ma. ASSOCIATES Submitted to the Environmental Protection Agencsv CONSERVATION AND REUSE METHODS A MANUAL OF NEW WASTE FOR THE ELECTROPLATING INDUSTRY

Transcription

1 A MANUAL OF NEW WASTE CONSERVATON AND REUSE METHODS FOR THE ELECTROPLATNG NDUSTRY Submitted to the Environmental Protection Agencsv ma. ASSOCATES A Company of Science Applications, nc.

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3 A MANUAL OF NEW WASTE CONSERVATON AND REUSE METHODS FOR THE ELECTROPLATNG NDUSTRY Prepared for: Environmental Protection Agency 401 M Street, S.W. Washington, D.C Prepared under: EPA Contract No JRB Project No Edward R. Saltzberg, Ph.D. Vice President JRB Associates 8400 Westpark Drive McLean, Virginia October 22, 1982

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5 TABLE OF CONTENTS ABOUT THE AUTHOR WRODUCT ON 1. OVERVEW OF THE ELECTROPLATNG PROCESS 1.1 Process Steps 1.2 Sources of Water Pollution Rinsewater Spent Process Baths Other Sources 2. CO"T0NAL N-PROCESS CONTROLS Page i Common n-process Controls Proper Rinse Tank Design and Operation Conventional Water and Waste Reduction 2-4 Methods High Technology Controls 2.2 End-of-Pipe Treatment Costs of Treating Electroplating Wastewater Using Conventional Control Technologies NEW METHODS OF PLATNG WASTE CONTROL FOR ELECTROPLATERS Drag-out Controls Using Multiple Drag-out Tanks The Factors Controlling Concentrations in Kinse Water Managing Drag-out Solution Water Conservation Through Reactive Rinsing ntraprocess Reactive Rinsing nterprocess Reactive Rinsing Cost Savings of Reactive Rinsing 3.3 Summary - A Strategy for Applying the New Methods of Plating Waste Control Pollution bad Characteristics of Rinse Water Pollution bad Characteristics of Spent Solution CASE STUDES OF THE NEW METHODS OF PLATNG WASTE CONTROLS Typical Results at a Medium-sized Plating Shop Case 1: Case 2: Results of Using Standard Methods Moderate Use of the New Methods Case 3: Extensive Use of the New Methods 4.2 Results of the New Methods at a Large Job Shop 4-4 in New England Waste Control Program APPENDX Cost Savings 4-7 B BLOGKAPHY

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7 LST OF TABLES Page 2-1 End-of-Pipe Treatment Costs 3-1 Direct Drag-out Recovery Potential of Electroplating Processes 3-2 Effluent Characteristics of 22 Cleveland Electroplating Shops 3-3 Analysis of Typical Spent Process Solutions From Cleveland Platers 4-1 Comparison of Waste Reduction Uternatives For a Medium Size Electroplating Firm 4-2 Effectiveness of the Waste Reuse Program at a Large Metal Finishing Job Shop in New England 4-3 Cost of the Waste Reuse Program at a Large Metal Finishing Job Shop in New England LST OF FGURES Overview of the Electroplating Process Anodizing Line Proper Rinse Tank Design Counterflow Rinsing Single Drag-ut Method Block Diagram of a Conventional Electroplating Treatment System Multiple Drag-out Effects of Control Parameters on Rinse Tank Discharge Co ncent ra t io n Concentration in Combined Discharge ntegrated Treatment for Chromium and Cyanide Plating Rinse Water The Polish Waste Recovery System Batch Treatment System ntraprocess Reactive Rinsing Example of nterprocess Reactive Rinsing System Daily Rinsewater Consumption at a Large Metal Finishing Job Shop in New England Page

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9 NTRODUCTON

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11 NTRODUCTON 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 in July, 1982 show that of the 34 industries covered by EPA's toxic wastewater regulations, metal finishers contribute 57 percent of the metals released to sewers (EPA, 1982b P. 1-3). &st of the constituents in metal-finishing wastes (e.g. metals, cyanides, and acids) are toxic and potentially threaten aquatic life and human health. 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 threshhold levels for these chemicals in surface waters. Typical EPA clean water targets, or water quality criteria for metals, range from 96 mg/l for nickel to.025 mg/l for cadmimm (EPA, 1980b), but the concentrations of these metals in untreated metal finishing discharges are several times these values (EPA, 1982a, p. 3). Even when diluted by the volume of the receiving water, metal-finishing discharges can severely degrade water quality. STANDARD PLATNG WASTE TECHNQUES HAVE HGH ECONOMC MPACT As a result of the toxicity of pollutants in metal-finishing wastes and of the volume of these discharges, this industry is subject to national wastewater regulations established by EPA. 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. Most people agree that sound, cost-effective control of toxic pollutants, like those found in metal-finishing wastewaters, i JRB Associates

