Process for the production of highly concentrated phosphoric acid.

摘要

In the production of highly concentrated phosphoric acid by digesting phosphate rock with a mixed acid consisting essentially of sulfuric acid and phosphoric acid to obtain a slurry comprising phosphoric acid and hemihydrate (CaSO4.1/2H2O), then filtering the slurry taking out highly concentrated phosphoric acid as product acid and subsequently hydrating the hemihydrate to obtain gypsum (dihydrate: CaSO4.2H2O), the improvement which comprises: mixing phosphate rock into a portion of the slurry comprising highly concentrated phosphoric acid and hemihydrate after completion of the digestion to obtain a premixed slurry; forming a recycle acid using substantially the bulk of washed filtrate obtained by washing hemihydrate cake from a hemihydrate filter and also using a portion of the highly concentrated, phosphoric acid as product acid; adding concentrated sulfuric acid to the recycle acid to obtain a mixed acid consisting essentially of sulfuric acid and phosphoric acid and having a sulfuric acid concentration in this mixed acid adjusted to less than 60% by weight; adding this mixed acid to the premixed slurry and allowing digestion to take place at a temperature in the range of 70 DEG to 85 DEG C to obtain a slurry comprising highly concentrated phosphoric acid having a concentration of at least 40% P2O5 by weight and including a concentratoin of at least 0.5% excess H2SO4 by weight and hemihydrate; recycling a portion of this slurry just obtained as a premixing slurry to a premixer, while filtering another portion thereof by the hemihydrate filter to separate it into the highly concentrated phosphoric acid as product acid and hemihydrate; using a portion of the product acid just obtained for the formation of the recycle acid; while washing the hemihydrate cake on the hemihydrate filter with mother filtrate provided from a dihydrate filter; using substantially the bulk of the resulting washing filtrate from a hemihydrate filter for the formation of the recycle acid; while slurrying the washed hemihydrate cake using washed filtrate obtained by water-washing dihydrate cake from dihydrate filter and also using a two- to five-fold amount of the same slurry, based on the amount of dihydrate slurry obtained after the completion of hydration; additing concentrated sulfuric acid to the resulting slurry and allowing hydration of the hemihydrate to take place under strong agitation to obtain a dihydrate slurry; filtering this dihydrate slurry by the dihydrate filter followed by water-washing; and using the resulting mother filtrate for washing hemihydrate cake on the hemihydrate filter.