Characterization of Szego-milled Hemp Fibers · Characterization of Szego-milled Hemp Fibers ......

Characterization of Szego-milled Hemp Fibers

By

Ali Zarei

A thesis submitted in conformity with the requirements for the degree of

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

© Copyright by Ali Zarei 2010

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Abstract

Characterization of Szego-milled Hemp Fibers

Ali Zarei

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2010

Hemp, one of the fast-growing plants with strong, durable fibers is being used for industrial

applications, including papermaking, as a biodegradable reinforcing material for plastics as well

as in construction. A series of tests has been conducted grinding hemp with the SM-220 Szego

Mill equipped with three helically grooved, 30 cm long rollers with 8*8 or 4*4 mm grooves. A

range of flow rates and rotational speeds has been covered in both dry and wet operation to

determine the best processing conditions for various potential applications of refined hemp

fibers. Particle size distributions and power consumption have been measured. Fiber length and

diameter measurements allow an evaluation of fiber aspect ratio. For dry grinding, these are

relatively low, typically in the 6-10 range. Wet grinding generally favors higher aspect ratios

due to reduced fiber cutting and greater clearing action in the mill. Measured values are in the

60-80 range.

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Contents 1. Introduction .............................................................................................................................................. 1

2. Litreture review ......................................................................................................................................... 2

2.1 Size reduction and grinding conception .................................................................................................. 2

2.1.1 Comminution concept .................................................................................................................. 2

2.1.2 Size reduction machines................................................................................................................... 3

2.1.2.1 Tumbling ball mills ................................................................................................................... 3

2.1.2.2 Hammer mills ............................................................................................................................ 4

2.1.2.3 Disk mills .................................................................................................................................. 5

2.1.2.4 Szego Mill ................................................................................................................................. 5

2.2 Particle size distribution and fiber quality .......................................................................................... 8

2.2.1 Sieving and size distribution data ................................................................................................ 8

. 2.2.2 Fiber quality and coarseness ...................................................................................................... 9

2.3 Natural fibers as reinforcement ........................................................................................................ 11

2.3.1 Characteristics of natural fibers ................................................................................................. 11

2.3.2 Processing and properties of composites with natural fibers ..................................................... 11

3. Material and experimental procedure .................................................................................................... 12

3.1 Material ............................................................................................................................................. 12

3.1.1 Hemp fiber ................................................................................................................................. 12

3.2 Fiber Quality Analyzer (FQA) ............................................................................................................. 13

3.3 Optical microscope ........................................................................................................................... 14

3.4 Power consumption .......................................................................................................................... 15

4. Results and discussion ............................................................................................................................ 15

4.1 Dry grinding ....................................................................................................................................... 15

4.1.1 Particle size distribution ............................................................................................................. 15

4.1.2 Length distribution ..................................................................................................................... 16

4.1.3 Diameter distribution ................................................................................................................. 18

4.1.4 Coarseness and aspect ratio ........................................................................................................ 20

4.1.5 Effect of operational variables on grinding hemp fiber ............................................................. 22

4.1.5.1 Effect of feed rate on untreated hemp fiber………………………………………………….22

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4.1.5.2 Effect of feed rate on treated hemp fiber………………………………………………………………………… 25

4.1.5.3 Effect of rotational speed on grinding hemp fiber……………………………………………………………..27

4.1.6 Effect of roller geometry ............................................................................................................ 29

4.2 Wet grinding ...................................................................................................................................... 31

4.2.1 Particle size distribution ............................................................................................................. 31

4.2.2 Fiber analysis………………………………………………………………………………….32

4.2.3 Fiber lenth and size distribution ................................................................................................. 32

4.2.4 Morphology…………………………………………………………………………………………………………………………..35

4.3 Power consumption comparison………………………………………………………………………………………………….37

5. Conclusions………………………………………………………………………………………………………………………………………..39

6.References…………………………………………………………………………………………………………………………………………40

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List of Tables

Table 4.1 Operating conditions 17

Table 4.2 Fiber length, diameter and aspect ratio 32

List of Figures

Figure 2.1 Ball mill grinding forces 4

Figure 2.2 Szego Mill 6

Figure 2.3 Geometry of roller grooves 7

Figure 4.1 Size distributions of Szego-milled hemp fibers 16

Figure 4.2 Length- weighted fiber lengths for screen sizes 106-250 and 250-355 µm 17

Figure 4.3 Szego-milled hemp fibers; 4*4 rollers; 1200 rpm; 430 gram/min 19

Figure 4.4 Average diameters for screen sizes 106-250 and 250-355 µm 19

Figure 4.5 Average coarseness for screen sizes 106-250 and 250- 355 µm 21

Figure 4.6 Average aspect ratio for screen sizes 106-250 to 500-1000 22

Figure 4.7 Effect of feed rate on untreated hemp, 1000 rpm 23



Figure 4.10 Pretreatment of hemp fibers 25

Figure 4.11 Effect of feed rate on treated hemp fiber 26

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Figure 4.12 Effect of feed rate on specific energy 26

