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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:

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

ii

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 dont 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.

http://epswww.unm.edu/xrd

LEADERS IN SIZE REDUCTION EQUIPMENT Glen Creston Limited, Lombard Road, London SW19 3TZ

Tel: (+44) 0208 545 9140 Fax: (+44) 0208417 0857 e-mail: [email protected] www.glencreston.com

McCrone Micronising Mill

-Instructions For Use

http:www.glencreston.commailto:[email protected] Text

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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. (Sintered 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 pouring lid

1.2.7. Brochure and instructions

1.3 Locate and identify each item. Shortages must be notified to the manufacturer within 5 days of del ivery.

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 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.

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 product slurry is poured off at the completion of a grind .


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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 sample. This corresponds to a weight of 10 grams for a material having a density of 5 grams per millilitre, or 2 grams for a material having a density of 1 gram per milli litre.

3.3 Approx. 4ml of sample can be milled and approximately 7ml of grinding liquid (water, propan2-01, 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 the cylinder and place the other grinding element on top of the sample.

4.4 Place the steel rod in the cyl inder 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.


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.


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, crosscontamination 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.


5.3.19. 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 Maintenance

6.1 The mill requires little attention beyond the occasional application of a little lubricating oil to the motor bearings.

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 coupling .

6.2.3. Slide off flexible coupling from each shaft end in turn . (It often helps to free the motor by removing the 4 x M6 bolts. If this is done after flexible coupling installed, re-align shafts and flexible mountings and make sure flyweight 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 fibreglass cover and re-connect electrical supply. Run short test with grinding elements etc. in place.


6.3 FLEXIBLE MOUNTING 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. )

r 6.3.3. Insta ll replacement flexible mountings

6.3.4. Replace fibreglass cover and re-connect electrical supply. Run short test with grinding elements etc. in place.

RIlS'd N .5a.i 8401 -ng fin) Reg' Office: Lombard Rond, l ondon, SWI9 3TZ U


The McCrone Sample Preparation KitThe McCrone Sample Preparation Kit was designed to rapidly and easily reduce large particles to suitable sizes for the McCroneMicronizing Mill. It consists of a Percussion Mortar, Sieve and Sieve brush.

The Percussion MortarThe 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 preventedin the same manner. Operation is simple. The steel base and cylinder are screwed together. The dry sample, of maximum dimensions 12mmdiameter and 40mm length, is loaded into the cylinder between the two polished 12mm diameter cylindrical, polycrystalline corundumcrushing elements. The steel piston is loaded above these. A series of taps with a one-kilogram hammer will reduce most samplesto a powder of which a considerable proportion will pass through a 0.4mm aperture sieve. Larger particles can be returned to themortar for further treatment. Internal contamination has been reduced to a minimum. Polycrystalline corundum has a hardness approximately equal to that ofcobalt-bound tungsten carbide. The sintered corundum anvil surfaces do not flake or pit during the course of normal use even with themost coherent materials. Analyses of the reduced samples have shown that iron pickup is negligible. The steel cylinder serves only tocontain the sample. The base and cylinder can be unscrewed for easy cleaning and rapid oven drying.

DimensionsHeight 108mmMaximum diameter 38mmWeight 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 aluminumring. The metal sieve disc is not of woven mesh, but has 0.4mm square openings. This aperture size is slightly less than the maximumsize of particles acceptable to the McCrone Micronizing Mill. A sieve brush with coarse bristles and 8 additional polycrystalline corundum crushing elements complete the Sample PreparationKit.

Fig. 1 The McCrone percussion mortar Plate 2. The McCrone Sample Preparation Kit








Features of the McCrone Micronizing MillThe problems associated with preparing solid samples for infrared absorption and x-ray diffraction analysis can be summarized asfollows: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.

Applications of the McCrone Micronizing MillPREPARATION 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 totalsample recovery, eliminates manual element and vessel cleaning and reduces sample oxidation and cross contamination. It is superiorto 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 linebroadening measurements. Sample weight, grinding element type and grinding time are the only variables that need to be specifiedwhen 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 alkalifusion.

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 withrespect 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 becalculated 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 mostphysical 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 gsample could be used for milling).Choice of Grinding Elements: Corundum and agate elements are available which allow the Volborth Dual-Grindtechnique1 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 aremost often used. These jars suffer little abrasion from hard samples, provided that the sample is passed through a 0.4mmaperture 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 ispermanently displayed.




