COAL WASHING. Coal washing con sists in the removal from coal of the impurities which it contains in nature. Some of these cannot be removed because they are part of the coal itself ; for example, some of the ash, of the phosphorus and of the sulphur. Others, as slate, bone or impure coal, and pyrite, can be removed to a certain extent by washing, because they are mechanically mixed with the true coal. The need for washing lies in the fact that when the slate is removed a ton of coal has greater heating power and is worth more to the con sumer, and when pyrite is removed the coal has less sulphur and it is more advantageous for making coke and from it iron and steel. Coke for the iron business should not contain over 1 per cent sulphur nor more than 6 to 10 per cent of ash., Bituminous The operation of wash ing bituminous coals consists of three steps: crushing, screening and washing. Of the many breakers the Bradford breaker has been much used. It consists (see Fig. 1) of a revolving cylinder with perforated walls for screening the coal and with longitudinal lifting plates which lift and drop the lump coal and thus break it. The hard lumps consist mainly of pyrite balls and slate and are sent direct to the dump as waste; the fine coal is then ready for the washer. Crushing rolls with inserted teeth (see Fig. 2) are used in many collieries for crushing the coal, the two rolls revolving toward one another brealc the coal which is fed in between them. The closer the rolls are set together and the finer the teeth, the smaller will the coal be broken.
The Jeffrey-Robinson coal washer (see Fig. 3) consists of a cone which is fuced and is fed from the water which is returned to do more washing. The New Century Nut Coal Jig (see Fig. 4) has a screen in a tank with two compart ments, the screen occupies one compartment while a plunger occupies the other. As the plunger is pushed down it causes a rush of water up through the screen, the coal and heavy impurities resting on the screen are made liquid and mobile by the rising water, and the heavy particles qtucldy find their way to the bottom while the light pure coal rises to the top and overflows at the end and is saved; the heavy slate and pyrite are discharged under a gate and over a slig,ht dam and are discharged as refuse beneath. The Pittsburgh
jig (see Fig. 5) has no plunger to actuate the water, but It has its screen mounted in a large box and this box moves up and down in the water tank; as the box moves down the water rushes up through the intalce valves and up through the perforated metal bottom or screen and liquifies the coal and slate, allowing the with its crude coal through the spout, g; it has revolving arms and from the arms are vertical revolving stirring rods, b, b; a current of water rising through the coal is fed in from the pump, p, and the annular distributing pipe and screen, i. This rising current loosens the coal which the revolving rods still further stir, and the heavy slate and pyrite settle to the bottom and are drawn into the pocket, d, by the upper gate valve, e, and later are discharged by opening the lower pte valve, f. The clean coal over flows at the top at j, is scraped over the coarse screen, k, by the scraper conveyor and discharged at m; the fine coal which passes through k is scraped forward on the fine screen, 1, by its scraper conveyer and discharged at m'; the fine clay and sludge pass through this fine screen and are separated coal to rise to the top and the slate to settle to the hottom; the cleaned coal overflows and is earned off by a drag conveyor while the slate is let out from time to time by opening the slate gate. It goes to the bottom of the tank and is removed mechanically from there.
For cleaning very fine coals, bumping wash ing-tables are used. The Campbell table (Fig.6) is one of these; it is operated by what is called the cam spring and bumping post mechanism; the cam pushes the table down hill gently and is followed by the spring which pulls the table up hill more rapidly until it strikes the bump ing post; at this instant all the particles on the table are jarred forward, the slate being heaviest is beneath, while the coal which is lightest is above; on the return. stroke the water is acting to float this coal down hilL The end point is that the slate is all discharged at the upper end while the coal is discharged at the lower end. To do this the table is sus pended on rods. The upper part of the table slopes slightly more than the lower part and the surface has thin cleats on it to prevent the slate from moving down hill.