Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 553 of 707

Fig. 8. Ion exchange schematic. partment is utilized for the corrosive process solution and the adjacent com- partments, which are separated by ion exchange membranes from the center com- partment, operate as catholyte and anolyte compartments. During operation, anolyte/catholyte-maintenance solutions are recirculated through their corre- sponding 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 appli- cations for the ME technology in surface-finishing operations include regener- ation of etching and stripping solutions; purification and regeneration of chromium plating baths; recycling and maintenance of chrome conversion coat- ing 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 refin- ishing) 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 electro- dialysis or ME, DD does not require an electrical potential across the mem- brane to effect separation. A flow schematic of a typical DD system is illustrat- ed in Figure 7. The separation mechanism utilizes the concentration gradient between two liq- uids—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 552

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