12 -"- is a national priority. However, Federal wastewater regulations could have a severe economic impact on the metal-finishing industry. n 1979, EPA estimated that up to 20 percent of all electroplating firms may close because of wastewater regulations alone (EPA, 1977, P-100). 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. This might be a high price to pay for the amunt of environmental improvement that would result. NEW METHODS OF PLATNG WASTE CONTROL HAVE LOW ECONOMC MPACTS Fortunately, the economic outlook for cleaning up metal-finishing waste- water is not as dismal as EPA expected. Kecently, some electroplating firms have found that very inexpensive changes in processing and waste control methods can greatly reduce the amunt 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 actually pay for them- selves 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; the quickest, easiest and cheapest way to save energy is to avoid using it. Likewise, the quickest, easiest pollution control costs is not to pollute in the first place. ---A cheapest way to keep down Accordingly, the new methods of pollution control for the electroplating industry rely on conservation practices - usually low technology methods that are lnex?ensive and easy to ii JRB Associates -

13 use. The new methods depend rmre 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 treat- ment technologies for compliance only when absolutely necessary. PURPOSE OF TU MANUAL The purpose of this manual is to demonstrate how to use the new waste control techniques so that mre electroplaters can greatly reduce their pollution control costs. These techniques are very easy to understand, and many electro- platers will be able to use them based only on the information provided in this document. The manual was designed to help electroplaters apply the methods rather than to derive the mathematics that prove their application. t is not intended to be a rigorous scientific evaluation of in-process waste control measures; the manual contains no equations, mathematical proofs, or complicated charts and tables. nstead, it presents common sense approaches for minimizing the generation of wastewater that are easy to understand and implement. The manual also provides examples of how to use the methods to solve specific waste control problems, and it presents field data on the cost and technical ef fectlveness of the various methods. Readers interested in additional examples of theoretical applications of commn sense methods to control plating wastes should read Water and Waste Control for the Plating Shop, by Joseph Kushner. This book is a compendium of clever ways to reduce pollution control problems at their source. Two recent EPA publications also contain useful theoretical L nforma t io n : Enviro nmen t 31 Po 1.1 ut io n Co nt ro 1 Alternatives ( EPA iii JRB Associates

14 and Control and Treatment Technology for the Metal Finishing ndustry (EPA 625/ ). These publications present some of the mathematics of waste control not included in this manual (See Bibliography for complete citations of these pub licat io ns 1. ORGANZATON OF THE MANUAL This manual is divided into four chapters. Chapter 1 provides background information on electroplating processes. t covers the steps involved in plating and describes how waste is generated at electroplating shops. Chapter 2 discusses the conventional waste conservation measures that electroplaters have used and describes the standard methods of waste treatment. These methods are hardware intensive and, compared with the new methods, are very costly. Chapter 3 describes the new methods of waste control that enable electro- platers to meet EPA discharge standards at mini&l expense. Data documenting the costs and the technical effectiveness of the new methods are sparse; there are no case studies, either in previous EPA documents or in the general literature of firms using the new methods. Chapter 4 narrows this information gap by illustrating the cost effectiveness of the new methods in two ways. First, an example electroplating firm is used to compare the costs of standard treatment practices to the costs of the new methods. The comparison is based on actual unit cost data, but it is still theoretical. Second, the results of actually using the new methods at a large electroplating job shop are reported, and the costs of the new methods are compared with the costs the firm would have faced if it used standard methods. The cost comparisor iv JRB Associates

15 indicates that the entire water pollution control program for the electro- plating job shop pays for itself in less than two years just through savings in water use that result from using the new methods of plating waste control. L JRB Associates - V

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17 1. OVl3RVEW OF THE ELECTROPLATNG PROCESS

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19 1. OVERVEW OF THE ELECTROPLATNG 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 comnly used items are electroplated. For examplc automobile bumpers and door handles are often chrome plated; printed circuit boards are copper plated; and watch bands and necklaces can be gold or silver plated. 1.1 PROCESS STEPS Most electroplating processes can be divided into three principal work steps as shown in Figure 1-1: 1. Surf ace Preparation Surface preparation involves steps to clean 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 remve corrosion. Both the alkaline and the acid dip are followed by rinsing in running water. 2. Plating Application n the second work step, a metal coating is applied from a solution containing the plating metal in dissolved 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. 3. Post Treatment Some plating steps are followed by post treatment of the work piece to color it or to add corrosion resistance. Chromate, for example, is a commn 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. 1-1 JRB Associates

