METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES

Transcription

1 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES Prepared for: Process Technology 88 The Key to Hazardous Waste Minimization August 15-18, 1988 Sacramento, California Prepared by: Edvard R. Saltzberg, Ph.D. Science Aplications International Corporation 8400 Westpark Drive McLean, VA 22102

2 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES EDWARD R. SALTZBERG, PH.D SCIENCE APPLICATIONS INTERNATIONAL CORPORATION 8400 WESTPARK DRIVE, MCLEAN, VA (703) TOPIC NUMBER: 1000 The majority of metals and cyanide discharged by industry into the Nation s waterways comes from metal-finishing facilities, primarily from electroplating processes. The U.S. Environmental Protection Agency (EPA) figures released a few years ago, show that of the 34 industries covered by EPA s toxic wastewater regulations, metal finishers contribute 57 percent of the metals released to sewers. The degree of risk posed by these discharges is hard to determine because it depends on site-specific factors; however, small amounts of the types of chemicals discharged by metal finishers are toxic enough to cause EPA to set stringent, safe threshold levels for these chemicals in surface waters. I Because of the toxicity of pollutants in metal-finishing wastes and the amounts of these discharges, EPA established national wastewater regulations for this industry. The goal of these regulations is to reduce the contaminants in metal-finishing discharges to levels that are environmentally acceptable while remaining technically feasible and affordable for the industry. However, Federal wastewater regulations could have a severe economic impact on the metal-finishing industry. EPA has estimated that up to 20 percent of all electro- -1-

3 METHODS TO HINIMIZE WASTES FROM ELECTROPLATING FACILITIES 2 plating firms may close due to wastewater regulations. Since the conventional methods used to treat metal-finishing wastewater produce sludges that are subject to costly hazardous waste regulations, even a larger percentage of this industry than was estimated by EPA could go out of business. Fortunately, the economic outlook for cleaning up metal-finishing wastewater is not as dismal as EPA expected. Recently, some electroplating firms have found that very inexpensive changes in processing and waste control methods can greatly reduce the amount of wastewater and hazardous waste they generate. These techniques enable plating shops to avoid much of the cost of waste treatment originally estimated by EPA. Moreover, these techniques can actually pay for themselves in a very short period of time because they save large quantities of water and process chemicals. As a result, far less than 20 percent of the firms in the electroplating industry should close because of EPA's wastewater regulations. The new methods of plating waste controls that are being used and accepted by the industry are based on what energy conservationists have been proclaiming for more than a decade: and cheapest way to save energy is to avoid using it. the quickest, easiest Likewise, - the quickest, easiest and cheapest way to keep down pollution control costs is not to pollute in the first place. Accordingly, the new methods of pollution control for the electroplating industry rely on -2-

4 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES conservation practices - usually low cost technology methods that are inexpensive and easy to use. The new methods depend more on a change in attitude toward production than they do on expensive hardware. The central theme of the new methods is to strictly conserve and reuse water and chemicals, and to employ wastewater treatment technologies for compliance only when absolutely necessary. The new methods depend on reducing the drag-out of chemicals from plating baths and low rinse rates, but the techniques are slightly different, more effective, and less costly than those traditionally used in the industry. The principal measures used in the new methods are: (1) Multiple drag-outs -- a variation on single drag-out tanks; (2) Reactive rinsing -- a technique for reusing rinsewaters. Rather than reducing the size of end-of-pipe treatment equipment, the new methods of plating waste controls use multiple drag-out tanks and apply reactive rinsing techniques in order to avoid the need for end-of-pi-pe treatment equipment altogether, resulting in very small pollution control costs. In instances where some final waste treatment is still necessary, the cost of the treatment is still far less than it would be using standard techniques. -3-

