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The method of operation has a considerable effect upon the nature of the product When minimum fines are a requirement, the hammermill should always be operated in closed circuit with a screen, or screens; the circulating load should be fairly high; speed should be as low as is consistent with physical structure of the material; and grates should be widely spaced or, in some cases, dispensed with entirely.
Although the hammermill usually produces a relatively high percentage of fine material, it does not usually make an inordinately high proportion of what are commonly designated as "extreme-fines" unless the mill is adjusted and operated with that end in view. For example, if we are turning out an inch. (top size) product in one of these machines we will usually find the proportion of particles in the range between % in. and about 30 or 40 mesh to be high, as compared to the product of pressure-type crushers; but unless the material is very friable and the natural grain size very fine the percentage of minus 100 and 200-mesh particles will usually compare quite closely. Here again we have the fact that the impact effect falls off directly with the particle mass, to explain an apparent inconsistency.
It is easy to understand why the hammermill should turn out a cubical product. The impact action is ideally adapted for breaking flat or elongated spalls. If the material contains numerous parallel cleavage planes the initial blows
delivered by the hammers may knock off flat spalls; but the chances are very much against such spalls getting out of the machine before they are struck several times and broken into particles of more cubical proportions. This is especially true of machines which have a long impact zone ahead of the grate section, as no particle can possibly be discharged until it has passed through this impact section, where it will almost certainly be struck several blows. |
