Using the McCrone Micronising Mill

Using the McCrone Micronising Mill
in the X-ray Diffraction lab in Earth & Planetary Sciences at the University of New Mexico
Instructional materials assembled by Jim Connolly ([email protected]) on 27-Feb-2013
Introduction
The McCrone Micronizing mill installed in the X-ray Diffraction lab in Room B25 of Northrop Hall is used
for reducing specimens to uniform fine powders (typically 1 to 4 m – in some cases less with special
treatment) with the goal of minimizing preferred orientation and creating uniformly sized powders from
samples with an uneven size distribution of different constituents.
This documentation what users need to know to operate the Mill safely and effectively and consists of
three parts:
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These two introductory pages
Pages 1-8 (page number on bottom of page not the Acrobat PDF page number) are the
Instructions for use from McCrone that explains how to:
o Install the Mill (already done)
o Use the percussion mortar to reduce the starting powder to the requisite <0.5mm size
prior to grinding
o Operate the mill in both wet grinding (preferred) or dry grinding modes
o Do basic maintenance (for lab administrators)
Pages 9-15 (page number on bottom of page not the Acrobat PDF page number) are
informational in nature, but all users are encouraged to read them because they provide useful
insights into capabilities and operations of the system. Included are:
o The sample prep kit including the percussion mortar
o Features and applications of the mill
o Discussion of operational capabilities of the mill
o Two experimental case studies
o A variety of analytical questions answered (dry vs. wet grinding, type of abrasives best
for different materials, use for XRD, IR and XRF analysis, etc.)
o A reference list of work involving the micronizing mill
Inventory of Items
The mill is mounted on the low cabinet (currently) located on the south wall behind the sink; because of
vibration associated with operation, it is important NOT to store items in the drawer or shelves in this
cabinet. The items used with the micronizing mill are stored in the top drawer immediately behind the
sink, and include the following:
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
Two polypropylene grinding jars with gray lids each containing 48 sintered corundum grinding
elements
Two polypropylene grinding jars with gray lids each containing 48 agate grinding elements
i
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
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Four spare agate grinding elements)
One white polythene pouring lid (with two round holes)
A black plastic “jig” to assist in stacking the grinding elements for replacement in the grinding
jars when they are removed for cleaning
Parts Kit including: one spare 3-amp Fuse, a rubber jar gripper, 0BA/2BA spanner wrench, 3/32”
Allen key, 1 spare flexible coupling, 4 spare flexible rubber mountings, 1 plain polythene lid,
spare polythene pouring lid.
Percussion Mortar Kit including: percussion mortar (3 machined stainless steel parts, including
piston, cylinder and base), 8 ea. sintered corundum grinding elements, 0.5mm mesh screen, and
small stiff brush for use in cleaning and removing particles from the sieve.
This documentation (17 pages) in a report binder folder.
If anything is missing, please contact the XRD Lab Manager immediately. This documentation in Acrobat
PDF form will also be found online on the XRD Lab web page (http://epswww.unm.edu/xrd) by clicking
on the “About our Lab” menu link.
Notes about Cleaning, Acetone and the Choice of Grinding Element type
This advisory section supplements materials in the remainder of the documentation:

Cleaning: If grinding wet (with deionized H20) cleaning is accomplished by successive decantings
of grinding/rinse water (see p.12) until the fluid poured off is clear. When subsequent grinds are
to be done with the same type fluid, the next sample may be added without drying or even
removing the elements from the grinding jar. If doing dry grinding or using another fluid (like
isopropanol) then the elements need removed and the jar and elements air dried before using
again. The reason we have multiple sets of containers and elements is to allow relatively rapid
sample throughput by switching the grinding set used between samples if needed.

Important Caution: Acetone should never, never, ever be used as a grinding fluid. It will

react and fuse the lid to the grinding jar, making it an expensive piece of useless plastic. It will
also react with and make useless the polythene pouring lid. Acetone may be used with the
ground powder as part of the separation and decanting process, but only in glass beakers or
other glass items used for that purpose. An item on p. 6 cautions about using acetone, but as
one who has destroyed some very expensive plastic equipment with it, please don’t get it
anywhere near the grinding containers.
Corundum or Agate? The choice of which grinding elements to use (agate vs. sintered
corundum) is discussed at several pages in this manual. In general, the corundum is probably
best for general purpose wet grinding for most materials. For the hardest materials, agate is
generally the best because it is less brittle (and tends to chip less) when grinding materials that
approach the hardness of corundum. In some cases (typically grinding for XRF analysis where
trace element contamination is an issue and of much less importance in XRD) grinding splits of
the sample in each type of element and analyzing separately is a good way of analyzing for
contamination. The different grinding elements and their uses are addressed in this manual on
pages 10, 11, 13.
ii
LEADERS IN SIZE REDUCTION EQUIPMENT
Tel: (+44) 0208 545 9140
Glen Creston Limited, Lombard Road, London SW19 3TZ Fax: (+44) 0208417 0857 e-mail: [email protected] www.glencreston.com McCrone Micronising Mill -Instructions For Use­
1
McCrone Micronising Mill Instructions for Use. 1 Unpacking.
