ABSTRACT
The use of solvent extraction as a unit operation in hydrometallurgy now extends to a wide range of metals from a variety of feed materials including low-grade ores, scrap and waste, and dilute aqueous solutions. The technology was pioneered in the 1940s for the extraction of uranium from its ores and, later, for the treatment of wastes from spent reactor fuel, still an important use of the technique today (see Chapter 12). The knowledge gained led to processes for the recovery of other high-value metals and the separation of elements such as the rare earths, zirconium-hafnium, and niobium-tantalum that, before the introduction of this technology, could be separated only by lengthy batch techniques with many recycle steps to obtain the desired purity. Gradually, solvent extraction was seen to have applications for the recovery of other less valuable, but important, metals such as cobalt and nickel. At the time, the process was confined to rather small operations. It was only after the development of selective chelating acidic reagents in the 1960s that liquid-liquid extraction was seen to be a commercially viable addition to the unit operations of hydrometallurgy and was able to compete with alternative processes like cementation.†Liquid-liquid extraction is now an economic alternative to pyrometallurgy for metal extraction, especially when physical beneficiation of the ore to provide a suitable concentrate for smelting is difficult. The alternative hydrometallurgical route (Fig. 11.1) involves leaching of the ore to provide a leachate that, after any necessary solid-liquid separation, can be fed to the extraction circuit. Control of relative liquid flows allows the concentration of the desired metal and, thereby, economic processes for the recovery of metals from dilute aqueous wastes such as mine waters and wash solutions from the metal-plating industries.
