions Test 6 Test 7 Test 8

Chemical Dosage (g/t) 1000 700 500

Stirrer Speed (m/s) 5.42

Mill Power (kW) 5.2 5.16 5.43

Feed Material Temp (°C) 53

Product Temp (°C) 92

Feed Rate (kg/h) 308

Feed Size, F50 (µm) 14.11

Blaine of feed (cm2/g) 3277

n (RRBS slope of feed) 1.00

The experimental results Test 6 Test 7 Test 8

Specific Energy (kWh/t) 16.88 16.78 17.63

Product Size, P50 (µm) 10.58 10.54 11.12

Blaine (cm2/g) 4208 4135 4026

n (RRBS Slope) 1.01 0.99 0.99

70

Figure 4.3. The size distributions obtained from the EPCT-01 tests

Table 4.8. The results of strength tests for EPCT-01

The results obtained from EPCT-01 tests showed that, the slope of the feed and

product size distributions were the same for each of the test works. Furthermore, it

was observed that changing chemical dosage had no influence on size reduction

performance of the mill. Despite of obtaining similar size reductions, the power

draw and the specific energy consumption of the mill changed considerably with

increasing chemical dosage. From 500 g/t to 700 g/t, energy saving around 4.8%

was obtained. Regarding to cement properties, the cement strength of each

ground product was increased at different rates with respect to feed material. The

highest increase in ultimate strength (28 days) was obtained with 18.75% at 700

g/t which was considerably higher than 500 g/t with 11.69% and close to 1000 g/t

with 17.94%. The tests with EPCT-01 also showed that the water demand of the

ground material changed slightly compared to feed material.

2 Days Strength (MPa)

7 Days Strength (MPa)

28 Days Strength (MPa)

Water Demand %

Feed 29.6 40.8 49.6 28.2

1000 31.7 46.6 58.5 28.6

700 30.9 46.3 58.9 28.2

500 30.8 44.2 55.4 28.2

71

4.1.4. Comparison of the Chemical Performances

The test studies performed with different types of chemicals indicated that there

were distinctive differences between them regarding to their provided energy

efficiency to the mill and improving cement properties. While comparing chemical

performances, the shape of product size distributions, specific energy

consumption of the mill, product fineness, cement strength development, water

demand of cement mortar parameters were taken into consideration. In Figure 4.4

and Figure 4.5, chemical performances at the same dosages are compared

graphically.

Figure 4.4. Performance assessments of the chemicals at 1000g/t

Figure 4.5. Performance assessments of the chemicals at 700g/t

As can be understood from the figures;

Regarding to the mill performance, no evident difference is observed

between 700 g/t and 1000 g/t for each of the chemicals. The specific energy

consumptions and product fineness are close to each other.

The chemicals have no effect on the shape of product size distributions

since calculated n parameters (Eq. 1.1) are the same for all.

72

EPCT-01 contributes more to energy saving operation of the mill compared

to EPCT-04 and EPCT-02.

When the strength development with respect to feed material are taken into

consideration, EPCT-01 has the highest rate with 17.94% at 1000 g/t and

18.75% at 700 g/t.

Finally, EPCT-02 changes the water demand of the cement mortar slightly

while EPCT-01 does not at 500 g/t and 700 g/t.

These assessments on chemical performances lead to the selection of EPCT-01

(TIPA-based chemical) type grinding chemical for dry horizontal stirred mill

application owing to its improving effects on energy efficiency of the mill and

cement properties. In the next study, the influences of different chemical dosages

on mill inside material amount were investigated with EPCT-01. In this context,

four test studies were arranged at varying dosages (0-1000 g/t) and the mill was

crash stopped at each case soon after the steady state conditions were provided.

Finally mill chamber was removed to weigh mill inside material. In this study, the

material with d50 of 20 µm was subjected to grinding tests and cement strengths

were not determined. The milling conditions adjusted during the tests are given in

Table 4.9.

Table 4.9. The milling conditions adjusted to measure mill inside material amount

with EPCT-01 chemical

Media Size (mm) 4

Media Filling (%) 50

Stirrer Speed (m/s) 4.34

As can be noticed from the table above, lower media filling and stirrer speed

conditions were adjusted in order not to cause any operational deficiency during

non-chemical test study. The test results given in Table 4.10 implies that grinding

chemical reduces the material amount inside the mill therefore, efficient grinding

conditions are provided and consequently specific energy consumption of the mill

is lowered. From 0 to 700 g/t, material amount is reduced by 34.5% and specific

energy saving of 7.5% is obtained however further increase is not beneficial (1000

g/t). The interaction of chemical with cement played an important role in obtaining

73

these results. It is thought that when the chemical is introduced into the system,

the material become fluidized initially then particles that are already in desired

fineness now discharges from the mill thus product gets finer. Owing to decreased

amount of mill inside material, the power and specific energy decreases as well

because of the less resistance on the discs and on the shaft.

Table 4.10. The change in specific energy consumption and material amount

inside the mill

Chemical Dosage (g/t)

Specific Energy (kWh/t)

Material Amount Inside the Mill (kg)

Mean Product Size, d50 (µm)

0 21.15 17.1 14.64

500 20.18 12.2 14.07

700 19.55 11.2 13.71

1000 19.31 11.3 13.65

It should be emphasized that, owing to its proven operational benefits, EPCT-01 at

the dosage of 700 g/t was used in the further studies.

4.2. The Effects of Air Flow Rate

As indicated previously, air supply into the mill improved material transportation

when using fine media size (3 mm) in the meantime when grinding coarser

material (>d50:47 µm). In order to investigate the influences of air flow rate on

grinding performance, series of grinding tests were performed. The test conditions

and the related results are given in Table 4.11 and Figure 4.6.

Table 4.11. The milling conditions and the obtained results for determining the

effects of air flow rate

The experimental conditions Test 9 Test 10 Test 11

Media Size (mm) 3

Media Filling (%) 50

Stirrer Speed (m/s) 4.34

Feed Rate (kg/h) 270

Feed size F50 (µm) 23.34

Chemical Dosage (g/t) 700

Air Amount (L/h) 0 500 1000

74

Table 4.11 (Cont.). The milling conditions and the obtained results for determining

the effects of air flow rate

The experimental results Test 9 Test 10 Test 11

Specific Energy (kWh/t) 9.78 9.67 9.77

Product d50 (µm) 19.1 19.27 19.11

Amount of material inside the mill (kg) 16.2 15.8 15.9

Figure 4.6. The size distributions obtained from the air flow rate tests

Test results given in Table 4.11 and Figure 4.6 implied that, increasing air flow rate

had no evident effect on milling performance since the same specific energy

consumptions and the product size distributions were obtained. The studies

conclude that, air supply improves the material transportation but it is not a

parameter influencing the grinding performance of the mill.

4.3. The Effects of Stirrer Speed

Referring to the Section 2.4.1, stirrer speed mainly increases the probability of

media to particle collision by creating high energy intensity environment. The test

conditions and test results for investigating the effects of stirrer speed are given in

Table 4.12, Figure 4.7 and Figure 4.8.

75

Table 4.12. The milling conditions and the obtained results for determining the

effects of stirrer speed

The experimental conditions Test 12 Test 13 Test 14 Test 15

Stirrer Speed (m/s) 2.17 3.25 4.34 6.5

Media Size (mm) 4

Media Filling (%) 60

Feed Rate (kg/h) 400

Feed Size, F50 (µm) 22.09

Blaine of feed (cm2/g) 2356

n (RRBS slope of feed) 1.02

The experimental results Test 12 Test 13 Test 14 Test 15

Specific Energy (kWh/t) 4.29 6.08 8.23 19.14

Product Size, P50 (µm) 18.73 18.27 17.55 14.92

Reduction Ratio 1.18 1.21 1.26 1.48

Blaine (cm2/g) 2514 2597 2613 2950

n (RRBS slope) 1.03 1.03 1.03 1.05

Amount of material inside the mill (kg)

19.2 19.52 19.61 N.D.*

Material Load (%) 102 104 104 N.D.*

* Not determined

Figure 4.7. The size distributions obtained from stirrer speed tests

76

Figure 4.8. The relationships between stirrer speed, product fineness and specific

energy developed from stirrer speed tests

Test results given in Table 4.12, Figure 4.7 and Figure 4.8 indicate that grinding at

lower stirrer speeds (Test 12 and Test 13) does not make evident contribution on

size reduction. Despite of the change in the specific energies (from 4.29 kWh/t to

6.08 kWh/t), the reduction ratios (1.18, 1.21) and the surface area measurements

are close to each other. This may be due to the smaller size of media, which is not

able to create high stress intensity environment. Jankovic [8] in zinc grinding tests

pointed out that stirrer speed was a parameter influencing the performance of

grinding operation of IsaMill®, however the effect was so small when finer media

was used (0.85-0.6 mm). In his study, 1.44 reduction ratio was obtained for both

3.6 m/s and 5.2 m/s stirrer speeds at an energy level of around 5 kWh/t. Jankovic

[8] also concluded that higher stirrer speeds were required to improve the grinding

efficiency of smaller sized media. Another conclusion at lower stirrer speed tests

is, the amount of material left inside the mill was not affected as the temperature of

mill chamber did not increase evidently (reached max 40°C).

In Figure 4.8 and Figure 4.7, distinctive difference in size distributions at stirrer

speeds of 4.34 m/s (Test 14) and 6.5 m/s (Test 15) can be seen. Therefore,

another test work was arranged to study the effects of maximum stirrer speed. The

test conditions and the related results for investigating the effects of maximum

stirrer speed are given in Table 4.13, Figure 4.10 and Figure 4.9.

77

Table 4.13. The milling conditions and the obtained results for determining the

effects of maximum stirrer speed

The experimental conditions Test 16 Test 17 Test 18 Test 19

Stirrer Speed (m/s) 4.34 6.5 8.67 9.76

Media Size (mm) 4

Media Filling (%) 60

Feed Rate (kg/h) 250

Feed Size, F50 (µm) 29.42

Blaine of feed (cm2/g) 1420

n (RRBS slope of feed) 1.02

The experimental results Test 16 Test 17 Test 18 Test 19

Specific Energy (kWh/t) 13.33 21.88 30.23 37.17

Product Size, P50 (µm) 19.61 17.98 17.14 16.32

Reduction Ratio 1.50 1.63 1.72 1.80

Blaine (cm2/g) 2380 2667 2764 2864

n (RRBS slope) 1.09 1.08 1.07 1.06

Amount of material inside the mill (kg)

19.7 19.8 20.93 21.46

Material Load (%) 105 105 111 114

Figure 4.9. The size distributions obtained from the maximum stirrer speed tests

78

Figure 4.10. The relationship between stirrer speed, product fineness and specific

energy developed from maximum stirrer speed tests

The results given in Table 4.13, Figure 4.10 and Figure 4.9 indicate that as the

stirrer speed is increased, both the specific energy consumption and the material

amount inside the mill increases. Figure 4.9 shows that, the change in size

distributions is becoming insignificant at higher stirrer speeds due to the inefficient

grinding operation. The Blaine surface area measurements also support this

finding.

The reduced energy efficiency at high stirrer speeds were observed by Zheng et

al. [32] who calculated the grinding efficiency from specific surface area

development, energy consumption and volume of the ground material. Fadhel and

Frances [34] also indicated that higher stirrer speeds did not make an evident

difference in terms of size reduction (median size) due to the inefficient grinding

conditions. Therefore, they recommended mills to be operated at an energy level

that was just enough to break the particles because the excess energy

transformed into heat reduced the efficiency of grinding. This behaviour has also

been encountered in this study. High stirrer speed increased the temperature of

the mill chamber (75°C at 8.67 m/s tip speed, 90°C at 9.76 m/s tip speed) which

affected the material load directly and reduced the efficiency of grinding. Although

the specific energy was changed considerably from 13.33 kWh/t to 37 kWh/t, the

size reduction value changed from 1.50 to 1.80 which was not noteworthy.

Stirrer speed tests showed that the slopes of feed and ground materials were quite

similar to each other. In contrast to IsaMill® operations where the product size

79

distribution becomes narrower due to the selective grinding of coarse particles, the

slope remained constant.

4.4. The Effects of Feed Rate

Referring to the Section 2.4.4, in any kind of grinding operation, specific energy is

one of the most important parameters related to the product fineness. Feed rate is

one of the parameters affecting energy consumption directly. The test conditions

and the related results for investigating the effects of feed rate are given in Table

4.14, Figure 4.11 and Figure 4.12.

Table 4.14. The milling conditions and the obtained results for determining the

effects of feed rate

The experimental conditions Test 20

Test 21

Test 22

Stirrer Speed (m/s) 4.34

Media Size (mm) 4

Media Filling (%) 60

Feed Rate (kg/h) 230 310 400

Feed Size, F50 (µm) 25.95

Blaine of Feed (cm2/g) 1780

n (RRBS slope of feed) 1.02

The experimental results

Test 20

Test 21

Test 22

Specific Energy (kWh/t) 15.33 11.9 8.8

Product Size, P50 (µm) 17.1 18.83 20.85

Reduction Ratio 1.51 1.38 1.24

Blaine (cm2/g) 2770 2571 2391

n (RRBS slope) 1.06 1.06 1.04

Amount of material inside the mill (kg) 13.68 15.26 19.43

Material Load (%) 73 81.4 104

80

Figure 4.11. The size distributions obtained from the feed rate tests

Figure 4.12. The relationships between feed rate, product fineness and specific

energy developed from feed rate tests

The feed rate has mainly two major effects on grinding performance which are; the

specific energy consumption and the product fineness. As can be understood from

Table 4.14, Figure 4.11 and Figure 4.12, the change in feed rate from 230 kg/h to

400 kg/h reduces the specific energy consumption from 15.33 kWh/t to 8.8 kWh/t

and at the same time decreases both the reduction ratio from 1.51 to 1.24 (Figure

4.11 and Figure 4.12) and Blaine values from 2770 cm2/g to 2391 cm2/g. The

difference in Blaine values is around 14%. The MaxxMill study performed by Wang

et al. [3] indicated that at 350 rpm stirrer speed, increasing feed rate from 300 kg/h

to 500 kg/h decreased the BET surface area (m2/g) by approximately 14%.

Dikmen [33], in wet stirred mill study, obtained coarser product when the feed rate

was changed from 557 kg/h to 916 kg/h.

81

Furthermore, test results indicate that the feed rate and the amount of material

inside the mill are directly proportional, i.e. the lower the feed rates, the lower the

amount of material left inside the mill. Test results also show that the slopes of the

feed and product size distributions are very close to each other.

4.5. The Effects of Media Filling

Referring to the Section 2.4.3, media filling is a parameter affecting ball to material

ratio, thus the product fineness as well. In this study, several test works were

performed in order to investigate the effect of media filling on grinding

performance. The test conditions and the related results for investigating the

effects of media filling are given in Table 4.15, Figure 4.13 and Figure 4.14.

Table 4.15. The milling conditions and the obtained results for determining the

effects of media filling

The experimental conditions Test 23 Test 24 Test 25 Test 26

Stirrer Speed (m/s) 4.34

Media Size (mm) 4

Media Filling (%) 30 40 50 60

Feed Rate (kg/h) 400

Feed Size F50 (µm) 26.78

Blaine of feed (cm2/g) 1851

n (RRBS slope of feed) 1.01

The experimental results Test 23 Test 24 Test 25 Test 26

Specific Energy (kWh/t) 4.26 5.29 6.67 8.8

Product Size, P50 (µm) 25.94 23.95 22.12 20.85

Reduction Ratio 1.03 1.12 1.21 1.28

Blaine (cm2/g) 1893 1952 2112 2357

n (RRBS slope) 1.01 1.03 1.03 1.04

Amount of material inside the mill (kg) 18.91 18.82 18.84 19.12

Material Load (%) 200 150 120 102

82

Figure 4.13. The size distributions obtained from the media filling tests

Figure 4.14. The relationships between media filling, product fineness and specific

energy developed from media filling tests

The test results shown in Table 4.15, Figure 4.13 and Figure 4.14 imply that there

is a systematic size reduction depending on the increase in media filling. It was

observed that, the change in media filling from 30% to 60% increased both the

specific energy consumption from 4.26 kWh/t to 8.8 kWh/t and the reduction ratio

from 1.03 to 1.28. In the meantime, the Blaine value was changed from 1893

cm2/g to 2357 cm2/g. Sivamohan and Vachot [45] found that the increase in media

filling from 29% to 57% increased the surface area from 2 m2/g to 6 m2/g. Persson

and Forssberg [41] investigated the effect of media level (centimetres) on specific

surface area (m2/g) and indicated that at constant specific energy (50 kWh/t) the

83

change in level from 40 cm to 60 cm produced material having higher surface area

(from 1 m2/g to 1.7 m2/g).

