A study has been made of wet, autogenous grinding of disseminated copper ores, including testing of a large number of samples from Kennecott Copper Corporation’s Chino mine. The efficiency with which the various samples could be ground was found to vary over an extremely wide range, even among samples taken from different locations within the same ore body. These variations in grindability can be qualitatively correlated with the composition of the rock and the fracturing and alteration that had occurred.
Fully-Autogenous Pilot Grinding Tests on Various Kennecott Ores
Typical test results obtained in the 6-foot Cascade Mill with samples from Kennecott’s Ray Mines Division, Chino Mines Division, Utah Copper Division, and a development property designated an Mine A are presented in Figure 2. As shown, the ores that were ground most efficiently in the pilot-scale mill were composite mine samples obtained from the Chino Mince Division, Test results indicated that these ores could be ground as efficiently, energy-wise, in an autogenous circuit as in conventional crushing and ball milling circuits. The size distribution of the finished product obtained from the autogenous circuit was substantially the same as that obtained from a conventional grinding circuit.
Limited flotation test work conducted on flotation pulps prepared by dry grinding in an Aerofall Mill indicates a different metallurgical response. Continuous pilot-scale flotation testing showed copper recoveries were from two to three percentage points higher than recoveries from ore ground to the same nominal fineness by conventional methods. The tests were made on a copper ore in which chalcocite occurred as replacement coatings on pyrite. Examination of the flotation concentrate showed that a greater amount of these grains were recovered intact, that is, they had not been fractured transversely during grinding.
Fully-Autogenous Pilot Grinding Tests on Chino Ore
An extensive pilot test program was completed recently at the Chino Mines Division to further assess fully-autogenous grinding for that division’s milling operations.
Briefly describing each of the rock types:
Magnetite Garnet ore contains, typically, 75 to 85 percent magnetite, 6 to 11 percent silicate minerals (garnet and epidote), 6 to 8 percent quartz, and 3 to 6 percent sulfides.
Garnet Epidote ore contains, typically, approximately 80 percent silicate minerals, 9 percent sulfides, 5 to 7 percent magnetite, and 4 percent clay.
Estrella Sill ore is a diorite. It consists of fine to medium grained plagioclase feldspar, biotite, and hornblende. Clay and sericite account for approximately 12 percent of the rock volume.
Estrella Sediment ore consists of interbedded shales and mudstones which have been intensely baked and silicified. Clay and sericite account for approximately 8 percent of the rock volume.
8-Pump Sill ore is a quartz diorite. The mineralogy is similar to Estrella Sill but contains less silica alteration and a higher content of clay minerals.
In some cases wide ranges in grinding characteristics are indicated within each rock type. From the data presented, all ores except those designated as Estrella Sill, Estrella Sediment, and 8-Dump Sill could be expected to be ground autogenously with power consumptions reasonably close to those required in conventional crushing and ball milling circuits.
Since certain of the ores in the Chino mine could not be ground efficiently in the fully-autogenous Cascade Mill by themselves, testing was undertaken to determine if the adverse grinding characteristics could be mitigated by mixing two or more rock types as feed to the mill. Results of testing showed that only the Magnetite Garnet would provide sufficient grinding media to improve the grinding of media-deficient ores.
Including grinding advantages obtained from Magnetite Garnet, average, autogenous grinding performance of the ores in the Chino mine, based upon reserve percentages, is reasonably good. Mill throughput rate of all ores in the mine for a 24-foot by 8-foot Hardinge Cascade Mill driven by a 1500 horsepower motor is projected to average 146 tph. This projection was estimated by extrapolation of all pilot mill test data proportionately to reserves and using the formula:
NP = p (D/d)2.5 (L/T) were NP, D and L are respectively, the net power consumption, diameter, and cylinder length of the large mill, and p, d, and l are, respectively, the power consumption, diameter, and length of the small pilot mill. Dimensions are taken inside liners. Extrapolation by the formula assumes that the large and small mills are operated at the same percentages of critical speed and the same percentages of mill volume are occupied by the grinding charge.