An experimental study of the solubility and speciation of tantalum in fluoride-bearing aqueous solutions at elevated temperature

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• 作者：A. Timofeev^a ; ^{alexander.timofeev@mail.mcgill.ca} ; Art. A. Migdisov^b ; A.E. Williams-Jones^a
• 关键词：Tantalum solubility and speciation ; Hydrothermal systems ; Niobium and tantalum mobility ; Nb/Ta ratios
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2017
• 出版时间：15 January 2017
• 年：2017
• 卷：197
• 期：Complete
• 页码：294-304
• 全文大小：1204 K
• 卷排序：197

文摘

The solubility of Ta2O5 (solid) and the speciation of tantalum in HF-bearing aqueous solutions have been determined at temperatures of 100–250 °C and vapour-saturated water pressure. Tantalum is transported as the species Ta(OH)50 at low HF concentration and pH ∼1–3. At higher HF concentration, tantalum mobility is controlled by the species TaF3(OH)3− and TaF50; the presence of TaF50 is only evident at ⩽150 °C. Equilibrium constants range from −17.4 ± 0.45 to −16.4 ± 0.12 for the formation of Ta(OH)50 from crystalline Ta2O5 and from −8.24 ± 0.64 to −8.55 ± 0.68 for the formation of TaF3(OH)3− at 100 and 250 °C, respectively. For TaF50, they were determined to be 0.13 at 100 °C and −0.35 at 150 °C.In many respects, the behaviour of tantalum in acidic fluoride-bearing solutions is similar to that of niobium. The solubility of Ta2O5 (solid) is not dependent on HF concentration in fluoride-poor fluids, but rises rapidly at higher HF concentration. However, at the conditions of our experiments, namely a pH of ∼2, temperature up to 250 °C, and a wide range of HF concentrations, Ta2O5 (solid) solubility is almost invariably lower than that of Nb2O5 (solid). Modelling of Nb and Ta leaching confirmed the preferential mobility of niobium under most conditions expected in natural fluoride-rich hydrothermal systems. This modelling also demonstrated that both niobium and tantalum are rapidly deposited upon removal of fluoride from an acidic brine. As a result of hydrothermal alteration, the Nb/Ta ratios of secondary minerals may increase relative to those of the primary mineral, or remain largely unaffected, depending on the pH of the fluid.