Solvent extraction in the primary and secondary processing of zinc

The traditional roast-leach-electrowin(RLE) zinc production route was developed for the processing of sulphide ores. It can prove inefficient for some complex sulphides and is not readily applicable to the treatment of other ore types. Inherent to this route is the generation of SO2 gas that has to be fixed for environmental reasons, usually resulting in the production of sulphuric acid. These limitations have encouraged the search for alternative processing routes for both sulphides and other ores. Approximately 30% of global zinc production arises from recycled zinc. With increasing awareness of secondary zinc materials as a valuable resource and stricter environmental legislation that restricts dumping of these hazardous materials, interest in their recycling has increased1. Direct retreatment of such materials via the process of origin is not always cost effective and can deleteriously affect the operation of the primary plant due to high levels of impurity species. The development of process routes for the treatment of both primary and secondary sources of zinc has been hindered by the very stringent requirements for produciton of highpurity zinc by electrowinning (EW)2. Zinc EW from sulphate medium is extremely sensitive to the presence of trace impurities and requires a highly purified electrolyte. The selectivity of modern solvent extractants, an improved understanding of the process chemistry, and engineering innovations, have today enabled SX to provide unique advantages as a purification step ahead of the EW circuit in hydrometallurgical process flowsheets. In most of the new applications involving SX processing in sulphate media, di(2ethylhexyl)-phosphoric acid (D2EHPA) is used. This extractant is selective for zinc over most of the species deleterious to EW (Cu, Cd, Co, Ni, and the halides) and is readily stripped by acid concentrations typical of the spent tankhouse electrolyte (~180 g/l H2SO4). Circuit configurations generally include an organicphase scrub to further ensure electrolyte purity. Iron build up in the organic phase is controlled by a concentrated HCI treatment. The selectivity of D2EHPA for zinc over selected base metal and alkali cations is illustrated in Figure 1. In contrast to the zinc industry itself, zinc SX has found wider application in the refining of other base metals, providing an efficient means for the removal of zinc as an impurity species. These applications are surveyed elsewhere4. This review focuses on recent installations and developmental projects featuring zinc SX. Some historical perspective is provided by brief descriptions of earlier processes.