1.1 Important: The mill is secured to the bottom of the wooden packing case by two bolts. Do not
invert the packing case. The bolts must be removed from underneath the packing case. This
can be achieved by carefully sliding the packing case to the edge of a bench so that one of
the bolts is just visible from underneath. Completely remove the bolt. Repeat the procedure
for the second bolt. The mill can then be lifted out without damage to its fibreglass cover.
1.2 A standard mill unit consists of the following items:
1.2.1. Mill
1.2.2. Two polypropylene grinding jars with grey lids containing 48 grinding elements. (Si ntered Corundum or agate, depending on customer order.) 1.2.3. One polythene pouring lid
1.2.4. Connecting lead, with mains plug
1.2.5. Spare one-ampere fuse (230v) or three ampere fuse (11 Ov)
1.2.6. Spares Kit comprising:
1.2.6.1. Rubber Jar Gripper
1.2.6.2. OBAl2BA Spanner
1.2.6.3. 3/32" Allen Key
1.2.6.4. Flexible Coupling x 1
1.2.6.5. Flexible Rubber Mountings (set of 4)
1.2.6.6. Plain Polythene Lid
1.2.6.7. Spare polythene pou ring lid
1.2.7. Brochure and instructions
1.3 Locate and identify each item. Shortages must be notified to the ma nufacturer within 5 days of
delivery.
2 Installation
2.1 To obtain the maximum operating efficiency, the mill must be fixed securely to a solid bench
or wall bracket.
2.2 Remove the mill cover by unscrewing the two Allen screws . Secure the mill by means of
substantial woodscrews or bolts to a rig id bench. Refit the cover. It is im portant that neither
the mill nor the bench vibrates during operation. All the vibrational energy available should be
directed to the grinding jar and holder and not to any other structure.
3 Set Up
3.1 The 48 cylindrical grinding elements within the grinding jar must always be kept in an ordered
array of 6 layers of 8 elements each. When wet grinding is used, the grinding elements need
never be removed. The twin hole pouring lid is used to prevent them failing out when the mill
prod uct slurry is poured off at the completion of a grind .
2
3.2 The volume of sample to be ground on each run should not exceed 5 millilitres. The optimum
grinding efficiencies are obtained with 2 millilitres of sam ple . This corresponds to a weig ht of
10 grams for a material having a density of 5 grams per millilitre, or 2 grams for a material
havi ng a density of 1 gram per millilitre.
3.3 Approx. 4ml of sample can be milled and approximately 7ml of grinding liquid (water, propan­
2-01, or cyclohexane).
4 Use of the Percussion Mortar
McCrone Sample Preparation Kit
(optional accessory)
4.1 Particles above O.5mm should be reduced in the Percussion Mortar.
4.2 Place one grinding element (either polycrystalline corundum {supplied} or Tungsten Carbide
{available separately}) in the stainless steel cylinder.
4 .3 Introduce the sample into th e cylinder and place the other grindi ng element on top of the
sample.
4.4 Place the steel rod in the cylinder on top of the second element and deliver a few sharp taps
to the top of the rod with a small hammer.
4.5 Remove the rod and empty the cylinder into a suitable container. Recover the grinding
elements.
4.6 Sieve the sample through the sieve provided, gently brushing the material to aid its passage
through the sieve.
4 .7 Any material not able to pass through the sieve may be returned to the mortar for further
treatment.
3
5 Operation
5.1 Before adding sample, ensure that grinding elements are in ordered array (6 rows of 8).
5.2 Place the crushed and sieved sample in the centre of the top layer of grinding elements taking
care that no sample particles fall or remain on the top lip of the jar.
5.3 WET GRINDING
5.3.1. For Wet Grinding pour approximately 7ml of liquid (water, propan-2-01 or
cyclohexane) over the sample, making sure that all sample particles are washed
down into the jar.
5.3.2. Screw the lid onto the jar making sure there is no leakage.
5.3.3. Pull the grinding jar clamp forward and rotate upward.
5.3.4. Insert the Jar into the jar carrier so that the collar on the jar body fits snugly against
the front edge of the jar carrier.
5.3.5. Rotate the jar clamp downward and allow the spring to pull the clamp into the groove
in the jar lid. Make sure the clamp is properly seated in the groove in the lid as this
prevents the jar from rotating during grinding.
