1. M. Jeljyaskova, I. Sariev, S. Koralska, S. Aneva, M. Pankova and M. Jantscheva: Chem. Tech. (Leipzig) Vol. 34 (1982), p.651.
2. G. Eugenia, S. Petru George, G. Constanta, Z. Florica, P. Sergiu, G. Dumitru, A. Nedelcu, S. Costel, S. Dino, B. Florica Maria and V. Nicolae Serban: Romanian Patent, RO 71056 B 19801205, (1983).
3. J. Chen and K. Huang: Hydrometallurgy Vol. 82 (2006), p.164.
4. K. Shams, M.R. Beiggy and A. Gholamipour Shirazi: Appl. Catal. A-Gen. Vol. 258 (2004), p.227.
5. A.A. de Sá Pinheiro, T.S. de Lima, P.C. Campos and J.C. Afonso: Hydrometallurgy Vol. 74 (2004), p.77.