Recovery of cobalt, nickel and copper from converter slag through roasting with ammonium sulphate and sulphuric acid

Abstract

Roasting of copper converter slag containing 4.03% copper, 1.98% nickel and 0.48% cobalt with ammonium sulphate open to atmosphere has been carried out in order to achieve sulphation of copper, nickel and cobalt followed by leaching of the metal values as soluble sulphates with water. The effect of parameters such as temperature (200-600° C), time (15-120 min), and amount of ammonium sulphate (0.5-2.5 times stoichiometric) has been studied. Under atmospheric conditions, using 2.5 times the stoichiometric requirement of ammonium sulphate, the recovery of copper, nickel and cobalt was found to be 85%, 81% and 85%, respectively. Similar studies were carried out with sulphuric acid. The influence of experimental variables such as the amount of sulphuric acid (0.25-2 times stoichiometric), roasting temperature (100-300° C) and time (15-120 min) has been studied. Under optimum conditions, i.e., at 150° C and a roasting time of 60 min with the stoichiometric amount of sulphuric acid, recoveries of copper, nickel and cobalt were 95, 90 and 99%, respectively, along with a contamination of 60-80% iron. Removal of most of the iron from the leach liquors has been effected with ammonia liquor and lime as precipitants. A two-stage roasting operation using sulphuric acid, first at 150° C and then at 650° C, has resulted in bringing the iron content down to about 3% in the sulphation product without much affecting the recovery of other metal values.