Microwave assisted chloride leaching of zinc plant residues

Microwave (MW) assisted chloride leaching was studied to remove valuable and heavy metals from two zinc plant residues, i.e. goethite sludge and Zn-leach product. For both materials, NaCl leaching parameters, such as temperature, NaCl concentration, leaching time and addition of acid, were optimized. For goethite sludge, the best efficiencies for Cu (45-47 %), Pb (83-90 %), and Zn (47-58 %) extraction, with a minimal dissolution of matrix elements, were obtained at 200 °C, 300 g/L NaCl and L/S 10. At short leaching times (5 min) the maximal leachability of Cu and Zn was reached, while at longer leaching time (60 min) the Pb extraction increased to 90 ± 1%. Zn leaching was limited due to the presence of stable franklinite (ZnFe₂O₄). NaCl (280 g/L) leaching of Zn-leach product required the addition of 1 M HCl to improve metal leaching to Ag 52 ± 3 %, Bi 83 ± 1 %, Cd 82 ± 4 %, Sb 39 ± 1 %, Zn 71 ± 2 % at 200 °C, L/S 10 for 30 min. Consequently, matrix dissolution was enhanced. Metal associations in Zn-leach product were statistically investigated. The environmental impact of the MW leached materials was evaluated by a one stage leaching test, which showed a significant overall reduction in heavy metal leachability compared to untreated materials.