Peter G. Moleux Peter Moleux & Associates 44 Wheeler Road, Newton Centre, Massachusetts 02159

Transcription

1 A Case Study & Update Using Diffusion Dialysis to Purify an Acid in a Plant Which Fabricates Printed Circuit Boards Peter G. Moleux Peter Moleux & Associates 44 Wheeler Road, Newton Centre, Massachusetts To be presented at the American Waste Management Association Meeting in Nashville, TN June,

2 INTRODUCTION The use and disposal of chemicals are a significant cost to companies supplying printed circuit boards (PCB s). A continuing objective is to reduce the production cost for making (also called fabricating) PCB s without adversely affecting the quality of the final product. In addition, the prevention of pollution continues to be a national and a corporate goal. Commonly used acids in this industrial category include nitric acid, fluoboric acid, sulfuric acid, and hydrochloric acid. For example, nitric acid is used to remove metals that have been deposited on the racks that are used to carry a number of panels through the electroplating operation. One or more PCB s (e.g., that are used in computers) are contained in each panel. This paper presents the results achieved by one PCB fabricator to reduce their production cost in one of their unit operations. An acceptable continuous in-line bath purification system is a process that will: achieve a payback of one year or, hopefully, less time, increase the bath performance, require a minimal floor space and low maintenance requirements, continuously remove the impurities from the bath while it is in operation, and return most of the acid back into the process. The removal (also referred to as stripping ) of solder and tin coatings, using methane sulfonic acid (MSA), was the unit operation selected for evaluation. A pilot study was conducted to determine if diffusion dialysis would be an acceptable in-line bath purification system for this application. Following the study, an automated bath purification system was installed. Other chemical applications suitable for this equipment, just within this industrial category, include: the 20% MSA Solder-On bath that precedes the MSA solder electroplating bath, the nitric acid rack stripping bath, the proprietary tin and solder stripping baths using nitric acid based chemistries, and the purification and recycling of fluoboric acid. At present, studies are being conducted at other PCB facilities, with their chemical suppliers, to determine the recovery rate of the proprietary additives (in addition to nitric) used in tin and solder stripping baths. While there are other acids (used to produce PCB s) that can be purified, the payback is somewhat longer. PCC elected not to use the alternative expensive resin bead system that required frequent regeneration and periodic replacement of the resin. 2

3 THE BACKGROUND AND DIFFUSION DIALYSIS The treated wastewater from Printed Circuit Corporation (PCC), in Woburn, Massachusetts, enters a publicly owned treatment works (POTW). PCC uses a chemical precipitation type wastewater treatment system to meet its pretreatment requirements. That treatment system could not properly pretreat the spent MSA bath (MSA is an organic chemical) after the bath was discarded. That spent bath had to be drummed and then transported off site by a licensed hauler for proper disposal. PCC conducted their on-site pilot demonstration study, using a manual acid purification system, to reduce their cost and acid discharged during the summer of The acid purification system was supplied by Pure Cycle Environmental Technologies, Inc. (Pure Cycle) from Palmer, Massachusetts. The technology, as supplied by Pure Cycle, is diffusion dialysis which is a mass transfer operation using anionic permeable membranes. In simple terms, a membrane is positioned to separate waste acid, containing metals, on one side (of the membrane) from water on the other side. Water is used to receive the acid (transported through the membrane) and this recovered acid stream is returned back to the process bath. In a single transfer stage, a constant membrane surface area allows the transfer of one or more acids (at a discrete rate) from the waste acid side into the water side. A membrane stack contains several single transfer stages plumbed in series. Increasing the number (or the dimensions) of each stage increases the membrane contact time with the waste acid. More contact time provides an opportunity for some metals in the waste acid side to leak through the membrane into the water side and for some water to pass through the membrane. This was closely observed during the pilot study and is monitored periodically since that study. Diffusion is where a solute (e.g., a chloride ion) migrates from a high concentration to a lower concentration. Dialysis is where a solute (e.g., an acid) passes through a membrane. As the proton associated with an acid is so small, the proton also diffuses through the membrane. The metal cations in the waste acid are mostly rejected by the membrane and remain in the diluting acid stream. Figure 1 is a visual explanation of diffusion dialysis. Figure 2 is a process flow diagram for plumbing a single acid purification module to a single solder strip tank. Figure 3 is the mass balance after one year of continuous operation. EPA reported their observations at PCC in reference numbers 4 and 6. The patented Pure Cycle diffusion dialysis system (U.S. Patent Number 5,264,123) provides a longer contact time compared to competitive systems, thereby allowing a higher mass of acid to be recovered in a single pass. The Application PCC uses a 35% MSA bath to electrolytically strip a tin or a tin / lead (solder) coating that has been previously deposited on top of a copper part on a PCB being processed. Typical operations that deposit various thicknesses of tin or solder onto copper surfaces include Hot Air Solder Leveling 3

