224 CLAYS AND CLAY INDUSTRY. 



is to be used, dry pans are often employed. These consist of a 

 circular pan, in which there revolve two iron wheels on a hori- 

 zontal axis. The wheels turn because of the friction against the 

 bottom of the pan, the latter being rotated by steam, power, and 

 in turning they grind by reason of their weight, which ranges 

 from 2000 to 5000 pounds. The bottom of the pan is made of re- 

 movable, perforated plates, so that the material falls through as 

 soon as it is ground fine enough. Two< scrapers are placed in 

 front of the rollers to' throw the material in their path. The diam- 

 eter of a dry-pan may range from 6 to> 9 feet. For a 9-foot pan 

 with rollers 48 inches in diameter and 12 inches face, the total 

 weight would be about 20 tons, while the weight of the two' rollers 

 with their shafts, and boxes is about 6^2 tons. From 12 toi 16 

 horsepower are required to operate the pan. The capacity of such 

 a machine will depend on the size of the screen meshes, and char- 

 acter of raw material, whether hard or soft, dry or moist. For a 

 medium shale, it is possible to grind 8 tons per hour through a 

 3/£ -inch screen and about 12 tons through a 34 -inch screen. 



Disintegrators represent a third type of machine used for break- 

 ing up clay or shale, and, where used, are commonly found to^ be 

 quite effective. Their capacity is large, but much power is also 

 required to drive them. A disintegrator has several drums, or 

 knives on axles, revolving rapidly within a case, and in opposite 

 directions. As the lumps of clay are dropped into the machine 

 they are thrown violently about between the drums and also strike 

 against each other, thus pulverizing the material completely and 

 rapidly. Such machines can pulverize from 8,000 to 28,000 

 pounds of material, such as shale or gypsum, in one hour, and 

 require from two and one-half to four horsepower per ton per 

 hour. 



Rolls (PI. XXIV, Fig. 2) are often employed for breaking up 

 clay and pebbles, and, where dry material is used, they are quite 

 effective, but if damp clay is put through them, as is done at some 

 yards,, the lumps are simply flattened out. Rolls are also supposed 

 to break up any stones that may be present in the clay. The sur- 

 face of the rolls is smooth, corrugated or toothed, or tapering. 

 The two; rolls revolve in opposite directions and with differential 

 velocities of 500 to 700 revolutions per minute. 



