Manufacture of wet process phosphoric acid

Claim 1

IN THE PROCESS FOR THE PRODUCTION OF PHOSPHORIC ACID ACCORDING TO THE WET PROCESS WHEREIN A FEED OF PHOSPHATIC MATERIAL AND SULFURIC ACID ARE CONTROLLED, SAID PROCESS INCLUDING THE STEPS OF INITIALLY REACTING PHOSPHATIC MATERIAL WITH RECYCLED WEAK PHOSPHORIC ACID, REACTING THE RESULTING MEANS WITH CONCENTRATED SULFURIC ACID TO OBTAIN A SLURRY OF PHOSPHORIC ACID AND GYPSUM, FILTERING SAID SLURRY IN A FILTRATION ZONE TO OBTAIN A STRONG ACID FILTRATE COMPRISING STRONG PHOSPHORIC ACID AND A REMAINING MASS COMPRISING GYPSUM, WASHING THE REMAINING GYPSUM MASS WITH A WASH MEDIUM TO OBTAIN A FIRST WEAL FILTRATE, WASHING THE NOW REMAINING MASS WITH WATER TO OBTAIN A SECOND WEAL ACID FILTRATE, USING SAID SECOND WEAK ACID FILTRATE AS SAID WASH MEDIUM, AND RECYCLING WEAK FILTRATE FOR USE AS SAID RECYCLED WEAK PHOSPHORIC ACID, THE IMPROVEMENT COMPRISING ANALYZING THE FREE SULFURIC ACID CONTANT IN A STREAM RESULTING IN THE PROCESS AND CONTAINING FREE SULFURIC ACID BY DILUTING A PORTION OF SAID STREAM WITH WATER, SUBSEQUENTLY NEUTRILIZING THE RESULTING DILUTED PORTION WITH STANDARD SODIUM HYDROXIDE BASE TO A PH OF ABOUT 4.8 AND THEN TO A PH OF ABOUT 9, CORRELATING THE DIFFERENCE IN THE AMOUNTS OF SAID BASE USED IN THE NEUTRALIZATIONS TO OBTAIN AN INDEX OF THE FREE SULFURIC ACID CONTENT OF SAID STREAM, AND REGULATING, AS REQUIRED, THE AMOUNT OF SAID CONCENTRATED SULFURIC ACID REACTED WITH SAID PHOSPHATIC MATERIAL AND RECYCLED WEAK PHOSPHORIC ACID MASS AND THE AMOUNT OF THE WASH WATER, AS REQUIRED, IN ACCORDANCE WITH THE DETERMINED FREE SULFURIC ACID CONTENT SO AS TO MAINTAIN THE DESIRED AMOUNT OF FREE SULFURIC ACID IN THE RECYCLED WEAK PHOSPHORIC ACID.