5 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES EDWARD R. SALTZBERG. PH.D. This paper explains how to use these new methods. Before examining these new methods, a brief description of electroplating is presented to provide some background to those unfamiliar with this industry. OVERVIEW OF THE ELECTROPLATING PROCESS Electroplating is a process for applying a thin metal coating such as zinc, copper, nickel, chromium, etc. to the surface of metal parts, which are usually made of iron, steel, brass or aluminum. The coatings serve to protect the metal from corrosion, to build up the surface thickness, or to decorate the piece. Many commonly used items are electroplated. Automobile bumpers and door handles, for example, are often chrome plated; printed circuit boards are copper plated, and watch bands and necklaces can be gold or silver plated. PROCESS STEPS Most electroplating processes can be divided into three principal work steps as shown in Figure 1. Surface Preparation Surface preparation involves cleaning the part before it is plated. Cleaning is usually accomplished by placing the work piece in a tank containing a solvent or alkaline solution, and then in an acid dip to remove corrosion. Both the alkaline and the acid dip are followed by rinsing in running water. Plating Application In the second work step, a metal coating is applied from a solution containing the plating metal in dissolved -4-

6 Port Treatment PICUlfK 1 OMXVIKY OF ELZC IPOPUTINC PBocllSS -5-

7 HETHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES form and other chemicals. The part to be plated is placed in the solution and charged with electricity to attract the dissolved metal to its surface much like a magnet attracts iron filings. Plating is followed by rinsing with water to flush the process solution from the work piece. Post Treatment Some plating steps are followed by post treatment of the work pieces to color it or to add corrosion resistance. Chromate, for example, is a common post treatment for zinc and cadmium plating. Post treatment steps are also followed by rinsing in running water. Some electroplating processes are complete after the plating step and do not require post treatment. Sometimes the configuration of an electroplating process line will appear to be complicated. Because of space constraints, work flow requirements, ancillary components of a work step, or poor tank layout, the three plating steps can be difficult to recognize. For example, Figure 2 illustrates the layout of an anodizing line in the shop of an east coast plater. Anodizing is an electrolytic process for finishing aluminum. The three work steps are not easy to distinguish because of the positioning of the tanks and because several components comprise each work step. Nonetheless, there are still only three steps as identified below. The surface preparation step in the anodizing process begins with degreasing (Tank 1) in a hot solvent. Next, parts are further -6-

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9 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES cleaned in Tanks 2 and 3, which contain alkaline cleaners, and then etched in Tank 4. The plating step, in this case the anodizing of aluminum, can take place in any of five process tanks (8, 9, 10, 11, 12). Finally, post treatment of aluminum takes place in the dye tanks (18, 19, 21, 24, 25, 27, 29), and sealing of the dyes takes place in tanks 32, 33, and 34. Although the layout and number of the tanks complicate the plating sequence, there are still only three steps to this and all electroplating process lines. Usually, every component of each step is followed by rinsing in running water. SOURCES OF WASTE Wastes from electroplating shops originate in several ways. One source of pollution is from "drag-out," which is processing solution that clings to the work piece as it is removed from the plating bath. The amount of pollutants contributed by drag-out is a function of many factors including, the design of the racks or barrels carrying the parts to be plated, and the shape of the parts. Plating procedures and several interrelated parameters of the process solution, such as concentration of toxic chemicals, temperature, viscosity, and surface tension also affect drag-out rates. Rinsewater Large volumes of rinsewater are usually needed to clean the drag-out from the work with conventional rinsing techniques. Rinsing actually serves two purposes: -6-

10 WETHODS TO WINIMIZE WASTES FROM ELECTROPLATING FACILITIES 1. It cleans the part, which prevents staining and other quality control problems: 2. It protects subsequent process baths from "drag-in" contaminat ion. Because of high flow rates used in conventional rinsing techniques, rinsewaters are contaminated with relatively dilute concentrations of process solutions. Typically, rinsewaters that follow plating solutions contain between 15 and 100 milligrams per liter (mg/l) of the metal being plated.' Host job shops operate several plating lines such as zinc, copper, nickel, cadmium, and chromium. The rinsewaters discharged from each line are usually combined in a common pipe or floor trench, and the concentrations of the individual metals from each process are diluted in the entire volume of the shop's wastewater, usually to less than 40 mg/l for each metal.3 Spent Process Baths Another source of contamination from electroplating shops is used or spent process solutions. Platers routinely discard spent cleaners, acids, and bright dips. Although these solutions are not usually made up of metals, it is not uncommon to find cyanide and heavy metals in concentrations of several thousand milligrams per lirer in these solutions. This contamination is caused by drag-in from previous process cycles and from metals leached or dissolved from the work by the process chemicals. Plating solutions and other process chemicals containing high metal -9-

