Removal of fluorine compounds from phosphoric acid

Claim 1

IN THE MULTI-STAGE METHOD FOR CONCENTRATING "WETPROCESS" PHOSPHORIC ACID WHEREIN SAID ACID IS CONCENTRATED UNDER VACUUM IN A PRIMARY CONCENTRATOR TO PRODUCE VAPOS CONTAINING WATER VAPOR AND FLUORINE COMPOUNDS MAINLY IN THE FORM OF HF AND SIF4, WHEREIN SAID ABSORBING LIQUOR TO ABSORB SAID COMPOUNDS IN SAID PRIMARY LIQUOR, AND WHEREIN SAID ACID IS TRANSFERRED TO A SECONDARY CONCENTRATOR, AN IMPROVEMENT FOR AVOIDING PRECIPITATION OF SILICA DURING RECOVERY OF FLUORINE COMPOUNDS FROM SAID VAPORS, SAID IMPROVEMENT COMPRISING THE STEPS OF: ADDING HF TO SAID PRIMARY FLUORINE COMPOUND-ABSORBING LIQUOR IN QUANTITIES SUFFICIENT TO PROVIDE AN EXCESS OF HF IN SAID PRIMARY LIQUOR AFTER ABSORPTION OF SAID VAPORS WHEREBY PRECIPITATION OF SILICA IS AVOIDED; FURTHER CONCENTRATING SAID ACID UNDER VACUUM IN SAID SECONDARY CONCENTRATOR TO PRODUCE ADDITIONAL VAPORS CONTAINING A STOICHIOMETRIC EXCESS OF HF OVER SIF4; CONTACTING SAID ADDITIONAL VAPOS WITH A SECONDARY FLUORINE-COMPOUND-ABSORBING LIQUID WHEREBY AN EXCESS OF HF IS ABSORBED BY SAID SECONDARY LIQUOR; AND TRANSFERRING A PORTION OF SAID SECONDLARY LIQUOR TO SAID PRIMARY LIQOUR TO SERVE AS A SOURCE OF HF FOR SAID PRIMARY LIQUOR