Electroplating Effluent Polishing

Transcription

1 247 Electroplating Effluent Polishing Using Tertiary Ion Exchange: A Case Study I John Lindstedt, President Artistic Plating Company, Inc. and Ernestine Wagner, Environmental Manager Artistic Plating Company, Inc.

2 249 Electroplating Effluent Polishing Using Tertiary Ion Exchange, A Case Study By: John Lindstedt, President Ernestine Wagner, Environmental Manager Artistic Plating Company, Inc. ABSTRACT As the surface finishing industry moves into the decade of the OS, the trend toward more stringent regulations is clear. The reduction of categorical limits is inevitable. To meet this environmental demand, polishing systems added to conventional treatment systems will become more commonplace. A case study is provided which details the retrofitting of a tertiary ion exchange system to a conventional alkalinechlorination heavy metal precipitation treatment system. The requirements to consistently remove metals from a plating effluent stream to the fractions of parts-permillion range demands an integrated system of polishing filtration, oxidant removal, and ion exchange. The system design,, operation, learning experiences, and system capabilities will be detailed. 1

3 INTRODUCTION Since the enactment of the Clean Water Act Amendments of 1972, industry has been under an escalating obligation to reduce the amount of pollutants discharged in its effluent. This trend will continue through the decade. Traditional wastewater treatment methods such as alkaline chlorinatiodmetal hydroxide precipitation have been dependable and have served our industry well. However, these methods cannot reduce pollutants to the degree required by more stringent regulation. Therefore, traditional treatment will need to be retrofitted with new technologies in order to further polish effluent to meet higher Staadards. DEFINITIONS For purposes of this report, all pretreatment systems initiated at the plating tank will be considered as primary systems. This includes such items as counterflow rinse tanks, flow,restriction valves, electrowinning of dragout. tanks, evaporation, and electrodialysis. See Figure 1. The function of these primary systems is to return the chemicals directly to the plating bath or to convert the material to a readily recyclable form. Y+ Traditional pretreatment is referred to as the secondary system and includes alkaline chlorination, metal hydroxide precipitation, clarification, and solids dewatering. The function of the secondary system is to capture and remove pollutants which have eluded the primary systems. The discharge of the secondary system is a solid F006 sludge which can be landfilled or recycled and a wastewater effluent which meets the requirements for discharge to a POW. See Figure 2. Any adjunctive treatment of effluent after the secondary system will be referred to as the tertiary system. Examples of this are reverse osmosis or ion exchange. This study involves a tertiary ion exchange system. BACKGROUND This is a case study of Artistic Plating Company, a job shop electmplater with rinse waters which contain regulated levels of cyanide, copper, nickel, zinc, lead, silver, and cadmium. Wastewater pretreatment was instituted to comply with federal, state, and local regulation in At that time, a traditional secondary pretreatment system of alkaline chlorination and metal hydroxide > precipitation was installed. All rinse waters were equalized and treated. Through the ensuing years, many additions and refinements were made to the secondary systems including: 1) in-house studies to select optimal flocculation polymer and coagulant; 2) ph ranges were tested and selected to enhance metal hydroxide precipitation; 3) a dewatering system was installed with a clarifier, solids separator, and filter press. Primary pretreatment was modified and improved by: Dragout tanks were provided for all electroplating process baths. Rinse tanks were converted to the counterflow type and flow restriction valves were installed. Drip guards were placed between plating baths to return dragout solution to the tank. Electrowinning systems were installed for all gold and silver dragout baths. 2

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5 SCHEMATIC OF SECONDARY PRETREATMENT SYSTEM Figure 2 Outfall of Primary Pretreatmen a. AI kaline Chlorination Sludge. For Recycle 4