11 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES EDWARD R. SALTZBERG. PH.D. concentrations are rarely discarded. Instead, they are deconta- minated or rejuvenated in place so they are not usually a hazardous vaste problem. Other Sources Accidental spills, leaks, and floor drips of process solutions also can contribute to effluent contamination. Additional - pollution sources include sludges from the bottoms of plating baths generated during chemical purifications, backwash from plating tank filter systems, and stripping solutions. Although the contribution from all pollution sources varies from shop to shop, in almost every case the most significant pollution problem is drag-out and the resultant contaminated rinsewater. A recent survey in Cleveland underscores this point. The average rate of rinsewater discharged from 22 Cleveland electroplating shops was 18,500 gallons per day (gpd), whereas spent process solution accounted for only 60 gpd. 3 DRAG-OUT MINIMIZATION USING MULTIPLE DRAG-OUT TANKS Minimizing the amount of plating solution that is dragged from work pieces upon their removal from the process tank reduces the amount of contamination in the rinse tanks. A single drag-out tank, installed immediately following the plating process, will capture some of the contamination. Two or more drag-out tanks will capture most of it. The multiple drag-out technique is similar to counterflow rinsing - a common water conservation method - because it uses several rinse -10-

12 METHODS TO MINIMIZE WASTES PROW ELECTROPLATING FACILITIES tanks in series. The difference is that instead of a single drag- out tank and two or more running rinses, the multiple drag-out method uses several drag-out tanks and single running rinse, as Figure 3 illustrates. Host of the drag-out is captured in the first tank, leaving the second tank less contaminated than the first. As a result, the concentration of pollution in the discharge from the running rinse tank is lower than it would be if only one drag-out tank is used. More drag-out tanks lower the discharge concentration even further. As a rule of thumb, each drag-out tank reduces the discharge concentration by 50 percent. Accordingly, two drag-outs are twice as effective as one, and three drag-outs are four times as effective as one. The concentration of pollutants in the running rinse tank does not remain constant. As pollution builds up in the drag-out tanks, it also increases in the running rinse tank. However, the more dragout tanks used, the slower the buildup of contaminants in the running rinse. This is the principle behind multiple drag-out tanks and a key feature of the new methods of plating waste control. Using two or more drag-out tanks, the concentration of pollutants in the discharge from the running rinse tank can be held below effluent limits for extended periods of time. The length of time depends on five factors: (1) Concentration of the process solution: (2) Rate of drag-out; -11-

13 Work Flow [ Plating Tank 1 c 1 D<:y 1.1 D ~ ~ ~ u c Rinse Tank I PICW 3 MULTIPLE DRAG-Om To ph Neutralization -12-

14 METHODS TO MINIMIZE WASTES PROM ELECTROPLATING FACILITIES (3) Number and size of the drag-out tanks: (4) Rinse rate; (5) The number of rinse tanks in the plating shop. These factors and the handling of discarded drag-out solutions are examined below. THE FACTORS CONTROLLING POLLUTION CONCENTRATIONS IN RINSE-WATER Figure 4 is a graph illustrating the effect of the factors identified above on the concentration of pollutants in a running rinse tank. The upper solid curve plots the discharge concentration over time from a rinse station without any drag-out tanks, and the lower solid curve plots the concentration for one with two drag-out tanks. The curve for one drag-out tank would fall between the two curves shown in Figure 4. It takes longer for a two drag-out tank system to reach a certain Concentration such as "CR" than a system without a drag-out tank or with only one drag-out. The curve for three or more drag-out tanks would fall below the curve for two drag-outs and, consequently, take longer to reach a concentration of C, in the discharge from the rinse tank. The strength of the process solution also affects the concentration of pollutants in the rinse discharge. A high strength solution will pollute more quickly than a low strength solution and, consequently, -13-

