The actual method of grinding, and consequently the machinery employed for the purpose, vary not only with the material to be ground, but with the method of leaching adopted, as it is essential that the mass of ground material should be completely permeated by the liquor employed in leaching; and if it be ground too finely, or subjected to too much pressure on account of the height to which it is piled in the leaches, it is apt to form a compact and clay-like mass, the interior of which remains unextracted.
Fig. 68.—Cone-Mill.
In the laboratory, where thorough extraction must be completed in a few hours, the material can hardly be too fine; but on the larger scale a much coarser product must be used, and leaching requires days, or sometimes even weeks, and is then seldom successful in removing all the tannin. It is probable, however, that in the future these mechanical difficulties of extraction will be overcome; and the material will then be as finely divided, and as completely extracted on the large scale, as it is in the laboratory at the present time.
One of the earliest methods of grinding oak-bark, and which is still used for sumach ([p. 271]) consists in crushing it under large circular edge-stones, frequently turned by a horse. This process was very slow and inefficient for barks, and both it and horizontal millstones similar to those used for wheat were long ago superseded by iron or steel mills on the same principle as the ordinary coffee-mill.
These mills, [Fig. 68], consist of a “bell” or inner cone, covered with blades or teeth arranged at a slight angle to the vertical section of the cone, and which are made finer and increased in number towards its lower and wider part. This cone rotates within an outer hollow cone or casing, also provided with blades or teeth which are sloped slightly in the opposite direction to those of the inner cone, so as to meet them at an angle, like the cutting-blades of a pair of scissors, and the angles of the cone are so chosen that the blades approach each other more closely towards their base. The outer cone is fixed, and is provided with a hopper like a coffee-mill, while the inner cone is so rotated on its axis that bark placed in the hopper is screwed down between the two, and cut finer and finer till it reaches the lower edge, when it drops out. The blades or teeth are usually cast in one piece with the metal cones, and sharpened when required by chipping with cold chisels. This operation should not be conducted in the mill-house, or small chippings of iron may get mixed with the bark, and cause stains on the leather. This form of mill, which is run in England at about 30 revolutions per minute, and at nearly three times this speed in America, works very well with dry material, but clogs badly if it be appreciably damp. On this account it is always well to run the mill with a fairly slack belt which will slip before exerting sufficient pressure to break the machine, as in such operations as grinding, safety clutches are of but little use.
A type of mill varying somewhat from the above, consists of a pair of discs or very obtuse cones, the inner one of which runs on a horizontal axis. The teeth are generally arranged in concentric rings and interlock with each other. The material to be ground is fed at or near the centre of the fixed disc, and escapes at the edges. The construction of this class of mill will be easily understood from [Fig. 69]. Very small pieces of iron or steel which get caught between the teeth will often result in the breaking of the latter, and the formation of iron dust, which is a serious objection to the employment of this type of mill (to which the Schmeija “Excelsior,” the Glaeser “Favorita,” and the “Devil Disintegrator” of the Hardy Patent Pick Co. belong) for grinding barks.
Fig. 69.—“Excelsior” Mill.
Myrobalans and mimosa-barks have proved especially troublesome to grind, the former from the hardness of the stones of the fruit, and a tendency to clog the mill, and the latter from their combined hardness and toughness. “Disintegrators” of various patterns are now made, which are capable of grinding both these materials satisfactorily, and but for their liability to cause fire, and the large proportion of fine dust which they make, are usually to be preferred to toothed mills. In spite of their disadvantages, however, they have come very largely into use, on account of their efficiency in grinding obstinate materials. Disintegrators work on the principle of knocking or beating the material to powder, by means of very rapidly revolving beaters, which, in the smaller machines, are driven at 2500 to 3000 revolutions per minute.