SIZE OF FEED AND DISCHARGE
The feed for a tube mill may vary between 2- and 40-mesh and the discharged product contains usually at least 70 per cent.-200-mesh. The size of ore particles fed to the machine varies with the diameter of the tube mill and the efficiency of the intermediate crushers (such as stamps) to grind to a particular size. The experiments at El Oro proved that "the efficiency of the tube mill increases with the coarseness of the sand fed to the mill," within limits.
It was found at the West End, Tonopah, that the crushing efficiency could be increased 25 per cent, by the addition of sufficient coarse material to the feed, together with the fines, to permit the coarse material to travel almost to the discharge end of the tube mill. To accomplish this 10 stamps crushed through 10- mesh, and 10 through 6-mesh.
One of the peculiar features of a tube mill is that a reduction in the size of the ore fed to the mill does not give a corresponding increase in the number of tons ground per day. This is accounted for by the fact that when ore particles are ground by the impact of pebble on pebble the spaces between the pebbles allow different size pieces of ore to be ground with the same blow. This is not so apparent when the ore is ground by the rubbing of the pebbles for then the particles at the point of contact only are ground. Referring to Fig. 16, A represents two large-size pebbles with pieces of ore between the surfaces, B a small and a large pebble and C two small pebbles. It can easily be seen that two large pebbles will crush larger grains of ore than one large and one small or two small pebbles, so the larger the pebbles the larger size ore can be fed to the tube mill with the expectation that the capacity will not be seriously reduced. There is a limit to this increase which can be determined only by experiment.
W. J. Pentland 1 relates the following experience to show the increased capacity of stamps and tube mills in combination by increasing the size of the ore particles fed to the tube mills:
"We had 200 stamps, cone classifiers and thickeners, two tube mills 5 by 20 ft. and seven tube mills 4 by 22 ft. We used 35- and 40-mesh screens on the batteries, yielding a product containing about 60 per cent. 200-mesh solids. From 730 to 750 tons could be treated, giving a final pulp of 80 per cent. 200-mesh. By substituting 25-mesh screens on 100 stamps and 30- or 35- mesh on the other hundred, we were able to raise the tonnage from 830 to 850 tons, but this was the most we could get through the tube mills. With cone thickeners we had to give constant attention to keep the moisture down 3 to 38 per cent.
"We thpn hiinir nn 100 stnmns nut Particles of ore arc crushed stamps, pu between pcbblcs of different 8- and 10-mesh screens on the remain- sizes. ing 100, rearranged and improved the cone system of classification and thickening, and added three tube mills 5 by 24 ft. This system enabled us to increase the capacity to 1,050 tons daily; while battery, crusher and conveyor costs that used to total 35 cts. went down to 17 cts. . . .
"With coarse feeds we could get lower moisture, greater tonnage and better ground pulp; with the finer feeds it was almost impossible to get low moisture, the tonnage was reduced and the product never contained as much 200-mesh solids as was secured from the coarser feeds."
