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Pollution Prevention and Control Technologies for Plating Operations

Section 5 - Substitute Technologies


Chlorinated solvents have been used extensively in the metal working and metal finishing industry for degreasing and cleaning. The most commonly used degreasing solvents include: 1,1,1-trichloroethane (or TCA), trichloroethylene (or TCE), perchloroethylene (or PERC), chlorofluorocarbons, (or solvent 113) and methylene chloride. These solvents are used in a variety of methods, but most frequently in vapor degreasers, immersion or spray operations, and hand wiping. Solvent use has declined during recent years and especially since 1990, due to regulations restricting the use of ozone-depleting substances, voluntary actions by industry under EPA's 33/50 Program (Industrial Toxics Project) (ref. 203), rising costs for the purchase of solvents and the disposal of solvent wastes, and concerns over the health effects of solvents. However, as late as 1989, an estimated 73,000 U.S. firms (includes all types of industry) still used vapor degreasing with TCA (ref. 79).

Solvent use among the respondents to the Users Survey is summarized in Exhibit 5-1. Column 1 indicates the shop code number (PS 001 to PS 318). Column 2 shows the type of solvent used. Column 3 indicates the percentage of the shop's work that is job shop-type. Where 100% is shown, that business is entirely a job shop. Where 0% is shown, that business is entirely a captive shop. Where a different percentage is shown, that shop does both job and captive-type work. Column 4 indicates the year that the business commenced plating operations. Column 5 indicates the year that solvent use was eliminated. Where no year is shown in column 5, the solvent process was still in use at the time of the survey. Columns 6 and 7 indicate the quantity of solvent (in gallons or pounds) that is currently used or was used in cases where the applications were eliminated. Column 8 indicates the type of application (vapor degreasing, cold dip, spray or hand wipe). The last column indicates the most sophisticated method that is used (or was used in the case of eliminated applications) for controlling solvent emissions. The following abbreviations are used (shown in increasing levels of sophistication):

  NONE       no controls
  FREEBOARD  increased freeboard          degreaser
  ROLLTOP    automatic rolltop for        degreaser
  LIFTSPEED  controls to limit vertical   speed of degreaser hoist
  REFRIGER   refrigeration zone to        supplement conventional cooling coils       

Exhibit 5-2 shows that among respondents to the Users Survey, the number of solvent users has changed since 1980. In Exhibit 5-2, the shops are divided into three groups: (1) those in existence in 1980; (2) those established from 1981 to 1985 (inclusive); and (3) those established from 1986 to 1993 (inclusive). For the older shops, the number of solvent users remained approximately the same from 1980 to 1985 and then declined substantially from 1986 to 1993. In 1980, 53% of the shops used solvent and by 1993 only 39% used solvent. Therefore, 26% of the solvent users in 1980 have eliminated its use. For shops established from 1981 to 1985, the frequency of solvent use was below that of the older shops in 1985 and then from 1986 to 1993, the percentage declined similarly to the declining use rate of the older shops. Thirty-one percent of the shops established from 1981 to 1985 that originally used solvent have eliminated its use. The most recently established shops (1986 to 1993) presently have approximately the same percentage of solvent use as the shops established in 1981 to 1985.

Exhibit 5-2 indicates that shop age had only a small bearing on the frequency of solvent use. This is somewhat surprising since older shops are often thought of as being less likely to change their operational procedures and processes.

Exhibit 5-3 shows that the quantity of solvent use per application has also declined during the time period 1980 to 1993 and that this decline has been restricted to the time period of 1986 to 1993. A solvent application is defined as the use of a given type solvent by a given shop. Therefore, shops can have more than one solvent application. Combined with the fact that there are fewer applications, overall solvent use has declined significantly, probably by 50 percent or more from 1986 to 1993. All of the solvent use data are contained in the database and users are encouraged to utilize the database and draw their own conclusions.

Solvent use reduction has been achieved by survey respondents by improving the designs of vapor degreasers and the operating practices used with these units and by substituting aqueous and semi-aqueous cleaners for solvent degreasing. The most frequent vapor degreaser design and operating changes made by survey respondents focus on reducing solvent emissions, and include: increasing freeboard, adding a rolltop, controlling hoist speed and adding a refrigeration zone (see Exhibit 5-1). Of the 90 vapor degreasing applications in use in 1993, at least 75 percent were equipped with one or more of these design and operational changes. Various substitutions for solvent have been made by the survey respondents and these are discussed later in this section.

Although the numerical survey data provide insight to the changes regarding solvent use in the metal finishing industry, the comments submitted by respondents give an even clearer picture. The remainder of this subsection is devoted to their comments.

