190 ANNUAL REPORT 
crushed material makes a complete circuit, the fine material being con- 
stantly removed and the coarser particles being even carried along with the 
fine grindings from the buhr mill, since they insure better working of the 
screens. 
Separating Tailings from 
Finishing “Stone (208 ale 
E/evotor for Crusher and Cracker Stone Product: 
> 
Separating Coarse rom eK 
(GEES. Frailings 4 Ss 
RY 
u cs) 
I< 
\ Q 
we 
Coarse Ss 
Tailings ; 85 
v 
Sg : 
One ~ . 
Gre (ere Product, corusher 
A 
Fig. 21. Berthellet system of screening. . 
Fine Grinding. —The mill most frequently used is the old-fash- 
ioned buhr mill, from 3 to 5 feet in diameter. One of the large size mills 
will grind about 1o barrels of cement per hour. These mills require, 
however, frequent dressing, which is equivalent to considerable expendi- 
ture of time and money. Emery mills are now being used in considerable 
numbers, which have proven more efficient, as they need less dressing. 
A 42 inch emery mill will grind from 18 to 20 barrels of rock per hour, 
95 per cent. passing the 100 mesh sieve. ‘The power required is about 15 
to 18 horsepower. ‘The grindings from the finishing mill are usually 
screened and delivered to the hopper, from which chutes take the cement 
to the packing room. Usually the rock cement is not stored, but shipped 
at once. ‘This, of course, is a matter depending on the composition of the 
rock. Very basic cements must be stored. 
