Regardless of the category, all tumbling mills operate on the same principles:
■ The total weight of the charge in the mill—the sum of the weight of the grinding media, the weight of the material to be ground, and any water in the mill—is a function of the percentage of the volume of the mill it occupies.
■ The power the mill draws is a function of the
— Weight of the charge in the mill
— Percentage of volumetric loading of the mill
— Percentage of critical speed, which is the speed in revolutions per minute at which the outer layer of the charge in the mill will centrifuge.
— The length of the lever arm from the center of the mill drum to the center of gravity of the charge
— The angle above horizontal at which the tumbling of the charge starts
■ All the elements of the rotating drum go through a complete stress cycle from compression to tension and back to compression during each rotation of the mill.
The development of tumbling mills was heavily influenced by three industrial processes:
■ The grinding of clinker for the cement industry (up to about 1900)
■ The cyanide process, which was developed to remove gold from finely ground gold ore (1900-1910)
■ The flotation process, which was developed to remove sulfide minerals from mineral ores that had been ground fine enough to liberate the mineral particles (1910-the present)
A discussion of these influences is necessary to put the development of tumbling mills and fine grinding into context (Coghill 1929).