Figure 4.13 Effect of rotational speed on treated hemp fiber 28

Figure 4.14 Effect of rotational speed on specific energy 28

Figure 4.15 Effect of roller geometry on size distribution, 4*4 and 8*8 rollers 30

Figure 4.16 Effect of roller geometry on size distribution, 8*8 and 16*8 rollers 30

Figure 4.17 Size distribution of sample 1 (4 mm rollers, 1200 rpm, 20%) 33



Figure 4.20 Size distribution of sample 4 (8 mm rollers, 1200 rpm, 20%, 1 kg/min.) 34



Figure 4.23 Microscopic images of pulp fibers with a 181 mm grade 35

Figure 4.24 Microscopic images of pulp fibers with a 0.1 mm grade 35

Figure 4.25 Mill power; 8*8 mm rollers, dry and wet grinding; 1 kg/min; 1000 rpm 38

Figure 4.26 Mill power; 8*8 mm rollers, dry and wet grinding; 1kg/min; 1200 rpm 38

Figure 4.27 Mill power; 8*8 mm rollers, wet grinding at different consistencies;

1kg/min; 1200 rpm 39

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1. Introduction

Worldwide interest in high performance bio-based plastics and materials is increasing due

to high demands for environmentally friendly materials for industrial applications. Natural fibers

are known as a renewable, cheap and nonabrasive material. They typically have a high aspect

ratio, a high strength to weight ratio, low density, and good tensile strength. For defibrillating the

fibers or opening up the fiber bundles into individual strands of fibers, shear forces are required

in order to break the bonds. An important aspect of high performance polymer composites

manufacturing is to tailor-make natural fine fibers of high strength, at a low cost and in an

environmentally friendly manner.

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2. Literature review

2.1 Size reduction and grinding conception

2.1.1 Comminution concept

Comminution is a complicated process which defines the change of particle size and its

distribution for materials such as grains and other solid particles to finer particle sizes. The unit

operation of size reduction of solids by crushers and mills involves many aspects of powder

technology such as cutting, crushing, and grinding by different kind of equipment for varied

industrial applications, for example those in mineral processing and chemical industries.

There are several aspects to the basic fracture mechanisms for comminution processes.

Specific fracture energy of the particles plays the significant role for determining the properties

of the material. Some particle are brittle, others show elastic–plastic behavior. Further, particles

with elastic–viscous behavior require very high stressing velocities at room temperatures or they

have to be cooled down for efficient comminutetion. Particles with mostly elastic behavior are

not influenced by temperature and can be broken down readily at lower stressing velocities.

Industrial application for comminution processes does not have high efficiency for energy

consumption by the operating equipment. Energy efficiency depends on the minimum energy

required to break the particles for the surface area produced. Size reduction operations are known

as energy intensive processes. Improvement in the efficiency of the milling circuit will lead to

better economic benefits.

There has been much progress in developing various models for the unit operation of size

reduction, dealing with every mode of grinding operation. Usually, these models are based on the

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material breakage functions in mills which are used in rate processes for the application of size-

mass balances or population balances.

Comminution as a unit operation is thoroughly described in Reference 1.

2.1.2 Size reduction machines

Different types of operations require a reduction of the material from larger size to a fine

powder, possibly less than 100 µm in size. Size reduction may not be achieved for many

different desirable size ranges with high efficiency in a single machine. Therefore, each machine

is designed for efficient operation on a particular feed size.

Machines which are used for breakage of large lumps are known as crushers. These are

followed by mills which are used for smaller size applications, with different ranges of overlap

where either a fine crusher or a coarse mill can be used for different kind of applications.

Different types of machines are available in the market for size reduction. The size reduction

ratio is known as the medium feed size divided by the medium product size. It is typically of the

order of 10 or lower and is not constant but variable, considering the adjustable gap setting of the

equipment used.

Different types of size reduction machinery are described in Reference 1.

2.1.2.1 Tumbling ball mills

The tumbling ball mill is one of the famous retention mills which has been used widely in

both dry and wet grinding for size reduction in different type of applications over a wide range of

particle sizes compared to other mills. It is using collision and compressive forces for breaking

the large particles and also attrition for small particles.

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Figure 2.1 Ball mill grinding forces.

2.1.2.2 Hammer mills

Typical hammer mills have rows of swinging hammers extended by centrifugal force which

form a cross-arm pattern for the impact of the larger particles with the hammers. After the initial

impact, particles are further reduced also by shear and attrition.

These mills are used in different types of applications for mostly brittle materials, and also

fine dry grinding for which grinding action by shear between hammer tips and the case is

important. Horizontal-shaft hammer mills or hogs are useful for processing wood residues into

fuel.

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2.1.2.3 Disc mills

Disc mills are comprised of rotating surfaces with various contours moving at high speed in

opposite directions, with the gap controlled by springs for good compressive and shearing forces.

Disk mills are used for mechanical refining, of wood pulps and are utilizing mainly shearing and

attrition forces.

2.1.2.4 Szego Mill

The Szego Mill used in this study is a planetary ring-roller mill which shown in Figure 2.2.

It has a simple structure which consists principally of a stationary, cylindrical grinding surface

inside which a number of helically grooved rollers rotate. These rollers are suspended between

upper and lower flanges connected to a central drive shaft.

