Pollution Prevention and Control Technologies for Plating
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).
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