272 ANNUAL REPORT 
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al 29,500 3,000 | 12 to 16 bbls. per hour 30 to 40 H.P. 
8 41,100 | 4,500 18 to 24 bbls per hour 40 to 50 H.P. 
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In starting the mill, from 100 to 120 per cent. additional power is 
required. When pulverizing to pass all through 20 mesh, from 30 to 40 
per cent. will pass a 100 mesh sieve. In grinding limestone, the capacity 
is greater and averages, for the larger size of mills, 5 tons per hour to 
20-mesh sieve. It was found that in grinding 102,400 barrels of cement 
the steel balls lost 2,959.0 pounds of metal, or 0.139 pounds of metal per 
ton crushed. At one cement mill in Europe a ball-mill of 64 inches inside 
diameter, making 17 revolutions per minute, with slits 0.4 inch wide, and 
requiring 17 horsepower, working a hard limestone, showed an average 
capacity of 7,700 pounds per hour. At the works of William Krause and 
Sons, Martin’s Creek, Pa., accurate measurements showed the actual aver- 
age power required to drive one ball-mill to be 39 horsepower. As has 
been said, some European manufacturers use the ball-mill for fine grinding, 
separating the fine powder either by means of screens or by blowing air 
through the material. The first kind is represented by the mill built by 
Siller and Dubois, in which the first sieve is a steel screen, with one mm. 
slit perforations, while the fine sieve is a brass screen, 12 meshes to the 
linear inch. The mill, 64 inches inside diameter, was found to produce 
with 17 revolutions per minute and 15 horsepower 2,794 pounds of cement 
per hour, while another mill, of the same make but 72 inches inside diam- 
eter, showed an average capacity of 2,970 pounds per hour, requiring 17 
horsepower. The fineness of the ground cement is, however, not stated. 
The second class of ball-mill with wind separator, having no fine 
sieves, as built by Pfeiffer, has an average capacity of 1,430 pounds of 
fine cement, leaving a residue of from Io to 13 per cent. on a go-mesh 
sieve, per hour, requiring 12 to 14 horsepower, or producing 110 pounds 
of cement per horsepower hour. Since the sieves are very troublesome 
to take care of and the wind separator does not possess a sufficient 
capacity, the American practice of using the ball-mill purely as an 
intermediate grinder without the fine sieves is the best, as it leaves the 
fine grinding for a typical fine grinding machine. As a type of inter- 
mediate grinding machine the ball-mill has the advantage of thoroughly 
blending the materials, but shows excessive expenditure of power and 
high repair cost. The fact that it produces 30 to 40 per cent. of fine 
material does not help the case where this fine size cannot be separated 