20 .- WORK STEPS Surfaa Preparation Fresh Water,+ Rinse ToDrain Plating Applio~tion Post Treatment Figure 1-1. Overview of the Electroplating Process 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 1-2 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 cleaned in 1-2 JRB Associates

21 1 c L> -- Water Water Rinse Rinse Sook Etch Cdd Alkalr Water Ckmer clepnn Dip Rinse L (Chromr a a Phorphorr) (Nitru) Wme ACN m D u) v) s. u) 3 Figure 1-2. Anodizing Line

22 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. 1.2 SOURCES OF WATER POLLUTON Contaminants in the effluent from electroplating shops originate in several ways. The most obvious source of pollution is from "drag-out", which is processing solution that clings to the work piece and contaminates the rinse water. The amunt of pollutants contributed by dragaut 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 af fect drago ut rates Rinsewater Large volumes of rinse water are usually needed to clean the drag-out from the work with conventional rinsing techniques. Rinsing actually serves two purposes: (1) t cleans the part, which prevents staining and other quality control problems; (2) t protects subsequent process baths from "drag-in" contamina t io ne Because of high flow rates used in conventional rinsing techniques, rinse waters are contaminated with relatively dilute concentrations of process 1-4 J RB Associates

23 solutions. Typically, rinse waters that follow plating solutions contain between 15 and 100 milligrams per liter (mg/l) of the metal being plated (EPA, 1982a, P. 2). Most job shops operate several plating lines such as zinc, copper, nickel, cadmium, and chromium. The rinse waters discharged from each line are usually combined in a commn pipe or floor trench, and the concentrations of the individu metals from each process are diluted in the entire volume of the shop's wastewate usually to less than 50 mg/l each (EPA, 1982a, P.3) Spent Process Baths Another souree of contamination from electroplating shops is used or spent process solutions. Platers 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 liter in these solutions. This contamination is caused by drag-in from previous process cycles and from metals leached from the work by the process chemicals. Plating solutions and other process chemicals containing high meta concentrations are rarely discarded. nstead, they are decontaminated or rejuve- nated in place so they are usually not a hazardous waste problem Other Sources Accidental spills, leaks, and drips of process solutions also can contribute to effluent contamination. Additional pollution sources include sludges from the bottoms of plating baths generated during chemical purification, 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 1-5 JRB Associates

24 rinse water. A recent survey in Cleveland underscores this point. The average rate of rinse water discharged from 22 Cleveland electroplating shops was 18,500 gallons per day (gpd), whereas spent process solution accounted for only 60 gpd (EPA, 1982a, P-3). 1-6 J RB Associates

25 2. CONVENTONAL N-PROCESS CONTROLS

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27 2. CONVENTONAL POLLUTON CONTROL TEC,HNOLOGES The capital cost of standard equipment to treat electroplating waste- water depends upon the volume of water being treated. To reduce this cost, platers have for several years used techniques to reduce rinsing rates without affecting plating quality. The operating costs of the treatment equipment depend upon the amount of contamination in the wastewater, and platers have, therefore, used methods to reduce the drag-out from process baths to the rinse tanks. These conventional rinse reduction and drag-out techniques were designed to minimize the cost of wastewater treatment. Although they are effective, they fall far short of the results achieved by the new methods discussed in Chapter 3. The difference is that the conventional methods attempt to minimize waste treatment while the new methods try to avoid waste treatment altogether by using the natural resources of the plating shop. The conventional methods of waste treatment are described in this chapter. n Chapter 3, the newer methods are compared against them. The first part of Chapter 2 describes the conventional methods of reducing rinsing rates and drag-out. The second part examines the conventional end-of-pipe treatment technologies used to treat electroplating discharges 20 1 COMMON N-PROCESS CONTROLS This section is divided into three parts: (1) (2) Proper rinse tank design and operation Conventional water- and waste-reduction methods (3) rugh technology recovery processes. Approximate costs for these controls are provided in th-3 section for comparison purposes and are based on rhe author's experience unless otherwise referenced 2-1 JRB Associates

