Except in unusual circumstances, copper ores must be beneficiated prior to smelting. In leaching however, ores are often processed without prior beneficiation, and beneficiation may not even be considered as a preparatory step.
With mixed ores, flotation has been used to segregate the oxide and sulphide components1 to avoid sulphur deficiency in the smelter, but for such processing, the chief oxide mineral must be malachite. Oxide ores of chrysocolla and other copper silicate minerals unfortunately cannot be concentrated by conventional upgrading techniques.
Only two examples are available, each having unique and unusual features. At nchanga, zambia,2 low-grade (4-10% cu) oxide concentrates and high-grade (0.4-0.6% cu) tailing material consisting of chrysocolla and cupriferous vermiculite were stock-piled for many years, to await future processing. Today, modern solvent extraction technology is employed to economically recover copper from these materials.
At twin buttes, arizona,3 flotation can be used to recover up to 80% of the copper in oxide ores at a ratio of concentration of 5 to 10. this result was considered unacceptable, with acid consumption varying between 200 and 400 lb per ton, and so an alkali leach process was developed. Currently the availability of cheap acid allows direct leaching with sulphuric acid. The twin buttes experience suggests that the concentrategrade constraint should be examined. Certainly, a decline in acid consumption together with a reduction in leach plant size might justify the addition of a beneficiation plant. However, the process conditions at which beneficiation is viable need to be defined.
In this light, this paper presents an economic model for treating oxide copper ores. The model is derived from available data on current operating plants and examines two cases where beneficiation could be applied: 1) in the development of a new orebody, and 2) by the addition of a beneficiation plant to an existing operation.
Capital cost items
The capital costs used are shown in table 1. all costs have been inflated into 1979 us dollars using the marshall and stephens equipment index from chemical engineering.
Mining costs were obtained from bell copper in british columbia.6 this cost estimate can be viewed as high, because an oxide deposit of similar size would be unlikely to require the amount of site preparation required at the bell copper mine, where preproduction stripping totalled 10 million st.
Crushing and grinding costs are taken from the gibralter mine in british columbia.7 the work index of an oxide ore would probably be much lower than gibralter ore, but the required product size would be finer. Thus scale-up is related to tonnage rate alone. In addition, for simplicity, the crushing and grinding requirements for direct leaching have been assumed to be equivalent to those for beneficiation.
The capital costs of the beneficiation process are based on data for the magnex process.8 since this item is a key issue, the sensitivity of the model to this variable is examined.
Leach plant costs are based on the tailing treatment plant commissioned in 1973 at nchanga in zambia.9 this source was especially useful, because separate costs were available for acid handling facilities. As a result the capital costs have been scaled according to tonnage rate and acid consumption.
The costs for the solvent extraction plant and electrowinning tankhouse are also taken from nchanga. In this case, the cost is scaled to copper production rate.
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