Separation of niobium and tantalum from Mozambican tantalite by ammonium bifluoride digestion and octanol solvent extraction

Abstract

Tantalite ore from Mozambique was digested using molten ammonium bifluoride (NH4HF2) as an alternative to hydrofluoric acid. Niobium and tantalum were extracted from the water-leached filtrate using 1- and 2-octanol as extractants. The main process variables are time, temperature and the bifluoride-to-ore mass ratio, for the bifluoride treatment step, along with the sulphuric acid concentration of the aqueous filtrate, and the solvent-extraction contact time. The optimum parameters obtained for the bifluoride treatment were a reaction time of 3h, a reaction temperature of 250°C, and a tantalite-to-bifluoride mass ratio of 1:30. Under these conditions, up to 97.24% of Nb and 98.66% of Ta were recovered. Introduction of sulphuric acid (H2SO4) into the aqueous phase increases the extraction of Nb and Ta with octanol. It is shown that a 5M to 6M sulphuric acid concentration results in good extraction of tantalum. An acid concentration higher than 5M must be used for efficient extraction of niobium. A contact time of 10min between the feed solution and the solvent is required. McCabe–Thiele equilibrium diagrams constructed from the experimental data suggest that two and three extraction stages are sufficient for almost complete extraction of Ta and Nb respectively, at optimum extraction conditions.