McCrone Mill Control Panel
5.3.6. Set the timer for the required grinding time. This time needs to be determined
empirically but is generally in the range 2 - 30 minutes. The red line indicating the set
time does not move during operation, allowing the grinding time to be repeated as
many time as required.
4
5.3.7. Press the red On/Off rocker switch.
5.3.8. When the grinding cycle has finished remove the jar by reversing the operations for
clamping.
5.3.9. Remove the grey lid and replace with the two hole pouring lid. If the lid is stiff, a
rubber jar gripper is provided.
5.3.10. Pour contents of jar into a beaker or dish.
5.3.11. To clean the jar, add approximately 15ml of liquid, replace the grey lid and return the
jar to the mill. Vibrate for a further 15 seconds and then remove the jar and pour the
contents into the beaker or dish.
5.3.12. Repeat as many times as necessary to clean the jar. (Usually one more clean will
suffice.)
5.3 .13. Allow the product and washings to settle in the beaker or dish and decant off the
clear liquid. If water is used as the grinding liquid, some acetone can be added after
the decanting stage and the mixture decanted again. This replacement of water by
acetone greatly speeds up the decanting and drying stages. With a volatile grinding
liquid, a dried product can quickly be obtained
5.3.14. To ensure there is no cross contamination of one sample by another either a blank
sample or a small portion (say < 0.1 ml) of the next sample can be milled for
approximately a minute and then discarded. The jar is then ready to receive the
prepared second sample
5.3.15. When grinding insoluble, highly pigmented or black materials, particularly if they are
sectile or unctuous like graphite or some heavy metal sulphides, the polythene jar
and elements become stained. The staining often persists even when the mill is then
used for grinding harder or more abrasive material. Because of this staining, it is
often believed that it represents a serious source of cross contamination. This is not
so. The very persistence of the staining is in itself evidence that the pigment is not
being passed on to subsequent samples.
5.3.16. If the work is concerned with trace elements in the parts-per-million range, separate
jars and elements can be set aside for that particular task. However, in the case of
geochemical investigations where the accurate estimation of trace concentrations is
especially important, it has been found that with good cleaning between runs, cross­
contamination effects are negligible.
5.3.17. When the grinding surfaces are hard alloys or metal-cemented carbides or borides,
pigmented areas whilst present are normally invisible. It is particularly instructive to
examine the surfaces of such a mill in ultra-violet light, after it has been used to grind
an ultra-violet-excited phosphor.
5.3 .18. N.B. Certain chemicals, such as cyclohexane, may distort the PVC lid making it
impossible to remove. In this case polythene lids can be supplied by Glen Creston.
5
5.3.19. Acetone may damage the PVC lid and so contact should be avoided between them.
Acetone is not recommended as a grinding liquid but can be used after grinding to
speed evaporation of the slurry.
5.4 DRY GRINDING
5.4.1.
Slightly agitate the jar to ensure that the sample falls into the body of the jar and is
not left on the top surface of the grinding elements.
5.4.2.
Screw the lid, insert it into the mill as described above and grind for the required time.
5.4.3.
When the grinding cycle is finished, remove the jar from the mill and remove the lid.
5.4.4.
To recover the sample from the jar, the grinding elements must be removed from the
jar for cleaning (forceps are particularly useful for this). When the jar and elements
have been cleaned, the elements must be reloaded into the jar using the loader
supplied.
6
6 Maintenance
6.1 The mill requires little attention beyond the occasional application of a little lubricating oil to
th e motor bearing s.
6.2 INSTALLING REPLACEMENT FLEXIBLE COUPLING
r
Flexible coupling between motor and
jar carrier.
6.2. 1. Disconnect electrical supply and remove fibreglass cover.
6.2.2. Using Allen key provided, remove 2 screws from flexible cou pling .
6.2.3. Slide off flexi ble coupling from each shaft end in turn . (It often helps to free the motor
by removi ng the 4 x M6 bolts. If this is done after flexible coupling installed, re-align
shafts and flexible mountings and make sure flyweig ht shaft does not foul timer body
when vigorous grinding.)
6.2.4. Install replacement flexible coupling replacing screws (using Allen key provided).
6.2.5. Replace fibreg lass cover and re-connect electrical supply. Run short test with
grinding elements etc. in place.
7
6.3 FLEXIBLE MOUN TING REPLACEMENT
6.3.1 . Disconnect electrical supply and remove fibreglass cover.
6.3.2.
