Jigs.—The jigs used in washing coal are modifications of the or dinary Hartz jig used in ore-dressing, differing only in size, capacity, and minor details of construction. The principle of coal-washing, moreover, is identical with that of ore-dressing. except that in the lat ter heavy mineral is separated from lighter gangue, which is thrown away, while in the former light coal is to be separated from heavier slate or pyrites. The coal-jigs in general use are invariably of the side-piston type, and consist of a single compartment. In the jigs used at the Drifton breaker (Fig. 4) the sieves are 5 ft. long and 3 ft.. wide, awl the pistons of the sante size. The bottom of the jig is semi circular. The coal to be washed is fed on to the jig at the side of the sieve next the piston, over an adjustable plate (6), the lower end of which is placed as near the sieve as is consistent with a free discharge of the coal. The coal passes out under this, spreading, over the sieve, its constituents arranging themselves according to their specific grav ities—the slate and pyrites at the bottom and the pure coal at the top. At the outside of the sieve the pure coal is skimmed off from the top by a series of flat strips of iron carried on two rows of link-helt chains, running over a wheel (34), or by some similar device. The coal is thus dragged up an inclined plane and discharged, the water carried with it draining back to the jig. The slate passes out through an opening in the side of the jig just above the sieve, which is regulated by an adjustable slide, into a flat east-iron hopper (9). The hottom of this hopper is closed by a gate, which allows neither slate nor water to escape. This gate is opened at proper intervals, the upper opening from the sieve to the hopper being closed at the same time, and the accumulate:I slate discharged from the hopper into a trough, whence it is removed by a suitable conveyor after having been inspected.
For jigging line coal similar jigs are used, but the sieves are bedded with feldspar or like material of approximately the same specific: gravity. In jigs of this class the slate discharges through it goose-neck outlet instead of one of the kind shown in Fig. 4. or else through the bedding nail sieve into the hutch below, whence it can be drawn through a proper gate.
A ulowelic Slale-Pickers.—These depend for their action upon the fact that, while the coal generally breaks into cubical :misses, the pieces of slate of the same length and width are of very much less thickness. Hence, if a quantity of slate and coal which has been passed through a screen and properly sized, the slate, if placed edgewise, would drop through a slit over which the coal would pass. There are two types of automatic slate-pickers : one, intended to be placed in a chute and to he fixed; and the other, to be placed in the discharge-slip of a gyrating screen and gyrated.
The fixed slate-picker consists essentially of a series of V-tronghs of iron cast in one piece, one side of the V being shorter and at right angles to the other. The lower half of
the casting has a taper slit in the short side. The slit is so arranged that anything lying on the long side of the trough and of not too great height can slide out through it. Any lump which is thicker than the height of the slit will of course be retained in the trough. The slits widen as they approach the lower end. and the part of the casting below the cross-bar hangs freely, so that there is nothing to stop a piece from sliding through the slit. This slate-picker is placed in an ordinary trough or chute down which the coal slides. It receives pitch enough to allow the coal to slide over freely, but with not too great velocity. As the coal and slate come down the chutes, each lump places itself in one or other of the grooves or troughs, which are made a little wider than the largest lump of the size for which the slate-picker is to he employed. As the lumps slide down, all the flatter pieces tend to pass out through the slit on the side, while the cubical lumps go over. Should a piece catch in the slit in consequence of the increase in height to ward the end, some one of the pieces which follow will generally knock it loose, so that it does not remain and block the slits. The slits if made parallel would soon clog. The fiat pieces, which are mostly slate, and which fall through the taper slit, pass over a chute or picking-table or any convenient place, where they are examined by a boy, who takes out any that coal that come through with the slate. The size and taper of the slit, the pitch of the picker, the width of the troughs, the length of the upper and the lower portion of the casting, vary with the size of the coal, nature of slate. etc.
The Gyrating Automatic Slate-Picker is made in the same way, with this exception. that only the part with the slit is used. This is placed on the attached to gy rating screen. The pickers are made in two patterns, to be according as the screen gyrates in one direction or the other. They must be so arranged that the gyrating motion of the screen has a tendency to throw the coal and slate against the short high side. In this way the latter is thrown out and passes to a jig or picking-table.
A third method of removing slate mechanically is used in several breakers in the Wyo ming region. It consists essentially of an inclined plane, down which the lumps of coal and slate are allowed to slide freely. The plane may be covered with iron, stone, or slate. The angle is such that the slate will slide down uniformly while the velocity of the coal increases. There is a gap at the end of the inclined plane. over which the coal jumps by virtue of the greater velocity acquired in sliding down the plane, while the slate, moving slowly, drops into it. There are a number of devices for changing the pitch of the chute, the form of the open ing. etc.