520 CEMENTS, LIMES, AND PLASTERS. 



for coal-grinding, the performances quoted being equivalent to power 

 consumptions of from 5 to 15 H.P. hours per ton of coal ground. Mr. 

 C. D. Bartlett, in discussing * this question, states that a good mill will 

 handle slack at the rate of four tons per hour, crushing it to 80-mesh 

 and using about 25 H.P. in doing so. For kiln use, of course, the fine- 

 ness must be considerably greater than this. 



Total cost of coal preparation. The total cost of crushing (if neces- 

 sary), drying, and pulverizing coal, and of conveying and feeding the 

 product to the kiln, together with fair allowances for replacements and 

 repairs, and for interest on the plant, will probably range from about 20 

 to 30 cents per ton of dried coal for a four-kiln plant. This will be 

 equivalent to a cost 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 remembered that careful drying and fine pulverizing enable 

 the manufacturer to use much poorer, and therefore cheaper, grades of 

 coal than could otherwise be utilized. 



The coal used at American plants costs from 80 cents to $2.50 per 

 ton delivered at the mill, according to the quality of the coal and the 

 location of the mill. In the West, where good coal is far more expensive 

 than stated, oil is used in its place. 



It is probably safe to say that if a plant is so located that coal will 

 cost over $4 per ton, and no oil or gas is obtainable, the rotary 

 kiln is too expensive for use. Under such fuel conditions it is probably 

 best to install stationary kilns of one of the improved designs described 

 in Chapter XXXII. This is particularly the case if a wet mix be used 

 in the kilns. 



Fire and explosion risks. The coal-handling end of the plant is 

 subject to two quite distinct, though related, kinds of risks from 

 explosion and fire respectively. Precautions must be taken to guard 

 against both of these dangers. 



Explosions may occur when finely divided powdered coal is given 

 free access to air. In order to keep as little powdered coal on hand as 

 possible, the coal-mill is usually run so as to just supply the kilns. This 

 has some inconveniences, but it lessens the risk. During grinding 

 care must be taken to prevent the use of exposed lights or even motors, 

 which are apt to spark, in the coal-pulverizing building. The methods 

 of supplying coal to the kiln should give as little access to air as possible. 

 Separation by blowing is, of course, inadmissible, as was emphasized 

 by the fatal results at the Edison plant in 1903. 



* Journ. Assoc. Engineering Societies, vol. 31, pp. 44-48. 1903. 



