Modern Gasworks Practice

GAS-MAKING AND OTHER COALS 369 (a) Treatment of coal in an ascending current of water, whereby the lighter particles are lifted and the heavier particles sink. (b) Exposure of coal alternately to upward and downward currents of water. (c) Sluicing down an inclined surface, whereby the larger and lighter particles are washed down more quickly than the smaller and heavier particles, or the ligliter particles are caused to ride over weirs, whereas the heavier particles are held up by the weirs. (<Z) Treatment in jigger travs whereby the small heavy particles are j erked forward more quickly than the lighter ones. In conjunction with the jigger effect a stream of water is frequently employed for assisting separation. The Baum washer is operated on. the principle of “ first wash, then classify, and is shown in Fig. 257. Any coal below 3|-inch. mesh is washed without previous screening in one washer box, the specifically heavier products going to the bottom and the lighter products (i.e. the coal) going away with the water. The Baum washer is capable of dealing with. any quantity of coal up to 150 tons per hour, and the application of the water is efEected by air at a pressure of about 2 Ib. per square inch. The air valve is provided with a sleeve piston which is raised and lowered by an eccentric. When the valve is in the lower position air passes direct from the blower and exerts its füll pressure on the surface of the water. When the valve is in the upper position the air supply is cut off from the washer box, which. is simul-taneonsly opened to the atmosphere. In this way, alternate depression and rebound of the water take place. The coal to be washed, lying on the grid R, is thus subjected to the pulsations of the water, and the dirt falls to the bottom, whence it is removed by the elevator, Q. The worm conveyor, S, collects towards the elevator any dirt deposited furt her along the washer box. The coal, along with the middles and light dirt, overflows into a second section of the washer box, where it undergoes furtlier treatment, the good products overflowing at N to be dealt with according to require-ments. The washing water enters at T, and passes with the washed coal over the outlet N. It is afterwards clarified before being returned to the box. The Coppée washer (Kg. 258) deals with each size of coal in a separate box, the classification usually being as follows :— (1) Nuts..............................................f in. to | in. (2) Beans.............................................i in. to J in. (3) Peas..............................................i in. to 1 in. (4) Small.............................................0 in. to | in. Pulsation of the water is produced by plungers driven by adjustable eccentric rods, the stroke of the rods being regulated to suit the dass of coal undergoing treat-ment. The pulsation gives rise to thorough agitation of the coal in the water, the dirt falling to the bottom of the box, while the coal is carried away by the over-flowing water. For the smallest coal a machine of slightly different design is employed, but the principle made use of is the same. The Draper washer (Fig. 259) employs the principle of upward-current Classification. B B