Alkaline process for the manufacture of cryolite

Claim 1

A PROCESS FOR THE MANUFACTURE OF CRYOLITE OF SPECIFICATION GRADE WHICH COMPRISES PREPARING AN IMPURE AMMONIUM FLUORIDE SOLUTION CONTAINING FLUORIDE, PHOSPHORUS, SILICON AND IRON COMPOUNDS IN SUCH PROPORTIONS THAT THE WEIGHT RATION F:P2O5IS 25:1 TO 125:1, THE WEIGHT RATIO OF F:SIO2 IS AT LEAST 60:1, AND THE WEIGHT RATIO OF F:FE2O3 IS AT LEAST 100:1? HEATING THE SOLUTION TO BOILING, ADDING SODIUM HYDROXIDE AND A SOLUBLE ALUMINUM SALT TO THE SOLUTION IN SUCH PROPORTIONS AS TO BRING THE SODIUM AND ALUMINUM CONTENT OF THE SOLUTION INTO THE RANGE FROM ABOUT 90 TO 100 PERCENT OF THE QUANTIES THEORETICALLY REQUIRED TO FORM NA2ALF6 WITH ALL FLUORINE PRESENT IN THE SOLUTION, ADJUSTING THE QUANTITIES OF SAID ADDED SODIUM AND ALUMINUM IN DEPENDENT RELATIONSHIP TO THE WEIGHT RATIO F:P2O5 SUCH THAT SUBSTANTIALLY 90 PERCENT OF SAID THEORETICAL QUANTITIES IS PRESENT AT A WEIGHT RATIO OF ABOUT 25:1 AND THAT THE QUANTITIES OF SAID ADDED SODIUM AND ALUMINIUM ARE SIMULTANEOUSLY INCREASED IN PROPORTIONATE RELATIONSHIP SUBSTANTIALLY CORRESPONDING TO PROPORTIONAL INCREASE IN THE WEIGHT RATIO F:P2O5 IN SAID SOLUTION TO A MAXIMUM OF APPROXIMATELY 100 PERCENT OF THE THEORETICAL QUANTITY AS THE WEIGHT RATIO F:P2O5 BECOMES GREATER THAN ABOUT 125:1, NEUTRALIZING THE SOLUTION TO THE PHENOLPHTHALEIN END POINT BY ADDITION OF CARBON DIOXIDE: COOLING THE SOLUTION, SEPARATING A RESULTING PRECIPITATE OF NA3AIF5: WASHING THE PRECIPITATE, AND RECEIVING A CRYOLITE OF SPECIFICATION GRADE.