9000吨每天线路板废水处理工艺设计【含CAD图纸+文档】
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OPTIMIZED WASTEWATER TREATMENT FOR A PRINTED CIRCUIT BOARD FACILITYLarry G. Stuart & Lawrence A. GreenbergPost, Buckley, Schuh & Jernigan, Inc.2416 Hillsboro Road, Nashville, Tennessee 37212In 1987, Hughes Aircraft Co. (HAC) decided to open a new printed circuit board facility in South Carolina. This facility would manufacture state-of-the-art fine line boards with up to 16 layers meeting military specifications. In order to preclude the impact of having to temporarily shut down key production operations due to waste treatment problems, an extensive evaluation of the waste requirements was made. A task force was assembled to perform process evaluations to minimize water usage and waste generation. A complete set of Material Data Safety Sheets, tank make-up instructions, batch dump volumes and expected rinse flowrates was also compiled to provide the designers with the best possible information. The process evaluations resulted in the following:1. Counter-flow rinses on all lines and stand-alone equipment.2. Selection of filtration units on all scrub stations which would allow water reuse of over 90%.3. Precious metals recovery at each process. For example, silver from artwork development was captured using cannister resins4. Elimination of chrome by replacing the chromic back-etch with plasma desmear.5. Selection of sulfuric/peroxide for all microetchant processing, such as preplate clean and oxide preparation. These solutions are constantly recirculated through a crystallizer where excess copper is removed. Since these microetchants are used prior to solder plate, no lead contamination is present as is the case when sulfuric/peroxide is used to etch the outer layers of the circuit boards. The resultant high-purity copper pentasulfate can then be sold as is or redissolved and the copper recovered in an electrowinning unit which will be described later.6.Selection of cupric chloride process for innerlayer etching, which accounts for over 80% of the circuits produced. In this process, the copper-bearing rinses are returned to the etchant chamber and used as a reagent. The only waste produced is the spent etchant which can be shipped for copper recovery.Even with the water conservation and waste minimization efforts of IIAC, severalthousand gallons per week of concentrated solutions and roughly 200 gpm of rinsewater has to be handled by the wastewater treatment plant. These wastes must be treated to meet the City of Orangeburgs effluent limits which are considerably more stringent than the EPA Categorical Pretreatment Standards, as can be seen in Table 1.TABLE 1 Maximum Effluent Concentrations (mg/L)City of Orangeburg,South CarolinaEPA CategoricalPretreatmentParameterDailyMonthlyDailyMonthlyChromium,total CopperLeadNickelZincIron2.77 1.610.011.612.613.5 1.711.00.071.01.482.52.773.380.693.982.61N/A1.712.070.432.381.48N/AIn order to meet these discharge requirements, three distinct treatment systems wereselected: (1) ion exchange for chelated and copper-bearing waste, (2) membrane filtrationfor other rinses requiring treatment and as backup for the ion exchange, and (3) batchtreatment of concentrated wastes.1 Ion Exchange With Copper ElectrowinningThis treatment system contains the ion exchange (IX) process known as Reciprocating Flow Ion Exchange combined with a unique electrowinning recovery system.Both of these unit systems incorporate relatively recent advances in design and have several advantages compared to conventional recovery technologiesl. In ion exchange columns, the exchange process takes place in a narrow band called the “exchange zone”. The resin on either side of this narrow band is inactive. The resin upstream of this zone is exhausted and downstream it is still in its regenerative state.The exchange zone moves through the column until all the resin becomes saturated.Unlike conventional ion exchange columns, the reciprocating flow system selected used a column which is only slightly larger than the exchange zone. This means the resin bed is less than a foot thick versus several feet for traditional systems. This process is made possible by incorporating design features such as fine mesh resins, low exchanger loadings, counter-current regeneration, packed resin columns and continuous metal analysis on the effluent.Fine mesh resins provide a faster exchange rate which reduces the length of the exchange zone, allows the use of higher flow rates and reduces the regeneration rinse.These factors are of key importance for printed circuit board operations since a selective chelating resin is required to treat complexed copper. These resins tend to have poor exchange kinetics and are generally limited to slower flowrates. Due to the small bed size, the cost of these resins is kept to a minimum.By using only readily accessible exchange sites near the resin particles surface or roughly 15% of