| 摘要 |
The invention relates to a process for producing simultaneously food-grade a nd fodder-grade phosphoric acid by crystallizing phosphoric acid hemihydrate, H3PO4 .times. 0.5 H2O, 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 concentra te 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 &lt ; 6, c) the phosphoric acid is concentrated, the solids precipitate is removed, a nd the F ions are evaporated, d) the feed acid obtained from step c, having a concentration of > 58% P2 O5, 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 < 0.2%, is crystallized at a steady crystal growth rate of < 10 .mu.m/min, the temperature difference in the first crystallization being < 17.degree.C, and the crys tals 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 ou t as in step d, the temperature difference being < 8.degree.C, and the crystals are wa shed with an undersaturated solution of phosphoric acid, and f) optionally the phosphoric acid crystallized in step e is melted, is dilut ed to a concentration of < 63% P2O5, seed crystals are added, and crystallization is carried out as in step d, the temperature difference being < 6.degree.C, and the crystals are washed with an undersaturated washing solution prepared from product crystals.</SDO AB>
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