Phosphoric acid production

A process for producing phosphoric acid from rock phosphate which comprises:

• a. continuously feeding finely divided rock phosphate and phosphoric acid into a first mixing zone to form a slurry;
• b. continuously withdrawing said slurry from said first mixing zone and passing about 90 percent of the withdrawn slurry into a second mixing zone and the remainder of the withdrawn slurry into a third mixing zone;
• c. mixing the slurry in the second mixing zone with concentrated sulfuric acid in a stoichiometric excess of that required to precipitate the calcium ion contained in the slurry;
• d. withdrawing slurry from the second mixing zone and passing about 80-90 percent of the withdrawn slurry into the first mixing zone and the remainder of the withdrawn slurry into the third mixing zone;
• e. continuously withdrawing slurry from the third mixing zone and filtering it in a filtering zone to separate a filter cake and filtrate;
• f. withdrawing a portion of the filtrate as product;
• g. washing the filter cake with water and adding the washings to the remainder of the filtrate and returning the resultant mixture to the first mixing zone;
• h. periodically determining the calcium ion concentration in the slurry of the first mixing zone and sulfate ion concentration in the second mixing zone;
• i. adjusting the quantity of slurry returned from the second mixing zone to the first mixing zone to maintain the calcium ion concentration in the first mixing zone in the range of approximately 1.0% to 2.5% by weight;
• and
• j. adjusting the rate of sulfuric acid addition to the second mixing zone to maintain sulfate ion concentration in the slurry in said mixing zone in the range of approximately 2.0% to 4% by weight.