Unbolt and remove the four flexible mountings. (one either side of jar carrier and two
beneath the jar carrier. )
6.3.3. Install replacement flexible mountings
6.3.4. Replace fibreglass cover and re-connect electrical supply. Run short test with
grinding elements etc. in place.
r
RIlS'd N .5a.i 8401 -ng fin)
Reg' Office: Lombard Rond, l ondon, SWI9 3TZ U
8
The McCrone Sample Preparation Kit
The McCrone Sample Preparation Kit was designed to rapidly and easily reduce large particles to suitable sizes for the McCrone
Micronizing Mill. It consists of a Percussion Mortar, Sieve and Sieve brush.
Fig. 1 The McCrone percussion mortar
Plate 2. The McCrone Sample Preparation Kit
The Percussion Mortar
The effort, tedium and losses associated with grinding by mortar and pestle have been eliminated by the McCrone Percussion Mortar.
The particle size of hard, tough materials is reduced within seconds.
No sample loss occurs, because the steel cylinder contains all crushed fragments. External contamination of samples is prevented
in the same manner.
Operation is simple. The steel base and cylinder are screwed together. The dry sample, of maximum dimensions 12mm
diameter and 40mm length, is loaded into the cylinder between the two polished 12mm diameter cylindrical, polycrystalline corundum
crushing elements. The steel piston is loaded above these. A series of taps with a one-kilogram hammer will reduce most samples
to a powder of which a considerable proportion will pass through a 0.4mm aperture sieve. Larger particles can be returned to the
mortar for further treatment.
Internal contamination has been reduced to a minimum. Polycrystalline corundum has a hardness approximately equal to that of
cobalt-bound tungsten carbide. The sintered corundum anvil surfaces do not flake or pit during the course of normal use even with the
most coherent materials. Analyses of the reduced samples have shown that iron pickup is negligible. The steel cylinder serves only to
contain the sample. The base and cylinder can be unscrewed for easy cleaning and rapid oven drying.
Dimensions
Height – 108mm
Maximum diameter – 38mm
Weight – 410g
Also supplied with the Percussion Mortar are:
The McCrone Percussion Mortar Sieve, which consists of a 50mm diameter disc, mounted on a 25mm deep anodized aluminum
ring. The metal sieve disc is not of woven mesh, but has 0.4mm square openings. This aperture size is slightly less than the maximum
size of particles acceptable to the McCrone Micronizing Mill.
A sieve brush with coarse bristles and 8 additional polycrystalline corundum crushing elements complete the Sample Preparation
Kit.
9
Features of the McCrone Micronizing Mill
The problems associated with preparing solid samples for infrared absorption and x-ray diffraction analysis can be summarized as
follows:
1. Control of particle size distribution.
2. Introduction of crystal lattice disturbances.
3. Contamination from grinding elements and cross contamination.
4. Sample loss.
5. Oxidation, hydrolysis or other chemical degradations of the particles.
6. Prolonged grinding times.
The McCrone Micronizing Mill has been designed specifically to minimize or overcome these problems.
It rapidly reduces the particle size of troublesome samples by a unique grinding action. Each cylindrical element moves with
respect to its neighbors, so as to produce line contact blows and planar contact shears.
Wet grinding in airtight containers reduces crystal lattice deformation and oxidation.
Virtually the whole sample is recovered. If required, the very low levels of contamination by the grinding elements can be
calculated for precise quantitative analysis1.
The Mill was designed to reduce the size of particles from 0.5mm diameter to the fine micrometer sizes required for most
physical analytical techniques. It is widely used in sample preparation prior to quantitative analysis.
Sample Capacity: the Mill will handle a sample volume up to 4 ml (e.g. if the sample has a density of 2.5g./ml, a 10 g
sample could be used for milling).
Choice of Grinding Elements: Corundum and agate elements are available which allow the Volborth Dual-Grind
technique1 to be used to obtain the true composition of samples.
Inert polypropylene jars allow a wide choice of grinding liquids to be used, although water, propan-2-ol or cyclohexane are
most often used. These jars suffer little abrasion from hard samples, provided that the sample is passed through a 0.4mm
aperture sieve prior to grinding. This is provided with the Sample Preparation Kit.
The process timer is calibrated in one minute intervals up to 35 minutes. Processing is started by switching the red,
illuminated rocker switch to the “ON” position. A red dot shows the elapsed time during a run and the initial time set is
permanently displayed.
Applications of the McCrone Micronizing Mill
PREPARATION OF SAMPLES FOR QUANTITATIVE ANALYSIS
X-Ray Diffraction, X-Ray Fluorescence and Infra-red Spectroscopy:
An upper limit on particle size is undoubtedly the most critical factor in accurate quantitative analytical techniques using XRD, XRF
(pressed self-bonded disc) and IR.
X-Ray Diffraction and Infra-red Spectroscopy:
Wet grinding results in the least damage to the samples’ crystal structure, which is crucial for XRD and IR. It also makes for easier total
sample recovery, eliminates manual element and vessel cleaning and reduces sample oxidation and cross contamination. It is superior
to dry grinding in that it yields much smaller particles, narrower particle size distributions and gives more uniform phase distributions.
