The total feed to the ball-mill was therefore 11.54 T. per hr. or 179 per cent, of the original feed. It was evident that the ball-mill was greatly underloaded in this test, but so much trouble developed in the classifier, due to the tendency of the sands to slip down the slopes, that a more rapid feed was not attempted at this time.

The theoretical critical speed of a ball mill is the speed at which the centrifugal force is sufficiently large to cause a small particle to adhere to the shell interior for a full revolution of the mill. The common range of mill speeds is 65% to 80% of critical, depending on mill type, size and the application.