Process for extending the life of a displacement plating bath

A process for displacement plating a substrate metal surface with an other metal comprising the steps:

• (a) providing a reservoir of aqueous plating solution comprising;
• (i) a metal ion of a free metal, which is present in its lowest oxidation state wherein the free metal is different from the metal of the substrate surface;
• (ii) a complexing agent;
• and
• (iii) an acid;
• (b) directing a stream of the aqueous plating solution onto the substrate metal surface;
• whereby a portion of the metal ions of (i) are oxidized to ions in a higher oxidation state, and another portion of the metal ions of (i) are reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate metal ion complex dissolved in the reacted aqueous plating solution at the surface of the substrate metal;
• and
• (c) returning the plating solution to the reservoir;
• (d) repeating steps (b) and (c) for a series of substrate metal surfaces whereby the concentration of the substrate metal ion complex in the plating solution reaches a high level of about 80% to 95% of its saturation concentration which is below the concentration at which the ion complex precipitate is formed;
• and when the concentration of the substrate metal ion complex in the plating solution reaches the high level,
• (e) withdrawing a portion of the volume of the aqueous plating solution from the reservoir;
• (f) cooling the withdrawn portion to a temperature at which the substrate ion complex is insoluble and the complexing agent is soluble so that the substrate ion complex precipitates from the solution;
• (g) removing the substrate metal ion complex precipitate from the solution;
• (h) returning the solution to the reservoir;
• and
• (j) repeating steps (e) through (h) a sufficient number of times until the concentration of the substrate metal ion complex in the plating solution drops to a predetermined low level of about 80% lower of the saturation concentration.