28 2.1.1 Proper Rinse Tank Design and Operation The measures described in this subsection are basic to all waste control programs for electroplaters. Regardless of whether the firm uses the conven- tional conservation practices discuseed in Section or the more innovative techniques described in Chapter 3, all rinse tanks should be properly designed and should contain the gquipment described below. Figure 2-1 illustrates a properly designed rinse tank. Uw.e Tagk Features A properly designed rinse tank should be supplied with water from a distri- butlon pipe or sgardulator at the bottom of the tank. through holes drilled in the pipe at 3-inch intervals. Fresh water enters the tank The incoming water creates a rolling action, to help scour the work clean, and the entire tank volume dilutes the drag-out. A dam-type discharge also improves rinsing. The tank shown in Figure 2-1 has both of these features. Flow Controllers At many plating shops the shop personnel use more water than is necessary to adequately rinse parts. Mechanical flow control valves solve this problem by restricting the amunt of water that can be introduced to a rinse tank. Flow controllers maintain a specified flow rate independent of the faucet setting. The valves remve the control of rinse rates from the operator and water is not wasted. Flow controllers are rated in 1/2 gallon per minute (gpm) intervals. Most plating applications require valves in the 1.0 to 5 gpm range. Mechanical flow control valves cost about $10 each, including installation. Alternatively, electronic controllers that regulate rinse 2-2 JRB Associates

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30 rates based on the conductivity of the rinsewater are also marketed. However, they are expensive (about $200 each), sensitive to operate, require maintenance, and, for mst applications, are not much mre effective than mechanical valves in reducing water use. &ir Agitation The effectiveness of rinsing can be improved by turbulating the water in the tank. Too often, turbulation is accomplished by high rinsing rates, which is wasteful. &r agitation is an alternative to that practice. Air bubbles introduced into the rinse tank help to dislodge the plating solution from the work piece. The amount of air required depends upon the surface area of the tank. The bubbles materially improve the rinsing efficiency of each unit of water by increasing agitation. Air is generally supplied from a low pressure blower and introduced through a pipe distributor located diagonally across the bottom of the tank, as shown in Figure 2-1. Air agitation costs about $60 to install in a rinse tank. Longer Drainage Periods Allowing work pieces to drain into the plating solution for longer periods of time will reduce drag-out losses. As mre plating solution drains from the work piece while it is suspended over the plating tank, less contam- ination gets into the rinse water. A drip bar can be stationed over the tank so that the platers can lean the work pieces against it to help increase draining time. The bars can be fabricated easily in the plating shop by welding support legs onto the sides of the rinse tanks; the cost is about $100 per tank Conventional Water- and Waste-Reduction Methods The techniques described in this section are currently industry stmdards. EPA assumed that electroplating firms would use these techniques when they 2-4 / 1 JRB Associates

31 estimated the cost of complying with electroplating wastewater regulations and the number of closures expected in this industry Water Coaservation Through Counterflow Rinsing Electroplaters have long reduced water use by employing several rinse tanks connected in series. Fresh water flows into the rinse tank located farthest from the process tank and overflows, in turn, to the rinse tanks closer to the process tank. This technique is called counterflow rinsing because the mrk piece and the rinse water move in opposite directions. Figure 2-2 illustrates a two-stage counterflow rinsing system. Over time, the first rinse becomes contaminated with drag-out and reaches a stable concen- tration which is lower than the process solution. The second rinse stabilizes at an even lower concentration which enables less rinse water to be used. The more counterflow rinse tanks (three-stage, four-stage, etc.), the lower the rinse rate needed or adequate remval of the process solution because pollution concentration decreases dramatically in the final rinse tank when mre rinse stages are added. Benefits The rinse rate needed for adequate cleaning is governed by an exponential equation that depends on the concentration of contaminants in the drag-out, the amount of contamination that can be tolerated in the final rinse tank before poor plating results, and the number of counterflow rinse tanks. As a rule of thumb, each added rinsing stage enables the rinsing rate to be reduced by 50 percent. Typically, a single rinse runs at 4 gpm, a double counterflow rinse at 2 gpm, and a triple counterflow rinse at 1 gpm. The details of calculating rinse rates can be found in several publications listed in the Bibliography so they are not addressed in this report. 2-5 JRB Associates

