Somewhere between plating and waste treatment is a process desired by a variety of industries...
Somewhere between electroplating and waste treatment exists a process rather ambiguous, yet desired by a variety of industries--In-Process Recycling. This new-wave process has been increasingly demanded by industry in an attempt to reduce waste and to avoid continually tightening restrictions concerning waste disposal. Although this process is highly sought by industry, it is extremely misunderstood. To determine if in-process recycling is a myth or a reality, let's consider all the possibilities the process has to offer for an electroplating operation.
In-Process Recycling Potential Advantages. One advantage is the ability to recycle rinse water. This offers immediate economic savings by significantly reducing the volume of water used. However, one must determine the impact on the sludge collected in the waste treatment tank if a substantial amount of the water was required to produce the sludge. Additionally, what is the economic impact on the sludge's value if a recoverable metal such as copper is removed?
If a printed wiring board (PWB) shop produces 15 tons of wet sludge per year and maintains a copper percentage greater than 20 pct of total solids, the cost for sludge removal is approximately $895 per ton. If the copper percentage in the sludge falls below 20 pct, sludge removal cost rises to approximately $1,300 per ton. The recycler will also guarantee the PWB shop that if the copper percentage remains at a certain level he will not landfill the sludge and will provide a clean F006 report.
A second advantage is the ability to recycle the actual plating solution. This gives one the ability to reintroduce a highly concentrated solution (2,000 - 3,000 ppm) back into the plating tank. Although there is tremendous economic value, most veteran electroplaters would not even think of adding a recycled solution to a precious metal plating bath.
Why? Is there a scientific reason for not reintroducing recycled solution to the plating bath or is this process simply "not the way it has been done?" Technically, there is no scientific reason why a highly concentrated, recycled solution cannot be added to a plating bath.
If one plates copper sulfate, the initial plating solution consists of copper sulfate, sulfuric acid, hydrochloric acid and various chemical additives such as brighteners, levelers and carriers. A recycled, highly concentrated copper solution would consist of copper sulfate and sulfuric acid with metal concentrations in the 2,000 to 3,000 ppm range.
A third advantage is the ability to actually decontaminate your spent plating bath. If one could remove metals from prior operations, decomposed additives and other organics, the frequency of bath "dumps" could be reduced and a considerable economic gain realized.
In-Process Recycling Parameters. Many electroplaters do not completely understand in-process recycling. Ask yourself. "How do I characterize an acceptable incoming rinse water?"If your answer is "water purified through deionization (DI water)," then consider the following.
In a study conducted at a client's PWB manufacturing facility, Faraday Technology, Inc. (FaraTech) scientists took samples from a primary copper sulfate rinse tank for three months and found that the metal concentration (copper), TDS (Total Dissolved Solids), and pH of used rinse water ranged from 1.53 to 131 ppm, 180 to 2,800 ppm, and 2.2 to 6.4, respectively. The test helped determine an average parameter for each of the three variables. The parameters could be used to develop an in-process recycling system for recycling rinse water to the PWB shop's copper sulfate rinsing operation. The results of this study are in Table I.
The results of these tests show the dramatic difference in these three variables over three months between samples taken at different times during the operation. The results indicate that acceptable metal concentration, TDS, and pH limits for incoming rinse water are far higher than those found in DI water.
This suggests that in-process recycling can be a viable process if we look at electroplating from a scientific rather than an artistic perspective. There are, however, additional considerations to investigate before a clear determination can be made. These include 1) how to incrementally introduce in-process recycling into your operation; 2) which application in your total process is the most appropriate starting point for introducing in-process recycling; and 3) how do various technologies compare in capabilities to achieve in-process recycling.
