Kinetics and Sequential Leaching of Copper and Gold from Malachite-Bearing Oxide Ore Using a Two-Stage Sulfuric Acid and Thiosulfate System

In the treatment of gold-bearing copper oxide ores, the focus is often on copper extraction, neglecting the full potential of the other valuable metals present in these ores, such as gold. In this study, a two-stage atmospheric leaching process was developed to recover copper (Cu) and gold (Au) from a gold-bearing copper oxide ore. Acidic leaching (H2SO4) was initially employed to selectively extract Cu, followed by alkaline leaching using sodium thiosulfate (Na2S2O3) to extract Au from the solid residue. Various parameters were studied to achieve mild optimal leaching conditions for enhanced copper and gold extraction. Copper extraction of 96% was obtained after 2 h of leaching at 60 °C with a sulfuric acid concentration of 0.5 M. Under the same conditions, the majority of the iron was immobilized in the solid residue. In the second stage, ammoniacal thiosulfate leaching was applied to extract gold from the residue. Under optimized conditions with the addition of ammonium dihydrogen phosphate (ADP), gold extraction reached 88% after 4 h of leaching. The selective leaching of Cu and Au is due to the difference in their electrochemical behavior, in which gold remains in its metallic state across a wide range of acidic conditions due to its high redox potential, while copper, with a lower potential, dissolves readily in sulfuric acid. Kinetic study showed that the dissolution of malachite was represented by a shrinking core model with a diffusion-controlled mechanism. The activation energy (Ea) of the leaching reaction was calculated to be 21.8 kJ/mol. To further interpret the mechanism of gold dissolution in thiosulfate medium, kinetic modeling was performed. Based on these findings, a process flow was proposed for the recovery of copper and gold from a gold-bearing copper oxide ore. Graphical Abstract