15 . CP Concentration in Rinse Tank c* No Drag-out Tanks Days FICW 4 EFFECTS OF CONTROL PAibUfSTgPs ON RINSE TANK DISCEARCE CONCEHlgAIIOll -14-

16 METHODS TO I(INIl4IZE WASTES FROM ELECTROPLATING FACILITIES the rinsewater will reach C, sooner. Likewise, the higher the drag-out rate, the quicker the rinsewaters become contaminated. Figure 4 illustrates these effects for the two drag-out tank case. The solid curve is shifted upward for higher drag-out rates or highly concentrated process solutions and is illustrated by the upper dashed curve. Accordingly, the time to reach a specific discharge concentration is reduced. Lower values for these factors would shift the curve downward and increase the time to reach C,. Rinse rates have just the opposite effect. At higher rinse rates, the curve shifts downward because dilution in the rinse tanks is increased. As a result, the time to reach C, lower dashed curve illustrates this effect. is increased. The If C, is the effluent limit for the metal being plated then "T,", shown in Figure 4, indicates the time it takes a double drag-out rinse tank discharge system to reach this limit. This is also the time at which the drag-out solutions must be replaced or purified if C, is to be maintained in the discharge from the running rinse. Using two drag-outs, T, usually ranges from 2 to 16 hours, depending on the process concentration and the rinse rate. Actually, drag-out solutions need replacement far less frequently because government limitations apply to the firm's combined rinse- waters from all processes and not to individual tank discharges. -15-

17 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES Most processes have four rinse tanks (cleaner, acid, process, and post-treatment), so there is a four to one natural dilution within each process. Since plating shops generally provide several processes, there is additional natural dilution within the rinsewaters of the shop. Electroplating shops with five or more processes are not uncommon, and natural dilution can increase T, by a factor of "five times 4" or at least 20. Accordingly, the time for a particular drag-out solution (T,) to be replaced in order to keep a firm's combined discharge below a government limitation (C,) will range from 5 to 40 working days (assuming 8 hours of production per day). Figure 5 illustrates this effect. If the drag-out solution is replaced on schedule, then the rinsewater from that process can be discharged without further treatment because it will meet government wastewater limits. Accordingly the firm will save tens and even hundreds of thousands of dollars because it will not need a conventional end-of-pipe treatment system. In most cases, the ph of the discharge will still have to be regulated, but ph control for dilute rinsewater costs only a few thousand dollars even for large flow rates. 3 Replacing each drag-out solution every 1 to 8 weeks is not a very large burden on a plating shop and, considering the savings in pollution control costs, it is certainly worth the extra effort. Cleaners and acids, for example, are replaced on a similar schedule anyway, so drag-out control can easily become part of a firm's -16-

18 3 Processes Concentration in Combined Discharge (MI) / -* 10 Processes I I I I 1 1 IO Days PICURB 5 CONCE~TION IA COWIWED DISCUCI -17-

19 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES routine process maintenance program. What to do with drag-out solution (generally listed or characteristic hazardous waste) once it is discarded is addressed below. MANAGING DRAG-OUT SOLUTIONS There are two techniques for discarding drag-out solution. In the first, the entire volume of the first drag-out tank is drained before time "TR" is reached. It is replaced with the entire volume of the second tank, and tank 2 is filled with freshwater (in a two-tank system). If there are more than two drag-out tanks, then the last one is filled with freshwater and upstream tanks are replenished with solutions from the downstream tank. Alternatively, drag-out solution can be trickled from the appropriate drag-out tank as in a very slow counterflow rinse tank system. The advantage of this method is that a steady concentration is maintained in the running rinse instead of cycling from zero to the value that assures that C, is not exceeded in the firm's combined discharge. This technique is more expensive than the first, because it requires electronic controls to maintain the proper trickling rate. However, considering the convenience, it may be the preferred approach at many plating shops. There are three ways to handle discarded drag-out solutions: -18-

