Metal Finishing Guide Book


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Fig. 8. Ion exchange schematic. partment is utilized for the corrosive process solution and the adjacent compartments, which are separated by ion exchange membranes from the center compartment, operate as catholyte and anolyte compartments. During operation, anolyte/catholyte-maintenance solutions are recirculated through their corresponding cells and storage tanks. The purified process solution is pumped via a designated pump from the process tank back into the process bath. Depending on the chemistry and the specific application, ME systems are designed either with cation or with anion exchange membranes. Typical applications for the ME technology in surface-finishing operations include regeneration of etching and stripping solutions; purification and regeneration of chromium plating baths; recycling and maintenance of chrome conversion coating solutions; and reactivation and metal removal from deoxidizing solutions. Benefits of the ME technology are consistent performance and quality of etching agents and acids; constant production speed; accurate high-quality etching and chrome conversion coating results; reduced reject rate (no costly refinishing) reduced manpower requirement because of process automation; and reduced wastewater treatment and waste disposal result in lower operating cost. Diffusion Dialysis Diffusion Dialysis (DD) is also a membrane technology for separating and recovering clean acid from used or spent acid solutions. Compared to electrodialysis or ME, DD does not require an electrical potential across the membrane to effect separation. A flow schematic of a typical DD system is illustrated in Figure 7. The separation mechanism utilizes the concentration gradient between two liquids���deionized (DI) water and the used process acid���separated by a specific anion exchange membrane, which allows natural diffusion of highly dissociated acid (anions) through the polymeric membrane structure while cations (metals) are 653

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