Retsch: The art of Milling

with detailed

information on

materials

An expert guide to neutral-to-analysis size reduction

and homogenization in the laboratory

The Art

Milling

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any way without written consent of RETSCH GmbH nor may be

modified, copied or distributed with the help of electronic systems.

Content

Page

1. Why Size Reduction? ............................ 4

2. The Size Reduction Process ....................... 7

2.1. Sample Preparation ........................... 7

Sample Division ............................ 8

Drying .................................. 9

Metal Separation .......................... 10

Embrittlement (liquid nitrogen or dry ice) ......... 10

2.2. Size Reduction Principles....................... 11

Size Reduction of hard / brittle materials ......... 12

Size Reduction of soft, elastic, fibrous materials .... 13

2.3. Grinding Tools .............................. 14

2.4. Grinding Aids ............................... 18

3. RETSCH's Product Range ......................... 20

3.1. Jaw Crushers ............................... 21

3.2. Rotor Mills ................................. 22

3.3. Cutting and Knife Mills ........................ 25

3.4. Disc Mills .................................. 27

3.5. Mortar Grinder .............................. 28

3.6. Ball Mills .................................. 28

4. Conclusion .................................... 34

Annex: Tips & Tricks for Ball Milling ............. 36-39

Annex: Materials ............................ 41-65

Annotations ................................ 66-69

The Art

Milling

1. Why Size Reduction?

A reliable and accurate analysis can only be guaranteed by reproducible

sample preparation. The "Art of Milling" describes the process of turning a

laboratory sample into a representative part sample with homogeneous

analytical fineness.

For this task RETSCH offers a comprehensive range of the most modern laboratory

mills and crushers for coarse, fine and ultra-fine size reduction of almost any mate-

rial. The wide selection of grinding tools and accessories not only ensures contam-

ination-free sample preparation but also adaptation to the specific requirements of

such different areas of application as construction materials, metallurgy, foodstuffs,

pharmaceuticals or environment.

Particle size reduction of solids or bulk materials is required when the particles are

too coarse or the sample is too inhomogeneous for subsequent processes such as

analysis, division, mixing or further processing. The standard deviation of any sub-

sequent analysis can be minimized drastically by particle size reduction and homog-

enization of the analysis sample (see diagram below).

A rubber sample was ground to different particle sizes. The degree of homogenization

increased with decreasing particle size. The subsequent measurement of the sulfur con-

tent (measured with an ELTRA CS-580 analyzer) showed that the more homogeneous a

sample was (red) the lower was the standard deviation, i. e. the more reliable were the

analysis results.

3 mm: 2.12% ± 0.11% (5.2%)

300 µm: 2.05% ± 0.02% (0.9%)

S[%]

2.3

2.2

2.1

2.0

1.9

Measurement No.

0 1 2 3 4 5 6 7 8

1. Why Size Reduction?

Required Fineness

A frequent requirement is to "grind the sample to fine powder". The term "powder",

however, is not precise. Washing powder, coffee powder or baking powder are bulk

materials which are all characterized as "powders", although they have very differ-

ent particle size distributions.

Another typical request is to have the sample ground "as fine as possible". This

involves a high input of energy and time which in turn increases costs. A much

more effective approach is not to grind as fine as possible but only as fine as

necessary. This parameter is typically determined by the subsequent analysis.

Homogenization

Usually, only a few gram or milli-

gram of a laboratory sample are

required for an analytical method. If

the sample is inhomogeneous, this

small amount does not represent

the entire sample as some ingre-

dients may be over-representative

or missing altogether. Therefore,

the sample must be homogeneous

to ensure reliable and representati-

ve analytical results.

Analytical fineness

The required analytical fineness of

the sample material depends on the

analytical method or further pro-

cessing and can vary greatly. Most

methods require a fineness in the

size range from 20 µm to 2 mm.

As product properties (e.g. extrac-

tion, filtration or absorption capaci-

ty) are often influenced by the par-

ticle size, size reduction on a labo-

ratory scale is also essential for the

development of new products or

production processes.

Only a homogenized analytical sample can repre-

sent the total initial sample and lead to reliable

and reproducible results.

Aspects which influence the selection of a suitable mill

When searching for a suitable mill and grinding tools, one has to keep in mind that

the sample properties to be determined (such as moisture or heavy metal content)

must not be altered in any way during the process.

To make the right choice not only precise knowledge of the instruments is required

but also profound experience in the preparation of different materials.

To find the best suited mill for a specific application, the following aspects should be

considered in advance:

n Quality/characteristics of sample

(e.g. dry, tough, abrasive, fibrous, brittle, hard, soft, temperature-sensitive

etc.)

n Feed size

n Required final fineness

n Sample volume

n Sample throughput

n Subsequent analysis

(which type of contamination by abrasion of grinding tools is acceptable?)

n May the sample be dried or embrittled before grinding?

n Not to forget: Is the laboratory sample representative?

Depending on the quality of the material different size reduction principles are

applied to obtain the required fineness. Large particles cannot always be ground to

analytical fineness in one step. In some cases it is possible to carry out coarse and

fine grinding in the same mill with different settings; in other cases two mills or

crushers are required.

1. Why Size Reduction

2.1. Sample Preparation

Before starting the actual grinding process it must be ascertained that the sample

can be processed without further treatment. Moisture, agglomerations, segrega-

tions or impurities could disturb the process and falsify the grinding result.

The sample quantity is also an

important factor. How much is

needed for analysis? How big is the

total sample amount in comparison

to that and what is the grain size?

These parameters determine the

minimum amount which is needed

for the part sample to be represen-

tative. Representative means that

the composition of the part sample

is exactly the same as that of the

total sample.

2. The Size Reduction Process

To produce a size reduction effect, the comminution principle of the mill

should be matched to the breaking behavior of the respective material

(see chapter 2.2).

When selecting a suitable instrument and before beginning the prepara-

tion process, a thorough evaluation of the material is necessary. Proper-

ties such as density, hardness, consistency, residual moisture or fat con-

tents have to be examined. The success of the grinding process can also be

influenced by temperature stability, tendency to agglomerate or surface

reactions.

In any case, the requirements of subsequent analysis should always be

taken into account when carrying out a particular grinding task.

Different models

of sample dividers

2. The Size Reduction Process

2.1. Sample Preparation

Sample Division

Most laboratory samples consist of an inhomogeneous mix-

ture. Different particle sizes can lead to segregation during

transportation of the sample. If the sample is not ground

completely, a part sample should be extracted.

If the initial grain size is too big, preliminary size reduction

must be carried out before the sample can be divided. The

selection of the division method and the instrument depends

on the sample material and quantity. Dry, free flowing sam-

ples can be fed via vibratory feeders to rotary tube dividers

and sample dividers with a rotating dividing head whereas

sample splitters are used for materials with a low flowability.

Manual random sampling is only acceptable if the sample is

homogeneous with regards to material and grain size. How-

ever, without preliminary examination, this is difficult to

ascertain.

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Loss of weight [%]

Mean value of 10 samples after

random sample division

Standard deviation:

10.1%

of total value

Mean value of 10 samples after

sample division in PT 100

Standard deviation:

1.1%

of total value

The standard deviation e.g. in a plastic sample analyzed for its moisture content can be

decreased drastically by correct sample division using a sample divider.

2.1. Sample Preparation

Drying

To grind moist or even wet sample

materials is not possible without both-

ersome side effects, especially in jaw

crushers, rotor or disc mills. Moist

materials tend to block the ring and

bottom sieves which can lead to a

blockage of the machine. As a conse-

quence, material is lost and much time

and effort has to be spent on cleaning

the mill.

There are a few exceptions: colloidal

grindings can only be carried out in ball

mills by adding a liquid. Fresh fruit and

vegetables can be homogenized in knife

mills without material loss.

However, in most cases, moist samples have to be dried (e.g.

leaves) before they can be subjected to size reduction. When

choosing the drying method and temperature, care must be

taken that the properties of the sample to be determined are

not altered in any way. That is especially important with

regards to volatile components such as furans, polychlorinat-

ed biphenyls (e.g. PCBs) and dioxins. Usually, these types of

sample can only be air-dried at room temperature.

RETSCH's TG 200 is suitable for gentle and quick drying. It

uses the fluidized bed drying method, similar to that of indus-

trial dryers. For many products the drying time is as little as

5 to 20 minutes.

Further methods include vacuum and freeze drying as well as

drying in drying ovens.

Fluid Bed Dryer

TG 200

2.1. Sample Preparation

Metal Separation

Samples such as industrial waste, recyclable waste and sec-

ondary fuels often contain metallic components which cannot

be pulverized with laboratory mills. On the contrary, metallic

objects such as steel nails or iron screws can damage the

grinding tools which can lead to a considerable deterioration

of the mill's performance. Therefore, it is necessary to sepa-

rate the metal components before grinding (e.g. with mag-

netic forces). If required, they have to be evaluated sepa-

rately.

Embrittlement (with liquid nitrogen or dry ice)

Cooling the sample material often

improves its breaking behaviour. There-

fore, temperature-sensitive materials,

such as some types of plastics, have to

be cooled directly before they can be

subjected to preliminary or fine size

reduction. One way is to embrittle the

sample in liquid nitrogen (N

, LN) before

grinding. At a temperature of

-196 °C even soft rubber becomes so

hard and brittle that it can be ground

without problems. Another possibility is

to mix the sample with dry ice (CO

For indirect cooling, the grinding jar is

placed in liquid nitrogen.

If the sample contains volatile substances which must be pre-

served during grinding, cryogenic grinding is also the method

of choice.

However, materials which must not become moist should not

directly be treated with cooling agents. The reason for this is

that the steam in the air is frozen and is precipitated as water

when it unfreezes.

Cooling agents should not be used in closed grinding tools as

evaporation causes overpressure in the jar.

Cooling a jar

in liquid nitrogen

2.2. Size Reduction Principles

Laboratory mills work with different size reduction principles.

