An article from the Journal of Environmental Regulation, about successful pollution prevention strategies in a Zinc Die-Casting Plant.
Eugene Park, Richard T. Enander, and
Richard G. Girasole Jr.
Various types of business struggle with meeting regulatory requirements related to their wastewater discharges. Often, the only alternatives available for these companies are to clean; up the restricted wastes or to reduce or eliminate them from the waste stream through redesigned processes. This article describes how pollution prevention methods paid off in the case of the plant that manufactures miniatures die casts. Not only have a number of the chemicals from the original production process been eliminated, but also product rejection rates have actually decreased as a result of process changes.
Since the late 1980s, EPA has administrated grants to state governments and universities to promote pollution prevention at the state level. The Rhode Island Department of Environmental Management (DEM) received a such a grant in 1989 to begin a pollution prevention program and has adopted a nonregulatory approach to solving waste problems by offering free technical assistance to Rhode Island companies. In conjunction with the University of Rhode Island's (URI's) Chemical Engineering Department, the DEM pollution prevention section has assisted more than 200 companies as of March 1995, resulting in the elimination of over 52 million gallons of industrial waste and wastewater. The impact of pollution prevention on Rhode Island industry has been significant.
Eugene Park Ph.D., is a research professor at the Chemical Engineering Department of the University of Rhode Island. Since 1989, he has provided pollution prevention technical assistance to numerous Rhode Island companies. Richard T. Enander is a principal environmental scientist and pollution prevention program manager with the Rhode Island Department of Environmental Management's Office of Environmental Management's Office of Environmental Coordination. Richard G. Girasole, Jr., is an engineer with the Rhode Island Department of Environmental Management's Pollution Prevention Program. He has performed over 150 company assessments, providing pollution prevention assistance to state manufacturers.
Vibratory and tubbing operations are metal-finishing techniques that are found in a variety of industries. Typical industry types include jewelry manufacturing and machine tool manufacturing. The size of the metal parts to be finished range from small jewelry pieces that measure a fraction of an inch to large, heavy, metal parts that are several inches long.
In order to clean and/or polish metal parts, aqueous-based chemicals and ceramic or plastic media are typically used in either a vibrating basin or a rotating barrel. Occasionally, metal-based media like steel shot is used. In most cases, the waste effluent from these operations usually consists of soapy water, pulverized media, and both soluble and insoluble metals of the type found in the metal parts being processed. Traditional waste treatment techniques have included chemical flocculation and settling to remove metal contaminants prior to sewer discharge. In most instances, where other types of metal-finishing and plating operations exist in the plant, the vibratory waste effluent is mixed with other metal-bearing waste streams prior to treatment and disposal.
Pollution prevention for these operations generally consists of source segregation, the elimination of treatment chemicals, the reduction of sludge generation, water/soap conservation, and metal recovery. Though no two processes are exactly the same, this article profiles a typical tubbing operation and the types of technologies and methods that can be used by companies to reduce operating/environmental costs and improve overall operating efficiency.
Since its founding in the early 1960s, Miniature Casting Corp. has been involved in the manufacture of precision miniature zinc die castings. As a fully integrated facility, the company is responsible for the design and construction of its own die-casting equipment as well as the production of castings serving a variety of industries. In the early days of operation, all effluents of the die-casting process were discharged into the city sewer system including, 1,000 gallons per day of noncontact cooling water and soapy discharge from the vibratory finishing operation. When the company relocated in 1988 to its existing location in Cranston, Rhode Island, a conscious effort was made to undertake an effective environmental management/pollution prevention program. In 1989, the company contacted the Rhode Island DEM's pollution prevention section. Miniature Casting sought to determine the most efficient and cost-effective means of reducing and managing its vibratory waste effluent. In late 1990, DEM's pollution prevention program helped Miniature Casting applying for and win a $24,000 U.S. EPA small-business pollution prevention grant to carry out a year-long study on vibratory solution recycling using membrane technologies.
Pollution prevention consists of source segregation, the elimination of treatment chemicals, the reduction of sludge generation, water/soap conservation and metal recovery.
As part of the manufacturing process, precision zinc die castings are cleaned and deburred with vibratory finishing equipment. For heavily soiled parts, about 10 percent of the die castings had to be precleaned with mineral spirits prior to vibratory finishing. All of the effluent from the tubbing machines was mixed and diluted with noncontact cooling water and discharged directly to the publicly owned treatment works (POTW) (Exhibit 1). The objective of the EPA-funded study was to incorporate membrane technologies, such as ultrafiltration, into a working pollution prevention program for the cleaning operation. Principal investigators anticipated that the mineral spirits could also be eliminated and replaced with an aqueous cleaner; membranes could be used to clean and recycle all of the spent cleaning solution. An additional objective was to recover all die-cast metal concentrate for off-site recovery.
Ten tons of zinc alloy bars are melted down every month in the die-casting operation of the manufacturing process. Approximately 10 percent of the die casts exhibited noticeably more carbon deposits as a result of high temperatures, pressures, and the presence of burning oils, than the remaining 90 percent of the parts. Although all parts are cleaned in the vibratory machines, 10 percent of the pieces had to undergo an additional precleaning step with mineral spirits prior to the vibratory cleaning. The mineral spirits cleaner was changed over every month by an outside solvent supplier. This practice was costly, required the use of a hazardous waste manifest, and had potential liabilities associated with it. On a daily basis, 200-300 gallons of water were used with approximately two gallons of soap concentrate in the vibratory operation. The effluent was filtered through a ten-micron cartridge filter and mixed with 1,000 gallons per day of noncontact cooling water prior to discharge. The local POTW required a sewer discharge permit with monitoring and treatment requirements. Though the discharge met permit requirements, the company knew that its practice of "dilution" would not be allowed in the future.
