Arsenic removal from arsenic-containing copper and cobalt slag using alkaline leaching technology and MgNH4AsO4 precipitation

•Arsenic leaching rate was 94.5% using NaOH-H2O2 leaching method.•The leaching residues could be recovered more efficiently by metallurgy process.•Arsenate was selectively precipitated as MgNH4AsO4 from alkaline solution..•The purified alkaline solution could be circulated for the leaching process.•MgNH4AsO4 residues contributes to reducing arsenic pollution. In this study, a novel alkaline cycle leaching and MgNH4AsO4 precipitation technology was used to remove arsenic in arsenic–containing copper and cobalt slag. The phase analysis results indicated the arsenic species mostly existed in the original slag as Cu-As alloy. By adopting NaOH-H2O2 leaching method, the arsenic leaching reached as high as 94.5% under the optimum conditions. The XRD results indicated that copper in the Cu-As alloy was oxidized to be Cu(OH)2 in high concentration NaOH solution. And the optimum conditions were established as: NaOH concentration, 2 mol/L; leaching temperature, 25℃; H2O2 concentration, 5%; and liquid-solid ratio (L/S), 9:1. Thermodynamic analysis showed that it was feasible to selectively precipitate arsenate in alkaline leaching solution in the form of MgNH4AsO4. The XRD results confirmed that the arsenic in the filter residue mainly existed in the form of MgNH4AsO4·6H2O, which has good crystallinity, contributing to a better arsenic and alkali separation. Under the optimum conditions, the content of arsenic in the purified liquid and purified residue were 39 mg/L and 24.15%, respectively. Thus, the purified alkaline solution could be circulated for the leaching process, which is beneficial to the cost reduction and environment protection.