the resins capacity, this system can operate at higher flowrates and can be quickly regenerated. Although this usage rate requires more frequent regeneration, using counter-current regeneration of a packed column with no free space above the resin allows the resin bed to be regenerated in a few minutes versus hours for a conventional system. The overall chemical consumption is also reduced by roughly half due to higher efficiency. Since the major cost in operating an ion exchange system is the regenerant acid, this efficiency results in a substantial cost savings.Continuous metal analysis of the ion exchange effluent is made using an indirectcalorimetric monitor. A reagent is added to a continuously flowing sample and readautomatically by a calorimeter. If the copper level exceeds the set point, the unitautomatically goes into regeneration. The indirect calorimeter can detect levels as lowas 0.2 mg/L of copper.The regenerant solution from the ion exchange is a copper sulfate solution with a copper level of 10 to 20 g/L. Conventional electrowinning works well for solutions with high concentrations of copper, but not down to 1 g/L or less. Since any remaining copper has to be reprocessed and will, therefore, use a percentage of the ion exchange resin, levels of 10% or less are desired. The electroplating system selected has several modifications to allow efficient plating down to these levels. These modifications include uniform air agitation at the cathode surface, close anode to cathode spacing, and closed-loop rectifier control by a direct copper-reading calorimeter. In addition, ease and costefficient operation is provided by features such as insoluble anodes, reuseablc stainlesssteel cathodes, and the use of current shadowing to prevent deposition on the outer edges of the cathode. A tandem IX system with dual 100 gpm ion exchange columns was provided. Although the second column was not absolutely necessary, redundancy and flexibility provide a greater level of assurance that compliance is maintained.The electrowinning unit was sized to provide removal of 5 lbs/hr of copper. As seen in Figure 1, several solutions are bled into the chelated or copper-bearing rinses. These solutions include electroless copper, nitric acid rack strip, and numerous spent acidic process baths.A spent chelated cleaner/conditioner from the electroless copper plating line is mixed directly with the rinses. Capacity has also been provided to process the sulfuric/peroxide microetchant in the event that bath contamination occurs. A provision for processing the spent copper sulfate in the electrowinning system was also incorporated. Most circuit board shops rarely, if ever, dump their plating tanks. Due to the high quality which must be maintained to produce state-of-the-art hardware and the relatively high dollar value at this level of processing, HAC remakes their plating tanks an average of once each year. Figure1 ion exchange and electrowining system2 Membrane FiltrationIn a gravity settling system, polymer or another flocculating agent is added after the metals have been precipitated to form larger particles or “floc” which will settle readily. If the wastewater changes in either volume or concentration, the dosage must be adjusted to obtain optimum settling. Even at optimum dosage, settling will not occur if air bubbles are contained in the solids as sometimes occurs when foaming agents, such as detergents, are treated. Since the type of floc formed also depends on the metals being treated and the reagents chosen, the selection of a polymer and establishment of the correct dosage is more of an art than an exact science. Sometimes a polishing filter is added to these systems. Disposable cartridges become irreversibly fouled, and sand filters allow particles less than a micron to pass through. Use of a fine-grain filtering media, such as diatomaceous earth, requires reprocessing of filter media.With a membrane filtration system, settleable particles do not have to be formed, The membrane provides a physical barrier to small particles, so a flocculating agent is not needed. The membrane filtration system selected has several design features that are improvements over conventional membrane systems.Typical ultrafiltration (UF) and reverse osmosis (RO) systems have low flux rates, i.e., the rate water passes through the membrane, due to the small pore size used. The pore size, generally less than 0.1 microns, also requires higher operating pressures. Both of these factors contribute to the fact that these systems would be prohibitively expensive in wastewater applications.Most commercially available membranes are made of cellulose derivatives or polymeric materials that are not able to withstand the extreme corrosive and oxidative conditions encountered in industrial wastewater.The short life expectancy of these materials makes them unsuitable for this type of application. Some membrane fouling also