THE LOAD OF PEBBLES
The weight of pebbles in a tube mill affects capacity by providing surfaces of contact on which the ore is ground. We must not only provide grinding surfaces but we must provide the right amount to obtain the maximum grinding effect with the minimum power. It takes the least amount of power to revolve an empty mill where there is no grinding ; as the mill is filled with pebbles the consumption of power increases until the mill is half full; when the pebble load is carried over the center, the power required to revolve the mill increases but slightly until the mill is completely full when there is no grinding action. At some point between the center and full load there is the greatest amount of grinding for the amount of energy put into the mill. This point in the cylindrical tube is usually from 2 to 3 in. above the center although there are cases where the load has at times been kept 6 in. above the center.
H. H. Seeber shows, Fig. 20, that the power required to revolve a Hardinge conical mill increased with the amount of pebbles until the mill was half full with 9,950 Ib. of pebbles and from this load up to 13,000 Ib. the power required to revolve the mill fell off slightly.
H. A. Megraw1 states that at the West End Con. mill, Tonopah, charging the mill 6 in. above the center, while the power required increased slightly, the tonnage increased from 100 to 150 tons a day, thereby allowing a coarser screen on the stamp batteries.
The following table will be of use for determining the weight of pebbles in a cylindrical mill. Divide the figure under the internal diameter of the mill by 22 and multiply by the length of the particular mill, in feet, to obtain the weight of pebbles in tons.
The size of the pebbles for use in a tube mill should bear some relation to the diameter of the mill, for presumably a mill of large diameter will be used for grinding large pieces of ore, and would then require large pebbles, the limit of size being a 7-in. pebble, while pulp coming from the average stamp mill with a 10- or 20-mesh screen would require a tube mill of medium diameter with 3 to 4-in. pebbles. While the maximum grinding effect is no doubt secured by small pebbles which give the most grinding surface per ton of pebbles, we must adapt the size to the character of the work. A soft ore or one easily reduced in its smaller sizes should be ground with pebbles 2^ to 3^ in. in diameter, while an ore that offers more resistance to being broken will take larger pebbles.
In the order of their efficiency as grinders in tube mills we have Danish, French and Newfoundland flint pebbles. In a class by themselves we have flints from Texas, pebbles from Manhattan, Nev., pebbles from the Pacific Coast beaches, quartz pebbles from mountain streams, rhyolite and basaltic lava blocks and mine rock.