32 -- u Work Flow Plating Tank m First Running Fresh Water Second Running Rinse To Drain Figure 2-2. Counterflow Rinsing. Drawbacks Counterflow rinsing systems are not without drawbacks. For one, they are expensive. The cost of a counterflow rinse tank depends on the number of rinse tanks and their size. For reference, a single 300 gallon rinse tank costs about $600. A double counterflow tank with two 300 gallon compartments costs about $1,200, and a triple counterflow tank costs about $1,600. Another problem with counterflow rinsing is that space limits the appli- cation of counterflow rinsing because extra room is needed to install additional tanks, and many platers already work in cramped quarters. Third, the added rinsing steps slow production. However, this is not usually a significant problem, because rinsing is not the slowest production step. The process step takes much more time than rinsing Drag-out Control Platers often use a stagnant rinse tank to capture drag-ut before the plated parts are cleaned in running rinses. A diagram of this method is shown in Figure 2-3. fresh water. The stagnant or drag-out rinse tank iu initially filled with Work pieces are rinsed in this tank immediately after processing, and much of the process solution is captured before the piece is cleaned in 2-6 JRB Associates

33 -.- Work Flow Fresh Water t To Drain Figum 2-3. Single Drag-out Method the running rinse. As a result, the amunt of pollution in the firm's dis- charge is reduced considerably. Over time, the contaminant concentration in the drag-out tank will increase until it no longer effectively captures process solution. When this occurs or before it occurs, the drag-out solution must be replaced with fresh water and the cycle begun again. Now, the discarded drag-out solution is potentially a hazardous waste problem. For some processes, the drag-out solution can be recycled back to the plating process where it will no longer be a waste treatment problem. This is practical for heated processes such as nickel, chromium and certain copper plating processes because evaporation is sufficient to make room for the drag-out. Some platers use an evaporation system on the drag-out tank to reduce the volume enough to return drag-out to cold plating processes (such as zinc and cadmium). is not cost effective for most plating shops. However, evaporative recovery This point is further discussed in Section under High Technology Controls. f the drag-out can not be recycled then it is a waste treatment problem. Therefore, platers generally use drag-out tanks after heated solutions when the drag-out could be returned to the process tank. 2-7 JRB Associates

34 2.1.3 Rtgh Technology Controls High technology waste treatment methods can be used for end-of-pipe treatment of wastewater and for reducing contamination during plating processes - This subsection reviews the high technology controls commonly proposed for application in plating shops. ation of electroplating wastes. They are very effective in minimizing the gener- However, these controls are very capital intensive. Moreover, they can be difficult to operate because they are extremely sensitive to variations in temperature and ph, and they require frequent maintenam to run efficiently. As a result, their application as an in-process control technique is limited to very lagge shops (uwally captive shops) and to high priced, specialty plating, like gold and rhodium. The technologies are addressed below on Exchange on exchangers create a chemical reaction between a solid and a fluid to interchange ions from one substance to another. The solid is known as an exchange resin and is usually made from organic compounds. Generally, in electroplating processes, ion exchange units are used to extract polluting chemicals from the drag-out tank as the solution is cycled through the unit. A cationic section in the deionizer remves metals, hydrogen, and ammonium, and an anionic section removes nonmetals such as sulfates, carbonates, and chlorides. Water discharged from the ion exchange unit is deionized and can be recycled back to the drag-ut tank. on exchange has some drawbacks. First, deionized water is purer than is needed in most plating shops. Second, the exchange resin must be regenerated 2-8 JRB Associates

35 frequently by backwashing with chemicals to remove the metals, and the back- wash solutions must still be disposed of. Third, resins eventually lose their effectiveness and must be replaced periodically. on exchange units cost about $31,000 including installation (EPA, 1979a, P. 58) and are a very expensive way to eliminate a single metal from electroplating discharges Reverse Osmosis n reverse osmosis, pressure is applied to the surface of a solution (wastewater) to force pure water through a semipermeable membrane too dense to permit the passage of the contaminant. Reverse osmosis can be used on drag-out tanks in order to recirculate purified water to the drag-out tank, and, at the same time, concentrate the process solution and retuxn it to the process tank. However, the use of reverse osmosis has several drawbacks: o The permeable membrane may let a particular ion go through the water, while another ion may be inhibited upon the application of pressure. Thus, the desired purity would not be obtained. o The membrane is extremely sensitive both to temperatures greater than 100 F and to strong acid or alkaline solutions. o The membrane requires attentive maintenance to avoid clogging by dirt and oil. o At pressures over 600 psi, the interior structure of the membrane deteriorates, and less permeate is allowed to pass. A8 in the case of ion exchange, reverse osmosis is also an expensive pollution control method. ncluding installation, an RO unit for a drag-out tank costs about $20,000 (EPA, 1979a, P. 51) Electrodialysis Electrodialysis uses electricity and semipermeable, chemically treated, plastic membranes to separate soluble minerals from water. By applying an electrical current, cations are drawn to a cathode, and anions are drawn to an anode, leaving purified water in a center cell. 2-9 Electrodialysis JRB Associates -

