The speed of cylindrical mills
A. M. Merton1 says in regard to the speed of cylindrical mills :
"The speeds of from 360 to 400 ft. per minute will cover most cases, and the mean of 380 ft. may be confidently adopted in a new plant. The best speed at which a tube mill should run in a given case may be determined in a very simple way. The method is to operate the mill at various speeds determining the power consumed at each speed. By dividing the power consumed by the number of revolutions a factor is obtained. which is really the power absorbed at each revolution. The higher the factor the most effective the crushing power of the pebbles.
Before applying this method it is necessary first to determine the most economical load of pebbles for the mill, because we may fill the mill above the center with pebbles with a small increase or even less power consumption than when the pebbles are below the center and so do more grinding with a lower factor.
At some mills the operator judges by the temperature of the tube mill discharge whether the mill is grinding properly, a high temperature indicating that the mill is doing good work. While the theory that the amount of heat generated in the mill and absorbed by the pulp might give an indication of the amount of work being done on the ore, it is not a safe theory to bank on. The following notes on the subject are given to indicate the method of calculating and the result of one experiment.
The proper speed of rotation might be determined by the amount of heat generated in the mill if all the heat generated represented useful work. By taking the temperature of the material entering and discharging from the mill we can determine the amount of work represented by these heat units, but we have no means of calculating the amount of heat lost in radiation, friction or sound. These unmeasurable losses of heat will no doubt be fairly constant for a given atmospheric temperature so that if we measure the heat units generated we will no doubt find that the greater the amount of heat generated the greater the grinding efficiency, but not necessarily in the same ratio.
The following data were obtained from two different mills on the same day and while the figures are approximate the result shows that the method may have some merit.
The figures for mill No. 1 show that 60 per cent, of the energy put into the mill has been returned as heat in the pulp and only 7.5 per cent, of that put into mill No. 2. Had mill No. 1 been run with less moisture, no doubt as much as 65 per cent, of the energy would have appeared as heat in the pulp. Dowling gives the efficiency of tube mills on the Rand at 57.4 per cent. The lesson to be learned from the fact that mill No. 2 has returned in the pulp but 7^2 per cent, of the energy put into the mill is that the mill is running too slow which is likewise shown by the tonnage which should be at least 65 tons per day, instead of 22 tons.
W. J. Pentland says :
"When a tube mill is grinding its full load, a distinct rise of temperature of the shell can be noted. If the pebble load is light, or the moisture content of the pulp so high that the increased temperature is not apparent, the mill is not doing the work it can be made to do."
