VOLUME OF FEED
Experiments at El Oro proved that the capacity of a tube mill " decreased proportionally to the rate of feed." This is partially true, for the increased capacity due to increased feed has its limits. The capacity of a tube mill increases with the amount of feed to a point where the returned oversize is sufficient either to overload the classifier or to increase the circulating load to the extent that the feed must be decreased. Feeding above this point provides the mill with too much coarse material to do efficient work.
V. B. Sherrod found at the Guerrero Mill, Pachuca, "That the grinding efficiency increases with the quantity of ore fed per minute, to a point which varies with the diameter of the mill, and that beyond this point capacity is not gained, and within reasonable limits there is no loss except that due to the return to the mill of an excess of oversize. The limit of profitable feed varies with the character of the pulp, and can be determined only by experiment."
An experiment at the Goldfield Con., Nevada, showed that with a medium feed 0.74 tons of ore were ground per horsepower, while with a heavy feed the same mill ground 0.97 tons per horsepower per day to 200-mesh.
In a case noted the cost of grinding, elevating and classifying with a light feed was $1.88 per ton of initial sand fed. With a medium feed the cost was reduced to 97 cts. and with a heavy feed, the cost dropped to 68 cts. per ton.
When designing a fine-grinding plant it is often necessary to figure the capacity of the classifier return feed which returns the oversize to be reground. This varies in different mills but may be placed at from 75 to 100 per cent, of the initial feed. At the Tonopah-Belmont the initial feed is 67.18 tons while the return feed is 60.20 tons or in the ratio of 0.91 to 1.
R. T. Mishler, assistant general manager of the Tigre Mining Co., Senora, Mexico, made experiments on the effect of varying the amount of ore fed to the tube mills at their plant and being of more than passing interest the entire paper is here reproduced. 1
"The fine-grinding equipment of the Lucky Tiger plant consists of five 5 by 14-ft. tube mills. They are lined with manganese-steel liners, of a modified El Oro type, which reduce the effective dimensions to 4>^ by 133^ ft. The feed ends of the tube mills are equipped with spiral sand feeders, and the discharge end with worm-pebble feeders. Each mill is driven, through gearing, by a separate 60-hp., slip-ring induction motor. There are ammeter and watt-hour meter connections to each motor, making it possible to ascertain the power used by any mill. The speed of the mills is constant at 27 r.p:m.
"Danish flint pebbles of the best grade are used, the pebble load being kept fairly constant at 7 tons. This is accomplished by regulating the feed of pebbles by the readings of the ammeter, an amperage of 58 being maintained. With a voltage of 450, and a power factor of 0.75, this indicates a horsepower of 45.5 per mill.
"The tube mills receive from the concentrating -plant and from old dumps, an average of 150 dry ton per day of clean, coarse sands. After passing through the tube mills, these sands are raised in bucket-elevators, passed over Wilfley tables, and then distributed to four 16-ft. drag* classifiers. The sand discharged from these machines is returned to the tube-mills, and the slimed overflow is sent to agitation-tanks, thus being removed from the tube -mill circuit. An average of 80 per cent, of the overflow passes a 200-mesh screen, the remaining 15 per cent, being finer than 150-mesh.
