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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.6 ELECTRODIALYSIS
3.6.1 Overview
The electrodialysis technology employs ion-permeable and selective membranes under an applied d.c. potential difference to separate ionic species from an aqueous solution. Its primary application for chemical recovery is nickel plating, where it competes with recovery rinsing (drag-out tanks), evaporation technologies and ion exchange. As such, it has not seen widespread use, despite being commercialized for this application for nearly fifteen years. The results of the Users Survey indicate that only three respondents (less than one percent) have employed electrodialysis for recovery. All of these units, which were used for nickel recovery, are no longer in operation.
When used for nickel recovery, a concentrated rinse solution is pumped into an electrodialysis unit that is comprised of a stack of alternating cation- and anion-selective membranes, each separated by a spacer through which the solutions are allowed to flow (Exhibit 3-48). Due to the electrical potential applied across the stack, cations (Ni2+) in solution migrate toward the cathode, and anions (SO42- and Cl-) toward the anode. When a cation encounters a cation-selective membrane, it passes through. As it continues to migrate, it will subsequently encounter an anion-selective membrane through which it cannot pass and will remain in that compartment. Similarly anions migrate toward the anode, pass through anion selective membranes and their travel is halted when they encounter a cation-selective membrane. This process results in alternating compartments between consecutive membranes becoming increasingly enriched (concentrate) and increasingly depleted (diluate) in ion concentration (ref. 399). Based on data presented in the literature, the feed stream (typically from a drag-out tank) usually contains 2 to 5 g/l Ni, the concentrated return stream contains 30 to 60 g/l Ni and the depleted stream contains less then 200 mg/l Ni (ref. 32). The return stream is less concentrated than the bath (typically 70 to 90 g/l Ni). However, surface evaporation from the plating bath tank is generally sufficient to volumetrically accommodate complete assimilation. The percentage of nickel recovered from the feed stream is typically 90 to 95%.
Exhibit 3-48. Diagram of an Electrodialysis Process for Nickel Recovery A potential advantage of electrodialysis over other concentrating and return methods of nickel recovery (e.g., evaporation and reverse osmosis) is its ability to selectively retard the recovery of certain organic materials that tends to buildup in nickel plating baths, while more freely permitting the transport of a desirable organic bath constituent (saccharin) and nickel salts. This aspect of the process could reduce the frequency of bath purification as compared to other recovery schemes. Bath purification causes the loss of 10 to 15% of the bath per cycle, which significantly reduces the pollution prevention benefits of recovery (ref. 28).