Which type of mill is used for a particular size reduction task

always depends on the breaking properties of the sample mate-

rial. Hard-brittle materials are best pulverized with impact, pres-

sure and friction whereas soft and elastic substances require cut-

ting and shearing effects to be successfully comminuted.

Size reduction machines for large particle sizes above 40 mm are

known as crushers or shredders while particle sizes below this

are processed with mills.

The next pages show the most common mechanisms for the size

reduction of solids.

2.2. Size Reduction Principles

Size Reduction of hard / brittle materials

Pressure

Force is applied between two solid sur-

faces that either represent the grinding

tool surfaces directly or may be the

surfaces of adjacent particles. Pressure

is exerted by the grinding tools.

Examples:

jaw crushers, toggle crushers.

Impact effects

Force at a solid surface. This could

either be that of a grinding tool, or be

represented by other particles. Strain

by impact is mainly caused by one-sid-

ed and opposing particle acceleration.

Examples:

mixer mills, planetary mills, impact

mills, jet impact mills, drum mills.

Friction

Force between two solid surfaces.

Caused by the vertical pressure of one

surface and the simultaneous move-

ment of the other surface.

Examples:

mortar grinders, disc mills,

hand mortars, rod mills.

2.2. Size Reduction Principles

Size Reduction of soft, elastic, fibrous materials

Usually, various size reduction principles are combined in

one mill, such as impact and friction in planetary ball mills or

shearing and impact in rotor mills.

Shearing

Force between two or more solid sur-

faces moving in opposing directions

which results in a shearing effect. At

least one fixed and one moving surface.

Examples:

rotor beater mills, cross beater mills,

ultra centrifugal mills.

Cutting

Force between two or more sharp-

edged surfaces. At least one fixed and

one moving cutting edge.

Examples:

shredders, cutting mills, knife mills.

2.3. Grinding Tools

Each RETSCH mill comes with grinding

tools that are optimized with regards to

their functionality and handling. How-

ever, due to the wide range of applica-

tions, the requirements may differ

greatly. Therefore, RETSCH offers a

great variety of accessories. For ball and

disc mills, for example, the grinding

sets are available in various sizes. By

using distance sieves and rotors it is

possible to process temperature-sensi-

tive materials in rotor mills.

All grinding tools are available in differ-

ent materials.

Materials

The materials used for RETSCH grinding tools can be grouped

as follows:

n Metal (steel, cast iron, titanium)

n Ceramics (tungsten carbide, zirconium oxide, sintered

aluminium oxide, hard porcelain, silicon nitride)

n Natural stone (agate)

n Plastics (PTFE)

The chemical and physical properties of a material determine

whether it is available for a particular type of mill. Grinding

tools made of steel are available for all mills.

Grinding jars

“comfort” and

grinding balls for

Planetary Ball Mills

2.3. Grinding Tools

The table below gives a survey of parameters such as hard-

ness, energy input, wear resistance and possible contamina-

tion through abrasion:

Materials

Hardness Density Energy

input*

Wear

resistance*

Possible contami-

nation through

abrasion

Stainless

Steel

48 - 52 HRC

(approx.

550 HV)

7.8 g/cm

very high

good

(to certain

extent)

Fe, Cr

Hardened

Steel

58 - 63 HRC

(approx.

750 HV)

7.85 g/cm

very high good

Fe, Cr, C

(less than

SS)

Tungsten

Carbide

approx.

1250 HV

14.8 g/cm

extremely

high

very good

W, C, Co

(marginal)

Agate

hard and

brittle

6.5 - 7 Mohs

(approx.

1000 HV)

2.65 g/cm

very low

good

(to certain

extent)

SiO

Sintered

Aluminum

Oxide

hard and

brittle

8 - 8.5 Mohs

(approx.

1750 HV)

3.9 g/cm

low

good

, SiO

(low),

no contami-

nation with

Fe, Cr, Ni or Co

Zirconium

Oxide

hard and brit

tle, tougher

than agate

7.5 Mohs

(approx.

1200 HV)

5.9 g/cm

high very good

ZrO

and Y

(marginal),

insignificant

for analyses

Silicon

Nitride

approx.

1500 HV

3.2 g/cm

low excellent

, Y

PTFE

elastic

Shore Hardness

D 56

2.1 g/cm

very low poor

contamina

tion with

F, C

*e.g. for ball mills

2.3. Grinding Tools

When choosing a suitable grinding set, several factors have

to be considered such as the hardness of the sample mate-

rial and its breaking properties. The material of the grinding

set should be harder than the sample to avoid excessive

wear. For example, silica sand should not be ground with

agate tools but with zirconium oxide or silicon nitride as these

are much harder.

Abrasion resistance is also an important parameter. Tung-

sten carbide, zirconium oxide and silicon nitride are highly

resistant against abrasion. However, the amount of abrasion

also depends on the properties of the sample and the size

reduction principle of the mill.

In mechanical size reduction processes, abrasion cannot be

completely avoided. Therefore, when choosing a material it

should be taken into account if possible contamination will

have a negative influence on the product or the subsequent

analysis (e.g. abrasion of chrome or nickel influences subse-

quent heavy metal analysis). Thus, neutral-to-analysis mate-

rials should be chosen.

Breaking jaws BB

(left)

Rotors and sieves

ZM (right)

2.3. Grinding Tools

Another important feature of ball mills and vibratory disc mills is the energy input

generated by the different materials. Grinding balls of tungsten carbide, for exam-

ple, generate a much higher energy input, and thereby a better size reduction

effect, due to the higher density of the material than balls of the same size of

other materials.

Application examples:

n If soil samples are to be analyzed for iron, chrome or cobalt, grinding tools of

stainless or hardened steel are not suitable as they contain the elements which

are to be determined.

n If, however, calcium or silicon dioxide are to be determined in cement clinker,

grinding jars of steel are suitable.

n PTFE, zirconium oxide, silicon nitride and glass can be sterilized;

therefore, they are often used for preparing food or microbiological samples.

n Abrasion is also an issue for the further processing of samples, even if there is

no subsequent analysis. Homeopathic products, active agents in lactose and

pharmaceuticals, for example, should only be ground in ceramic or agate mor-

tars in order to avoid contamination of the sample.

Please refer to the annex for detailed information on the

materials of the grinding tools.

2.4. Grinding Aids

Many grinding tasks which are known from the field of mechanical process engi-

neering can be solved by using one of the various mill types with a suitable size

reduction principle. However, some applications cannot be carried out successfully

with common laboratory mills despite the wide range of accessories. Difficult sam-

ples are, for example, moist materials which must not be dried or soft and elastic

materials and those which contain fat or oil. To produce ultra-fine powders by

mechanical energy input, it is often necessary to add a liquid.

In the above cases, the use of a grinding aid can be helpful. Grinding aids are addi-

tives which activate, accelerate and also improve chemical or physical processes.

Before using a grinding aid for the preparation of solids it must be ensured that the

additive does not influence the subsequent analysis or further processing of the

sample in any way.

Grinding aids are differentiated according to their state of aggregation:

n Solid (powder, granulate, pellets) to bind fat and/or moisture (e.g. talcum)

n Liquid (water, alcohol, benzine) to avoid agglomeration

n Gaseous (inert gas, cooled air, e.g. argon)

Another grinding aid is:

n Tempering (cooling with liquid nitrogen or dry ice, heating)

2.4. Grinding Aids

Solid Additives

When preparing samples for XRF analysis, neutral-to-analysis aids, such as Spec-

tromelt pellets (based on cellulose) are often added to the sample material during

grinding in planetary ball mills or vibratory disc mills. If they are mixed in the cor-

rect ratio, they support the size reduction effect and help to avoid caking of the

material inside the grinding jar. When pelletizing the sample material afterwards,

this grinding aid also serves as a binding agent.

The addition of sodium sulfate is a common method to bind fat or moisture that is

to be determined afterwards (e.g. when grinding insects, small marine animals or

moist soils). Trituration is carried out in mortar grinders which guarantees 100 %

sample recovery.

Liquid Additives

To homogenize oil seeds such as rape seed, soy beans or mustard seeds in ball

mills or mortar grinders, it is helpful to add benzine. Benzine is used as extraction

liquid for the determination of oil contents.

The production of ultra-fine powders, e.g. in the ceramics industry, powder metal-

lurgy or mineralogy, can often only be realized by wet grinding. Usually, water or

isopropanol are used as dispersants. Ball mills and mortar grinders are especially

suitable for wet grinding.

Air or Inert Gas

If a size reduction system is sufficiently ventilated, e.g. through a cyclone or a filter

system, the frictional heat is continuously discharged. This helps to reduce the

heating up of the sample material and to increase the throughput.

Gassing with inert gas during grinding prevents the reaction of surface active par-

ticles with oxygen (= oxidation).

3. RETSCH's Product Range

The RETSCH product range covers applications from the preliminary size

reduction of particles of several centimeters to fine grinding down to the

nano range. The following criteria are decisive when selecting a suitable

laboratory mill:

n Material properties of the sample (e.g. breaking behavior)

n Feed size of the sample

n Required final fineness

n Feed quantity

If the initial particle size of the sample is coarse, it might be necessary to

use two mills, one for preliminary size reduction and one for fine grinding,

to achieve analytical fineness. To choose suitable grinding tools is also

part of the selection process (see chapter 2.3). Here the important criteria

are hardness, abrasion resistance, possible contamination and, for ball

mills, the energy input.

Interdependence of instrument, sample mate-

rial and working principle

Device

Hard and

brittle

materials

Soft,

elastic

and

brous

materials

Working principle

Jaw Crusher Pressure

Ultra Centrifugal Mill Impact, shearing

Cyclone Mill Friction, shearing

Cross Beater Mill Impact, shearing

Rotor Beater Mill Impact, shearing

Cutting Mill Shearing, cutting

Knife Mill Cutting, impact

Mortar Grinder Pressure, friction

Disc Mill Pressure, friction

Mixer Mill, CryoMill Impact, friction

Preliminary size

reduction

Planetary Ball Mill, E

max

Impact, friction

Drum Mill Impact, friction

Fine grinding Rod Mill Friction

3.1. Jaw Crushers

Jaw crushers are used for the prelimi-

nary size reduction of hard-brittle, dry

materials, such as ores, minerals, slag

and coal.