X-Ray Diffraction:
Dry grinding can be used to induce microstrains in the crystal lattice for the determination of ultrastructural damage by XRD line
broadening measurements. Sample weight, grinding element type and grinding time are the only variables that need to be specified
when describing the amount of induced lattice deformation.
Atomic Absorption Analysis:
In the case of samples that are difficult to dissolve, grinding with the mil is found to greatly facilitate subsequent acid digestion or alkali
fusion.
10
The McCrone Micronizing Mill
The McCrone Micronizing Mill was designed to reduce a few grams of material to micrometer size particles of a narrow size range,
minimizing contamination, time, cost, and mess.
What kind of grinder is it?
What charge and particle size sample does it take?
It is a vibratory laboratory mill powered by a 1/30 HP motor. The
grinding vessel consists of a 125 milliliter capacity polypropylene
jar fitted with a screw-capped, gasketless, polythene closure. The
jar is packed with an ordered array of identical, cylindrical, grinding
elements. The elements normally supplied with the mill are of
fine-grained, nonporous, polycrystalline corundum. They are
packed in 6 regular layers of 8 elements each, making a total of 48
in each jar. Agate grinding elements are also available.
It can accept up to about 4 ml of material. This corresponds to a
sample weight of 20g if the material has a particle density of 5g/
ml, or to a sample weight of 4g if the particles have a density of 1g/
ml. However, an optimum grinding efficiency is usually achieved
with 2 ml of sample.
The largest particles presented to the mill should not exceed 0.5
mm diameter. Any sample fraction remaining on top of a No. 30
mesh British Standards sieve should be crushed in a mortar to
pass through this aperture size.
Is it necessary to maintain this configuration of grinding
elements?
Absolutely necessary. During grinding, each element moves with
respect to its neighbors, grinding between the plane ends and along
the cylindrical sides of the elements. The powder continuously
circulated between these surfaces is ground much more rapidly
than in a ballmill, for example, with its point contacts. The edges
of the Micronizing Mill grinding cylinders have been chamfered to
reduce point contact damage to the elements.
What is the smallest particle size produced?
Almost all substances can be reduced to sub-micrometer sizes.
Soft materials, like some plastics and metals, cannot be
successfully ground to these sizes. Even in the presence of suitable
liquids they show a tendency to reweld. However, several authors
have reported success with some metals (nickel, iron, cobalt), and
the difficult platey minerals (mica, talc, graphite). Through the use
of selected grinding aids (certain inorganic salts) and grinding
liquids, it has been found possible to reduce these to 0.1µm2,3.
The mill must be secured by substantial wood screws or bolts to a
rigid bench. It is highly important that neither the mill nor the bench
vibrates during operation. All the vibrational energy available
should be directed to the grinding jar and holder and not to any
other structure. Indeed, the mill will operate at its greatest efficiency
if the rubber pads on the base are removed before bolting it tightly
to the bench. Ideally, vibration of the mill unit should scarcely be
detectable.
How long does it take to grind a sample?
On wet grinding runs, the mill is operated for periods ranging
from 2 to 30 minutes, depending on the fineness of product
required and the fineness, volume and grinding resistance of the
starting sample. Changes in grinding time affect the particle size
distribution of the product. The mill is fitted with a process timer,
graduated in one minute intervals up to 35 minutes, to ensure
reproducibility of sample size.
Does it work with really hard substances?
Fig. 2 Cross section of grinding jar, showing movement of
grinding elements
Few substances cannot be ground. Even silicon carbide and
various metal carbides, nitrides and borides can be ground
effectively, although as they abrade the corundum grinding
elements, agate grinding elements are recommended.
11
What is a typical performance?
Is grinding done dry or in a liquid slurry?
EXPERIMENT 1
Conditions: Charge of 1.0g of biotite mica, passed through 400µm
aperture sieve; 10 ml of propan-2-ol. Ground for 10 minutes with
corundum grinding elements.
Results: 1.0g of dried product recovered. Average diameter of
product was 5µm. Largest particle present was 10µm.
Plate 3. Biotite mica before grinding
Either way, but slurry grinding has advantages over dry grinding.
It is now the preferred method in most laboratories.
First of all, a liquid slurry helps ensure that none of the sample
compacts into corners where it escapes the grinding elements.
Comparisons of dry and slurry grinding show that slurry-ground
products always have the narrowest particle size ranges.
Secondly, for comparable grinding times, slurry grinding
produces the finer product.
