DRESSING OF ORES 



83 



fall, a double conical trommel is employed. Its use is, however, more for separating 

 or dividing stuff for subsequent sizing, than for sizing directly to jiggors. 



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Cascade Trommels. A system of six sizing trommels is shown in Fig. 659. Each 

 trommel is 42 inches long, 21 inches diameter at the large end, and 18 inches dia- 



meter at the small end. At a speed of 20 revolutions per minute, the six trommels 

 will dispose of from two to three tons per hour. The volume of water required per 



658 



trommel to keep the holes clear is from two to three gallons per minute. The order 

 of sizing is from great to small grains ; that is, the first or uppermost trommel 

 has the coarsest holes ; each succeeding trommel having holes of lesser diameter. 

 Through this arrangement the smallest holes and thinnest plates get only a minimum 

 amount of wear. In order to convey the stuff from one trommel to another sheet-iron 

 or wooden hoppers are employed. 



Each trommel is light, portable, and readily replaced. Instead of distributing the 



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