In this study it was also concluded that the change in media filling had no effect on

the slope of the product size distributions (constant n parameter).

In another study, series of test works were arranged to compare the grinding

performances of different media fillings (30%, 40% and 60%) separately. In these

tests, material with d50 of 57 µm was fed into the mill and the grinding tests were

conducted over a range of energy levels. Instead of presenting the operating

conditions of all the tests separately, the range of the parameters is given in Table

4.16. The test results are presented graphically in Figure 4.15.

Table 4.16. The milling conditions adjusted for 30%, 40% and 60% media filling

tests

The experimental conditions Value

Stirrer Speed (m/s) 2.17-6.50

Feed Rate (kg/h) 100-400

Figure 4.15. Comparison of grinding performances at different media fillings

Even though a wide range of milling conditions were arranged to perform the tests

at different specific energy levels, no considerable difference in size reductions

were obtained with 30% filling where only a small difference was observed with

40% filling. The test results conclude that 30% and 40% media fillings are not

energy efficient conditions. In other words, higher specific energies are required

for lower fillings to obtain the size reduction that higher media fillings (60%)

achieve. Similar results were also obtained by Sivamohan and Vachot [45] in

84

muscovite and wollastonite grinding. In their study, the mill load was changed

gradually and the development in surface area was followed in different times of

grinding. The grinding results indicated that the lowest media filling (30%) had a

small effect on surface area evolution as the surface area was increased from 2

m2/g to 4 m2/g in 40 minutes. On the other hand, at the same time of grinding the

surface area of 14 m2/g was achievable at maximum media filling (83%).

4.6. The Effects of Ball Size

Referring to the Section 2.4.2, the selection of proper size of media improves the

efficiency of grinding operation. In this study, the grinding performances of 3 mm,

4 mm, 6 mm and 8 mm steel media sizes were compared and the results are

presented in the following sections.

4.6.1. 4-6 mm Comparison

The test conditions and the related results for investigating the effects of media

size (4 & 6 mm) are given in Table 4.17 and Figure 4.16.

Table 4.17. The milling conditions and the obtained results for determining the

effects of media filling (4 & 6 mm)

The experimental conditions Test 27 Test 28

Stirrer Speed (m/s) 4.34

Media Size (mm) 4 6

Media Filling (%) 60

Feed Rate (kg/h) 400

Feed Size F50 (µm) 22.32

Blaine of feed (cm2/g) 2212

n (RRBS slope of feed) 1.03

The experimental results Test 27 Test 28

Specific Energy (kWh/t) 8.38 11.01

Product Size, P50 (µm) 18.74 19.41

Reduction Ratio 1.19 1.15

Blaine (cm2/g) 2628 2557

n (RRBS slope) 1.02 0.99

Amount of material inside the mill (kg) 17.55 30.2

85

Figure 4.16. The size distributions obtained from the media size tests (4 & 6 mm)

The test results presented in Table 4.17 and Figure 4.16 imply that 4 mm media

produces slightly finer product (with a reduction ratio of 1.19) by consuming 23.9%

less specific energy compared to 6 mm media size. It is clear that using finer

media brings energy efficiency.

4.6.2. 4-6-8 mm Comparison

In another test work, the grinding performances of 4 mm, 6 mm and 8 mm media

sizes were compared using relatively coarser material (feed size, F50:51.03 µm)

compared to Section 4.6.1. The test conditions and the related results for

investigating are given in Table 4.18 and Figure 4.17.

Table 4.18. The milling conditions and the obtained results for determining the

effects of media filling (4 & 6 & 8 mm)

The experimental conditions Test 29 Test 30 Test 31

Stirrer Speed (m/s) 5.42

Media Size (mm) 8 6 4

Media Filling (%) 50

Feed Rate (kg/h) 110

Feed Size F50 (µm) 51.03

Blaine of feed (cm2/g) 706

n (RRBS slope of feed) 0.84

86

Table 4.18 (Cont.). The milling conditions and the obtained results for determining

the effects of media filling (4 & 6 & 8 mm)

The experimental results Test 29 Test 30 Test 31

Specific Energy (kWh/t) 43.65 38.5 31.68

Product Size, P50 (µm) 21.09 22.25 22.66

Reduction Ratio 2.42 2.29 2.25

Blaine (cm2/g) 2333 2219 2180

n (RRBS slope) 1.05 1.03 1.02

Figure 4.17. The size distributions obtained from the media size tests (4 & 6 & 8

mm)

The test results given in Table 4.18 and Figure 4.17 show that although the

obtained size reductions are close to each other (2.42-2.25), the specific energy

consumptions vary considerably. In other words, at the same product d50’s, 27.4%

less energy was utilized by 4 mm media compared to 8 mm media. As a

conclusion, using finer media brings energy efficiency.

4.6.3. 4-3 mm Comparison

The tests have been performed so far showed that the finer media draws less

power, thus utilizes less specific energy. In order to find out the minimum media

size that could be used in dry stirred milling operation, another test work was

performed in which the grinding performances of 3 mm and 4 mm media sizes

87

were compared. The test conditions and the related results are given in Table 4.19

and Figure 4.18.

Table 4.19. The milling conditions and the obtained results for determining the

effects of media filling (4 & 3 mm)

The experimental conditions

Test 32 Test 33

Stirrer Speed (m/s) 4.34

Media Size (mm) 3 4

Media Filling (%) 50

Feed Rate (kg/h) 160

Feed Size F50 (µm) 21.02

Blaine of feed (cm2/g) 2340

n (RRBS slope of feed) 1.01

The experimental results Test 32 Test 33

Specific Energy (kWh/t) 16.38 16.65

Product Size, P50 (µm) 15.78 15.49

Reduction Ratio 1.33 1.36

Blaine (cm2/g) 2931 2968

n (RRBS slope) 1.04 1.04

Figure 4.18. The size distributions obtained from the media size tests (4 & 3 mm)

As a result of the studies, no difference is observed between 3 mm and 4 mm

media sizes in terms of their providing grinding performances, therefore it can be

88

concluded that the media size of 4 mm is the lowest practical limit for efficient

grinding operation.

The test results presented in previous sections (Section 4.6.1-Section 4.6.3)

indicate that as the media size decreases, less energy is utilized at the same

target size. The experimental results show that, approximately 23.9% less energy

is consumed when grinding F50 of 22.32 µm material and 27.4% energy saving is

achieved when grinding F50 of 51.03 µm in case of using finer media. Mankosa et

al. [38] also declared the energy efficiency of using finer media and indicated that

50% energy savings were achievable in producing 5 µm mean product size.

Jankovic [8] in his vertical stirred mill study found that it was possible to utilize 14%

less energy with finer media to grind F80 of 20 µm down to P80 of 10 µm. In some

of his tests, energy utilization of finer media reached one third of the coarser one.

Mende et al. [40] also indicated that the finer media utilized one fourth of the

energy consumed by the coarser one.

In this study, 4 mm media was found as the minimum size that affects the grinding

process. In the literature similar findings had been obtained. Schollbach [42]

declared that using too fine or too coarse media was not beneficial and concluded

that the effective media size was between 2.5 mm and 4 mm.

In this study, it should also be emphasized that the media size has no effect on

slope of the product size distributions. Feed and product size distributions are

parallel to each other.

4.7. The Effects of Feed Size Distribution

This section aimed at investigating the mill performance at varying feed size

distribution. In this context, sufficient amount of material collected from each

stream (Figure 3.11) was subjected to grinding test studies. As a result of the

studies, the performance of dry stirred milling on each stream was evaluated by

considering the energy consumption and size reduction relationship. Then the

obtained trends were compared with each other and the effects of feed fineness

on grinding performance were investigated.

89

4.7.1. Grinding Tests Performed with Final Product Stream

Final product stream has the finest size distribution of the entire grinding circuit.

The size distribution curve and the parameters related to it are given in Figure 4.19

and Table 4.20 respectively.

Figure 4.19. Whole size distribution of the final product stream

Table 4.20. Size distribution parameters of the final product stream

n (RRBS) 1.018

d50 (µm) 14.81

d80 (µm) 32.44

Mill conditions adjusted for grinding test studies are given in Table 4.21. Due to

insufficient amount of mono-size media, media mixture was prepared and filled

into the mill (Table 4.21).

Table 4.21. The milling conditions of the final product stream grinding tests

Mill Chamber Volume (L) 42

Media Type Steel

Stirrer Type Disc

Chemical Type EPCT-01

Chemical Amount (g/t) 700

Air Flowrate (L/h) 1000

Media Mixture (mm) 60% from 4 mm ; 40% from 3 mm

90

The test plan is given in Table 4.22. As can be seen from the table, the mill was

tried to be operated at higher media fillings (70%) to find out whether the improved

performance was obtained. The cross-marked tests shown in the table imply that

they are incomplete due to operational problems. The operating problems were

mainly due to high intense milling conditions, i.e., high media filling, high stirrer

speed, that increased mill chamber temperature so high that either the use of

grinding chemicals or air supply did not bring any benefits.

Table 4.22. Grinding test plan arranged for final product stream

The experimental results obtained from the grinding tests are presented in Table

4.23. As seen from the table, mean product size, n and specific energy parameters

were calculated for each case. Test results imply that specific energy and product

mean size (P50) parameters are inversely proportional to each other. Moreover, the

same n parameters for feed and product size distributions indicate that, they are

parallel to each other independently of any operating conditions. This behaviour of

the mill is completely different from the wet stirred mills (IsaMill®) where coarser

particles are ground selectively, thus steeper product size distribution is obtained.

Table 4.23. The experimental results obtained from final product grinding tests

Feed Rate (kg/h) Power (kW) Specific Energy

(kWh/t) P50

(µm) n

(RRBS)

Test 34 102.96 2.82 27.39 11.21 0.97

Test 35 96.00 4.22 43.96 9.37 0.954

Test 36 97.20 6.26 64.40 8.38 0.921

Test 37 262.80 2.72 10.35 12.57 1.035

Test 38 271.00 4.22 15.57 11.49 1.007

Test 39 266.40 6.26 23.50 10.19 0.954

Test 40 424.00 2.69 6.34 13.37 1.009

Test 41 418.00 4.32 10.33 12.60 1.001

Test 42 425.50 6.28 14.76 11.14 0.942

Feed Rate (kg/h) 4.34 m/s 6.5 m/s 9.76 m/s 4.34 m/s 6.5 m/s 9.76 m/s 4.34 m/s 6.5 m/s 9.76 m/s

400 T40 T41 T42 T47 T48 T18 T51 T26 T27

250 T37 T38 T39 T45 T46 T15 T50 T23 T24

100 T34 T35 T36 T43 T44 T12 T49 T20 T21

50% Filling %60 Filling %70 Filling

91

Table 4.23 (Cont.). The experimental results obtained from final product grinding

tests

Feed Rate (kg/h) Power (kW) Specific Energy

(kWh/t) P50

(µm) n

(RRBS)

Test 43 100.80 4.07 40.38 9.38 0.963

Test 44 102.96 7.15 69.44 8.05 0.913

Test 45 259.56 3.89 14.99 11.87 1.034

Test 46 266.76 6.23 23.35 10.64 0.987

Test 47 430.56 3.72 8.64 12.93 1.046

Test 48 416.88 6.2 14.87 11.51 1.004

Test 49 96.48 5.71 59.18 8.50 0.931

Test 50 271.44 5.31 19.56 10.68 0.993

Test 51 411.12 5.1 12.41 11.64 1.016

With the data produced throughout this study, the influences of operating

parameters grinding process were investigated as well. Figure 4.20 and Figure

4.21 show the relationships between the parameters. Figure 4.20 illustrates the

influences of feed rate and stirrer speed parameters on power draw of the mill. It

implies that power draw increases with increasing stirrer speed. Additionally, the

feed rate had an adverse effect on power draw, in particular when the mill was

operated at higher media fillings. The measurements at 50% filling indicated that

no matter how high the feed rate was adjusted, the power draws of each stirrer

speed were the same (Figure 4.20a). However this behaviour changed at 60%

filling (Figure 4.20b) and became more evident at 70% filling (Figure 4.20c). That

is, increase in feed rate decreased power consumption. When the media filling

reached to the level of 70%, change in feed rate from 100 kg/h to 400 kg/h

reduced the power drawn by the mill by 10.7%. Inverse relationship between feed

rate and power draw is also observed in tumbling mills. Austin et al. [74] stated

that low powder load, which was a function of feed rate, had given significantly

higher power than normal powder loads in tumbling mills.

92

Figure 4.20. Effect of feed rate and stirrer speed on power draw during grinding

tests with final product stream

Figure 4.21. Effect of media filling and stirrer speed on power draw during grinding

tests with final product stream

The relationships given in Figure 4.21 show that media filling and stirrer speed

parameters are directly proportional to power draw of the mill.

c. a. b.

93

Figure 4.22 illustrates energy and size reduction relationship of final product test

results. As can be seen from the figure, there is a steadily upward trend between

the parameters. Besides it is seen that grinding results of each media filling are in

the same trend.

Figure 4.22. Specific energy consumption and size reduction relationship

developed from grinding tests with final product stream

4.7.2. Grinding Tests Performed with Separator Reject Stream

Separator reject stream has relatively coarser material when compared to other

streams (filter return and final product). In Figure 4.23 and Table 4.24 particle size

distribution of the stream and the parameters defining the size distribution curve

are presented respectively.

Figure 4.23. Whole size distribution of the separator reject stream

94

Table 4.24. Size distribution parameters of the separator reject stream

n (RRBS) 1.573

d50 (µm) 66.19

d80 (µm) 120.84

Test studies on separator reject stream aimed at investigating the stirred mill

performance with relatively coarser material. In a typical cement grinding process,

high amount of material is recirculated back into the mill via separator reject

stream owing to its coarseness. Therefore, an energy-efficient stirred milling

operation on this stream is expected to improve overall circuit performance since

more fine material could be produced and sent directly to final product bin.

The milling conditions adjusted throughout the grinding tests are presented in

Table 4.25. Major difference compared to final product stream milling conditions

(Table 4.21) is; grinding chemicals were not added as it was proved that they were

ineffective within this particle size range.

Table 4.25. The milling conditions of the separator reject stream grinding tests

Mill Chamber Volume (L) 42

Media Type Steel

Stirrer Type Disc

Air Flowrate (L/h) 1000

Media Mixture (mm) 60% from 4mm ; 40% from 3mm

A comprehensive test plan arranged for separator reject stream are given in Table

4.26. This plan aimed at pushing the grinding limits of the mill so as to determine

operational boundaries. As can be noticed from the table, some of the milling

conditions (cross-marked on the table) resulted in stopping the grinding operation

owing to drastic increase in mill chamber temperature that ultimately influenced

material transportation adversely despite of using maximum available amount of

air.

95

Table 4.26. Grinding test plan arranged for separator reject stream

Experimental results obtained from the grinding tests are presented in Table 4.27.

As presented in the table, specific energy, mean product size and n parameters

were calculated for each of the test studies. Afterwards, the correlations between

the parameters were plotted. In other words, the effects of operating parameters

on power utilization and obtained size reduction values were examined.

Table 4.27. The experimental results obtained from separator reject grinding tests

Feed Rate (kg/h) Power (kW) Specific Energy

(kWh/t) P50

(µm) n

(RRBS)

Test 52 118.80 3.75 31.57 31.99 1.074

Test 53 123.12 5.7 46.30 28.30 0.988

Test 54 119.52 8.43 70.53 22.49 0.883

Test 55 251.28 4.43 17.63 47.20 1.322

Test 56 250.56 7.2 28.74 38.09 1.148

Test 57 252.72 9.55 37.79 31.28 1.021

Test 58 371.52 6.06 16.31 51.29 1.317

Test 59 119.52 5.37 44.93 20.55 0.979

Test 60 122.40 8.7 71.08 15.14 0.903

Test 61 252.72 5.94 23.50 32.13 1.104

Test 62 254.52 9.2 36.15 23.72 1.01

Test 63 380.16 6.71 17.65 39.27 1.201

Test 64 126.36 7.36 58.25 14.37 0.972

Test 65 250.56 7.67 30.61 24.44 1.031

Firstly, the change in n parameter was investigated to find out whether operating

conditions had influence on the shape of the product size distribution. The

calculations showed that, in contrast to the results obtained from the previous

studies, n parameter was influenced by operating conditions, i.e. feed rate, media

filling and stirrer speed. It decreases with increasing stirrer speed and media filling

Feed Rate (kg/h) 4.34 m/s 6.5 m/s 9.76 m/s 4.34 m/s 6.5 m/s 9.76 m/s 4.34 m/s 6.5 m/s 9.76 m/s

400 T58 T8 T9 T63 T17 T18 T25 T26 T27

250 T55 T56 T57 T61 T62 T15 T65 T23 T24

100 T52 T53 T54 T59 T60 T12 T64 T20 T21

50% Filling %60 Filling %70 Filling

96

owing to the fines production. Conversely, it increases with increasing feed rate.

