Mixed acid acidulation of phosphate rock and growth of gypsum crystals

Claim 1

IN A METHOD OF PRODUCING A CHEMICAL FERTILIZER IN WHICH ROCK PHOSPHATE IS ACIDULATED WITH A MIXED ACID SYSTEM TO FORM A SLURRY, SAID MIXED ACID SYSTEM BEING SELECTED FROM THE GROUP CONSISTING OF NITRIC ACID-SULFURIC ACID, NITRIC ACID-SULFATE, AND NITRIC ACID-SULFURIC ACID-SULFATE, THE SLURRY BEING THEN NEUTRALIZED, CONCENTRATED, GRANULATED, DRIED, AND COOLED TO FORM THE FINAL PRODUCT, AN IMPROVEMENT COMPRISING ADDING A FIRST PORTION OF THE ROCK PHOSPHATE IN A FIRST STAGE TO THE MIXED ACID IN AN AMOUNT WHEREIN THE QUANTITY OF ROCK PHOSPHATE CONTAINS CALCIUM IN AN AMOUNT BETWEEN ABOUT 80 TO 120% OF THE EQUIVALENT AMOUNT OF SULFATE ION IN THE MIXED ACID TO FORM A SLURRY IN SAID FIRST STAGE WHEREIN GYPSUM CRYSTALS ARE FORMED AND GROWN IN SAID SLURRY, AND THEN ADDING IN A SECOND STAGE THE REMAINDER OF THE ROCK PHOSPHATE TO BE TREATED TO SAID SLURRY TO COMPLETE THE TREATMENT OF THE ENTIRE QUANTITY OF ROCK PHOSPHATE AND FORM A SLURRY OF RELATIVELY LOW VISCOSITY CONTAINING GROWN GYPSUM CRYSTALS, AND SEPARATING SAID GROWN GYPSUM CRYSTALS FROM SAID SLURRY BY VIRTUE OF THEIR INCREASED PARTICLE SIZE.