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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
4.7.3 Applications and Restrictions
Potential applications of this technology for plating shops are shown in Exhibit 4-27. Application ME-1 is the typical bath purification scheme and the most widely used application for this technology. Application ME-2 shows the use of this process for several functions related to bath maintenance and chromic acid recovery.
The bath maintenance process in ME-1 is typically operated on a continuous basis. Process solution is pumped to a tank holding the electrolytic cell (alternatively the cell can be placed into the process tank) or through a stacked membrane system and returned to the bath. With each pass of the process solution, a portion of the cations are transported through the membrane and into the catholyte solution. This application describes the use of membrane electrolysis systems that operate with either an acidic or caustic catholyte (see catholyte discussion in Section 4.7.4).
Application ME-2 is a theoretical configuration showing: (1) conventional bath maintenance; (2) reformulation of cation regenerant; and (3) reformulation of a combined anion regenerant/caustic strip solution. The first two applications use the two cell membrane electrolysis configuration (one anode located in an anolyte and one cathode located in a catholyte) and the third uses a more complicated three cell arrangement (anode, reactor and cathode compartments). With this particular application of a three cell arrangement, sodium dichromate solution is reformulated into sodium hydroxide and chromic acid. Other applications requiring three compartments include phosphoric acid and conversion coating bath purification (ref. 384).
The functions of the two and three compartment cells are described in Exhibit 4-28. With the two cell arrangement, the chromic acid bath is the anolyte and the tramp metals present in that solution are transported into the catholyte, where they precipitate as metal hydroxides. With the three compartment cell, the anolyte is the sodium dichromate solution. The cations in the anolyte are transported into the center compartment (reactor compartment) where the multi-valent cations are precipitated as metal hydroxides. The mono-valent cations are further transported to the cathode compartment where the caustic is reformed. This process, which includes the precipitation of metals, is patented by Ionsep.
For the plating industry, membrane electrolysis is most applicable to the maintenance of chromic acid solutions, including hard chromium and decorative chromium plating, chromic acid etching, chromic acid anodizing and chromic acid stripping (e.g., copper). These are the only applications that were identified during the NCMS/NAMF Users Survey. Other potential applications include sulfuric and nitric acid and sodium hydroxide based solutions (e.g., pickle, etch, strip, rust removal), chromate conversion coating, and sodium dichromate deoxidizer (ref. 384).