University of Minnesota
Minnesota Technical Assistance Program
http://www.mntap.umn.edu
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Pollution Prevention in Your Cleaning and Rinsing Processes

Overview

Metal finishing often begins with steps to prepare the metal prior to coating. The first step is generally cleaning, which most often consists of a water-based alkaline cleaner for plating processes. However, cleaning can also consist of acid cleaner or a relatively neutral pH bio-cleaner. In a few cases, solvent degreasing is first required to deal with heavy oil or grease soiling. Stripping may be required for products that need rework. The stripping process involves very strong chemicals to remove plated coating, and should be avoided as much as possible by addressing the root causes of quality and rework issues. Rinsing is generally required after every significant chemical processing step to avoid cross-contamination of baths which would shorten bath life and increase waste. By implementing some pollution prevention opportunities, your facility can reduce the amount of waste generated, save money, and lessen the risk for employee exposure.

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Eliminate Contaminants & Change Cleaning Processes

The first step for reducing pretreatment waste is to assess how clean your parts are prior to the pretreatment process. Determine the sources of contaminates and consider to what degree surfaces become contaminated with substances such as oil from machining, dirt from the manufacturing environment, and oil from people's skin, and identify the age of those soils. Once contaminant sources are identified and cleanliness standards are set, determine if the contaminants that are within your control that can be eliminated, or talk to customers about contaminates that may be reduced or elimated before the parts enter your facility. Review the Get it Plated Right series for tips for customers on problem soils.

Consider maintaining cleaners with long lives to reduce chemical use. Two examples of effective general purpose cleaners for plating operations are: cleaners that allow oils to float to the surface for removal and bio-cleaners that break oils down over time. For more information about cleaning processes, see the discussion on dragout reduction, concentration control, and bath life extension in the plating Web page.

Cleaning Resources

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Rinsing

Metal finishing processing leaves a film of liquid, or dragout, on parts leaving each tank. This is a major concern due to chemical loss, waste treatment requirements, and large volumes of water consumption used to remove this film. The water is used for rinsing to avoid cross-contaminating bath chemistries and to leave parts surfaces clean at the end of the process. Dragout reduction is the starting place for efficient rinsing. See the discussion on dragout reduction, on the Plating Web page for more information. Spray rinses and dead rinses can be used to recover chemicals for reuse in the process bath. Water use can be further reduced by:

  • Measuring and controlling the flow to prevent over use. Use flow meters and control valves, or flow restrictors to control flow. Flow restrictors work well if parts load is steady, while flow meters with valves work better if flow needs to be adjusted in steps for different types of parts. A good control valve set-up consists of a gate valve or needle valve used to set flow with a ball valve for easy on/off flow control. Set the gate valve and leave it alone as much as possible.
  • Counter-current, cascaded rinses add rinse tanks and allow a 3 to 10-fold reduction in water use for each tank added, to produce the same rinse purity in the final rinse stage.
  • Conductivity control turns rinse flow off when the rinse water remains pure enough for adequate rinsing and turns rinse flow on when higher rinse purity is needed. Conductivity control works well when parts or contaminant/dragout flow is uneven. Conductivity control using electrodeless probes tend to require less maintenance and thus have lower cost and higher reliability over their life.
  • Timers and solenoid valves can also be used when the parts or contaminant load is steady but intermittent.
  • Rinse water can be purified and recycled to further reduce water use. Reverse osmosis and electro dialysis are the most common rinse purification methods and pay off when water costs are high either because of scarcity or because the rinse water purity is high for precision cleaning operations.

Rinsing Resources

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Phosphatizing

Phosphatizing prepares the surface of metal parts for coating. The volume of water used to maintain the phosphatizing bath solution can be reduced by analyzing and controlling each bath's temperature, chemical concentration, and pH level; and by recirculating the solution or rinse water to other baths where possible. An added benefit is the potential for reduced chemical use. Read the MnTAP fact sheet, Metal Phosphatizing Operations, for more information.

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Solvent and Vapor Degreasing

There are a number of reasons to tightly control vapor degreaser emission. Solvent degreasers can cause high evaporation rates and toxic exposure to workers. Traditional solvents like trichloro-ethylene (TCE), methylene chloride (MeCl) and perchloroethylene (perc) are expensive to use due to regulations and chemical costs. A National Emissions Standard for Hazardous Air Pollutants (NESHAP) limits emissions from degreasers using these traditional solvents. Additionally, large uses of traditional solvents, over 10,000 pounds a year, require reporting under the Superfund Amendments and Reauthorization Act (SARA)/Toxic Release Inventory (TRI) regulations and pollution prevention planning under the Minnesota Toxic Pollution Prevention Act (TPPA).

Solvent use can be reduced by process and procedure improvements. Releases can be made safer by switching to a less hazardous chemical. Solvent use can sometimes be eliminated through the use of aqueous products.

MnTAP has an instrument to measure real-time chlorinated solvent emissions. We can use this instrument to help you identify opportunities for solvent emission reduction.

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Solvent and Vapor Degreasing Resources

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