FULLY AUTOGENOUS GRINDING AT UNKI MINE CONCENTRATOR … · FULLY AUTOGENOUS GRINDING AT UNKI MINE...

The Southern African Institute of Mining and Metallurgy

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T. Nyakudarika, S. Makumbe, C.Rule

FULLY AUTOGENOUS GRINDING AT UNKI MINE CONCENTRATOR – A

CASE OF SUCCESSFUL VALUE ENGINEERING

T. Nyakudarika DRA Mineral Projects (Pty) Ltd

S. Makumbe Anglo American Platinum Limited

C. Rule Anglo American Platinum Limited

Abstract

The Unki Platinum Mines concentrator, employing an MF2 circuit configuration which includes

fully-autogenous primary milling of run-of-mine (ROM) ore, was commissioned in November

2010. The project was Anglo American Platinum’s first PGM concentrator in Zimbabwe and

would form a significant contribution to the group’s platinum production. The project financials

demonstrated that the project was operating-cost sensitive.

After reviewing the metallurgical testwork, mineralogical examinations, and the history of

similar operations, it became apparent that the Unki ore characteristics rendered it amenable

to autogenous milling. The primary ball mill, structurally designed for 30 per cent steel load,

could be operated at the required initial phase throughput with fully-autogenous primary

milling, realizing savings in both power requirements and grinding media consumption. The

original mill sizing had been done with fully-autogenous milling in mind, but allowing for

flexibility and mitigation in case of varied mining fragmentation.

Since attaining steady state, target grinds have been achieved. This paper looks at the Unki ore

characterization programme, design considerations, and the operational and control philosophy

for a successful comminution circuit, and discusses the design to produce a flexible, low opex,

mitigated comminution and overall MF2 flowsheet that is easily expanded in line with the

potential for optimum exploitation of the orebody.

Introduction

The Unki Mines platinum project is the first Anglo American Platinum operation in Zimbabwe to

exploit the rich platinum resources of the Great Dyke. Due to the size of the ore resource,

future expansion of the scale of the Unki operation is an important consideration. The circuit

design considered this aspect carefully. The exploitation strategy adopted for the Unki ore

resource was to start the operation at 120 kt/month and ramp up to 240 kt/month after five

years.


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It was imperative for the design to produce a flexible, low operating cost, but fully risk-

mitigated comminution and overall MF2 flowsheet that is easily expanded in line with the

potential for optimum exploitation of the orebody. Figure 1 is the locality map for the Unki

Mine.

Figure 1- Map of Zimbabwe showing the location of Unki on the Great Dyke

A MF2 circuit configuration employing run–of-mine (ROM) primary ball milling, followed by

secondary ball mill regrinds, was initially proposed after a full risk evaluation, based on the

comprehensive testwork programme conducted on borehole cores and bulk samples from the

property. Target grind for primary milling was 35 to 40 per cent passing 75 µm (corresponding

to a P80 of 212 µm), with a final grind of a minimum 80 per cent passing 75 µm. The target

platinum recovery for this circuit was 82 per cent. The financial evaluation of the project

demonstrated that the project was primarily operating-cost sensitive.

The ore characteristics from drop weight tests conducted on geological and metallurgical

samples and the estimated ’as is’ ROM ore size distribution presented an opportunity for

changing the primary milling circuit from ROM primary ball milling to fully autogenous (FAG)

primary milling. An important interpretation for FAG milling utilization is the expected

fragmentation from blasted ore mined by mechanized bord and pillar’ mining. This would result

in a saving in grinding media and power in the primary milling stage.


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By suitably sizing the mills, future circuit flexibility could be incorporated in the design for

minimal initial cost.

There was concern regarding this approach, as testwork had previously indicated that the Unki

ore exhibited some degree of bimodality from a perspective of broken ore particle size, and this

could result in critical size build-up in the mill, and as such represented a risk to the success of

autogeneous milling.

The Unki concentrator has achieved the design target of throughput and grind with the

projected savings in grinding media and primary mill power by employing a FAG primary milling

circuit.

The paper discusses the testwork programme, design, and evaluation of the plant performance.

Test work

Extensive metallurgical test work over a prolonged period was conducted to characterize the

orebody and to formulate the plant flow sheet. Laboratory and pilot-plant testwork were

conducted at Geomet (Harare), AARL (Crown Mines), ARC (Anglo Research Centre, Germiston),

Mintek (Randburg), and DML (Anglo American Platinum’s Divisional Metallurgical Laboratory,

Rustenburg). Eurus Mineral Consultants (EMC) validated the laboratory and pilot-plant scale

testwork results using their ’Kincalc’ and ’Supasim’ simulation software. Significant evaluation

of the Unki ore resource commenced in 1992 with testwork conducted on 22 drill cores over

the mining area and six bulk ore samples from a trial mining shaft. Further test work was

continued from the early 2000s up to the time of project execution. The test work from 2002 to

2008 was on more drill cores and two bulk ore samples (2005 and 2008).