RETSCH offers eight sizes: the floor

models BB 100, BB 200, BB250 XL,

BB 300, BB 400 XL, BB 500 XL,

BB 600 XL and the benchtop model

BB 50.

The BB 600 XL crushes particles of up

to 35 cm while the maximum feed size

for the BB 50 is approx. 4 cm. The final

fineness achievable with a BB 600 XL is

approx. 6 mm, the BB 50 produces par-

ticles of 0.5 mm and smaller. The final

fineness is determined by the (adjustable)

gap width of the breaking jaws.

With increasing size of the jaw crusher,

the potential throughput of sample

material also increases as the BB 200

and the BB 300 achieve a high through-

put of more than 100 kg/h and the

BB 600 XL even a throughput of up to

3.5 t/h (depending on the sample

material).

Four models can be integrated in pro-

cess lines: BB 200, BB 300, BB 500 XL

and BB 600 XL.

The jaw crushers of the XL line can be

equipped with an automated sorting

machine.

The breaking jaws and the wearing

plates are available in five different materials (depending on

the model):

Manganese steel, stainless steel, steel for heavy metal-free

grinding, tungsten carbide and zirconium oxide.

Jaw Crusher

BB 50

Jaw Crusher

BB 600 XL

3.2. Rotor Mills

The Ultra Centrifugal Mill ZM 200 is

used for the rapid fine size reduction of

soft, medium-hard, brittle and fibrous

materials such as fertilizer, plastics or

feed pellets. For the size reduction of

elastic plastic materials it may be nec-

essary to embrittle the sample in liquid

nitrogen before grinding.

Size reduction in the ZM 200 is effected

through impact and shearing forces

between ring sieve and rotor. The maxi-

mum feed size is up to 10 mm. Depend-

ing on the material, a final fineness

down to 40 µm and below can

be achieved. The final fineness is determined by the

exchangeable ring sieves. The speed of the

ZM 200 ranges from 6 000 to 18 000 min

-1

. The patented cas-

sette principle guarantees 100 % sample recovery and easy

cleaning.

For an automatic and uniform feed of larger amounts of free

flowing materials the Vibratory Feeder DR 100 is recom-

mended. A cyclone with a 3 liter or 5 liter collector can also

be used for larger amounts as well as for temperature-sensi-

tive materials. The frictional heat that is generated during the

grinding process is partly discharged through the cyclone.

The use of distance sieves instead of standard ring sieves

also helps to reduce frictional heat due to the greater gap

between sieve plate and rotor.

Accessories for the ZM 200 include ring sieves and rotors of

titanium for heavy-metal-free size reduction. If hard and

abrasive materials are to be ground, a rotor with abrasion

resistant coating is required. For processing very small

amounts of sample, e.g. of pharmaceuticals, RETSCH offers a

mini-cassette with matching 316 L stainless steel rotor and

various ring sieves.

Ultra Centrifugal

Mill ZM 200

3.2. Rotor Mills

The Cyclone Mill TWISTER is specially

designed for the processing of foods

and feeds for subsequent NIR analysis.

It processes fibrous and soft products

quickly and gently to the required ana-

lytical fineness. The mill is ideally suited

for grinding feeds, forage and cereals

as well as various types of food.

The TWISTER is equipped with a rotor

and grinding ring with sieve insert. The

high speed of up to 14 000 min

-1

and

the optimized grinding geometry of

rotor and grinding chamber generate an

air stream which carries the sample

through the integrated cyclone into the

sample bottle. The cyclone provides

additional cooling of the sample and the

grinding tools. This prevents loss of

moisture and thermal degradation ensuring preservation of

the sample properties to be determined. The ground material

is separated in the cyclone and collected in a sample bottle

for complete recovery.

The provided sieves guarantee an optimum particle size dis-

tribution so that it is not necessary to recalibrate the NIR

spectrometer. The rotor speed can be adjusted in 3 steps

allowing for perfect adaptation to the sample requirements.

For most products the air stream effects a complete discharge

of the material from the grinding chamber, particularly if a

vacuum cleaner is connected, so that hardly any cleaning is

required. This helps to avoid cross contaminations and is

especially convenient when processing a series of samples.

Cyclone Mill

TWISTER

The Rotor Beater Mill SR 300 is used

for the preliminary and fine size reduc-

tion of soft, medium-hard and brittle

materials with a maximum feed size up

to 25 mm. The final fineness is deter-

mined by the aperture size of the

exchangeable ring sieves. The SR 300

can achieve a fineness down to 50 µm

and below, depending on the properties

of the sample material. Typical applica-

tions include the grinding of grain, coal,

chemicals and soils.

Size reduction in the rotor beater mill is

effected by impact and shearing forces

between rotor and ring sieve. Rotor

beater mills are usually equipped with ring sieves. To achieve

an additional size reduction effect through impact, a 180°

grinding insert can be used for medium-hard materials.

The speed of the SR 300 is adjustable between 3 000 and

10 000 min

-1

. A higher speed does not only generate a higher

throughput but also more frictional heat which can affect the

sample material. For temperature-sensitive materials the use

of a distance rotor is recommended. The larger grinding gap

ensures a reduction in frictional heat. Moreover, a cyclone is

available which also reduces generation of heat by dragging

ground particles faster out of the grinding chamber and gen-

erating a cooling air flow.

The Cross Beater Mill SK 300 is suitable for the preliminary

and fine size reduction of medium-hard/brittle materials such

as ores, minerals and cement clinker with feed sizes up to

25 mm. The final fineness is determined by the exchangeable

bottom sieves with various aperture sizes. Size reduction is

effected through impact and shearing forces between the

baffle plates of the rotor and the toothed grinding insert.

Rotor Beater Mill

SR 300 with

cyclone attached

3.2. Rotor Mills

Particles which are ground to the

desired fineness pass the bottom sieve.

The SK 300 is available in cast iron,

stainless steel, hardened steel and steel

1.1740 (for heavy-metal-free size

reduction). An optional cyclone is also

available.

The speed of the SK 300 is adjustable

in steps of 200 min

-1

between 2 000

and 4 000 min

-1

3.3. Cutting and Knife Mills

The Cutting Mill SM 100, SM 200 and

the Heavy-Duty Cutting Mills

SM 300 and SM 400 XL are used for

preliminary size reduction of bulky, soft

and fibrous materials such as branches,

straw and plastics. Due to an additional

flywheel mass on the drive shaft the

SM 300's performance equals that of

motors with twice the power rating.

Therefore, it is also suitable for the size

reduction of solid and tough materials

such as leather, thick pieces of rubber

and secondary fuels or electronic scrap.

The speed of the SM 300 is freely

adjustable between 700 and 3 000 min

-1

The final fineness which can be achieved with the cutting

mills depends on the aperture size of the exchangeable bot-

tom sieve and the breaking properties of the sample materi-

al. For all members of the Cutting Mills family three types of

rotors are available: a parallel section rotor which is espe-

Cross Beater Mill

SK 300

3.3. Cutting and Knife Mills

Cutting Mill

SM 300

cially suitable for soft, elastic and fibrous materials. And a

6-disc rotor with replaceable and reversible tungsten carbide

cutting plates. For the SM 300 the V-rotor is available which

is especially suitable for tough, soft and fibrous material.

Further accessories include bottom sieves and rotors made of

steels which are suitable for heavy-metal-free size reduction.

The Knife Mills Grindomix GM 200

and GM 300 are suitable for the size

reduction and homogenization of sam-

ples with a high fat, oil or water con-

tent. The mills are frequently used in

food control laboratories.

Whereas the GM 200 can process sam-

ple volumes from 300 - 700 ml, the

GM 300 homogenizes sample amounts

of up to 4 500 ml.

A wide range of accessories is available

for the knife mills. The standard con-

tainer of the GM 200 is made of poly-

propylene. Additionally containers of

stainless steel, polycarbonate and borosilicate glass are avail-

able which can be sterilized and autoclaved. The GM 300 fea-

tures containers from polycarbonate and stainless steel which

can also be autoclaved.

When using a gravity lid the volume of the container is

reduced and automatically adapted to the sample amount.

For samples with a high liquid content gravity lids with over-

flow channels are best suited, as the liquid contents of the

sample ascend the container walls and are returned to the

center of the container.

For heavy-metal-free grinding and homogenization processes

neutral-to-analysis knives are available.

Knife Mills Grindomix

GM 200 and GM 300

3.3. Cutting and Knife Mills

3.4. Disc Mills

The Vibratory Disc Mills RS 200 and RS 300 XL are fre-

quently used for the sample preparation to XRF analysis.

They are suitable for the fine size reduction of medium-hard

and hard materials such as slag, ore, cement clinker and

minerals with a maximum feed size up to 20 mm. Depending

on the material and the selected parameters a final fineness

down to 20 µm can be achieved. A fineness of <100 µm can

usually be obtained after only 1 - 2 minutes of grinding. Grind-

ing sets are available in the sizes 50, 100, 250, 800, 1000 and

2000 ml in up to six different materials (depending on model).

Size reduction in the RS 200 is effected by pressure and fric-

tion between the grinding jar wall, disc and ring which move

in horizontal circles. The speed can be selected in a range

from 700 to 1 500 min

-1

The preliminary and fine size reduction of hard-brittle materi-

als is effected in the Disc Mills DM 200 and DM 400 through

pressure and friction between two vertical grinding discs, one

of which is fixed and the other one rotates. The maximum

feed size is up to 20 mm. The final fineness is determined by

the gap width setting of the grinding discs and is around

0.1 mm. The grinding discs are available in 4 different mate-

rials.

Vibratory Disc Mill

RS 200 (left)

Disc Mill

DM 200 (right)

3.5. Mortar Grinder

Mortar grinders are used for grinding,

mixing and triturating soft, hard and

brittle materials. Mortar grinders are

dust-tight and therefore ideally suited

for wet grindings and trituration of

pastes.

