1. A. G. Babkin, V. G. Maiorov, and A. I. Nikolaev, The Extraction of Niobium, Tantalum, and Other Elements from Fluoride Solutions (Nauka, Leningrad, 1988) [in Russian].
2. S. N. Bhattacharyya and B. Ganguly (Nandi), Solvent Extr. Ion Exch. 2, 699 (1984).
3. V. G. Mayorov and A. I. Nikolaev, Hydrometallurgy 66, 77 (2002).
4. G. M. Ritcey and A. W. Ashbrook, Solvent Extraction. Principles and Applications to Process Metallurgy (Elsevier, Ansterdam/Oxford/New York, 1979).
5. A. I. Nikolaev, Processing of Nontraditional Raw Materials of the Kola Peninsula (Apatity, 1991) [in Russian].