AMMONIA LEACHING PROCESS FOR ESCONDIDA COPPER CONCENTRATES (REPRINTEDFROM EXTRACTIVE METALLURGY OF COPPER, NICKEL AND COBALT, VOL 1, 1993)

A novel hydrometallurgical process for the treatment of copper concent rates from Escondida mine, Chile, has been developed by The Minerals L aboratory of BHP. The Escondida process consists of concentrate-leachi ng, solution-purification, electrowinning and residue-flotation stages . Part of the product will be sold as solvent-extracted, electrowon co pper of high purity and part as high-grade concentrate. In contrast to previous attempts to develop a hydrometallurgical process for copper concentrates, this patented process does not advocate the total dissol ution of copper, aiming to extract only the portion that is readily so luble in an ammoniacal or acidic lixiviant under mildly oxidizing cond itions. Both types of lixiviant were studied, but ammonia was preferre d 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. Th e mild leaching conditions also prevent the dissolution of unwanted im purities and result in the production of electrowon copper ofultra-hig h purity. Solution purification is carried out by solvent extraction w ith LIX-54. The very high loading capacity of this organic agent allow s a compact solvent-extraction circuit that consists of two loading st ages, one scrubbing stage and one stripping stage. As LIX-54 is select ive for simple copper ions, the copper ammnine complex is broken down and free ammonia is regenerated and recycled for leaching. Copper cath odes of high purity are produced in a conventional sulphuric acid-base d electrowinning system. One scrubbing step is needed between the ammo niacal and acid circuits. Limiting copper dissolution to less than 50% also provides an opportunity to produce high-grade concentrates by fl otation of the leach residues. Copper recoveries in excess of 98%, wit h precious-metal values reporting to the final concentrate, can readil y be obtained, Complete breakdown of the copper mineral-pyrite middlin gs results in a final concentrate grade that can be higher than that o f the original feed material. The Escondida process has also been desi gned to accept lower-grade copper concentrates, producing improved cop per recoveries in the mill and yet high-quality concentrates for marke ting.