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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.6 ION TRANSFER
4.6.4 Technology/Equipment Description
This subsection contains a description of commercially available ion transfer equipment that is manufactured and/or sold by respondents to the Vendors Survey. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.
The PPS1 and PPS2 (Hard Chrome Plating Consultants) are the simplest commercial versions of the ion transfer technology. Both units are single cell systems, the difference being their size and capacity. The PPS1 is the larger of the two units and is recommended by the manufacturer for tanks containing up to 3,000 gal. of chromium plating solution. The PPS2 is recommended by the manufacturer for shops with less than 700 gal. of plating solution. Each unit consists of a porous pot, anode, cathode and connecting bars. The pots have a 40% porosity and a pore size of 1 micron or less. The cathode is made from lead alloy mesh (ref. 371). These units are designed for use with the 2-bus bar, reversible rack plating tank configuration. However, they can be rigged for other bus bar configurations. The units are located directly in the plating tank during use, where they consume approximately six to twelve inches of tank length and have a maximum working depth of 25 inches. No pumps, piping or power supplies are usually needed to operate the unit. The porous pot is powered by the tank rectifier. The manufacturer also sells a PPS-4 unit that is applicable to cadmium, nickel copper and other solutions.
Before using the PPS1 or PPS2, the porous pot is filled with plating solution or fresh chromic acid solution, without catalyst (approximately 2 gallon for the PPS1). The unit is then lowered into the plating bath. By design, the lip of the pot is above the solution level of the plating tank to avoid having direct exchange of solution. When the unit is energized (smaller unit draws about 70 amps and the larger unit 210 amps) (ref. 371), Cr+3 in the anolyte (plating tank) is oxidized to Cr+6 and cations in the anolyte are electrically driven through the pores of the pot into the catholyte. Some cations are deposited on the cathode, most remain in solution. It is unclear whether the cations are electrolytically deposited or precipitated. One source suggested the latter method of deposition occurs because hydrogen evolution is occurring at the cathode which will result in a rise in pH (ref. 371). The chrome present in the catholyte is reduced to Cr+3. As these changes occur, the effectiveness of the unit diminishes and eventually the catholyte is replaced. The frequency of catholyte replacement is usually in the range of 8 hours to 3 days (ref. 370).
The E.P. Technology unit (patent formally owned by Cosmos Minerals Corporation and subsequently by Pfaudler Co. and later by E.P. Technology) consists of a tank holding four to eight pots and an equal number of sets of anodes and cathodes. Basically, this is the same technology as the Hard Chrome Plating Consultants PPS1 or PPS2, but on a much larger scale. The pots hold the catholyte solution and cathode. Plating solution is pumped to the tank on a continuous basis; it circulates around the pots and is returned by gravity flow to the plating tank. The cells are powered by a rectifier (1,000 to 2,000 amps depending on the number of pots) dedicated to the purification unit. The unit requires lateral exhaust ventilation similar to a chrome plating tank, since chromic acid mist is generated by the release of oxygen at the anode and hydrogen at the cathode. It is not known if the EP Technology unit is still marketed. No recent advertising or installations were located during this project.
Artec manufactures four models of the polyfluorocar-bon membrane ion transfer technology for chromium bath maintenance (APS series) and seven models for chromium recovery from rinse water (ARS series) (marketed by i3). Three of the APS models are progressively larger units of similar design (Exhibit 4-23) and the fourth is a Unicell model. The three larger Artec units operate in a manner very similar to the EP Technology porous pot unit. Solution is pumped from the plating tank to the unit, circulated past the cells and returns by gravity to the plating tank. Within the unit, the chromic acid solution circulates around modules (3 to 12), which extract cationic impurities. The solution collected in the membrane compartment which contains iron, trivalent chromium, and other cation impurities is discharged to waste treatment. System specifications for the Artec units are given in Exhibit 4-24. The Artec Unicell is a single module unit that fits into a plating tank, and therefore functions like the single porous pot cell units.