36 units are difficult to maintain, having tens to hundreds of compartments located between a single set of electrodes. Membranes are a significant maintenance problem, subject to deterioration and clogging. Costs for this method are in line with the costs for other high technology systems (EPA, 1979a, P. 61) Evaporation Using evaporation systems platers can return dragvut wastes of higher concentrations than the original bath. There are basically three types of evaporators: vacuum evaporators, thin-film evaporators, and atmspheric evapo ra to rs. (1) Vacuum evaporators lower the boiling point of the solution to reduce its volume. (2) The thin-film or rising-film evaporators provide a very fast rate of heat input to a thin film of solution. (3) Atmspheric evaporators are normally operated below the boiling temperatures using a vent fan to pass large volumes of air through a packed column where the warm solution is sprayed. The savings and economics for evaporation are dependent on the concentration of rinse water being evaporated and volume of drag-out. Plating solutions do contain contaminants which, when evaporated and returned to the bath, can result in bath failure. The use of a cation exchanger before the wastes are concentrate in an evaporator is therefore recommended. Evaporation systems cost about $35,000 (EPA, 1979% P. 50). 2.2 END-OF-PPE TREATMENT The xmst commn end-of-pipe treatment method for electroplating wastewater is to precipitate and separate the metals from the wastewater, neutralize the wastewater and then discharge it. Chromium and cyanide waste streams are treated separately in this method and then combined with the main waste stream for metals remval. Figure 2-4 illustrates this system JRB Associates

37 Cyanide Water Cyanide Treatment - t Chromium Rinse Water L, chromium Treatment Metals * Precipitation - Aad/Allralai Rinse Water Clarification Purified Sludge Filter or Centrifuge Thickened Sludge to Disposal Figure 2-4. Block Diagram of a Conventional Electroplating Treatment System 2-11 JRB Associates

38 After precipitation, wastewater is piped to a clarifier to separate the precipitated metals from the purified water. Clarifiers are rated by flow handling capacity and are available in standard sizes that cover the range of discharges commn in the electroplating industry. Two types of clarifiers are in general use: lamella plates and tube-type settlers. to separate the sludge is the same for either technology. The principle used Liquid enters at the bottom of the apparatus and flows upward slowly enough past the lamella plates or tubes to allow the solids to settle downward. A flocculating agent is often added to the incoming wastewater to increase the size of the solid particles 60 that they settle faster. The treated liquid from the clarifier (now essentially contaminant free) is neutralized and discharged. The sludge from the clarifier can be hauled away in its raw form of 1 to 2 percent solids, or it can be concentrated to a 15 to 25 percent solids by centrifuging or filtering. 2.3 COSTS OF TREATNG ELECTROPLATNG WASTEWATER USNG CONVENTONAL CONTROL TECHNOLOGES Standard treatment systems are effective, but they are also expensive. Typical treatment costs compiled by EPA in 1979 for small, medium, and large electroplating shops (measured by the daily volume of wastewater generated) are shown in Table 2-1 (EPA, 1979b, P. 302). The conventional flow and drag-out reduction measures discussed earlier in this chapter were designed to reduce end-of-pipe treatment costs. A typical me- dium sized electroplating firm, for example, discharges about 50,000 gpd of waste- water. Table 2-1 indicates that this firm would face a wastewater control bill of $145,000 to install standard end-of-pipe treatment equipment and about $20,000 each year to operate the equipnient. f the same firm installed counterflow 2-12 JRB Associates

39 rinse tanks and drag-out tanks it could reduce its water use to about 20,000 gpd. nterpolating from Table 2-1, the firm's end-of-pipe treatment costs would drop from $145,000 to about $100,000. However, the additional rinse tanks and ancillary equipment would cost about $25,000. Therefore, the firm would only save $15,000 in equipment costs or 10 percent of its capital costs. Operating costs drop about 50 percent, from $20,000 to $10,000 per year. TABLE 2-1 END-OF-PPE TREATMENT COSTS TOTAL FLOW RATE (Gallons/Day) 16,665 50, ,000 LEAST COST SYSTEN Batch Batch Continuous NVE- COSTS: Wastewater Treatment Sludge Handling Total nvestment $ 96,142 22,600 $118, ,166 39, , , , ,143 ANNUAL COSTS: Capital Costs $ 7,576 Depreciation 23,748 Operation & Maintenance Costs (Excluding Energy & Power Costs 7,555 Sludge Handling 0-4,278 Total O&M 11,833 Sludge Hauling 1,557 Energy & Power Costs 627 Total Annual Costs $ 45,341 11,750 36,833 19,731 4,278 24,009 4,680 1,341 78,613 43, ,628 33,267 63,000 46,267 46,800 1, ,018 Source: (EPA, 1979b, P. 302) These savings are certainly notable, but they are not especially dramatic. n comparison, the new methods for waste management described in the next chapter save firms much uore mney JRB Associates -

