. Machinery for metalliferous mines : a practical treatise for mining engineers, metallurgists and managers of mines. y be made cylindrical, as in fig. 185,or conical, as in fig. 186, which is a compound trommel having an insidescreen of perforated sheet metal and an outside one of wire gauze, thusscreening to two sizes at one operation. In the case of cylindricaltrommels the shaft must be inclined so as to allow the mineral to progressthrough without blocking, but the shaft of conical trommels may be level,as the angle of the screen allows for the onward motion of the ore. In the set of tromm
. Machinery for metalliferous mines : a practical treatise for mining engineers, metallurgists and managers of mines. y be made cylindrical, as in fig. 185,or conical, as in fig. 186, which is a compound trommel having an insidescreen of perforated sheet metal and an outside one of wire gauze, thusscreening to two sizes at one operation. In the case of cylindricaltrommels the shaft must be inclined so as to allow the mineral to progressthrough without blocking, but the shaft of conical trommels may be level,as the angle of the screen allows for the onward motion of the ore. In the set of trommels shown in fig. 185, the ore from the crusher entersat a, and the coarse ore for recrushing travels through the trommel andout at d, to be sent back by means of an elevator. Assuming that thistrommel has holes of 20 mm. diameter the whole of the mineral passingthrough there goes on to No. 2 trommel by the shoot (c). This trommelhas holes of 15 mm., and consequently rejects all ore above that size,which falls down the shoot (d) and is fed on the coarse jigger destinedfor treating all stuff between 15 and 20 Fig. 1S6.—Series Trommel. The ore under 15 mm. passes through the screen, and by the shoot (e)to the trommel, No. 3 having holes of 10 mm. In this all ore over 10mm. is rejected at/ and goes to the jigger, which treats ore between 10and 15 mm. The fine stuff passes through the mesh and on by the shoot(g) to another trommel, and so on down to i mm. holes, after which thefurther operation is one of classification by means of water. TROMMELS AND SCREENS. 281 Sometimes the space and fiiU will not allow of the trommels being thusarranged in a cascade, and in such a case the operation, instead ofproceeding from coarse to fine, is reversed, and goes from fine to coarse,the whole of the rough ore falling first upon the fine sieves, an arrangementwhich, if possible, should be rigorously avoided. When these conditionsare a necessity, a long continuous trommel, such as the one
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