The material is fed into the top of the mill and is discharged at the bottom. The particles are

repeatedly crushed between the rollers and the stationary grinding surface. The crushing force is

created mainly by the radial acceleration of the rollers.

Szego Mill has several design variables which can be changed for different product

requirements. The significant variables are the number of rollers, their mass, diameter and

length, shape, size and number of starts of the helical grooves on the rollers. The product

becomes finer by increasing the number of rollers. Heavier rollers and higher rotational speeds

create greater crushing forces which is required for hard or tough materials. The operating

variables for the mill are considered with two main parameters such as material feed rate and

rotational speed of the rotor; however, wet grinding has another well known variable, solids

consistency.

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Figure 2.2 Szego Mill (Ref. 2)

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Compression and attrition are important for cutting the fibers in wet grinding. Size

reduction is carried out also by shear; hence, the possibility of fiber bundles opening into fibers,

or fibrils.

The Szego Mill has the advantage of dynamics under specific conditions - grinding wet in

the paste mode, a condition between dry grinding and wet grinding process. There is a kind of

restriction of interest for the paste grinding regime where individual particles get crushed but do

not influence the roller path except by slurry or paste viscosity and density. A brief description

of the dynamite model of mill performance in wet and paste operation is given in Reference 3.

Figure 2.3 Geometry of roller grooves (Ref. 3)

Most of the experimental runs were made with SM-220 Szego Mill. It has an internal

diameter of 220 mm and is equipped with three helically grooved, 30 cm long rollers. The rollers

are 9 cm in diameter with 8*8 or 4*4 mm grooves. Some experiments were also done using 16*8

mm grooves. A winged, push- down top plate was used for the 4*4 mm roller experiments, for

ease of feeding the fluffy, fibrous material. Air flow induced by a vacuum cleaner at the mill exit

eliminated airborne dust and eased flow through the mill. Grinding runs lasted from about 2 to 4

minutes. The ground hemp fibers were dry sieved by using a Ro-Tap shaker. The sieve aperture

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sizes used were chosen according to the product size, ranging from 1mm down to 106 µm. 30

gram hemp fiber samples were used for each test taken from the well-mixed product drum. Each

sieve run time was about 20 min. Sampling of the product from the drum was performed after

each run by manual mixing from different locations.

2.2 Particle size distribution and fiber quality

2.2.1 Sieving and size distribution

All products from a comminution process are characterized by means of the „average‟

particle size as well as the variation of size among the particles. The particle size distribution of a

material is important for understanding the physical and chemical properties of the material.

Sieving as the simplest and most widely used methods for particle size analysis determines

the separation of fine material from coarse material of the material.

Sieving is carried out by stacking sieves in ascending order of aperture size and placing the

sample on the top sieve. The stack is vibrated for a fixed time and the residual weight on each

screen is determined for each sample. Results are usually reported in the form of a cumulative

percentage of passing sizes. Test sieves are round and made from stainless steel or brass with

wire mesh. The standard frame sizes are three, six, eight, ten, or twelve inches diameters. In

order to produce reliable and consistent results, sieving methods require appropriate procedures,

including sampling, timing, weighing and proper choice of sieves.

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Different normal size openings are available in inches, millimeters, and sieve mesh

numbers, depending on opening size.

Wet sieving is appropriate for adequate separation between the individual fractions and

requires a clamping cover with spray nozzle and a collector with outlet. This process is carried

out by water from the spray nozzle which is located above the sieve. The material may be mixed

with water to provide a suspension and facilitate passage of the material.

. 2.2.2 Fiber quality and coarseness

The arithmetic average based on the number of particles is one of the methods for

measuring the fiber length. It emphasizes the effect of the short fibers. There is another,

commonly used method, the length weighted average of fiber length (LWAFL). which puts the

emphasis on the longer fibers. Fiber-length has a strong influence on overall strength, stability

and stiffness of the material. Therefore, it plays an important role in the processing and

mechanical properties of fiber-based products such as paper.

As has mentioned above, the length of the fibers has a strong influence on strength, stiffness

and impact resistance of the component. However, the desirable length of fibers is strongly

dependent on the purpose of the production. It is important to determine the distribution of the

fibers and their lengths after the production process in order to estimate the quality of the

resulting product (4).

Fiber length over diameter of the fiber is known as aspect ratio. It is important to know that

low length-to-diameter (L/D) ratios within small ranges cause stress concentrations leading to

decreased strength. There is a possibility to realize the strength and stiffness with increasing fiber

length when adhesion between wood fibers and the plastic matrix is good and fibers are

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uniformly dispersed in the matrix. Good orientation of the fibers is also important. The benefits

of high aspect ratio fibers to the mechanical properties can be negated by using wood fibers

without effective dispersion or coupling agents.

The benefits of the fiber length on mechanical properties can be reduced by several

processing challenges such as difficulty for feeding because of low density and poor flowability,

potential degradation of fibers in the extrusion process, or a tendency for fibers to agglomerate in

the dry mix which will result the poor quality of fiber dispersion in the matrix.

