Both methods are suitable but wet
grinding (as a slurry) is the preferred method for sample
preparation in the McCrone Micronising Mill in most
laboratories.
Advantages are:
1) Slurry grinding helps ensure that
none of the sample compacts into corners where it can escape the
action of the grinding pellets. Comparisons of dry and slurry
grinding show that slurry ground products always have the narrowest
particle size ranges.
2) For comparable grinding times,
slurry grinding produces a finer product.
3) Much less microstructural damage
occurs both to the product and to the grinding pellets. 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
4) Various inert liquids can be chosen
to protect the sample from unwanted reactions arising from the
presence of moisture carbon dioxide or oxygen.
WET GRINDING allows
total recovery of the ground sample and cleaning of the jar to be
combined in one set of operations.
When grinding has finished remove the
lid from the jar and replace it with the pouring lid that has two 6
mm holes. The ground slurry can then be poured out through one of
the holes and the jar, with the elements still in place, is washed
a further two or three times with intermediate shakings. This
procedure yields the combined pourings and washings together with a
clean jar and pellets without having to remove the pellets from the
jar.
To obtain a dry sample clear
supernatant fluid can be safely decanted and the rest of the liquid
is evaporated.
In the case of water, the remaining liquid can be replaced by
acetone which is lighter and less viscous than water thus allowing
the ground powder to settle out more rapidly. After decanting off
the clear acetone layer the remainder can be evaporated under an
infra red lamp. Acetone should never be used in the jar, during the
wet grinding process.
The densities of practically all liquids used in wet grinding are
greater than that of the polypropylene jar material. Any jar
material transferred to the sample during grinding will therefore
float on the surface of the carrier liquid and can be removed when
the supernatant liquid is decanted off the ground sample.
Theoretically, complete removal of any
traces of corundum from the sample can be achieved by the use of a
heavy liquid suspension centrifuging technique but this would only
be used in the preparation of micronised products where the highest
purity is required.
In practice the material transfer from
the jar and grinding pellets to the sample is minimal.
DRY GRINDING may be preferable when the relationship
between amount of mechanical work put into a sample and the amount
of ultrastructural damage produced needs to be studied.
The work of Burton (1) on the production of gram quantities of
materials with unusual properties for research and development
purposes is a good example.
Increasing attention is also being given to the examination of
various crystallographic transformations and tribochemical
reactions induced by prolonged dry grinding
Lewis and his colleagues (2) 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 Micronising Mill is the appropriate
quantitative tool for such studies.
THE TRUE OR ORIGINAL ELEMENTAL COMPOSITION OF A ROCK,
GLASS, CEMENT OR CERAMIC SAMPLE can be obtained by
subjecting the sample to two parallel grinding operations, each
introducing entirely different kinds of contamination. The sample
should be split into equal volumes and placed into two jars; one
containing corundum pellets and one containing agate pellets. Each
ground product is completely analysed using any of the appropriate
methods, such as wet analysis, ultraviolet emission analysis or
x-ray fluorescence analysis. By simple calculation, the true
composition is then derived. Dual grinding is almost a mandatory
procedure when analyses of the very highest quality are
required.
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Reference
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1) T.G Burton
Changes in the state of solids due to
milling processes
Trans Inst Chem Engineers 44 (1966), 37
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2) D.Lewis & E.J Wheeler
The Effect of Temperature on Microstrains and crystalline growth
in alumina
Jour. Materials Science 4 (1969), 681
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