Recovery of zinc and copper from copper smelter flue dust. Optimisation of sulphuric acid leaching

Copper smelter flue dust is a residue, rich in zinc, originating from the pyrometallurgical process of copper, and constitutes a potential raw material for zinc production. In order to valorise this waste by hydrometallurgical processes, its leaching with sulphuric acid at constant pH is studied. Experimental conditions (temperature, pH, and pulp density) are optimised by design of experiments in order to maximise Zn and Cu extractions while minimising the leaching of Fe. The experimental results are fitted to a full-quadratic second-order equation model by applying multiple regression analysis for extractions of Zn, Cu, and Fe. In order to determine the optimum conditions, the response surfaces are overlaid and the intersection of the plot of the three responses identifies conditions of pH and pulp density that satisfy recoveries of Zn higher than 95%, recoveries of Cu around 70%, and Fe extractions lower than 35%. Under the best conditions, that is, pH = 2, 10% of pulp density, and at 25°C, a liquor rich in zinc and copper, and a lead concentrate are obtained.