4 (HASL), solder electroplating and tin electroplating. For technical information on these processes please refer to the 4th edition of the Printed Circuits Handbook (see reference 5). The electrolytic strip process using MSA occurs before nickel and then gold conductive coatings are applied to the tips of the PCB. Metals (copper, tin and lead) enter the bath as the solder coating and a portion of the copper layer are stripped. As the bath is used, the bath becomes contaminated with an increasing concentration of metals, losing its efficiency, and is periodically discarded. Original Solder Strip Costs The tin strip unit operation had a significant cost to PCC. The annual cost of the MSA concentrate was over $17, The 70% MSA concentrate is diluted (to 35%) in a 840 liter (220 gal.) process tank with soft water. The cost for a 70% concentrate MSA solution is $5.55 / liter ($21.00 / gal.). The bath was drummed and hauled off site once every six (6) weeks at an annual disposal cost (in 1994) of $6, (not including inhouse labor). Including disposal, the annual chemical and hauling cost was about $23, THE PILOT STUDY PCC s average bath waste volume was about 28 L per day (about 7 gal. per day). Pure Cycle leased a manual pilot unit capable of processing up to 20 L / day (about 5 gal. per day) for testing purposes. The pilot system was used to assess its capability to purify and to reduce (or eliminate) the need to discard the bath. The purpose of a pilot study was to: 1) become familiar with the equipment 2) determine the mass of acid being transferred per day 3) determine if any bath chemistry changes occurred that effected its performance, 4) determine the mass of metals being rejected from the bath and to 5) determine the effect on the above with changing the quantity of acid processed per day. The diffusion dialysis pilot system processed waste acid for 24 hours per day (including weekends). Steady state conditions were reached after 24 hours of continuous operation. The pilot unit was leased from Pure Cycle at a cost of $l, per month. Diffusion Dialysis Operation In an automated system, a magnetic drive transfer pump periodically sends a small volume of acid from the process bath, through a filter, to fill one of the two low volume chambers on top of the purification module. A liquid level sensor, positioned in the spent acid chamber, controls the transfer pump. In the 4

5 manual pilot system used at PCC during the pilot study, an individual had to manually start and stop the transfer pump. The particular pilot unit used at PCC in 1994 was not equipped with liquid level sensors, alarms, or filters. Since that time, Pure Cycle has upgraded most of their pilot units. The second chamber receives approximately an equal volume of soft water, again based upon a liquid level sensor positioned in its receiving chamber. The level sensor is used to activate and deactivate a solenoid valve for the incoming water. Each input flows by gravity (not under pressure) from its respective chamber down to the membrane stack that is positioned underneath the two chambers. An automated system does not require continuous monitoring and can be operated when production was shut down (e.g., overnight). As long as sufficient volume is placed in its appropriate chamber, the manual system can also operate overnight. The purification module produces two (2) continuous streams: A 1) recovered (also referred to as purified ) acid stream, which is high in acid content and low in metal content, and a 2) dilute waste stream, which contains a high concentration of metals and a very low concentration of acid. Two (2) LMI metering pumps are used to adjust the continuous flow rates. One pump is used to transfer the recovered acid output. The second pump is used to transfer the dilute acid output (please refer to Figure 2.). The suction side of each metering pump withdraws from the appropriate outlet connection on the membrane stack. Floor Space & Purchasing Actions Following the Pilot Study Based upon the results achieved, a 40 L per day, an automated Model AJ-10, Bath Purification System, was purchased from Pure Cycle Environmental Technologies, Inc. The Model AJ-10 occupies about 0.6 square meters (6.25 square feet) of floor space and is 1.37 meters tall (4 6 high). OPERATING EXPERIENCE AFTER ONE YEAR OF OPERATION Savings After One Year and the Return of PCC s Investment The system was purchased (1994) for $10, and began operation in October, The system was installed in one day for under $1, A single phase electrical plug was located close to the purification system, which was positioned close to the tin strip bath. Two (2) days were required for start up and operator training. As PCC was familiar with the system, PCC did not have to purchase additional start up time for the larger automated system. The annual MSA cost has been reduced from $17, to $2, This is an 85% reduction. PCC did not discard their bath during the first year of operation. Previously, this acid bath had to be drummed for off site disposal once every 6 weeks at an annual cost of over $6, That cost was eliminated. The $23, annual chemical and hauling cost is now $2,500.00, saving $20, per 5