Size reduction in mortar grinders is

effected by pressure and friction

between mortar and pestle. The pestle

pressure can be set manually and the

position of scraper and pestle is adjust-

able.

The RETSCH Mortar Grinder RM 200

features a useable volume of 10 to 190

ml. The maximum feed size is up to 8 mm. Depending on the

properties of the sample material, a final fineness down to 10

µm can be achieved. Grinding tools are available in seven dif-

ferent materials.

3.6. Ball Mills

Ball mills are frequently used for the fine size reduction of

hard-brittle materials. A crucial advantage of ball mills is their

great versatility. Grinding jars and balls are available in vari-

ous sizes and materials, e.g. agate and ceramics such as zir-

conium oxide. This is important if the sample is analyzed for

heavy metals.

The grinding tools for ball mills always consist of a grinding

jar and grinding balls made of the same material (exception:

XRD mill). The following rule of thumb can be applied for the

jar filling: 1/3 is filled with balls, 1/3 with sample material.

When choosing the ball size, the feed size of the sample and

the desired final fineness have to be taken into account.

30 mm grinding balls, for example, are suitable to reduce

Mortar Grinder

RM 200

3.6. Ball Mills

particle sizes of approx. 10 mm while

for colloidal grindings which produce a

fineness down to the submicron range

grinding balls of only 2 or 3 mm are

used. Due to the dust-tight closure of

the grinding jars, ball mills are also

suitable for wet grindings.

RETSCH offers 4 models of Planetary

Ball Mills: the PM 100 and PM 100 CM

with one grinding station, the PM 200

with two grinding stations and the floor

model PM 400 with four grinding sta-

tions. Grinding jars are available in six

different materials with volumes from 12

to 500 ml. The maximum feed size is

10 mm. Depending on the material properties a final fineness

down to 100 µm can be achieved with dry grinding.

A final fineness below 10 µm down to the nano range can

only be achieved with wet grinding. Usually, grinding balls

with small diameters (e.g. 3 mm) are used for this. Approxi-

mately 60 % of the jar volume is filled with grinding balls. A

dispersing agent such as water or alcohol is added to the

sample. Care must be taken that the dispersing agent does

not dissolve the sample or change its

chemical properties. For wet grinding it

is recommended to use a safety closure

device, because the long grinding times

(of several hours) increase the temper-

ature inside the jar which can lead to

the buildup of overpressure.

The grinding jars are arranged eccentri-

cally on the so-called sun wheel. The

rotational movement of the sun wheel

is opposite to that of the grinding jars

in a ratio of 1:-2. Size reduction is

effected through impact and friction.

Planetary Ball Mill

PM 100

Planetary Ball Mill

PM 400

Measurement

System

PM GrindControl

3.6. Ball Mills

The Planetary Ball Mill PM 100 CM operates in centrifugal

mode, i.e. the speed ratio of sun wheel to grinding jar is 1:-1.

Size reduction with the PM 100 CM is effected through friction

and pressure, which leads to a more gentle size reduction pro-

cess with less abrasion.

RETSCH's planetary ball mills can also be used for mechanical

alloying where mixtures of metals and metal oxide powders

are ground for several hours to form new materials with new

properties. A speed ratio of 1:-2 is usually sufficient for ductile

metals whereas for hard-brittle materials, such as covalent-

bonded semiconductors, a higher energy input is required.

For this type of material the PM 400 MA with a speed ratio of

1:-2.5 or 1:-3 is the most suitable.

To monitor the processes which occur during grinding, RETSCH

offers the measurement system PM GrindControl which is

used to document pressure and temperature inside the grind-

ing jar.

The Mixer Mills MM 200 and MM 400

are suitable for grinding small sample

quantities. The grinding jars perform

radial oscillations in a horizontal posi-

tion. Size reduction is effected through

impact forces. When used with a great

number of small grinding balls (for an

increased inner friction), mixer mills are

also suitable for the disruption of bio-

logical cells.

For the preparation of pellets for XRF

analysis, the previously ground sample

can be mixed and homogenized with

wax (binding agent) using polyamide or steatite balls.

The MM 400 has a greater oscillation radius than the MM 200

which results in an approx. 30 % higher energy input. This

means that a greater fineness can be achieved with shorter

grinding times. Grinding jars for the MM 400 have a size range

from 1.5 to 50 ml. They have a screw-top lid which makes

them suitable for wet grinding. Another option is the use of

Mixer Mill

MM 400

3.6. Ball Mills

different adapters which hold up to 20 x 2 ml or 6 x 5 ml

reaction vials or 8 x 50 ml centrifugation tubes; those single

use vials can all be used for efficient cell disruption (bead

beating) or homogenization of biological tissue material like

liver.

The MM 400 can also be used for cryogenic grinding. The

stainless steel grinding jars which are filled with balls and

sample material are cooled for 2-3 minutes in liquid nitrogen

and are then fastened in the quick-clamping device.

The CryoMill has been specially

designed for cryogenic grinding. The

grinding jar is continually cooled with

liquid nitrogen from the integrated

cooling system before and during the

grinding process. Thus the sample is

embrittled and volatile components are

preserved. The liquid nitrogen circu-

lates through the system and is contin-

ually replenished from an autofill sys-

tem in the exact amount which is

required to keep the temperature at

–196 °C. This results in reduced con-

sumption and guarantees reproducible

grinding results.

Various grinding parameters can be

stored which helps to simplify routine tasks. LEDs in the dis-

play indicate the current state of operation, e.g. cooling or

grinding.

The size reduction principle is the same as that of the MM 400.

With a vibrational frequency of 30 Hz the CryoMill grinds

most materials very effectively in a few minutes. It is

equipped with one grinding station for grinding jar volumes of

25 ml, 35 ml and 50 ml. Another option is the use of an adapt-

er which holds up to six 2 ml grinding jars.

The CryoMill can also be operated without cooling which

makes it suitable for a vast range of applications.

CryoMill

3.6. Ball Mills

The High Energy Ball Mill E

max

is an

entirely new type of ball mill for high

energy milling down to the nanometer

range.

The novel size reduction mechanism of

the E

max

unites the advantages of dif-

ferent mill types: high-frequency

impact (mixer mill), intensive friction

(vibratory disc mill) and controlled cir-

cular jar movement (planetary ball mill)

allow for unrivalled grinding perfor-

mance. This unique combination is gen-

erated by the oval shape and the move-

ment of the grinding jars.

The grinding jar brackets are mounted

on two discs each which turn in the

same direction. As a result, the jars

(sizes 50 or 125 ml) move on a circular course without chang-

ing their orientation. The interplay of jar geometry and move-

ment causes strong friction between grinding balls, sample

material and jar walls as well as rapid acceleration which lets

the balls impact with great force on the sample at the round-

ed ends of the jars. This significantly improves the mixing of

the particles resulting in smaller grind sizes and a narrower

particle size distribution than achieved in ball mills.

Grind sizes on a nanoscale can only be achieved by wet grind-

ing. For this method a large number of grinding balls with

diameters of 0.1 mm to 3 mm is used to create as much fric-

tion as possible. The resulting grinding energy is extended

even further by the high speed of 2 000 min

-1

in the E

max

The high energy input is fully exploited as the unique liquid

cooling system quickly discharges the frictional heat.

Depending on the sample characteristics and grinding mode,

cooling breaks of approx. 60% of the total grinding time are

recommended for conventional planetary ball mills to prevent

overheating. The E

max

, on the other hand, is suitable for con-

tinuous grinding without breaks thanks to its efficient liquid

cooling system.

High Energy

Ball Mill E

max

The XRD-Mill McCrone was specially

developed for the preparation of sam-

ples for subsequent X-ray diffraction

(XRD).

The XRD-Mill McCrone carries out size

reduction mainly by friction. 48 cylindri-

cal grinding elements are placed into

the grinding jar in eight rows of six ele-

ments each.

During operation, the jar’s circular

motion causes the elements to grind

the sample from < 0.5 mm to the low

μm-range (typically < 10 μm).

Thanks to the very gentle size reduction process, the crystal

lattice of the sample is preserved. This makes the XRD-Mill

McCrone the instrument of choice for the sample preparation

for subsequent X-ray diffraction analysis

The Drum Mill TM 300 XL is used for

the preparation of granules and pow-

ders.

The grinding process is performed either

in dry or wet conditions with variable

speed up to 80 min

-1

. The drum mill can

be operated either as a Ball or as a Rod

Mill by using the corresponding module.

A sufficient number of balls or rods is

required for an effective grinding pro-

cess. Typically, a final fineness below

20 µm is obtained.

3.6. Ball Mills

XRD-Mill McCrone

Drum Mill

TM 300 XL

4. Conclusion

For many applications size reduction is an important

step in the process of sample preparation. In this

expert guide we have presented the different size

reduction principles and have discussed which type of

mill is best suited for which type of sample.

4. Conclusion

New challenges in research and applications technolo-

gy as well as increasing precision of instrumental anal-

ysis have lead to increasing requirements and a con-

tinuous optimization of size reduction instruments and

grinding tools.

4. Conclusion

Annex: Tips & Tricks for Ball Milling

How to find the correct grinding ball size?

When choosing the ball size, the initial feed size and the required final fineness of

the sample have to be taken into account. Some rules of thumb help to find the

adequate ball sizes:

n The grinding balls should be 3 x bigger than the biggest particles of the sample

to provide enough energy to crush the particles. Consequently, particles with

an initial size of e.g. 10 mm are crushed most effectively with grinding balls of

30 mm in diameter.

n Note: If very hard sample material is to be pulverized, it may be advantageous

to use bigger balls which generate a higher energy input. If, however, the

sample tends to cake, less energy input is required which means smaller grind-

ing balls.

n Generally speaking, the final fineness which can be obtained with grinding balls

of a certain diameter is approximately 1/1000 of that size. This means that

30 mm grinding balls are required when the initial particle size is 10 mm, and

a final fineness of approximately 30 µm can be obtained. To achieve smaller

particle sizes, two or more grinding steps with different ball sizes may be nec-

essary.