It can be concluded that it is not the particle length that affects strength properties

comparing the aspect ratio value; however, increasing the aspect ratio can increase the strength

and rigidity of the composites.

Fiber coarseness is mass of fibers per unit length (C=M/NL) where M is a very small mass

of fibers, L is the arithmetic mean length of the fibers and N is the total number of fibers in the

mass M. The accuracy of coarseness depends on different factors in the equation. For example,

if the optical fiber length analyzer provides accurate values for L and N , then the accuracy of the

coarseness measured depends on the accuracy of M, etc.

There are different types of methods for measuring the coarseness based on the principle of

the statistical geometry of a fiber network; therefore, coarseness C is related to the number N of

fibers and length L of a scan line in a random network. Accurate measurement of the mass of

fibers deposited per unit area in the network is very important as well as the large number of

fiber crossing along the scan lines has to be counted to obtain fiber coarseness with sufficient

accuracy.

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2.3 Natural fibers as reinforcement

2.3.1 Characteristics of natural fibers

Natural fibers are a bundle of simple fibrils covered and bonded together by the lignin that

influences the structure, properties and morphology of the natural fibers. Comparing with glass

fibers which are isotropic and can be produced with a definite range of properties, natural fibers

have complicated, though similar structure, like most biological materials. They can be defined

like elementary fibers which are bound together by pectin, forming the next level of

microstructure with 50-100µm diameters. Natural fibers are these bundles of individual strands

of fibers held together by means of an interface of pectin and lignin.

2.3.2 Processing and properties of composites with natural fibers

Natural fibers, as reinforcement materials have attracted the attention of researchers because

of their advantages over the other type of materials. They are environmentally friendly, fully

biodegradable, abundantly available, renewable and cheap with low density by replacing

synthetic fibers as reinforcement in various matrices.

Some composites with natural fibers and thermoplastics are produced by extrusion using

single screw or twin screw extruders. The fibers should be ground in order to obtain a well

defined size and length distribution.

Natural fibers have significant advantage over synthetic fibers in different applications as

reinforcing fillers for plastics composites. Many types of natural fibers have been investigated

for use in plastics including flax, hemp, jute straw and wood.

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Mechanical properties of the composites and physical properties of the natural fibers vary

considerably depending on the chemical and structural composition, fiber type and growth

conditions (5).

Automotive parts industry has attractive applications for the composites reinforcement by

using the materials which are based largely on polypropylene or polyester matrices,

incorporating fibers such as flax and hemp. Thus cars of the future may be produced from

cashew nut oil and hemp (6)!

3. Materials and experimental procedure

3.1 Materials

3.1.1 Hemp fiber

Hemp is a lignocellulosic raw material from a high-yield herbaceous annual plant

(Cannabis Sativa L) grown around the world with an interesting economical benefit. Hemp

provides long and strong fiber that can be used for different applications, such as paper products,

housing materials, and auto interior parts.

Several advantages including excellent fiber quality and physical properties of strength,

durability, absorbency and as a potential energy source are important because of the conversion

the hemp plant by bio-processing into many bio-energy products, such as fuel pellets, liquid

fuels, and gas, as well as fibers. This type of fibers have nonwoven composites applications as

well as woven composite application that are ideal for the production of bio-based auto interior

parts.

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Hemp seeds are being used directly in the production of food products. Hemp oil extracted

from hemp seeds is the richest known source of poly-unsaturated fatty acids that are excellent

ingredients for personal care products.

The chemical constituents of the hemp plants cell wall are very interesting as they consist

not only of cellulose, but also of hemicelluloses, pectin and lignin. The properties of these

constituents can contribute to better understanding of the overall properties of the fiber. An

elementary fibril determines the smallest building element of the cellulose. It has a size range

between 5 and 10 nm in diameter and its length varies from 100 nm to several micrometers.

There is a migration of lignin to the surface of the fiber after heating the hemp fibers above the

glass transition temperature, depending of the source of the cellulose.

The raw material used for this experiment was hemp fiber (Cannabis Sativa L.) grown in

Ontario, field retted and mechanically processed. It was supplied by Hempline Inc. Hempline

contracts the work of hemp plantation and harvesting in Southwestern Ontario near the City of

London. These fibers are approximately 12.7 mm in length and average 170 µm in diameter.

They are very hard to handle, like cotton wool, and require preprocessing before feeding to the

Szego Mill.

3.2 Fiber Quality Analyzer (FQA)

The fiber quality analyzer is a hydraulic, optical and image processing system which

consists of a novel flow cell that uses hydrodynamic focusing to orient the fibers which is helpful

for precise measurement in the imaging region. A dilute suspension of hemp fibers is

transported past the optical imaging system. Fiber quality analyzer determines the quality of

cellulose fibers accurately, rapidly and automatically

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Fiber length is calculated as the sum of individual fiber lengths squared divided by the sum

of the individual fiber lengths. The length weighted average of fiber length is appropriate for

better comparison of different samples because fines do not have a significant effect on that

result. The system has a fully integrated sensor unit such as the optics, control and measurement

electronics, and the pneumatic and liquid systems.

