Manufacture

Phosphoric acid for food products can be produced either from elemental phosphorus or by the purification of wet-process phosphoric acid (9,11). Elemental phosphorus is produced by the reduction of phosphate rock with coke in the presence of silica in an electric furnace. Thermal (furnace) phosphoric acid is manufactured by the combustion and subsequent hydration of elemental phosphorus. For food use, thermal acid is treated with sulfide to remove traces of arsenic.

The North American production of technical- and food-grade phosphoric acid has shifted in the 1990s from almost exclusively thermal acid to a sizable proportion as purified wet-process acid. This shift was a continuation of a trend that had already taken place in many other parts of the world. In the wet process, phosphoric acid is made by reacting phosphate rock and sulfuric acid. The resultant acid contains percent levels of sulfate, fluoride, and metal impurities (Fe, Al, etc) and is therefore not suitable for food use. It is purified by a combination of solvent extraction and chemical treatment to yield a food-grade phosphoric acid. Phosphoric acid made by either the thermal or the wet acid purification route is typically available as 75 to 85% H3PO4.

The orthophosphate salts are made by neutralizing phosphoric acid with the appropriate base (Na2C03, NaOH, KOH, NH3, CaC03, Ca(OH)2, Mg(OH)2, etc) and separating the product by crystallization or drying. The condensed phosphate salts are made by thermal dehydration of the orthophosphate (9,11).

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