A fluoride-free liquid-liquid extraction process for the recovery and separation of niobium and tantalum from alkaline leach solutions

Abstract

The production of high purity niobium (Nb) or tantalum (Ta) compounds still relies on processes that require highly acidic conditions and concentrated HF or NH4F solutions. Recent progress in the chemistry of group V elements, along with the evolution of the environmental concerns, call for the development of more sustainable industrial processes for these strategic elements. In sharp contrast with the historical fluoride approach chosen by the Nb and Ta refining industry, there has been a recent surge in the literature concerning the chemistry of these elements in diluted alkaline or oxalate media. We here propose an innovative liquid-liquid extraction process that is amenable to separate Nb and Ta from alkaline feed solutions, can be operated at low acidity and that does not require any fluoride-based chemical. The developed strategy is as follows: first, Nb and Ta are co-extracted from their alkaline solution (pH 12) using a quaternary ammonium salt (Aliquat® 336) dissolved in an organic diluent; then, Nb is selectively back-extracted in a mixture of diluted nitric and oxalic acids; and finally, Ta is stripped in a diluted nitric media. The process also affords purifying Nb from Ti, Fe and Na. Following an optimization at laboratory scale, the full process has been validated during continuous running tests (160 h at 1.4 L h−1) using industrial Nb-Ta caustic solutions coming from the development of the so-called Maboumine process. The developed process works at room temperature, low acidity ([H+] < 1 M), exhibits high recovery yields, and allows producing Nb2O5·nH2O with purity higher than 99.5%. The process also yields a tantalum product (Ta2O5·nH2O with >20 wt% Ta) even when starting from a diluted feed (39 ppm Ta). The proposed strategy paves the way for new Nb and Ta hydrometallurgical processes operating under mild chemical environments and represents a concrete alternative to the current fluoride-intensive industrial methods.