AUTOMATION OF THE METAL PRECIPITATION PROCESS AT CHALLENGER ELECTRICAL USING ORP ELECTRODES

Transcription

1 AUTOMATION OF THE METAL PRECIPITATION PROCESS AT CHALLENGER ELECTRICAL USING ORP ELECTRODES 8, Kristine S. Siefert, CEF Nalco Chemical Company Naperville, Illinois Steven C. Hegerle Challenger Electrical Equipment Corporation Jackson, Mississippi Background Challenger Electrical Equipment Corporation, a wholly owned subsidiary of Westinghouse Electric Corporation, manufactures electrical circuit breaker boxes, loadcenters, and meterbreakers for commercial and residential use. Electroplating of the current-carrying aluminum bus bars and electrocoat painting of the external enclosures is conducted at the Jackson, Mississippi plant. These processes generate metal-bearing waste streams that require treatment before discharge to the sewer. Description of Plating and Painting - Processes The aluminum bus bars are plated with tin to prevent corrosion. The process involves cleaning, caustic etching, nitric acid pickling, phosphoric acid dip, and immersion in cyanide zincate solution before plating with a cyanide copper strike, followed by the final acid tin plate. After plating, the bus bars are removed from the racks, and the racks along with any rejected parts are stripped in nitric acid solution to remove plating before reuse. The cold roll and galvanized steel enclosures are pretreated in a six-stage washer prior to electrocoat painting. The parts are cleaned, phosphated, and then sealed in a hexavalent chrome solution. Overview of the Waste Treatment System The -waste treatment system is designed to treat a continuous flow of 300 gallons per minute (GPM). Plating and paint pretreatment rinse waters are segregated in sump tanks and pumped to the appropriate treatment tanks- Spent cyanide and acid solutions are segregated into separate storage tanks to be pumped gradually to the treatment system (see Figure 1). Prior to the precipitation of metals, three pretreatment steps must occur: cyanide destruction, chrome reduction, and ph neutralization. Non-metal-bearing rinses require only neutralization in the "Final ph" adjustment tank. The cyanide destruct system (CN#l and CN#2) treats approximately 20 GPM of cyanide rinse water and spent cyanide zincate solution (700 gallons every two months). This waste stream contains copper, zinc, and nickel (from the zincate bath) and is treated in two stages. Bleach (sodium hypochlorite) and caustic are 1

2 I i The Proceedings of the 79th AESF Annuul Technicai Conference SUIZ/F~NQ S M N The American Electroplaters and Surface Finishers Society, Inc. (AESF) is an international, individualmembership, professional, technical and educational society for the advancement of electroplating and surface finishing. AESF fosters this advancement through a broad research program and comprehensive educational programs, which benefit its members and all persons involved in this widely diversified industry, as well as govemment agencies and the general public. AESF disseminates technical and practical information through its monthly joumal, Plating and Surface finishing, and through reports and other publications, meetings, symposia and conferences. Membership in AESF is open to all surface finishing professionals as well as to those who provide services, supplies, equipment, and support to the industry. According to the guidelines established by AESF's Meetings and Symposia Committee, all authors of papers to be presented at SUWFIN@have been requested to avoid commercialism of any kind, which includes references to company names (except in the title page of the paper), proprietary processes or equipment. Statements of fact or opinion in these papers are those of the contributors, and the AESF assumes no responsibility for them. All acknowledgments and references in the papers are the responsibility of the authors. Published by the American Electroplaters and Surface Finishers Society, Inc Research Parkway Orlando,FL Telephone: 407/ Fa: 407/ Copyright 1992 by American Electroplaters and Surface Finishers Society, Inc. AI1 rights reserved. Printed in the United States of Amer.* ica. means, electronic. mechanical. ohotocodvinq. recording, or otherwise without the prior written permission of AESF, Research Parkway, Orlando, FL Printed by AESF Press SUWFIN*is a registered trademark of the American Electmplaters and Surface Finishers Society, Inc.