Mineralogy

The mineralogical work indicated that the ore was typical of the Great Dyke ores.

Methods of investigation used involved:

• X-ray diffraction (XRD) for comparative semi-quantitative gangue mineral investigation

• Scanning electron microscopy (SEM) and optical microscopy to determine the mass

distribution of sulphides

• Mineral Liberation Analyser (MLA) and QEMSEM for analyses of mineral associations

• X-ray fluorescence (XRF) scans to identify other valuable constituents of the ore samples

• ICP scans to check for variations in composition between the drill core and bulk samples.

The ore has high talc (8 to 12 per cent) and pyroxene (approximately 70 per cent) contents. The

predominant platinum group minerals (PGMs) are sperrylite and Pd-Bi Tellurides. The bulk of

the platinum group elements PGEs occur as free PGMs. The platinum-bearing PGMs are mainly

associated with pyrrhottite (approximately 66 per cent) and lesser amounts (approximately25

per cent) with pentlandite, while the palladium-rich minerals are associated with gangue

(approximately 79 per cent). The gold particles are mostly associated with chalcopyrite1, 2


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The base metals sulphides are predominantly pyrrhotite, pentlandite, chalcopyrite, and pyrite.

The average size of the PGMs varies from 5 to 30 µm with a maximum of 90 µm. The base

metal sulphides vary from 5 to 20 µm. The detailed head analysis for the ore at a mining cut of

1.8 m is as follows:

Pt 1.83 g/t, Pd 1.56 g/t, Rh 0.16 g/t, Au 0.30 g/t, Ni 0.21%, and Cu 0.15%.

The mineralogical data formed the basis for optimizing key design parameters and also for

benchmarking against the other Great Dyke operations.

Comminution

AARL, ARC, Mintek, and later DML conducted a series of drop weight test (DWT) and Bond

Work Index (BWI) test work on core and bulk samples of the Unki ore. JKMRC analysed the

Mintek work. Comparison between Unki and other Great Dyke ores was also done. The Unki

ore characteristics are typical of the Great Dyke ore. The ore displays moderately hard range of

resistance to impact breakage and medium abrasion range. Test methods used included:

• Calibrated AARL Rod Mill by AARL

• JKRMC standard procedure for the DWT and the Warren Spring Laboratory standard

procedure for BWI by ARC.

• Mintek used their batch mill (which conforms to the standard Bond batch ball) to

determine the Bond ball mill Work Index

• The DML’s Magotteaux pilot mill demonstrated that the Unki ore was amenable to

autogenous primary milling and there was no critical size build-up. A target grind of 35

to 40 per cent -75 µm was achieved at a coarse to fine ore ratio of 25:75

The results for the ARC test work are summarized in Table I:

Table I- Unki BWI test work results benchmarked against Great Dyke ore

Limiting size (μm)

Sample 300 150 106

Unki ore BWI (kWh/t) 11.6 17.7 17.6

Other G Dyke tests: BWI (kWh/t) 15.1 16.5* 18.1

* calculated


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Mintek results are shown in Table II:

Table II-Mintek BWI test work results for Unki Ore

Sample Limiting screen

(µm)

F80

(µm)

P80

(µm)

Work Index

KWh/t

Unki ore 75 1906.18 57.56 18.2

150 1906.18 114.02 15.0

The above confirms that the design BWI for the primary ball mill is between 15 kWh/t and 18.2

kWh/t. As will be shown later, for sizing the primary mill motor the Bond method is used. The

design Bond Work Indices for the primary and secondary ball mills are 16.5 kWh/t and 20.5

kWh/t respectively. Inefficiency factors to mimic industrial applications were considered.

Flotation

AARL, ARC, and Mintek conducted laboratory flotation test work on core and bulk samples to

determine the metallurgical response of the Unki ore and develop a flow sheet. The test work

also compared the performances of MF1 and MF2 circuit configurations. The challenge the Unki

ore posed was the presence of a footwall shear containing high levels of altered silicates, raised

levels of talc, and swelling clays. The impact of treating ore that included the footwall material

was also investigated and quantified in the DML pilot plant. The recoveries obtained for the

MF1 and MF2 circuits from different test work campaigns are shown in Table 3.

Table III- MF1 and MF2 recovery

Test work

4E Recovery (%)

MF1 MF2

Mintek pilot 84.6 87.0

DML pilot 77.9 80.4

EMC (simulation) 78.3 84.0

DML’s pilot trials indicated that the 4E PGM recovery dropped by as much as 7.7 per cent and

3.7 per cent due to the higher hydrophobic, talcose component of the shear zone foot wall

material. This and the above results favour an MF2 circuit configuration.

