Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/97

Rh SURFACE ARRANGEMENTS.] COAL t carried to an elevator or Jacob s ladder, and discharged into rotatory drum sieves of about ^-inch aperture, pro ducing a second size of saleable coal, known as nuts, and FIG. 22. Elevation. FIG. 23. Plan. FIG. 24. Transverse Elevation. Fias. 22-24. Surface arrangements of Colliery. slack, which is sent away to the coke ovens attached to the colliery. The whole of the labour required in the screening the output of 600 tons in the day of ten hours is performed by one engineman, who has charge of all the mechanical arrangements, and nine boys, who pick out any large lumps of stone from the coal as it passes the first screens. The engine driving the screens and elevators is in charge of a special engineman. Fig, 25 represents one of a pair of pits at Pemberton Colliery, near Wigan, having the pit frames constructed in wrought iron lattice truss-work instead of wood. The screens for large coal (S) are arranged symmetrically on the landing platform, three on each side of the pit top, and discharge directly into waggons on the railway below. The small coal from these screens is passed by a screw creeper C, like those used in flour mills, to a bucket elevator E, which delivers it at the top of the second set of screens R, where the nuts and slack are separated. The platform, as in most of the new collieries in this district, is roofed over to protect the workmen from the weather. The second pit, which occupies a corresponding position on the oppo site side of the engine-house, is in every respect similar. The large collieries in the steain-coal district of North umberland are among the most productive ; thus, at Bed- lington, near Morpsth, 1200 tons are raised daily, and at North Seaton from. 1500 to 1800 tons. When the coal is very much mixed with shale, the slack Coal- often contains so much mineral matter as to be quite worth- washing less, until at least a partial separation has beeu effected. maclunes - This is now done by means of coal-washing machines, which were first adopted in France, but have now become general in other countries. There are many different forms, but the most usual is a fixed sieve plate, upon which the slack is received and subjected to the action of a current of water forced through the holes by the action of a fast-moving short-stroke plunger pump, which puts the Elevation tfvr /m 1 1 r 3 3 [ R 3 c Fro. 25. Surface arrangements, Pemberton Pit, Wigan. whole of the materials into suspension, and allows them to fall through the water at each stroke. By this means the coal, being the lighter material, travels to the surface, and the heavier shale and stone going to the bottom are dis charged through a valve there. The apparatus is in fact a form of the hydraulic jigging hutch used for the dressing of lead and other ores, except that in this case the lighter and not the heavier part is the valuable mineral. In another form of coal-dressing machine introduced by Mr Evrard, the jigging action is produced by a jet of steam acting directly upon the water instead of a plunger piston. Washed slack when suitable is used for conversion into coke, but in France and Belgium it is now generally employed in the production of agglomerated fuel, or bri quettes, or what is usually known in England as patent fuel. These consist of coal dust mixed with a sufficient amount of gas-pitch to be moulded into coherent bricks or cylinders, which are afterwards dried at a high tem perature, but below the point of carbonization. The con solidation of the slack may also be effected by the use of starch or dextrine, or even by cement or clay. This class of fuel is much used upon the French railways, being con venient for stowage and economical in use; but as a rule it is disagreeable to the passengers from the large amount of coal-dust carried off by the exhaust steam, and the unpleasant vapours produced by the burning pitch. The