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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.1 INTRODUCTION
Chemical solution maintenance includes a range of pollution prevention practices and technologies that preserve or restore the operating integrity of metal finishing process solutions, thereby extending their useful lives. Some forms of solution maintenance, such as filtration, have been utilized nearly as long as metal finishing itself. However, due to rising costs for chemicals, energy and treatment/disposal and increasingly more stringent environmental requirements, solution maintenance has become a greater priority to plating shops and the methods and technologies they employ have increased in sophistication. Today, firms are willing to expend significant amounts of capital and operating funds for equipment and methods that primarily reduce the disposal frequency of their baths.
In addition to extending bath lives, solution maintenance often improves the average operating efficiency and effectiveness of a process solution and therefore has a positive impact on production rates and finish quality.
Metal finishing solutions are subjected to a variety of forces that cause them to become unusable. The key contributing factors are: (1) depletion of bath chemicals; (2) chemical break-down of process chemicals or chemical side reactions; (3) contamination from impurities in make-up water, chemicals or anodes; (4) anodic/cathodic etching of parts and inert electrodes; (5) corrosion of parts, racks, bussing, tanks, heating coils, etc.; (6) drag-in of non-compatible chemicals; (7) buildup of by-products (e.g., carbonates); (8) breakdown of maskant, fume suppressant and wetting agents; (9) errors in bath additions; and (10) airborne particles entering the tank.
Solution maintenance replaces the practices of: (1) using a chemical solution until it is degraded and replacing it with fresh solution or (2) decanting a portion of a degraded solution and replacing it with fresh solution. In both cases, the spent solution is usually either treated on-site or transported to a treatment/disposal facility. On-site treatment is not always possible because concentrated wastes may upset treatment facilities designed primarily for treating dilute rinse waters. In some cases, shops are able to reuse spent solution for either: (1) a less critical process application or (2) as a treatment reagent (e.g., spent acid cleaner used in place of sulfuric acid for pH adjustment). The former of these uses is regarded as a pollution prevention option by EPA. The latter method may reduce the overall use of chemicals at a shop, but because it involves treatment, it is not considered "pollution prevention" by EPA. EPA's definition of pollution prevention is presented and discussed in Section 2.
Two major categories of solution maintenance have been identified during the NCMS project: preventative and corrective. Within this text, preventative solution maintenance refers to the practices that avoid bath contamination or involve monitoring and adjusting of solution chemistry. Corrective solution maintenance refers to the practice of removing contaminants from the bath, whether they are dissolved or particulate, organic or inorganic. Both preventative and corrective solution maintenance involve the use of methods, techniques and technologies. Methods and techniques are typically procedural in nature or low capital items that can be implemented quickly and have an almost immediate payback. Technologies are generally equipment packages that have a moderate to high capital cost and payback periods of one year or greater. Most preventative measures are either methods or techniques. However, some technologies such as an electroless nickel bath automatic replenishment system would also fall into this category. Corrective measures include both methods/techniques such as dummy plating and technologies such as microfiltration.
Methods of preventative and corrective solution maintenance that are commonly applied by plating shops, and therefore do not require a detailed discussion, are reviewed in Section 4.2. The corrective technologies, which are generally less familiar to platers, are covered in detail in Sections 4.3 through 4.8. Exhibit 4-1 presents the results of the Users Survey that show the frequency of use for each of the common corrective bath maintenance techniques.
The information contained in Sections 4.3 through 4.8 is derived from the results of the Users Survey, Vendors Survey and literature search. The Users Survey requested platers to provide detailed technical, performance and operating cost data for bath maintenance technologies. The vendors were requested to provide technology descriptions, operating data and capital cost data. As a result of obtaining data from these two sources, plus the information from the extensive literature review, Section 4 contains a substantial quantity of information for the following corrective bath maintenance technologies: microfiltration, ion exchange, acid sorption, ion transfer, membrane electrolysis and diffusion dialysis. A separate subsection of the text is devoted to each of these technologies. Within each subsection, the following are provided: technology overview; development and commercialization; applications and restrictions (with diagrams showing different potential configurations); technology/equipment description; capital costs; operating costs; performance experience; and residuals generation. The capital cost curves contained in Section 4 are based on data collected from the technology vendors and the operating cost curves are based mainly on data from platers. Both the capital and operating cost information are expressed in 1993 dollars. A labor cost of $25 per hour (includes overhead) and an electricity cost of $0.10/kWh have been used, where applicable, in calculating operating costs.