Fiber quality is defined in terms of fiber length and shape. Fiber length is normally

described as the contour length, two common measurements of fiber shape are curl and kink.

Curl is the gradual and continuous curvature of a fiber and kink is the abrupt change in fiber

curvature.

This instrument has touch screen controls with appropriate user menus. The operator can

select the desired result output. And it obtains mean fiber length as an arithmetic, length

weighted, and weight weighted average, the average kink and curl.

3.3 Optical microscope

The Olympus PME3 optical microscope with a high resolution digital camera and the

CLEMEX vision professional‟s software were used for measuring the diameter and length of

each size–consist of the dry-ground fibers. 25 to 30 fibers were measured for each operating

condition.

For the wet ground sample analysis, the Olympus CH microscope was used with two square

grades, 1 mm for fiber length and 0.1mm divisions for fiber diameter measurements. 100 random

fibers were used for length and 50 for diameter measurements for each sample.

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3.4 Power consumption

The energy requirement for running the rollers without contact with the wall or material

next to the wall has a significant effect on total power consumption. The energy supplied is

consumed in rotating the rollers for all normal operating conditions. The material is stressed, the

particle bed behaves as a deformable solid between the rigid surfaces and most of the strain

energy is consumed by inter particle friction, elastic deformation without breakage. The major

goal of this study was to compare power measurement for the two different grinding methods,

wet and dry, and for different operating conditions.

A rectifier/meter was installed on the pilot Szego Mill. It allowed control of the rotational

speed of the mill as well as recording and calculation of the power drawn by the mill at intervals

of one second.

4. Results and discussion

4.1 Dry grinding

4.1.1 Particle size distribution

Size reduction of fibrous materials in the Szego Mill depends on the compression forces

created by the roller ridges and on shear forces. In most cases, a rotational speed of at least 1000

rpm is required. Most of the experiments were done using 8*8 mm rollers which have 8 mm

wide ridges separated by 8 mm wide grooves, 8 mm deep. In order to understand the importance

of groove configuration, two other types of rollers were also used: 4*4 mm rollers and 16*8 mm

rollers. 4*4 mm rollers have 4 mm wide ridges and grooves and 16*8 mm rollers have 16 mm

wide grooves with 8 mm wide ridges. The 16*8 mm rollers, of which there were only two in the

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mill, were used for pretreatment of hemp fibers in order to improve their handling properties.

Large grooves aid transport of material through the mill.

It is observed in Figure 4.1 that the finest product is obtained with the 4*4 rollers (1200

rpm). The feed rate at 1200 rpm rotational speed (8*8 mm rollers) does not have a significant

effect whereas the lower rotational speed of 1000 rpm does result in a coarser product, as

expected. The 16*8 rollers with smaller grinding surface give the coarsest product, shown as

feed material for the other experiments. Roller geometry has the main significant effect on mill

performance in dry grinding.

Figure 4.1 Size distributions of Szego-milled hemp fibers

4.1.2 Length distribution

The length of each fiber was measured with the Fiber Quality Analyzer (FQA) for each run

and each screen aperture to understand the effect of grinding conditions and size distribution.

The average length of the Szego-milled hemp fibers for two screen sizes is shown in Figure 4.2

and the grinding conditions are given in Table I. It is surprising to observe that even in the same

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screened size range, the 4*4 mm rollers give a lower average length compared to the 8*8 mm

rollers. It shows that the length distribution of fibers with 8*8 mm rollers at different feed rates

and rotational speeds are more similar; yet do exhibit small differences which line up with

grinding conditions, as the milder ones yield slightly longer fibers. The strong effect of the

screen aperture size is, of course, expected. The microscope measurement of a total of about 200

fibers gives average length values lower than the length – weighted FQA values, obtained from

thousands of fiber measurement.

1 4*4 rollers; 1200 rpm; 430 gram/min




Table 4.1 Operating conditions

Figure 4.2 Length- weighted fiber lengths for screen sizes 106-250 and 250-355 µm

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4.1.3 Diameter distribution

After the fibers had been ground, the average fiber diameter was measured depending upon

the features of the fiber. For example, in some fibers, there was just one strand of the fiber but in

some other cases, there were many strands at one end but only few were running all the way to

the other end. In such cases, the average diameters were taken of those strands which were

running all through. As seen in Figure 4.3, Szego-milled hemp fibers are like individual strands

or bundles; however, there are some particles that would cause a lower overall aspect ratio, such

small particles or fines have not been measured with the optical microscope.

The average diameter of the fibers for different screen sizes is shown in Figure 4.4, for the

same operating conditions given in Table 1. The average diameter values for the coarser mesh

size are higher than those of the finer one. The milder grinding condition No.4, 1000 rpm with

8*8mm rollers, does give fibers of larger diameter. The diameters obtained from FQA coarseness

measurements, have been calculated, assuming the fiber specific gravity to be 0.6. These values

are considerably higher, particularly so for the larger screen size.

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Figure 4.3 Szego-milled hemp fibers; 4*4 rollers; 1200 rpm; 430 gram/min

Figure 4.4 Average diameters for screen sizes 106-250 and 250-355 µm.

