1. A.R. Tron:Niobium Mining Ann. Rev., 1986, p. 74.
2. W.W. Albrecht and W. Rockenbauer:Modern Extraction of Tantalum and Niobium with Special Emphasis on the Production of High Purity Compounds, in Proc. Int. Symp. on Mining, Processing, Applications and New Developments in Tantalum and Niobium, Nov. 7-9, 1988, Orlando, FL, pp. 219–40.
3. T. Sugai and R. Watanabe:Extraction of Niobium Metal by Chlorine Process, in Proc. Int. Symp. on Mining, Processing, Applications and New Developments in Tantalum and Niobium, Nov. 7–9, 1988, Orlando, FL, pp. 241–54.
4. Nabuaki Sato and Michio Nanjo:Metall. Trans. B, 1985, vol. 16B, pp. 639–44.
5. B.R. Steele and D. Geldart:Extraction and Refining of Rare Metals, Proc. Symp., Inst. Min. Metall., London, 1956, pp. 287–309.