Sulfide copper, zinc and iron concentrates are recovered in the 5000 ton per day flotation plant operated by Cities Service Company at Copperhill, Tennessee. The heavy-sulphide ore is first ground to 50 pct. minus 200 mesh and a bulk concentrate floated. The bulk concentrate, over 60 pct. of the ore weight, is reground to 72 pct. minus 200 mesh and then treated in a selective flotation circuit to recover copper and zinc concentrates. The tailing of selective float is the iron sulphide material source for the manufacture of sulfuric acid and iron sinter.
The collector addition in the bulk rougher float is a sodium ethyl xanthate solution. The pH at the head of the circuit is 5.5 but neutralization by calcite in the ore raises the pH to 7.0 at the tailing end. Sulfuric acid is the regulator.
The xanthate concentration in the tailing solution was found to be between 1 and 2 ppm. This was about as expected. The 59.6 ppm. residual xanthate in the concentrate pulp after the lime slurry addition confirmed an interpretation of the finding of Bushell and Malnarich. The tests indicated the xanthate put into solution by a rise in pH would go back on to a mineral surface by a lowering of the pH. Copper recovery was 98.4 pct. in the standard test and 98.7 pct. with the test solution. Iron recovery was 93.3 pct. in the standard test and 93.4 with the reclaimed xanthate.
To test this thinking, bulk cleaner concentrate was refloated at a pH of 7.0 and 11.8. The concentrate and tailing product were then screened.
It was recalled that in the plant a circuit was run adding lime slurry to the copper cleaner concentrate, pH 9.7, to raise it to 11.5 and then refloating. The copper grade of the final concentrate was raised 2 pct. but there was an operational problem handling a 400 pct. circulating load of coarse high-grade mineral. In the light of the xanthate removal effect by increase in pH it seemed likely that the coarse copper mineral with less surface area lost enough xanthate so its specific flotation rate approached zero faster than did the specific flotation rate of the finer particles.