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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.2 COMMONLY APPLIED PREVENTATIVE AND CORRECTIVE BATH MAINTENANCE TECHNIQUES
4.2.2 Common Corrective Measures
4.2.2.1 Filtration
4.2.2.2 Carbon Treatment
4.2.2.3 Electrolysis (Dummying)
4.2.2.4 Carbonate Freezing
4.2.2.5 Precipitation
Filtration is the most commonly applied method of corrective bath maintenance. It is used to remove suspended solids from plating and other metal finishing solutions. Suspended solids in plating solutions may cause roughness and burning of deposits. Various equipment are used for filtration, with the most common being cartridge filters and precoat (diatomaceous earth) filters. Sand or multimedia filters are also employed. Cartridge filters are available with either in-tank or external configurations, with the former used mostly for small tanks and the latter for larger tanks. Most cartridges are disposable, however, washable and reusable filters have been recently commercialized. Several respondents reported use of the reusable filters (PS 180, PS 265, PS 229). Precoat filters are used mostly for large tank applications. Filter media are selected based on the chemical composition of the bath. Filtration systems are sized based on solids loading and the required flow rate (turnovers per hour). Typical flow rates for plating solution applications are 2 to 3 bath turnovers per hour. Other operating criteria for filtration processes can be found in the literature (ref. 421).
A total of 1,020 applications of bath filtration were reported by the 318 respondents of the Users Survey or an average of 3.2 per shop. Approximately two thirds of these filtration applications involved the use of the less expensive in-tank devices and one-third the external devices. The distribution of filtration applications among the different types of plating baths was approximately the same as the usage of the baths themselves (see Exhibit 4-1 which indicates for each process the total number of plating processes and the number of filtration applications identified). One notable exception was chromium plating, where filtration was not frequently applied.
Cost data for common bath maintenance techniques were not specifically requested during the Users Survey. However, four shops provided the following data for filtration.
PS 160 reported a capital cost of $14,000 (1991 costs, includes $1,000 of installation costs), annual non-labor costs of $2,000 and an annual labor requirement of 50 hours for an external sand filtration system (Techmatic) that serves three zinc chloride tanks containing 7,600 gal of solution. They reported that the sand bed requires manual stirring once per month and that the sand must be replaced once each year. They indicated that the downtime of the unit was 2% and reported a high satisfaction level for the filter.
PS 191 reported that they have purchased multiple filter units (Serfilco) that had a total capital cost (purchased during various years dating back to 1965) of $40,940 (includes $3,000 of installation costs) to filter two nickel plating tanks totaling 7,200 gal. They employ standard filter cartridges, but rather then discard them after a single use, the spent cartridges are washed in a standard washing machining (inexpensive Sears unit). They have found that by washing the cartridges (after first soaking them in a tank), they can reuse them 3 to 4 times. They expend approximately 30 man-hrs per year washing cartridges. They first implemented this procedure in 1989 and had to replace the washer in 1992. The discharge from the soak tanks and the washer is sent to their conventional treatment system. They originally implemented this procedure in order to dispose of the filters as non-hazardous waste, but after seeing the results of the cleaning process, decided to reuse them. Used anode bags are processed in the same manner.
PS 114 reported the purchase of used filtration equipment for $500 with installation costs of $350 (1992) for filtering an electrocleaner and acid pickle (105 gal each). They indicated that annual non-labor costs are $100 and the labor requirement is only 10 hrs/yr. They estimated that by using filtration, they extend the life of the electrocleaner by 10 times and the life of the acid pickle by 5 times.
PS 253 reported cost data for a carbon treatment/filtration system (see Section 4.2.2.2).
Carbon treatment of plating baths is a common method of removing organic contaminants. The carbon adsorbs organic impurities that are present as a result of oil introduction or the breakdown of bath constituents. It is used on both a continuous and batch basis. Various application methods are available, including carbon filtration cartridges (contain up to 8 oz of carbon and are restricted to use on small applications), carbon canisters (up to 10 lbs of carbon), precoat filters, and bulk application/agitation/filtration (ref. 421). Typical dosages are 1 to 4 pounds of carbon per 100 gallons of solution (ref. 341, 421).
A total of 505 applications of carbon treatment were reported by respondents to the Users Survey. The most frequently cited applications were nickel electroplating (mostly Watts nickel and nickel sulfamate), which accounted for 50% of all carbon treatment applications. The other most common applications were copper electroplating (18% of all applications) (mostly copper cyanide and copper sulfate), zinc plating (10% of all applications) and cadmium cyanide plating (4% of all applications). Survey respondents used continuous (46%) and batch (54%) carbon treatment methods on an almost equal basis.
Cost data were not requested in the Users Survey for common bath maintenance techniques. However, two shops provided the following data for carbon treatment. PS 253 reported a capital cost of $14,200 (installed cost for external Mefiag 6500 SS filter/carbon treatment system, 1986), non-labor annual costs of $616 and 39 hours/yr of labor for maintaining a 3,600 gal Watts nickel bath. The bath is continuously pumped through a carbon filter at a rate of 55 gpm. This operation generates 125 lbs/mth of spent filter pads which are sent to off-site disposal. PS 253 indicated that they are very satisfied with the unit and that it has a downtime of 0%. PS 160 reported a capital cost of $120 (1992) for an in-tank unit (Flo-King) and annual operating costs of $240 (includes both labor and non-labor costs). They indicated that they are satisfied with the unit and that it has a downtime of only 2%.
