The Allis-Chalmers adjustable ball mills
In the Allis-Chalmers adjustable ball mills the "lining is of manganese steel, with stepped corrugations. The mechanism of the machine includes a diaphragm near the discharge end, having a number of concentric holes, which may be closed by wooden plugs. With all the holes open the mill will discharge within 3 or 4 in. of the periphery, and by closing the holes the discharge is raised accordingly. By this means the pulp level is regulated to suit the material crushed, and to effect the ratio of fineness of product that may be required."
In mills where the load of pebbles is kept near the center line the hollow trunnions are lined with an internal screw or helix which, when the mill revolves, causes the coarse material or pebbles to progress into the mill and provides means for feeding pebbles at this end. Fig. 39 illustrates the idea. In combination with the helix we may have a conical lining to the trunnion which somewhat answers the purpose of the conical end of the Hardinge mill, presumably acting as a classifier, of limited extent and utility.
A device, the object of which is to increase the capacity of ball or tube mills, was recently tried out at the Tough-Oakes mill, Kirkland Lake, 1 on a 6-ft. Hardinge conical ball mill that was installed last year to handle 100 tons of ore per day.
"The ore, generally speaking, is a hard and rather tenacious reddish porphyry with gold disseminated throughout in minute joints and small quartz stringers. This ore proved comparatively refractory to disintegration by ball mill treatment, for after a thorough test extending over a period of 3 months it was found that the ball mill had an average daily capacity of only 82.68 tons for total time and a maximum capacity of only 90.62 tons for actual running time, instead of a 100 tons daily. Many experiments were conducted by ball mill experts to increase the capacity of this mill during the period mentioned, including increase and decrease of revolutions, load, etc., but the capacity failed to reach the prescribed figure.
"Finally, the idea was conceived of a less obstructed discharge by permitting the whole product, oversize and undersize which was inclined to accumulate in the conical section of the mill, to pass through the conical revolving screen inserted and fastened in the discharge, as shown in Fig. 40, thereby classifying the product and returning the oversize to the feed end of the mill by means of a conveyor. This arrangement, as will be seen, permitted a much heavier load to be carried and developed consequently a greater discharge area. Later there was installed a stationary water pipe with a series of water jets arranged so as to assist the discharge of both the oversize and undersize.
"After a 5-month trial of the device it was found that the capacity of the mill had been increased so as to average for that period 102.27 tons per day and for actual running time 110.41 tons per day, thus giving an increase in tonnage of 23.7 per cent, for total running time and 21.8 per cent, for actual time. The power required to operate the mill before the installation of the device was about 40 hp. ; after the installation it was 35 hp., showing an actual decrease of 12.5 per cent. By taking into consideration the increased tonnage for the actual running time, the actual decrease in power consumption per ton of ore was about 28.1 per cent.
"The device makes it possible to raise the level of the load in the ball mill without discharging the grinding medium and thus increase the load from 8,500 lb. to 11,500 lb., or about 35.3 per cent. This feature also decreased the ball consumption from 2.45 lb. per ton to 1.8 lb. per ton. Attached to the tube mills the device permitted ready discharge and classification of oversize ore particles and undersize pebbles, which tend to accumulate and thus congest the tube mill circuit. By its use about 2 tons of oversize is removed daily from the tube mill circuit and returned to the ball mill feed. Partly by substituting ore masses and partly by the use of the device, the pebble consumption was reduced from 16.1 Ib. to 3.9 Ib. per ton. The accompanying table is based on averages over a period of 3 months without and 5 months with the device."
