Pretreatment of copper anode slime with alkaline pressure oxidative leaching

• The joint process of alkaline pressure oxidation and H2SO4 leaching was proposed.
• [NaOH], temperature and oxygen partial pressure had significant effects on APOL.
• Arsenic and selenium were removed fully in alkaline pressure oxidation leaching.
• Lead sulfate was transformed into lead hydroxide in APOL, but barium sulfate was not.

Pretreatment is crucial in the process of extracting precious metals from copper anode slime. In this study, alkaline pressure oxidative leaching (APOL) followed by sulfuric acid leaching (SAL) was employed to pretreat copper anode slime. The effects of variables in APOL on leaching ratios of different metals were investigated in detail. Under the optimum conditions using NaOH solution of 2.0 mol/L, a temperature of 200 °C, an oxygen partial pressure of 0.7 MPa, a time of 3 h, a liquid-to-solid ratio of 5:1 and a stirring speed of 750 r/m, the leaching ratios of selenium and arsenic reached up to 99.0%, and that of lead was only 3.0%. Copper, tellurium and silver were not dissolved in alkaline solution. The alkaline residue was then treated with sulfuric acid solution at fixed conditions, and the leaching ratios of copper, tellurium and silver were 95.0%, 77.5% and 10.0%, respectively. The analysis results of X-ray diffraction (XRD) analysis and calculation revealed that the phase of lead sulphate in copper anode slime was easily transformed into lead hydroxide, but barium sulfate did not.