Silver, gold and palladium leaching from electronic scrap using bromine- bromide solution

Abstract

This study investigated the thermodynamics and kinetics of gold and accompanying noble metals, such as silver, palladium and platinum, leached from the electronic scrap by bromine-bromide solutions. Theoretical and practical analysis of the behavior of gold in the traditional Br2 - Br- - H2O system confirmed the fact that in a neutral and slightly acidic medium, when bromine hydrolysis proceeds slightly, a bromine in bromide solution can be used to leach gold. In this case, bromine-bromide leaching compared with cyanidation is more favorable in terms of kinetics and selectivity. In particular, when brominated in a neutral medium as a result of the formation of a passivating copper monovalent copper (Cu2O) film on the surface of copper particles, copper does not go into solution, while copper forms stable soluble anionic complexes with cyanide. For the first time in order to stabilize the pH in the alkaline area the conditions for gold leaching in the presence of a phosphate buffer solution (NaH2PO4) were investigated. The effect of the pH of the leach solution, the concentration of active bromine, bromide ions and the concentration of the buffer solution on the leaching kinetics of gold was studied. It was established experimentally that a noticeable dissolution of gold begins at pH ≤ 8, and at pH 6, almost all gold passes into the solution. A sufficient concentration of active bromine under these conditions can be considered 6.6 g · dm-3 Br2 at a bromide concentration of 20 g · dm-3 NaBr. Leaching with the productive solution turnover made it possible to reduce the consumption of bromine from 89-95 to 20-32 kg Br2 per ton of scrap and raise the concentration of gold from 76 to 195 mg · dm-3. In this paper, it was first shown that silver and palladium in the system under study (pH = 5-6) begin to dissolve noticeably only at high concentrations of bromide ions. Complete dissolution of palladium is achieved with an excess of bromide ions and pH values of 1.5-2.0. Platinum turned out to be more resistant to bromine-bromide leaching because of the formation of a sparingly soluble platinum dibromide on its surface.