A novel hydrometallurgieal process for the treatment of copper concentrates from Escondida mine, Chile, has been developed by The Minerals Laboratory of BHP. The Escondida process consists of concentrate-leaching, solution-purification, electrowinning and residue-flotation stages. Part of the product will be sold as solvent-extracted, electrowon copper of high purity and part as high-grade concentrate. In contrast to previous attempts to develop a hydro-metallurgical process for copper concentrates, this patented process does not advocate the total dissolution of copper, aiming to extract only the portion that is readily soluble in an ammoniacal or acidic lixiviant under mildly oxidizing conditions. Both types of lixiviant were studied, but ammonia was preferred because of the fast kinetics at ambient temperatures and pressures. By controlling the oxidation potential the formation of sulphate ions and, consequently, of an ammonium sulphate by-product is precluded. The mild leaching conditions also prevent the dissolution of unwanted impurities and result in the production of electrowon copper of ultra-high purity. Solution purification is carried out by solvent extraction with LIX-S4. The very high loading capacity of this organic agent allows a compact solvent-extraction circuit that consists of two loading stages, one scrubbing stage and one stripping stage. As LIX-54 is selective for simple copper ions, the copper ammine complex is broken down and free ammonia is regenerated and recycled for leaching. Copper cathodes of high purity are produced in a conventional sulphuric acid-based electrowinning system. One scrubbing step is needed between the ammoniacal and acid circuits. Limiting copper dissolution to less than 50 percent also provides an opportunity to produce high-grade concentrates by flotation of the leach residues. Copper recoveries in excess of 98 percent, with precious-metal values reporting to the final concentrate, can readily be obtained. Complete breakdown of the copper mineral-pyrite middlings results in a final concentrate grade that can be higher than that of the original feed material. The Escondida process has also been designed to accept lower-grade copper concentrates, producing improved copper recoveries in the mill and yet high-quality concentrates for marketing.
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