The McCrone Micronizing MillThe 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?It is a vibratory laboratory mill powered by a 1/30 HP motor. Thegrinding vessel consists of a 125 milliliter capacity polypropylenejar fitted with a screw-capped, gasketless, polythene closure. Thejar is packed with an ordered array of identical, cylindrical, grindingelements. The elements normally supplied with the mill are offine-grained, nonporous, polycrystalline corundum. They arepacked in 6 regular layers of 8 elements each, making a total of 48in each jar. Agate grinding elements are also available.

Is it necessary to maintain this configuration of grindingelements?Absolutely necessary. During grinding, each element moves withrespect to its neighbors, grinding between the plane ends and alongthe cylindrical sides of the elements. The powder continuouslycirculated between these surfaces is ground much more rapidlythan in a ballmill, for example, with its point contacts. The edgesof the Micronizing Mill grinding cylinders have been chamfered toreduce point contact damage to the elements.

Fig. 2 Cross section of grinding jar, showing movement ofgrinding elements

What charge and particle size sample does it take?It can accept up to about 4 ml of material. This corresponds to asample 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 achievedwith 2 ml of sample.

The largest particles presented to the mill should not exceed 0.5mm diameter. Any sample fraction remaining on top of a No. 30mesh British Standards sieve should be crushed in a mortar topass through this aperture size.

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 besuccessfully ground to these sizes. Even in the presence of suitableliquids they show a tendency to reweld. However, several authorshave reported success with some metals (nickel, iron, cobalt), andthe difficult platey minerals (mica, talc, graphite). Through the useof selected grinding aids (certain inorganic salts) and grindingliquids, it has been found possible to reduce these to 0.1m2,3.

The mill must be secured by substantial wood screws or bolts to arigid bench. It is highly important that neither the mill nor the benchvibrates during operation. All the vibrational energy availableshould be directed to the grinding jar and holder and not to anyother structure. Indeed, the mill will operate at its greatest efficiencyif the rubber pads on the base are removed before bolting it tightlyto the bench. Ideally, vibration of the mill unit should scarcely bedetectable.

How long does it take to grind a sample?On wet grinding runs, the mill is operated for periods rangingfrom 2 to 30 minutes, depending on the fineness of productrequired and the fineness, volume and grinding resistance of thestarting sample. Changes in grinding time affect the particle sizedistribution of the product. The mill is fitted with a process timer,graduated in one minute intervals up to 35 minutes, to ensurereproducibility of sample size.

Does it work with really hard substances?Few substances cannot be ground. Even silicon carbide andvarious metal carbides, nitrides and borides can be groundeffectively, although as they abrade the corundum grindingelements, agate grinding elements are recommended.



What is a typical performance?

EXPERIMENT 1Conditions: Charge of 1.0g of biotite mica, passed through 400maperture sieve; 10 ml of propan-2-ol. Ground for 10 minutes withcorundum grinding elements.Results: 1.0g of dried product recovered. Average diameter ofproduct was 5m. Largest particle present was 10m.

Plate 3. Biotite mica before grinding

Plate 4. Biotite mica after grinding

EXPERIMENT 2Conditions: Charge of 2.0g of graded Belgian optical glass silicasand of particle size 285m + 15m; 10 ml of water. Ground for10 minutes with corundum grinding elements.Results: 2.0g of dried product recovered. Average diameter ofproduct was 6m. Largest particle present was 14m.

Plate 5. Belgian optical sand before grinding

Plate 6. Belgian optical sand after grinding

Is grinding done dry or in a liquid slurry?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 samplecompacts into corners where it escapes the grinding elements.Comparisons of dry and slurry grinding show that slurry-groundproducts always have the narrowest particle size ranges. Secondly, for comparable grinding times, slurry grindingproduces the finer product. Thirdly, much less microstructural damage occurs both to theproduct and to the grinding elements. This product damage isless, probably because of the presence of a thermally conductingliquid which limits the momentary local high temperatures andpressures produced at impact sites. Thus, rewelding of particlesor the formation on their surfaces of amorphous Bielby layers isless likely to happen4. (See diagram below) Lastly, various inert liquids can be chosen to protect thesample from unwanted reactions arising from the presence ofmoisture, carbon dioxide or oxygen.

