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

In this work, a production process for Zn powder by alkaline treatment of a n oxidized Zn ore, smithsonite (ZnCO3), 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 degrees C using 5 M Na OH solution as leaching agent. The dissolution of impurities such as Fe, Ca , etc., was negligible. Leaching of Pb can be improved remarkably with addi tion of NaCl to the leaching systems. Typical composition of the leach solu tion is 22-25 g/L Zn, 2.96-3.05 g/L Pb and 0.5-0.7 g/L Al. Zn and Pb conten ts 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 additi on of sodium sulfide. The Pb-free solution is then used for the electrolysi s of metallic Zn using stainless steel electrodes. Zn metal powder with pur ity higher than 99.5% is obtained, The specific energy for the Zn electroly sis 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 p rocess of Zn electrolysis in acidic sulfate electrolytes. The process devel oped 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 degrees C temperature for 1.5 h at a phase ratio [NaOH solution added (ml) to the ore leached (g)] o f 6-7. After filtration, the leach solution is subjected dto electrolysis f or the production of metallic Zn after Pb is separated by sodium sulfide. T he NaOH solution is recycled to the next leaching operation after most of t he Zn is electrowon. To save energy and keep heat loss minimal, all operati ons can be conducted in an adiabatic environment, including leaching, filtr ation 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 wi th dilute NaOH solution and water. However, loss of NaOH is dependent on th e type and composition of the ores. The presence and leaching of silicates, carbonates and phosphates in ores would increase the loss of NaOH and dete riorate 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. (C) 20 00 Published by Elsevier Science B.V. All rights reserved.