Important: Never mix different ball sizes in one grinding step. The bigger ones

would crush the smaller grinding balls, leading to contamination of the sample.

Always use grinding jars and grinding balls of the same material (e.g. stainless

steel) to minimize wear of the grinding tools.

Initial particle size

Grinding balls 3 times bigger

than the biggest particle

Required particle size

3 mm balls Ã down to 3 µm particles

...

0.1 mm balls Ã down to 0.1 µm particles

Maybe two or more grinding steps are needed to obtain very fine parti-

cles , including intermediate change(s) of the ball size.

Annex: Tips & Tricks for Ball Milling

For final sizes <10 µm wet grinding is the only option. Due to their significantly

enlarged surface in relation to their volume, small particles are attracted to each

other by their electrostatic charges. With the addition of dispersants such as water,

buffer or alcohol the charges on the surfaces can be neutralized.

Recommendations for dry and wet grinding (colloidal grinding)

Dry grinding - The following rule of thumb should be obeyed when filling the jar

for dry grinding: 1/3 is filled with balls, 1/3 with sample material, 1/3 remains

empty to leave enough room for the ball movement. Mostly, balls >3 mm are used.

The impact forces needed to crush larger particles increase with the size of the

grinding balls.

Wet grinding - A final fineness below 10 μm can only be obtained by wet grinding

for which small grinding balls of 0.1 - 3 mm (colloidal grinding) are used. Approxi-

mately 60 % of the jar volume is filled with grinding balls to increase the friction

surface required for producing very fine particles. Grinding balls made of an abra-

sion-resistant material like zirconium oxide are recommended for wet grinding to

minimize wear and abrasion. 30 % of the grinding jar volume is filled with sample

material.

A dispersing agent like water or alcohol is added to the sample until the mixture

shows a consistency like engine oil. It is important to choose a dispersing agent

that does not dissolve the sample or change its chemical properties. If the sample

has a tendency to swell, add more dispersant right from the beginning or after 5 or

10 minutes of grinding.

Neutralization of charged particles by adding a buffer (electrostatic stabilization, left)

or by adding long-chained molecules (steric stabilization, right)

Annex: Tips & Tricks for Ball Milling

Removal of the grinding jar

Care should be taken when removing grinding jars from planetary ball mills after

grinding as they may be very hot (~ 150 °C). Heat is generated during the grinding

process and pressure inside the grinding jar increases. Therefore, it is advisable to

use the optional safety closure device for the “comfort” grinding jars of the PM

series which allows for safe removal of the jar. After finishing the grinding process

the jar should cool down for a while.

The E

max

jar is already equipped with an integrated safety closure. Moreover, the

effective cooling system of the E

max

prevents the jars from heating up too much.

The jars for the Planetary Ball Mills and the E

max

can be equipped with optional

aeration covers which allow working under inert atmosphere.

How to separate the grinding balls from the sample after wet grinding

Pour the content of the jar on a test sieve (with aperture sizes 20 % to 50 % small-

er than the balls) with collecting pan. The vibratory movement of the sieve shaker

helps to separate the sample material from the balls. Wet material tends to stick to

the balls due to capillary forces. Flushing with more dispersant to wash away the

sample helps to increase sample recovery.

Attached safety closure device and aeration lid of PM jars (left) and E

max

jar with integrated

safety closure (right).

Annex: Tips & Tricks for Ball Milling

Selection of grinding tools:

size and material of

grinding jar and balls,

grinding balls 3 x larger than biggest

particle of the sample

Grinding jar lling:

60 % grinding balls, 30 % sample

material, addition of dispersant until a

consistency like engine oil is obtained

Dispersion media:

e. g. isopropanol, ethanol,

mineral turpentine, sodium

phosphate, diaminopimelic acid

Grinding process

Cooling down:

grinding jar should not be opened

until it has reached room temperature

Particle size analysis

Grinding process and parameters for wet grinding.

Annex: Tips & Tricks for Ball Milling

Annex: Materials

A-1. Metals .......................................40

A-1.1. Steel .................................... 40

A-1.2. Cast Iron / Malleable Iron ..................... 47

A-1.3. Titanium ................................. 48

A-2. Ceramics ....................................49

A-2.1. Tungsten Carbide (Hard Metal).................. 49

A-2.2. Zirconium Oxide ............................ 51

A-2.3. Sintered Aluminium Oxide ..................... 53

A-2.4. Hard Porcelain ............................. 55

A-2.5. Silicon Nitride.............................. 56

A-3. Other Materials ................................58

A-3.1. Agate (natural stone) ........................ 58

A-3.2. Glass.................................... 60

A-3.3. PTFE (Plastic).............................. 62

A-3.4. Polypropylene (Plastic) ....................... 64

A-3.5. Polycarbonate (Plastic) ....................... 65

A-4. Annotations ...................................66

A-4.1. Hardness ................................. 66

A-4.2. Chemical components ........................ 67

A-4.3. Hardness table ............................. 68

The percentages of the chemical components of the materials are

rounded. Only the major or characteristic components have been

considered. Therefore, the percentage values do

not necessarily add up to 100 %.

More detailed information, also listing the minor components, can be

found in the pdf-document “Material Analyses of Grinding Tools”

on www.retsch.com.

A-1. Metals

A-1.1. Steel

What is steel?

Steel is an alloy whose major component is iron with a carbon

content that is usually less than 2 %.

Varying the amount of alloying elements (such as chrome or

manganese) and their distribution in the steel controls quali-

ties such as the hardness, elasticity, ductility and tensile

strength of the resulting steel. There are various procedures

for manufacturing steel which, however, will not be discussed

here.

RETSCH use the following types of steel for their instruments

and accessories:

n Stainless steel

n Hardened steel (“chrome steel”)

n Steel for grinding without heavy metal contamination

n Manganese steel (“precision-cast manganese steel”)

Stainless steel

The term “stainless steel” refers to corrosion-resistant steel.

The corrosion resistance of this type of steel is due to a very

thin, not visible oxidic protective film which is formed when

the chrome content is >12 %. The corrosion resistance

increases with the chrome content. However, if the chrome

content is more than 20 %, the steel loses its hardenability

and thus its wear resistance.

Hardness

48 to 52 HRC

(Rockwell)

Density

7.8 g/cm

Composition

Fe: 62 – 84.5 %

Cr: 13 – 19.5 %

Ni: <14 %

A-1.1. Steel

Constant subjection to mechanical stress, as is the case with

grinding tools, can lead to the destruction of the protective

film and can also roughen the surface. This can result in cross

contamination as well as the forming of corrosive spots on

the material. These are not a sign of low quality but are

caused by wear.

To remove corrosion stains, dirt or rust from the grinding

tools standard brushes or cleaning agents for metals can

be used. After wet cleaning it is recommendable to clean

the parts with isopropanol or acetone and to dry them thor-

oughly.

Hardened steel (“chrome steel”)

Hardened steel, just like stainless steel, belongs to the group

of chrome steels. Its chrome content however is not more

than 12 %. Consequently, corrosion resistance is not the

strong point of this type of steel but its great hardness.

Steel for grinding without heavy metal contamination

(e. g. steel 1.1750, St 1203)

These steels are chromium- and nickel-free and can be used

for sample preparation for analysis of heavy metals, provided

that possible contamination with iron is acceptable.

Heavy-metal-free steel has a high carbon content which

allows for a hardness up to 62 HRC (Rockwell). Due to the

high iron content of 97 - 99 % it is not resistant to corrosion

and should therefore be cleaned and stored with great care.

This is especially important after the tools have come into

contact with wet sample materials to avoid the formation of

Hardness

58 – 63 HRC

(Rockwell)

Density

7.85 g/cm

Composition

Fe: 84 – 85 %

Cr: 12 %

Hardness

up to 62 HRC

(Rockwell)

Density

7.85 g/cm

Composition

Fe: 98 – 99 %

A-1.1. Steel

rust. Grinding tools which have not been used for a long time

can be protected with a thin oil or fat film against corrosion.

Manganese steel (“precision-cast manganese steel”)

Manganese steel is also called Hadfield steel after its inventor.

The manganese content lies between 12 - 14 %, the carbon

content between 1 - 1.2 %.

After the 1100 °C metallic melt is quenched, the material still

shows a very ductile structure. Subjected to impulsive pres-

sure, e.g. in jaw crushers, a part of the structure could

be modified. Thus manganese steel can obtain a hardness

of 600 HV (approx. 55 HRC). The technical term for this pro-

cess is called strain hardening.

Application areas of steel grinding tools

Stainless steel

Grinding tools of stainless steel are frequently used for min-

eral and metallurgical samples. Further suitable materials are

coal, coke and slag as well as all raw materials and intermedi-

ate products for the production of cement and lime.

As chrome and nickel are substantial parts of the alloy, stain-

less steel is not suitable for the preparation of samples for the

subsequent determination of heavy metals.

When used with hard and abrasive samples, grinding tools of

stainless steel may not have the necessary degree of wear

resistance, especially when the main stress mechanism is fric-

tion.

Hardness

up to 55 HRC

(Rockwell)

Density

7.2 g/cm

Composition

Fe: 85 %

Mn: 13 %

Applications:

e.g.

n Mineralogy

n Construction

materials

n Plastic

n Fabrics

n Secondary fuels

etc.

Sample materials:

n Minerals

n Ores

n Cement

n Polymers

n Circuit boards

n Paper

n Feed pellets

A-1.1. Steel

Hardened steel (“chrome steel”)

Hardened steel grinding tools are used for applications where

stainless steel would wear too quickly, e.g. for the cutting

plates of cutting mills or, generally, when hard, abrasive sam-

ple materials have to be pulverized. They are also highly suit-

able for mills which pulverize with friction.

