Treatment of salt solutions to

Claim 1

THE METHOD OF TREATING AN ACID SULFATE SOLUTION OF A METAL OF THE GROUP CONSISTING OF ZINC, NICKEL AND COPPER CONTAINING FERRIC IRON AS IMPURITY AND AT LEAST ONE MINOR CONTAMINANT IN WHICH THE IRON IS PRESENT IN AN AMOUNT SUBSTANTIALLY NOT LESS THAN 0.5 GRAM PER LITER SO THAT UPON SUBSEQUENT PRECIPITATION OF THE IRON AS A FERRIC IRON COMPOUND THE FILTER RATE OF THE SULFATE SOLUTION WILL BE INCREASED, WHICH COMPRISES AS A FIRST STEP ADDING TO SAID SOLUTION A SOLUBLE BASE AND A PHOSPHATIC COMPOUND WHICH SUPPLIES (PO4)= IONS, SAID PHOSPHATIC COMPOUND BEING ADDED PRIOR TO THE POINT AT WHICH ANY ADDITIONAL BASE WILL BEGIN TO PRECIPITATE THE FERRIC IRON, THE AMOUNT OF SUCH ADDITION OF SAID PHOSPHATIC COMPOUND BEING LESS THAN THE STOICHIOMETRIC EQUIVALENT (AS FEPO4) OF THE FERRIC IRON IN SOLUTION AND THEN ADDING AN ADDITIONAL QUANTITY OF BASE IN A SECOND STEP SUFFICIENT TO PRECIPITATE SUBSTANTIALLY ALL OF THE FERRIC IRON IN SOLUTION, FORMING A PRECIPITATE CONTAINING SAID FERRIC IRON AND CONCOMITANTLY REMOVING SAID CONTAMINANT SUBSTANTIALLY COMPLETELY FROM THE SOLUTION BY SAID PRECIPITATE.