The differences in n values are more obvious at 50% media filling condition. In this

case, n parameter ranges between 1.322 and 0.883.

In Figure 4.24 and Figure 4.25, the trends showing the correlations between the

parameters are illustrated. In Figure 4.24, the effects of feed rate and stirrer speed

parameters on power consumption are illustrated graphically. Similar to previous

observations, as the stirrer speed is increased more power is utilized by the mill.

On the other hand, feed rate parameter acts in a different manner compared to

final product test works. The trends imply that power consumption increases to

some extent when the mill is operated at higher feed rates.

In Figure 4.25, the trends showing the effects of media filling on power draw are

illustrated. As expected, media filling and power consumption parameters are

directly proportional to each other.

Figure 4.24. Effect of feed rate and stirrer speed on power draw during grinding

tests with separator reject stream

97

Figure 4.25. Effect of media filling and stirrer speed on power draw during grinding

tests with separator reject stream

Stirred mill performance on separator reject stream grinding can be summarized

by plotting obtained reduction ratio values against specific energy consumption

(Figure 4.26). Such a correlation enables calculating how much energy is

demanded for a given reduction ratio.

Figure 4.26. Specific energy consumption and size reduction relationship

developed from grinding tests with separator reject stream

As can be seen from Figure 4.26, degree of size reduction increases with specific

energy consumption. Moreover, it is obvious that, each of the media fillings has

different grinding performance. In other words, at the same degree of size

reduction less energy is consumed when the mill is operated at higher media

fillings [36; 41; 45].

4.7.3. Grinding Tests Performed with Mill Filter Return Stream

As explained in previous sections, the mill filters are used to collect already ground

particles from inside of the ball mill and to keep the temperature of the mill

chamber at a constant level. Mill filter stream has a size distribution that remains in

98

between separator reject and final product streams. In Figure 4.27 and Table 4.28

typical particle size distribution and parameters defining the size distribution curve

are presented respectively.

Figure 4.27. Whole size distribution of the mill filter return stream

Table 4.28. Size distribution parameters of the mill filter return stream

n (RRBS) 1.112

d50 (µm) 24.84

d80 (µm) 49.07

In general, the fineness of this material does not meet the final product

specifications therefore it is fed to the separator feed stream again. It is thought

that stirred mill application on this stream would be highly beneficial with regards

to reducing energy consumption of the overall grinding circuit by producing more

fines that can be sent directly to final product bin. Mill conditions adjusted during

grinding studies are presented in Table 4.29. As can be noticed, grinding chemical

together with air was supplied into the mill in order to provide efficient grinding

environment.

99

Table 4.29. The milling conditions of the mill filter return stream grinding tests

Mill Chamber Volume (L) 42

Media Type Steel

Stirrer Type Disc

Chemical Type EPCT-01

Chemical Amount (g/t) 700

Air Flowrate (L/h) 1000

Media Size (mm) 4

In contrast to test schedules arranged for final product and separator reject

streams, test works with filter return material were not performed systematically.

Therefore it is unlikely to present a testing matrix. With filter return stream, over 50

test works were carried out at different time intervals and milling conditions. These

test works comprise both investigating the effects of operating parameters

(presented in Section 4) and some additional studies evaluating the mill

performance. The range of experimental results obtained from the tests are given

in Table 4.30.

Table 4.30. The ranges of experimental results obtained from filter return grinding

tests

Ball Filling (%)

Tip Speed (m/s)

Feed Rate (kg/h)

Specific Energy

(kWh/t)

d50

(µm) No. of Tests

60 2.17-9.76 180-410 4.3-37 14-21 26

50 1.08-6.5 45-406 6.7-37.8 13-22 25

40 4.34-5.42 116-390 5.29-15.6 18-24 4

30 4.34-5.42 36-400 4.26-36.6 18-26 4

Considerable amount of data produced during the test studies enabled to examine

influences of operating parameters on power consumption of the mill. In Figure

4.28, effects of stirrer speed together with media filling on power consumption are

illustrated graphically. Similar to previous findings it is understood that stirrer

speed and power consumption are directly proportional to each other. Additionally

100

it is observed from the figure that media filling ratio directly affects power

consumption.

Figure 4.28. Effect of media filling and stirrer speed on power draw during grinding

tests with filter return stream

In Figure 4.29, relationship between size reduction and specific energy

consumption parameters at different media fillings are illustrated. As expected

there is a steadily upward trend between them that is, degree of size reduction

gets higher as the energy level increases. However as noticed from the figure,

each of the media fillings behaves in a different manner. The studies concluded

that 30% and 40% ball fillings are not efficient grinding conditions since higher size

reductions are obtained at 50% and 60% filling at the same energy level. When

the trends between 50% and 60% fillings are compared it is noticed that there is

no evident difference in terms of grinding performances. This behaviour is quite

similar to what was observed with final product test works (Section 4.7.1).

Figure 4.29. Specific energy consumption and size reduction relationship

developed from grinding tests with filter return stream

101

4.7.4. Performance Evaluation of Stirred Mill at Different Feed Fineness

Comprehensive test schedules arranged for each stream made it possible to

investigate the influences of feed size distribution on stirred mill grinding

performance. In general as particles get coarser, discontinuities they have

increase thus energy consumption to cause breakage action decreases. On the

other hand the number and size of the discontinuities decrease with increasing

fineness therefore applied stress should be increased gradually to cause breakage

action [75; 76; 77]. As a conclusion, fine material milling requires higher energy

inputs compared to coarse material to obtain same degree of size reduction.

Mill conditions of the grinding tests at each stream had been given in Sections

4.7.1, 4.7.2 and 4.7.3. As can be noticed, 50% and 60% ball filling conditions are

in common for each of the cases therefore grinding results gathered from these

test works were used to investigate the influences of feed size distribution. The

attention should be drawn to media size selected during grinding test works.

Grinding tests with separator reject and final product streams were carried out at

the same media composition (60% 4 mm and 40% 3 mm) in order to push the

limits of the mill (Table 4.21, Table 4.25) while filter return stream tests were

performed with mono-size (4 mm) media. It is thought that such a difference would

not contribute any significant changes in the grinding performances as previous

studies (Section 4.6) proved that 4 mm and 3 mm media sizes had the same

grinding performance regarding to size reduction and specific energy relationship.

In Figure 4.30, energy utilization and size reduction relationship of each grinding

stream is illustrated. Figure 4.30a and Figure 4.30b show 50% and 60% ball filling

conditions respectively.

Figure 4.30 Grinding performance of the dry stirred mill at varying feed size

a. b.

102

As can be seen from the figure, as the feed size gets coarser much more rapid

size reduction is obtained that directly affects the slope of the trend. The trend

obtained from the separator reject stream has higher slope compared to the rest.

The studies conclude that at the same degree of size reduction, less energy is

demanded by coarser material.

In the following study, energy consumptions of each of the feed sizes to achieve

the same degree of size reductions were evaluated. In Figure 4.31, graphical

representation of the results is illustrated. In this graph, mean feed sizes lie on the

x-axis, including final product, filter return and separator reject streams from left to

right, while their energy consumptions are on y-axis. At the same feed size,

increasing energy consumption increases the obtained size reduction as well.

Moreover, it is observed that for the size below F50 of 25 µm, the energy

consumption increases drastically. Therefore, this size can be specified as the

minimum feed d50 that efficient cement grinding takes place for a given milling

conditions.

Figure 4.31. Effect of feed size on dry stirred mill performance

In the literature, similar conclusions have been reported. Figure 4.32, represents

the work done by Yue and Klein [78]. In their study they used wet operated

horizontal stirred mill manufactured by Netzsch and performed cycle tests on

quartz with F80 of 83 µm. The study concluded that size reduction decreased with

feed size and got close to 1 at the end of the tests. Ultimately, they concluded that

103

for a given grinding condition grinding limit existed where no evident breakage

action occurred any further.

Figure 4.32. Effect of feed size on size reduction obtained [78]

In a stirred mill, like any kind of ball mills, the characteristic of media has a

prominent role on performance of grinding action. Among other properties such as

chemical composition, strength, wear etc. the selection of proper size of media

comes forward to achieve improved milling performance. In other words, the size

of media should be accordant with material feed size to carry out an efficient

grinding operation. In the literature many studies investigating the relationship

between ratio of media diameter (Dball) to material feed size (Dparticle) and specific

breakage rate of the mill have been reported so far.

Referring to Section 2.4.2, Mankosa et al. [38] in their study observed that

breakage rate increased with Dball/Dparticle ratio until 20:1 and beyond this point

breakage rate started decreasing. This behaviour was attributed to small size of

media that was barely able to nip the particles. In another study Zheng et al. [32]

specified Dball/Dparticle ratio of 12:1 as an optimum point that efficient grinding action

took place.

Within the scope of the thesis, F50 of 25 µm was determined as the optimum feed

size that efficient grinding took place (Figure 4.31). When the top size of this

material (150 µm) was divided by the media size used in grinding tests (4 mm), the

ratio of Dball/Dparticle was found as 27:1. The difference with the literature comes

from the mill design, material selection and media properties.

104

4.8. The Effects of Mill Geometry

Prior to presentation of the test results, it is beneficial to indicate the differences in

mill chamber geometries. In Table 4.31 mill dimensions are given.

Table 4.31. Dimensions of the two chambers

23 L Mill 42 L Mill

Effective Diameter (cm) 20.4 26.4

Effective Length (cm) 74.0 75.0

Disc Stirrer Diameter (cm) 16.8 16.8

Ratio of Length to Dia. 3.63 2.84

Ratio of Mill Dia. to Stirrer Dia. 1.21 1.57

In this section, the performances of 42 L mill and 23 L mill were compared on final

product stream grinding. The data of 42 L mill on final product stream were

presented in Section 4.7.1. Therefore in this section, initially a test schedule for 23

L mill was arranged and then the tests were performed, afterwards the results

were compared with 42 L mill.

Table 4.32 presents the test plan for 23 L mill performed at the same milling

conditions as 42 L mill (Table 4.21). In these tests it should be emphasized that

water was circulated through the water jacket to observe whether the use of it is

beneficial in milling operation.

Table 4.32. Grinding test plan arranged for final product stream with 23 L mill

Table 4.32 implies that, the tests at 70% filling with 9.76 m/s tip speed are

incomplete because of the operational problems. When this test plan is compared

with Table 4.22 (42 L mill test plan) it is understood that the use of water jacket

enables performing the tests at 60% filling, 9.76 m/s tip speed and at 70% filling,

6.5 m/s tip speed. These observations indicate that the rise of mill chamber

temperature is a serious problem for the milling operation and it can be avoided

with the use of water jacketed mill chamber.

Feed Rate (kg/h) 4.34 m/s 6.5 m/s 4.34 m/s 6.5 m/s 9.76 m/s 4.34 m/s 6.5 m/s 9.76 m/s

400 T74 T75 T78 T14

250 T71 T72 T73 T15

100 T66 T67 T68 T69 T70 T76 T77

%50 Filling %60 Filling %70 Filling

105

The experimental results obtained from grinding tests are presented in Table 4.33.

By using the given data, relationships investigating the effects of parameters on

power draw of the mill were developed.

Table 4.33. The experimental results obtained from final product grinding tests

with 23 L mill

Feed Rate (kg/h)

Power (kW)

Specific Energy (kWh/t)

P50 (µm)

n (RRBS)

Test 66 106.56 2.25 21.11 12.11 0.992

Test 67 101.16 3.2 31.63 11.22 0.961

Test 68 103.68 2.66 25.66 10.82 0.965

Test 69 102.24 4.2 41.08 10.05 0.94

Test 70 93.60 7.1 75.85 8.84 0.88

Test 71 259.20 2.49 9.61 12.88 0.996

Test 72 263.16 3.9 14.82 12.22 1.003

Test 73 262.80 6.31 24.01 11.44 0.972

Test 74 400.68 4.02 10.03 12.70 1.003

Test 75 380.52 6.1 16.03 12.40 0.992

Test 76 94.32 3.25 34.46 10.01 0.967

Test 77 95.04 6.4 67.34 8.59 0.899

Test 78 380.16 5.4 14.26 11.40 0.993

In Figure 4.33 and Figure 4.34 the influences of stirrer speed, media filling and

feed rate parameters on power draw are illustrated. The relationships given in

Figure 4.33a imply that as the stirrer speed increases power drawn by the mill

increases. Moreover, the adverse effect of feed rate on power draw can be

followed from the graph as well. In particular, at the tip speed of 9.76 m/s, a

noteworthy difference is observed between 100 kg/h and 390 kg/h. However, the

effect of feed rate becomes more evident when the media filling reaches to a level

of 70% (Figure 4.33b). In this case, power drawn by the mill is reduced by 15.3%

when the feed rate is changed from 95 kg/h to 380 kg/h. The cushioning effect of

material bed is thought to be effective in reducing the power draw of the mill. Test

results also indicate that the slopes of the feed and product size distributions are

106

close to each other, which means operating parameters have no influence on the

shape of the size distribution curves.

Figure 4.34 illustrates signature plots showing the effect of media filling on power

draw. It is understood that, when the other parameters are constant, the higher the

ball charge the higher the power draw.

Figure 4.33. Effect of feed rate and stirrer speed on power draw during grinding

tests with final product stream (23 L mill)

Figure 4.34. Effect of media filling and stirrer speed on power draw during grinding

tests with final product stream (23 L mill)

The grinding performance of 23 L mill can be evaluated with the aid of a

relationship developed between specific energy consumption and obtained size

reduction (Figure 4.35). In the following figure, the trends at each media fillings are

illustrated.

a. b.

107

Figure 4.35. Specific energy consumption and size reduction relationship

developed from grinding tests with final product stream (23 L mill)

According to Figure 4.35, it is obvious that the grinding performances of each

media filling differ considerably. That is, at the same degree of size reduction (from

Test 67 to Test 78) 54.9% decrease in energy consumption is achievable at higher

media fillings. This behaviour of the mill is different from 42 L mill (Figure 4.22)

where different media fillings produce similar grinding results. Certainly, the use of

water jacketed chamber was the major factor that contributed obtaining these

results. It directly affects grinding performance of the mill.

The literature reports on discussing the influences of mill geometry on grinding

performance had previously been explained in Section 2.4.5. In this section, the

compatibility of the obtained results with the literature was also discussed. As

given in Table 4.31, mill length to mill diameter ratio and mill diameter to stirrer

diameter ratio of the two chambers are evidently different from each other and it is

thought that these differences affect grinding performance directly. Figure 4.36

illustrates the grinding results obtained from 42 L mill and 23 L mill test studies.

According to the figure, 42 L mill utilizes less energy compared to 23 L mill

operated at the same milling conditions. The difference in grinding performances

comes from the variations in mill design.

108

Figure 4.36. Comparison of grinding performances of the two mill chambers

Literature presented in Section 2.4.5 implies that changing aspect ratio of mill

internal parts and mill chamber has a considerable influence on grinding

performance. Test works performed by Zheng et al. [32] showed that as the ratio

of mill diameter to stirrer diameter decreased, milling operation displayed improved

performance. However, when the results obtained in this study are considered

(Figure 4.36) it is observed that better performance is obtained with higher ratio of

mill diameter to stirrer diameter. Such a conflict with the literature may either be

due to the differences in milling operations, as Zheng et al. [32] in their study used

batch operated wet mill, or different length to diameter ratios of the two mill

chambers hampered to observe the effect of mill diameter to stirrer diameter ratio

properly.

4.9. The Effects of Stirrer Type

No load power draws of each stirrer given in Section 3.1.2 implied that, the use of

disc type stirrer was advantageous over the other types in particular at higher

stirrer speeds. In this section the influences of the stirrer designs on grinding

performance are investigated. The milling conditions, the test plan and the related

results are given in Table 4.34, Table 4.35 and Table 4.36 respectively.

109

Table 4.34. The milling conditions of the stirrer type tests

Media Size (mm) 4 & 6 mixture

Feed Size F50 (µm) 66.17

The test plan arranged for the grinding tests are given in Table 4.35. Cross

marked tests in the table represents the conditions that the mill stopped due to

overheating of the mill chamber. From the table it can be distinguished that, the

grinding operation with disc type stirrer could be performed at intense milling

conditions since the tip speed could be adjusted up to 6.5 m/s without having any

operational difficulties. The test results are presented in Table 4.36.

