Fluorine (and silicon) can be removed from technical phosphoric acid (30% P205) and recovered quantitatively as a by-product by adding a sodium or a potassium salt preferably the carbonate and boiling for a few minutes. The corresponding fluorosilicate precipitate, e.g. 2Na+ + SiF6- → Na2SiF6 can be separated by filtration. If this procedure is followed, no precipitation will take place in the evaporators during concentration of acid to 40% P2O5.