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

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.