1. US Pat. 3180746 (1965).
2. US Pat. 3454410 (1969).
3. US Pat. 3442671 (1969).
4. US Pat. 3493401 (1970).
5. D. Barby, J. A. R. Griffiths andD. Pawson, in “The Modern Inorganic Chemicals Industry”, edited by R. Thompson (Chemical Society, London, 1977) p. 320.