I. Metal Cleaning
Metal cleaning is an essential function in manufacturing, repairing, and maintaining parts and equipment in industries such as furniture, machinery, electric and electronic equipment, instruments and metal fabricating.
The purpose of metal cleaning is to remove impurities from the metal surfaces in preparation of finishing processes such as electroplating and/or coating; a clean product diminishes the potential for rejects during further processing steps. The cleaning methods and materials used depend upon the type of contaminants, which typically are soils, rust, paint, and oil. Usually, because more than one type of contaminant is present on the metal surface, a series of cleaning operations is required to fully prepare a metal surface for its final treatment.
Below is a review of basic waste minimization options applicable to metal cleaning operations including strategies for developing pollution prevention techniques for most metal cleaning applications.
II. General Waste Minimization for Parts Cleaning
Each cleaning process generates a specific type of waste that must be managed and disposed. The costs of proper waste treatment and disposal, more stringent environmental regulations, and increased liability are forcing industries to reevaluate wastes generation practices.
Waste minimization in metal cleaning operations can be accomplished through source reduction, material reuse, and product recycling. The following steps will assist in reducing wastes from most metal cleaning operations.
- Preventing Surface Impurities
The primary step for control of surface impurities is to identify the sources of the pollutants. Surface pollutants can be incoming soils applied by metal vendors or contaminants such as coolants, stamping fluids, and rust inhibitors applied inhouse.
Peel coatings or shrink wrap may substitute for protective coatings of grease or paint which require solvent removal. These coverings will prevent dust and oils from depositing on the metal surfaces during shipment, storage, and handling. Because the used wrapping materials may be sellable to a recycler, revenues may be generated and disposal fees may be eliminated.
Moisture which causes rust to form can also be eliminated or reduced by properly drying the parts between operations or by storing them indoors away from condensation or rain.
- Selecting the Least Hazardous Cleaning Agent
Selection of a cleaning process for specific parts is determined by several factors including physical and chemical properties of contaminants, substrate surface, and cleaning media; the amount of contaminant to be removed; the required level of cleanliness; the complexity of the surface to be cleaned; the quantity of parts to be cleaned; the costs of materials/equipment/labor; and worker/environmental protection.
In the search for a less polluting cleaning process, the user should select the least toxic and most environmentally friendly medium that performs with the desired efficiency. Below are cleaning media and the order in which they should be evaluated:
- Water or air
- Abrasive media with water or air as carrier
- Aqueous solutions
- Acidic or caustic solutions
- Solvents
A new cleaning process or any proposed process changes must be carefully evaluated for potential effects on substrate and downstream processes such as anodizing, plating, and painting. Additionally, all regulations and existing permits that the facility must comply with should also be evaluated for the effects of a process change.
- Optimizing Cleaning Efficiency
If cleaning steps cannot be eliminated and the least toxic/hazardous materials are being used, the absolute smallest amount of cleaner necessary to obtain an acceptable product should be applied. Also, proper worker training and retraining are essential to ensure the materials are applied with the greatest achievable efficiency. Employees should also be given the opportunity to identify wastes and determine ways to eliminate and reduce them.
- Segregating Wastes and Reuse/Recycling
Source segregation will minimize the amount of wastes generated and disposed and also save on purchases of raw materials. In the metal cleaning industry, separating, reusing, and recycling used solvents result in enormous economic and environmental savings. With careful attention to the following handling and safety tips, a facility can profit from successful waste segregation.
- The solvents should not be allowed to mix with contaminants such as water, solids, and garbage.
- Hazardous materials should be separated from nonhazardous so that the latter will not be misclassified as hazardous.
- All used solvent containers should be properly labeled, covered, and stored in a restricted area that is kept cool and provides containment in the event of a release.
- Employees working with any type of material whether new or spent should be trained on proper handling practices.
- To comply with applicable regulations and prevent a spill incident, the facility must ensure that required procedures are followed and personnel safety equipment is worn when workers handle any regulated raw materials or wastes.
Facility conditions such as space for materials storage and/or recycling equipment; costs of equipment, training, and manpower; and suitability to processes determine whether spent solvents may be segregated and recycled on or offsite.
For more information on solvent recycling from metal cleaning operations or strategies for reducing wastes in all media, call the NC Office of Waste Reduction at (919) 715-6500; FAX to (919) 715-6794; or e-mail OWR
References
Waste Minimization in Metal Parts Cleaning. Office of Solid Waste and Emergency Response, U. S. Environmental Protection Agency. EPA/530SW89049. August 1989.
The North Carolina Office of Waste Reduction provides free, non-regulatory technical assistance and education on methods to eliminate, reduce, or recycle wastes before they become pollutants or require disposal. Industrial Pollution Prevention Section. September 1993.