Kinetics and Iron Selectivity of Copper and Zinc Extraction from Fayalitic Copper Smelter Slag by Atmospheric Sulfuric Acid Leaching

The dump slags of copper smelters contain considerable amounts of Cu and Zn, but hydrometallurgical valorization is hindered by the co-dissolution of iron. The present work investigates the effects of acid concentration, temperature and hydrogen peroxide on the distribution of Cu and Zn relative to Fe during atmospheric sulfuric acid leaching of a fayalitic dump slag from the Balkhash copper smelter (Kazakhstan; 0.9 wt% Cu, 3.7 wt% Zn, 36.7 wt% Fe). At 1.5 mol/L H2SO4, 60 °C and 180 min, 87.1% Cu and 76.4% Zn were extracted together with 16.5% Fe, corresponding to a Cu/Fe extraction ratio of 5.3. Raising the temperature to 80 °C nearly doubled Fe dissolution (32.3%), whereas 2.0% (v/v) H2O2 enhanced Cu and Zn extraction by just 5.7 and 9.9 percentage points, respectively, but enhanced Fe extraction to 52.7%, reducing the Cu/Fe ratio to 1.8. The first stage (X < 0.65) followed the chemical-control variant of the shrinking-core model with apparent activation energies of 29.6 (Cu) and 26.9 kJ/mol (Zn) compatible with mixed control. Eyring analysis showed ΔH‡ of 27.0 and 24.2 kJ/mol and ΔS‡ of −236 and −244 J/(mol·K); the similar rate constants and statistically indistinguishable activation energies for Cu and Zn are compatible with release via a single rate-limiting process. For this slag type, metal-to-iron selectivity is highest for non-oxidative leaching at moderate temperatures rather than oxidant-assisted leaching.