Steel for grinding without heavy metal contamination

(e. g. steel 1.1750, St 1203)

Crushers and grinders whose stress mechanism does not

allow for ceramic materials – such as jaw crushers, cutting

mills or cross beater mills – are usually equipped with heavy-

metal-free grinding tools.

Heavy-metal-free steel can be used for the preparation of

samples which will be analyzed for their heavy metal con-

tents provided that possible contamination with iron is

acceptable. Examples: Size reduction of secondary fuels in

cutting mills or of soils in jaw crushers/cross beater mills.

Manganese steel (“Precision-cast manganese steel”)

Manganese steel is used for the breaking jaws in jaw crush-

ers as it is ideal for wear-resistant tools which are subjected

to pressure.

Advantages

– Wide range of applications

due to different types

of steel with different

properties

– Very high energy input

– Low price

Disadvantages

– Not absolutely wear-

resistant (contamination

with iron and other com-

ponents is possible)

A-1.1. Steel

Available grinding tools made of steel:

Stainless steel

– Jaw Crushers BB 50, BB 100, BB 200, BB 300

– Ultra Centrifugal Mill ZM 200

– Rotor Beater Mills SR 200, SR 300

– Cross Beater Mill SK 100

– Cutting and Knife Mills GM 200, GM 300, SM 100,

SM 200, SM 300

– Mortar Grinder RM 200

– Planetary Ball Mills PM 100, PM 200, PM 400

– High Energy Ball Mill E

max

– Mixer Mills MM 200, MM 400, CryoMill

Hardened steel (“chrome steel”)

– Cross Beater Mill SK 100 (baffle plates, grinding inserts)

– Cutting Mills SM 100, SM 200, SM 300

(knife for parallel section rotor)

– Disc Mills RS 200, RS 300 XL, DM 200, DM 400

– Mortar grinder RM 200

– Planetary Ball Mills PM 100, PM 200, PM 400

– Mixer Mills MM 200, MM 400, CryoMill

Steel for grinding without heavy metal

contamination (e. g. steel 1.1750, St 1203)

– Jaw Crushers BB 50, BB 100, BB 200, BB 250 XL, BB 300,

BB 400 XL

– Cross Beater Mill SK 100

– Cutting Mills SM 100, SM 200, SM 300

Manganese steel (“precision-cast manganese steel”)

– Jaw Crushers BB 50, BB 100, BB 200, BB 250 XL, BB 300,

BB 400 XL, BB 500 XL, BB 600 XL

– Disc Mills DM 200, DM 400

A-1.2. Cast Iron / Malleable Iron

What is cast iron / malleable iron?

Cast iron is a ferrous alloy with a carbon content of >2 % and

a silicon content of >1.5 %. In the so-called grey cast iron

carbon is present in the form of fine irregularly distributed

graphite flakes. The term “cast iron” refers to the production

process of this material which involves casting a metal melt

into forms.

As opposed to cast iron malleable iron is tempered for sev-

eral days at a temperature of approx. 1000 °C in oxidizing

atmosphere.

Application areas of cast iron/malleable iron

grinding tools

Cast iron is an inexpensive alternative to steel. However, due

to its low hardness it is not used frequently. Malleable iron is

more ductile than cast iron and allows for minor plastic defor-

mations without breaking. Therefore, it is mainly used for

tools which have to bear hammering effects such as the cross

beater in the cross beater mill SK 100. The baffle plates which

are attached to the cross beater are made of steel because

stress and wear are especially strong there.

Cast iron / malleable iron grinding tools

are available for

– Rotor Beater Mill SR 200 (housing, inlet, outlet)

– Cross Beater Mill SK 100 (grinding insert, cross beater)

Tensile strength

Cast Iron:

200 – 300 N/mm

Malleable Iron:

approx. 370 N/mm

Density

Cast Iron:

8.25 g/cm

Malleable Iron:

7.2 g/cm

Composition

Cast Iron:

Fe: 94 %

C: 4 %

Malleable Iron:

Fe: 95.5 %

C: 3 %

Applications:

e.g.

n Mineralogy

n Construction

n Agriculture

Sample materials:

n Minerals

n Ores

n Cement

n Feed stuffs

Advantages

– Insensitive to hammering

effects

– Very high energy input

– Low price

Disadvantages

– Limited wear resistance

(contamination with Fe

and other components

possible)

– Lower hardness than steel

A-1.3. Titanium

What is titanium?

Titanium (Ti) is a chemical element with the atomic number 22.

It belongs to the group of transition metals and is very strong

despite its relatively low density. Titanium forms a protective

oxide coating which makes it resistant to corrosion. In its

purest form it is ductile. In order to improve its mechanical

properties (embrittlement), other elements such as oxygen

and nitrogen can be added.

Titanium is one of the most abundant elements in the Earth's

crust but it mostly occurs in low concentrations in the form of

oxides. It is mainly extracted from the minerals rutile (TiO

)

and ilmenite (FeTiO

Application areas of titanium grinding tools

RETSCH supplies titanium grinding tools for the Ultra Cen-

trifugal Mill ZM 200 and for the Knife Mill Grindomix GM 200

for grinding without heavy metal contamination. Ceramic

materials are not suitable for these mills as they could break

easily.

Titanium grinding tools are available for

– Ultra Centrifugal Mill ZM 200

(rotor, sieve, cassette)

– Knife Mills Grindomix GM 200, GM 300

Hardness

6 Mohs

Density

4.5 g/cm

Composition

Ti: 99.5 %

Applications:

e.g.

n Pharmaceutics

n Food

n Heavy metal

analysis

Sample materials:

n Vegetables

n Fruit

n Meat

n Drugs

n Cables etc.

Advantages

– No heavy metal

Disadvantages

– Expensive

– Less hard than steel

A-2. Ceramics

A-2.1. Tungsten Carbide

What is tungsten carbide?

Tungsten carbide is a hard metal. Pure tungsten carbide, a

mixed crystal of tungsten and carbon, is very brittle. There-

fore, 6 - 10 % of cobalt is added which increases the tough-

ness of the material and reduces abrasion. The appropriate

material composition of this hard metal depends on the tech-

nical requirements of the application.

Tungsten carbide is produced by sintering tungsten and car-

bon. The components are heated under high pressure and

carbon is incorporated into the crystal structure of the tung-

sten. The resulting carbide increases the melting point and

the hardness of the metal.

The extreme hardness (even at high temperatures) and wear

resistance are characteristic for hard metals. Depending on the

composition, hard metal can be as hard as diamond. Therefore,

it is frequently used for chipping tools and non-cutting mould-

ing tools. It is also used for tools which are subjected to fric-

tional abrasion. These include grinding tools for mechanical

size reduction as used in RETSCH mills and crushers.

Application areas of tungsten carbide grinding tools

Due to their wear-resistance and hardness, hard metals are

brittle and therefore sensitive to impact effects. Also, sudden

temperature changes, e.g. during cryogenic grinding, can

cause the material to break. Therefore, not all our mills can

be equipped with grinding tools of hard metal (e.g. Rotor

Beater Mills).

The composition of the hard metal as well as the production

process determine to a certain extent properties such as

pressure resistance, resistance to bending, toughness, high

Hardness

approx. 1250 HV

(Vickers)

Density

14.8 g/cm

Composition

WC: 90 – 94 %

Co: 6 – 10 %

Applications:

e.g.

n Mining and

metallurgy

n Chemistry

n Mineralogy

Sample materials:

n Ores

n Ferro alloys

n Metal oxides

n Cement clinker

n Charcoal

n Minerals

A-2.1. Tungsten Carbide (Hard Metal)

density and hardness. As a result, the percentage of tungsten

carbide and cobalt in our grinding tools can vary. However, it

is always ideally adapted to the stress mechanism of each

mill or crusher. Grinding tools for planetary ball mills, for

example, have to withstand friction and impact effects and

therefore have a higher tungsten carbide content. Breaking

jaws in jaw crushers, however, have to withstand high pres-

sures and therefore contain more cobalt to increase their

pressure resistance.

Grinding sets made of hard metal are available for

– Jaw Crushers BB 50, BB 100, BB 200, BB 250 XL, BB 300,

BB 400 XL, BB 500 XL, BB 600 XL

– Cutting Mills SM 100, SM 200, SM 300 (reversible cutting

plates for 6-disc-rotor)

– Disc Mills RS 200, DM 200, DM 400

– Mortar Grinder RM 200

– Planetary Ball Mills PM 100, PM 200, PM 400

– Mixer Mills MM 200, MM 400

– High Energy Ball Mill E

max

Advantages

– High wear resistance and

elevated-temperature

hardness

– Excellent grinding results

due to high material densi-

ty (especially in grinding

processes with free-to-

move grinding balls)

– High energy input

Disadvantages

– Cannot be used, if possible

contamination with tung-

sten or cobalt (metal)

influences the subsequent

analysis

– If the material is soft, the

sample may cake during

grinding

– Warming of the sample

material

A-2.2. Zirconium Oxide

What is zirconium oxide?

The main component for the production of zirconium oxide

(ZrO

) is the mineral zircon (ZrSiO

). By melting it with coke

and lime (reduction of SiO

) ZrO

is won on an industrial level.

The resulting powder is mixed with aggregates and dry

pressed into the desired form. The formed part is transferred

to the raw product by sintering and is then abraded and pol-

ished according to its intended purpose.

The sintering process can be carried out under atmospheric

pressure as well as under high pressure. The formed parts

receive their actual properties during the sintering.

Zirconium oxide occurs in different crystal modifications,

depending on the temperature. These have different

volumes. By adding yttrium oxide, zirconium oxide is pre-

vented from turning into the modification which is more sta-

ble at room temperature – it is thus kept in the partially sta-

bilized form. If there's a micro crack, the zirconium oxide

changes at that specific point by volume extension into the

more stable modification. This effect causes the crack to

close immediately.

Zirconium oxide is traditionally used as a refractory ceramic.

Due to its properties and high bio-compatibility zirconium

oxide is increasingly used as a ceramic for implants and den-

tal prostheses. In addition, there are many technical applica-

tions for partially stabilized zirconium oxide. It is highly resis-

tant to thermal, chemical and mechanical influences which

makes it very suitable for grinding tools.

