The diffusion dialysis process separates acid from its metal contaminants via an acid concentration gradient between two solution compartments (contaminated acid and deionized water) that are divided by an anion exchange membrane. Acid is diffused across the membrane into the DI water whereas metals are blocked due to their charge and the selectivity of the membrane. A key difference between diffusion dialysis and other membrane technologies such as electrodialysis or reverse osmosis is that diffusion dialysis does not employ an electrical potential or pressure across the membrane. Rather, the transport of acid is caused by the difference in acid concentration on either side of the membrane.
| Some Additional Diffusion Dialysis Resources
|
The process uses ion exchange membranes which are assembled in a membrane stack. The membrane separates two liquids: (1) acid contaminated with metal and (2) deionized water. The physical laws of diffusion and electroneutrality cause material in high concentration to move to an area of low concentration without an imbalance of electrical charge. Because of the presence of the anion membrane, the metals in the concentrated solution are unable to pass from the concentrate to the DI water. However, anions in the concentrate (e.g., chlorides, sulfates, nitrates, phosphates) are permitted passage. Also, hydrogen ions, although positively charged, diffuse along with the disassociated acid (anions). The passage of hydrogen, which is key to the success of this process, is due to the small size of the hydrogen molecules and their mobility. The passage of the positively charged hydrogen ions satisfies the law of electroneutrality, preventing an imbalance of ionic charge on either side of the membrane (ref. 79, 80).
Diffusion dialysis, like other membrane technologies, is not 100 percent efficient; not all of the acid will be recovered and some leakage of metal will occur. In the laboratory, the process has yielded acid recovery efficiencies as high as 99% with 98% metal removal. In the manufacturing environment, the practical limits are 80% to 95% acid recovery with 60% to 90% of the metal contaminants removed. Also, the recovered acid may be of insufficient concentration to permit direct reuse. In such cases, vacuum evaporation may be needed to increase its concentration (ref. 81), although the economics of a concentration step are questionable. One source indicates, based on 1.5 years of experience with diffusion dialysis, that it is more efficient and economical than acid sorption for certain applications (e.g., recovery of mixed acid pickling baths) (ref. 80).
The diffusion dialysis membrane material is relatively resistant to chemicals commonly used in the PWB shop. However, contact with solvents could cause swelling of the membrane and strong oxidizing agents can deteriorate the membrane material (ref. 82). The process is tolerant of feed solution temperatures up to 50°C (ref. 82).
One survey respondent reported the use of this technology (ID #955099). Their unit is successfully used for maintenance of a solder strip solution. The unit was purchased for $10,000 in 1994 and requires approximately 50 man-hours per year to maintain. However, use of the technology in PWB shops has been reported elsewhere (ref. D, F). One shop successfully applies diffusion dialysis to a methane sulfonic acid (MSA) solder electrostrip to continuously remove metals. As a surface finish, this shop uses solder-mask-over-bare-copper with hot-air-solder-leveling. This outer layer finish prevents copper oxidation and facilitates solderability during the assembly process. Before panels can then undergo nickel/gold tab plating (also called finger plating, connector plating, or microplating) for electrical conductivity and environmental resistance, the tin/lead solder must be stripped from the panel. In the stripping process, they use methane sulfonic acid (MSA) and apply a reverse electrical current to dissolve tin and lead from the boards.
In the past, the shop changed the acid every 30,000 ends (one pass of a circuit panel), or approximately every 6 weeks depending on production schedules. MSA is an expensive acid (~$21/gal.), and accounted for an average of $17,000/year in raw material costs. Spent solution was sent off-site for disposal at a cost of approximately $5,600/year. The shop recognized an opportunity to conserve acid, prevent hazardous waste generation, and lower employee exposure to corrosive materials using a relatively simple and efficient in-process recycling technology called diffusion dialysis.
At this shop, the diffusion dialysis recycling unit is hard-piped to the MSA tab stripping bath. The company first evaluated a 5 gallon/day recycling unit in an off-line pilot test. They assessed parameters such as acid recovery and metal rejection rates, as well as the stripping rate of the recovered acid. The shop then proceeded to evaluate the system on-line. After working with the vendor to fine-tune metal rejection and acid recovery rates, they were able to maintain a constant solution level in the stripping bath. Based on the projects costs and savings, the payback on the investment was approximately 6 to 7 months (ref. 74).
The same shop is investigating use of diffusion dialysis for maintaining their nitric acid/ ferric nitrate tin/lead etch-resist strip solution. The nitric acid solution is used to strip the tin/lead layer, and the ferric nitrate component is necessary to remove the intermetallic layer that forms when the tin and copper diffuse into each other. These solutions also contain wetting agents, copper etching inhibitors, and anti-tarnishing agents (ref. 74). The investigation involves the use of diffusion technology to separate the stripped metals from the stripping solution, rendering it reusable. This would be a continuous, on-line recycling system similar to that used for their MSA recovery. The major roadblock to this process is the presence of an iron component in the proprietary stripping solution. They anticipate that the diffusion dialysis process will reject from the spent solution all metals, including the iron, which is essential to the stripping process. However, the shop believes it may be possible to determine the rate of loss of iron from the diffusion dialysis process and replace the iron with a concentrated replenisher. The difficulties here include adjusting for the losses of the other components, since rejection of organics and non-metal inorganic materials varies, depending on the charge and size (ref. 74).
In order to make these determinations, the shop contacted its solder strip chemical vendor and arranged a meeting with the company's process engineers, representatives from its chemical vendors, and the diffusion dialysis equipment vendor. Together they designed an off-line pilot system to test the acid reclaim efficiencies and metal rejection rates at various ratios of virgin to spent solder strip. The shop is awaiting further test results from its chemical vendor on parameters such as solder stripping rates, intermetallic removal, copper etching inhibition, and anti-tarnish capability. Based on the findings, the chemical vendor will be able to determine the additive package of chemical constituents that would replace the components lost from the diffusion dialysis process (ref. 74).