The invention relates to a process for producing simultaneously food-grade
and fodder-grade phosphoric acid by crystallizing phosphoric acid
hemihydrate, H.sub.3 PO.sub.4.times.0.5H.sub.2 O, 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 SO.sub.4
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%
P.sub.2 O.sub.5, solids concentration of <0.05%, Mg ion concentration
of <1.5%, SO.sub.4 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 .mu.m/min, the temperature difference
in the first crystallization being <17.degree. 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% P.sub.2 O.sub.5, seed crystals are added, and
crystallization is carried out as in step d, the temperature difference
being <8.degree. 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% P.sub.2 O.sub.5, 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.
L'invention concerne un procédé pour produire simultanément l'acide phosphorique de catégorie alimentaire et de fourrage-catégorie en cristallisant le hemihydrate d'acide phosphorique, H.sub.3 PO.sub.4.times.0.5H.sub.2 O, à partir de a prepurified l'acide d'alimentation, qui est purifié et cristallisé avec l'aide des étapes suivantes, a) après que l'étape de la mousse-flottaison du concentré de phosphate, le concentré soit dirigée vers une étape fortement magnétique de séparation afin de diminuer la quantité d'ion de magnésium, b) le concentré de phosphate est lixiviée dans un mélange d'acide sulfurique et d'acide phosphorique selon le processus humide, le SO.sub.4 précipité et pendant que des ions sont enlevés, et une source de silicium est ajoutée afin d'ajuster le rapport molaire de F/Si à 58% P.sub.2 O.sub.5, concentration en solides de