Table 4.35. The test plan for determining the effects of stirrer type

60 % Media Filling

The Disc Type 3.25 m/s 4.34 m/s 6.5 m/s 9.76 m/s

400 kg/h T 83 X X

250 kg/h T 81 T 82 X

100 kg/h T 79 T 80 X

The Cross Type 3.25 m/s 4.34 m/s 6.5 m/s 9.76 m/s

400 kg/h T 87 X X X

250 kg/h T 86 T 85 X X

100 kg/h T 84 X X

The Wing Type 3.25 m/s 4.34 m/s 6.5 m/s 9.76 m/s

400 kg/h T 90 X X X

250 kg/h T 89 X X

100 kg/h T 88 X X

110

Table 4.36. The experimental results obtained from stirrer type tests at the same

milling conditions

The Disc Type T 79 T 80 T 81 T 82 T 83

Power (kW) 5.37 8.7 5.94 9.2 6.71

Feed Rate (kg/h) 119.52 122.4 252.72 254.52 380.16

Specific Energy (kWh/t) 44.93 71.08 23.50 36.15 17.65

Product d50 (µm) 20.55 15.14 32.13 23.72 39.27

The Cross Type T 84 T 85 T 86 T 87

Power (kW) 8.03 8.84 4.78 5.01

Feed Rate (kg/h) 114.84 263.52 256.32 385.92

Specific Energy (kWh/t) 69.92 33.55 18.65 12.98

Product d50 (µm) 18.17 28.44 37.61 41.93

The Wing Type T 88 T 89 T 90

Power (kW) 8.6 8.9 5.61

Feed Rate (kg/h) 108.36 257.76 382.32

Specific Energy (kWh/t) 79.37 34.53 14.67

Product d50 (µm) 16.19 26.04 38.40

By using the obtained results given above, the power draws of each stirrer type

were compared. The trends developed at the same milling conditions (60% media

filling, 4.23 m/s stirrer speed) indicate that the stirrers with edged shaped (wing

and cross types) consume more power compared to rounded shaped disc stirrer

(Figure 4.37). The differences in power draws come from the media movement

along the mill since more media is lifted by the edged shaped stirrers thus more

load is exerted by the shaft. As a conclusion, power draw of the disc type is found

to be the least among the stirrer designs. Such a finding emphasizes the

importance of stirrer design on the efficiency of grinding operation.

111

Figure 4.37. Comparison of power draws of each stirrer type at the same milling

conditions

In Figure 4.38, specific energy-size reduction relationship developed from the test

studies are illustrated. The trends indicate that, at higher energy levels (>20 kWh/t)

wing and cross type stirrers utilize more energy to achieve the same degree of

size reduction as the disc type. In other words, disc type stirrer provides higher

degree of size reduction at higher energy levels when compared to the others.

Figure 4.38. Comparison of grinding performances of each stirrer type at 60%

filling

112

5. MODELLING of DRY HORIZONTAL STIRRED MILL

Perfect mixing model considers a ball mill as a perfectly stirred tank. In this

modelling approach, process can be described in terms of transport (discharge

rate) and breakage events (breakage rate) occur in the mill. The discharge of ith

size fraction from the mill can be calculated by Equation 5.1 where, si is the mass

of size fraction I (tons) in the mill hold-up (S) as tons, pi is the mass flow rate of

particle fraction i out of the mill as product [79].

pi=di.si (5.1)

Ball mills are modelled by back calculating the ratio of breakage rate to discharge

rate parameters (r/d function) as the mill hold-up parameter cannot be determined

precisely. Equation 5.2 is used to determine r/d function.

i

j

i

i

i

i

j

jiji pd

rp

dj

rpaf

1

0 (5.2)

Where;

fi : Mass flowrate of size fraction i in the mill feed

pi : Mass flowrate of size fraction i in the mill discharge

ri : Specific breakage rate of size fraction i

aij : Mass fraction of size j that appear in size i after breakage

di : Specific discharge rate of size fraction i

si : Mass of size fraction i in the mill hold-up

In case of stirred media mill, r/d function can be used in modelling of the mill [33].

However material characterization plays an important role to have an accurate

model. For this purpose, a method developed by Ekşi [80] determining the

breakage characteristics of the fine particles was used and tested in stirred mill

modelling studies with JK-SimMet software. Ekşi [80] applied a bed breakage

method to determine breakage characteristic of clinker finer than 3.35 mm by

using a drop-weight tester. The beds prepared in this study were 2 cm in diameter

and 1 cm in height. The samples in bed were broken under 1 kWh/t energy level

and then the particle size distributions of broken products were determined by

113

sieve analyses. Unbroken parts of the products were eliminated and then

converted into breakage distribution.

Within the stirred mill modelling studies, the breakage distributions of clinker

determined for (-0.850+0.600), (-0.150+0.102), (-0.102+0.072), mm size fractions

were used. These size fractions were selected by considering top size of the feed

materials came into the mill. The breakage distributions are presented in Table

5.1. The data sets were directly taken from Ekşi [80].

Table 5.1. The breakage distribution of (-0.850+0.600), (-0.150+0.102), (-

0.102+0.072) mm size fractions

Particle Size (mm)

(-0.850+0.600) mm

(-0.15+0.102) mm

(-0.102+0.072) mm

0.600 0.0000 0.0000 0.0000

0.425 0.3605 0.0000 0.0000

0.300 0.1558 0.0000 0.0000

0.212 0.1097 0.0000 0.0000

0.150 0.0947 0.0000 0.0000

0.102 0.0891 0.0000 0.0000

0.072 0.0557 0.5063 0.0000

0.050 0.0454 0.2155 0.5191

0.036 0.0289 0.1371 0.2036

0.025 0.0210 0.0697 0.1600

0.018 0.0110 0.0236 0.0640

0.012 0.0094 0.0200 0.0211

0.0086 0.0053 0.0093 0.0119

0.0044 0.0066 0.0109 0.0117

0.0026 0.0028 0.0031 0.0041

0.0018 0.0013 0.0014 0.0014

In modelling studies, r/d* functions were determined initially then correlated with

stirrer speed, feed rate, media filling and feed size distribution parameters. The

accuracy of the model fitting studies were measured by plotting the measured

cumulative passing percentages against the calculated values.

114

In stirred media mill, the breakage rate of the particles is mainly affected by stirrer

speed, media filling and media size while the discharge rate is influenced by

throughput of the mill. Figure 5.1 illustrates the influence of stirrer speed on r/d*

function of the mill. As can be seen from the figure, increasing stirrer speed

increases the breakage rate of the particles therefore finer product is obtained. It

should be emphasized that the tests were performed at the same feed rate thus

discharge rate was constant. The accuracy of the model fitting studies was

evaluated by drawing the measured and calculated size distributions (Figure 5.2).

Figure 5.1. Effect of stirrer speed on r/d* function

Figure 5.2. The measured and calculated size distributions of stirrer speed fitting

studies

115

Figure 5.3 shows the effect of feed rate on r/d function at constant stirrer speed,

media filling and media size. In other words, the investigations were carried out at

constant breakage rate of the particles. As can be seen from the figure, feed rate

influences the r/d* values slightly. That is, lower feed rate resulted in obtaining

higher r/d function owing to decreased discharge rate. The measured and

calculated particle size distributions are illustrated in Figure 5.4.

Figure 5.3. Effect of feed rate on r/d* function

Figure 5.4. The measured and calculated size distributions of feed rate fitting

studies

116

The effect of media filling on r/d function is illustrated in Figure 5.5. The tests were

performed at constant feed rate that is, discharge rate remained constant

throughout the studies. As can be seen from the figure, increase in the amount of

media in the mill increases the breakage rate gradually. As a result, finer product

is obtained. The results of the model fitting studies were evaluated by plotting the

measured and calculated size distributions (Figure 5.6).

Figure 5.5. Effect of media filling on r/d* function

Figure 5.6. The measured and calculated size distributions of media filling fitting

studies

117

Within the modelling studies, the effects of feed size distribution on r/d function

were also investigated (Figure 5.7). In r/d* calculations, the grinding tests

performed for three different size distributions at the same operating conditions,

i.e., 60% filling, 4.34 m/s stirrer speed, 4mm media size and 400 kg/h feed rate,

were taken into consideration. The graph illustrated below indicates that as the

feed gets coarser, r/d of the mill decreases. The figure also implies that, increase

in r/d value get slower below F50 of 25 µm size range. The measured and

calculated size distributions of the model fitting studies are illustrated in Figure 5.8.

Figure 5.7. Effect of feed size distribution on r/d* function

Figure 5.8. The measured and calculated size distributions of feed size fitting

studies

118

So far, the variations of r/d* values with operating parameters have been

presented. In the following study, a comparison between the r/d values of dry

stirred mill and 2nd chamber of various two-compartment ball mills during CEM I

42.5R production is presented. It is thought that such a comparison would be

beneficial to find out the effective grinding size for dry stirred mill technology. 2nd

chamber of the ball mill was chosen due to its fine grinding operations. Typically,

material with 80% passing size of 0.8 to 1 mm is processed in this chamber to

obtain a product with 80% passing size of 100 µm.

Within the study, ball mills having similar diameters and second compartment

lengths were chosen to be compared with stirred mill. In order to evaluate the

performance of the ball mills, extensive sampling campaigns were arranged by

Hacettepe University Mining Engineering Department. The samples were collected

from inside the 2nd chamber after the mills were crash-stopped following a steady

state condition. Technical specifications of the ball mills and size distributions of

2nd compartments are presented in Table 5.2 and in Figure 5.9 respectively.

Table 5.2. The specifications of the ball mills used in the study

Ball Mill #1 Ball Mill #2 Ball Mill #3

Production Type CEM I 42.5R

Bond Work Index of Clinker (kWh/t) 14.37 14.37 16.2

Diameter of the Mill (m) 3.66 3.66 3.4

Length of 2nd Chamber (m) 6.4 6.4 7

Ball Charge % 30.75 32.5 31

Median Ball Size (mm) 25 25 30

119

Figure 5.9. 2nd chamber feed and product size distributions of the ball mills

The 2nd compartment of the ball mills were modelled by using the breakage

distribution of clinker determined for (-1.7+1.18) mm size fraction as the feed

entered into these compartments had top size of 1.18 mm. The breakage

distribution is given in Table 5.3. The data sets were directly taken from Ekşi [80].

Table 5.3. The breakage distribution of (-1.7+1.18) mm size fraction

Particle Size (mm) Ball Mill (-1.7+1.18) mm

1.18 0.0000

0.85 0.3069

0.600 0.1355

0.425 0.0948

0.300 0.0934

0.212 0.0894

0.150 0.0630

0.102 0.0686

0.072 0.0455

0.050 0.0326

0.036 0.0197

0.025 0.0155

0.018 0.0099

0.012 0.0085

0.0086 0.0048

0.0044 0.0059

120

In Figure 5.10, r/d* values of ball mill and stirred mill are illustrated graphically. As

can be seen from the figure, r/d* values ball mills are close to each other owing to

having similar mill design and operating parameters (ball charge, median ball

size). When compared with stirred mill, it is observed that, the r/d* values are

intersecting each other at 80 µm particle size (for fine feeding). Over this size

range, the breakage rate of ball mill is considerably higher than that of stirred mill.

For coarse feeding (F50 of 66 µm), the r/d* values crosses at 200 µm size range

which is still advantageous over the ball mills. In the literature, energy efficient

operations of stirred media mills over the ball mills have been reported by many of

the studies [7; 8; 9]. This study also supports the findings reported in the literature.

In the next section simulation studies performed to unveil the benefits of dry stirred

mill are presented.

Figure 5.10. Comparison of r/d* functions of ball mill and dry horizontal stirred mill

121

6. SIMULATION STUDIES

Possible applications of dry stirred mill had previously been presented in Section

1. It is seen that wide range of applications would exist if stirred milling is found

viable in terms of energy consumption and size reduction performance. This

section aimed at evaluating the implementation of dry stirred mill into existing

cement grinding circuits with the aid of simulation techniques. For this purpose two

types of circuit designs were selected which were open circuit and closed circuit

operations. Initially, sampling campaigns was arranged around the circuits.

Afterwards, the samples were subjected to material characterization studies and

their particle size distributions were determined. Size distribution measurements

together with operating parameters were used to perform mass balancing studies.

Finally, model structures of each unit existing in the circuits were developed to

prepare a simulation platform for further studies.

6.1. Selected Circuit Configurations for Simulation Studies

The simplified flow sheets of the sampled circuits are illustrated in Figure 6.1 and

Figure 6.2. As seen from the figures, the assessments were done for both open

and closed circuit ball milling. These two grinding circuits, which are owned by a

cement plant, are operated in parallel to each other at the same cement quality

(CEM I 42.5R).

Figure 6.1. Closed circuit cement grinding operation sampled for simulation

studies

122

Figure 6.2. Open circuit cement grinding operation sampled for simulation studies

Figure 6.1 shows closed circuit cement grinding operation where a two-

compartment ball mill, mill filter and air separator are operated. On the other hand,

Figure 6.2 illustrates open circuit operation that is composed of single

compartment ball mill and mill filter. Technical specifications of the units are listed

in Table 6.1.

Table 6.1. Technical specifications of the units operating in Figure 6.1 and Figure

6.2

Open Circuit Ball Mill

Closed Circuit Ball Mill

Ball Mill

Ball Mill

Effective Diameter (m) 3.4

Effective Diameter (m) 3.4

Effective Length (m) 11

Length of 1st Chamber 4.25

Installed Motor Power (kW) 2000

Length of 2nd Chamber 7.0

Mill Filter Fan

Installed Motor Power (kW) 1696

Max. Air Quantity (m3/h) 35000

Mill Filter Fan

Fan Motor Power (kW) 40

Max. Air Quantity (m3/h) 35000

Fan Motor Power (kW) 55

Air Separator

Rotor Diameter (mm) 1860

Rotor Height (mm) 1150

Max. Rotor Velocity (rpm) 327

Installed Rotor Power (kW) 110

Max. Air Quantity (m3/h) 110000

Fan Motor Power (kW) 250

123

Prior to performing mass balancing studies followed by simulations, the sampling

campaigns around the circuits were arranged initially. Sampling survey is the

starting point of further studies therefore it should be performed carefully to

minimize errors that may occur in the following steps. In general, sampling study of

a grinding circuit is conducted at its optimum operating conditions. In other words,

the circuit is operated at its maximum allowable throughput rate that target

fineness is achieved.

Within the study, the circuits were operated around two hours until steady state

conditions were provided. Then each stream shown in the figures above was

sampled. Afterwards, the particle size distributions of the streams were determined

and then graphed to check whether the sampling surveys were performed reliably.

The mean values of operating conditions during sampling surveys are given in

Table 6.2. Moreover, Figure 6.3 and Figure 6.4 illustrate the measured size

distributions of closed circuit and open circuit milling respectively.

Table 6.2. Mean operating conditions during sampling surveys

Closed Circuit Milling Open Circuit Milling

Cement Production Type CEM I 42.5R CEM I 42.5R

Feed Rate (t/h) 34 27.4

Separator Reject Tonnage (t/h) 78 -

Separator Rotor (%) 77.6 -

Separator Rotor Power (kW) 82 -

Separator Air Fan Quantity (m3/h) 97000 -

Separator Air Fan Power (kW) 213 -

Mill Filter Fan Valve % 100 68

Mill Filter Fan Power (kW) 48 34

Mill Power (kW) 1467 1640

Elevator Amperage (A) 28.1 -

Chemical Name Sika 870 -

Chemical Dosage (g/t) 500 -

Cement Product Temperature (˚C) 103.5 113

Water Addition (L/h) - 779

124

Figure 6.3. Measured particle size distributions of the closed circuit sampling

campaign

Figure 6.4. Measured particle size distributions of the open circuit sampling

campaign

As seen from the figures above, the size distribution of each stream was

determined starting from their top size down to 1.8 µm. It is understood from the

figures that the sampling surveys were accomplished reliably as the obtained size

distributions around units are logical.

125

6.2. Mass Balance and Model Fitting of the Circuits

The measured particle size distributions were then used through the mass

balancing studies (with JK-Simmet) in order to calculate the flow rates of each

stream and to disperse errors that occurred during sampling campaigns. In Figure

6.5 and Figure 6.6, the flow rates of closed and open circuit milling are illustrated

respectively. Additionally Figure 6.7 and Figure 6.8 show comparison between

measured and calculated particle size distributions.

Figure 6.5. Calculated flow rates of closed circuit ball milling

Figure 6.6. Calculated flow rates of open circuit ball milling

126

Figure 6.7. Measured and calculated particle size distributions of closed circuit

milling

Figure 6.8. Measured and calculated particle size distributions of open circuit

milling

Good agreement between measured and calculated size distributions indicates

that the collected samples represent the milling conditions properly thus mass

balancing studies were performed with minimum error. Table 6.3 gives the

performances of the two circuit configurations with regards to energy consumption

and final product size.