Design

The Unki concentrator MF2 flow sheet is shown in Figure 2. The flow sheet included an in-

circuit crushing (ICC) or pebble-milling circuit to mitigate potential comminution problems. The

ICC circuit was planned to consist of pebble transfer system to a cone crusher or HPGR. A

pebble stockpile and reload facility would be allowed to provide optimum mill load control.

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Platinum 2012

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Process description

The Unki concentrator design is for a nominal capacity of 120 kt/month using a 20 foot

diameter by 28 foot length primary mill with 5.2 MW installed mill motor power as a ROM AG

mill in closed circuit with a classification screen (with 0.63 mm to 0.8 mm aperture opening)

and a 20 foot diameter by 28 foot length secondary mill with 5.2 MW installed mill motor

power as a ball mill in closed circuit with a classification cyclone cluster for secondary regrind.

Mills were sized to enable a future twin-module primary milling configuration – future

alternative milling equipment could then be fully evaluated.

The flotation circuit consists of a short residence-time, primary roughing stage comprising

3x70m3 flotation cells and primary cleaning and re-cleaning stages. Primary rougher tails report

to the secondary mill ahead of scavenger flotation, and cleaning and re-cleaning. Two

concentrates are produced, a high-grade concentrate from the primary re-cleaner flotation

circuit and a medium-grade concentrate from the secondary re-cleaner circuit. These combine

to form the final concentrate. Open- or closed-circuit cleaning in the scavenger circuit is an

option

Concentrate handling consists of a Magra thickener and a conventional clarifier, and Larox high-

pressure filtration of the thickened concentrate. Tailings thickening circuitry includes guard

cyclones, a high-rate thickener, and clarifier. Final tails are pumped to a tailings storage facility

about 5 km away from the plant.

The original design concept offers great flexibility and expansion potential for Unki. The layout

provided for a future ICC to allow a sequential expansion of a single module, from 120 to 140

kt/month using suitably oversized 20 foot diameter mills and then by utilizing ICC technology to

potentially achieve a throughput of as much as 180 kt/month, once FAG operation had been

proven. This is achieved through an F80 of 300 mm and P80 of 250 µm in the primary AG mill,

with a circuit grind of 36 per cent and 80 per cent -75 µm in the primary and secondary mill

respectively. It is worth noting that mitigation in this design is by:

1. Oversized 20 foot diameter mills

2. Mill and mill drive designed for 30 per cent steel loading, allowing for a SAG and high-

steel ROM ball milling option. The mill was designed for 75 per cent critical speed and 10

per cent circulating load

3. ICC circuit addition in the event of critical size build-up and retention of FAG. The circuit

will be designed for 20 to 30 per cent pebble generation. The primary mill grate

discharge grate will be modified to include ±65 mm pebble ports (design to be

confirmed from operational data)

4. Possibility of converting the mill drive to VSD as mitigation for throughput and FAG

constraints due to unsuitable ore fragmentation.


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Ore characterization

Competent ore and a sufficient coarse fraction component are prerequisites for successful

autogenous milling. An extensive review of literature of all previous and current test work and

operations treating similar ore from mechanized and non-mechanized mined operations was

conducted. Ore hardness and grindability tests demonstrated that the ore could be classified as

hard. The results are shown in Table IV. These results were also used to estimate power

requirements for comminution circuit design.

Table IV- Summary of comparative ore hardness test and operational results

SAMPLE DWT BWI (kWh/t)

A b A*b Ta 106 µm 75 µm

2004 Test

work 67.6 0.62 41.9 0.38 19.2 20.5

May 2011 78.5 0.67 52.6 0.37 19.7 21.1

Analysis of t10 (size passing 10 per cent of the original particle size) and unit specific energy

(Ecs) established that the Unki ore displays the normal trend of decreasing slope with

decreasing energy (Ecs values). A study of the distribution of the relative density of particles

showed no evidence of bimodality (that is, no evidence of dense particles that could build up in

the mill and cause mill load problems).

Two measures were put in place to cater for possible risk of failing to achieve the target primary

grind. An in-circuit crusher circuit was provided for in the layout, but its installation was

deferred. The primary mill liners were designed for both FAG and SAG operation; the first set

was designed conservatively, as is normal practice.

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Platinum 2012

372

Figure 7- Example of projected trajectories for the Unki primary mill

Operation and post-commissioning

Mill control

The application of Anglo American Platinum’s APC (Advanced Process Control System)

guaranteed the performance of the mill. The installation of a mill feed silo with enough buffer

capacity (>20 hours) ahead of the mill also enhanced the control solution.