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41 3. NEW METHODS OF PLATNG WASTE CONTROL FOR ELECTROPLATERS JRB Associates

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43 3. NEW METHODS OF PLATNG WASTE CONTROL FOR ELECTROPLATERS The previous chapters discussed the traditional methods that electro- platers have been using to reduce drag-out, minimize rinse water volume, and treat wastewater. This chapter presents the new methods of waste controls that are coming into widespread use in the plating industry. several ways, these methods are similar to the traditional techniques. n For one, they retain the basic features of traditional waste control discussed in Chapter 2 such as properly des-igned rinse tanks, longer draining, and, in some instances, counter flow rinses. The new methods also depend on reduc- ing drag-ut and rinse rates, but the techniques are slightly different, mre effective, and less costly than those described in Chapter 2. The principal techniques used in the new methods are: (1) (2) Multiple drag-outs - a variation on single drag-out tanks Reactive rinsing - a technique for reusing rinse waters. Rather than reduce the size of end-of-pipe treatment equipment, the new methods of plating waste ccntrols use multiple drag-out tanks and apply reactive rinsing techniques in order to avoid the need for end-of-pipe treatment equip- ment altogether. This can result in very small pollution control costs. n instances where some final waste treatment is still necessary, the cost of the treatment is far less than it would be using standard techniques. Chapter 3 is divided into three sections. n the first section, the multiple drag-out technique is described and contrasted with traditional drag-out control measures. This section focuses on applying this technique. 3-1 JRB Associates

44 r- Using multiple drag-out tanks, the concentration of pollutants in a firm's discharge often can be reduced below acceptable effluent limits without further waste treatment. n the second section, reactive rinsing is explained and compared with counterflow rinsing, which is the standard flow reduction technique. Some of the situations in which reactive rinsing should be used are examined in this section. n the third section, the two techniques are integrated in a strategy for controlling wastes at plating shops. The strategy is essentially a framework for applying the new methods in order to control plating rinse waters and spent process solutions. 3.1 DRAG-OUT CONTROLS USNG MULTPLE DRAG-OUT TANKS Controlling the amount of plating solution that is dragged from work pieces upon their removal from the process tank reduces the amount of contam- ination in the rinse tanks. As described in Chapter 2 a single drag-out tank;, installed immediately following the plating process will capture some of the contamination. Two or mre dragout tanks will capture most of it. The multiple drag-ut technique is similar to counterflow rinsing, be- cause it uses several rinse tanks. The difference is that instead of a sin- gle drag-out tank and two or more running rinses, the multiple drag-out method uses several drag-out tanks and a single running rinse, as Figure 3-1 illustrates. Most of the drag-out is captured in the first tank, leaving 3-2 JRB Associates

45 Work Flow b 1 1 T> lm Plating Tank Dtag4ut Drag-out Tank 2nd Eigure 3-1. Multiple Drag-out Rinse Tank b To ph Neutralization the second tank lese contaminated than the first. As a result, the concentration of pollution in the discharge from the running rinse tank is lower than it wuld be if only one drag-out tank were used. More drag-out tanks lower the discharge eoncentra.tion even further. As a rule of thumb, eaeh drag-out tank reduces the disarge 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 ncreases in the running rinse tank. However, the more drag-ut tanks used the slower the buildup of contaminants in the running rinse. This is the principal behind multiple drag-out tanks and a key feature of the new methods of plating waste control. Using two or rmre drag-out tanks, the concentration of pollutants in the discharge from the running rinse tank can be controlled below effluent limits for extended periods of time. The length of time depends on five factors: 3-3 J RB Associates

46 (1) Concentration of the process solution (2) Rate of drag-out (3) Number and size of the drag-out tanks (4) Unse 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 PoVution Concentrations in Rinse-water Figure 3-2 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 the figure. t takes longer for a two tank drag-out system to reach a certain concen- tration 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 tm drag-outs and consequently, take longer to reach a concentration of CR 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 mre quickly than a low strength solution and, consequently, the rinsewater will reach CR sooner. Likewise, the higher the drag-out rate, the quicker the rinsewaters become contaminated. Figure 3-2 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. 3-4 JRB Associates