20 NETHODS TO NININIZE WASTES FROM ELECTROPLATING FACILITIES (1) Recycle drag-out solution to the plating bath; (2) Treatment in place; (3) Batch treatment on-site or off-site. The application of these methods depends on the chemistry and operating conditions of the processes and the size and geographic location of the firm. Each technique is discussed below. Recycle Drag-out Solution to the Plating Bath The best alternative for managing drag-out solution is to return it to the process bath from which it came. Total recycle eliminates the drag-out from being a hazardous waste problem and conserves process chemicals. Returning drag-out directly to the process is possible if the process bath is hot and there is sufficient evaporation to make room for the amount of solution dragged out each day. For example, nickel and chromium plating are both operated at or above 130'F and drag-out solution can be returned to these processes. Returning drag-out solution to the plating bath is a technique that should not be used indiscriminately because it can impact plating quality. For example, copper pyrophosphate plating is a heated process but contaminants build up in the bath because of chemical reactions that occur in the process solution. There are many plating processes on the market, and one cannot generalize about the application of drag-out recovery to all the -19-

21 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES processes. However, in general, the technique can be used after most heated plating baths. The manufacturer of the individual process chemicals will usually advise platers on the application of drag-out recovery to their processes. Unfortunately, many plating baths are operated at room temperature, and there is little evaporation from them. Accordingly, drag-out cannot be returned directly to these processes. In instances in which drag-out cannot be recycled directly, high technology recovery techniques such as reverse osmosis and evaporation have been used in plating shops to concentrate the drag-out into volumes small enough to be returned to cold process baths. However, there are two primary drawbacks to these techniques: o o They are very expensive; These techniques not only concentrate the excess solution in the drag-out tank, but they also concentrate the impurities in the drag-out. When the concentrate is returned to the process, the concentrated impurities can contaminate the plating solution and impair the quality of the plating. The concentrate can be purified using ion exchange to selectively remove impurities, but this drives up the already high cost-of recovery. In general, the following methods of handling drag-out solutions which cannot be returned to the plating bath are more cost effective than high technology controls. -20-

22 HETHODS TO HINIHIZE WASTES FROM ELECTROPLATING FACILITIES Treatment in Place When the drag-out solution cannot be returned to the plating tank the next best alternative is treatment in place. A process called integrated treatment was developed several years ago by Lancy Laboratories, Zelienpole, Pennsylvania. It was primarily designed to complete in-process the first step in the two-step treatment of chromium and cyanide. The second step would be carried out in the end-of-pipe treatment system. Electrolytic recovery of metals from the drag-out tank has become cost-effective in the last 2 years. This technique works best on zinc, cadmium, copper, silver, gold and tin processes. It is not effective on chromium and nickel. Batch Treatment on Site If the drag-out solution cannot be returned to the process tank or treated entirely in place, it has to be chemically treated, usually in a batch treatment system. In these cases, the drag-out solution can be handled like other spent electroplating process baths. In addition to drag-out solutions, spent baths from plating shops include cleaners, acids, and post treatment solutions. (Platers rarely discard plating solutions.) Spent baths can be treated on-site in the firm s own bath treatment system or shipped off-site to a private waste treater. Central processing facilities that recover metals may be able to refine drag-out solutions and certain other spent plating baths

23 HETHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES A diagram of a batch treatment system for spent plating baths (including drag-out solutions) is shown in Figure 6. The solutions to be treated are discharged on a schedule, and the size of the treatment system is designed for the schedule specific to each plating shop. Below is a typical weekly schedule for dumping plating baths at a medium-sized shop. o AciddAlkalies 300 gallons/week o Cyanides 100 gallondweek o Chrome 100 gallons/week o TOTAL 500 gallons/week For this schedule, the firm would need: o o o o o Three 5000 gallon holding tanks (one for each type of solution); A 750 gallon treatment tank; A filter to consolidate the sludge; A few chemical feed tanks; Some chemical control equipment. The system would cost about $60,000 including engineering and installation. If the firm used the new methods of plating waste control, this would be the major portion of the firm s waste treatment bill. If it didn t, the bill would be several hundred thousand dollars

