Method of controlling the aluminum content of aqueous acid aluminum coating solutions

Claim 1

IN THE ART OF COATING A SUCESSION OF ALUMINUM SURFACES BY TREATING THEM WITH AN AQUEOUS ACID BATH CONSISTING ESSENTIALLY OF A SOLUTION OF HYDROFLUORIC, CHROMIC AND PHOSPHORIC ACIDS; THE METHOD OF CONTROLLING THE COMPOSITION OF THE BATH WHICH COMPRISES ADDING SAID INGREDIENTS AS REQUIRED TO MAINTAIN THE CONCENTRATION THEREOF AS DESIRED AND, AS THE BATH IS CONTINUED IN OPERATION, RETAINING THEREIN A PORTION OF THE ALUMINUM DISSOLVED FROM THE SURFACES BEING TREATED, BY WITHDRA WING SOLUTION FROM THE BATH AND DELIVERING IT TO A PRECIPITATION TANK, ADDING TO THE WITHDRAWN SOLUTION SODIUM AND POTASSIUM IONS IN AN AMOUNT WHICH IS STOICHIOMERTRICALLY SUFFICIENT TO PRECIPITATE ONLY THAT QUANTITY OF THE DISSOLVED A ALUMINUM WHICH IT IS NECESSARY TO REMOVE IN ORDER TO LEAVE IN THE SOLUTION A SUFFICIENT QUANTITY TO PRODUCE A UNIFORM COATING OVER AN EXTENDED PERIOD OF TIME, THE COMBINED MOLS OF SODIUM AND POTASSIUM BEING THREE TIMES THE MOLS OF ALUMINUM WHICH IS TO BE PRECIPITATED BUT NOT MORE, SEPARATING THE SO-TREATED SOLUTION FROM THE PRECITITATE AND RETURNING IT TO THE BATH, AND REPEATING THE PRECIPITATING PROCEDURE AS MAY BE NECESSARY TO MAINTAIN IN THE BATH THE SAID DESIRED QUANTITY OF ALUMINUM DISSOLVED FROM THE WORK.