6 5. Dragout spray rinse heads were used to promote the recovery of nickel plating solution. 6. Nickel was also recovered with the use of an evaporation unit. 7. An electrodialysis unit was installed also for nickel recovery. The_ Third Land Ban regulation was promulgated in May of At that date, landfills were prohibited from disposing of waste which contains in excess of 590 mg/kg of cyanide. Understanding the implications of the impending regulation, a Cyanide Elimination Program was instituted in The results of the program included: 1) All cyanide strips were replaced with acid strips. 2) Copper cyanide plating baths were converted to alkaline copper plating processes. 3) All cyanide-based activation baths were removed and replaced with sodium hydroxide products. Replacement non-cyanide gold and silver baths were investigated, but not were found to be of, acceptable quality. Therefore, a treatment scheme was developed which would remove the cyanide metal complexes prior to secondary treatment. Management of these complexed metal cyanides was accomplished with ion exchange. The ion exchange system was installed on silver and gold cyanide plating rinses. The system efficiently removes those complexed gold and silver cyanides which would previously pass through alkaline chlorination untreated and on to the wastewater effluent andlor the F006 sludge. All cleaners and acid salts were scrutinized for the presence of complexors (such as ammonium) which interfere with efficient hydroxide precipitation treatment of metal cations. When possible, these products were replaced with nonproprietary acids and basis. A program was instituted which requires suppliers to authorize the return of all unused proprietary product found to be incompatible with the wastewater pretreatment processes. Filtration chambers were installed on acid pickles. This effort has increased the useful life of these baths from 1 month to approximately 3 months. Arbitrary tank dumping was eliminated by setting standards as to when baths need to be replaced. Tank dumping authority was restricted, dump lines were Iocked, and responsibility for dumping decisions was assumed by plant management. Under judicious operation and maintenance of the primary and secondary systems, the quality of wastewater effluent was compliant. See Table 1. See Figure 3. Although the primary and secondafy systems were functioning well, there was an inherent dependency on the clarifier which is sensitive to hydraulic and chemical surges, and this affected system efficiency. These surges caused effluent discharge values to creep uncomfortably close to discharge permit levels. Therefore, a method of consistently and dependably polishing the effluent, regardless of the status of the clarifier, was required to assure the final effluent quality. 5

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8 EFFLUENT RANGES FROM SECONDARY PRETRIZATMENT IN 1992' Table 1 *Analysis performed by in-house analytid laboratory using acid digestion and atomic absorption specthphotometer. 7

9 8 ENGINEERING STUDY An engineering firm was consulted to design a system which would meet the following requirements: 1. Lower pollutant metals in anticipation of ever-decreasing effluent limits. 2. Provide protection against clarifier upsets. 3. Set the stage for a future in-house water reuse program. The collaborative effort of the engineering firm and Artistic Plating Company resulted in the selection of ion exchange as the most plausible tertiary treatment method. See Figure 4. Bench studies were undertaken to determine the proper resin choice. Chelating and weak acid resins were studied with regard to efficiency of removal of copper, chromium, cadmium, nickel, silver, zinc, and lead. A weak acid cation exchange & Haas DP-1) in the sodium form was chosen. Laboratory studies revealed the following removal efficiencies: Copper 90-95% Nickel 90-95% zinc 90-95% Lead 2040% Cadmium, lead, and silver concentrations are routinely quite low in the effluent samples, and no conclusion could be made regarding the effectiveness of the resin on these metals. Chromium in the samples was found to be in the hexavaient (anionic) form and was unaffected by the resin. This resin was selected for the tertiary system because of excellent removal of the metals of interest, that is, copper, nickel, and zinc. Bench studies revealed dependable regeneration of the resin to be accomplished in two steps. A 30% sulfuric acid solution is used to release the metal ions from the resin. Sodium ions are replaced on the resin with 50% sodium hydroxide solution. SYSTEM DESIGN, INSTALLATION & OPERATION. All pretreatment efforts prior to the tertiary system are directed toward converting metal ions into precipitate. To maximize the benefits of this effort, a sand filter was installed. This sand filter system was designed with the capacity to recirculate the wastewater through the sand repeatedly for several cycles. The water is, therefore, polished by this filter system. With volume controls, the water is discharged from the recircuhtion tanks. See Figure 5. The sand filter captures the metal hydroxide particles which may escape from the clarifier. The metal hydroxides build up on the sand andincrease the efficiency of the filter until the flow through the filter is restricted. The sand filter must then be backwashed with water. The metals are flushed out of the filter and back into the secondary pretreatment system to be cycled through the pretreatment system again. The sand filter also removes any particles which may clog the resin bed or cause channels to develop in the resin. This continuous prefiltration is necessary to keep the resin bed loose and free of debris and to assure efficient resin usage. Organic resin materials are readily oxidized which render them ineffective for ion exchange. Therefore, the resin must be protected from contact with any oxidizing 8