Thirdly, much less microstructural damage occurs both to the
product and to the grinding elements. This product damage is
less, probably because of the presence of a thermally conducting
liquid which limits the momentary local high temperatures and
pressures produced at impact sites. Thus, rewelding of particles
or the formation on their surfaces of “amorphous” Bielby layers is
less likely to happen4. (See diagram below)
Lastly, various inert liquids can be chosen to protect the
sample from unwanted reactions arising from the presence of
moisture, carbon dioxide or oxygen.
Plate 4. Biotite mica after grinding
EXPERIMENT 2
Conditions: Charge of 2.0g of graded Belgian optical glass silica
sand of particle size 285µm + 15µm; 10 ml of water. Ground for
10 minutes with corundum grinding elements.
Results: 2.0g of dried product recovered. Average diameter of
product was 6µm. Largest particle present was 14µm.
Fig. 3 Comparison of results after slurry grinding and dry
grinding5
Plate 5. Belgian optical sand before grinding
Plate 6. Belgian optical sand after grinding
12
Surely wet grinding is more trouble than dry grinding?
Surprisingly, no. With wet grinding, the total recovery of the ground
sample and the cleaning of the jar and grinding elements is simpler.
It is accomplished by removing the closure at the end of the
run and replacing it with a similar closure but having two
diametrically-opposed holes of about 6mm diameter. The ground
slurry is then poured out through one of these and the jar with the
elements in place is washed two or three times with intermediate
shakings. This procedure yields the combined pourings and
washings, together with a clean jar, and with the grinding elements
clean and their packing unaltered.
The conversion of a slurry into a dry powder is not such a
messy step as it appears. Clear supernatant liquid can be safely
decanted. If water is the liquid, the remainder can be replaced by
acetone. As the acetone is a lighter and less viscous liquid than
water, the ground powder settles out more rapidly. After decanting
off the clear acetone layer, the remaining small amount of acetone
can be evaporated off in a few minutes under an infrared lamp.
Some low boiling organic liquids may be used directly. One
of the best liquids for grinding Portland cements for analysis is
propan-2-ol. Cyclohexane is also used.
The low density polypropylene jar is inert to most non-polar
hydrocarbons and alcohols.
The work of Burton 6 on the production of gram quantities of
materials with unusual properties for research and development
purposes is a good example of this.
Increasing attention is also being given to the examination of
various crystallographic transformations and tribochemical
reactions induced by prolonged dry grinding 7, 8, 14.
Lewis and his colleagues9 have made use of the x-ray line
broadening effects observed when powders are dry-ground for
different times. They were able to measure the amount of lattice
microstrain produced in brittle substances as diverse as calcite,
lithium fluoride, corundum and tungsten carbide. Such
measurements have been shown to be of great value in
fundamental studies of the sintering of metal powder compacts.
The McCrone mill has the virtue that close control can be
maintained over every aspect of the strain-inducing milling
operation.
With the quantity of sample and the grinding time as the only
variables, the McCrone Micronizing Mill is the appropriate
quantitative tool for such studies.
What about contamination from the grinding elements
and the container?
In all grinding operations, some abrasion of both parts will inevitably
occur no matter how hard or tough the materials are.
The densities of practically all the liquids used in wet grinding
are greater than that of polypropylene, the container material. As
a consequence, any abrasion particles from this source will appear
as a faintly visible layer on top of the supernatant liquid. This is
easily removed. In practice, the jar seldom needs replacement as
a result of wear.
The density of the corundum abrasion product is 3.7g/ml. If
the densities of the constituent phases of the sample product are
different from 3.7, then complete removal of the traces of corundum
particles is theoretically possible. This is done by using a heavy
liquid suspension centrifuging technique. It would only be
appropriate in the preparation of extremely pure micronized
products.
How then can the true or original elemental composition
of a rock, glass, cement, or ceramic sample be obtained?
Plate 7. Pouring slurry from grinding jar
Is there any occasion when dry grinding is preferable?
Yes. When it is required to study the relationship between the
amount of mechanical work put into a sample with the amount of
ultrastructural damage produced.
To do this, the sample has to be subjected to two parallel grinding
operations, each introducing entirely different kinds of
contamination. The most convenient method involves grinding in
two separate jars, one filed with the standard corundum elements
and the other with agate elements. A 5 ml sample, say, is split into
equal volumes and ground in the separate jars. Each ground
product is then completely analyzed, using any of the appropriate
methods, such as classical wet analysis, ultraviolet emission
analysis or X-ray fluorescence analysis. By simple calculation,
the true composition is then derived1. Dual grinding is almost a
mandatory procedure when analyses of the very highest quality
are required.
13
Can this mill produce samples suitable for quantitative
x-ray diffraction analysis?
Yes. Indeed, the development of the McCrone Micronizing Mill
arose out of a sponsored study. The study was undertaken in an
attempt to improve the hitherto poor performance of X-ray
diffraction as a strictly quantitative tool.
