A typical flat sifting-machine, with eccentric jig motion, is illustrated in [Fig. 21]. The machine is fed through a hopper provided with feed rollers, the rate of feed being adjustable. The screened product is discharged through a shoot at one side of the machine, and the residue at the opposite side, into boxes, etc., placed underneath.
For materials that give off a large amount of dust, the machine can be enclosed in a dust-proof casing, in which event the product and residue are delivered into drawers. The machine is easily cleaned and the sieves quickly changed, and is well adapted for dealing with a succession of different materials. The hopper can be fitted with a pair of adjustable crushing rollers.
Fig. 22.
[Fig. 22] is a drum sifter, which is fed by means of a hopper and worm; and the drum can be covered with wire or silk gauze. The sifted product falls into a worm conveyor in the bottom of the casing and is discharged at the side. This may be replaced by a series of mouths for discharging direct into bags, or the machine can be adapted to deliver into an elevator, worm conveyor or other means of transport to a distance.
The screenings are discharged through a shoot at the back of the machine, and can be handled in various ways. A beater is provided to clear the drum and increase the output.
Fig. 23.
[Fig. 23] illustrates a centrifugal sifting-machine for producing very fine powder in large quantities without any escape of dust. It contains a screening drum, the frames of which are detachable and facilitate changing the sieves. A beater revolving inside the drum projects the powder against the sieves, such portions as pass through being taken up and discharged by a worm conveyor; this, however, can be replaced by a bagging device, etc.