Particle Size Reduction is a method for breaking up materials that need to be made smaller in order to be used. In its crude form it has been done for centuries. Mills have ground grain from time immemorial. Apothecaries in the old days crushed healing minerals and herbs with mortar and pestle. Many coffee lovers still grind their own beans.
The modern applications of this technique are extremely numerous. Certain drugs are still reduced by a grinding process in the modern pharmaceutical industry. In mining, minerals must be separated from the surrounding rock and rid of impurities. Food processing also breaks down grains and other ingredients to reach an edible end product.
The machines employed for reducing these masses of raw matter are many and varied. In general, the material to be crushed (called the feed) is fed into the device at one end and comes out the other. The process used must be strong enough to break the bonds that hold the mass together. Therefore the hardness of the substance is the first thing to be considered.
The Mohs scale is a measuring standard for the hardness of an object. Talc, familiar in its commercial form as talcum powder, rates a 1 on the scale, while quartz is a 7 and diamond, the hardest material known, scores a 10. The harder the feed, the more powerful is the equipment needed to crush it.
Whether or not a material is brittle is the next quality that affects the choice of equipment. A substance that is hard but liable to fragmentation is easier to grind down than one is tightly bound in its natural state.
Another aspect of the milling process that merits attention is the production of dust or other small particles that are too fine to be of use. In fact, these particles (sometimes called fines) may actually obstruct the reduction process. Sometimes chemicals are introduced into the mix so as to cause these fines to bind together in slightly larger masses.
There are many kinds of mills. A very common type is the ball mill. This device consists of a cylinder filled with steel balls that grind material fed into the cylinder when the latter is rotated at high speed. A related kind used in mining and mineral extraction uses pebbles in a rotating drum.
Another example of such a device is a hammermill. In this case, the material is crushed and broken by rods that swing at high speed. A familiar variation of this type is a tree or brush chopping machine that reduces pruned branches to wood chips quickly and efficiently.
The story is quite different in the pharmaceutical industry. There a problem may arise with drugs that do not dissolve well enough in water. Nanotechnology is used to reduce particles at a molecular level. These particles are to small to be seen without a microscope.
Whether it helps bring food to the table, minerals to the factory or medicine to the suffering patient, Particle Size Reduction plays a crucial role in daily life.
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