55 



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

 cent, of water in winter or wet season. Usually it will run 

 from 3 to 8 per cent. To secure good results from the crush- 

 ing machinery it is necessary that this water should be driven 

 off. For coal drying, as for the drying of raiw materials, the 

 rotary dryer seems best adapted to American conditions. It 

 should be said, however, that in drying coal it is usually consid- 

 ered 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 dryer, but an evaporation of 6 to 8 pounds of water per 

 pound of fuel coal can still be counted en with any good dryer. 

 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 ob- 

 tained from it in the kiln ; and the poorer the quality of the coal 

 the finer it is necessary to pulverize it. The fineness attained 

 may therefore vary from 85 per cent, through a loo-mesh sieve, 

 to <)5 per cent, or more, through the same. At one plant a very 

 poor but cheap coal is pulverized to pass 98 per cent, through 

 ;' loo-mesli sieve, and in consequence gives very good results 

 in the kiln. 



Coal pulverizing is usually carried on in two stages, the ma- 

 terial being first crushed to 20 to 30 mesh in a Williams mill or 

 ball mill, and finally reduced in a tube mill. At many plants, 

 however, the entire reduction takes place in one stage, Griffin 

 or Huntington mills being used. 



Total cost of coal production. The total cost of crushing (if 

 necessary), drying and pulverizing coal, and of conveying and 

 feeding the product to the kiln, together with fair allowance for 

 replacements and repairs, andl for interest on the plant, will 

 probably range from about 20 to 30 cents per ton of dried coal, 

 for a 4-kiln plant. This will be equivalent to a ccst of from 3 

 to 5 cents per barrel of cement. While this may seem a heavy 

 addition to the cost of cement manufacture, it should be remem- 

 bered that careful drying and fine pulverizing enable the manu- 



