Fig. 72.—Section of Williams’ Crusher.
When myrobalans or valonia is to be used for leaching, it is generally better to crush it between toothed or fluted rollers, rather than to grind it finely, as the cellular structure is just as completely broken up, and the flakes formed by crushing allow of much freer percolation than when the material is powdered by the disintegrator, while the consumption of power is also less. The general construction of the machine will be easily understood from [Fig. 73], and it is only necessary to point out that the small upper roller acts mainly as a “feed” to the larger crushing rolls.
In the best mills, the rollers are made up of a series of toothed steel discs on a square axis, and are on this account easily replaced or sharpened when they have become broken or worn.
Fig. 73.—Myrobalans Crusher.
Several mills have been introduced in America in which the bark is sawn or rasped by toothed discs like circular saws, but these are only capable of dealing with barks of a brittle nature, and are immediately choked by tough materials like the bark of the mimosa or oak. A better form of mill, but one which is, to some extent, subject to the same disadvantage, is the “shaving-mill,” in which blades are fixed like plane-irons upon a disc, cones or cylinder, and are rotated at a high speed against the material which is fed against them by toothed rollers at such an angle that the shavings are cut diagonally to the grain. These shaving-mills are largely in use in America for hemlock-bark, with which they are particularly successful. The principle of the machine is exactly the same as that of the machines used in cutting oakwood, quebracho, and the different dye woods. One type of shaving-mill is illustrated in [Fig. 74].
Fig. 74.—Shaving Mill.
It frequently happens that the material is delivered from the mill in a very unequal state of division, and it is sometimes necessary to screen it and thus separate the coarser portion either for use in the leaches or for re-grinding, while the finer portion is more suitable for “dusting.” With disintegrators, which deliver the bark with considerable impetus, the screening can be accomplished by placing a screen diagonally below the mill, through which the finer parts are projected. It is, however, essential that this screen should be quite smooth on its upper surface and very strong, as ordinary wire gauze is immediately cut through by the impact of the material. What are called “locked wire screens” in which the wires are supported by being actually twisted round the transverse bars are very suitable. Where the circumstances will not permit of screening in this way, cylindrical rotating screens, or nearly horizontal screens vibrated by an eccentric may be used. The latter are cheaper to erect and have the advantage that they take up less room, and by having lengths of wirework or perforated steel of different coarseness, the material may be separated into more than one degree of fineness.