EXPERIMENTAL STUDY OF Ta₂O₅ SOLUBILITY UNDER HYDROTHERMAL CONDITIONS

RELATED TO THE PROBLEM OF TANTALUM DEPOSITS IN GRANITES

Kotova N.P., Zaraisky G.P.

Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow region, Russia

kotova@iem.ac.ru, zaraisky@iem.ac.ru

This investigation is directed to solving a specific task of determining the oxide tantalum (Ta₂O₅-tantite) solubility in fluoride solutions at high temperatures and pressures for quantitative estimation of the possible hydrothermal mass transfer and precipitation of tantalum during formation of its deposits that are associated with albitized and greisenized granite (apogranite) (Beus, 1962).

The experiments on studying the dependence of pentoxide tantalum (Ta₂O₅) solubility on F-ion concentration have been carried out in HF, KF solutions of 0.01-2.0 mole/kg H₂О, NaF solutions of 0.01-1.0 mole/kg H₂О and LiF solutions of 0.01-0.3 mole/kg H₂О at Т=550^oС, P=1000 bar (buffer Co–СоО) (Zaraisky, 2005). Experiments were carried out by a capsule technique in a hydrothermal high pressure apparatus. The run duration was 15 days. The quenched solutions were analyzed for tantalum and impurities by ICP/MS and ICP/AES methods.

It has been established that the Ta₂O₅ solubility isotherms have a similar behavior in all fluoride solutions (HF, NaF, KF, LiF) and have strongly positive trend at low concentrations of fluorides (0.01 and 0.1 mole/kg Н₂О). Tantalum solubility is not very high (10^-6-10^-5 mole/kg H₂O). However, Ta₂O₅ solubility strongly increases with the increasing F-ion content. It provides the possibility for mass transfer of Ta by hydrothermal solutions. The highest tantalum content was detected in HF solutions (up to 10^-1 mole/kg Н₂О) at high concentrations of fluorides (1.0-2.0 mole/kg Н₂О). The dependence of Ta₂O₅ solubility on HF concentration is close to linear. Ta₂O₅ solubility was slightly lower in KF solutions (up to 10^-2 mole/kg Н₂О in 2.0m KF). Ta₂O₅ solubility is also high enough in NaF and LiF solutions. The equilibrium concentration of tantalum is within the limits of 10^-2 mole/kg H₂O in 0.5m NaF and 0.5∙10^-3 mole/kg H₂O in 0.1m LiF.

Temperature dependence of pentoxide tantalum solubility has been studied in HF, KF, NaF solutions with concentration 0.1; 1.0 mol/kg H₂O and in LiF solutions with concentration 0.03; 0.1 mole/kg H₂O at Т=300, 350, 400, 450, 500, 550^oС, Р=1000 bar and low oxygen fugacity (CO–CoO buffer). The run duration was 30 days at Т=300, 350^o С, 21 days at Т=400, 450, 500^oС, 15 days at Т=550^oС.

The obtained experimental data have shown that in HF solutions oxide tantalum has congruent solubility throughout the investigated range of temperatures, and its solubility is practically independent on T. The equilibrium concentration of tantalum is within the limits of 10^-2 mole/kg H₂O in 1.0m HF and 10^-4 mole/kg H₂O in 0.1m HF (Fig. 1).

Temperature dependence of Ta₂O₅ solubility in KF solutions is also slightly expressed, as well as in HF solutions (Fig. 2). However, in KF solutions, oxide tantalum has incongruent solubility. After the runs the solid products were examined by X-ray diffraction procedures and microprobe investigation. It has been shown that fluorides with formula K_xTaO_2+xF_1-x, where х is within the limits from 0.4 up to 0.6 are formed. The equilibrium concentration of tantalum is approximately 10^-3 mole/kg H₂O in 1.0m KF solutions. It is by the order of magnitude lower than that determined in 1.0m HF solutions

In NaF solutions the equilibrium concentration of tantalum does not practically change with the increasing temperature, and it is within the limits of 10^-3 mole/kg H₂O in the whole investigated range of temperatures (Fig. 3). In NaF solutions, oxide tantalum has incongruent solubility, as well as in the case of KF. Na-tantite (Na₂Ta₄O₁₁) and fluoride of Na-tantalite (Na₂Ta₂O₅F₂) are formed.

In LiF solutions at T= 300-450^oС Ta₂O₅ solubility strongly increases with the increasing temperature (Fig. 4). A temperature increase from 300 to 450^oC leads to an increase in tantalum content up to 10^-4 mole/kg H₂O in 0.03m LiF and 10^-4,5 mole/kg H₂O in 0.1m LiF. Then it practically stops to change with temperature increase. In the whole investigated range of temperatures, Ta₂O₅ is congruently dissolved.