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Modeling the cyanide heap leaching of cupriferous gold ores Coderre, Francois


A mathematical model of cyanidation heap leaching of cupriferous gold ores which includes the speciation of all cyanide, cyanate, thiocyanate, hydroxide and sulfate complexes, and their corresponding copper and gold precipitates, was developed to facilitate the treatment of these complex ores. The concepts of back mixing and solution retention are represented by two fundamentally different fluid elements: i) the bulk solution flowing through the heap, and ii) a stagnant solution reservoir adjoining and enclosed by the ore particles. Four 1.5 m tall column tests simulating actual heap leaching behavior under different nominal influent cyanide concentrations were operated for a period of up to seven months, along with semi-batch coarse ore and batch fine ore leaching tests. The resulting experimental data were used for the calibration and validation of the model. It was found that, in the presence of significant copper sulfate mineralization, the pH of the solution would naturally buffer to approximately 4.6. Further pH decrease to 3.5 is provoked by the oxidation of cyanide to cyanate by cupric ions. At such low pH levels, it is thought that cuprous and aurous cyanide salts tend to precipitate out of solution both within and around porous ore particles. As a result, neither dissolved gold nor dissolved copper from secondary copper sulfides appear in the column effluent for a significant period of time, depending on the influent cyanide concentration and the height of the column. Once dissolved again, gold appears in the column effluent, whereas its leaching kinetics are a strong function of leaching kinetics of secondary copper sulfide minerals such as covellite and chalcocite. Furthermore, the combined effect of covellite leaching and copper sulfate discharge is synergistic in that both promote the precipitation of copper hydroxide. This leads to the dispersal of a Cu(OH)₂(s) reserve throughout the column which has the effect of increasing the long term cyanide consumption. This, in turn, affects the leaching of secondary copper sulfides, and hence, of gold.

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