First, reconsider the issue of recycling rinse water; however, this time look at recycling rinse water from the PWB shop's lead/tin fluoborate solder operation. The solder plating operation is a much smaller part of the typical PWB plating process, hence the volume of metal contaminants and water used in a rinse operation are substantially lower than those for copper plating. If, using in-process recycling, the shop could eliminate the lead/tin content of its end-of-pipe sludge and reduce its water consumption by reusing the water from its lead/tin rinsing operation, what would the impact be on the end sludge collected in waste treatment? Certainly the overall volume (water reduction) has decreased and two of the constituents of the sludge (lead and tin) have been removed. Therefore, the overall volume of the sludge in tons is lower, decreasing the overall cost of sludge removal in total tons. Also, the percentage of copper in the sludge has increased, making the remaining sludge more valuable for the recycler. The shop can continue to enjoy the value of a clean F006 report and no liability for sludge removal.
Preparing Spent Rinses for Discharge. The above scenario assumes, however, that the shop is large enough to use an off-site recycler. If your shop is small, perhaps you are simply interested in preparing your spent rinse water for discharge. If this is the case, then in-process recycling, or "closing-the-loop" is also a viable solution. Treating the rinse water from your plating operation helps you avoid discharge completely. Your rinse water can be returned to the rinsing operation, eliminating wasted water. The highly concentrated solution generated from the in-process recycling system can be reintroduced to the plating tank (scientifically), eliminating wasted plating solution.
To completely understand the impact of introducing in-process recycling to your plating operation, you must address its effect on plating bath integrity; water consumption; acceptable incoming rinse water parameters (metal concentration, TDS, and pH); waste treatment; and end-of-pipe recycling or discharge considerations.
Finally, if you have multiple applications in your plating operation, it is probably not economically or logistically feasible to close-the-loop on all of your applications simultaneously. Therefore, it is extremely important to analyze the impact of incrementally introducing in-process recycling to each application within the total plating operation.
In-Process Recycling Emerging Technology. FaraTech is developing an Electrochange In-Process Recycling System for a variety of electroplating operations. The patented technology integrates electrowinning and ion-exchange into one system. It combines the benefits of both: 1) treatment of contaminated rinse water from high to low metal concentrations, 2) removal of anions for rinse water recycling, 3) current use for plating metal contaminants for recovery, and 4) electrochemical regeneration. Using Modulated Current (MC) for process control, the system provides cost-effective recycling capabilities for both rinse water and plating solution. During the treatment step, contaminated rinse water from an overflow rinse tank is treated by the system and returned directly to the rinse operation. During the regeneration step, the recovered metal can either be returned to the plating operation in a highly concentrated solution form, or recovered as a metallic foil for recycling.
The system has been Beta tested for copper sulfate and is currently being developed for the cost-effective recycling of lead/tin solder for PWB applications. A key feature of in-process recycling is the use of integrated ion-exchange electrodes (IIXTM). The cathode and/or anode consists of an intimate blend of high-surface-area carbon and cation or anion exchange resin, respectively.
The ion-exchange resin chemically removes the ionic contaminate from the rinse water and concentrates it near the carbon electrode bed. Heavy metal cations, such as copper, nickel, lead/tin and silver, are then electrochemically removed from the ion-exchange resin and deposited in the carbon component of the IIXTM electrode bed. During regeneration, the modulated electric field is reversed causing the cations and anions to reform into a highly concentrated solution for recycling to the plating operation. The MC electrolysis balances the electrochemical and ion-exchange processes and is used to enhance the transport of contaminants to the electrode.
Technology Benefits. The system can treat contaminated rinse water from high- to low-metal concentrations (ppb range). The actual parameters are based on clients' requirements and their specifications for an acceptable incoming rinse water. Also, electrowinning cannot remove anions from the contaminated rinse water; therefore, the rinse water cannot be recycled to the rinse operation. Second, the system does not require a strong acid, such as hydrochloric acid, or a strong base, such as sodium hydroxide, for chemical regeneration of the contaminants. The system uses rinse water as a starter solution during the electrochemical regeneration step. Also, the resin is used for a short time to store the metal contaminants, because the system's cathode actually plates the metal contaminants onto the carbon bed for storage until regeneration.
As previously stated, when exploring the introduction of in-process recycling to your plating shop, you should analyze all the aspects of your operation. If you think in terms of incremental improvements in water consumption, end-of-pipe sludge reduction and reuse of plating solution, the challenge of "closing-the-loop" should be much easier to accomplish.