There are seven factors involved in X-ray diffractometry that
must be kept under tight control if good quantitative performances
are to be obtained.
The first four are respectively:
a) the degree of preferred orientation of the crystals.
b) the specimen’s X-ray absorption characteristics.
c) the X-ray beam geometry.
d) the X-ray intensity stability.
Less attention has been given to the remaining three factors.
These are:
e) the degree of primary and secondary extinction.
f) the depth of the non-crystalline layer of the crystal.
g) the degree of “spottiness” of the Debye-Scherrer lines.
The effect of (e) and (f) is well illustrated in the graph Fig. 4
shown below. It is taken from a report 10 on a study of the variation
of the relative peak intensity of a strong diffraction line of quartz
with the crystal size of the diffracting particles. It shows that line
intensity is only constant over a relatively short interval of crystal
size, i.e. between approximately 3 and 30µm. Above a size of
30µm, the effect is due to extinction and below 3µm it is due to the
presence of non-crystalline layers on the crystals.
Grinding techniques and sample origin influence factor (f).
Factors (e) and (g) have one feature in common, the effect of
size. If the size of the particle is less than 30µm, and preferably
below 10µm if a non-rotating specimen holder is used (factor g),
then the errors from these two factors are eliminated. It is not too
much to claim that by far the largest share of errors in quantitative
X-ray diffractometry arises from these two factors.
Why then, are particle size considerations so often neglected?
Partly because of a common failure amongst diffractionists to
appreciate the almost dominating importance of particle size. This
failure has, no doubt, arisen because conventional powder
diffractometers have photon counters which are unable to reveal
the presence of crystals between 40µm and about 150µm diameter
in the stationary or rotating specimen. Only a diffractometer having
arrangements for photographic as well as counter recording would
be able to do this effectively11.
(The Nelson X-ray Diffractometer has been designed for both
counter and photographic recording. For further details: contact
McCrone Research Associates.)
Can the mill be used to prepare specimens for quantitative
infrared absorption analysis?
Yes. Two kinds of specimen are usually used, the thin, selfsupporting, pressed KBr or Csl disc containing the embedded
specimen particles or the Nujol mull type.
In both types, if the particle size is much greater than about
5µm, heavy radiation losses occur due to optical scattering, and
quantitative measurements are insensitive and unreliable. There
is also a likelihood of the spectra showing spurious peaks arising
from the Christiansen effect. With the KBr or Csl pressed disc
type, optical homogeneity of the finished disc is particularly
important. To ensure this, the KBr powder and the previously
ground specimen powder can be milled together.
A good example is an account of the successful quantitative
IR determination of water in granites12. The authors state that,
ideally, the particle size should be below 3µm (i.e. the wavelength
band of the OH-stretching modes in silicates). They also stressed
the point that this size would give a much needed control of the
mull thickness between the KBr windows.
Fig. 4 Relationship between X-ray diffraction (peak) intensity and
Fig. 5 Relationship between infra-red absorption and particle size
particle size (quartz 0.181nm reflection)
(quartz)
The effects of particle size on x-ray diffraction and infrared absorption. The quartz was from many sources and was compared with a
Belgian sand (Snowit). Size determination was by microscope and when necessary size fractioning was by sedimentation.
Figs. 4 and 5 above are Crown copyright and were supplied by Health and Safety Executive’s Laboratories in Sheffield.
14
Is sample size reduction an important factor in x-ray
fluorescence analysis?
Yes. Specimen preparation remains an area where attention to
tighter particle size control could yield considerable improvements
in speed, convenience and accuracy.
Specimen preparation techniques in common use are of two
general types:
A direct analysis of ground, pressed powder compacts1,
and
B analysis of glasses produced by fusion of the sample at 1100º C
in a graphite crucible with a flux such as lithium tetraborate. The
fused glass buttons are crushed, ground and pressed into specimen
compacts.
Method A was the earlier scheme. It was, and still is, used by
many workers. The objection to it, in spite of its great sensitivity,
was that such specimens gave unreliable and erratic
measurements. This was claimed to be linked to the presence of
platey or lamellar constituents (micas, clays).
The accuracy was restored by converting the sample into a
homogeneous, glassy specimen. The improvement was obtained,
however, at the cost of sensitivity, speed and convenience.
There appears to be a movement back to the direct method,
with a realization of the need for a much more rigorous control of
specimen particle size.
Other Applications:
Whilst designed originally as a highly specialized laboratory tool,
in general it can deal with any task that requires small quantities
of materials reduced in size, dispersed in liquid or very intimately
mixed together.
Illustrations of recent successful applications are:
(1) Preparation of specimens for the quantitative analysis of
Portland cements by x-ray diffractometry, infrared analysis and xray fluorescence analysis.
