Ask the Expert Question-and-Answer Archive
by George Cushnie
Removal of Iron from Pickling and Plating Baths
Q. Im looking for proven technologies for the removal of Iron from Sulphuric Acid Pickling and Tin Plating Baths. There should probably be examples in the Chrome Plating industry that would apply, but I dont know where to start looking for literature or vendors of such equipment. Any info you could provide would be much appreciated.
A. As you know, when acid pickling solutions are contaminated with dissolved iron, the free acid concentration decreases and the pickling efficiency drops. There are two proven and relatively widely used technologies for removing iron from pickling baths, acid sorption and diffusion dialysis.
- Acid sorption is a separation process where an acid is separated from its salts by using an ion exchange column containing a strongly basic anion exchange resin of a specific porosity and particle size. It is marketed by Eco-Tec in Ontario Canada.
- Diffusion dialysis is an ion exchange membrane technology. The diffusion dialysis process separates acid from its metal contaminants via an acid concentration gradient between two solution compartments (contaminated acid and deionized water) that are divided by an anion exchange membrane. Acid is diffused across the membrane into the DI water whereas metals are blocked due to their charge and the selectivity of the membrane. This technology is sold by several companies, including Pure Cycle and Zero Discharge Technologies.
The primary source of tin and tin–lead bath contamination is metal impurities from anodes. These contaminants can be partially avoided by purchasing high-quality/low-impurity anodes.
Dissolved metal impurities can be removed using low current density electrolysis (dummying). Stannic ion is an unavoidable contaminant that must be maintained at relatively low levels by performing continuous filtration and chemical treatment. Fluoborate baths are particularly subject to buildups of sludge. Therefore, continuous in-tank or external filtration during operation is recommended with these solutions. Organic bath additives are decomposed during plating and will concentrate in the bath. A carbon treatment can be performed on tin–acid and tin–lead baths to eliminate all organics. Fresh organic additive is then reintroduced.
Please let me know if Ive answered your questions and if this information was useful or not.