Phosphoric acid manufacture

Claim 1

A PROCESS FOR THE DIRECT MANUFACTURE OF CONCENTRATED WET-PROCESS TYPE PHOSPHERIC ACID IN WHICH GROUND PHOSPHATE ROCK IS REACTED WITH SULFURIC ACID, ANDC CALCIUM SULFATE HEMIHYDRATE IS PRODUCED AS A BYPRODUCT, WHICH COMPRISES THE STEPS OF SIMULTANEOUSLY INTRODUCING GROUND PHOSPHATE ROCK AND WEAK PHOSPHORIC ACID INTO A PREMIXER VESSEL, SAID WEAK PHOSPHORIC ACID INTRODUCED FROM A LATER-MENTIONED FILTERING STEP; MIXING SAID GROUND PHOSPHATE ROCK AND SAID WEAK PHOSPHORIC ACID IN SAID PREMIXING VESSEL; INTRODUCING SAID MIXED PHOSPHATE ROCK AND WEAK PHOSPHORIC ACID INTO A SLURRY AND BENEATH A LAYER OF FOAM MAINTAINED OVER THE SURFACE OF SAID SLURRY IN A DIGESTER VESSEL, SAID SLURRY AND SAID FOAM FROM LATERMENTIONED SLURRY AND FOAM MAINTENANCE SEPS, RESPECTIVELY; MAINTAINING IN SAID DIGESTER VESSEL A SLURRY RESULTING FROM THE REACTION OF SULFURIC ACID, SAID WEAK PHOSPHORIC ACID AND SAID PHOSPHATE ROCK, SAID SULFURIC ACID INTRODUCED INTO SAID DIGESTER VESSEL FROM A LATERMENTIONED ADDITION STEP; MAINTAINING A LAYER OF FOAM ON THE SURFACE OF SAID SLURRY IN SAID DIGESTER VESSEL, SAID FOAM BEING GENERATED BY THE LIBERATION OF GASES BY THE REACTION BETWEEN SAID SULFURIC ACID AND SAID GROUND PHOSPHATE ROCK; SIMULTANEOUSLY SUPPLYING SAID SULFURIC ACID TO SAID DIGESTER VESSEL IN THE ABOVE-MENTIONED ADDITION STEP BY APPLYING SAID SULFURIC ACID ONTO THE UPPER SURFACE OF SAID FOAM LAYER IN A MANNER DESIGNED TO MINIMIZE PENETRATION THERETHROUGH; SIMULTANEOUSLY SLOWLY AGITATING SAID FOAM LAYER AND SAID SLURRY WHEREBY THE AGITATION PROVIDED THERETO IS GENTLE ENOUGH TO AID IN THE DISTRIBUTION OF SAID SULFONIC ACID IN SAID FOAM LAYER AND TO KEEP THE SOLIDS IN SAID SLURRY SUSPENDED, BUT NOT SO VIGOROUS AS TO CAUSE RAPID MIXING OF THE SULFURIC ACID IN THE FOAM WITH THE SLURRY PHASE REMOVING SAID SLURRY FROM SAID DIGESTER VESSEL; FILTERING SAID REMOVED SLURRY; RETURNING WEAK PHOSPHORIC ACID FROM THE ABOVE-MENTIONED FILTER STEP TO SAID PREMIXER VESSEL; AND RECOVERING FROMS SAID FILTERING STEP A WET-PROCESS PHOSPHORIC ACID PRODUCT CONTAINING UP TO ABOUT 50 PERCENT P2O5, SAID PROCESS CHARACTERIZED BY THE FACT THAT THE CALCIUM SULFATE BYPRODUCT CRYSTALS THEREFROM ARE PRODUCED SUBSTANTIALLY IN THE HEMIHYDRATE FORM, SAID HEMIHYDRAE FORM BEING STABLE AND RESISTIVE TO HYDRATION DURING THE SUBSEQUENT FILTERING AND WASHING THEREOF.