Plant performance

The Unki concentrator has been able to operate at above design throughput (approximately

130 kt/month versus design of 120 kt/month) with a primary grind meeting design specification

(approximately 42 per cent -75 µm versus design of 35 to 40 per cent -75 µm) and final grind of

approximately 80 per cent -75 µm. The oversize (>19 mm) material generated from the primary

mill is much less than predicted. Figure 8 is a typical size distribution curve for the Unki

concentrator comminution circuit.

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Platinum 2012

375

Primary mill liner wear

The first set of liners were of an abrasion-resistant, air-hardened pearlitic chrome-molybdenum

alloy steel approximately 2% Cr). These liners were deliberately selected for possibility of

adding steel (SAG operation) due to ’critical size potential’ and were expected to last between 9

and 12 months. The liners were replaced after nine months. After demonstrating that FAG

primary milling is feasible, the liner material has been changed to a martensic abrasion-

resistant high-chrome iron (16.5-24.5% Cr). This material is not tolerant to steel media. The

high-chrome iron liners are approximately 32 per cent more expensive than the Moly-Cr liners,

but last approximately twice as long. This has been confirmed to date by monitoring of liner

wear. Table V demonstrates the benefit of using a more durable and expensive liner for the

primary mill operated in FAG mode. The throughput for the new liners is a projected from wear

rates over a six month period.

Table V-New and old mill liner comparison

Liner type Throughput to

replacement (t)

Liner cost (R) Liner cost

(R/t)

Liner life

(months)

Old (Moly-Cr : Low

Cr)

725 012 6 392 585 8.82 9

New (High Cr)* 2 625 000 10 010 291 3.81 21

Estimated from liner wear monitoring on plant

Platinum recovery

The current ROM material contains significant footwall shear zone material, which is depressing

recovery due to raised hydrophobic gangue levels and hence greater competition with slower-

floating value minerals. Target recoveries are for mechanized mining cut excluding the FWSZ.

Figure 11 represents a cross-section through the typical Unki mineral width (mining cut of 2.2

m), with platinum grades peaking at the Main Sulphide Zone (MSZ) and decreasing into the

hangingwall and footwall. The zone in the footwall is characterized by presence of hydrophobic

altered silicate talcose material that impacts negatively on plant PGM and BMS recoveries.

T

T

The

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Platinum 2012

380

Acknowledgements

Anglo American Platinum for permission to publish the paper.

My co-authors Chris Rule and Sarah Makumbe.

Unki Platinum Mines concentrator team for living the FAG primary mill dream and making it a

success story.

References

1. Hey. P. V. Platinum Group Mineral Characterisation of Various Channel Samples from the

Unki Deposit, Great Dyke Mineralogy. Report 02/116. Anglo Research Centre, Germiston,

South Africa, 2003.

2. Schouwstra, R. P. Mineralogical Characteristics of PGE Deposits of the Great Dyke

Zimbabwe (Literature Research). Anglo Research Centre, Germiston, South Africa, 2003.

3. Nyakudarika, A. Unki Mine - Process Design Criteria 120 ktpm Concentrator Plant Design -

RoM AG Milling Option. Anglo American Platinum - Process Technolgy Division, Nov.

2007.

4. Sweet, J. and Powell, M.S. Assessment of design of Unki Comminution Circuit. Julius

Kruttschnitt Minerals Research Centre (JKMRC), University of Queensland, Australia,

August 2007.

5. Napier-Munn, T.J., Morrell, S., Morrison, R.D., and Kojovic, T.. Mineral Comminution

Circuits : Their Operation and Optimisation. Julius Kruttschnitt Minerals Research Centre

(JKMRC), University of Queensland, Australia, 1999.

6. Powell, M.S. Unki liner and discharge design review. ulius Kruttschnitt Minerals Research

Centre (JKMRC), University of Queensland, Australia. Report 1. JKTech, August 2007.


Platinum 2012

381

The Author

Tony Nyakudarika, Principal Process Engineer, DRA Mineral Projects (Pty) Ltd

Tony Nyakudarika is Principal Process Engineer with DRA Mineral Projects. He has been in this

role since October 2011. His responsibilities are to manage process design and commissioning

of client concentrator projects. Prior to joining DRA, Tony was involved in the mining industry

for over sixteen years as a Divisional Engineer, Project Services Manager, and Lead Process

Engineer for Anglo American Platinum in Zimbabwe and South Africa. Over the last seven years

Tony has been involved in design and commissioning of platinum concentrators for both

greenfield and brownfield projects as a Senior/Lead Process Engineer. Some of the projects

Tony was involved in were Amandelbult UG2 Upgrade, Amandelbult Tailings Disposal Upgrade,

and the Unki Concentrator from PFS to operation.

Tony also spent more than fifteen years working for multinational companies in thecChemicals

and household and personal hygiene goods industries in engineering and operation roles in

Zimbabwe and Tanzania.


Platinum 2012

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