A procedure for the production of spectrographically pure niobium and tantalum oxides from columbite-tantalite concentrates was developed in this study. The procedure involves a caustic fusion process and the separation of tantalum and niobium by liquid-liquid extraction.
Pierret, J. A. and Wilhelm, H. A., "Caustic fusion of columbite-tantalite concentrates with subsequent separation of niobium and tantalum" (1956). Ames Laboratory ISC Technical Reports. 152.