Process for production of phosphoric acid by crystallization of phosphoric acid hemihydrate

摘要

The invention relates to a process for producing simultaneously food-grade and fodder-grade phosphoric acid by crystallizing phosphoric acid hemihydrate, H₃PO₄×0.5H₂O, from a prepurified feed acid, which is purified and crystallized with the help of the following steps,;a) after the step of froth-flotation of phosphate concentrate, the concentrate is directed to a strongly magnetic separation step in order to decrease the Mg ion amount,;b) the phosphate concentrate is leached in a mixture of sulfuric acid and phosphoric acid according to the wet process, the precipitated SO₄ and As ions are removed, and a silicon source is added in order to adjust the F/Si molar ratio to 6,;c) the phosphoric acid is concentrated, the solids precipitate is removed, and the F ions are evaporated,;d) the feed acid obtained from step c, having a concentration of 58% P₂O₅, solids concentration of 0.05%, Mg ion concentration of 1.5%, SO₄ ion concentration 1%, As ion concentration of 8 ppm and F ion concentration of 0.20%, is crystallized at a steady crystal growth rate of 10 &mgr;m/min, the temperature difference in the first crystallization being 17° C., and the crystals are washed with the undersaturated mother liquor of the subsequent recrystallization step,;e) the phosphoric acid crystallized in step d is melted, is diluted to a concentration of 63% P₂O₅, seed crystals are added, and crystallization is carried out as in step d, the temperature difference being 8° 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, is diluted to a concentration of 63% P₂O₅, seed crystals are added, and crystallization is carried out as in step d, the temperature difference being 6° C., and the crystals are washed with an undersaturated washing solution prepared from product crystals.