occurs during the operation of a membrane system which reduces the flux rate.Periodic cleaning is required to restore operating efficiency.Due to the susceptability ofmost membrane materials tocorrosive chemicals, the cleaning methods are also limited.The membrane system selected has a pore size of roughly 0.1 micron and is made ofinert polymers. The larger pore size results in considerably higher flux rates even at lower operating pressures. The use of fluorocarbon polymers for the construction of the membrane allows the use of harsh acids or bleaches for cleaning. This system also uses an annular design with the wastewater inside the tubular membrane and the permeate passing to the outside. By maintaining turbulent conditions inside the tube, fouling is reduced and the solids are continually scoured from the surface. 2 The solids removed are returned to the feed tank and reprocessed until the percent solids is high enough, generally 3%, to be dewatered using a filter press.A membrane filtration system with two segregated trains, one 200 gpm and the other 100 gpm, was provided. The larger train handles general rinses. The spent oxide preparation bath is bled into one of the neutralization tanks. This solution contains chlorides and can generate a toxic gas if exposed to acidic conditions. The smaller train serves as a back-up for the larger membrane system and also for the IX system as seen in Figure 2. 3 Batch Treatment of Concentrated WastesOnly two aqueous solutions, both photoresist strippers, were found that could not behandled by either the ion exchange or membrane systems. Photoresist or “resist” is aphotosensitive plastic maskant used in the production of printed circuits. The photoresiststrippers are organic solutions that can contain small pieces of resist, both of which would tend to foul the ion exchange or membrane system. The level of copper, which can rangefrom 1 to 200 mg/L, contained in the resist strippers is dependent on the formulation of the stripper solution, the resist selected, and the usage rate prior to disposal. The control of this level by means of process selection would be overly restrictive.Generally, these solutions are bled into the rinses which bypass the treatment process and are therefore not a problem. In some cases, such as for NPDES discharges, the organic constituent and the associated copper are precipitated using an acid reagent. This treatment results in relatively large volumes of sludge, but does reduce the high levels of BOD and COD associated with this type of waste. Since only the metal level was of primary concern in this application, a treatability study was conducted to determine a means to precipitate the copper without removing the organic constituents which would minimize the sludge volume.Since the resist strippers are slightly alkaline and lowering the pH results in precipitation of the organic component, a treatment was found which could be effective in the pH range of 8 to 10. Normally, copper hydroxide would form under these conditions, but complexing agents are present, In order to overcome the complexing agents and produce a filterable solid, sulfide chemistry was determined to be the best alternative.The high reactivity of sulfides with heavy metal ions and the insolubility of heavy metal sulfides over a wide pH range has several advantages when compared to hydroxide treatment. Sulfide treatment can treat complexed waste streams and can achieve considerably higher removal levels. Sulfide precipitation employs either a soluble sulfide, such as sodium sulfide or sodium hydrosulfide or an insoluble sulfide such as iron sulfide. The use of sulfides requires design constraints to assure that these reagents are not exposed to acidic conditions which results in the generation of toxic hydrogen sulfide fumes.Soluble sulfide treatment requires either enclosed ventilated tanks or control of reagent addition so that overdosing does not occur which creates an odor and exposure problem. In a conventional gravity system, special polymers arc also required to effectively flocculate the fine sulfide particles formed.A patented process was developed in the late 1970s using ferrous sulfide (FeS).3 Aferrous sulfide slurry is prepared by reacting ferrous sulfate and sodium hydrosulfide. Inthis process, FeS is the source of the sulfide and, due to its low solubility level of 0.02 ppb, no noticeable odor is present. As sulfide is consumed, more FeS will disassociate. Since most metals encountered in metal finishing wastes will form a sulfide with a lower solubility than FeS, they will selectively displace iron. The iron ions liberated then precipitate as a hydroxide. Insoluble sulfide chemistry appeared promising for this application. Jar tests were conducted using this reagent and compliance levels readily achieved. Unfortunately, the cost of the patented batch system was not as appealing.Another insoluble sulfide can be produced by