6 year. The return of their investment occurred within seven (7) months. Safety, Health, & Bath Performance Benefits By maintaining a low and uniform concentration of metals in the strip bath, better process control (allowing the operator to maintain the constant bath immersion time for parts in the bath as a function of time and bath age ) was achieved. PCC did not have to decrease the tin strip machine conveyor speed as the bath becomes loaded (which is a common practice)! One safety benefit for reclaiming an acid bath, by recycling that bath (through a purification unit), includes the reduced risk of exposure to employee s who had to previously transfer (drum) and dispose of the spent acid bath. In addition to the reduced risk, a reduction in labor cost results from the fact that little labor is required to operate each recovery unit and little downtime of the bath occurred. Only 1.5 hours of maintenance time has been required per week. Time to periodically analyze the strip bath for its acid and metal content has been included in their standard operating procedures. Update Figure 3 presents the factual mass balance after one year of continuous operation. The dilute (7%) acid containing metals from the bath is being adequately processes in PCC s continuous chemical precipitation system. The Pure Cycle system at PCC produced a dilute acid waste output stream containing the mass of metals equal to that being stripped by the process bath. This may not occur in other applications. The rates for recovery can always be adjusted by changing the two output flow rates that are controlled by the metering pumps. Pure Cycle and PCC are working together using another pilot plant to demonstrate the purification of a specific tin stripping bath. This project is partially funded by the State of Massachusetts. OTHER APPLICATIONS FOR PCB FABRICATORS Nitric acid based proprietary chemicals are also used to strip a tin/lead coating from the panels during the fabrication operation. The cost of using alternative proprietary nitric based tin/lead (solder) strip solution may range from $7.50 to $13.00 per gal. including disposal and/or waste treatment. Some PCB fabricator s using this chemistry in conveyorized equipment may consume 15,600 gal. or more per year. Testing of these solutions is presently under way at various facilities. Reference number 4 addresses these issues. 6

7 Reference number 2 describes systems used, since 1991, at Pitney Bowes, Stamford, CT, for purifying and recovering their nitric acid parts strip bath, which is very similar to the rack strip bath used by fabricators of PCB s. CONCLUSIONS The installation of this system provided a seven (7) month Return of their Investment and their production costs (on a per panel basis) were significantly reduced. By maintaining a relatively constant concentration of metals in the stripping bath, a constant stripping rate was achieved and continues to be repeatable thereby producing less rejected panels. This increases the bath s productivity without slowing down their production rate. Prevention of pollution was achieved by recycling 85% of the MSA. The pilot plant was an effective tool for demonstrating this recovery system. Diffusion Dialysis is a cost effective tool to purify acid baths such as methane sulfonic acid. BIBLIOGRAPHY 1. D. Bailey, and C. Ford, Acid Recycling for Tab Solder Stripping, presented at the January, 1995, Toxic Use Reduction Institute Workshop held in Worcester, Massachusetts. 2. D. Bailey, and D. Offinger, Acid Recycling at a Captive Finishing Shop, Products Finishing Magazine, pages 62-66, December, P. Moleux, Acid Purification Applications Using Diffusion Dialysis, presented at the Eighth Annual Pollution Prevention Conference Partnerships sponsored by the New York State Department of Environmental Conservation, Pollution Prevention Unit, in Albany, New York. May 19, Opportunities for Acid Recovery and Management, EPA Design for the Environment (DfE) Printed Wiring Board Case Study 3, EPA Document 744-F , Draft dated October, Printed Circuits Handbook, edited by C. Coombs, Jr., McGraw Hill Book Company, New York City, 1988 (3rd edition), pages to 12.33, pages 15.1 to The 4th edition will be published with updated information in Printed Wiring Board Pollution Prevention and Control: Analysis Of Survey Results, EPA Document 744-R , September, 1995, pages

8 Figure 1. Diffusion dialysis allows acids to pass through an anionic permeable membrane while rejecting dissolved metal cations. 8

9 Figure 2. This figure displays the actual incoming flow path for both the soft water and the waste acid. The pump that transfers the waste acid to the 40 L. chamber on top of the module is controlled by a level sensor on that module. This pump may operate for just a short period of time to fill the chamber. Gravity flow is used to allow the waste acid (and water) to enter the membrane stack. The metering pumps on the discharge side of the stack are used to control the flow rates. 9

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