Recycle, Recovery, and Bath Maintenance Technologies
One section of the PWB survey form was devoted to gathering information concerning pollution prevention and recovery technologies that are applied for the purposes of recovering and recycling chemicals and improving the life-span of process solutions (see Appendix A, Section 5 of the survey form). Seventy-six percent (76.3%) of the survey respondents completed Section 5 of the form. These respondents reported use of a range of technologies, including ion transfer, electrowinning, ion exchange, diffusion dialysis, membrane electrolysis, evaporation, and solvent extraction. The individual technologies reported in use by the respondents are briefly discussed in this section. A more in-depth technical discussion can be found in Reference 1.
6.2 Technologies in Use by Respondents
6.2.1 Ion Transfer/Porous Pot
Electrolytic regeneration of permanganate desmear baths using a porous pot (or similar ion transfer designs) is a pollution prevention technology employed by 32% of all respondents. This relatively inexpensive and simple technology is used for bath maintenance (i.e., extending the useful life-span) of permanganate desmear baths. In the conventional permanganate process, the permanganate ion is reduced by heat and contact with PWBs and is replaced by chemical addition. Also, during operation of this bath, by-products (including the manganate ion) accumulate in concentration causing a sludge to form and frequent disposal is necessary. The porous pot can be used to maintain a sufficient low concentration of contaminants and thereby reduce the frequency of disposal.
The common porous pot design consists of a rectifier, a ceramic pot that houses a cathode (protecting the cathode from direct contact with the process solution), and an anode, which surrounds the pot and is in direct contact with the bath. At startup, the pot is immersed into the bath (with the top remaining above the solution, preventing it from flowing into the cathode compartment) and filled with an electrolyte, usually sodium hydroxide. With the bath shielded from the cathode, the primary reaction that occurs is the anodic re-oxidation of the manganate ion back to permanganate. Using the porous pot, a bath-life extension of ten-fold or more can be realized.
Capital costs for this technology are low and some respondents reported leasing the equipment from chemical vendors rather than purchasing it. Of those who did purchase the equipment, the median price was $900 per unit. The reported installation and operating costs were also low.
Ninety-two percent (92%) of the respondents who operate ion transfer units indicated they are satisfied with the technology. A somewhat lower percentage of respondents (67%) indicated that in the future they would buy the same technology from the same vendor if faced with a similar situation.
6.2.2 Ion Exchange
Ion exchange is a versatile technology that is applied by PWB manufacturers for various, sometimes overlapping purposes, including: chemical recovery, water recycle, solution maintenance, and waste treatment.j For example, twenty-six percent (26%) of the respondents reported using ion exchange as a water recycle/chemical recovery technology. Many of these respondents reported the same system as a component of their waste treatment system. Therefore, in some cases it is difficult to distinguish one function from another. In general, most of the waste streams discharged from the PWB process are compatible with ion exchange, and many shops mix several similar rinse streams and treat them with a single ion exchange unit (e.g., sulfuric acid dips, micro-etch, and copper electroplating rinses are frequently combined). The ion exchange effluent may be discharged and the regenerant processed using electrowinning, thereby making ion exchange both an end-of-pipe waste treatment and a component of a metal recovery system.
Besides the application of ion exchange as a raw water treatment method (i.e., water softening or deionization of city water), ion exchange is most often used by PWB shops in one of two configurations. If wastewater is to be recycled for re-use as rinse water, both cation and anion resins are required and wastewater passing through the units is completely de-ionized (impurities of 10-40 mg/l usually remain, consisting mostly of carbonates and silicates). A second strategy is to remove only certain cations from the wastewater and discharge the treated water. For many streams, the only regulated ions are the metallic species (typically, Cu, Ni, and Pb) and these may be selectively removed by special cation resins that allow common monovalent species (Na, K) to pass through. This cation-only configuration is employed as a stand-alone treatment system (in association with electrowinning) or as a polishing step for the effluent of a conventional precipitation system. Multiple columns of resin are common for either configuration to allow for continuous operation (one column regenerates while the other handles the waste stream).
Ion exchange resin operates by exchanging a H+ ion for a cation in the waste stream, or in the case of anion resins, an OH- ion for an anion in the waste stream. When most sites have exchanged their base ion, the resin must be regenerated. During the regeneration phase, an acid is passed through the cation resin (a base is passed through the anion resin) and cations previously removed are exchanged for the base H+ ion. Metal ions present in the regenerant (in concentrations of a few grams per liter) are commonly removed using electrowinning, or the regenerant may be sent to a conventional precipitation treatment system.
