Nitric Acid Route to Processing Copper Concentrates

The process parameters for effective utilization of nitric acid as an oxidant for copper-iron sulfides have been developed. Leaching variables found to be important were acid concentration, temperature, time, atmosphere, pressure, mineral type, particle size, and percent solids. Particular attention was devoted to analysis and control of fumes evolving during the oxidation. Data were recorded on metal extraction, elemental sulfur formation and oxidation, product gas composition and evolution rate, reagent consumption, and residue flotation response. The study was concluded by laboratory testing a flowsheet based on the foregoing observations. Ninety-eight percent of the copper contained in a chalcopyrite-bornite concentrate was recovered, requiring 1.6 lb of nitric acid to be regenerated externally per pound of copper extracted. Iron remained in the leach residue as hydrogen jarosite and was ultimately rejected from the system by froth flotation. Sixty percent of the sulfur contained in the copper concentrate reported in the elemental form; the remainder was oxidized to sulfate and eliminated with the hydrogen jarosite in the flotation tails or by neutralization of the solvent extraction raffinate with lime. Nitrogen was evolved primarily as NO and NOD; up to 3% N2O also was present. Gases containing sulfur were absent except when pyrrhotite was solubilized in the absence of soluble copper. In this case, HAS was evolved.