Process for the production of highly concentrated phosphoric acid

A process for the production of highly concentrated phosphoric acid, comprising digesting phosphate rock with a mixed acid consisting essentially of sulfuric acid and phosphoric acid to obtain a slurry comprising phosphoric acid and hemihydrate, calcium sulfate CaSO.sub.4 .multidot.1/2H.sub.2 O, then filtering the slurry, taking out highly concentrated phosphoric acid as product acid and subsequently hydrating the hemihydrate to obtain gypsum dihydrate, CaSO.sub.4 .multidot.2H.sub.2 O, the process 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 tee 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.degree. to 85.degree. C. to obtain a slurry comprising highly concentrated phosphoric acid having a concentration of at least 40% P.sub.2 O.sub.5 by weight and including a concentration of at least 0.5% excess H.sub.2 SO.sub.4 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;
• adding 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.