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12 agents. Upstream alkaline chlorination causes excess hypochlorite ion to be present in the pretreated water entering the tertiary system. An automatic dechlorination module was installed in one of the recirculation tanks of the sand filter. This unit effectively reduces the hypochlorite species by utilizing sodium bisulfite combined with continuous OW metering control. After dechlorination and sand filtration, the wastewater is acidified to convert the metals into the ionic form necessary for ion exchange treatment. To accomplish this, a surge tank was installed where the ph is adjusted using sulfuric acid. This tank is under continuous automatic ph control. See Figure 6. Two ion exchange columns were then installed with a resin capacity of 15 cubic feet per vessel. The system was designed to operate with one column as the primary unit (lead) and the other column as the polishing unit (lag). When the primary unit is exhausted, as evidenced by cation pass through on analysis by atomic absorption, it is taken off-line arid regenerated. The polishing column then becomes the primary vessel. Regeneration is initiated manually by the operator, and the regeneration cycle proceeds under automatic conqd. A 30% sulfuric acid solution is drawn through the column to remove metal ions from the resin. See Figure 7. The metal-laden sulfuric acid regenerate is directed back into the secondary system for metal hydroxide precipitation. After rinsing the resin with deionized water, a 50% sodium hydroxide solution is drawn through the column to recharge the resin with sodium ions. The highly alkaline sodium hydroxide solution regenerate is also directed back into the pretreatment system for ph adjustment. A final deionized water rinse completes the regeneration. After regeneration, the column is placed back on-line as the polishing unit. Systematic routine analysis must be maintained to determine the status of the columns. Upon exiting the ion exchange columns, wastewater ph must be adjusted to meet effluent limitations. This is accomplished in a separate tank using sodium hydroxide. Wastewater treatment operators of traditional systems require intensive training. The addition of the tertiary system increases the complexity of operation and also the operator training requirements. One operator has been assigned to the system on a full-time basis. Three supervisory level employees have been given in-depth training on the system. These supervisors are called upon to assist the operator if necessary and to attend the system in the absence of the operator. The tertiary system was designed with the flexibility to address as many potential situations as feasible. Either the sand filter or ion exchange columns, or both, may be bypassed, if necessary. Bypass results in effluent discharge from the secondary system clarification tank, as was done prior to installation of the tertiary system. See Figure 8. Storage tanks were installed and sized to collect rinse water from one shift. The system can, therefore, be shut down for one shift, and rinse water can be collected for pretreatment at a later time. This water can be introduced to the secondary system of alkaline chlorination or metal hydroxide precipitation, whichever is appropriate. This holding capacity is an advantage because an

13 SAND FILTRATION, DECHLORINATION, & PH CONTROL PRIOR TO ION EXCHANGE Figure 6 - I R e ci r c u I at i o n 12

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15 SCHEMATIC OF INTEGRATED PRETREATMENT SYSTEM Figure 8 Effluent 14

