1、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. Thi
2、s 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 mad
3、e. 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 infor
4、mation. 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 devel
5、opment 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 crystallize
6、r 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 redissolv
7、ed 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 reagen
8、t. 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
9、 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 CategoricalPretreat
10、mentParameterDailyMonthlyDailyMonthlyChromium,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 copp
11、er-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 Exchan
12、ge 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 “exc
13、hange 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 reci
14、procating 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 lo
15、adings, 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 impo
16、rtance 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 ne
