REQUISITES AND TREATMENT OF KILN FUELS. 515 



128 a set of 24"X18" plain-faced disintegrating rolls are shown. These 

 will handle coal up to say, 4 to 6 inch size, and reduce it economically 

 to f or \ inch. Finer than this it is hardly profitable to go, for -inch 

 coal is readily dried and is of convenient size for either ball, Griffin, 

 or Williams mills. 



Drying coal. Coal, as bought, may carry as high as 15 per cent 

 of water in winter or wet seasons; usually, it will run from 3 to 8 per 

 cent. To secure good results from the crushing machinery it is neces- 

 sary that this water should be driven off. For coal drying, as for 

 the drying of raw materials, the rotary drier seems best adapted to 



FIG. 128. Rolls for coal-crushing. (Allis-Chalmers Co.) 



American conditions. Several types of these driers are discussed on 

 pp.400 and 649. It should be said, however, that in drying coal it is 

 inadvisable to allow the products of combustion to pass through 

 the cylinder in which the coal is being dried. This restriction serves 

 to decrease slightly the possible economy of the drier, but an evap- 

 oration of 6 to 8 pounds of water per pound of fuel coal can still 

 be counted on with any good drier. The fuel cost of drying coal 

 containing 8 per cent of moisture, allowing $2 per ton for the coal 

 used as fuel, will therefore be about 3 to 4 cents per ton of dried product. 

 Pulverizing coal. Though apparently brittle enough when in large 

 lumps, coal is a difficult material to pulverize finely. For cement-kiln 

 use, the fineness of reduction is very variable. The finer the coal is 

 pulverized the better results will be obtained from it in the kiln, and 

 the poorer the quality of the coal the finer it is necessary to pulverize 

 it. The fineness attained in practice may, therefore, vary from 85 per 

 cent through a 100-mesh sieve to 95 per cent or more through the 

 same. At one plant a very poor but cheap coal is pulverized to pass 

 98 per cent through a 100-mesh sieve, and in consequence gives very 

 good results in the kiln. 





