Process for manufacture of phosphoric acid

Claim 1

IN A PROCESS OF CONVERTING A CALCIUM SALT OF PHOSPHORIC ACID INTO PHOSPHORIC ACID AND A PRECIPITATE OF CALCIUM SULFATE BY REACTION WITH SULFURIC ACID, THE IMPROVEMENT WHICH COMPRISES SEPLARATING THE MIXTURE OF PHOSPHORIC ACID AND PRECIPITATE OF CALCIUM SULFATE INTO A PHOSPHORIC ACID PORTION SUBSTANTIALLY FREE FROM PRECIPITATE AND A RESIDUE IN THE FORM OF A SLURRY CONTAINING CALCIUM SULFATE PRECIPITATE AND A SUBSTANTIAL AMOUNT OF LIQUID PHOSPHORIC ACID, SUBJECTING SAID RESIDUE TO THE ACTION OF A SULFURIC ACID SOLUTION, MAINTAINING THE MIXTURE OF SULFURIC ACID SOLUTION AND SAID RESIDUE FOR A SUFFICIENT LENGTH OF TIME TO CONVERT THE CALCIUM SULFATE PRECIPITATE TO CALCIUM SULFATE ANHYDRITE AT TEMPERATURES WITHIN THE RANGE OF THE BOILING POINT OF THE SULFURIC ACID SOLUTION TO 60*C., THE TEMPERATURE OFR THE MIXTURE BEING SO CORRELATED WITH THE CONCENTRATION OF SULFURIC ACID IN THE SOLUTION THAT AT A TEMPERATURE OF THE BOILING POINT OF THE SOLUTION SAID SOLUTION CONTAINS AT LEAST 15% SULFURIC ACID WITH THIS MINIMUM LIMIT INCREASING TO 33% FOR A MIXTURE AT 60*C., SEPARATING SAID ANHYDRITE FROM THE LIQUOR, AND RETURNING SAID LIQUOR TO THE CALCIUM SALT OF PHOSPHORIC ACID FOR FURTHER CONVERSION INTO PHOSPHORIC ACID AND A PRECIPITATE OF CALCIUM SULFATE.