16 additional shift of production may be scheduled as needed and the rinse water can be treated by the wastewater operator during the day shift. The following items are essential to the successful addition of a tertiary ion exchange system: Excellent filtration prior to tertiary ion exchange is imperative. The resin may need to be replaced if it becomes sufficiently clogged, which is very costly. It is necessary that low micron filter chambers be installed just after the sand filter to assure that debris does not pass through into the resin. Analytical in-house laboratory support is required to accurately determine the status of the ion exchange columns. Advanced level of training for personnel to maintain and properly use this complex integrated system. Proper water pressure is essential to accomplish complete and consistent high quality regeneration. RESULTS OF TERTIARY SYSTEM 4 The entire tertiary system went on-line on January 4, Daily 24 hour flow proportionate sampling and effluent analysis by atomic absorption with acid digestion has been done to trace the status of the ion exchange columns. Cation break-through signals that resin regeneration is imminent. Column regeneration has been required approximately every four weeks. It has been found that by directing the sulfuric acid and sodium hydroxide regenerate solutions to the storage tanks and allowing for equalization, these waters may be easily fed through the metal hydroxide precipitation stage of pretreatment. Analytical tracking of effluent discharge concentrations has indicated a relatively steady decline since See Figure 9. This trend reflects the improvements made to the primary and secondary systems (reported earlier in this article) and the installation of the sand filter component of the tertiary system. January of 1992 signifies the operational debut of the completed tertiary system. Copper, nickel, and zinc are illustrated because they have the most significant concentrations in the influent, and therefore the effluent, wastewater stream. This trend of metals reduction in effluent is also shown in Figure 10 which graphically represents copper, nickel, and zinc concentrations for 1990, 1991, and The daily analytical samples are shown as the mean effluent concentration for each metal for each year. Review of the daily analytical data, collected from January 1992 through September of 1992, discloses metal hydroxide precipitation to be a very efficient means of pretreatment for pollutant metals. See Table 2. Efficiencies for precipitation range from 61 % to 94%. The efficiency of cadmium precipitation was indeterminate because influent and effluent levels were not above the detection limit. The addition of the tertiary system resulted in further metal removal, but certainly at a less dramatic rate than shown by hydroxide precipitation. The tertiary system shows removal efficiencies of 57% for zinc, 38% for nickel, and 30% for copper. See Table 3. 15

17 COMPARISON OF AVERAGE MONTEtt,Y EFFLUENT CONCENTRATIONS BEFORE (1991) AND AFTER (1992) "IZRTIARY PRETRl3AnMENT INSTALLA'IION Local P OW Limit - Copper (2.7 PPM) o Local POW Limit - Zinc and Nickel (2.6 PPM) Nickel Coppar zinc 16

18 COMPARISON OF AVERAGE ANNUAL - EFFLUENT CONCENTRATIONS Figure Copper Nickel I

19 PRETREATMENT EFFICIENCY OF SECONDARY SYSTEM Table 2 c Influent to I x Secondary System Effluent from Secondary System % Efficiency - t I R Silver PPm \.10 Copper PPm 3.50 Nickel Lead Zinc PPm PPm PPm I R c c C < X "C.02 c ** < < < < % ID I 67% <*03-1*2 I.1&4,0 I C C % I 93% I 61% I 94% Note: 1) All analyses conducted by in-house analytical laboratory using acid digestion and atomic absorption. 2) ID = indeterminate. * Calculated as.02 for this statistical study, ** Calculated as.05 for this statistical studv. 18

20 Influent to Tertiary System Artistic Plating Co. Effluent Data State Certified Laboratory Effluent Data PRETREATMENT EFFICIENCY OF TERTlARY SYSTEM Table 3 Local POW Effluent Data Effluent Grand Average % Efficiency jf Silver <.02 <.al < <.01 < < < <.002.M <.002 <.05 <.002 <.05 <,002 <.05 PP*.30 Nickel I Lead PPm I PPm < < < < I c I < < < < < < I <.05 < ll.ll < < *< < c I <.lo % 57% Note: *Calculated as.01 for this statistical study. 1) All analyses conducted by in-house analytical laboratory using acid digestion and atomic absorption unless otherwise noted. E 2) ID = indeterminate. 19

