Patent ID: 6814949
Filing Date: 2004-11-09
Classification: C01B

Abstract:
A process for the production of food-grade phosphoric acid by crystallization of phosphoric acid hemihydrate, H3PO4Ã—0.5H2O, from a prepurified feed acid, comprising:purifying the feed acid used for crystallization by the following steps a, b and c: a) after a froth-flotation step of a phosphate concentrate, the concentrate is directed to a strongly magnetic separation step for the decreasing of an Mg ion amount, b) the phosphate concentrate is leached in a mixture of sulfuric acid and phosphoric acid according to a wet process, precipitated SO4 and As ions are removed, and a source of silicon is added in order to adjust an F/Si molar ratio to <6, c) the phosphoric acid is concentrated, a solids precipitate is removed and F ions are evaporated, and d) the feed acid obtained from step c, having a concentration of >58 wt. % P2O5, a solids concentration of <0.05 wt. %, an Mg ion concentration of <1.5 wt. %, an SO4 ion concentration of <1 wt. %, an As ion concentration of <8 ppm, and an F ion concentration of <0.2 wt. %, is crystallized at a steady crystal growth rate of <10 &mgr;m/min, corresponding to <25 wt. % crystals/hour, when seed crystals are added in an amount of at maximum 2 wt. % in the form of a 40 wt. % crystal slurry, in which an Fe ion concentration is <500 ppm and seed crystal size is <200 &mgr;m, the temperature difference in a first crystallization being <17Â° C., and the crystals are washed with an undersaturated mother liquor of a subsequent recrystallization step, e) the phosphoric acid crystallized in step d is melted, diluted to a concentration of <63 wt. % P2O5, seed crystals are added, and crystallization is carried out as in step d, the temperature difference being <8Â° C., and the crystals are washed with an undersaturated solution of phosphoric acid, and f) optionally the phosphoric acid crystallized in step e is melted, diluted to a concentration of <63 wt. % P2O5, seed crystals are added, and crystallization is carried out as in step d, the temperature difference being <6Â° C., and the crystals are washed with an undersaturated washing solution prepared from product crystals.