Character of product delivered by the hammermill

Two broad general statements may be made about the character of product delivered by the hammermill:

(1) The percentage of fines, in a product of similar "top size," will usually be substantiality higher, as compared to products of pressure type crushers.

(2) On almost all materials for which the hammermill is economically suited, the product will be of a cubical nature-with a minimum of flats and splinters.

The shattering effect of the blows delivered by hammers traveling at velocities as high as 200 f.p.s. is conducive to both of these results. It is natural to expect that gradation of the hammermill product would vary somewhat for materials of differing friability, and results verify this expectation. Furthermore, speed has a definite influence upon product gradation; high speeds increase fines, and vice versa. The effect of impact at extremely high speed is, on friable material, almost explosive, the action being more aptly designated as "pulverizing," rather than crushing. Lower impact velocities have a more moderate breaking effect, and if the material is able to clear the crushing chamber before it is struck too many times, the low speed hammermill will turn out a fairly uniformly graded product on material of average friability.

The design of the crushing chamber will also affect product gradation. In general, those machines which perform most of their work by straight impact action will turn out a more uniformly graded product than milIs which depend upon interaction between hammers and grates for most of their reduction. This is only natural in view of the fact that attritional grinding is minimized in the former type of mill.