1. DT-OS 2.162.951; B-PS 792.928; DL-PS 102.391; GB-PS 1.407.257; ES-PS 409.725.
2. I. Chem. E. Symposium Hydrometallurgy, Series No. 42;Kroebel,1975
3. Lewatit OC 1026, brochure Bayer AG, Leverkusen; Order No. OC/I 20208.
4. Private communication Dr. Roever, Duisburger Kupferhütte.