Hardness

approx. 7.5 Mohs

(approx. 1200 HV)

Density

5.9 g/cm

Composition

ZrO

: 94.5 %

: 5 %

A-2.2. Zirconium oxide

Application areas of zirconium oxide grinding tools

Zirconium oxide has a high density and mechanical wear

resistance. It is free of heavy metals and can therefore be

used for sample preparation to heavy metal analysis. Further

areas of application are biology, the preparation of plant and

animal tissue, as well as human medicine, where any kind of

contamination can be hazardous. Zirconium oxide only leaves

neutral contamination, if at all, for this type of application.

RETSCH uses zirconium oxide for ball mill and disc mill grind-

ing tools. The high density of the material allows for a high

energy input. As zirconium oxide is a rather brittle ceramic, it

is important to follow the recommendations for sample

amount and maximum feed size to avoid damages to the

material. It is also used in the jaw crusher BB 50 for heavy-

metal-free grinding. However, brittleness and low resistance

to impact are the reasons why zirconium oxide is not used in

the bigger jaw crushers or in rotor mills.

Zirconium oxide grinding tools are available for

– Jaw Crusher BB 50

– Disc Mills RS 200, DM 200, DM 400

– Mortar Grinder RM 200

– Planetary Ball Mills PM 100, PM 200, PM 400

– Mixer Mills MM 200, MM 400

– High Energy Ball Mill E

max

– XRD-Mill McCrone

Applications:

e.g.

n Biotechnology,

medicine

n Analysis of trace

elements and

heavy metals

Sample materials:

n Soils

n Sewage sludge

n Compost

n Plants

n Bones

n Teeth

n Tissue

Advantages

– No heavy metal

– High density

– High wear resistance

Disadvantages

– Expensive

– Limited suitability for mills

which comminute by

impact and hammering

A-2.3. Sintered Aluminium Oxide

What is sintered aluminium oxide?

Sintered aluminium oxide (Al

) is a synthetic ceramic mate-

rial. In nature Al

occurs in form of corundum, the second

hardest mineral after diamond. Due to its hardness Al

often is used as abrading medium.

When producing sintered aluminium oxide, pressed alumina

powder is fired to 1300 °C. The powder is an intermediate

product in the process of winning aluminium from bauxite.

The main reasons for using sintered aluminium oxide

in grinding tools are its considerable hardness and purity but

also the reasonable price. Due to the relatively low density of

sintered aluminium oxide and the low energy input which

results from this, the sample material is hardly subjected to

thermal stress during grinding. This can be an advantage

with sensitive materials such as plants.

Application areas of sintered aluminium oxide

Ceramics

The abrasion which is to be expected from sintered alumini-

um oxide can be considered as “neutral” as it is pure Al

That is why one of the main areas of application for these

grinding tools is the preparation of ceramic materials and

glazes as found in sanitary and household ceramics. Refrac-

tory ceramics are also often prepared with grinding tools

made of sintered aluminium oxide.

Homeopathic substances

As sintered aluminium oxide has a greater wear resistance

than hard porcelain, it sometimes replaces this material in

mortar grinders when homeopathic ingredients are prepared.

Hardness

approx. 8 - 8.5

Mohs

1750 HV (Vickers)

Density

3.9 g/cm

Composition

: 99.7 %

Applications:

e.g.

n Biology

n Microbiology

n Ceramics

n Homeopathy

Sample materials:

n Ceramics

n Soils

n Grass

n Conifers

n Compost

A-2.3. Sintered Aluminium Oxide

Heavy metal analysis

The relevant standards for heavy metal analysis in soils often

recommend the use of ball mills with grinding sets made of

zirconium oxide as suitable tools for the sample preparation.

Due to the lower costs, grinding sets made of sintered alu-

minium oxide are also frequently accepted from an analytical

point of view. Al

has a hardness of 9 - 9.5 on Mohs' scale

but less wear resistance than zirconium oxide. Therefore, it is

not recommended to use them in planetary ball mills at high

speeds.

Grinding sets made of sintered aluminium oxide are

available for

– Mortar Grinder RM 200

– Planetary Ball Mills PM 100, PM 200, PM 400

– Mixer Mills MM 200, MM 400

– XRD-Mill McCrone

Advantages

– No heavy metal

– Low heat build-up /

sample doesn't cake

– good wear resistance

- Reasonable price

Disadvantages

– Susceptible to shock by

impact and temperature

– Low energy input

A-2.4. Hard Porcelain

What is hard porcelain?

Hard porcelain is a silicate ceramic which is composed of the

raw materials kaolin (non-ferrous clay), potash, feldspar and

quartz. It consists of 25 - 70 % Al

and 30 - 75 % SiO

. Hard

porcelain is fired to a temperature of approx. 1400 °C.

In the middle of the 19th century, porcelain was first used as

engineering ceramics, mainly in the electronic industry where

it remains an important material until today (e.g. as insula-

tors).

Application areas of hard porcelain grinding tools

A major field of application is the trituration of substances

used in homeopathy and medicine with the help of mortars.

Therefore, RETSCH only uses hard porcelain for the grinding

tools in the mortar grinder RM 200.

Hard porcelain grinding tools are available for

– Mortar Grinder RM 200

Hardness

approx. 6 – 7 Mohs

(approx. 1200 HV)

Density

2.4 g/cm

Composition

SiO

: 68.5 %

: 26 %

Applications:

e.g.

n Pharmaceutical

industry

n Medicine

n Biology

Sample materials:

n Plants

n Pharmaceuticals

n Oil seeds

n Pastes

Advantages

– Reasonable price

– Heavy-metal-free

Disadvantages

– Low wear resistance

against abrasive sample

materials

-> rough surface

A-2.5. Silicon Nitride

What is silicon nitride?

Silicon nitride is a non-oxidic ceramic which stands out by

extreme fracture toughness and wear resistance. The material

does not occur naturally and is produced in different ways. One

of the most popular manufacturing methods is reaction bond-

ing: a compact of silicon powder is heated in a nitrogen atmo-

sphere. The reaction in which the silicon powder is forming

nitrides starts at 1200°C. Since silicon nitride decomposes at

standard pressures and temperatures above 1700°C the pow-

der has to be sintered under very high pressures. Silicon

nitrides are characterized by a homogeneous grey colour.

By adding sinter additives (AL

and Y

) the material hard-

ness can be even increased whilst tensile strength remains.

These ceramics are then termed SiAlONs.

Because of its high abrasion resistance silicon nitride is par-

ticularly well suited for the use in bearing technology and as

cutting tool.

Application areas of silicon nitride grinding tools

The incredibly high abrasion resistance of silicon nitride qual-

ifies it for heavy metal free fine grinding.This makes it per-

fectly suitable for the use in RETSCH Ball Mills.

Despite the high stability of the material, grinding jars should

be filled and handled according to RETSCH recommendations

in order to avoid unnecessarily high wear or abrasion.

Hardness

1500 HV

Density

3.2 - 3.4 g/cm

Composition

92.5%

4.5%

2.5%

Applications:

e.g.

n Biotechnology,

medicine

n Analysis of trace

elements and

heavy metals

Sample materials:

n Soils

n Sewage sludge

n Ceramics

n Compost

n Plants

n Bones

n Teeth

n Tissue

A-2.5. Silicon Nitride

Advantages

– No heavy metal

– high abrasion resistance

– Very good impact and

shock resistance

– Excellent resistance to

temperature changes

Disadvantages

– Low density

– More expensive than steel

Silicon nitride grinding tools are available for

– Planetary Ball Mills PM 100, PM 200, PM 400

A-2.5. Silicon Nitride

A-3. Other Materials

A-3.1. Agate (Natural Stone)

What is agate?

Agate is a quartz-mineral (silicon dioxide SiO

) and belongs to

the group of semi-precious stones. It is formed in cavities of

volcanic rocks through precipitation of SiO

from solutions.

Agate was named from the river “Achates” (today “Drillo”) in

Sicily where it was found for the first time. Today's habitats

include Bohemia, Brazil, Sicily and Uruguay.

At the beginning of the 20th century agate was first used as

a technical material. Pharmacists and homeopaths soon rec-

ognized that agate hand mortars offered considerable advan-

tages for the trituration of powders and pastes due to their

pure SiO

composition and their wear resistance. Agate does

not cause abrasion and is easy to clean due to its smooth

surface which also helps to avoid cross contamination. These

properties distinguish agate from brass or hard porcelain

mortars.

The procedures for processing agate have improved through

the years by using diamond tools which finally allowed for its

use in a variety of grinding instruments. As a consequence,

hard porcelain is gradually losing its significance as a mate-

rial for mechanical size reduction.

Application areas of agate grinding tools

As mentioned before, agate consists mostly of silicon dioxide.

This makes it ideal for grinding instruments which are used

for neutral-to-analysis sample preparation as is required in

fields such as food, biology, medicine and pharmaceutics.

Further areas of application are sample preparation for heavy

metal determination, trituration of pigments (metallic abra-

sion can lead to changes in the colour quality) or trituration of

Hardness

6.5 – 7 Mohs

(approx. 1000 HV)

Density

2.65 g/cm

Composition

SiO

: 99.9 %

A-3.1. Agate (Natural Stone)

pasty substances and pharmaceutic ointments.Agate is resis-

tant to pressure and friction but is susceptible to impact.

Therefore, it is mainly used for mortar and pestle in mortar

grinders. It can also be suitable for ball mills as, due to its

relatively low density, it is resistant to the impingement effect

which occurs in ball mills. The vibratory disc mill features a

sensor which recognizes agate grinding sets and automati-

cally reduces the speed to 700 min

-1

so as to avoid premature

damage of the grinding tools.

Agate is a natural product, therefore its quality can vary.

Before processing it further, the raw material is checked for

hollows and impurities. However, no uniform and homoge-

neous structure – like that of artificially produced materials

– can be guaranteed. High mechanical and thermal stress

over a long period of time can lead to premature material

fatigue and even breaking of the grinding set. That is the

reason why planetary ball mills should only run at 60-70 % of

the maximum speed when used with agate grinding jars.

