Fluoride-free, simple, and environmentally friendly extraction of mixed oxides of niobium and tantalum from the Nigerian and Rwandan columbite-tantalite concentrates

Abstract

We report a fluoride-free comparative study with promising results of an alkali-assisted roasting with KOH and water-based leaching of the Nigerian and Rwandan columbite-tantalite concentrates and application of guanidine carbonate to extract Nb and Ta as mixed oxides. The results proved that the alkali-assisted roasting the sample with KOH at optimum conditions, (250 °C, 100 % flux-to-concentrate ratio, 1 h), facilitated open-up of the (Nb–Ta)-minerals and induced their water solubilities. However, the Nigerian sample seems to be highly soluble relatively to the Rwandan sample. In all investigated cases, most of tin and iron in form of cassiterite and hematite and other impurities remained in the residue and their dissolutions decreased as temperature increased. Guanidine carbonate was used to extract both Nb and Ta dissolved in the pregnant leach solution of the Nigerian and Rwandan columbite-tantalite concentrate samples. The (Nb–Ta)-guanidine precipitate with purity of more than 95.0 % was recovered from leach liquor solution of −250 µm size fraction containing Nb (48.5 %), Ta (47.5 %), with Fe (2.6 %) and Mn (0.1 %) for N-Nass Con sample. For R-KMY Con sample, similar precipitate with purity of more than 94.0 % with chemical compositions of Nb (28.45 %), Ta (65.75 %) along with Fe (3.32 %) and Mn (0.77 %) was obtained from leach liquor of +250 µm size fraction. The amorphous (Nb–Ta)-guanidine precipitate was then calcined at 900 °C and converted the metals into their mixed oxides with high purity (more than 99%) and removed the guanidine as evidenced with their crystallographic peaks. The study presents a potential alternative to HF and promising opportunity for the sustainable extraction of Nb and Ta while the world is shifting toward clean energy.