Fluorine from phosphoric acid manufacture

The large amount of fluorine values released from phosphate rock in the manufacture of fertilisers (qv) gives a strong impetus to develop fluorine chemicals production from this source (see Phosphoric acid and the phosphates). Additional incentive comes from the need to control the emission of fluorine-containing gases. Most of the fluorine values are scmbbed out as fluorosiUcic acid, H2SiPg, which has limited useflilness. A procedure to convert fluorosihcic acid to calcium fluoride is available (61). 

In a recent document [25] presenting techniques adopted by the French for pollution prevention, a new process modification for steam segregation and recycle in phosphoric acid production is described. As shown in Figure 9, raw water from the sludge/fluorine separation system is recycled to the heat-exchange system of the sulfuric acid dilution unit and the wastewater used in plaster manufacture. Furthermore, decanted supernatant from the phosphogypsum deposit pond is recycled for treatment in the water filtration unit. The claim was that this process modification permits an important reduction in pollution by... 

The hexafluorosilicic acid solutions produced as a byproduct in the production of phosphoric acid by the digestion of apatite [Ca (P04)2 Cap2 with 2-4% fluorine content, byproduct silicon dioxide] with sulfuric acid are important raw materials for the manufacture of fluorochemicals (e.g. manufacture of sodium fluoride NaF). The reserves of available fluorine from fluorapatite are estimated to be 327 10 t Cap2 (of which 32 10 t is in the USA), the fluorine quantities available in fluorapatite being therefore considerably greater than the fluorspar reserves. However, the industrial exploitation is still negligible. The reasons therefor are that ... 

Most phosphate rocks of the fluoroapatite - type contain a significant quantity of fluorine, usually 3%-4% F by weight. In some cases up to 60% of this can be evolved during the manufacture of wet-process phosphoric acid. The remainder of the fluorine is retained in the gypsum (depending mainly on the rock composition), and most of remainder is in the filter acid 130], The fluorine is evolved from various stages of phosphoric acid processes from the reactor slurry surface, in the flash cooler, and in the concentration plant. In a dihydrate process the proportions are ------ --------------... 

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