Treatment of by-product gypsum

Claim 1

IN THE WET PHOSPHORIC ACID PROCESS WHICH INCLUDES THE STEPS OF REACTING PHOSPHATE ROCK WITH SULFURIC ACID IN A FIRST REACTION STATION TO YIELD A SLURRY OF BY-PRODUCT GYPSUM SOLIDS IN LIQUID PHOSPHORIC ACID AND FILTERING SUCH SLURRY AT A FILTRATION STATION TO RECOVER PHOSPHORIC ACID AS FILTRATE AND IMPURE BY-PRODUCT GYPSUM AS FILTER CAKE, SAID GYPSUM CONTAINING RESIDUAL SOLUBLE P2O5, AN IMPROVED METHOD OF TREATING SAID BY-PRODUCT GYPSUM TO REMOVE IMPURITIES AND TO RECOVER RESIDUAL P2O5 THEREFROM, SAID IMPROVEMENT COMPRISING THE STEPS OF REMOVING SUCH FILTER CAKE FROM THE FILTRATION STATION, REPULPING SAID CAKE WITH LIQUOR TO FORM A SUSPENSION HAVING FREE SETTLING CHARACTERISTICS WHILE EFFECTING REMOVAL OF SOLUBLE P2O5 FROM SAID CAKE BY DISSOLVING SAID P2O5 IN SAID LIQUOR, SUBJECTING SAID SUSPENSION TO HYDRAULIC CLASSIFICATION TO YIELD AN OVERFLOW FRACTION COMPRISING FINE SOLID IMPURITIES SUSPENDED IN ONE PORTION OF SUCH P2O5 BEARING LIQUOR AND AN UNDERFLOW FRACTION COMPRISING SUBSTANTIALLY PURE GYPSUM SUSPENDED IN ANOTHER PORTION OF SUCH LIQUOR, RECOVERING SUBSTANTIALLY PURE GYPSUM FROM SAID UNDERFLOW FRACTION, SEPARATING SAID FINE SOLID IMPURITIES FROM SAID OVERFLOW FRACTION TO YIELD A SUBSTANTIALLY SOLIDS-FREE LIQUOR CONTAINING SOLUBLE P2O5, TRANSFERRING SAID SOLIDS-FREE LIQUOR TO SAID FILTRATION STATION AND THERE PASSING IT THROUGH SAID IMPURE GYPSUM FILTER CAKE TO WASH THE SAME THEREBY TO EFFECT RECOVERY OF RESIDUAL PHOSPHORIC ACID, AND THEN FLOWING THE RESULTING FILTRATE CONTAINING P2O5 AND PHOSPHORIC ACID-BEARING LIQUOR TO THE FIRST REACTION STATION FOR FURTHER REACTION WITH SULFURIC ACID.