4.2.2.3 Electrolysis (Dummying)
Dummy plating is an electrolytic treatment process in which metallic contaminants in a metal finishing solution are either plated out (low current density electrolysis) or oxidized (high current density electrolysis). Dummy plating has been documented to be in use since 1916 (ref. 339).
Low current density (LCD) dummy plating is applied to a range of plating and other metal finishing processes. The contaminant metals that are most frequently removed by dummy plating are copper, zinc, iron and lead. Dummy plating is usually performed using a corrugated steel sheet cathode with an anode to cathode spacing of approximately 4 in. The optimal current density will depend on the metal contaminants being removed. The normal range is 2 to 8 ASF. The duration of treatment is typically 2 to 5 amp-hr/gal. Agitation is essential for speedy removal of contaminants and air agitation should be used if the type of bath permits (ref. 270, 339, 340, 341).
LCD dummy plating can be performed on a batch or continuous basis. Batch treatment is usually performed in the process tank and requires down-time. Continuous treatment is usually performed in a side-tank and cathodes are typically sized to permit 0.05 amp/gal of solution (ref. 339, 341). The solution is preferably returned to the process tank through a filter (ref. 314). An example of a continuous LCD dummy plating application used by PS 118 on a closed-loop nickel plating line is shown in Section 2.4.
High current density (HCD) dummy plating typically refers to the practice of oxidizing trivalent chromium to hexavalent chromium in chromic acid baths (e.g., chromium plating and chromic acid anodizing). It is also used to gas-off chloride as chlorine.
The HCD process requires an anode to cathode ratio of between 10:1 and 30:1. Lead or lead alloy anodes are typically used in the process. A lead peroxide film is formed on the anode which functions as the oxidation agent. Current densities of between 100 to 300 ASF are used. The rate of conversion is controlled by the overall cathode and anode areas and current flow.
Respondents to the Users Survey employed dummy plating most frequently with nickel electroplating (48% of all electrolysis applications), chromium electroplating (13%), zinc electroplating (11%), copper electroplating (10%) and cadmium electroplating (7%).
Cyanide baths are adversely affected by carbonate buildup. Carbonates are formed by the breakdown of cyanide (especially at high temperatures), excessive anode current densities and the adsorption of carbon dioxide from the air. Excessive cabonates cause increased resistance in the bath, yielding low plating current densities, which normally accentuate the poor appearance that metallic impurities cause. An excessive carbonate concentration can affect the smoothness of deposits, plating efficiency and plating range. Both sodium and potassium baths are affected by carbonates. However, the sodium bath is affected at a much lower concentration (14 oz/gal vs 40 oz/gal) (ref. 340, 482).
Sodium cyanide baths can be treated for excessive carbonate buildup by "carbonate freezing" or crystallization. Potassium cyanide baths must be treated by precipitation, which is also applicable to sodium cyanide baths (see Section 4.2.6). Carbonate freezing takes advantage of the low solubility of carbonate salts in the sodium cyanide bath. The method involves lowering the bath temperature to approximately 26F (-3C) where hydrated salt/Na2CO3 10H2O crystallizes out. This treatment procedure will also remove sodium sulfate and sodium ferrocyanide (ref. 339).
A total of 111 applications of carbonate freezing were reported by the respondents to the Users Survey. The vast majority of these applications were for cadmium cyanide plating, copper cyanide plating and copper cyanide strike baths.
Various chemical treatments of plating baths are performed to remove bath contaminants via precipitation. Precipitation is generally a batch process that is often performed in a spare tank where the solution is chemically treated and filtered and returned to its original tank. Electrolytic purification is sometimes applied following chemical treatment to remove metal contaminants less affected by precipitation (e.g., following purification of a Watts nickel bath to remove copper, zinc and cadmium). Precipitation is an alternative method to carbonate freezing for cyanide baths and is especially applicable to potassium cyanide baths. Chemicals used for this purpose include: barium cyanide, barium hydroxide, calcium hydroxide, calcium sulfate or calcium cyanide. The least expensive of these chemicals, calcium sulfate, forms a bulky precipitate that is less easily removed. Other relatively common uses of precipitation include lime addition for the removal of carbonates from silver cyanide baths, sodium sulfide treatment of cyanide baths for zinc and lead removal and nickel carbonate (1 to 2 lbs/100 gal) or nickel hydrate treatment of nickel plating baths to remove miscellaneous metal contaminants (e.g., iron, aluminum, silicon). The latter of these methods is termed "high-pH treatment." Peroxide is sometimes added during high-pH treatment to enhance precipitation and destroy organics (ref. 340). Hydrogen peroxide is also used for oxidizing soluble ferrous iron to insoluble ferric hydroxide in acid chloride zinc baths (ref. 340, 482). Two respondents reported the use of potassium permanganate (oxidizing agent) for iron removal from zinc baths (PS 076, PS 268). The precipitated iron is removed by filtration. Also, lime is sometimes used to remove phosphorus compounds from electroless nickel solutions; however, this process is not widely applied (ref. 286). One respondent reported the use of silver oxide to precipitate chloride from a decorative chromium bath (PS 162).
Respondents to the Users Survey reported 35 applications of precipitation and another 42 applications of high pH treatment. The use of high-pH treatment was restricted mostly to Watts nickel and sulfamate nickel baths. Other precipitation applications in order of decreasing frequency include: zinc plating, nickel plating, silver plating, copper plating, bronze plating, chromium plating and cadmium plating.