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ArticleName Research of equilibrium in the system (NH4)3ScF6 – NH4HF2 – H2O
DOI 10.17580/tsm.2015.11.08
ArticleAuthor Sokolova Yu. V., Pirozhenko K. Yu.

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

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


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).

keywords Scandium, leaching, scandium fluoride, ammonium hydrofluoride, ammonium hexafluoscandate, incongruent solubility, congruent solubility

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