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4.1.4 Coarseness and aspect ratio

Determination of fiber coarseness has always been a challenge. One of the commonly used

methods is the measurement of the total length of a known mass of hemp fibers with the fiber

quality analyzer (FQA). Coarseness is obtained by dividing the hemp fiber mass by the total

measured length of the fibers. The mass of fibers deposited per unit in the network has to be

known accurately, and a large number of fibers flowing along the scan lines have to be counted

to obtain fiber coarseness with sufficient accuracy. Typically, 10,000-40,000 fibers were

processed in each measurement.

As seen in Figure 4.5, the average coarseness of measured mesh size 250-355 µm for all

different operating conditions is higher than that measured for the mesh size 106-250 µm. The

average coarseness for the 4*4 mm a roller is, again, lower than that for the 8*8 mm rollers.

Condition 4 for the larger screen size is doubtful as the operation of the FQA became difficult.

With the measurement accuracy judged from data scatter, no distinction can be made between

the different 8*8 mm roller experiments.

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Figure 4.5 Average coarseness for screen sizes 106-250 and 250- 355 µm.

Figure 4.6 shows the average aspect ratio obtained from optical microscope measurements,

also for the larger size cuts. The aspect ratio is higher for the larger screen size; however, there

is not much difference between the aspect ratio with grinding conditions.

In the same plot which shows the aspect ratio from the FQA after measuring thousands of

fibers, the result are most consistent and the aspect ratio value higher for the smaller sizes. Data

from the larger, 500-100 µm size range are dubious, as the FQA malfunctioned. Another important

point is that the length -weighted measurement by FQA favored longer fibers for larger fiber

comparing the manual measurement with optical microscope. From both sets of results it is

apparent that coarser fibers have higher aspect ratio (except the dubious FQA results) i.e. that

finer dry grinding with the Szego Mill does reduce fiber length more than its diameter.

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Figure 4.6 Average aspect ratio for screen sizes 106-250 to 500-1000 µm.

4.1.5 Effect of operational variables on grinding hemp fiber

Experiments were performed using feed rates ranging from 80 grams per minute to 1000

gram per minute and constant rotational speed of 600, 800, 1000 and 1200 rpm. Figures 4.7 to

4.14 pertain to these experiments.

4.1.5.1 Effect of feed rate on untreated hemp fiber

The fact that we get coarser product with higher feed rates can be explained with the

appearance of particle bed. With less material in the mill, the roller ridges make contact with the

material multiple times. With more material in the mill, the „bed‟ of material is formed and

ridges can make contact with only a fraction of hemp or only for a limited time.

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Unlike the applying of crushing force to a brittle material, applying the same force to hemp

fiber does not have as significant an effect on size reduction. Primarily, size reduction of fibrous

material in the Szego Mill depends on the compression forces induced by the roller ridges and on

shear forces as the material moves down along the mill. Therefore, size reduction performance is

highly dependent upon contact of the material with the roller ridges.

A feed rate of 200 gram per minute was about the maximum which the mill was capable of

handling without becoming serious obstructed while there was minimum air flow through the

mill with 4*4 mm rollers. It is possible to increase these feed rates by increasing air flow or pre-

processing with wider grooves but this would cause an unwanted decrease in residence time and

result in a broader size distribution.

Figures 4.7 to 4.9 show also that the lower rotational speeds give coarser products. Figure

4.10 gives pretreatment results using the 2-roller, mill with 16x8 mm rollers.

Figure 4.7 Effect of feed rate on untreated hemp, 1000 rpm

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25

Figure 4.10 Pretreatment of hemp fibers.

4.1.5.2 Effect of feed rate on treated hemp fiber

Experiments were accomplished by using feed rates ranging from 430 gram per minute to

1000 gram per minute at a constant rotational speed of 1200 rpm. Figure 4.11 shows the size

distribution curves versus screen aperture, indicating a minimal difference. Figure 4.12 gives

specific energy (mill power over feed rate) as a function of feed rate. This plot shows that with

increasing the feed rate the specific energy decreases even though mill power rises with

increasing feed rate.

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Figure 4.11 Effect of feed rate on treated hemp fiber

Figure 4.12 Effect of feed rate on specific energy

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4.1.5.3 Effect of rotational speed on grinding hemp fiber

Experiments here were conducted with a feed rate of about 1000 grams per minute and

rotational speed values of 1000 and 1200 rpm. Figure 4.13 shows the effect of rotational speed

on particle size distribution for these experiments. It can be readily seen that size reduction

increases noticeably with higher rotational speeds. In most cases, a rotational speed of at least

1000 rpm is required.

It is interesting to note that when the rotational speed of 1000 rpm is used, the size

distribution under 500 µm is almost the same as size distribution when the rotational speed of

1200 rpm is used; however, there is a slight difference for the coarser size distribution that is

over 500 µm. This is best observed in the grinding of hemp fiber because of the effect of

rotational speed. This does not appear to be the result of sieve analysis error since multiple

analyses were performed here. The combination of high feed rate and low rotational speed of the

mill may have caused more of a buildup of coarse hemp fiber. It is interesting to note that

specific energy slightly increase by increasing the rotational speed, Figure 4.14 illustrates this

fact along with the relation between mill power and rotational speed.