127

Table 6.3. Performance evaluation of closed and open circuit ball milling

Closed Circuit

Milling Open Circuit

Milling

Total Power Utilization (kW) 1810 1674

Throughput Rate (t/h) 34 27.4

Specific Energy Consumption (kWh/t) 53.24 61.09

Final Product, P50 (µm) 13.74 15.68

Final Product, P80 (µm) 31.52 39.08

As can be understood from the table above, despite of being operated at the same

production types the production rates of the two circuits vary considerably. The

open circuit configuration is the more problematic one since there is a temperature

rising problem in the ball mill. The differences in product temperatures between

open and closed circuit can be followed from Table 6.2. The temperature problem

is due to the processed material and target fineness. In CEM I 42.5R type

production, 90% of the feed is composed of clinker having high temperature as it is

a product of pyrometallurgical operation (rotary kiln). Therefore it is inevitable to

obtain high product temperature levels as the feed material comes in ball mill is

already hot and target product is too fine for open circuit design. Because of that,

production rate is not allowed to be increased any further. In general, the

temperature in ball mills is controlled by providing sufficient amount of mill

ventilation however in open circuit designs ventilation directly affects the product

fineness. Therefore mill ventilation is limited with target fineness. Open circuit

design some drawbacks in cement production.

Mass balancing studies were followed by model fitting of each unit. For closed

circuit milling, model structures of ball mill and air separator were developed while

only ball mill model was fitted in open circuit milling. Within the study, ball mill

model was developed by using perfect mixing approach (Equations 5.1 and 5.2).

In Figure 6.9, back-calculated r/d parameters by using JK-SimMet software are

illustrated graphically.

128

Figure 6.9. r/d function of the ball mill in closed circuit (left) and open circuit (right)

Air separator model was fitted by using Whiten’s efficiency curve approach exist in

JK-SimMet. The mathematical expression of the equation, which uses overflow

efficiency of separation process, is given in Equation 6.1 [1].

(6.1)

Where;

X : d/d50c

d : Particle size

d50c : Corrected cut size

β : Parameter that controls the initial rise of the curve in fine sizes

α : Sharpness of separation

β* : Parameter that preserves the definition of d50c ; d=d50c when E=(1/2)C

C : Fraction subjected to real classification; (1-Bypass)

Eoa : The actual efficiency to overflow

The efficiency curve of air separator and back-calculated parameters are given in

Figure 6.10 and Table 6.4 respectively.

2)exp()exp(

1exp1*

*

X

XCEoa

129

Figure 6.10. Efficiency curve of air separator

Table 6.4. Efficiency curve parameters of air separation process

C 68.29

α 1.05

ß 1.58

d50c 0.04

ß* 2.82

6.3. Simulation Scenarios Prepared for Dry Stirred Mill Operation

6.3.1. The Use of Dry Horizontal Stirred Mill in Finish Grinding

The simulation scenario prepared for the finish grinding application was to push

the entire circuit to produce coarser product initially then performing final grinding

with a stirred media mill to achieve target fineness. While choosing the grind size

of closed cement grinding circuit, specific energy consumption of stirred media mill

was taken into consideration. In fact, it was aimed to consume around 8-10 kWh/t

of specific energy in stirred mill, which corresponds to RRd80 of 1.26 (Section

4.7.3, Figure 4.29), therefore the closed circuit system was pushed to produce P80

of 39 µm. Simulation studies showed that if the grinding circuit was operated at 50

t/h production rate, it would produce P80 of 39 µm (Figure 6.11). This size of

material was then fed to stirred mill to achieve target P80 of 31 µm (Figure 6.12).

130

Figure 6.11. The differences in product size distributions obtained from closed

circuit ball milling at different throughput rates

Figure 6.12. Simulation results of closed circuit operation

For closed circuit operation, another alternative including the use of a second

separator was developed. The second separator was commissioned ahead of the

existing one with the aim of collecting already finished particles and reducing the

work load of stirred mill. This alternative brings extra separator investment cost

however, leads to manufacturing of stirred mill with less capacity compared to the

previous one. The economic facts should be considered to find out which

alternative is feasible, either building a larger scale stirred mill (50 t/h) or

purchasing air separator and 30 t/h stirred mill. The simulation results are

illustrated in Figure 6.13. As can be seen from the figure, overall circuit production

rate was increased from 34 t/h to 55 t/h. With this alternative, 10% more increase

131

was achieved compared to the Figure 6.12. While assessing the energy

consumption of the circuit, motor size of the commissioned air separator was

considered as well. Half size of motor and fan of the existing separator was

thought to be sufficient.

Figure 6.13. Simulation results of closed circuit operation with double separator

The simulation study for open circuit operation was performed in the same

manner. Initially, the ball mill was simulated so as to produce P80 of 48 µm.

Simulation studies indicated that production rate of ball mill could be increased up

to 55 t/h (Figure 6.14 and Figure 6.15). Then the product of ball mill was fed to

stirred mill where desired product P80 of 39 µm was produced. In stirred media

milling operation size reduction of 1.24 was achieved that equalled to around 9

kWh/t specific energy consumption. Further assessments are presented in the

following tables.

Figure 6.14. The differences in product size distributions obtained from open

circuit ball milling at different throughput rates

132

Figure 6.15. Simulation results of open circuit operation

The practicability of stirred media milling operation was assessed by comparing

specific energy consumption of the entire circuit before and after the simulation

studies. In energy calculations, it was assumed that the power utilization of each

unit was independent of throughput rate. In other words, the same values of power

utilizations were used for simulated conditions. In addition to ball mill circuit,

specific energy of stirred milling was taken into consideration as well. As indicated

previously, 8-10 kWh/t of energy was required for stirred milling to obtain 1.24-1.27

of reduction ratio. Table 6.5 summarizes the obtained result from simulation

studies.

It is understood from Table 6.5 that the use of stirred media mill has brought two

main advantages which are; increased overall capacity and reduced specific

energy consumption. The calculations showed that 15.1% of energy saving was

achievable for closed circuit configuration where energy saving could reach up to

35.45% in open circuit design. With regards to capacity improvements, closed

circuit operation could increase its capacity by 47.06% while 103.7% increase is

possible for open circuit design (when compared with data in Table 6.2).

133

Table 6.5. Energy assessments of the simulation results obtained from closed and

open circuit operations

Closed Circuit Open Circuit

Alter. #1 Alter. #2

Total Power Utilization of Ball Mill Circuit (kW) 1810 1958 1674

Throughput Rate of Ball Mill Circuit (t/h) 50 55 55

Spec. Ener. Ball Mill Circuit (kWh/t) 36.20 35.59 30.44

Stirred Mill Feed, F80 (µm) 39 49 48

Stirred Mill Product, P80 (µm) 31 40 39

Spec. Ener. of Stirred Media Milling (kWh/t) 9 9 9

Total Energy (Ball Mill Circuit + Stirred Mill) kWh/t 45.2 44.59 39.44

Total Energy (Existing) kWh/t 53.24 53.24 61.09

Energy Saving of Stirred Mill Implementation (%) 15.1 16.24 35.45

It is thought that open circuit designs would benefit more from stirred mill

technology, in particular when a plant struggles with operating at high production

rates. With stirred mill implementation, open circuit design would not have

temperature rising problem anymore as coarser material would be produced from

the ball mill and fine grinding would be performed by stirred milling. Consequently,

more flexible grinding operation would be provided as the production type and

target fineness could easily be changed.

As a conclusion, the simulation studies showed that the finish grinding at dry

stirred mill is applicable as it improves capacity of circuit and decreases overall

specific energy consumptions. The obtained results so far are promising to assure

cement manufacturers to apply this technology into their existing cement grinding

circuits.

6.3.2. The Use of Dry Horizontal Stirred Mill on Filter Return Stream

The aim of using stirred mill on filter return stream is to produce cement with

desired specifications that could be sent directly to product bins. With this

application, considerable increase in overall throughput is expected as more fines

would be produced from filter product. In order to assess stirred mill application on

filter return stream, the simulation studies were carried out with the closed circuit

134

grinding operation (Figure 6.1), which was already model fitted. In addition to

existing models, the stirred mill model was introduced into the circuit with JK-

SimMet software. Stirred mill model was developed with the data produced from

Test 14 by using perfect mixing approach.

Once the model of each unit was developed, the studies focussed on simulating

the entire circuit at the same product fineness. Figure 6.16, Figure 6.17 and Table

6.6 summarize the results of simulation studies. As can be seen from the figure,

the production rate of 34 t/h (Figure 6.5) increased to 42 t/h at the same product

fineness in case stirred mill is used on filter return stream. According to the

calculations, dry stirred mill processes 14 t/h material.

Figure 6.16. The calculated flow rates of simulation studies

Figure 6.17. Product size distributions of the studies

The simulation results given in Table 6.6 imply that the use of stirred mill on filter

return stream contributes a 23.5% increase in production rate in the meantime

135

3.7% decrease in specific energy consumption of the grinding circuit. It seems the

stirred mill application is viable for filter return stream operations.

Table 6.6. Energy assessments of simulation results obtained from filter return

studies

Before

Simulation After

Simulation

Total Power Utilization of Ball Mill Circuit (kW) 1810 1810

Production Rate of the Circuit (t/h) 34 42

Final Product, P50 (µm) 14 14

Spec. Energy Consumption of Ball Mill Circuit (kWh/t) 53.24 43.09

Spec. Ener. Con. of Stirred Media Milling (kWh/t) - 8.2

Total Energy (Ball Mill Circuit + Stirred Mill) kWh/t 53.24 51.29

136

7. RESULTS and DISCUSSIONS

Within the context of the study, a dry horizontal stirred mill was developed, with the

partnership of Netzsch-Feinmahltechnik GmbH, to be used for cement grinding

purpose. As the dry horizontal stirred mill was newly developed technology, some

problems occurred owing to transportation difficulties of material along the mill,

then were solved with the use of water jacketed mill chamber, introduction of

grinding aids (adjusting the rheology of bulk material) and air supply from the feed

inlet. Ultimately, sustainable grinding operation was provided (Section 3.3). These

two parameters (grinding aid and air flow) were of crucial importance therefore

initial studies focussed on optimizing them. The results of the test studies were

presented in Section 4.1 and Section 4.2. After that, the rest of the test works

(stirrer speed, feed rate etc.) were performed at optimized conditions.

Stirred mill performance is affected by many parameters and in this study many of

them were investigated. The studies concluded that;

Increase of the stirrer speed produced finer material up to a point that further

addition of energy was converted into heat causing decreased efficiency of the

grinding operation.

Lower media fillings created inefficient grinding environment. In other words,

higher specific energies were required for lower media fillings to obtain higher size

reduction values.

The feed rate had mainly two major effects on grinding performance, which were

the specific energy consumption and the product fineness. Increasing feed rate

decreased specific energy consumption thus coarser product was obtained as

expected.

The selection of media size is another parameter affecting the grinding operation.

The experimental results showed that the use of finer media was advantageous

over the coarser one and 27% energy saving was achievable. Besides, it was

suggested that, with this design of the mill for cement grinding purpose, 4 mm

media size was the lowest limit as no difference in size reduction and energy

efficiency were observed compared to 3 mm media size.

137

The mill performance at varying feed size distribution was also investigated

(Section 4.7). The test results implied that obtained size reduction was much more

rapid for coarse grinding when compared with relatively fine feeding (Figure 4.30).

As a result of the test studies, F50 of 25 µm was determined as the optimum feed

size that efficient grinding took place (Figure 4.31). When the top size of this

material (150 µm) was divided by the media size used in grinding tests (4 mm),

Dball/Dparticle ratio was found as 27:1. The difference with the literature may be due

to mill design, material and media properties.

The studies showed that, the slope of the feed and product size distributions were

similar to each other for final product stream and filter return stream grinding. This

behaviour of the dry mill was different from IsaMill operations, where the

classification effect and the selective grinding of coarser particles were observed.

In contrast to results obtained from fine material test studies (final product and

filter return) separator reject stream test works indicated that that the slope of the

size distribution was affected by the operating parameters, i.e. feed rate, stirrer

speed, media filling. It is thought that further investigation would be beneficial to

understand whether the difference in n values of two size distributions, i.e. 1.3 and

0.8, have considerable effect on cement properties. Statistically there may not be

a significant difference between the two values however what really matter is its

effects on cement properties. In the literature some studies were carried out to

reveal the slope effect on cement strength [81; 82]. This was not in the scope of

this study.

Within the study, grinding performances of 42 L mill and 23 L mill were compared

on final product stream. The results concluded that 42 L mill utilized less energy

compared to 23 L mill operated at the same milling conditions. Furthermore the

benefits of using water circulation were revealed. It was understood that the use of

water jacketed mill chamber (23 L mill) enabled performing the tests at intense

conditions as well.

Stirrer design had effects on power draw thus specific energy consumption of the

mill. Within the study, the disc type stirrer was found to be an energy efficient

design in particular when operating at higher energy levels. However there could

138

still be room for the development therefore it would be beneficial to find out the

optimum stirrer design.

Throughout performance evaluation section (Section 4), surface area

measurements were performed as well (Section 4.1-4.7). The reliability of the

measurements were cross checked by drawing the trend between mean size of

the distributions (d50) and surface area. Figure 7.1 shows that as the product gets

finer, more surface area is obtained. The grinding action produces newly formed

surfaces therefore a direct correlation between the energy consumed and surface

area developed (Equation 7.1) is expected to be observed (Rittinger’s law).

Graphical representation of the results is illustrated in Figure 7.2.

Figure 7.1. The variation of Blaine with d50

Specific Energy = f (new surface area – old surface area) (7.1)

Figure 7.2. The relationship between specific energy consumption and surface

area development

139

As can be seen from Figure 7.2, surface area and specific energy consumption

parameters are directly proportional to each other. Decreasing slope at higher

energy levels indicates that the mill is getting close to its grinding limits while trying

to produce more surfaces. Similar trend was also reported by Pilevneli [83] who

conducted a Ph. D thesis on fine cement grinding with a vertical stirred mill.

Within the thesis dry stirred mill model was developed with the perfect mixing

approach. In these studies, r/d* values were back calculated (with JKSimMet

software) for a given feed and product size distributions as well as the breakage

distribution, which was determined by a novel technique developed by Ekşi [80].

The studies correlating r/d values with operating parameters concluded that, at

constant throughput (constant discharge rate) increasing stirrer speed and media

filling increased the breakage rate of the particles as well. Additionally, increasing

feed rate and feeding coarser material resulted in obtaining lower r/d function.

Similar conclusions have also been reached by Dikmen [33] who conducted a Ph.

D thesis on modelling of wet stirred mill.

Within the scope of the thesis, simulation studies were performed to find out

whether the use of stirred mill in existing cement grinding circuits was viable. For

this purpose, cement plant having two different circuit designs, open and closed

circuit, producing the cement with the same quality figures was selected. Initially

the sampling campaigns then the material characterization studies were

performed. Afterwards, the circuits were mass balanced and the models of each

unit were developed. Once the models were developed, varieties of simulation

scenarios were prepared. The simulation studies concluded that, the use of stirred

media mill had brought two main advantages which were; increased overall

capacity and reduced specific energy consumption. The calculations on finish

grinding applications of stirred mill showed that 15.1% of energy saving was

achievable for closed circuit configuration where energy saving could reach up to

35.45% in open circuit design. With regards to capacity improvements, closed

circuit operation could increase its capacity by 47.06% while 103.7% increase is

possible for open circuit design. Another simulation study was performed on filter

return stream application of stirred mill. The results implied that stirred mill

contributed a 23.5% increase in production rate in the meantime 2.2% decrease in

specific energy consumption of the grinding circuit. With its current performance,

140

stirred mill operation in cement grinding circuit seemed viable. However, further

improvements in performance could lead to lower energy figures that makes the

use of stirred mill inevitable in the future.

Although the simulation results are promising it is thought that dry stirred mill

performance would be improved. In this context future studies will be focussing on;

Finding optimum mill geometry

Further improvement in material transportation

Finding optimum stirrer design

For both cement producers and machine manufacturers wear rate of internal

components e.g., shaft, stirrers, media, is of crucial importance since it brings

extra operational and maintenance costs. Therefore, the future studies should

present the wear measurements as well.

In the final study of the thesis, a scaling up procedure was developed between 23

L mill and 42 L mill. Scaling up defines the probability of obtaining the same

grinding results irrespective of the mill dimensions. Kwade and Stender [60]

reported that the same grinding performance, in other words constant grinding,

could be obtained when the two of the net specific energy, stress intensity and

stress number parameters are set constant. Within the study, this methodology

was verified. The adjusted milling conditions for scaling-up tests are given in Table

7.1.