Exhibit 1. Original Plant Operation
See Data Source
The company contacted the DEM pollution prevention program in 1989. Assessment consisting of DEM personnel and URI engineers toured the facility and identified the problem areas listed previously. URI test equipment was brought to the site to determine its applicability as well as to train facility personnel on the technology's use and overall concepts of pollution prevention.
The project was divided into three phases, during which several membrane filters and acids were tried, as well as some major procedural modifications. Exhibit 2 depicts the process-flow scheme used in the first two phases of the project (i.e., when acids were used to adjust pH and facilitate sludge removal and the membrane system was used to recycle all rinse waters). Exhibit 3, which represents the final process design, shows how major procedural changes have vastly simplified the operation.
In Phase 1 (Exhibit 2), mineral spirits was eliminated entirely from the cleaning operation. An aqueous-based cleaner (Oakite M3) was used as an effective replacement. In Phase 2, different acids were tested for pH adjustment and sludge settling. In Phase 2, different acids were tested for pH adjustment and sludge settling. In Phase 3 (Exhibit 3), acid use and rinsing were also eliminated. As shown in Exhibit 3, the final process has been simplified considerably. Not only has metal-contaminated waste-water discharge to the sewer been eliminated, but also soap use has been reduced by almost 90 percent. Tap water is actually added to thesystem to make up for natural evaporation. A total of 100 gallons of solution is continuously recycled. The sludge is periodically removed and sent back to the metal supplier for reclamation; analysis of the sludge is shown in Exhibit 4.
The cleanliness of the parts resulting from the new process (Exhibit 30 was monitored in-house against normal production quality requirements. The following section provides a more detailed discussion of the resulted obtained.
The cleanliness of the parts resulting from the new process was monitored in-house against normal production quality requirements.
Although the original project objective included the use of acids and filtration methods of minimize waste discharges, developmental testing and improvements obtained throughout the year-long project indicated that acid was no longer required and that the need for membrane filtration was much less than originally anticipated. The company has continually reevaluated its process and reapplied the pollution prevention philosophy in order to use the most efficient and economical process it can. Pollution has been achieved through:
Exhibit 2. Flow Scheme for Vibratory Solution Recycle
(Phase 1 and 2) See Data Source
Elimination of mineral spirits to preclean parts by increasing the strength and broadening the use of the aqueous cleaner.
Elimination of acid used to settle and remove sludge. Although sludge could be more easily removed through pH adjustment and settling, it was discovered that eliminating the use of acid did not affect the cleaning and quality of the parts; in fact, the chemistry of the solution could be maintained longer while much less soap was required, and the sludge settled naturally as well.
Elimination of a 60,000 gal/year sever discharge of metal-bearing wastewater. All solutions have been continuously reused for 12 months with absolutely no effluent sewer discharge.
Recovery of zinc metal for off-site recycling. All sludge is recovered from the operation and is sent back to the metal for reclamation.
Recycling of an aqueous-based soap. The caustic cleaner is continually reused; small amounts are added periodically to maintain the necessary soap strength. Soap purchases have been reduced by approximately 90 percent.
The only filtration required is ultrafiltration for cleaning the entire system once every six months (Exhibit 3). Except as a prefilter for the membrane, cartridge filters are no longer used and the rinsing step has been eliminated. In addition, product rejection rates have actually decreased in comparison with rejection rates experienced before the project was initiated.
In Phase 3, the process was simplified and optimized: acid use was eliminated, the rinsing step was eliminated, the ten-micron cartridge filters used prior to the project and during Phase 1 and 2 were also eliminated, less sludge was created, membrane system operation was reduced by 90 percent, and the product quality had actually improved. In addition to the environmental benefits achieved, many costs savings were quickly realized. Exhibit 5 shows a direct cost comparison between the present recycling operation and the old, chemical treatment/sewer discharge operation of 1990. Not included in the exhibit are the costs incurred by the test program itself (i.e.,DEM staff, URI chemical engineers and analytical test costs) where were covered mostly by the EPA grant. Also not listed are the savings obtained by increased productivity that resulted from fewer rejected lots of product. Based on capital expenses and annual operating cost calculations, a significant payback was observed in less than one year.
Exhibit 4. Sludge Analysis
See Data Source
This project demonstrates a cost-effective environmentally sound solution to vibratory waste management. The project has demonstrated success in pollution prevention and the recycling of vibrating solution in a zinc die-casting industry. On September 29, 1992, Robert Piacitelli, president of Miniature Casting, received a recognition award for note-worthy contributions and special Administration Julie Belaga. Similar programs at other Rhode Island facilities have begun. Although the vibratory/tubbing operation is ubiquitous, many parameters exist that render each individual operation unique. The type of metal to be cleaned, the type of soap used, the desired finish on the metal, the type of vibratory media used, and even the incoming tap water supply must all be considered.
An economic analysis of this project displays a favorable payback. Although direct technology transfer has already been demonstrated in other Rhode Island companies, it is anticipated that the data collected and experience acquired from this project will facilitate programs at other companies to the extent that developmental work and costs are minimized. As of March 1995, more than 60 Rhode Island companies have successfully implemented similar programs at their facilities. If more information on this EPA-funded project is desired, a detailed technical paper is available from the Rhode Island DEM Pollution Prevention Office, 83 Park Street, Providence, Rhode Island 02903, (401) 277-3434
Exhibit 5. Economic Analysis of Vibratory/Cleaning Operation See Data Source
This work was carried out in conjunction with Miniature Casting Corporation of Cranston, Rhode Island, and funded by the Rhode Island Department of Environmental Management and a $24,000 U.S. EPA small-business pollution prevention grant.