Innovative processing technique to produce zinc concentrate from zinc leach residue with simultaneous recovery of lead and silver

Abstract

During the conventional zinc hydrometallurgical roast-leach-electrowin process lead and silver report in the neutral leach residue along with zinc ferrite. Lead and silver from the zinc plant leach residue are recovered by a froth flotation process followed by roast-leach-precipitation-reduction to obtain a highly pure silver. Inherent process constraints of water balance and build-up of problem impurities from the silver recovery plant to the main zinc circuit, low recovery of metal values (especially lead and silver), loss of sulphur values and high cost of flotation reagents make the Mitsubishi Materials Corporation's (MMC) silver recovery process through roast-leach-precipitation-reduction technique uneconomical. Attempts have been made to make the process more effective and attractive by removing the above process constraints. An alternative and innovative approach has been developed to achieve high recovery of lead and silver, effective utilization of sulphur values and better control of impurities. The process involves the treatment of silver concentrate generated in the flotation circuit at moderate temperature-sulfuric acid leach to partially solubilise the zinc and iron and enrich lead and silver in the leach residue. The residue is treated with brine solution to solubilise most of the lead and silver which are cemented out from the leach solution with the help of aluminium to obtain a high purity lead–silver sponge which will be fed to the silver circuit to recover the metal values through a pyro-refining route. The brine leach residue which is equivalent to high quality zinc concentrate containing high levels of zinc and sulphur and minimum levels of impurities finds application as a feed to the fluo-solid roaster. This approach is different from conventional practice. The paper describes the results of moderate temperature-sulfuric acid leaching, brine leaching, quality of zinc concentrate and lead–silver sponge obtained by this proposed route with other advantages. Based on the experimental results a conceptual flow sheet has been developed. The process is simple, eco-friendly and economically viable with excellent recoveries of lead, silver, zinc and sulphur values and can be easily implemented in the existing zinc hydrometallurgical plant.