SIZING SCREENS.—Sizing screens are a very important part of a concentrating mill, as the success of the subsequent separation of the various constituents of the ore to be treated by the hydraulic machines depends upon the proper sizing of the particles. The ordinary sizing screen or trommel consists of a series of spiders, keyed to a shaft, over which is stretched wire cloth or sheets of punched steel or iron plate. The number of trommels and the mesh of the screens on them is regulated to suit the character of the ore treated and the degree of separation desired. The general arrangement of the trommels used in concentrating mills is shown in Fig. 2. Each trommel is geared to the one next to it, so that the whole line may be driven from one point. The fine material from one screen passes to the next finer screen, and so on to the required number. The material remaining on each screen, and afterward discharged to the proper jig, is thus sized—i.e., it has passed through the perfora tion of the preceding screen and will not pass through the perforations of the one retaining it. In dressing works the ore is invariably screened wet. The water for this purpose is
sometimes fed through the shaft of the trammel, which is in this case made hollow, but usually from a perforated pipe hung above the trommel. Trommels are sometimes made of conical form, the axis being horizontal, and occasionally both cylindrical and conical screens are made with two sizes of wire cloth upon the same frame, making, in effect, a com pound trommel. Concentric trommels, which consist of drums of different mesh, one within another, are never used now, the difficulty of repairing them making them highly objection able in a mill. It is not usual in well-designed dressing works to use screens finer than 20 mesh, as the material which will pass that size is better prepared for the slime jigs and tables by hydraulic separators, and the finer screens wear out too fast, increasing expenses for repairs, and causing undue loss of time in patching or recovering them.