Production of Zn powder by alkaline treatment of smithsonite Zn–Pb ores

Abstract

In this work, a production process for Zn powder by alkaline treatment of an oxidized Zn ore, smithsonite (ZnCO 3), was studied. It was 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., was negligible. Leaching of Pb can be improved remarkably with addition of NaCl to the leaching systems. Typical composition of the leach solution is 22–25 g/L Zn, 2.96–3.05 g/L Pb and 0.5–0.7 g/L Al. Zn and Pb contents in the leaching residue were found to be lower than 2.4% and 0.05–0.2%, respectively. Pb present in the leach solution is separated with the addition of sodium sulfide. 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. The process developed is considered to be cost-effective, simple and easy to be operated and managed, and the flowsheet for the production of Zn from ores is presented. It is proposed that ores be broken into particles smaller than 0.1–0.5 mm. Then, leached with 5 M NaOH solution at a 90–95°C temperature for 1.5 h at a phase ratio [NaOH solution added (ml) to the ore leached (g)] of 6–7. After filtration, the leach solution is subjected to electrolysis for the production of metallic Zn after Pb is separated by sodium sulfide. The NaOH solution is recycled to the next leaching operation after most of the Zn is electrowon. To save energy and keep heat loss minimal, all operations can be conducted in an adiabatic environment, including leaching, filtration and electrolysis. Loss of NaOH was estimated to be less than 50 g for the production of 1 kg of Zn, assuming that most NaOH left in the leaching residues and precipitates of lead sulfide could be recovered by washing with dilute NaOH solution and water. However, loss of NaOH is dependent on the type and composition of the ores. The presence and leaching of silicates, carbonates and phosphates in ores would increase the loss of NaOH and deteriorate filtration of leach residues in the alkaline solutions. Therefore, most of the operational cost for this process comprises the consumption of electricity for electrolysis, NaOH, and energy required for heating.