Fig. 3 Comparison of results after slurry grinding and drygrinding5






Surely wet grinding is more trouble than dry grinding?Surprisingly, no. With wet grinding, the total recovery of the groundsample and the cleaning of the jar and grinding elements is simpler. It is accomplished by removing the closure at the end of therun and replacing it with a similar closure but having twodiametrically-opposed holes of about 6mm diameter. The groundslurry is then poured out through one of these and the jar with theelements in place is washed two or three times with intermediateshakings. This procedure yields the combined pourings andwashings, together with a clean jar, and with the grinding elementsclean and their packing unaltered. The conversion of a slurry into a dry powder is not such amessy step as it appears. Clear supernatant liquid can be safelydecanted. If water is the liquid, the remainder can be replaced byacetone. As the acetone is a lighter and less viscous liquid thanwater, the ground powder settles out more rapidly. After decantingoff the clear acetone layer, the remaining small amount of acetonecan be evaporated off in a few minutes under an infrared lamp. Some low boiling organic liquids may be used directly. Oneof the best liquids for grinding Portland cements for analysis ispropan-2-ol. Cyclohexane is also used. The low density polypropylene jar is inert to most non-polarhydrocarbons and alcohols.

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 theamount of mechanical work put into a sample with the amount ofultrastructural damage produced.

The work of Burton6 on the production of gram quantities ofmaterials with unusual properties for research and developmentpurposes is a good example of this. Increasing attention is also being given to the examination ofvarious crystallographic transformations and tribochemicalreactions induced by prolonged dry grinding 7, 8, 14. Lewis and his colleagues9 have made use of the x-ray linebroadening effects observed when powders are dry-ground fordifferent times. They were able to measure the amount of latticemicrostrain produced in brittle substances as diverse as calcite,lithium fluoride, corundum and tungsten carbide. Suchmeasurements have been shown to be of great value infundamental studies of the sintering of metal powder compacts.The McCrone mill has the virtue that close control can bemaintained over every aspect of the strain-inducing millingoperation. With the quantity of sample and the grinding time as the onlyvariables, the McCrone Micronizing Mill is the appropriatequantitative tool for such studies.

What about contamination from the grinding elementsand the container?In all grinding operations, some abrasion of both parts will inevitablyoccur no matter how hard or tough the materials are. The densities of practically all the liquids used in wet grindingare greater than that of polypropylene, the container material. Asa consequence, any abrasion particles from this source will appearas a faintly visible layer on top of the supernatant liquid. This iseasily removed. In practice, the jar seldom needs replacement asa result of wear. The density of the corundum abrasion product is 3.7g/ml. Ifthe densities of the constituent phases of the sample product aredifferent from 3.7, then complete removal of the traces of corundumparticles is theoretically possible. This is done by using a heavyliquid suspension centrifuging technique. It would only beappropriate in the preparation of extremely pure micronizedproducts.

How then can the true or original elemental compositionof a rock, glass, cement, or ceramic sample be obtained?To do this, the sample has to be subjected to two parallel grindingoperations, each introducing entirely different kinds ofcontamination. The most convenient method involves grinding intwo separate jars, one filed with the standard corundum elementsand the other with agate elements. A 5 ml sample, say, is split intoequal volumes and ground in the separate jars. Each groundproduct is then completely analyzed, using any of the appropriatemethods, such as classical wet analysis, ultraviolet emissionanalysis or X-ray fluorescence analysis. By simple calculation,the true composition is then derived1. Dual grinding is almost amandatory procedure when analyses of the very highest qualityare required.





Can this mill produce samples suitable for quantitativex-ray diffraction analysis? Yes. Indeed, the development of the McCrone Micronizing Millarose out of a sponsored study. The study was undertaken in anattempt to improve the hitherto poor performance of X-raydiffraction as a strictly quantitative tool. There are seven factors involved in X-ray diffractometry thatmust be kept under tight control if good quantitative performancesare to be obtained.The first four are respectively:

a) the degree of preferred orientation of the crystals.b) the specimens 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. 4shown below. It is taken from a report 10 on a study of the variationof the relative peak intensity of a strong diffraction line of quartzwith the crystal size of the diffracting particles. It shows that lineintensity is only constant over a relatively short interval of crystalsize, i.e. between approximately 3 and 30m. Above a size of30m, the effect is due to extinction and below 3m it is due to thepresence 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 ofsize. If the size of the particle is less than 30m, and preferablybelow 10m if a non-rotating specimen holder is used (factor g),then the errors from these two factors are eliminated. It is not toomuch to claim that by far the largest share of errors in quantitativeX-ray diffractometry arises from these two factors.

Fig. 4 Relationship between X-ray diffraction (peak) intensity andparticle size (quartz 0.181nm reflection)

The effects of particle size on x-ray diffraction and infrared absorption. The quartz was from many sources and was compared with aBelgian 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 Executives Laboratories in Sheffield.