(2) Preparation of industrial clay minerals and their products for
x-ray fluorescence analysis.
(3) Grinding and dispersion of a number of highly toxic organic
compounds in glycerol.
(4) Size reduction of various zirconia-based pottery pigments in
turpentine.
(5) Maceration of fibers (paper, straw, ramie, sawdust, asbestos,
high tensile carbon fibers) and liver and muscle tissue.
(6) Size reduction of sectile minerals such as shale, talc, mica,
vermiculite and graphite. These substances have proved to be
very difficult to grind in any other vibratory mill. The success with
the present mill is believed to be due to the planar shearing
contribution.
(7) Submicrometer grinding and milling of various metallic
silicides and glasses for phase equilibrium studies. Regrinding
the frits was done to further homogenize the final fired body. The
same kind of procedure was used in solid state reaction studies of
pure oxide mixtures with chromites and cerates.
(8)Used as a device for obtaining the abrasion pH13 of a series
of oxidizable, pure, inorganic compounds (various solid solutions
of manganous and ferrous carbonates).
(9) Preparation of various x-ray opaque dispersions for a study
of blood vessel distribution using x-ray stereo-microradiography.
(10) Crushing of the cell walls of penicillin mycelia to extract
the cell contents.
(11) Reducing the aspect ratio of crystals in pharmaceutical
products. Slurry grinding results in minimal crystalline damage.
(12) Grinding of segments of teeth and bone for X-ray diffraction
analysis.
Mill Specifications The mill measures 48 x 20 x 16.50 cm. It weighs
9 Kilograms when charged with grinding jar containing corundum
grinding elements. Models are available for use at either 220-240
volts 50Hz, or 110 volts 60Hz. Please specify voltage required
when ordering. The mill is supplied complete with separate
operating instructions.
Replacement of Parts & Accessories All spare parts are readily
available. Replacements of flexible mountings and polypropylene
grinding jars are occasionally required. Where the mill has been
used to reduce the size of exceedingly hard substances, agate
grinding elements have needed replacement only after an
estimated grinding time of 2,500 hours. The rubber flexible
couplings and corundum grinding elements are the only other
replacements requested in over 12 years of mill sales.
REFERENCES
1
A.Volborth
“Dual grinding and x-ray analysis of all major oxides in rocks to obtain
true composition”
Applied Spectroscopy 19 (1965), 1.
2 M. Quatinetz, R. J. Schafer and C. Smeal
“The production of submicron metal powders by ball-milling with
grinding aids”
Trans. Metall. Soc. A.I.M.E. 221 (1961), 1105.
3 A. Szegvari
“Preparation of colloidal dispersions by fine grinding”
Paper presented to the American Chemical Society, Atlantic City,
September (1959) (from Union Process Company, Akron).
4 F.P. Bowden & D. Tabor
“The friction and lubrication of solids”
Clarendon Press, Oxford (1950).
5 J.D. Stephens & W.M. Tuddenham
“Infrared analysis of minerals”
American Laboratory, November 1971, 8-13.
6 T.G. Burton
“Changes in the state of solids due to milling processes”
Trans. Inst. Chem. Engineers 44 (1966), 37.
7 J.C. Jamieson and J.R. Goldsmith
“Some reactions produced in carbonates by grinding”
Amer. Mineralogist 45 (1960), 818.
8 A.S. Fialkov
“Amorphous state of natural graphite powders”
Dokl. Akad Nauk., S.S.S.R. 153 (1963), 390.
9 D. Lewis and E.J. Wheeler
“The effect of temperature on microstrains and crystalline growth in
alumina”
Jour. Materials Science 4 (1969), 681.
10 R.L. Gordon & G.W. Harris
“Effects of particle size on quantitative determination of quartz by x-ray
diffraction”
Nature 175 (1955), 1135.
11 (a) J.B. Nelson
“An x-ray diffractometer for photography and counter recording”
Paper delivered at the Crystallography Group Conference of the Institute of
Physics, University of Hull April 1972.
(b) N.H. Hartshorne and G.D. Woodard
“Mesomorphism in the system disodium chromoglycate-water”
Molecular Crystals and Liquid Crystals 23 (1973) 343-386.
12 J.W. Aucott and M. Marshall
“Quantitative determination of water in granites by infrared analysis”
Mineralogical mag. 37 (1969), 256.
13 R.E. Stevens and M.K. Carron
“Simple field test for distinguishing minerals by abrasion pH”
American Mineralogist 33 (1948), 31.
14 Inagaki, Furuhashi, Ozeki and Naka
“Integrated intensity changes of XRD lines for crystalline powders by
grinding and compression”
Jour. Materials Science 8 (1973), 312-316.
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