Table 7.1. The milling conditions adjusted for constant grinding tests at different

mill geometries

Mill Volume (L) 23* and 42

Media Type Steel

Stirrer Type Disc

Chemical Type EPCT-01

Chemical Amount (g/t) 700

Air Flowrate (L/h) 1000

* Water was not circulated from 23 L mill chamber

The experimental conditions and the obtained results are given in Table 7.2. As

can be understood from the table, the test works were performed at two cases

141

having different specific energy values. Additionally, for each of the cases, the

tests carried out at the same stirrer speed, media size and media type indicating

stress intensity parameter was constant. In the meantime, the feed rate was

adjusted to achieve the same net specific energy values. As a conclusion, it is

expected that the fixed net specific energy and stress intensity could lead

obtaining the same grinding results. Figure 7.3 illustrates the feed and product size

distributions of each test works.

Table 7.2. The experimental conditions and the obtained results from scaling up

tests

Case #1 Case #2

The experimental conditions Test 1 Test 2 Test 3 Test 4

Mill Volume (L) 23 42 23 42

Stirrer Speed (m/s) 4.34 4.34

Media Size (mm) 6 6

Media Filling (%) 50 50

Feed Rate (kg/h) 154.4 266.4 66.6 112.3

Feed Size F50 (µm) 22.28 22.89

The experimental results Test 1 Test 2 Test 3 Test 4

Mill Power (kW) 2.26 3.15 2.32 3.2

No Load Power (kW) 1.12 1.12 1.12 1.12

Specific Energy (kWh/t) 14.64 11.82 34.83 28.49

Net Specific Energy (kWh/t) 7.38 7.62 18.02 18.52

Stress Intensity Const. Const. Const. Const.

Product Size, P50 (µm) 17.56 17.24 13.97 13.86

Amount of material inside the mill (kg) 12.5 17.68 11.3 12.5

Material Load (%) 80.73 114.18 72.98 80.73

142

Figure 7.3. The feed and product size distributions of scaling up tests

The studies concluded that scaling up of the dry stirred mill could be accomplished

by applying the methodology developed by Kwade and Stender [60].

143

8. CONCLUSIONS

Within the thesis study, a prototype dry horizontal stirred mill manufactured

by Netzsch Feinmahltechnik GmbH was tested on cement grinding area.

Although the operational problems occurred during initial grinding studies,

they were overcome with the use of grinding chemical and air from the feed

inlet.

The studies showed that 18% improvement in product quality was

achievable provided that the proper chemical was selected and air flow was

adjusted.

The effects of operating conditions were investigated and compared with

the related literature.

Performance evaluation data produced with dry horizontal stirred mill was

used to optimize the milling conditions. Up to 27% saving in energy

consumption was obtained when operating conditions were adjusted

properly.

The dry horizontal stirred mill can be modelled with the perfect mixing

approach and the relationships between r/d* functions and operating

conditions can be developed.

Simulation studies performed at open and closed circuit configurations

where stirred mills were employed indicated that energy saving up to 35%

was achievable. The thesis study concluded that the dry horizontal stirred

mill application was viable with different cement grinding circuit

configurations.

144

REFERENCES

[1] Napier-Munn, T.J., Morrell, S., Morriison, R.D, Kojovic, T. (1996). Mineral

comminution circuits-Their operation and optimization. Brisbane: JKMRC

monograph series in mining and mineral processing.

[2] Young, J.D. and Gao, M. (2000). Performance of the IsaMills in the George

Fisher flowsheet. Seventh Mill Operators' Conference, October 12-14,

Kalgoorlie, Australia.

[3] Wang, Y., Forssberg, E., Sachweh, J. (2004). Dry fine comminution in a

stirred media mill-MaxxMill. International journal of Mineral Processing 74,

pp. 64-75.

[4] Stehr, N. (1988). Recent developments in stirred ball milling. International

Journal of Mineral Processing 22 (1-4), pp. 431-444.

[5] Schilling, E. R. and Yang, M. (2000). Attritor grinding mills and new

developments. Panamerican Coatings Conference (Mexico City).

[6] Sepulveda, J. L. (1981). A detailed study on stirred ball mill grinding. Ph. D

thesis, Department of Metallurgy and Metallurgical Engineering, The

University of Utah, the USA.

[7] Corrans, I. J. and Angove, J. E. (1991). Ultra fine milling for the recovery of

refractory gold. Minerals Engineering 4 (7-11), pp. 763-776.

[8] Jankovic, A. (2003). Variables affecting the fine grinding of minerals using

stirred mills. Minerals Engineering 16 (4), pp. 337-345.

[9] Keith, R. S. (1990). Hilton inaugurated a new step in Mt. Isa's future.

Engineering and Mining Journal 191 (10), pp. 32-35.

[10] Shi, F., Morrison, R., Cervelin, A., Burns, F., Musa, F. (2009). Comparsion

of energy efficiency between ball mills and stirred mills in coarse grinding.

Minerals Engineering 22 (7-8), pp. 673-680.

[11] Svedala. (1993). Energy-saving ultra fine grinding with SALA agitated mill.

Zement-Kalk-Gips 46 (9), pp. 600-601.

145

[12] Lofthhouse, C. H. and John, F. E. (1999). The Svedala detritor and the

metals industry. Minerals Engineering 12 (2), pp. 205-217.

[13] Jankovic, A. (1999). Mathematical modelling of stirred mills. Ph. D Thesis,

University of Queensland, JKMRC, Brisbane, Australia.

[14] Lynch, A. J. and Rowland, C. A. (2005). The history of grinding. Society for

Mining, Metallurgy and Exploration, Inc. (SME).

[15] Eirich Corp. (2013). TowerMill. http://www.eirich.com/en/tower-mill.

[16] Svedala. (1995). "SAM" The SALA agitated Mill. 2 pages.

[17] Metso. (2005c). Stirred Milling, Vertimill grinding mills & Stirred media

detritor. www.metsominerals.com, Brochure No: 2357-02-10 MBL.

[18] Allen, J. (2010). Advances in stirred milling: Improving profitability of copper

ore processing. Mining Weekly.

[19] Metso. (2005a). Vertimill, fine and ultra fine wet grinding.

www.metsominerals.com, Brochure No: 1727-04-05-MPR/York.

[20] Metso. (2005b). Stirred Media Detritor. www.metsominerals.com, Brochure

No: 340 02-02.

[21] Nassetti, G. and Hessling, G. (2003). Application of the MaxxMill fro the

final grinding of unglazed porcellanato tile mixes. Special reprint from the

technical magazine "cfi ceramic forum international" (1-2).

[22] Gerl, S. and Sachweh, J. (2007). Plant concepts for ultrafine dry grinding

with the agitated media mill MaxxMill®. Minerals Engineering 20 (4), pp.

327-333.

[23] Günter, H., Theisenn, W., Wetzel, D. (2000). MaxxMill - dry and wet

grinding with stirred ball mills in comparison with conventional tumbling

mills. Aufbereitungs Technik 41 (6), pp. 278-283.

[24] Durr, H. (1976). Patent No. U.S. Patent 3 957 210.

[25] Pease, J. D., Young, M. F., Curry, D. C. (2005). Fine grinding as enabling

technology - The IsaMill. Brisbane, Australia: Crushing and Grinding

Symposium.

146

[26] Gao, M., Young, M., Allum, P. (2002). IsaMill fine grinding technology and

its industrial applications at Mt. Isa Mines. Proceedings of the 34th annual

meeting of the Canadian Mineral Processors, Ottawa, Canada, pp. 171-

188.

[27] Way, H. (1997). Particle size reduction of ceramic powders using a small

media mill. Research and Development Department of Netzsch

Feinmahltechnik.

[28] Clark, L. (2007). Grinding comparison test of IsaMills with Tower Mills using

magnetite. Xstrata technology, Technical Notes.

[29] Burford, B. D. and Clark, L. (2007). IsaMill technology used in efficient

grinding circuits. VIII International Conference on Non-ferrous Ore

Processing. Wroclaw, Poland.

[30] Becker, M. and Schwedes, J. (1999). Comminution of ceramics in stirred

media mills and wear of grinding beads. Powder Technology 105 (1-3), pp.

374-381.

[31] Gao, M. and Forssberg, E. (1995). Prediction of product size distributions

for a stirred ball mill. Powder Technology 84 (2), pp. 101-106.

[32] Zheng, J., Harris, C. C., Somasundaran, P. (1996). A study on grinding and

energy input in stirred media mills. Powder Technology 86 (2), pp. 171-178.

[33] Dikmen, S. (2008). Modelling of the performance of stirred media mills in

regrinding circuits. Ankara, Turkey: Ph. D Thesis, Hacettepe University.

[34] Fadhel, H. and Frances, C. (2001). Wet batch grinding of alumina in a

stirred bead mill. Powder Technology 119 (2-3), pp. 257-268.

[35] Pilevneli, C. C., Kizgut, S., Toroglu, T., Cuhadaroglu, D., Yigit, E. (2004).

Open and closed circuit dry grinding of cement mill rejects in a pilot scale

vertical stirred mill. Powder Technolgy 139 (2), pp. 165-174.

[36] Sadler III, L. Y., Stanley, D. A, Brooks, D. R. (1975). Attrition mill operating

characteristics. Powder Technology 12 (1), pp. 19-28.

147

[37] Farber, Y. B., Durant, B., Bedesi, N. (2011). Effect of media size and

mechanical properties on milling efficiency and media consumption.

Minerals Engineering 24 (3-4), pp. 367-372.

[38] Mankosa, M. J., Adel, G. T., Yoon, R. H. (1986). Effect of media size in

stirred ball mill grinding of coal. Powder Technology 49 (1), pp. 75-82.

[39] Kwade, A., Blecher, L., Schwedes, J. (1996). Motion and stress intensity of

grinding beads in stirred media mill Part 2: Stress intensity and its effect on

comminution. Powder Technology 86 (1), pp. 69-76.

[40] Mende, S., Stenger, F., Peukert, W., Schwedes, J. (2004). Production of

sub micron particles by wet comminution in stirred media mills. Journal of

Materials Science 39 (16-17), pp. 5223-5226.

[41] Persson, H. and Forssberg, E. (1994). Fine grinding of a magnetite ore with

a stirred ball mill. Aufbereitungs Technik 35 (6), pp. 307-320.

[42] Schollbach, A. E. (1999). Influence of the grinding media size on

comminution in stirred ball mills with additional introduction of vibrations.

Aufbereitungs Technik 40 (6), pp. 259-267.

[43] Wang, Y. and Forssberg, E. (2000). Product size distribution in stirred

media mills. Minerals Engineering 13 (4), pp. 459-465.

[44] Gao, W. and Forssberg, E. (1993). A study on the effect of parameters in

stirred ball milling. International Journal of Mineral Processing 37 (1-2), pp.

45-59.

[45] Sivamohan, R. and Vachot, P. (1990). A comparative study of stirred and

vibratory mills for the fine grinding of muscovite, wollastonite and kaolinite.

Powder Technology 61 (2), pp. 518-523.

[46] Strasser, S., Somani, R. A., Dembla, A. K. (1997). Improvements in the

production of raw meal and cement by the combined use of roller press and

V-Separator. ZKG 3 (50) , 140-146.

[47] Klimpel, R. R. and Manfroy, W. (1978). Chemical grinding aids for

increasing throughput in the wet grinding of ores. Ind. Eng. Chem. Process

Des. Dev. 17 (4), pp. 518-523.

148

[48] Rehbinder, P. A., Kalinkovskaya, N. A. (1932). Decrease in the surface

energy of solid-bodies and the work of dispersion during formation of an

adsorption layer. J. Tech. Phys. USSR (2), p. 726-755.

[49] Kapur, P. C., Healy, T. W., Scales, P. J., Boger, D. V., Wilson, D. (1996).

Role of dispersant in kinetics and energetics of stirred ball mill grinding.

International Journal of Mineral Processing 47 (1-2), pp. 141-152.

[50] Zheng, J., Harris, C. C., Somasundaran, P. (1997). The effect of additives

on stirred media milling of limestone. Powder Technology 91 (3), pp. 173-

179.

[51] Choi, H., Lee, W., Kim, D. U., Kumar, S., Kim, S. S., Chung, H. S., Kim, J.

H., Ahn, Y. C. (2010). Effect of the grinding aids on the grinding energy

consumed during grinding of calcite in a stirred ball mill. Minerals

Engineering 23 (1), pp. 54-57.

[52] Blecher, L., Kwade, A., Schwedes, J. (1996). Motion and stress intensity of

grinding beads in stirred media mill. Part I: Energy density distribution and

motion of single grinding beads. Powder Technolgy 86 (1), pp. 59-68.

[53] Theuerkauf, J. and Schwedes, J. (2000). Investigation of Motion in Stirred

Media Mills. Chemical Engineering and Technology 23 (3), pp. 203-209.

[54] Eskin, D., Zhupanska, O., Hamey, R., Moudgil, B., Scarlett, B. (2005).

Microhydrodynamics of stirred media milling. Powder Technology 156 (23),

pp. 95-102.

[55] Jayasundara, C. T., Yang, R. Y., Yu, A. B., Curry, D. (2006). Discrete

particle simulation of particle flow in the IsaMill process. Industrial &

Engineering Chemistry Research 45 (18), pp. 6349-6359.

[56] Yang, R. Y., Jayasundara, C. T., Yu, A. B., Curry, D. (2006). DEM

simulation of the flow of grinding media in Isamill. Minerals Engineering 19

(10), pp. 984-994.

[57] Westhuizen, A. P., Govender, I., Mainza, A. N., Rubenstein, J. (2011).

Tracking the motion of media particles inside an IsaMill™ using PEPT .

Minerals Engineering 24 (3-4), pp. 195-204.

149

[58] Jayasundara, C. T., Yang, R. Y., Guo, B. Y., Yu, A. B., Govender, I.,

Mainza, A., Westhuizen, A., Rubenstein, J. (2010). CFD-DEM modelling of

particle flow in IsaMills – Comparison between simulations and PEPT

measurements. Minerals Engineering 24 (3-4), pp. 181-187.

[59] Kwade, A. (1999 (b)). Jayasundara, C. T., Yang, R. Y., Guo, B. Y., Yu, A.

B., Govender, I., Mainza, A., Westhuizen, A., Rubenstein, J. Powder

Technology, pp. 382-388.

[60] Kwade, A. and Stender, H. (1998). Constant grinding results at scale-up of

stirred media mills. Aufbereitungs Technik 39 (8), pp. 373-382.

[61] Jankovic, A. (2001). Media stress intensity analysis for vertical stirred mills.

Minerals Engineering (14 (10), pp. 1177-1186.

[62] Kwade, A. (1999 (a)). Wet comminution in stirred media mills — research

and its practical application. Powder Technology 105 (1-3), pp. 14-20.

[63] Kwade, A. (2003). A Stressing Model for the Description and Optimization

of Grinding Process. Chemical Engineering Technology 26 (2), pp. 199-

205.

[64] Weller, K. R., Gao, M., Bowen, P. (1999). “Scaling-Up Horizontal Stirred

Mills From a 4-Litre Test Mill. Powder Technology Symposium.

Pennsylvania State University, USA.

[65] Curry, D. C., Clark, L. W., Rule, C. (2005). Collaborative Technology

Development – Design and operation of the World’s largest stirred mill.

Randol Innovative Metallurgy Conference. Perth, Australia.

[66] Karbstein, H., Müller, F., Polke, R. (1996). Scale up for grinding in stirred

ball mills. Aufbereitungs Technik 37 (10), pp. 469-479.

[67] Gao, M-W., Forssberg, E., Welleri K. R. (1994). Power predictions for a pilot

scale stirred ball mill. Procc. XIII Europian Comminution Conference,

Stocholm, pp. 698-709.

[68] Herbst, J. A. and Sepulveda, J. L. (1978). Fundamentals of Fine and

Ultrafine Grinding in a Stirred Ball Mill. International Powder and Bulk Solids

Handling and Processing: Proceedings held Rosemount Illinois, pp. 452-

470.

150

[69] Tuzun, M. A. (1993). A detailed study on comminution in a vertical stirred

ball mill. South Africa: Ph. D Thesis, University of Nadal.

[70] Weit, H. and Schwedes, J. (1987). Scale-up of Power Consumption in

Agitated Ball Mills. Chemical Engineering Technology (10), pp. 398-404.

[71] Chryso. (2008). CHRYSO Cem AMT 2X, Technical grinding-aid.

uk.chryso.com/upload/t_documents/Fichier_L1/43002/CHRYSOAMT2X.pdf.

[72] Sottili, L. and Padovani, D. (2002). Effect of grinding aids in the cement

industry. Petrocem (pp. 1-16). Cement Additives Division-MAPEI Italy.

[73] Allen, T. (2003). Powder sampling and particle size determination first ed.

Elsevier Science 682 p.

[74] Austin, L. G., Klimpel, R. R., Luckie, P. T. (1984). Process engineering of

size reduction:Ball milling, 561 pages. Society of Mining Engineers of the

American Institute of Mining, Metallurgical and Petroleum Engineers.

[75] Krajcinovic, D. (1996). Damage mechanics. UK: Elsevier.

