Experimental Study of the Solubility of Niobium Oxide and Pyrochlore in Fluoride Solutions at 300‒550°C, 50 and 100 MPa

The effect of physicochemical parameters (solution concentrations, temperature, pressure, and redox conditions) on the solubility of natural pyrochlore (Ca, Na)2(Nb, Ta)2O6(O, OH, F) and a Nb oxide (β-Nb2O5) is experimentally studied in aqueous fluoride solutions of HF, KF, NaF, and LiF at T = 300‒550°C, P = 100 and 50 MPa, and low oxygen fugacity (Co‒CoO buffer). The initial concentration of HF and KF solutions was varied from 0.01 to 2 m, NaF solutions from 0.01 to 1 m, and the concentration of LiF solutions from 0.03 to 0.1 m. A comparative analysis of equilibrium Nb contents in fluoride hydrothermal fluids with the dissolution of pyrochlore and niobium oxide was conducted. It is found that the presence of acidic fluoride solutions and reducing conditions favor the Nb solubility and transport by aqueous fluids. It is most important that the changes in temperature and pressure under chosen T–P–fO2 conditions have minor affect on the solubility of minerals.