Process for recovering strong hf from phosphate rock digestion processes

Claim 1

A PROCESS FOR THE RECOVERY OF FLUORINE VALUES IN THE FORM OF STRONG HYDROFLUORIC ACID FROM A WET PROCESS PHOSPHORIC ACID PLANT OR A PHOSPHATE FERTILIZER PLANT WHEREIN PHOSPHATE ROCK IS DIGESTED WITH ABOUT 70 TO 80% SULFURIC ACID AND THE FLUORINE VALUES ARE GIVEN OFF IN THE FORM OF SILICON TETRAFLUORIDE AND FLUOSILICIC ACID, COMPRISING: (1) CONTACTING SAID SILICON TETRAFLUORIDE AND FLUOSILICIC ACID WITH SUFFICIENT WATER TO CONVERT ALL OF SAID FLUORINE VALUES TO WEAK FLUOSILICIC ACID CONTAINING LESS THAN ABOUT 30% FLUOSILICIC ACID AND PRECIPITATING SILICA, (2) FILTERING THE RESULTING SLURRY TO REMOVE THE SILICA THUS FORMED, LEAVING A WEAK FLUOSILICIC ACID SOLUTION, (3) CONTACTING SAID WEAK FLUOSILICIC ACID SOLUTION WITH STRONG SULFURIC ACID IN AN AMOUNT SUFFICIENT TO YIELD A 70 TO 75% SULFURIC ACID BASED ON ALL OF THE WATER PRESENT IN THE AQUEOUS SOLUTION OBTAINED, SAID CONTACTING WITH STRONG SULFURIC ACID DECOMPOSING THE FLUOSILICIC ACID TO HF AND GASEOUS SILICON TETRAFLUORIDE. (4) RECYCLING SAID GASEOUS SILICON TETRAFLUORIDE TO THE SCURBBING OPERATION OF STEP 1, ABOVE, AND (5) SEPARATING THE HF FROM THE SULFURIC ACID AT A TEMPERATURE LOWER THAN THE BOILING POINT OF THE MIXTURE AT ATMOSPHERIC PRESSURE SO AS TO OBTAIN A HYDROFLUORIC ACID OF HIGHER CONCENTRATION THAN THE HFWATER AZEOTROPIC COMPOSITION AND A SEPARATE SULFURIC ACID SOLUTION HAVING A CONCENTRATION BETWEEN ABOUT 70 AND 80% H2SO4 SUITABLE FOR USE IN DIGESTION OF ADDITIONAL PHOSPHATE ROCK.