Alkaline Hydrometallurgy of Low-Grade Smithsonite Ores

Abstract

Low-grade smithsonite ores are leached using alkaline leaching solution. The zinc and lead concentrates can be obtained from lean oxidized zinc ores by alkaline leaching followed by two-step sulfide precipitation of lead and zinc sulfides. It is found that over 85% of both Zn and Pb and less than 10% of Al can be leached from the ore when the leaching operation is conducted at over 95 °C using 5 M NaOH solution as leaching agent. The dissolution of impurities such as Fe, Ca, etc. is negligible. Leaching of Pb can be improved remarkably with addition of NaCl to the leaching systems. Optimum conditions of lead precipitation are found to be 1.8 Na2S/Pb weight ratio, 90 °C, 60 min, and those for zinc precipitation are 2.4 Na2S/Zn weight ratio, 90 °C, 180 min. According to scale-up experiment, total extractions of both lead and zinc reach above 80% from the ores, and the quality of lead and zinc concentrates can meet industrial requirements for roasting process. The Pb-free solution is then used for the electrolysis of metallic Zn using stainless steel electrodes. Zn metal powder with purity higher than 99.5% is obtained. The specific energy for the Zn electrolysis in alkaline leach solution is around 2.4–2.6 kWh/kg Zn, which is lower than the energy consumption of 3.3 kWh/kg Zn required in the conventional process of Zn electrolysis in acidic sulfate electrolytes.