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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
4.7.4 Technology/Equipment Description
4.7.4.1 General
4.7.4.2 Commercially Available Equipment
Membrane electrolysis is the newest of the chromium bath purification technologies. In the most basic sense, these units consist of: (1) a tank containing an anode and cathode compartment separated by a selective membrane(s), and (2) a power source. The membranes are ion specific in a manner similar to ion exchange resin. They allow the passage of only positive or negative ions, depending on the type of membrane. This design element is used in this text to distinguish these units from ion transfer technologies that employ non-ion permeable membranes such as ceramic pots or a polyfluorocarbon material (Section 4.6). Cation specific membranes are used for chromic acid purification, the most common application for this technology. When current is applied to the cell, cations present in the anolyte flow through the cation permeable membrane. The anions in the catholyte are restricted by the membrane and remain in that compartment. Trivalent chromium present in the anolyte (plating bath) is reoxidized to the hexavalent state. Membrane electrolysis units can be configured with only cation or anion membranes or both.
It is important to note that Cr+3 is a cation, while Cr+6 combines with oxygen in solution to form CrO4 and acts like an anion. Therefore, in a membrane electrolysis system, the Cr+3 that is not oxidized to Cr+6 will behave like iron and aluminum ions and be transported through the cation membrane while the Cr+6 remains in the plating solution. The loss of Cr+3 in this manner is undesirable since Cr+3 can be restored to a useful state by simple oxidation at the anode. Further, the catholyte solution, into which the cations are transported, becomes more quickly spent when Cr+3 passes through the membrane. The cathode solution is subsequently discarded and its chromium content will increase disposal costs. Therefore, units with a rapid Cr+3 oxidation rate will generate less process residual and the residual will have a lower chromium content.
The available commercial membrane electrolysis units are discussed in Section 4.7.3.1. Four different membrane electrolysis manufacturers were identified during the literature search, Users Survey and Vendors Survey. A comparison of these technologies shows that there are several basic variations among the units. The following discussion highlights these differences.
The choice of catholyte solution is one major difference between the various commercially available units. Most of the units use mineral acid (typically sulfuric acid) as the catholyte, which was used by the Bureau of Mines in their early work. With an acidic catholyte solution, cations flow through the permeable membrane and into the acidic catholyte. The electroplatable cations deposit on the cathode and the non-electroplatable cations stay in that solution as salts. Periodically, the cathode is cleaned and the catholyte solution is discarded and replaced (370).
One of the available technologies uses a patented caustic catholyte (combination of alkali reagents). The caustic catholyte causes the precipitation of cations as they pass through the membrane. This overcomes any problem of plating-out the cations on the cathode. This catholyte converts multi-valent metal cations entering the catholyte solution into insoluble hydroxides. The hydroxyl ions needed to react with the metal cations are formed at the cathode of the cell. Precipitation of the cations prevents a loss of conductivity and eliminates the buildup of a deposit on the cathode; consequently, the operational period is extended to two or more weeks and there is no need to clean the cathode.
The overall appearance of the commercially available units varies significantly. Some of the units are configured similarly to the Bureau of Minesí prototype units. The chromic acid bath is pumped to a tank on either a batch or continuous basis. The tank contains multiple cathode cells, around which the anolyte (chromic acid bath) is circulated. Another commercial unit has a conventional electrodialysis design with ìstacksî of membranes that are kept together in a clamping unit which looks similar to a filter press. Anode and cathode electrodes are located at opposite ends of the stack. The plating solution is pumped through the stack. The pumps and other support equipment are located on a skid. A third configuration, places the cathode in a ìmembrane sock,î which is lowered into the plating tank. The catholyte is circulated from a side tank into the cathode compartment (ref. 384).
The capacities and capabilities of membrane electrolysis units should be measured in terms of the tramp metal removal rate (g/hr), which will vary over a range of tramp metal concentrations in the anolyte. Unfortunately, there is a limited amount of such data available. Exhibit 4-29 shows the results of a comparative test of a caustic catholyte membrane electrolysis system and a single cell porous pot unit.
4.7.4.2 Commercially Available Equipment
This subsection contains a description of commercially available ion specific electrochemical membrane technology equipment. 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.
Ionsep Corporation has manufactured electrochemical processing equipment since 1985 and has sold more than 150 units to metal finishing firms. Approximately two-thirds of their units were sold for chromic acid applications. The basic Ionsep unit consists of a electrolytic cell, a catholyte tank (located near and plumbed into the electrolytic cell ), and a power source (typically a 500 amp rectifier). The cell is inserted in a protective 6 to 9 in. diameter PVC pipe approximately 36 in. long that is kept in the plating bath during operation (alternatively, a side tank is used, especially with larger applications). The pipe is perforated to permit plating solution to enter and flow around the cell membrane. Within the PVC pipe is a cylindrical shaped anode, which is exposed to the plating bath, and a selective membrane ìsockî that creates an inner cathode compartment. A cathode and catholyte solution (an alkaline solution) are located in the sock. This configuration is termed a two compartment cell where the anode compartment is the plating bath and the cathode compartment is the space inside the sock.