[76] Schönert, K. (1991). Advances in comminution fundamentals and impacts

on technology. Aufbereitungs Technik 32, pp. 487-494.

[77] Tavares, L. M. and King, R. P. (1988). Single-particle fracture under impact

loading. International Journal of Mineral Processing 54, pp. 1-28.

[78] Yue, J. and Klein, B. (2005). Particle breakage kinetics in horizontal stirred

mills. Minerals Engineering 18 (3), 325-331.

[79] Genc, O. (2008). An investigation on the effects of design and operational

parameters on grinding performance of multi-compartment ball mills used in

the cement industry. PhD Thesis, Mining Engineering Department,

University of Hacettepe, Ankara, Turkey.

[80] Eksi, D. (2011). Development of a test method for determination of

breakage behaviour of fine particles. Ankara, Turkey: MSc. Thesis,

Hacettepe University.

[81] Kuhlmann, K., Ellerbrock, H.-G., Sprung, S. (1985). Particle size distribution

and properties of cement. Zement-Kalk-Gips 85 (6), p. 136-144.

151

[82] Tsivilis, S., Tsimas, S., Benetatou, A., Haniotakis, E. (1990). Study on the

contribution of the fineness on cement strength. Zement-Kalk-Gips 43 (1),

p. 26-29.

[83] Pilevneli, C. C. (2003). Investigation of fine sized clinker grinding in a stirred

bead mill. Zonguldak, Turkey: Ph. D thesis, Zonguldan Karaelmas

University.

152

APPENDICES

153

Appendix-1. Cumulative passing size (%) of chemical tests (Section 4.1)

EPCT-04 EPCT-02 EPCT-01

Particle Size (µm) Feed

Test 1 Product

Test 2 Product

Test 3 Product

Test 4 Product

Test 5 Product

Test 6 Product

Test 7 Product

Test 8 Product

150 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

102 99.94 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

72 97.78 100.00 100.00 100.00 99.83 99.96 100.00 100.00 100.00

60 95.13 100.00 100.00 100.00 99.42 99.74 100.00 100.00 100.00

50 91.26 98.70 99.04 98.41 98.44 98.97 99.62 100.00 100.00

42 86.50 96.08 96.34 95.67 96.48 97.19 98.57 99.50 98.41

36 81.57 92.52 92.56 92.16 93.55 94.38 96.44 96.90 95.36

30 75.10 86.97 86.79 86.79 88.43 89.36 92.00 92.80 89.84

25 68.37 80.31 80.00 80.25 81.94 82.94 85.87 86.21 83.05

21 61.98 73.30 72.95 73.30 75.01 76.01 78.99 78.93 75.97

18 56.47 67.05 66.74 67.10 68.67 69.62 72.46 72.30 69.53

15 50.13 59.95 59.65 60.00 61.15 62.00 64.48 64.40 61.87

12 42.79 51.95 51.59 51.93 52.29 53.02 54.95 55.03 52.87

10 37.41 46.14 45.71 46.08 45.81 46.45 48.00 48.16 46.32

8.6 33.54 41.87 41.40 41.82 41.14 41.73 43.03 43.24 41.65

7.4 30.23 38.08 37.62 38.07 37.17 37.72 38.86 39.08 37.72

6.2 26.97 34.13 33.73 34.19 33.27 33.79 34.80 35.03 33.87

5.2 24.21 30.61 30.29 30.74 29.97 30.47 31.41 31.62 30.62

4.4 21.86 27.49 27.26 27.68 27.15 27.63 28.52 28.70 27.84

3.6 19.20 23.93 23.78 24.16 23.94 24.39 25.22 25.36 24.63

3 16.86 20.81 20.72 21.06 21.08 21.49 22.26 22.37 21.74

2.6 15.04 18.44 18.39 18.68 18.84 19.22 19.93 20.02 19.46

2.2 12.93 15.73 15.71 15.96 16.21 16.55 17.17 17.25 16.77

1.8 10.46 12.62 12.60 12.82 13.11 13.39 13.91 13.96 13.58

154

Appendix-2. Cumulative passing size (%) of chemical dosage tests (Section 4.1)

EPCT-01 Dosage Tests

Particle Size (µm) Feed 0 g/t Product 500 g/t Product 700 g/t Product 1000 g/t Product

150 100.00 100.00 100.00 100.00 100.00

102 98.74 100.00 100.00 100.00 100.00

72 95.47 99.14 99.47 99.71 99.73

60 92.06 97.73 98.11 98.83 98.94

50 86.98 94.75 95.46 96.64 96.94

42 80.73 90.06 91.31 92.77 93.24

36 74.48 84.60 86.32 87.85 88.40

30 66.78 77.15 79.25 80.67 81.22

25 59.42 69.63 71.78 73.02 73.51

21 52.94 62.83 64.82 65.90 66.32

18 47.65 57.20 58.95 59.95 60.28

15 41.80 50.88 52.34 53.27 53.49

12 35.24 43.63 44.77 45.64 45.74

10 30.58 38.36 39.26 40.10 40.12

8.6 27.26 34.55 35.29 36.10 36.07

7.4 24.44 31.28 31.89 32.67 32.61

6.2 21.65 28.01 28.50 29.24 29.16

5.2 19.28 25.19 25.60 26.30 26.21

4.4 17.28 22.76 23.12 23.76 23.68

3.6 15.06 19.98 20.29 20.88 20.81

3 13.13 17.52 17.80 18.32 18.26

2.6 11.66 15.61 15.86 16.33 16.28

2.2 9.98 13.39 13.61 14.02 13.98

1.8 8.07 10.82 11.00 11.33 11.30

155

Appendix-3. Cumulative passing size (%) of air flow tests (Section 4.2)

Particle Size (µm) Feed Test 9 Product Test 10 Product Test 11 Product

150 100.00 100.00 100.00 100.00

102 98.61 99.77 99.78 99.89

72 93.87 98.23 98.24 98.19

60 89.40 95.84 95.88 95.88

50 83.27 91.45 91.45 91.54

42 76.00 85.17 85.04 85.21

36 68.94 78.36 78.11 78.32

30 60.54 69.60 69.27 69.50

25 52.78 61.13 60.79 61.04

21 46.09 53.76 53.43 53.71

18 40.71 47.84 47.50 47.83

15 34.86 41.40 41.05 41.41

12 28.47 34.24 33.89 34.22

10 24.03 29.16 28.83 29.10

8.6 20.96 25.60 25.30 25.50

7.4 18.45 22.65 22.38 22.55

6.2 16.08 19.85 19.61 19.77

5.2 14.18 17.60 17.38 17.57

4.4 12.64 15.77 15.57 15.78

3.6 10.99 13.79 13.60 13.85

3 9.59 12.08 11.91 12.17

2.6 8.52 10.78 10.61 10.89

2.2 7.31 9.27 9.12 9.39

1.8 5.92 7.53 7.40 7.64

156

Appendix-4. Cumulative passing size (%) of stirrer speed tests (Section 4.3)

Particle Size (µm) Feed

Test 12 Product

Test 13 Product

Test 14 Product

Test 15 Product

Test 16 Product

Test 17 Product

Test 18 Product

Test 19 Product

150 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

102 99.08 100.00 100.00 100.00 100.00 99.66 100.00 100.00 100.00

72 91.97 98.77 98.19 99.44 100.00 97.49 98.79 98.58 98.96

60 84.86 96.62 95.70 97.67 99.35 94.83 96.29 96.28 97.12

50 76.29 92.28 91.57 93.95 97.13 90.45 91.78 92.37 93.61

42 67.40 85.93 85.80 88.29 92.88 84.28 85.73 86.88 88.31

36 59.53 79.06 79.42 81.88 87.47 77.38 79.23 80.74 82.21

30 50.86 70.34 71.01 73.32 79.67 68.35 70.81 72.55 74.01

25 43.50 61.94 62.73 64.79 71.40 59.77 62.62 64.44 65.94

21 37.73 54.55 55.40 57.18 63.71 52.57 55.57 57.35 58.93

18 33.42 48.55 49.47 51.01 57.31 47.03 50.03 51.73 53.37

15 28.96 41.97 42.96 44.29 50.22 41.22 44.13 45.72 47.36

12 24.20 34.73 35.70 36.85 42.28 34.93 37.60 39.09 40.64

10 20.90 29.63 30.55 31.59 36.61 30.48 32.93 34.36 35.79

8.6 18.59 26.04 26.90 27.87 32.59 27.34 29.57 30.97 32.30

7.4 16.65 23.03 23.85 24.73 29.19 24.67 26.70 28.07 29.31

6.2 14.75 20.11 20.89 21.69 25.85 22.07 23.86 25.19 26.32

5.2 13.14 17.71 18.45 19.18 23.06 19.86 21.43 22.72 23.76

4.4 11.79 15.72 16.44 17.09 20.71 17.98 19.35 20.58 21.55

3.6 10.27 13.58 14.26 14.82 18.11 15.85 17.00 18.13 19.00

3 8.96 11.77 12.40 12.89 15.84 13.95 14.92 15.95 16.73

2.6 7.96 10.41 11.00 11.43 14.10 12.47 13.30 14.25 14.94

2.2 6.82 8.88 9.40 9.77 12.10 10.74 11.43 12.25 12.86

1.8 5.51 7.15 7.59 7.89 9.79 8.72 9.25 9.93 10.42

157

Appendix-5. Cumulative passing size (%) of feed rate tests (Section 4.4)

Particle Size (µm) Feed Test 20 Product Test 21 Product Test 22 Product

150 100.00 100.00 100.00 100.00

102 98.05 99.84 99.52 99.69

72 91.56 98.95 98.21 97.76

60 86.11 97.39 96.30 94.69

50 79.25 94.18 92.35 89.30

42 71.61 89.01 86.22 82.06

36 64.41 82.82 79.28 74.64

30 56.04 74.27 70.24 65.63

25 48.58 65.65 61.57 57.30

21 42.52 58.02 54.19 50.27

18 37.87 51.94 48.41 44.73

15 32.98 45.45 42.28 38.80

12 27.69 38.38 35.64 32.30

10 23.99 33.44 31.01 27.72

8.6 21.39 29.96 27.76 24.50

7.4 19.20 27.03 25.03 21.80

6.2 17.06 24.15 22.37 19.19

5.2 15.27 21.72 20.11 17.03

4.4 13.74 19.63 18.19 15.24

3.6 12.03 17.27 16.01 13.28

3 10.53 15.18 14.08 11.60

2.6 9.38 13.55 12.57 10.31

2.2 8.05 11.65 10.81 8.84

1.8 6.52 9.45 8.77 7.15

158

Appendix-6. Cumulative passing size (%) of media filling tests (Section 4.5)

Particle Size (µm) Feed Test 23 Product Test 24 Product Test 25 Product Test 26 Product

150 100.00 100.00 100.00 100.00 100.00

102 98.48 98.48 99.30 99.51 99.69

72 91.20 91.46 94.82 96.60 97.76

60 85.20 85.77 89.80 92.73 94.69

50 77.89 78.83 82.86 86.71 89.30

42 70.05 71.27 75.01 79.21 82.06

36 62.89 64.22 67.73 71.81 74.64

30 54.72 56.03 59.31 62.97 65.63

25 47.40 48.60 51.68 54.89 57.30

21 41.30 42.38 45.27 48.09 50.27

18 36.50 37.51 40.25 42.75 44.73

15 31.34 32.31 34.87 37.03 38.80

12 25.73 26.66 28.97 30.76 32.30

10 21.82 22.71 24.79 26.35 27.72

8.6 19.10 19.95 21.84 23.25 24.50

7.4 16.84 17.65 19.38 20.66 21.80

6.2 14.69 15.44 17.01 18.17 19.19

5.2 12.94 13.63 15.07 16.11 17.03

4.4 11.52 12.14 13.48 14.42 15.24

3.6 9.98 10.53 11.74 12.56 13.28

3 8.68 9.16 10.25 10.97 11.60

2.6 7.69 8.12 9.12 9.75 10.31

2.2 6.58 6.95 7.82 8.36 8.84

1.8 5.32 5.61 6.33 6.76 7.15

159

Appendix-7. Cumulative passing size (%) of media size tests (Section 4.6)

Particle Size (µm) Feed

Test 27 Product

Test 28 Product

Particle Size (µm) Feed

Test 29 Product

Test 30 Product

Test 31 Product

150 100.00 100.00 100.00

850 100.00 100.00 100.00 100.00

102 98.38 99.83 99.48

600 99.88 100.00 100.00 100.00

86 95.72 99.48 98.71

425 99.73 99.98 99.99 99.99

72 92.09 98.68 97.42

300 98.99 99.92 99.94 99.97

60 87.49 96.88 95.06

212 98.03 99.81 99.87 99.93

50 81.83 93.41 91.04

150 95.11 99.52 99.74 99.84

42 75.36 88.17 85.40

102 85.48 98.94 99.61 99.16

36 69.02 82.17 79.27

72 70.75 96.73 97.39 97.21

30 61.28 74.09 71.37

60 60.32 93.41 94.02 94.24

25 53.92 65.99 63.68

50 48.81 87.88 88.29 88.84

21 47.53 58.74 56.88

42 38.09 80.63 80.65 81.21

18 42.38 52.81 51.32

36 29.76 73.29 72.83 73.14

15 36.75 46.28 45.18

30 21.89 64.48 63.38 63.24

12 30.52 38.98 38.24

25 16.43 56.49 54.83 54.27

10 26.11 33.76 33.24

21 13.01 49.85 47.80 46.96

8.6 23.01 30.03 29.65

18 10.90 44.65 42.40 41.41

7.4 20.41 26.87 26.59

15 9.04 39.06 36.71 35.63

6.2 17.89 23.76 23.58

12 7.35 32.83 30.54 29.43

5.2 15.81 21.14 21.03

10 6.28 28.37 26.22 25.16

4.4 14.10 18.93 18.88

8.6 5.58 25.20 23.19 22.19

3.6 12.23 16.49 16.49

7.4 5.00 22.54 20.67 19.75

3 10.64 14.39 14.41

6.2 4.44 19.95 18.24 17.43

2.6 9.44 12.78 12.81

5.2 3.97 17.80 16.25 15.54

2.2 8.07 10.94 10.98

4.4 3.56 16.01 14.59 13.97

1.8 6.52 8.83 8.87

3 2.72 12.29 11.19 10.75

160

Appendix-7 (Cont.). Cumulative passing size (%) of media size tests (Section 4.6)

Particle Size (µm) Feed Test 32 Product Test 33 Product

150 100.00 100.00 100.00

102 100.00 100.00 100.00

72 96.98 99.81 99.69

60 92.89 99.01 98.84

50 86.87 96.84 96.76

42 80.00 92.66 92.76

36 73.40 87.08 87.32

30 65.19 78.77 79.15

25 57.12 69.96 70.47

21 49.97 61.89 62.52

18 44.20 55.30 55.99

15 38.00 48.14 48.82

12 31.31 40.25 40.89

10 26.70 34.69 35.30

8.6 23.50 30.78 31.36

7.4 20.81 27.51 28.05

6.2 18.22 24.34 24.84

5.2 16.07 21.72 22.17

4.4 14.29 19.52 19.92

3.6 12.36 17.09 17.43

3 10.73 14.98 15.26

2.6 9.50 13.35 13.58

2.2 8.11 11.46 11.66

1.8 6.53 9.28 9.44

161

Appendix-8. Cumulative passing size (%) of final product tests (Section 4.7.1)

Particle Size (µm) Feed

Test 34 Product

Test 35 Product

Test 36 Product

Test 37 Product

Test 38 Product

Test 39 Product

Test 40 Product

Test 41 Product

150 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

102 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 99.79

72 98.55 99.26 100.00 100.00 100.00 100.00 100.00 98.69 99.05

60 96.60 98.43 100.00 100.00 100.00 100.00 100.00 97.35 98.18

50 93.33 97.11 99.64 99.45 98.72 99.05 98.77 95.24 96.47

42 88.82 95.08 98.63 98.33 95.91 96.84 96.45 92.06 93.60

36 83.93 92.36 96.77 96.62 91.99 93.55 93.31 87.98 89.82

30 77.32 87.77 93.03 93.41 85.85 88.19 88.40 81.63 84.02

25 70.25 81.83 87.72 88.87 78.74 81.74 82.69 74.52 77.39

21 63.39 75.21 81.59 83.55 71.47 74.87 76.79 67.62 70.59

18 57.42 68.97 75.67 78.26 64.94 68.49 71.29 61.59 64.39

15 50.50 61.44 68.28 71.41 57.26 60.81 64.43 54.53 56.98

12 42.43 52.55 59.15 62.66 48.31 51.71 55.89 46.21 48.26

10 36.53 46.04 52.27 55.88 41.82 45.06 49.38 40.06 41.90

8.6 32.29 41.34 47.24 50.83 37.19 40.28 44.54 35.59 37.35

7.4 28.70 37.32 42.91 46.39 33.29 36.22 40.29 31.76 33.49

6.2 25.20 33.32 38.58 41.86 29.48 32.24 35.96 27.97 29.70

5.2 22.31 29.91 34.87 37.91 26.32 28.88 32.20 24.78 26.51

4.4 19.92 26.99 31.65 34.42 23.66 26.03 28.94 22.10 23.82

3.6 17.31 23.68 27.95 30.37 20.72 22.82 25.25 19.15 20.82

3 15.08 20.77 24.62 26.73 18.15 20.01 22.03 16.62 18.20

2.6 13.39 18.51 22.01 23.87 16.17 17.83 19.56 14.72 16.19

2.2 11.46 15.88 18.94 20.52 13.88 15.30 16.73 12.56 13.87

1.8 9.25 12.81 15.31 16.57 11.22 12.35 13.45 10.08 11.19

162

Appendix-8 (Cont.). Cumulative passing size (%) of final product tests (Section 4.7.1)

