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

Yu. V. Sokolova, Leading Researcher
K. Yu. Pirozhenko, Post-Graduate Student, e-mail: pirozhenkok@yandex.ru

Rough concentrates, containing from tenths to few percents of scandium (hydrate, carbonate or fluoride settings) are obtained on the first stage of accompanying scandium extraction during the complex processing of rare metal ores (tungsten, niobium and tantalum, uranium, titanium, zirconium) and other raw materials. Further processing of these concentrates shows the reasonability of application of the reagents, making possible the scandium leaching without transfer of the basic accompanying elements (aluminium, calcium, rare-earth elements, thorium, etc) into solution. Equilibrium in the system (NH₄)₃ScF₆ – NH₄HF₂ – H₂O at the temperatures of 18 and 90 °С was investigated for the purpose of definition of the parameters of the process of scandium leaching from its concentrates by NH₄HF₂ solution. Investigations used the (NH₄)₃ScF₆, obtained during the interaction of ScF₃ with NH₄F solution. Isotherms of (NH₄)₃ScF₆ solubility were made for the interval of concentrations of 0.05–3.50 mole/l of NH₄HF₂ and the temperatures of 18 and 90 °С. Equilibrium in the system was reached during 2.5–3 hours. There was made a definition that dissolution of (NH₄)₃ScF₆ in the investigated conditions is incongruent. The compositions of obtained settings of bottom phases were investigated. For the first time, a new phase of (NH₄)₅Sc₃F₁₄ compound was isolated from the solution; its existing areas and crystallographic characteristics were defined. In both cases, dissolubility isotherms have the expressed maximums. At the temperature of 18 °С, the maximum corresponds to the concentration of 1 mole/l of NH₄HF₂ and 0.137 mole/l of Sc. Setting at the maximum point is shown by (NH₄)₃ScF₆; while settings above and below the maximum point are shown by (NH₄)₅Sc₃F₁₄ and (NH₄)₃ScF₆, respectively. Dissolubility isotherm has two maximums at the temperature of 90 °С: concentration of 0.4 mole/l of NH₄HF₂ (0.253 mole/l of Sc, setting — (NH₄)₅Sc₃F₁₄; above the maximum point in the setting — NH₄ScF₄), and concentration of 1.5 mole/l of NH₄HF₂ (0.311 mole/l of Sc, setting — (NH₄)₃ScF₆ with impurity of (NH₄)₅Sc₃F₁₄; with NH₄HF₂ concentration growth in the setting – initial compound).

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