While giving a better size reduction, higher rotational speeds also increase power

consumption. This is apparent from the plots of specific energy consumption versus rotational

speed.

Nevertheless, benefits of having high rotational speeds usually compensate for the increased

power consumption by giving higher size reduction and better stability of the grinding system.

The upper limit of the rotational speed is determined by the mechanical strength of the mill. The

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practical limit for this particular mill is considered to be about 1200 rpm, as mechanical wear and

tear would increase rapidly at higher rotational speeds.

It has been noticed that more energy is required to grind a finer product than to grind a

coarser one. At 800 rpm the size reduction is minimal or even absent; therefore at least 1000 rpm

is required.

Figure 4.13 Effect of rotational speed on treated hemp fiber

Figure 4.14 Effect of rotational speed on specific energy

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4.1.6 Effect of roller geometry

As it has been mentioned most of the experiments were done using 8*8 mm rollers which

have 8 mm wide grooves. These 8*8 mm rollers have 3 starts or 3 separate helical grooves; such

configuration gives a roller better stability and provides better material transport through the

mill. In order to better understand the importance of groove configuration two other types of

rollers were used; 4*4 mm rollers and 16*8 mm rollers. 4*4 mm rollers have 4 mm wide grooves

and 16*8 mm rollers have 16 mm wide grooves. 16*8 mm rollers were used for pre-treatment of

hemp fibers in order to improve handling properties. The main goal of this set of experiments

was to establish the energy efficiency of grinding with different rollers.

Figures 4.15 and 4.16 show the relations between different roller geometry on size

distribution. Experiments were done using constant rotational speed of 1200 rpm and feed rate

430 grams per minute or higher. It can be concluded from these plots, that 4*4 mm rollers are

perfectly capable of giving fine products with modest energy consumption. Due to poor material

transport capability through the mill, 4*4 mm rollers increase residence time considerably thus

providing sufficient contact time between rollers and grinding cylinder. By increasing the feed

rate further, grinding becomes very unstable, due to the appearance of the „bed‟ of the material.

Feed rates over 1000 grams per minute create obstruction on the top, or, by using special „push-

down‟ top plate, the rollers will come into contact with central shaft and the shaft will stop

rotating.

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Large grooves act as „bed destroyers‟ giving the material with good flowability. After

pretreatment, grinding with 4*4 mm rollers makes more sense, because gravity then takes care of

preventing the mill from becoming obstructed.

Figure 4.15 Effect of roller geometry on size distribution, 4*4 and 8*8 rollers.

Figure 4.16 Effect of roller geometry on size distribution 8*8 and 16*8 rollers.

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4.2 Wet grinding

4.2.1 Particle size distribution

Wet grinding experiments were carried out by manual feeding of the wetted fibers. All had

been pretreated using the 2-roller mill with 8*16 mm ridge/groove rollers, dry, for ease of

handling. 20% of the feed was dry hemp, i.e. four times as much water, by weight, had been

added to the dry fibers and mixed well. The resultant feed was a moist “cake” with minimal

drainage.

Two roller designs were again used, the 4*4 mm ridge/groove rollers, at a rotational speed

of 1200 rpm. and a feed rate about 500g/min. The 8*8 mm rollers were used for the other

experiments. These were run at 1200 rpm. and at feed rates of about 500 and 1000 g/min. The

final experiment was run at 1000 rpm. and a feed rate of 850 g/min.

Some experiments were also carried out at 15% and 25% consistency, at the 1200 rpm and

500 g/min conditions.

Attempts were made to screen the wet-ground hemp fibers but, aside from removing a small

amount of fines, no meaningful results were obtained. The fibers were rather long and would

stay on the top screen. Therefore, only microscopic analysis is reported, from the measurement

of fiber length and diameter distributions.

4.2.2 Fiber Analysis

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Mean fiber length and fiber diameter, aspect ratio (defined as the ratio of fiber length to

diameter) and fiber distribution (number and volume) were analyzed (7).

Table 2 shows fiber length and diameter and the resultant aspect ratio for the six samples

analyzed. The aspect ratio was between 60 and 80. Sample 1 (4*4 mm rollers) was found with

lowest and sample 4 (1kg/min with 8*8 mm rollers) with the highest aspect ratio among all

samples. This suggests that more intensive grinding (e.g. sample 1) reduces the fiber length more

than the fiber diameter.

Table 2. Fiber length, diameter and aspect ratio of wet ground samples

Code Sample Mean Fiber

Length (mm) Mean Fiber

Diameter (mm) Aspect ratio

1 4mm 1200rpm 20% 2.57 0.043 59.49

2 8mm 1200rpm 15% 500g/min 3.33 0.042 79.38

3 8mm 1200rpm 20% 500g/min 3.03 0.044 69.16

4 8mm 1200rpm 20% 1kg/min 3.70 0.043 85.14

5 8mm 1000rpm 20% 3.69 0.052 71.24

6 8mm 1200rpm 25% 3.12 0.051 61.16

Mean fiber length and mean fiber diameter for all samples were similar to softwood pulp

fiber. The individual fiber length range was about 0.5 – 7mm and fiber diameters varied from

0.01-0.1 mm for all samples.

