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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.6 ION TRANSFER
4.6.2 Development and Commercialization
Chromic acid bath contamination has been a concern to platers since its development in the 1920's (ref. 367) and it continues to be a problem today. One of the surveyed shops offered this statement:
One of the biggest problems facing hard chromium platers, such as ourselves, is the purification of the plating baths. Contaminants in the plating tanks reduce the efficiency and quality of the chromium deposit, and once the contaminant level reaches a certain level, the solutions must be disposed of [sic]. (ref. PS 234)
The two key problems that plague chromic acid bath users are: (1) reduction of hexavalent chromium (Cr+6) to trivalent chromium (Cr+3) causing an unwanted buildup of Cr+3 and (2) buildup of dissolved metals (ref. 368, 369 and 370). The first of these two problems can be solved using the simple process of auxiliary electrolysis or high current density dummying (Section 4.2). However, dummy plating is a relatively slow and inefficient method of oxidizing Cr+3 (ref. 370). Since production plating cannot be performed during dummy plating, the tank being treated would be unusable during this time period. Therefore, for shops using this method, dummying is usually performed during weekends.
The shortcomings of dummy plating led to investigative work by Seegmoller and Lamb in 1948 (ref. 368) involving use of the porous pot. Prior to that time, and as early as 1840, the porous pot was employed in the plating industry as a "bladder" that separated zinc anodes from gold cyanide plating solution (ref. 371). The porous pot is also mentioned in U.S. patent literature in 1918 (ref. 371). The technology, however, was not widely applied until the late 1970's and 1980's with the commercialization of the Cosmos Mineral Corporation unit (latter sold by Pfaudler Co. and subsequently by E.P. Technology) and the Hard Chrome Plating Consultants' Porous Pot (PPS1 and PPS2). Before this time period, it is believed that homemade versions of the porous pot were in use. Hard Chrome Plating Consultants' indicated during the Vendors Survey that they have sold approximately 1,500 porous pot units to approximately 1,100 different plating shops.
Also, in the early 1980's, a polyfluorocarbon membrane version of the porous pot was developed (CatNapper). The polyfluorocarbon membrane acted like the ceramic pot, separating the anode and cathode reactions and products while permitting cations to migrate from the anolyte to the catholyte. The polyfluorocarbon membrane of the CatNapper is not anion or cation selective; the selectivity (i.e., migration of cations only into the catholyte) is a result of the electrical driving force. This fact distinguishes the technology from the more recently developed membrane electrolysis units that employ ion exchange membranes (Section 4.7). Another very significant difference between these technologies is that chromic acid is used as the catholyte with ion transfer technologies, whereas ion specific electrochemical membrane technology units typically use a non-chromic acid solution. This is a disadvantage of the ion transfer technologies if the catholyte, which accumulates the contaminants, is discarded. In such cases, ion transfer technologies would create a significant quantity of chrome-bearing residual. However, the use of a chromic acid catholyte would be viewed as an advantage in the event of a membrane leak. With a non-chromic acid catholyte, a membrane leak would result in significant bath contamination. The original CatNapper is no longer marketed. However, a similar technology (Artec) is now sold by i3.
It should be noted that ion exchange is a competitive technology for chromic acid bath maintenance. Ion exchange was used for this purpose as early as 1952 (ref. 368) and is still commercially available (ref. Eco-Tec file, Kinetico file). Use of ion exchange for chromic acid purification is discussed in Section 4.4.
Also, some hard chrome plating shops perform the preliminary reverse etching step (the surface of the part that is to be chrome plated is etched by reversing the normal polarity of the process tank which makes the part an anode and the lead alloy anodes become cathodes) in a dedicated etch tank. This practice reduces the introduction of iron (from the base metal) into the plating bath and delays the need to treat the bath (ref. 482).
The use of ion transfer as a chromium recovery technology probably began in the 1940's to 1950's, when the porous pot was being used for bath maintenance. However, there is no record of its use until the 1980's with the commercialization of the ChromeNapper by Innova (same manufacturer that made the CatNapper).