bckel.] METHODS OF MANUFACTURE. 57 
The coal as usually bought is either "slack" or "run of mine." In 
he latter case it is necessary to crush the lumps before proceeding 
farther with the preparation of the coal, but with slack this prelimi- 
lary crushing is not necessary, and the material can go directly to the 
irier. 
DRYING COAL. 
Coal as bought may carry as high as 15 per cent of water in winter 
>r in wet seasons. Usually it will run from 3 to S per cent. To obtain 
ood results from the crushing machinery this water must be driven 
aff. For coal drying, as for the drying of raw material's, the rotary 
Irier seems best adapted to American conditions. It should be said, 
lowever, that in drying coal it is usually considered inadvisable to 
dlow the products of combustion to pass through the cylinder in 
vhich the coal is being dried. This restriction serves to decrease 
lightly the possible economy of the drier, but an evaporation of to 
> pounds of water per pound of fuel coal can still be counted on with 
my good drier. The fuel cost of drying coal containing 8 per cent 
>f moisture, allowing $2 per ton for the coal used as fuel, will there- 
ore be about 3 to 4 cents per ton of dried product. 
PULVERIZING COAL. 
Though apparently brittle enough when in large lumps, coal is a 
iifficult material to pulverize finely. For cement-kiln use, the fineness 
)f reduction is extremely variable. The finer the coal is pulverized 
he better results will be obtained from it in the kiln, and the poorer 
he quality of the coal the finer it must be pulverized. The fineness 
bttained in practice may therefore vary from 85 per cent, through a 
OO-nmsh sieve, to 95 per cent or more, through the same. At one 
>lant a very poor but cheap coal is pulverized to pass 98 per cent 
h rough a 100-mesh sieve, and in consequence gives very good results 
n the kiln. 
Coal pulverizing is usually carried on in two stages, the material 
>eing first crushed to 20 to 30 mesh in a Williams mill or ball mill, and 
inally reduced in a tube mill. At many plants, however, the entire 
■eduction takes place in one stage, Griffin or Huntington mills being 
ised. 
TOTAL COST OF COAL PREPARATION. 
The total cost of crushing (if necessary), diying, and pulverizing 
oal, and of conveying and feeding the product to the kiln, together 
nth fair allowances for replacements and repairs and for interest on 
he plant, will probably range from about 20 to 30 cents per ton of 
Iried coal, for a 4-kiln plant. This will be equivalent to a cost of 
rom 3 to 5 cents per barrel of cement. While this may seem a heavy 
