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Factors influencing the recovery of molybdenum during the hypochlorite leaching of low grade molybdenite-copper concentrates Mounsey, Diana Mary


An investigation into the factors influencing the extraction of molybdenum from copper sulphide molybdenite concentrates in alkaline hypochlorite solutions has been carried out. Sodium carbonate buffer components were found to act as complexing agents for the copper, enabling it to exist in solution as a cupri-carbonate species several orders of magnitude more soluble than the thermodynamically stable copper phase at pH 9.0. Completely selective molybdenum extraction is therefore only possible from solutions containing no carbonate. The removal of the carbonate produced adverse side effects for the sodium hypochlorite lixiviant as a result of copper hydroxide formation on the mineral surface which led to rapid decomposition of the hypochlorite by heterogeneous catalysis. This hydroxide salt was found to consist at least partially of tri-valent copper and it was confirmed that sodium hypochlorite has a sufficiently high redox potential at pH 9.0 to enable the oxidation of copper[sup II] → copper[sup III] to take place. Experiment also showed that a tri-valent copper carbonate compound precipitated from leaching solutions containing copper after a suitable nucleation period. It was thus established that the copper present in carbonate containing hypochlorite solutions exists as the tri-valent complex Cu(CO₃)₃³⁻; that the induction time to precipitation is a function of the total carbonate content of the system, and that the solid precipitate is also an active catalyst for hypochlorite decomposition. The leaching of molybdenite - copper concentrates in acid solutions was studied, and results showed that such a process would not be feasible due to the formation and precipitation of copper molybdate, CuMoO₄. This compound is very insoluble, but its formation was found to be suppressed by the presence of sodium bicarbonate at pH values greater than 6.0. Several other insoluble molybdate compounds were found to be capable of forming in both acid and alkaline solutions as a result of hypochlorite dissolution of impurity elements contained in the copper sulphide ores. Potential-pH diagrams constructed to show the thermodynamic stability of the molybdate salts of copper, iron, calcium, zinc, lead and cadmium are included. Calcium was isolated as being the most detrimental impurity element in this respect, due to its common occurrence in copper porphyry ores as well as its solubility in chloride containing solutions. A limited study was carried out to show that this solubility increased in proportion to the chloride content and hence the hypochlorite concentration of leaching solutions. Calcium carbonate was found to precipitate in preference to calcium molybdate, but the use of silicates and phosphates as calcium suppressants was also considered as a means of avoiding copper - carbonate complexing while maintaining good molybdenum extraction.

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