4.2.3. Fiber length and size distribution make heavy, as 4.22

Fiber size distribution was calculated based on the number and volume of fibers. Figures 4.17 to

4.22 illustrate the length distribution and volume fraction of the samples. The distribution of each

sample was classified by one millimeter length intervals (7).

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Figure 4.17 Size distribution of sample 1 (4mm 1200rpm 20%)

Figure 4.18 Size distribution of sample 2 (8mm 1200rpm 15% 500g/min)

Figure 4.19 Size distribution of sample 3 (8mm 1200rpm 20% 500g/min)

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Figure 4.20 Size distribution of sample 4 (8mm 1200rpm 20% 1kg/min)



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4.2.4 Morphology The microscopic images show the fibers with different fiber lengths and diameters. The grade (1x1mm) was used to estimate the fiber length (7).

Figure 4.23 Microscopic images of pulp fibers with a 1x1mm grade The diameter of 50 fibers was determined using micrometer grade. The distance between two lines is 0.1mm.

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Figure 4.24 Microscopic images of pulp fibers with a 0.1mm grade

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4.3 Power consumption comparison

A rectifier was installed on the pilot Szego Mill used in this work. It allowed control of the

rotational speed of the mill and also recording and calculation of the power drawn by the mill at

intervals of one second. Power consumption comparison between wet grinding, at 20% dry

solids content, and dry grinding at different rotational speeds is shown in Figures 4.25 and 4.26.

Wet grinding appears to take more power for the same dry weight of feed; the fluctuations reflect

feed rate variations caused by manual feeding. For dry grinding with the lower rotational speed

of 1000 rpm, power consumption is significantly lower compared to wet grinding, Figure 4.25,

whereas at the higher speed of 1200 rpm, Figure 4.26, only a slight difference is observed if the

same feed rate is maintained. The higher rotational speed appears to avoid any mill blockage in

wet operation so feeding is easier, mill hold-up lower, and, hence, the motor load may indeed be

less than at the 1000 rpm rotational speed. All curves are reported for an interval time of 166

seconds. When there was mill blockage, or when a pass was too short, mill power was not

calculated.

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Figure 4.25 Mill power; 8*8 mm rollers, dry and wet grinding; 1 kg/min; 1000 rpm

Figure 4.26 Mill power; 8*8 mm rollers, dry and wet grinding; 1kg/min; 1200 rpm

When the solids content was varied by adding more or less water, it is seen in Figure 4.27 that the

highest content, 25%, took more power. This is clearly observed in the beginning and reasonably also

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during the rest of the experiment. Feed viscosity, hence, resistance to flow, caused this noticeable,

expected difference.

Figure 4.27 Mill power; 8*8 mm rollers, wet grinding at different consistencies;1kg/min;1200 rpm

5. Conclusions

A pilot size Szego Mill has been used to grind hemp fibers. From the results it is seen that

roller geometry, really roller groove structure has the most significant effect on size distribution

and fiber quality. Changing the operational variables such as feed rate and mill rotational speed

also led to variations but due to the limited range of these variables, particularly feed rate, these

are not as significant as roller groove structure. Power consumption has also been measured, for

both dry and wet grinding.

Wet grinding with Szego Mill is a useful process for defibrillating hemp fibers. In dry

grinding, fiber cutting predominates and fiber aspect ratios are low, in the 6-10 range. In wet

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grinding where fiber cutting is reduced and shearing is predominant, higher aspect ratios are

obtained, in the 60-80 range, comparable to wood pulps.

6. References

1. Austin, L.G. and Trass, O., “Size Reduction of Solids: Crushing and Grinding Equipment”,

Chapter 12 in Handbook of Powder Science and Technology, 2nd

edn., M. Fayed and L. Otten,

Eds., Chapman & Hall, N.Y., N.Y., pp. 586-634, 1997.

2. Trass, O, “Developments in Grinding Technology.” International Seminar on Chemical

Engineering and Science for Human Welfare”, Waseda University, Tokyo, Japan, March

2005.

3. Trass, O., Papachristodoulou, G.L. and Gandolfi, E.A.J., “Wet Grinding of Coal in the Szego Mill:

Limiting Predictions and Experimental Results”, Coal Preparation 16, 179-201 (1995).

4. Wang, B., Sain, M. and Oksman, K., “Study of Structural Morphology of Hemp Fiber

from the Micro to the Nanoscale”, Applied Composite Materials, January 2007.

5. Prasad, B.M., Sain, M. and Roy, D.N. “Properties of Ball-milled Thermally Treated

Hemp Fibers in an Inert Atmosphere for Potential Composite Reinforcement”, Jour.

Materials Science, May 2005.

6. Balatinesz, J and Sain, M., “Cars Made of Wood and Hemp Fibers? Why not? The

Forestry Chronicle, 482-484 (2007).

7. Sameni, J., “ Analysis of Pulp Fibers using Light Microscopy” Personal communication,

2010.

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