47 CP Concentration in Rinse Tank "/1) CR Higher Process Concentration Days Figure 3-2. Effects of Control Parameters on Rinse Tank Discharge Concentration 0 3 Processes Concentration in Combined Discharge b" 10 Processes CR Days Figure 3-3. Concentration in Combined Discharge JRB Associates

48 L lower values for these factors would shift the curve downward and increase Rinse rates have just the opposite effect. At higher rates, the curve sh&f&s downward because dilution in the rinse tank is increased. As a result, the time to reach CR is increased. effect. The lower dashed curve illustrates this f C, is the effluent limit for the metal being plated then "Tg", shown i n Figure 3-2, 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 CR is to be maintained in the discharge from the running rinse. Using two drag-outs, TR usually ranges from 2 to 16 hours, depend- ing on the process aoncentration and the rinse rate (Kushner, P-164). 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 rinse tank discharges. kst processes have four rinse tanks (see Chapter 2), so there is a four to one natural dilu- tion within each process. Since plating shops generally provide several processes, there is additional natural dilution within the rinse waters of the Shop. Electroplating shops with five or more processes are not uncommon, and natural dilution can increase TR by a factor of "5 times 4" or at least 20- Accordingly, the time for a particular drag-out solution (TR) to be replaced in order to keep a firm's combined discharge below a government limitation (CR) will range from 5 to 40 working days (assuming 8 hours of production per day). Figure 3-3 illustrates this effect. f the drag-out solution is replaced on schedule, then the rinse water from that process can be discharged Without further treatment because it will meet government limits. This will 3-6 J RB Associates

49 save firms tens and even hundreds of thousands of dollars. n most cases, however, the ph of the discharge will have to be regulated, but ph control for dilute rinse water costs only a few thousand dollars even for large flow rates (%PA, 1976b, P. 286). 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 sghedule anyway, so drag-out control can easily become part of a firm's routine process maintenance program. What to do with drag-out solution once it is discarded is addressed in the next section Manag-ing Drag-ut Solutions There ace two techniques for discarding drag-ut solution. n the first, the entire volume of the first drag-out tank is drained when time "TR" is reached. t is replaced with the entire volume of the second drag-ut tank, and this tank is filled with fresh water in a two stage system. f there are more than tm drag-ut tanks, then the last one is filled with fresh water and the upstream tanks are replenished with solutions from the next 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 Cg is not exceeded in the firm's combined discharge. This technique is more expensive that 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. 3-7 J RB Associates

50 There are three ways to handle discarded drag-out solutions: (1) Recycle drag-ut solution to the plating bath (2) Treatment in place (3) Batch treat on site. The application of these methods depends on the chemistry and operating condi- tione of the processes and the size and location of the firm. Each technique is discussed below Recycle Dragdut 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. Drag-out reduces this contamination. Since recycling drag-out returns the contaminants to the plating bath along with the process solution, drag-out recovery usually isn't used for this plating process. There are many plating processes on the market, and one cannot generalize about the application of drag-out recovery to all of the processes. in general, the technique can bz used after most heated plating baths. However, The 3-8 J RB Associates

51 manufacturer of the individual process chemicals will usually advise platers on the application of drag-out recovery to their process. 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. Table 3-1 lists town plating processes and their operating temperatures and identifies those that are candidates for returning drag-out to the process. n instances in which dragaut 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 0 They are very expensive as explained in Section 2.1. These techniques not only concentrate the excess solution in the dragout 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 impu- rities, but this drives up the already-high cost of recovery. n general, the following methods of handling drag-out solutions which cannot be returned to the plating bath are mre cost effective than high technology controls Treatment n Place When the drag-out solution cannot be returned to the plating tank the next best alternative is treatment ip place. A process called integrated treatment was developed several years ago by Lancy Laboratories, Zelienople, Pennsylvania. t was primarily designed to complete in-process the first step in the two-step treatment of chromium and cyanide. would be carried out in the end-of-pipe treatment system. The second step Figure JRB Associates

52 DLRECT DRAG-OUT TABLE 3-1 RECOVERY POTENTAL OF ELECTROPLATNG PROCESSES EtroGeSS All0 y Ant imny Arsenic Brass Bronze Cadmium Chromlum Copper Go d ndium ron Nickel Palladium Platinum Rhodium Silver Tin Zinc metaging 'JSemEe-rature 60 - loo'f Direct Recycle Of Drag-out S Yes No No S No Yes S Yes No S S S Yes Yes Yes S S S - Some Processes mrce: (b~, 19/ JRB Associates