24 - I Acid/Alkali ---h Holding Tank Cyanide Holding Tank Treatment * Tank Effluent to Drain 1 I Treatment Chemicals Chromium Holding Tank Sludge Filter To Disposal PICURX 6 BATCB TREA"T SYSTEM -23-

25 METHODS TO HINIMIZE WASTES FROM ELECTROPLATING FACILITIES EDWARD R. SALTZBERG. PH.D. WATER CONSERVATION THROUGH REACTIVE RINSING Reactive rinsing is a technique to reuse or recycle rinsewater one or more times before it is discharged. This technique takes advantage of the chemical reactivity of used rinsewater. Not only can a firm's water consumption be greatly reduced by recycling, but rinsing efficiency can actually be increased by this method, thus improving plating quality. Counterflow rinsing is the standard technique platers use to reduce flow rates. However, rinse tanks are expensive and require space for installation, while electroplating shops usually have trouble raising capital and are often located in tight quarters. Reactive rinsing is a very effective alternative to counterflow rinsing without the cost or logistical constraints. Reactive rinsing does not require additional rinse tanks, so it is less expensive than counterflow rinsing. It also does not use any additional space, so it has wider application than counterflow rinsing. INTRAPROCESS REACTIVE RINSING A typical nickel plating line is diagrammed in Figure 7. It is usually composed of three process steps: o o o An alkaline cleaning tank; An acid dip tank; A nickel plating tank. -24-

26 Work Flow- Fresh Water Fresh Water Fresh Wale c c FIGURE 7 INTBAPROCESS REACTIVE RINSIAG --- Reuse Flow Scheme -25-

27 HETHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES Each process step is followed by a running rinse tank, and each rinse tank has a separate freshwater feed line. The rinse tanks run at about 4 gpm, and the nickel plating line in Figure 7 uses a total of 12 gpm. Twelve gallons per minute is a lot of water. In a single 8-hour production day, this line alone accounts for 5,700 gallons of water. Other types of plating lines have more than three rinse tanks so they could use even more water than a nickel process. Therefore, it is little wonder that each plating shop employing standard rinsing methods uses tens of thousands of gallons of water each day. USING REACTIVE RINSING TO SAVE WATER The parts to be plated "drag-in" to the nickel tank whatever is in the previous rinse tank. If that rinse tank is fed with freshwater, the drag-in will be comprised primarily of a dilute acid solution, which will reduce both the nickel concentration in the process bath and, to a certain extent, the acidity of the bath. (Nickel solutions are slightly acidic.) Instead of using freshwater, the acid rinse tank could be fed with the discharge from the nickel rinse tank. Since the nickel rinse tank contains dilute process solution, it will feed the acid rinse tank with slightly acidic water containing nickel salts and other process additives. Accordingly, the drag-in from the acid rinse tank will partially replenish process chemicals in the nickel tank. This is an example of reactive rinsing. Nickel rinsewater does not harm the rinsing step after the acid bath, and it helps to -26-

28 HETHODS TO HINIHIZE WASTES FROM ELECTROPLATING FACILITIES conserve chemicals in the nickel plating tank and the freshwater feed line to the acid rinse tank can be turned off to save 4 gpm. This reactive rinsing application reduces water use and saves process chemicals without harming the rinse step. Some reactive rinsing applications actually aid rinsing and, therefore, improve plating quality. IMPROVING RINSING EFFICIENCY THROUGH REACTIVE RINSING A good example of how this technique can be used to improve rinsing efficiency is also illustrated in Figure 7. Cleaner solutions are alkaline--soapy and, therefore, difficult to rinse. Imagine trying to rinse off dishwashing detergent from dinner plates in cold water. It doesn t work very well because the detergent clings to the plates. In a kitchen, hot water is used to rinse dishes, but hot water is too expensive to use in plating cleaning lines because so much costly energy is needed to heat the water. Therefore, platers rinse cleaning solutions in cold water tanks and depend on the subsequent acid solutions to neutralize any cleaners still clinging to work. However, cleaner solutions are neutralizing agents and they reduce the useful life of the acid baths. Clearly, this is not a very efficient rinsing system. Acids are expensive, and their useful life should be prolonged, not reduced. Moreover, spent acid solutions are a costly waste manage- ment problem, which is a second compelling reason to lengthen the -27-