Why then, are particle size considerations so often neglected?Partly because of a common failure amongst diffractionists toappreciate the almost dominating importance of particle size. Thisfailure has, no doubt, arisen because conventional powderdiffractometers have photon counters which are unable to revealthe presence of crystals between 40m and about 150m diameterin the stationary or rotating specimen. Only a diffractometer havingarrangements for photographic as well as counter recording wouldbe able to do this effectively11. (The Nelson X-ray Diffractometer has been designed for bothcounter and photographic recording. For further details: contactMcCrone Research Associates.)

Can the mill be used to prepare specimens for quantitativeinfrared absorption analysis?Yes. Two kinds of specimen are usually used, the thin, self-supporting, pressed KBr or Csl disc containing the embeddedspecimen particles or the Nujol mull type. In both types, if the particle size is much greater than about5m, heavy radiation losses occur due to optical scattering, andquantitative measurements are insensitive and unreliable. Thereis also a likelihood of the spectra showing spurious peaks arisingfrom the Christiansen effect. With the KBr or Csl pressed disctype, optical homogeneity of the finished disc is particularlyimportant. To ensure this, the KBr powder and the previouslyground specimen powder can be milled together. A good example is an account of the successful quantitativeIR determination of water in granites12. The authors state that,ideally, the particle size should be below 3m (i.e. the wavelengthband of the OH-stretching modes in silicates). They also stressedthe point that this size would give a much needed control of themull thickness between the KBr windows.

Fig. 5 Relationship between infra-red absorption and particle size(quartz)


Is sample size reduction an important factor in x-rayfluorescence analysis?Yes. Specimen preparation remains an area where attention totighter particle size control could yield considerable improvementsin speed, convenience and accuracy. Specimen preparation techniques in common use are of twogeneral types:A direct analysis of ground, pressed powder compacts1,andB analysis of glasses produced by fusion of the sample at 1100 Cin a graphite crucible with a flux such as lithium tetraborate. Thefused glass buttons are crushed, ground and pressed into specimencompacts. Method A was the earlier scheme. It was, and still is, used bymany workers. The objection to it, in spite of its great sensitivity,was that such specimens gave unreliable and erraticmeasurements. This was claimed to be linked to the presence ofplatey or lamellar constituents (micas, clays). The accuracy was restored by converting the sample into ahomogeneous, 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 ofspecimen particle size.

Other Applications:Whilst designed originally as a highly specialized laboratory tool,in general it can deal with any task that requires small quantitiesof materials reduced in size, dispersed in liquid or very intimatelymixed together.Illustrations of recent successful applications are: (1) Preparation of specimens for the quantitative analysis ofPortland cements by x-ray diffractometry, infrared analysis and x-ray fluorescence analysis. (2) Preparation of industrial clay minerals and their products forx-ray fluorescence analysis. (3) Grinding and dispersion of a number of highly toxic organiccompounds in glycerol. (4) Size reduction of various zirconia-based pottery pigments inturpentine. (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 bevery difficult to grind in any other vibratory mill. The success withthe present mill is believed to be due to the planar shearingcontribution. (7) Submicrometer grinding and milling of various metallicsilicides and glasses for phase equilibrium studies. Regrindingthe frits was done to further homogenize the final fired body. Thesame kind of procedure was used in solid state reaction studies ofpure oxide mixtures with chromites and cerates. (8)Used as a device for obtaining the abrasion pH13 of a seriesof oxidizable, pure, inorganic compounds (various solid solutionsof manganous and ferrous carbonates). (9) Preparation of various x-ray opaque dispersions for a studyof blood vessel distribution using x-ray stereo-microradiography.

Mill Specifications The mill measures 48 x 20 x 16.50 cm. It weighs9 Kilograms when charged with grinding jar containing corundumgrinding elements. Models are available for use at either 220-240volts 50Hz, or 110 volts 60Hz. Please specify voltage requiredwhen ordering. The mill is supplied complete with separateoperating instructions.

Replacement of Parts & Accessories All spare parts are readilyavailable. Replacements of flexible mountings and polypropylenegrinding jars are occasionally required. Where the mill has beenused to reduce the size of exceedingly hard substances, agategrinding elements have needed replacement only after anestimated grinding time of 2,500 hours. The rubber flexiblecouplings and corundum grinding elements are the only otherreplacements requested in over 12 years of mill sales.

REFERENCES1 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.

(10) Crushing of the cell walls of penicillin mycelia to extractthe cell contents. (11) Reducing the aspect ratio of crystals in pharmaceuticalproducts. Slurry grinding results in minimal crystalline damage. (12) Grinding of segments of teeth and bone for X-ray diffractionanalysis.



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