reacting calcium chloride and sodiumhydrosulfide. This reagent is not as insoluble or stable as iron sulfide, but can be used forbatch treatment. Lime or caustic is also added to maintain the pH above 7.By adding calcium sulfide to the resist strippers, the complexing agents can be overcome. The dosage was set at five times the stoichiometric requirement based on laboratory analysis. Lime is then added as a filtering aid and the solids removed in a filter press. This small filter press (2 cu ft) is dedicated to processing the resist strippers. The filtrate was found to contain less than 1 mg/L copper. The filtrate is held and the copper concentration checked. If too high, the solution is refiltered. If acceptable, it is slowly bled into the final effluent so excessive levels of BOD and COD do not occur. Even with the filtering aid, the volume of sludge produced is less than 10% of the volume which would have occurred if the organic constituents were also precipitated.Batch treatment using this reagent was also determined to be the best procedure forhandling spent solder plate baths. Although this highly concentrated solution could be bled with the membrane system, batch treatment assures that the stringent lead effluent limitation of 0.10 mg/L is met. Since this solution contains a high concentration of fluoboric acid, it must first be diluted and then neutralized with sodium hydroxide. The treated bath forms a dilute sludge which is delivered to the sludge holding tank for dewatering in the larger filter press (17 cu ft) which is also used for the membrane systems.4 General Design FeaturesIn addition to careful selection of unit processes, a number of design features addressed segregation, the hold capacity of the collection system and instrumentation.In order to control batch dumps to the wastewater treatment plant, twelve dedicated lines were provided for concentrated solutions. Most treatment plants are susceptible to production dumping solutions at inconvenient times. For nine of the chemicals, hold tanks with differential pressure level transmitters and a minimum reserve volume of one week were provided. The waste treatment operator can process these wastes as time and conditions permit. The remaining lines were to deliver pH sensitive solutions directly to neutralization tanks to avoid the possibility of toxic gas generation or to allow installation of a hold/bleed tank in the future. This approach also makes it easier to switch treatment schemes if future chemical subsitutions occur as is often the case in state-of-the-art printed circuit board fabrication.Rinsewater was segregated into five lines. The genera1 rinse, chelated copperbearingrinse, and the metal-free rinse were segregated for current process reasons. In addition, a dedicated line was provided for ammoniated rinses, which are also bled into the ion exchange system. A line was also provided to the chelated membrane in case a chemistry was selected in the future which is incompatible with the ion exchange system. At least one spare line is desirable since trying to add a drain line in the future generally costs considerably more.The equalization tanks for the rinses were sized to provide a minimum reserve of one hour. All rinsewater pumping stations were equipped with a backup pump which is activated automatically by an abnormally high level. Mercury contact floats were used to activate these pumps. Even if one float failed, at least two pumps can still operate. Normally, some sort of resistivity/conductivity probe is used for level detection in wastewater applications. Since HAC uses deionized water for all of their processing, this type of probe is inappropriate due to the high resistivity of pure water.The instrumentation for this treatment plant was based on decentralized control for all unit processes, including pH adjustment, and centralized recording of all alarm conditions and tank levels. A personal computer with a graphics package was utilized to display this information.5 Star t-Up ExperienceIn general, the treatment systems have all performed well. Compliance levels can be maintained without using costly reagents such as carbamate or borohydride. Sludge levels are roughly a quarter to a third of that at a similar HAC circuit board facility using a conventional treatment system. The electrowinning system is producing over 200 lbs of copper per month with a purity level of over 97%.Some initial problems were encountered. One of the reagents for the indirect calorimeter on the ion exchange system was found to be sensitive to prolonged exposure to high temperature. This system has been predominantly used in northern climates and can not endure storage under typical summer conditions in South Carolina. With the degradation of the reagent, the calorimeter was not detecting increases in copper levels and continued to indicate an erroneous low level. This
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