3 .. automatically added to CN#l to an OW setpoint of 400 mv and ph setpoint of 11. In the second unit, CN#2, bleach and sulfuric acid are added to ph and OW setpoints of 10 and 600 mv, respectively. In operation, CN#2 intermittently overflows to PH#1, the first stage in the metal removal process. The chrome reduction unit (CHROME) treats approximately 2400 gallons of spent chrome and recirculated chromium-bearing rinse water every two weeks. This waste stream contains hexavalent chromium, Cr(VI), which must be reduced to Cr(II1) to enable subsequent removal by hydroxide precipitation. Reduction is accomplished by automated addition of sodium bisulfite solution to an ORP setpoint of 250 mv and sulfuric acid addition to ph 2.5. In the neutralization tank, PH#l, treated cyanide and chrome solutions as well as approximately 40 GPM of acid rinse water and spent acid solutions (1500 gallons per week) are adjusted to ph 6-9. The acid waste stream contains tin and copper. Wastewater requiring metal removal flows into the neutralization and precipitation units (PH#l and PH#2), through the flocculator and into a lamellartype clarifier. Precipitated metals settle to the bottom of the clarifier, and clear water overflows at the top. Clarifier effluent flows through a sand filter to remove any floc carryover, and then into the final neutralization tank (Final ph) before discharging into the city sewer and POW. The sludge containing precipitated metals is pumped to the Sludge Collection Tank, then to the filter press, and finally to a sludge dryer, eliminating as much water as possible before final transport to a recycling facility as F006 hazardous waste. Heavv Metal Removal Before the ORP Trial Copper, zinc, chromium, tin, and nickel were removed at ph 7-8 using three chemicals: a precipitant, a coagulant, and a flocculant. Precipitant was added in PH#l, the first neutralization tank, and coagulant was added in the transfer line between PH#1 and PH#2. PH#2 was utilized for ph adjustment and additional mixing. Flocculant was added to the water as it overflowed from PH#2 into a slow mixer, so that floc size could be increased to promote rapid settling in the clarifier. Prior to the OW trial, the three chemicals were added simultaneously based on the flow of water through the system. If any of the sumps were being pumped to the waste treatment system, a metering pump was activated to add the precipitant. Thus, the process did not compensate for the changing metal content of the waste stream. Hourly testing of the final effluent for copper was the only indication of inadequate precipitant addition. A manual adjustment would be made on the pump setting; however, this was one to two hours after the adjustment was needed. Fluctuation in effluent metals content created the undesirable possibility of discharge permit violations as well as increased chemical costs due to overfeeding

4 1 1 Oxidation Reduction Potential (OW) electrodes measure the flow of electrons in oxidation-reduction reactions by providing an active surface for electron exchange. The active electrode (surface) is typically platinum or gold, and a voltage difference or "potential" between it and a reference electrode is measured. Numerous components in metal-bearing wastewaters may be "active at the electrode," meaning that the OW electrode will measure a potential when one of these components is present. Among the active components are metal ions, chelated metal ions, certain organics (such as formaldehyde), strong oxidants (such as hypochlorite), and strong reducing agents (such as bisulfite). When several active components are present simultaneously, OW measures a "mixed" potential, a composite of the potentials, though one particular component may dominate the measured potential. ORE' electrodes are commonly used in the plating industry to automate control of cyanide destruction and chromium (VI) reduction processes; Challenger was already using OW electrodes to automate these processes. While the feasibility of OW electrodes for control of non-redox, metal precipitation processes was established in a previous study,l the applicability in a waste plant treating several different metals was unknown. Thus, a trial was conducted at the facility as part of Nalco Chemical Company's NALME~ program. ORP Trial Highlights The first stage of the trial involved monitoring of ORP potentials in the metal removal process. An OW electrode was installed in tank PH#l, opposite of the precipitant addition point, about 4 feet down from the top of the tank. The output of the ORP analyzer unit was recorded on a strip chart recorder. After several hours of operation in tank PH#l, it became apparent that brief episodes of high OW potential were periodically occurring. During these episodes, ORJ? potentials would be +150 to +250 mv, as compared to the "normal" operation with -20 to -30 mv. It was suspected that these episodes corresponded to periods of heavy flow from the cyanide destruction process (CN#2), exposing the ORP to high levels of unreacted bleach. To assess the response further into the process, the OW electrode was then moved to tank PH#2 for monitoring; precipitant was still being fed in tank PH#l. In PH#2, rapid changes in OW potential were not seen, but gradual changes were observed over time. Next, manual changes in precipitant feedrate were made to determine the impact on OW potentials in PH#2. It was apparent that higher feedrates resulted in lower ORP potentials. ~lso, grab samples taken from PH#2 during high feedrates were found to contain soluble precipitant product, indicating overfeed. Precipitant feedrate was then gradually decreased, resulting in gradually higher ORP potentials until no soluble precipitant was found in PH#2. Thus, a correlation between ORP potential and soluble precipitant concentration was seen in PH#2 when product was being fed in PH#1 (see Figure 2)

