National University of Science and Technology MISiS, Moscow, Russia:

Yu. V. Sokolova, Professor at the Department of Energy Efficiency and Resource Saving of Industrial Technologies, Doctor of Technical Sciences, e-mail: sokolova.yv@misis.ru

E. V. Bogatyreva, Professor at the Department of Non-Ferrous Metals and Gold, Doctor of Technical Sciences, e-mail: Нelen_Вogatureva@mail.ru

Calcium molybdate forms powellite, it is produced as a result of oxidizing roasting of off-grade molybdenum sulphide concentrates and other molybdenum materials with calcium additives (calcium oxides and hydro xides, calcium chlorides) in air at the temperatures of 550–600 ^oC. Use of Na₂CO₃ solutions enables an efficient recovery of Мо from CaMoO₄ and a quantitative removal of impurities. To determine the optimum conditions for this process, one would need data on CaMoO₄ leaching within a broad range of Na₂CO₃ concentrations and at high temperature and one would need to analyze the composition of the solid phase and the kinetic parameters of the process, i.e. rate and rate-controlling step. The authors look at the CaMoO₄ leaching kinetics in 1.0–2.5 mol/l Na₂CO₃ solutions at 60–90 ^oC. It was found that the process rate is dictated by the stirring intensity and tends to increase with a rising temperature and the reagent concentration rising in the range of 1.0–1.5 mol/l. A higher concentration of Na₂CO₃ has no effect on the reaction rate. An apparent reaction order was determined in the Na₂CO₃ concentration range of 1.0–1.5 mol/l. An equation is proposed for calculating the CaMoO₄ dissolution rate for the Na₂CO₃ solution and the temperature of 80 ^oC. It was established that a kinetic mode of leaching takes place in the soda concentration range of 1.0–1.5 mol/l amid intensive stirring. It is demonstrated that, within the studied Na₂CO₃ concentration range, calcite СаСО₃ is formed after vaterite, a less stable phase of calcite, with crystallization of double sodium-calcium carbonates Na₂Са(CO₃)₂·nH₂O (n = 0, 2, 5) occurring at the same time. With the concentration of soda being >1.5 mol/l, the process is controlled by internal diffusion. In this region, the leaching rate is independent of the Na₂CO₃ concentration. Formation of double carbonates is associated with an additional consumption of soda. Therefore, when using this system one should consider how CaMoO₄ typically dissolves in Na₂CO₃ solutions. The presence of these compounds in the soda solution after molybdenum leaching may impact the recovery of Мо from the solution using the known techniques. It may also hinder the recirculation of sodium carbonates going for the second leaching cycle.

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