This paper describes the development of a process for the utilisation of fluorine present in an aqueous solution of fluosilicic acid, obtained by scrubbing waste gases released during the processing of phosphate rock or phosphoric acid.
Among the many compounds which can be produced from fluosilicic acid, aluminium-fluoride, an indispensible raw material for the production of aluminium, was considered to be the most attractive.
A reaction scheme was chosen in which ammonia acted as the carrier of fluorine throughout the process. Ammonium fluoride, obtained by decomposition of fluosilicic acid with ammonia, is precipitated in the form of ammonium-fluo-aluminate or ammonium- cryolite, which in its turn is decomposed by calcination.
The aluminium-fluoride is produced in its instable II or variety which has catalytic properties.
Important features of the process are:
—its high yield;
—its complete absence of gaseous or liquid effluents;
—the fact that it is continuous.