5 k- Accurate control of precipitant feedrate was not possible with the OW installed in PH#2 and precipitant fed in PH#1, however. The gradual changes in OW potential that were observed during monitoring (under constant precipitant dosage) were actually the result of changing metals loading and inappropriate precipitant dosage in PH#l. By the time the OW electrode sensed this imbalance in PH#2, it was too late to correct the dosage. This lag time problem occurs when a sensor is too far from the chemical feedpoint. The logical location of both the precipitant feedpoint and the OW electrode, therefore, was the second tank, PH#2, with PH#l used as an equalization tank. In this way, intermittent flows from CN#2 containing high levels of bleach would be diluted before contacting the OW electrode, and the ORP response to changing metals loading could signal adjustments in precipitant feedrate. Operational changes were initiated by moving the precipitant feedpoint to the the transfer line between PH#l and PH#2, the coagulant feedpoint to PH#2, and the OW electrode installed in PH#2. Determination of OW setpoint for control parameters was done by manually turning on and off the precipitant feed pump, monitoring OW potentials, and analyzing PH#2 grab samples for soluble precipitant or soluble metals. A setpoint of +25 mv was found to correspond to a 5-10 ppm precipitant residual in PH#2 that virtually disappeared from final effluent samples. The precipitant feed pump was then wired to the system controller to provide on/off control of the pump. At the close of the trial, the metal removal system was operating under automatic control with an ORP electrode sensor. Difficulties in Imdementation Although O W control of precipitant dosing was successfully established, problems in the overall process soon arose: the final effluent was turbid, and metals had increased to unacceptable levels. To clear up the water, over a 100% increase in coagulant dosage was necessary. The additional coagulant solved this problem but created another, as there was a tremendous increase in sludge generation, overloading the holding tank and the clarifier. The extra sludge caused floc carryover in the clarifier, exacerbating the solids removal problem. Finding Success The problem appeared to be the instability of the water in the spent acid holding tank. The intermittent pumping of this acid caused a wide variation in the ph, OW, and metal content of the water in PH#1 and subsequently in PH#2. Problem resolution was accomplished by pumping a slow but steady stream of spent acid into PH#l, making the wastewater entering PH#2 more stable and consistent. Another action taken to improve the overall process was to divert non-metal bearing acid and caustic rinses from the acid sump, which goes to the metal removal system, to the non-metal sump, which goes directly to the final ph adjust tank. This refined the process to treat only metal-bearing water and allowed the 4 860

6 3RP electrode to accurately control precipitant dosage relative to metals loading. The overall result was consistently clear effluent water with acceptably low metals con tent. Demonstration of Good Control Several parameters were monitored over the next six months to confirm the benefits of OW control of the metal removal process. The evidence of good control is the following: 1. Comparable effluent metals levels before and after the trial using independent laboratory results. Ten 24-hour composite samples are tested monthly to comply with the discharge permit (see Figure 3). 2. Consistently low effluent metals from hour to hour over the course of a week, as opposed to the fluctuation observed before the trial. This is based on hourly effluent tests for copper2 on a Hach DR3000 spectrophotometer (see Figure 4). 3. Acceptably low effluent metals during treatment of spent cyanide zincate solution, a concentrated metals solution that is treated one week out of every two months, placing a high level of cyanide and metals into the system. Data taken during these treatment periods show precipitant usage increases while effluent copper levels stay constant, confirming OW response to changing metals loading (see Figure 5). 4. Decrease in annualized precipitant usage due to elimination of overfeed (see Figure 6). OW control results in savings on waste treatment costs while providing consistent compliance with effluent discharge permit levels. Summary A trial of OW electrodes for monitoring and control of the metal precipitation process was conducted at Challenger. Success was achieved by improving equalization of waste streams to the process, especially the overflow from the cyanide destruct system (containing bleach) and the spent acid stream. After six months of experience with ORP control, confidence in this automation step is high. Acknowledpements - The authors recognize the contributions of Nalco researcher David Picco who was a key participant in the trial, Nalco marketer Annette Pelegrin for her input both before and during the trial, and Nalco sales representative Leo Naeger for coordinating the trial and assisting in the implementation step. References 1. Siefert, K. S. and K. E, Lampert, "OW for Chemical Dosage Controi in Metal Precipitation," Proceedings of the 1991 AESF-EPA Conference, Orlando, F1. 2. Hach Company, Analytical Procedures, Bicinchoninate Method for copper in water, wastewater and seawater

7 _M Waste Treatment Schematic after ORP Trial (Figure 1) RO2CullnI I Cyanide Destruction (OKP) /Sulfuric odium Bisulfite (ORP) Acid (plq Son-.Metal Bearing Kinses (P.0.T.W) Final Sand Ella ubic Acid (ph) 6 862

8 Figure 2. ORP Potential (mv) versus Soluble Precipitant Concentration (ppm) h > y = ~ RA2 = i I '0 > mv -60 I I 0 10 ppm Figure 3. Effluent Metals Levels Before and After ORP Trial a v g 2.0 Before.d &I 0) a * 1.o 5 a 0.5 After Permit ".- nn COPPER ZINC NICKEL CHROME 8 3 Figure 4. Hourly Effluent Copper Fluctuations Before and After ORP Trial I 0

9 Figure 5. Precipitant Usage and Effluent Copper Levels During Treatment of Cyanide Zincate Solution t_ Precipitant cl( 0 &I.o pc Date: July - August " 333 aaa Figure Change - in Annualized Chemical Usage with ORP Control flocculant 4 (d v '2 coagulant 2 C I v change,