Particle Size (µm)

Test 42 Product

Test 43 Product

Test 44 Product

Test 45 Product

Test 46 Product

Test 47 Product

Test 48 Product

Test 49 Product

Test 50 Product

Test 51 Product

150 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

102 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

72 100.00 100.00 100.00 99.82 99.98 100.00 100.00 100.00 100.00 100.00

60 100.00 100.00 99.95 99.23 99.73 100.00 99.46 99.68 99.65 99.33

50 98.74 99.52 99.81 98.06 98.95 98.06 98.23 98.88 98.75 97.91

42 96.14 98.61 99.37 96.14 97.47 94.90 96.11 97.83 97.22 95.73

36 92.35 96.98 98.29 93.31 95.09 91.01 93.16 96.45 94.90 92.77

30 86.34 93.46 95.58 88.13 90.49 84.88 88.11 93.82 90.49 87.67

25 79.72 88.22 91.22 81.40 84.31 77.67 81.74 89.99 84.60 81.32

21 73.30 82.02 85.74 74.30 77.66 70.49 74.98 85.19 78.07 74.56

18 67.63 75.93 80.13 67.77 71.47 64.06 68.68 79.95 71.79 68.22

15 60.82 68.31 72.85 59.87 63.87 56.31 60.94 72.60 63.95 60.44

12 52.63 59.07 63.73 50.44 54.63 47.17 51.65 62.94 54.53 51.20

10 46.54 52.21 56.93 43.58 47.80 40.62 44.90 55.65 47.69 44.51

8.6 42.09 47.22 51.98 38.71 42.89 36.03 40.10 50.39 42.82 39.76

7.4 38.23 42.91 47.69 34.63 38.72 32.21 36.07 45.92 38.69 35.72

6.2 34.30 38.56 43.30 30.69 34.61 28.53 32.15 41.52 34.59 31.72

5.2 30.86 34.79 39.43 27.44 31.14 25.47 28.87 37.75 31.09 28.30

4.4 27.85 31.49 36.00 24.71 28.18 22.89 26.08 34.43 28.05 25.38

3.6 24.39 27.68 32.01 21.66 24.82 20.00 22.92 30.54 24.58 22.08

3 21.33 24.29 28.39 18.99 21.83 17.48 20.12 26.99 21.51 19.22

2.6 18.96 21.64 25.50 16.92 19.48 15.54 17.94 24.17 19.13 17.03

2.2 16.23 18.57 22.06 14.52 16.75 13.31 15.40 20.82 16.38 14.54

1.8 13.06 14.97 17.98 11.73 13.53 10.74 12.44 16.85 13.18 11.67

163

Appendix-9. Cumulative passing size (%) of separator reject tests (Section 4.7.2)

Particle Size (µm) Feed

Test 52 Product

Test 53 Product

Test 54 Product

Test 55 Product

Test 56 Product

Test 57 Product

Test 58 Product

Test 59 Product

850 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

600 99.80 100.00 100.00 100.00 99.97 100.00 100.00 99.95 100.00

425 99.38 99.98 100.00 100.00 99.92 100.00 100.00 99.87 100.00

300 98.33 99.97 99.99 99.95 99.78 100.00 99.97 99.69 99.99

212 95.80 99.92 99.95 99.91 99.45 100.00 99.89 99.23 99.99

150 89.89 99.79 99.74 99.74 98.40 100.00 99.47 96.89 99.94

102 73.61 97.42 97.33 97.82 92.42 95.38 97.44 86.31 99.45

72 55.06 87.89 89.85 91.96 76.92 83.71 88.26 69.79 96.29

50 34.81 71.30 75.05 79.24 53.40 64.12 71.58 48.64 86.51

42 26.70 62.52 66.73 72.22 43.70 54.76 62.92 39.81 79.46

36 21.02 55.15 59.63 66.12 36.75 47.46 55.86 33.40 72.64

30 16.10 47.45 52.13 59.33 30.38 40.16 48.41 27.45 64.53

25 12.67 41.07 45.85 53.24 25.54 34.27 42.08 22.98 57.07

21 10.38 35.99 40.76 48.07 21.98 29.78 37.07 19.70 50.75

18 8.87 32.07 36.76 43.88 19.39 26.47 33.31 17.31 45.72

15 7.50 27.89 32.39 39.20 16.77 23.03 29.30 14.90 40.25

10 5.42 20.09 23.90 29.69 12.05 16.68 21.58 10.71 29.71

8.6 4.85 17.82 21.36 26.75 10.72 14.84 19.30 9.53 26.58

7.4 4.37 15.93 19.22 24.22 9.60 13.31 17.39 8.54 23.93

6.2 3.86 14.07 17.10 21.68 8.53 11.82 15.53 7.57 21.31

5.2 3.40 12.53 15.33 19.52 7.65 10.58 13.97 6.73 19.09

3.6 2.56 9.82 12.12 15.54 6.06 8.37 11.14 5.25 15.08

3 2.19 8.58 10.63 13.65 5.32 7.35 9.80 4.57 13.23

2.2 1.62 6.55 8.14 10.48 4.09 5.63 7.54 3.47 10.14

164

Appendix-9 (Cont.). Cumulative passing size (%) of separator reject tests (Section 4.7.2)

Particle Size (µm)

Test 60 Product

Test 61 Product

Test 62 Product

Test 63 Product

Test 64 Product

Test 65 Product

850 100.00 100.00 100.00 100.00 100.00 100.00

600 100.00 100.00 100.00 99.99 100.00 100.00

425 100.00 99.99 100.00 99.96 100.00 100.00

300 100.00 99.97 100.00 99.89 100.00 100.00

212 100.00 99.91 99.98 99.69 100.00 100.00

150 100.00 99.59 99.88 99.01 100.00 100.00

102 99.88 98.07 99.41 96.25 100.00 100.00

72 98.47 89.85 95.69 83.99 100.00 95.99

50 92.12 72.37 83.36 63.38 96.88 83.09

42 86.71 62.76 74.76 53.38 92.64 74.45

36 81.24 55.06 67.24 45.96 87.14 66.69

30 74.24 47.21 59.06 38.87 79.29 58.16

25 67.21 40.64 51.90 33.16 71.28 50.83

21 60.82 35.41 45.96 28.69 64.09 44.91

18 55.58 31.47 41.33 25.35 58.18 40.30

15 49.72 27.36 36.30 21.87 51.58 35.31

10 37.91 19.76 26.51 15.64 38.56 25.73

8.6 34.27 17.59 23.64 13.87 34.63 22.93

7.4 31.15 15.77 21.23 12.36 31.29 20.59

6.2 28.04 13.98 18.87 10.85 28.00 18.33

5.2 25.37 12.48 16.86 9.57 25.21 16.46

3.6 20.46 9.79 13.27 7.32 20.10 13.11

3 18.09 8.56 11.64 6.33 17.68 11.54

2.2 14.01 6.56 8.94 4.77 13.58 8.89

165

Appendix-10. Cumulative passing size (%) of mill geometry tests (Section 4.8)

Particle Size (µm) Feed

Test 66 Product

Test 67 Product

Test 68 Product

Test 69 Product

Test 70 Product

Test 71 Product

Test 72 Product

Test 73 Product

150 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

102 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

72 98.93 99.89 100.00 100.00 100.00 100.00 99.19 99.76 100.00

60 97.10 99.36 100.00 100.00 99.65 100.00 98.00 99.17 99.28

50 93.82 97.86 98.67 99.33 98.90 99.32 95.93 97.74 97.76

42 89.64 95.04 96.26 97.54 97.43 97.81 92.80 95.13 95.24

36 85.20 91.31 93.01 94.66 95.10 95.48 88.94 91.57 92.03

30 78.98 85.58 87.70 89.60 90.78 91.41 83.11 85.87 86.94

25 72.00 78.96 81.20 83.23 85.06 86.20 76.40 79.08 80.73

21 65.06 72.13 74.42 76.43 78.73 80.46 69.56 72.08 74.12

18 58.99 65.89 68.33 70.19 72.75 75.02 63.38 65.73 68.00

15 51.99 58.44 61.09 62.71 65.39 68.25 56.07 58.20 60.59

12 43.86 49.67 52.47 53.84 56.46 59.94 47.50 49.34 51.79

10 38.00 43.28 46.10 47.34 49.83 53.75 41.26 42.88 45.41

8.6 33.80 38.72 41.48 42.66 45.04 49.22 36.79 38.28 40.84

7.4 30.20 34.85 37.54 38.68 40.95 45.25 32.99 34.39 36.93

6.2 26.62 31.05 33.64 34.75 36.91 41.15 29.26 30.60 32.98

5.2 23.58 27.85 30.34 31.40 33.46 37.49 26.13 27.43 29.57

4.4 21.01 25.12 27.51 28.50 30.46 34.23 23.48 24.75 26.62

3.6 18.20 22.04 24.27 25.16 26.99 30.40 20.53 21.74 23.30

3 15.81 19.34 21.38 22.17 23.84 26.92 17.95 19.09 20.39

2.6 14.01 17.23 19.11 19.82 21.35 24.15 15.97 17.03 18.16

2.2 11.98 14.79 16.45 17.05 18.41 20.87 13.69 14.63 15.60

1.8 9.68 11.94 13.30 13.78 14.90 16.99 11.05 11.83 12.65

166

Appendix-10 (Cont.). Cumulative passing size (%) of mill geometry tests (Section 4.8)

Particle Size (µm) Test 74 Product Test 75 Product Test 76 Product Test 77 Product Test 78 Product

150 100.00 100.00 100.00 100.00 100.00

102 100.00 100.00 100.00 100.00 100.00

72 99.92 100.00 100.00 100.00 100.00

60 99.26 98.95 100.00 100.00 100.00

50 97.19 96.80 99.13 99.81 99.15

42 93.72 93.55 97.57 99.08 97.09

36 89.63 89.75 95.47 97.49 93.87

30 83.70 84.17 91.73 94.02 88.34

25 76.97 77.69 86.43 88.97 81.62

21 70.13 70.96 80.08 83.06 74.64

18 63.97 64.84 73.84 77.31 68.32

15 56.63 57.51 66.06 70.13 60.80

12 47.99 48.85 56.73 61.34 51.94

10 41.75 42.57 49.98 54.80 45.47

8.6 37.28 38.09 45.15 50.05 40.81

7.4 33.47 34.25 41.01 45.96 36.82

6.2 29.69 30.42 36.82 41.82 32.86

5.2 26.46 27.14 33.15 38.20 29.48

4.4 23.73 24.34 29.95 34.99 26.57

3.6 20.70 21.22 26.31 31.19 23.28

3 18.07 18.52 23.11 27.67 20.38

2.6 16.06 16.47 20.63 24.85 18.14

2.2 13.77 14.13 17.76 21.47 15.55

1.8 11.15 11.44 14.43 17.43 12.53

167

Appendix-11. Cumulative passing size (%) of stirrer type tests (Section 4.9)

Particle Size (µm) Feed

Test 79 Product

Test 80 Product

Test 81 Product

Test 82 Product

Test 83 Product

Test 84 Product

Test 85 Product

Test 86 Product

Test 87 Product

850 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

600 99.80 100.00 100.00 100.00 100.00 99.99 100.00 99.98 99.97 99.94

425 99.38 100.00 100.00 99.99 100.00 99.96 100.00 99.96 99.92 99.86

300 98.33 99.99 100.00 99.97 100.00 99.89 100.00 99.93 99.80 99.68

212 95.80 99.99 100.00 99.91 99.98 99.69 100.00 99.85 99.62 99.33

150 89.89 99.94 100.00 99.59 99.88 99.01 100.00 99.30 99.09 98.29

102 73.61 99.45 99.88 98.07 99.41 96.25 99.79 98.27 96.07 93.72

72 55.06 96.29 98.47 89.85 95.69 83.99 97.45 91.58 84.29 79.54

50 34.81 86.51 92.12 72.37 83.36 63.38 89.19 75.85 64.58 59.11

42 26.70 79.46 86.71 62.76 74.76 53.38 82.87 67.13 55.40 50.09

36 21.02 72.64 81.24 55.06 67.24 45.96 76.46 59.78 48.02 42.96

30 16.10 64.53 74.24 47.21 59.06 38.87 68.60 52.03 40.50 35.82

25 12.67 57.07 67.21 40.64 51.90 33.16 61.22 45.51 34.54 30.26

21 10.38 50.75 60.82 35.41 45.96 28.69 54.85 40.26 30.06 26.16

18 8.87 45.72 55.58 31.47 41.33 25.35 49.71 36.19 26.73 23.16

15 7.50 40.25 49.72 27.36 36.30 21.87 44.03 31.79 23.20 20.03

10 5.42 29.71 37.91 19.76 26.51 15.64 32.89 23.36 16.64 14.38

8.6 4.85 26.58 34.27 17.59 23.64 13.87 29.54 20.86 14.77 12.80

7.4 4.37 23.93 31.15 15.77 21.23 12.36 26.68 18.77 13.22 11.48

6.2 3.86 21.31 28.04 13.98 18.87 10.85 23.87 16.71 11.69 10.18

5.2 3.40 19.09 25.37 12.48 16.86 9.57 21.49 14.99 10.40 9.07

3.6 2.56 15.08 20.46 9.79 13.27 7.32 17.14 11.89 8.12 7.07

3 2.19 13.23 18.09 8.56 11.64 6.33 15.08 10.44 7.08 6.15

2.2 1.62 10.14 14.01 6.56 8.94 4.77 11.59 8.01 5.38 4.67

168

Appendix-11 (Cont.). Cumulative passing size (%) of stirrer type tests (Section 4.9)

Particle Size (µm) Test 88 Product Test 89 Product Test 90 Product

850 100.00 100.00 100.00

600 100.00 100.00 99.96

425 100.00 99.99 99.91

300 100.00 99.97 99.77

212 100.00 99.93 99.48

150 100.00 99.77 98.72

102 100.00 97.85 94.97

72 97.64 90.91 81.95

50 90.91 78.46 62.77

42 85.12 71.41 54.10

36 79.06 65.01 47.27

30 71.43 57.69 40.38

25 64.33 49.08 34.79

21 58.24 45.55 30.40

18 53.31 41.14 27.07

15 47.80 36.30 23.53

10 36.63 26.84 16.90

8.6 33.19 23.99 14.98

7.4 30.26 21.58 13.37

6.2 27.35 19.21 11.83

5.2 24.87 17.20 10.55

3.6 20.18 13.60 8.33

3 17.88 11.92 7.31

2.2 13.85 9.13 5.62

169

CURRICULUM VITAE

Personal Information

Name Surname : Okay Altun

Birth Place : Ankara

Marital Status : Married

E-mail : okyaltun@hacettepe.edu.tr

Address : Ümit Mahallesi Meksika Cad. 2449. Sokak Defne 9 No:3 Blok Daire No:40 Ümitköy Ankara

Education

High School : Ayrancı High School (1996-1999)

Graduate : Hacettepe University Mining Engineering Department (1999-2004)

Master of Science : Hacettepe University Mining Engineering Department (2004-2007)

Doctor of Philosophy : Hacettepe University Mining Engineering Department (2007-2013)

Language

English/86.25 ÜDS

Job Experience

Research Assistant at Hacettepe University Mining Engineering Department : 2005-2013

Area of Experience

Performance evaluation, modelling and simulation of wet and dry comminution circuits.

Developed Projects from the Thesis

Publications from the Thesis

Okay Altun, Hakan Benzer, Udo Enderle, 2013, Effects of operating parameters on the efficiency of dry stirred milling, Minerals Engineering 43-44, pp. 58-66

170

Presentations from the Thesis

Effects of operating parameters on the efficiency of dry stirred milling, Oral presentation, Comminution’ 2012, Cape Town, South Africa