Thermal shocks which may occur when grinding with liquid

nitrogen in mortar and mixer mills should also be avoided as

this could lead to stress cracks in the material.

Agate grinding tools are available for

– Vibratory Disc Mill RS 200

– Mortar Grinder RM 200 and hand mortar

– Planetary Ball Mills PM 100, PM 200, PM 400

– Mixer Mills MM 200, MM 400

– XRD-Mill McCrone

Applications:

e.g.

n Pharmaceutics

n Food

n Medicine

n Biology

Sample materials:

n Plants

n Spices

n Cocoa

n Chocolate

n Drugs

n Pastes

n Oil seeds

n Biological tissue

and spores

Advantages

– No heavy metal

– Traditional natural product

(widely accepted)

– Hardly any thermal stress,

samples don't cake

Disadvantages

– Susceptible to shock by

impact and temperature

– Can be inhomogeneous

– Very low energy input

A-3.2. Glass

What is glass?

Glass is an amorphous, i.e. not crystalline, substance. It is

usually produced from a melt which cools down very rapidly

without sufficient time for a regular crystal lattice to form.

Glass mainly consists of SiO

. The properties of glass can be

modified with the help of various additives.

Laboratories mostly use borosilicate glass due to its high

chemical and temperature resistance. The chemical resis-

tance is determined by the contents of boron oxide which

amounts to approx. 13 %. Borosilicate glass is also known as

“Jenaer Glass” or “Duran Glass”.

Application areas of glass grinding tools

Containers of borosilicate glass are available as an alternative

to the standard polypropylene container for the knife mill

Grindomix GM 200. They are suitable for products which are

soft or have a high water or oil content. One of the crucial

benefits is that glass containers can be easily cleaned, steril-

ized and autoclaved which makes them ideal for preparing

food samples. Moreover, the grinding progress can be

observed through the glass.

Another application area of glass are grinding beads which

are used for cell disruption in the mixer mills MM 200 and

MM 400. As these glass beads are economically priced, they

can be used as disposable items together with the polypro-

pylene reaction vials. Thus, cleaning of the grinding tools is

no longer necessary.

Hardness

approx. 6 Mohs

Density

2.2 g/cm

Composition

SiO

: 80 %

: 13 %

Applications:

e.g.

n Food

n Pharmaceutics

n Medicine

n Biology

Sample materials:

n Vegetables

n Fruit

n Fish

n Pharmaceuticals

n Biological tissues

n Cell disruption

A-3.2. Glass

Glass grinding tools are available for

– Knife Mill Grindomix GM 200 (container)

– Glass beads for ball mills

Advantages

– Can be sterilized/

autoclaved

– Good chemical resistance

– Transparent

Disadvantages

– May break when subjected

to impact

Applications:

e.g.

n Pharmaceutics

n Medicine

n Biology

Sample materials:

n Plants

n Drugs

n Biological tissue

n Cell disruption

A-3.3. PTFE (Plastic)

What is PTFE?

PTFE or Teflon is a thermoplastic (polymer). The abbreviation

PTFE stands for polytetrafluoroethylene, a fluorine-carbon

compound. The term “Teflon” is a trade mark of the company

DuPont.

PTFE is practically inert which means that it is resistant to

many chemicals such as acids, bases, alcohol and benzine.

Therefore, PTFE coatings are often used as anticorrosive

against aggressive substances. Further applications of PTFE

are non-stick coatings (for pots and pans), high performance

fabrics (Gore-Tex), sealing materials and medical technology

(implants).

PTFE was developed in the 1930s while searching for a new

cooling agent for refrigerators.

Application areas of PTFE

Compared to other materials PTFE is less hard and should

therefore only be used for grinding soft sample materials

such as cell tissue. To generate a higher energy input, PTFE

grinding balls are equipped with an iron core.

PTFE grinding tools are often used for biological and pharma-

ceutical samples because the material is inert and, due to its

non-stick properties, PTFE can be easily cleaned. RETSCH's

PTFE grinding jars are not protected with an iron jacket and,

as PTFE is temperature-resistant, are therefore suitable for

cryogenic grinding.

Hardness

D 56 (Shore)

Density

2.1 g/cm

Composition

PTFE: 100 %

A-3.3. PTFE (Plastic)

PTFE grinding sets are available for

– Mixer Mills MM 200, MM 400

Advantages

– Inert (resistant to many

chemicals, also aggressive

ones)

– Temperature-resistant

from -200 °C to +260 °C,

suitable for cryogenic

grinding

– No heavy metal

– Non-stick properties

Disadvantages

– Low hardness

– Low energy input

– Possible contamination

with fluorine and carbon

– Low wear resistance

A-3.4. Polypropylene (Plastic)

What is Polypropylene?

Polypropylene (PP) is a thermoplastic resin (polymer) which is

produced by polymerization from the hydrocarbon propene.

Polypropylene is frequently used in the laboratory sector. It is

resistant to almost any organic solvent and fat as well as

acids and bases.

Application areas of polypropylene grinding tools

The standard container as well as the various lids for the

knife mill Grindomix GM 200 are made of polypropylene. The

material is suitable for products which are soft and soft-elas-

tic or which have a high content of water or oil. The polypro-

pylene container is an economically priced and break-proof

alternative to the glass container.

Reaction vials for cell disruption are also made of polypropyl-

ene. These are available in volumes of 0.2 ml, 1.5 ml and

2 ml thus allowing the processing of very small sample quan-

tities. The reaction vials are disposable items and therefore

don't need any cleaning.

Polypropylene grinding tools are available for

– Knife Mill Grindomix GM 200

– Mixer Mills MM 200, MM 400

(safe-lock reaction vials)

Applications:

e.g.

n Food

n Biology

n Medicine

Sample materials:

n Fruit

n Vegetables

n Cheese

n Sausage

n Tissue

n Cell disruption

Advantages

– Inert (resistant to many

[aggressive] chemicals)

– Economically priced

Disadvantages

– Low hardness

Density

0.89 - 0.92 g/cm

Composition

PP: 100 %

A-3.5. Polycarbonate (Plastic)

What is Polycarbonate?

Polycarbonate is a thermoplast (polymer), which is produced

by the esterification of carbonic acids with diols.

Due to its scratch and shock resistance polycarbonate is used

e. g. as a protective film for data layers on CDs and DVDs. It

is resistant to mineralic acids and most unpolar solvents, but

not to acetone.

Application areas of polycarbonate grinding tools

The standard container for the knife mill Grindomix GM 300 is

made of polycarbonate. A polycarbonate container is also

optionally available for the GM 200. In contrast to polypropyl-

ene polycarbonate offers the advantage that it is transparent

and therefore allows observation of the grinding process.

Additionally, polycarbonate can be autoclaved.

Polycarbonate grinding tools are available for

– Knife Mills Grindomix GM 200, GM 300

Advantages

– Transparent

– Can be autoclaved

– Reasonable price

Disadvantages

– Unstable against certain

solvents like acetone

Applications:

e.g.

n Food

n Feed stuffs

Sample materials:

n Fruit

n Vegetables

n Cheese

n Sausage

n Meat

n Cereals

Density

1.2 g/cm

Composition

PC: 100 %

A-4. Annotations

A-4.1. Hardness

The hardness of a particular material can be given with differ-

ent values, depending on the hardness scale to which this

value refers (e.g. Mohs or Brinell). The different hardness

scales have different origins. The Mohs' scale, for example,

classifies the scratch hardness of minerals on the basis of a

10-step scale. The scales of Brinell (HB), Rockwell (HRA /

HRB / HRC) and Vickers (HV) originate from the metallurgical

sector. The Shore hardness is frequently used in the plastics

industry.

Hardness is the mechanical resistance of a material against

the penetration of a foreign material. In materials testing the

hardness of a material is ascertained by determining the pen-

etration depth of a defined body under a given pressure.

To convert the hardness values to another scale is not always

possible. To facilitate a comparison of the scales of Mohs,

Vickers, Rockwell (HRA / HRB / HRC) and Brinell, we have

included the table on page 62/63.

Materials are also characterized by their tensile strength

which is sometimes given instead of the hardness, usually in

N/mm

. To determine the tensile strength, tensile force is

applied to the material until it breaks. Elastic/plastic materi-

als tend to deform (elongate) before breaking.

A-4. Annotations

A-4.2. Chemical components

The percentages of the chemical components of the materials

are rounded. Only the major or characteristic components

have been considered. Therefore, the percentage values do

not necessarily add up to 100 %. More detailed information,

also listing the minor components, can be found in the pdf-

document “Material Analyses of Grinding Tools” on

www.retsch.com.

A-4.2. Chemical components

A-4.3. Hardness table

A-4.3. Hardness table (approximate values)

Brinell

HRB

Rockwell HRC

Rockwell HRA

Vickers HV

50 100

20 30 40 50

65 70

100

200 400 600 800 1 000

75 80 82 84 86 88

60 70

100 300 630

1 4 5 6 7

Rockwell HRA

cast iron,

untermpered steel

titan,

glass,

hardened steel

zirconium oxide,

hard porcelain,

agate

non

ferrous

metal

tungsten carbide,

sintered corundum,

silicon nitride

Mohs

A-4.3. Hardness table

1 000

630

1 400 2 000 10 000

90 92 94

8 9 10

Mohs

Vickers HV

Rockwell HRA

zirconium oxide,

hard porcelain,

agate

hard materialstungsten carbide,

sintered corundum,

silicon nitride

Notes

RETSCH GmbH

Retsch-Allee 1-5

42781 Haan ∙ Germany

Phone +49 (0) 21 04 / 23 33-100

Fax +49 (0) 21 04 / 23 33-199

E-Mail [email protected]

Web www.retsch.com

Headquarters:

You will find a current list of all world wide

distributors at www.retsch.com.

Subject to technical modification and errors

99.100.1051/E-2017