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Pollution Prevention and Control Technologies for Plating Operations


Section 5 - Substitute Technologies

5.1 INTRODUCTION

As discussed in Section 2, pollution prevention includes a broad range of approaches and methods. Section 5 focuses on two aspects of pollution prevention: product changes and input material changes. The pollution prevention efforts and experiences of the NCMS/NAMF survey respondents with regard to these two topics have been categorized into six groups:

  • Product changes
  • Chlorinated solvent use reduction/elimination
  • Cyanide use reduction/elimination
  • Cadmium use reduction/elimination
  • Chromium use reduction/elimination
  • Other changes

Product changes made by survey respondents generally involved modifications to parts that resulted in a reduction of drag-out (e.g., adding drain holes to parts). A relatively low percentage (8%) of the shops surveyed have utilized this pollution prevention tool. Its limited use is most likely due to the fact that most of the shops surveyed were job shops, which usually have little or no control over the design of the parts they process.

However, the results of the Users Survey did show that respondents have made significant strides in reducing or eliminating the use of chlorinated solvents, cadmium, cyanide and chromium. Sometimes referred to as the four Cs, these materials have been identified by EPA as key targets for control within the metal finishing industry (ref. 415, 416). Although job shops do not have much control over part design, they usually have control over process chemicals. Some exceptions exist, such as instances where aerospace or military specifications require specific finishes, solutions and processing procedures; however, in spite of any limitations, approximately 60 percent of the shops attempted material input changes that potentially reduce or eliminate the use of one or more of the four Cs or another pollutant problem. Based on the comments received from respondents, these changes were made in an effort to reduce the impacts of their processes on the environment and worker health, to help meet environmental regulations and to reduce operating costs.

Although most of the material input changes attempted by survey respondents have been successful, there have been some failures and in many cases, even with successful changes, there have been adverse production impacts. An attempt has been made in this section to summarize the status of change in these areas and to convey the attitudes and concerns of the respondents.

One additional factor to keep in mind when reviewing this section, is that not all of the changes discussed in this section may be improvements. There currently exists a general feeling among regulators, the military and some private industry that all uses of cyanide, cadmium and chromium should be targets for substitution or elimination to meet health and safety and environmental goals. In some cases, the substitutes selected provide a lower quality finish and create new health and safety or environmental problems. In such cases, the question of whether it makes sense to improve the existing process through engineering or through making a substitution must be investigated.

As an example, the hard chromium process has been the target of reduction and/or elimination by the military and private industry (ref. 304, 418, 436). Potential substitutes for this process include: electroless nickel plating (ref. 418, 74), brush plating (ref. 69, 418), nickel alloys and metal spray coatings (ref. 418). In all cases, there is no single substitute that will meet all the requirements of hard chromium plating. Therefore, multiple substitutes must be used, and even then there will remain some applications for which no substitute is available.

Substitutes such as brush plating and nickel alloy plating may reduce the use of chromium, but they usually increase the net quantity of waste generated. These types of solutions are not amenable to recovery or bath maintenance and their by-products must be treated rather than recovered or reused. Conversely, the basic hard chromium bath (chromic acid/sulfate) can be operated in a closed-loop or zero discharge. Chromium can be recovered from rinse water (e.g., recovery rinsing, ion exchange, evaporation), air emissions (mesh pad mist eliminator), and strip solutions (membrane electrolysis) and the baths can be operated indefinitely without discharges (e.g., membrane electrolysis) (Sections 2, 3 and 4 provide information on methods and technologies for closed-loop operation of hard chromium plating). Additionally, parts plated with chromium generally last significantly longer than those plated with substitute coatings (ref. 304). Over time, this reduces the total number of parts that needs to be processed and further reduces the quantity of waste generated.

EPA recognizes the importance of considering all inputs and outputs in manufacturing and recommends a process termed life-cycle assessment to objectively identify and evaluate opportunities to reduce the environmental impacts associated with a specific product, process or activity. Life-cycle assessments take a holistic approach by analyzing the pollutant quantities and environmental impacts associated with: extraction and processing of raw materials, manufacturing, transportation, distribution, use/reuse/maintenance, recycling and final disposal (ref. 302). The life-cycle process should be kept in mind when reading this section, especially with regard to alternatives to cyanide, cadmium and chromium.


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