The Cities Service Company’s Pinto Valley mine and concentrator lie about 6 miles west of the town of Miami, Arizona at an elevation of approximately 4,000 feet above sea level. The concentrator started its first ball mill in July 74 and all six were available by November.
Every effort was made to reduce capital costs, consequently, each unit process was studied critically to eliminate excess capital costs without loss of efficiency. Equipment selection was made on the basis of proven dependability and operating costs rather than on low bids.
The concentrator is nominally 40,000 tons per day. The flowsheet is typical with 3 stage crushing, single stage closed circuit grinding, flotation, regrinding of rougher concentrate and cleaning of the rougher concentrate. T
Run-of-mine ore is delivered to the primary crusher, nominal capacity 4,100 t.p.h., in 150 ton Wabco trucks. The trucks discharge directly onto the crusher set in a nominal 300 ton dump pocket. Grizzleys are not used in the pocket to separate out the undersize. The crusher setting is adjusted hydraulically from the control room. At present, the setting is a nominal 7 inches.
The primary grinding circuit consists of six 18′ x 21′ Allis-Chalmers overflow ball mills driven by a 4,000 H.P. motor thru an air clutch. Each mill is in closed circuit with eight 26 inch Wemco cyclones of which 5 are in service at any time. The apex setting is 6-½ inches and the vortex 12 inches. The cyclone feed pump is 14 by 12 Warman driven by a 400 H.P. motor. The circulating load is approximately 500%. New feed rate is presently 370 tons per hour of minus 3/8 in ore.
The flotation circuit consists of 6 rougher rows of 14 cells each of 300 cubic foot Wemco flotation cells. The cleaners are two rows of 14 cells each of 300 cubic foot Wemco cells. The flotation cell are divided into two sections each fed by 3 ball mills through a Denver distributor. The rougher concentrate is reground in two 11 foot by 15 foot Traylor overflow ballmills.
The thickened copper-molybdenum concentrate is pumped to two conditioners in series to which is added the copper depressants. The conditioned concentrate is fed to a 9 cell row of 300 cubic foot cells which make a rougher concentrate. The rougher tailing is the final copper concentrate which goes to two 90 foot thickeners. The moly rougher concentrate is cleaned twice in another row of nine 300 cubic foot cells. The tailing is returned to the head of the rougher bank while the concentrate goes to a 40 foot thickener.
The final copper concentrate at 55 to 60% solids flows by gravity from the copper thickeners to either of two 238,000 gallon storage tanks. The slurry is withdrawn from the tanks by either of two 3″ x 3″ A-S-H centrifugal pumps. The slurry is passed through a test loop 170 feet long by 3.5″ inside diameter to determine its pumpability. The test loop is instrumented to indicate flowrate, density and pressure differential in the loop. The pressure differential across the test loop is the critical criterion in determining if the slurry is safe to pump.
Lime is received in a 240 ton bin, ground in a 6 foot by 10 foot ballmill and stored in two 100,000 gallon agitated storage tanks. The lime is pumped in a loop thru the mill and back to the storage tank.
Liquid flotation reagents are received in 9,000 gallon storage tanks and diluted as needed or used full strength. Reagents are pumped to head tanks in the mill.
A grizzley was not installed at the primary crusher. This greatly reduced the necessary structure. While dumping the undersize through the crusher may at times reduce the crusher capacity, in general, the crusher capacity is strictly related to the ability of the mine to deliver ore.
Ball consumption was estimated from results at Copper Cities for 1968 and 1969. The average consumption was 1.22 lbs. per dry ton new feed. The average KWH per dry ton was 5.693. Ball consumption would, therefore, be 1.22÷5.693 = 0.214 lbs. balls per KWH input to the mill. For the Pinto Valley concentrator ball consumption should, therefore, be 0.214 x 8.39 = 1.80 pounds per ton.
Ballmill liner consumption was again estimated from Copper Cities experience as 0.015 pound per KWH of primary grinding energy. To-date only the discharge and intake throat liners have been replaced. The estimate, at present, is that liner wear will be approximately 0.010 pounds per KWH of primary grinding energy. At the present time, it is expected that ball wear will be higher and liner wear less than with smaller mills.
Operating problems have hindered the work on determining the best liner pattern for grinding. The mills have, in general, operated on the same fixed tonnage. Also, the manufacturer of the cyclones has been very slow in supplying the necessary apex sizes.