Sixty-eight percent of the respondents that are presently using solvent identified that its elimination is a pressing environmental problem. These shops are at various stages in the process of elimination. Some shops indicated that they are just beginning to evaluate solvent substitution alternatives (PS 156, PS 165, PS 184) and other shops indicated that they have made progress and are continuing to work toward elimination (PS 124, PS 166, PS 176, PS 178, PS 181, PS 191). Only 11.7 percent of the solvent users gave an indication that they plan to continue the use of solvent. These shops gave a range of reasons why solvent use is necessary. These reasons include:

  • Solvent use is necessary to meet aerospace requirements (PS 121).
  • Unsuccessful at attempts to clean zinc die castings without solvent (PS 089).
  • Solvent is needed to remove polishing or buffing compounds (PS 098, PS 170, PS 171, PS 191, PS 205).
  • No single answer for replacing solvent degreasing (PS 049).
  • Some non-solvent products work well, but there is always a problem with long drying times for parts.
  • Have replaced TCA vapor degreasing with alkaline cleaning, but can't replace acetone used to strip maskant (PS 166).
  • Substitutes are too expensive (PS 217).
  • Citrus cleaners did not work (PS 248).
  • Tried six different safety solvents (citric based). None were satisfactory due to residue or unacceptable evaporation time (PS 271).

A number of problems with solvent substitution were cited by survey respondents. These include the following comments from both successful and unsuccessful substitution efforts:

  • Alkaline cleaners increase the amount of waste treatment performed and/or the labor for these efforts (PS 114).
  • More difficult to remove masking wax without solvent (PS 108).
  • Citric acid based emulsion cleaner leaves a residue that inhibits good plating and causes poor adhesion (PS 109, PS 271).
  • "Orange-Peel Cleaners" caused skin rashes on employees and excessive foaming in the waste treatment system (PS 021).
  • Caused oil fouling of ion exchange resins (PS 130).
  • Increased labor for degreasing and/or bath maintenance (PS 130, PS 262).
  • Created a new wastestream (e.g., oily waste, spent cleaner) in some cases that is more difficult to deal with than solvent waste (PS 153, PS 171).
  • Solvent substitutes simply do not perform as well as solvents (PS 163).Substitutes are too expensive (PS 217).
  • Citrus smell of substitute cleaners is objectionable.
  • Many substitutes contain chelating agents or surfactants that interfere with metal hydroxide precipitation (PS 296).
  • PS 021 indicated that they had a Òdifficult transitionÓ that took approximately 24 months and Òa lot of patience.Ó Their efforts included trials with terpene cleaners, Òterpene-like cleaners and non-ionic surfactant packages,Ó and finally they settled on a mildly alkaline aqueous cleaner with a non-ionic surfactant. They also use barium carbonate for post cleaning but eventually switched to pumice. These changes combined with "enough (cleaning) time" and "elbow grease" have resulted in "few cleaning problems."

Only two shops specifically indicated that there were no production related problems associated with the substitution for solvent degreasers (PS 153, PS 190).

Some respondents provided information with regard to successful substitute degreasers that they use in place of chlorinated solvents. The following responses were given:

  • Aqueous cleaner with glycol ether (PS 064).
  • Aqueous cleaner (PS 124, PS 166, PS 187, PS 292, PS 268).
  • Citri-Solv product and Safety Clean (PS 105).
  • Mildly alkaline, biodegradable aqueous cleaner with a non-ionic surfactant (PS 021).
  • Global 6 (PS 169).
  • Aqueous, citric, terpene cleaners (PS 174).
  • Emulsion cleaner (PS 181).
  • Alkaline power washer (PS 079, PS 197).
  • Alkaline non-etch ultrasonic clean, Enthone NS-35 (PS 221).
  • Increased concentration of existing cleaner (PS 253, PS 295), e.g., PS 295 eliminated solvent use (80,030 gal/yr CFC 113 and TCA) by increasing cleaner concentration from 10 oz/gal to 12 oz/gal.
  • Converted degreasers to soak cleaners and use 7-9 oz/gal Midstate 4310, 140 - 180 oF, alkalinity ratio 0.40 minimum, soil load 10% maximum (PS 273).
  • Water soluble solvents (PS 276).

A number of miscellaneous methods were employed to reduce solvent use; these include:

  • Use safety solvent cans with hand wipe operation (PS 069, PS 176).
  • Use low vapor pressure solvent blend in place of MEK for hand wiping.
  • Increase level of training and reviewed good operating practices in safety meetings (PS 191).
  • Increased use of hand wiping and decreased use of vapor degreaser (PS 191).
  • Added inspection of degreaser to regular monthly procedures (PS 191).
  • Working with customer on the cleanliness of parts (PS 248).
  • Increased agitation in cleaning bath.
  • Subcontract the 5% of workload that requires solvent degreasing.
  • Install solvent recovery stills (PS 124, PS 149).

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