In operation, a direct current is passed to the cell electrodes and the catholyte is continuously circulated from the catholyte tank (typically a 55-gal. plastic drum with a small circulation pump located on top) into the sock formed by the membrane. The cations (e.g., iron) are electrically driven from the plating bath through the selective membrane and into the cathode compartment. The metal cations precipitate as hydroxides. Most of the trivalent chromium present in the bath is converted to hexavalent chromium at the cellís anode. Some trivalent chromium, which is a cation, passes from the bath, through the membrane, and into the catholyte solution. The tramp metal removal rate for a small Ionsep unit (130 amp unit) is shown in Exhibit 4-29.
The two compartment Ionsep cell is applicable to hard chromium and chromic acid anodizing baths and other baths with similar chemistry. Baths with more complex chemistry often require cells with three or more compartments. For example, a caustic chromium stripper can be reformulated into sodium hydroxide and chromic acid with a three compartment cell. The resultant sodium hydroxide is reused as stripper and the chromic acid is reused in the plating bath.
Large Ionsep systems are configured differently from that described above. With the larger systems, the electrolytic cell is not located in the plating tank, but rather, inside of a skid mounted PVC piping system. With this configuration, the bath is pumped to the cell and circulates back to the plating tank. Many of the large systems also have a catholyte recycle capability, while in the small systems the catholyte solution is discarded weekly. The catholyte recycle system causes the separation of precipitated metals from the catholyte which permits the reuse of the catholyte.
Ionsep units and systems are available with capacities from 150 amps to 15,000 amps. In addition to the chromic acid bath maintenance (chromium plating, chromic acid anodizing and chomate conversion coating), their equipment has been applied to nitric acid strip, nitric-HF pickle, sulfuric acid pickle, hydrochloric acid, and caustic chem mill etch maintenance. They also manufacture a closed-loop rinse water recovery system that employs ion exchange and electrochemical regeneration to recover chemicals and enable the reuse of water.
GOEMA, a German-based company, manufactures a family of membrane technologies for water, wastewater, and bath maintenance. These technologies include membrane electrolysis, diffusion dialysis, ultrafiltration, and microfiltration. Their membrane electrolysis units are designed with a stacked membrane configuration. A ìmembrane stackî consists of a number of thin membrane compartments created by alternating layers of ion-specific membranes and spacers. The spacers are present to permit solutions (e.g., plating solution and catholyte) to flow between the membrane layers. Either anion- or cation-specific membranes or both are used to create a stack. For hard chromium plating and chromic acid anodizing bath purification, only cation membranes are used. The membranes and flow spacers are held together by a cell frame and clamping unit, giving the membrane stack the appearance of a plate and frame filter press.
In operation, plating solution and catholyte solution are pumped into the stacked cell. Separate inlets and outlets and flow channels are present for each solution. Anode (lead alloy) and cathode (stainless steel) electrodes are located at the ends of the stack in their associated electrolyte. When an electrical potential is applied across the stack, Cr+3 present in the anolyte (plating solution) is oxidized to Cr+6 and dissolved tramp metals migrate toward the cathode and pass through the cation membrane into the catholyte. Sulfuric acid (20 percent) is typically used as a catholyte for these applications.
GOEMA units can be purchased in various capacities. The key sizing variable is the number of compartments that make up the membrane stack. GOEMAís basic unit (KRME-100) has a rated metals removal capacity of 100 grams per hour. The membranes are approximately 4 ft2 each with a total membrane area of 20 ft2. The DC power source is a 1,000-amp rectifier. The flow rate of anolyte through the stack is approximately 6 gph (144 gpd). A single solution pass through the stack provides a high cation removal rate. Approximately one to two tank volumes of treatment will adequately purify a moderately contaminated bath. Assuming this rate of bath purification, a 500-gallon bath could be treated in approximately 1 week (two tank volumes x 500 gallons/144 gpd = 6.9 days) (Based on the manufacturerís rating for these units.) (ref. 384).
Graver Water Division of the Graver Company (Graver) is the distributor of Tokuyama Soda membrane electrolysis equipment. These units have been utilized for the following applications: nickel and zinc recovery from galvanizing, caustic recovery from aluminum etch, acid recovery from metal pickling, and aluminum recovery from nitric acid solutions.
Kinetic Recovery Corporation (KRC) manufactures the Membrane Electrolysis System 125 (Exhibit 4-30). This unit is designed for the oxidation of Cr+3 to Cr+6 and the removal of tramp metals from chromic acid. The basic unit is skid mounted and consists of: electrolysis container with a drip pan; anode compartment including anode (Ti/Pt expanded metal), inflow-assembly and electrical connections; stainless steel cathode; ion exchange membrane; anolyte and catholyte feed tanks; anolyte filter; catholyte pH control system; rectifier (15V, 150A); and control panel. The dimensions of the skid are 48" x 21" x 26" (H).