Seventy percent (70%) of the respondents using ion exchange indicated that they are generally satisfied with the technology and 60% indicated they would purchase the same unit from the same vendor if faced with a similar need. Twenty percent (20%) of the respondents using ion exchange indicated they would not purchase this technology for a similar need in the future.
Ion exchange capital costs are related to capacity, which is stated in terms of flow rate (typically gallons per minute). The median price paid by respondents who reported price information was $47,500. The range of capital costs was relatively wide, with the lowest being $5,000 and the highest being $100,000.
Electrowinning is a common metal recovery technology employed by PWB manufacturers to remove metallic ions from spent process fluids, ion exchange regenerant, and concentrated rinse water (e.g., drag-out rinses). Eighteen percent (18%) of the survey respondents reported using electrowinning as a recovery technology and 11% reported using electrowinning as part of their end-of-pipe system (i.e., in conjunction with ion exchange).
An electrowinning unit consists of a rectifier and a reaction chamber that houses anodes and cathodes. In the simplest design, a series of alternating cathodes and anodes are set in the reaction chamber and the electrolyte is made to flow past (or through) them. Metal ions are reduced onto the cathode. The rate at which metal can be recovered from waste fluids or ion exchange regenerant depends on several factors, including the concentration of metal in the electrolyte (the higher, the faster), the size of the unit in terms of current and cathode area, and the predominate species of metal being recovered.
Other than ion exchange regenerant, electrowinning can be applied to various spent process fluids. Three respondents indicated that they use this technology to process spent micro-etchant. Spent micro-etchant contains 15 to 30 g/l (i.e., 1 to 3%) of copper and is created at a relatively high pace from several process lines.k A common practice is to reduce remaining persulfate in the spent bath with a reducing agent (e.g., sodium bisulfite), adjust the pH to 1-3 and then commence electrowinning. Copper recovered by the process is often sold (it is not of sufficient quality to be reused in the PWB process), defraying the overall cost of operating this technology. Electrowinning is also commonly applied to spent gold solutions, gold drag-out and drip tanks, silver-bearing developer and fix solutions, and other copper- or lead-bearing spent solutions (such as strippers or acid dips). Recovery of precious metals rapidly recovers the cost of electrowinning equipment, but most PWB shops produce waste gold- or silver-bearing solutions only in small quantities. Solutions containing hydrochloric acid, or the chlorine ion in general, are usually not processed using electrowinning since electrolysis of these fluids can result in the evolution of chlorine gas. The concentration of metal ions in a solution can be readily reduced below 1 gram per liter using electrowinning, and lower concentrations (as low as 1 mg/l) are possible with some electrolytes. However, the efficiency of the electrowinning process (i.e., mass of metal removed per unit consumption of energy) steadily decreases as the metal ion concentration is depleted.
Eighty-nine percent (89%) of the respondents that have employed electrowinning as a recovery technology were generally satisfied with their unit. A lower percentage (63%) indicated they will buy the same technology from the same vendor if faced with a similar decision in the future. Thirty-eight percent (38%) indicated they will not purchase electrowinning at all in the future if faced with a similar need.
Electrowinning capital costs are dependent on the capacity of the unit, which is rated by most vendors in terms of current (maximum amperage range of common units is 20-1,000 amps DC), cathode area (ft2), and/or metal recovery capacity (lb/day). The median cost of the units for which data were provided was $15,000.
6.2.4 Other Technologies
No other technology was reported in use by more than one respondent. One respondent (ID# 133000) cited use of evaporation, which was employed to recover copper sulfate electroplating solution. One respondent reported using diffusion dialysis for bath maintenance on a tin-lead strip solution. Diffusion dialysis is a membrane technology that is typically employed to purify acids (e.g., nitric acid used in tin-lead stripping baths).
Membrane electrolysis is used by one respondent (ID# 946587). It is employed as an on-line regeneration method for cupric chloride etchant. The etching by-product, cuprous chloride, is brought in contact with the anode of the membrane electrolysis unit, where the cuprous ion is re-oxidized. At the cathode, copper is removed (in metallic form) from the solution at a rate roughly equal to the rate at with it is being introduced. A membrane separates the cathode and anode and allows the user to control the rate at which copper is removed from the etchant.
The same respondent also reported using solvent extraction technology for on-site regeneration of ammoniacal etchant (and drag-out recovery). With the solvent extraction process,l spent etchant containing dissolved copper is mixed with an organic solution containing an organic reagent in a mixer-settler tank. Some of the copper reacts with the organic reagent and forms a chemical complex which is more soluble in the organic phase than the etchant solution. The copper concentration of the etchant solution is thus reduced. The processed etchant solution is fed through a carbon filter and returned to the process. The organic reagent is processed using electrowinning to recovery the copper.