21 These results are low when compared to the engineering bench studies which predicted an efficiency of 93% for copper, nickel, and zinc. However, the water samples used in the engineering study were obtained from a sampling port just prior to the secondary pretreatment system. Therefore, the water which was studied had higher pollutant level concentrations which yielded higher removal efficiencies. The actual on-line tertiary system processes the outfall of the secondary system which has a much lower ionic loading. The degree of influent ionic loading directly affects the reduction efficiency available from ion exchange. More importantly, however, the degree of influent ionic loading does not affect the fmal effluent quality. Silver and lead concentrations in the influent and effluent are less than the analytical detection limit and are not statistically meaningfui. Therefore, no conclusion can be made from the data concerning these metals. The efficiency of chromium removal also cannot be quantified because the effluent concentration is below the detection limit of the analysis. It is notable that chromium is present in a detectable amount prior to tertiary treatment and was not detectable after tertiary treatment, revding an unquantifiable degree of removal. The overall removal efficiency of the integrated pretreatment sydfm was increased by 7% for copper, 4% for nickel, and 3% for zinc after the addition of the tertiary system. See Table 4. CAPITAL EXPENDITURES The tertiary system was installed at a total cost of $134,0o0. This compares with expenditures of $180,000 for installation of the secondary system. By extrapolating daily analytical results over one year at an a average flow rate of 7,250 gallons/day, it is demonstrated that there is significantly greater pollutant metal removal from the secondary system when compared to the tertiary system. See Table 5. Converting the cost of installation into the capital expenditure necessary for each pound of metal removed per annum reveals $657 to reduce the waste stream by one pound of metal for the secondary pretreatment system. Installation of the additional tertiary ion exchange systtim required an expenditure of $14,934 to mer reduce the waste stream by one pound of metal per annum. The monetary expenditures required for tertiary pretreatment are approximately 23 times greater than those required for secondary pretreatment. SUMMARY AND CONCLUSION Effluent from the comprehensive pretreatment system presented here is well below local limits for discharge compliance. This prohdes a reasonably wide safety margin for the rinse water stream variability which is inherent in electroplating job shops. The current combined system emits metals below the analytical detection limit in many instances. The system will withstand considerably more stringent local POW discharge limits than are presently imposed. The comprehensive pretreatment system is no longer solely dependent on clarifier functionality. Sand filtration readily removes particulate metal, and ion exchange effectively polishes the effluent of pollutant metal ions, regardless of the ionic loading from the clarifier. With the capacity to divert, store, analyze, and recycle wastewater through the pretreatment system at various stages, added assurance of effluent quality is accomplished. 20

22 ~ Silver PRETREATMENT EFFICIENCY OF SECONDARY & TERTIARY COMBINED SYSTEM - Table 4 I Cadmium I Chromium I Copper Influent to Combined ll System IR l f l Effluent from.10 < < < I I I ID I ID I ID I I I.34 I ID I.04 (( System It Combined System % Efficiency Combined System % Efficiencv Net Change I < ID <, < C ID ID 98% I D I I D I I D I + 7 % - ' < < < % I I D I +3% Note: 1) All analyses conducted by in-house analytical laboratory using acid digestion and atomic absorption unless otherwise noted. 2) ID = indeterminate. 21

23 COMPARISON OF SECONDARY AND TERTIARY PRETREATMENT SYSTEM INITIAL CAPITAL EXPENSES PER POUND OF mtal REMOVAL CAPACITY ANNUALLY Table 5-1 seconm hetreatment I Tertian Pretreatment Removed I # Metal Removed Metal "' Annually I ~aily I Annually Silver Cadmium II.0001.Chrome I1.01 Lead.005 Zinc II.08 Total Metal Removed Cost of installation II $1: ;O.OOO I $ Initial Capital Outlay per # of Metal Removal Capacity Annually oO ID ID I.03 ''-, I 8.97 $657 $14,934 Note: 1) (Average influent - Average effluent)(r;low) = Daily removed. 2) (Daily # of metal removed)@ays of operation) = Annually removed. 22

24 _- The resultant effluent is of sufficient quality to warrant planning an in-house water reuse program. Removal of nonregulated anions and cations would make this water acceptable for use in counterflow rinses. This would reduce water consumption and, more importantly, eliminate or drastically reduce wastewater discharge requirements. As POW effluent limits descend toward detection limits, laboratory analytical data and treatment efficiencies are increasingly difficult to statistically define. It is, however, apparent that every pound of pollutant metal which is removed from wastewater with technologies beyond those of traditional metal hydroxide precipitation causes monetary expenditures to rise dramatically. The question arises of the wisdom of such investments which may result in small environmental returns. Under the current environmental climate, however, there is no such question-it is the cost of remaining in business

25 'Doyle, M. and Lindstedt, J., "Silver Recovery with Ion Exchange and Electrowinning", Intemational Technical Conference Proceedings. AESF SUR/FzN 192, American Electroplaters and Surface Finishers Society, June 22-25, k 24

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