Nonstoichiometry of chalcocite in water-xanthate systems

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Virginia Polytechnic Institute and State University


Eh-pH diagrams were constructed from mass-balanced, computer calculations for the copper-sulfur-water system involving different Cu/S ratios that pertain to chalcocite, djurleite, anilite and covellite. Calculations were completed for cases where oxidation of the sulfur proceeded to i) elemental sulfur, ii) thiosulfate, iii) sulfate and iv) destabilized sulfate. Stability regions for each copper sulfide were shown to be dependent on both the Cu/S ratio in the system and the sulfur oxidation state.

Eh-pH diagrams were also constructed for chalcocite oxidation to metastable copper sulfides, both with and without xanthate. Stability regions for copper xanthates were also shown to be dependent on the sulfur oxidation state. As oxidation proceeded from elemental sulfur to sulfate, the copper xanthate stability region extended to lower potentials, directly dependent on the sulfide ion concentration.

IGP experiments at pH 1.1 suggested that chalcocite oxidation produced metastable nonstoichiometric copper sulfides while cyclic voltammetry indicated they formed at pH 1.1, 4.6, 6.8 and 9.2. XPS implied that copper sulfides may be solid solutions of chalcocite with variable amounts of copper disulfide: CU₂S·xCUS₂. The presence of djurleite in the chalcocite samples was confirmed by X-ray diffraction and may be responsible for the reduction reaction which occurred just prior to the reduction of chalcocite to metallic copper.

Reinterpreting cyclic voltammograms from a previous study indicated chalcocite reacted with xanthate to form cuprous xanthate and a nonstoichiometric copper sulfide near 0 mV. Chemisorbed xanthate formed at -295 mV which correlated well with the lower flotation edge determined in this and other studies. The standard free energy of the chemisorbed xanthate was determined to be -13.08 kcal/mole.