Homestake crushing and grinding-practice was completely revised during the period of 1951-1953. During the conversion, both crushing, plants at the head frames, the Ross and the Yates, were rebuilt. The flowsheet changes at each plant involved replacing the two 8K gyratories with a single thirty inch gyratory for primary crushing to four inch. The gyratory discharges to a seven foot Symons standard cone crusher, set at 1-1/8 inch opening. The cone discharge goes to Tyrock screens fed by vibrating feeders. Screen openings are ½ inch by 5 inches at the Ross Plant, and 3/8 inch by 5 inches at the Yates. Screen oversize goes to a seven foot Symons cone brusher converted to the short head type; the crusher setting is 5/16 inch. Crusher discharge is closed circuited with the Tyrock screens. The above crushing installation delivers a minus 5/8 inch rod mill feed to the mill bins.
Management’s decision was to proceed with the installation of a fourth milling unit at a minimum of capital expenditure. The unit consists of a 9 foot by 11 foot ball mill, identical to the three in use, except for feeding arrangement.
Since cyclone classifiers, operated at low inlet pressures, were giving excellent results at South African gold installations, this plus the lower capital cost, were the deciding factors in i selecting them for the closed circuit classification. Two 20 inch rubber lined cyclones receive mill discharge, which, after passing through the amalgamator box, is pumped from a receiving sump by an 8 inch rubber lined pump to the cyclones at 5 psi inlet pressure. A ball mill discharge trommel, with ¼ x ¾ inch openings, and equipped with spray nozzles, rejects a small amount of predominately quartz oversize to a 14 inch conveyor belt, which returns about a ton per hour of the oversize to the main feed belt.
Comparative performance of single and two stage grinding units— capacity and Grind. The fourth unit has been in operation since February 1956. Its performance has been all that was anticipated. The mill feed has averaged 800 tons per day (33.0 tons per hour), while a two stage unit averages 1350 tons per day (56.5 tons per hour).
