THE BALL MILL

The advantages to be gained by the use of the ball mill are the small amount of space required for the machine, the lesser first cost as compared with other intermediate crushers or grinders, the ability to grind through a wide range in one machine, the small amount of attention required during operation and the fact that the only repair necessary is to reline the mill, at long intervals. That the amount of attention required during operation is practically nothing accounts for the small labor cost, for the operator who takes care of the mill may look after rolls or other crushers, concentrators or other machinery.

I have been unable to obtain figures that show the ball mill to be an economical crusher for hard quartz ores, its sphere of usefullness being rather for ores of moderate hardness or soft friable ores. In Julian and Smart's "Cyaniding of Gold and Silver Ores' page 193, is given the horsepower per ton of ore crushed in several ball mills.

Von Bernewitz gives the data for a No. 5 Krupp ball mill at Kalgoorlie as follows: The mill consuming from 16 to 20 hp. crushed 40 tons of ore per day to 27-mesh with a load of 2,350 Ib. of steel balls and a consumption of steel amounting to 3J^ oz. per ton of ore ground. Converting this into horsepower per ton of ore ground we have Kalgoorlie, 0.40 to 0.50 hp. per ton of ore.

Objections may be made that the data here given for ball mills does not represent present-day practice. In a sense it does not,for the ball mills now being installed are being fed with a product from a disc crusher or an intermediate crusher between the coarse rock breaker and the ball mill which of course largely increases the output of the latter machine and consequently decreases its apparent power factor.