1. Vieille Montagne, Belgian Patent 724, 214, dated 20th November 1968.
2. Det Norske Zinkkompani A/S, Norwegian Patents 108,047 dated 20th April 1965 and and 123,248, dated 2nd October 1969.
3. Proceedings of the Institute of Mining and Metallurgy Symposium;Steintveit,1971
4. Shell, German Patent 2,404,185, dated 29th January 1974.