The results of a 2 phase evaluation program designed to determine the amenability of an oxide ore from the Butte, Montana mine of the Anaconda Company to a sulfuric acid leach is presented. In the phase 1 preliminary evaluation, using roll tests and percolation leaching in small columns, it is shown that the ore is highly extractable at low acid consumption ratios. In the phase 2 confirmatory tests in large columns on 1-1% tons of material, the amenability of the ore to the leaching process is proven and pregnant solution grade profiles from planned commercial heaps established.
Materials and Methods
For the Phase 1, preliminary evaluation, 40 lb of oxide ore were obtained and crushed to -3/8 in. The crushed material was screen assayed and each individual size fraction was taken for chemical assay. Composites having the same size distribution as the 3/8 in crushed material were made up for use in a roll test and for small column tests.
Ten 40-gallon drums of oxide ore were obtained from the mine for the Phase 2 study and brought to B.C. Research.
The small column leaching tests were performed in polyvinyl pipe sections of 3 in diameter and 3 ft long. The ore was held in place by a perforated PVC plate resting on two stainless steel rods inserted through the wall near the bottom of the column.
Column 1 – 60 g/l H2SO4
Column 2 – 30 g/l H2SO4
Column 3 – 15 g/l H2SO4
Column 4 – 5 g/l H2SO4
The barren leach liquor was pumped over the columns on a one-pass basis at a rate of 0.5 US gal/h/ft². The pregnant leach liquor was collected over a 24-hour period and assayed for pH, dissolved copper, dissolved total iron, dissolved ferric iron and normality. After measuring its volume, the pregnant liquor was discarded.
When the -3/8 in ore was leached with 60 g/l H2SO4, up to 88% extraction occurred during the first 8 hours. After 48 hours, the maximum extraction of 112% was reached. Most of the acid was consumed during the first 8 hours of the leach, during which time 90% of the copper was solubilized. After 24 hours, acid is consumed at a constant rate, whereas copper extraction rapidly decreases and finally stops after reaching a maximum value at 48 hours. At this time the acid consumption was 3.3 lb H2SO4 per pound of copper extracted or 72 lb sulfuric acid per ton of ore.
The extraction from the four small columns shows that increased rates of extraction are obtained when the acid content of the barren solution is increased. The time to 80% extraction was used as a basis for comparison and as expected, the highest grade sulfuric acid leach solution extracted copper at the fastest rate.
A total of 2,759.5 lb of -¾ in oxide ore at 6½% moisture content was charged to one column and 2,688.5 lb of as received oxide ore at 7.6% moisture to a second column. Screen assays of the “as received” and crushed material (Table 2) showed that 83.7% of the crushed ore and 87.5% of the “as received” ore was +16 mesh in size. About 37.9% of the “as received” ore was greater than 1 in in size. Leaching of both columns was started with a 60 gram/liter sulfuric acid solution which, in the earlier tests in the small columns, had been shown to give the fastest extraction rates.
The copper content of the pregnant solution declined steadily from the maximum of 15.8 gram/ liter to 1.0 gram/liter after 34 days at which time an extraction of 13½ lb/ton (66.2%) had been obtained. In an effort to improve the grade of the pregnant solution, the acid content of the barren solution was increased from 10 gram/liter to 20 gram/liter at day 37, and although the grade increased by up to 50%, the concurrent decrease of the pregnant pH showed that grade control, through variations in acid strengths of the barren solution, may not be practical.
The results of the large column tests showed that approximately 65% extraction can be obtained readily while producing pregnant solutions of significant copper content, that is, more than 1 gram/liter when leaching a 20-ft high column of ore. Size reduction of the ore to -¾ in did not significantly improve either rate of extraction or extent of extraction, suggesting that crushing of ore that is 88% -2 in is not necessary.