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

E. V. Bogatyreva, Professor of a Chair of Non-Ferrous Metals and Gold, e-mail: Nelen_Bogatureva@mail.ru
O. V. Khokhlova, Assistant of a Chair of Non-Ferrous Metals and Gold
E. A. Muraveva, Student of a Chair of Non-Ferrous Metals and Gold
A. V. Dolgov, Student of a Chair of Non-Ferrous Metals and Gold

Eudialyte raw material from Lovozero massif is a promising source of rare earth and zirconium raw materials in Russia, characterized by a high (2–10 times) level of moderate and severe groups of REE as compared to loparite and zirconium (up to 13% ZrO₂), hafnium, tantalum, niobium and titanium. Eudialyte is easily decomposable in acids, whilst a large amount of silicon (40–60% SiO₂) in eudialyte compositions leads to formation of amorphous silicon dioxide. It precipitates in the form of a poorly filtering gel which adsorbs REЕ and zirconium ions, thus reducing the level of their extraction into solution. In this regard, it is promising to develop alkaline decomposition of eudialyte concentrate, which enables extraction of silicon into solution, to obtain a concentrate, enriched in REM and PM, and suitable for subsequent efficient processing by using acid leaching or chlorination. To do this, it was first required — by using thermodynamic calculations — to determine which of the phases, formed during the process of alkaline composition and contain silicon, enable its most complete and selective extraction into an alkaline solution. A thermodynamic analysis on probability of alkaline decomposition of eudalyte concentrate phases — eudalyte with formation of NaZrO₃ and zirconosilicates (Na₂ZrSiO₅, Na₂ZrSi₂O₇, Na₄Zr₂Si₃O₁₂, Na₁₄Zr₂Si₁₀O₃₁, Na₄ZrSi₃O₁₀, Na₄Zr₂Si₅O₁₆, Na₂ZrSi₄O₁₁, Na₂ZrSi₆O₁₈) and also the accompanying minerals (albite, anorthite, nepheline, aegirine) was carried out in the temperature range of 298–423 K. The influence of molar ratios of Si/Zr, Na/Si and Na/Zr zirconosilicate on thermodynamic probability of their formation was determined. The extraction of silicon into solution during alkaline decomposition of eudialyte concentrate was estimated. The maximum extraction of silicon into solution (72.74%) is possible only when the Na₂ZrO₃ phase is formed. However, the formation of this phase is thermodynamically improbable. Theoretically, the level of extraction of silicon into eudialyte concentrate solution equals 23.56–27.81; 14.84–18.83; 41.54–45.78 and 50.52–54.77% during formation of thermodynamically most probable phases Na₁₄Zr₂Si₁₀O₃₁, Na₈ZrSi₆O₁₈, Na₄ZrSi₃O₁₀ and Na₂ZrSi₂O₇, respectively.

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