Removal of arsenic from Waelz zinc oxide using a mixed NaOH–Na2S leach

Arsenic was selectively leached from Waelz zinc oxide with a mixed NaOH + Na2S solution followed by hydrogen peroxide oxidation and lime precipitation. The effects of temperature, L/S ratio, leaching time and reagent concentration on the leaching of arsenic were investigated. With the mixed solution of 25 g/L NaOH and 25 g/L Na2S·9H2O and L/S ratio of 4:1, more than 90% As was extracted at 30 °C in 3 h, while the concentrations of lead and zinc in the leach solution were below 0.005 and 0.02 g/L, respectively. After recycling the leach solution and doubling the concentration of arsenic, the leach solution was treated by hydrogen peroxide oxidation followed by lime precipitation which removed 99.86% As, leaving 2 mg/L As in the solution. The filtrate was recycled with additional NaOH and Na2S·9H2O and the precipitate of calcium arsenate was solidified with cement and treated in landfill.