29 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES I BDWARD R. SALTZBERG, PH.D. time of service of acid baths. The answer to this problem is almost obvious -- use the acid rinse tank discharge to feed the cleaner rinse tank. If this is done, then neutralization of the drag-out from the cleaning process will occur in the cleaner rinse tank and not in the acid process tank. The parts get very well rinsed, and the life of the acid solution is prolonged. In this case, another 4 gpm of water is saved because the fresh feed to the cleaner rinse tank can be turned off. COST OF REACTIVE RINSING APPLICATIONS Reactive rinsing is not expensive. In this example, two thirds of the original flow rate vas saved without purchasing any additional tanks. Some plumbing is required but it is not extensive. The nickel rinse tank could be elevated slightly in order to gravity flow rinsewater to the receiving rinse tank. In this example, the nickel rinse would be raised above the acid rinse, which in turn would be raised above the cleaner rinse. Very little head pressure is needed to run rinsewater through the three tanks. The system will work if the nickel tank is raised or even tilted on standard building bricks, and the acid rinse tank is elevated half that distance. If it is not practical to elevate the tanks, then inexpensive submersible pumps can be used to move the rinsewater. In either case, reactive rinsing costs less than $250 per application. In contrast, counterflow rinsing, the conventional flow reduction technique, is many times more expensive than reactive rinsing, and its application is constrained by space limitations. -28-

30 METHODS TO WINIWIZE WASTES FROM ELECTROPLATING FACILITIES INTERPROCESS REACTIVE RINSING Interprocess reactive rinsing reuses rinsewater within a process and is a powerful technique for reducing water consumption. An even more powerful technique is interprocess reactive rinsing in which rinsewater is used across plating lines. Figure 8 diagrams this reactive rinsing concept. The Figure includes three plating lines: o Copper/nickel; o Zinc; o Anodizing. The plating shop in Figure 8 would require 12 freshwater feed lines (indicated by the numbers inside the rinse tanks). Using reactive rinsing, eight of these feed lines could be shut off. Some of the savings is from intraprocess reuse. For instance, used acid rinsewater is fed to the alkaline cleaning water rinse tanks in both metal-plating lines. Also, the anodizing rinsewater, which is very acidic, is used to feed the caustic etch rinse tank. The rest of the savings is due to interprocess reuse. The copper/nickel cleaning rinsewater is reused in the zinc cleaning steps and then again to feed the anodizing rinse tank for the nonetch cleaner. Rinsewater from the zinc process also is used in the rinse tank following the anodizing dye step. Rinsewater cannot be reused indiscriminately, because it could contaminate plating baths and effect plating quality. Accordingly, -29-

31 UP Non-Etch To Drain To Drain h t To Drain I $ Fresh Water Feed Lines FIGURE 8 IVE EXAHPLE OF INTERPROCESS BIRSIAG SYSTHn

32 METHODS TO ninihize WASTES FRON ELECTROPLATING FACILITIES EDWARD R. SALTZBERC, PH.D. reactive rinsing should be done with the guidance of a chemist or chemical engineer experienced with the firm s specific plating process. However, there are many applications for interprocess reactive rinsing that are safe similar to the ones shown in Figure 8. COST SAVINGS OF REACTIVE RINSING Reactive rinsing can save plating shops a considerable amount of money. The 12 rinse tanks in Figure 8, for example, would use 23,000 gpd if they each ran at 4 gpm. Reactive rinsing could cut the number of freshwater feed lines to four, and the shop would reduce its water use by 15,000 gpd. In Boston, Massachusetts, for example, water costs approximately $1.50 per gallons. Using this value, reactive rinsing would save $22.50 per day or $4,950 per year. The cost of the reactive rinsing program for this example is $1,750 ($250 per application) and would pay for itself in reduced water costs in less than 4 months. Clearly, reactive rinsing is a very good investment. SUMMARY -- A STRATEGY FOR APPLYING THE NEW METHODS OF PLATING WASTE CONTROL Electroplating firms typically discharge large amounts of rinsewater. In Cleveland, Ohio, for example, the average water consumption at 22 electroplating firms is 18,500 gpd. The discharge volumes at these firms range from 10,000 gpd at small shops to -31-

