Also known as fluorosilicic acid is an inorganic acid with chemical formula H2SiF6. It is formed during the production of phosphoric acid, due to the presence of fluorapatite in the phosphate rock, by the reaction between sulfuric acid and apatite. Commercially available as acid aqueous solution with concentration between 15-30% It has limited application, small quantities of FSA are used, in few countries, for water fluorination and, in some cases, to produce a few inorganic fluorinated products, such as low-density aluminium fluoride (LD-AlF3 ) or synthetic cryolite. Hence the most production is discharged into the sea or stockpiled as fluorosilicate and/or in mixtures with phosphogypsum, thus causing a serious environmental problem, being FSA an extremely toxic and polluting substance.
It is estimated that 50kg of FSA 100% is generated per every ton of phosphoric acid produced.