33 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES 150,000 gpd for very large electroplating operations. Conversely, electroplaters do not discharge large volumes of spent process solutions. The Cleveland firms each generate about 60 gpd of spent bath. However, the concentration of certain contaminants in spent baths is much higher than it is in plating wastewaters. The average concentration for each pollutant ranges up to 20 mg/l. A typical spent acid solution, for example, will contain three times the amount of zinc and nickel as a typical rinsewater discharge. Although there are other sources of pollution in plating shops such as leaks from process tanks and floor drippings, the major sources of pollution are drag-out in rinsewaters and spent process solutions. The most practical approach to controlling plating waste is, therefore, to concentrate on these two sources. However, the pollution load characteristics of rinsewater and spent baths differ. Rinsewaters are generally dilute but are large in volume, while spent solutions are usually small in volume but concentrated. Accordingly, these two pollution sources should be handled differently. It is the different characteristics of the two pollution sources that is the basis for the new methods of waste control for electroplating shops. POLLUTION LOAD CHARACTERISTICS OF RINSEWATER Typically, the concen- trations of individual metals in the untreated discharge from elec- troplating range from 10 to 20 mg/l. EPA regulations for individual -32-

34 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES metals are about 1 mg/1.6 Accordingly, if most of the drag-out is captured before the work piece is rinsed, then the rinsewater could meet EPA standards without treatment. Platers have long used a single drag-out tank to minimize the contamination of rinsewater and to conserve process chemicals. However, a single drag-out tank can rarely achieve enough drag-out control to meet discharge limits. The new methods carry the older drag-out control techniques a step or two further in order to achieve extra control of drag-out. Instead of one drag-out tank and counterflow rinse tank, the new methods use two or more drag-out tanks and one rinse tank. In addition, the new methods make extensive use of recycled rinsewater. The basic premise of the new methods for controlling wastewater is if the solution is dilute to start with, take steps to make it even less concentrated so that it can be discharged without end-of-pipe treatment. POLLUTION LOAD CHARACTERISTICS OF SPENT SOLUTIONS Spent process solutions are small in volume, but they are concentrated. There- fore, they should not be handled like rinsewaters. In a conven- tional treatment system, spent baths are trickled into the firm's rinsewater discharge for treatment with the rinses. In doing so, the concentrated solutions are first diluted in the rinsewater and then their contaminants are removed and reconcentrated. It is more straightforward to treat the concentrated solution separately through chemical physical means, or recover and reuse the solution. -33-

35 HETHODS TO HINIHIZE WASTES FROM ELECTROPLATING FACILITIES EDWARD R. SALTZBERG, PH.D. The premise for the new method of controlling spent baths is discharge is already concentrated, try to concentrate it further for reuse, batch treatment, or disposal. The goals of the new methods of plating waste control are to avoid wastewater treatment and to minimize hazardous waste problems while meeting environmental regulations. -34-

36 METHODS TO MINIMIZE WASTES FROM ELECTROPLATING FACILITIES EDWARD R. SALTZBERG. PH.D. LIST OF REFERENCES 'EPA, Assessment of the Impacts of Industrial Dischargers on Public'ly Owned Treatment Works, NTIS No. PB , February 902. 'EPA, Economic Analysis of Proposed Pretreatment Standards for Existing Sources of the Electroplating Point Source Category, EPA-230/ , December 'EPA, Control and Treatment Technology for the Metal Finishing Industry, EPA-625/ , January I EPA, Third Conference on Advanced Pollution Control for the Metal Finishing Industry, EPA-600/ , February EPA, Development Document for Existing Point Source Pretreatment Standards for the Electroplating Point Source Category, EPA-440/ , August Federal Register, Effluent Guidelines and Standards: Electroplating Point Source Category Pretreatment Standards for Existing Sources, Federal Register, 16(18): , January 28,