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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.5 ELECTROWINNING
3.5.3 Applications and Restrictions
Exhibit 3-32 shows the three basic configurations in which electrowinning was successfully applied by the shops responding to the survey.
The most common configuration (EW-1) employs an electrowinning unit connected directly to a drag-out tank. Alternatively, the solution from the drag-out tank can be periodically transferred to a holding tank that is connected to the electrowinning unit. Either of these arrangements can be used with flat plate or reticulate cathode units. The reticulate cathode types will maintain the rinse system at a lower metal concentration (in some cases below 1 mg/l) but, because the cathodes are not reusable, the operating costs will be higher. The operation of the flat plate cathode types are more significantly affected by fluctuations in the metal concentration of the electrolyte. Therefore, if the plating operation causes sharp fluctuations in the drag-out tank concentration, the user should consider the use of a side tank or a reticulate type of cathode. The HSA cathode units should be directly connected to the drag-out tank. This will permit them to maintain a low steady state concentration of metal in the drag-out tank.
Electrowinning removes metal from the drag-out solution, but does not remove all dissolved solids. For this reason, the drag-out solution must be occasionally discarded or purged to prevent the build-up of dissolved solids (e.g., acid). When this occurs, any residual metal in the drag-out solution will be lost.
The metal recovery efficiency (i.e., the percentage of metal recovered from drag-out) of the first configuration depends on two key factors: (1) the average concentration of metal in the drag-out tank and (2) the mass of metal in the purge. The concentration of metal in the drag-out tank is important because it determines the mass of metal that will be carried over by drag-out to the next rinse, which is treated. This factor points out the weakness of the flat plate cathode types. These units operate efficiently only when the metal concentration is high (usually 1 to 5 g/l of metal). Therefore, the drag-out tank must be operated until this level is achieved, which in turn increases the loss of metal to the free running rinse. The higher surface area of the reticulate and HSA units allow the user to operate the drag-out tank at a lower metal concentration and therefore reduce metal losses. Further, these types of electrowinning units generate a purge with a lower metal concentration.
The second configuration (EW-2) is a combination of ion exchange and electrowinning. This configuration potentially has a much higher metal recovery efficiency than the first configuration. It addresses both of the factors that impact metal recovery efficiency. The ion exchange unit maintains a low metal concentration in the final rinse, thereby almost eliminating drag-out losses. The ion exchange unit concentrates the metal into a regenerant stream and the electrowinning unit removes the metal. Residual metal in the regenerant is of less concern than the first configuration since it can be reconcentrated by the ion exchange unit. For the same reason, a flat plate cathode will suffice for this second configuration.
In some cases, the reticulate cathode units can be substituted for the second type of configuration. When such a unit maintains the drag-out rinse in the low mg/l range, the metal recovery efficiency of the process would approach that of the ion exchange/electrowinning combination. Some recent operating data for a copper recovery application using this configuration are presented in Section 3.5.4.3.
The third configuration shows the recovery of metal from a spent process solution. Either the flat plate or reticulate cathode type of unit can be used in this configuration. The reticulate cathode type will provide greater metal recovery efficiency because it can lower the metal concentration of the spent bath below that of the flat plate. Because the reticulate cathodes are not reusable, its higher recovery efficiency comes at an increased operating cost.
Electrowinning is applied to a wide variety of chemical solutions in the electroplating industry. The literature indicates that the metals that are most commonly recovered by electrolytic treatment are gold, silver, copper, cadmium, and zinc. The metal recovery applications identified from the Users Survey are shown in Exhibit 3-33. This exhibit indicates the number of survey respondents that applied electrowinning to each of the processes and the average satisfaction level of the technology for that application, based on a scale of 1 to 5 (1 equals the lowest satisfaction level and 5 equals the highest).
For practical purposes, the degree to which a metal can be recovered by electrowinning can be determined by its position in the Electromotive Series (see Exhibit 3-34). Metals that have more positive standard electrode potentials plate more easily than the ones with less positive potentials. As an illustration, the more noble metals, such as silver and gold, can be removed from solution to less than 1 mg/l using flat plate cathodes whereas with copper and tin, a concentration in the range of 0.5 to 1 g/l or more is required for a homogeneous metal deposit. Equations for accurately estimating the potential for a given application were presented by Brown (ref. 349) and Bailey and Chan (ref. 128).
It is interesting to compare the satisfaction levels in Exhibit 3-33 to the position of the metal in the electromotive series. The satisfaction levels for silver, copper, cadmium and zinc cyanide plating (the most common applications of the respondents) fall into nearly the exact order as the metal's position in the electromotive series.
Although copper, cadmium and zinc have a lower position in the electromotive series than precious metals and they received only moderate to low satisfaction levels from survey respondents, this is not to say that these applications cannot be successfully performed. With the application of proper engineering and good equipment selection these electrowinning applications can be highly successful, as indicated by some of the respondents. For additional data, Exhibit 3-35 groups potential electrowinning applications based on their frequency of use and success in industry and the general difficulty of the application. These rankings are based on input from electrowinning vendors and information from the literature. Included in this exhibit are a much broader range of metals than those identified in the Users Survey.
Although there are limitations for electrowinning nearly every metal, chromium is the only commonly electroplated metal that is not recoverable using electrowinning. Nickel recovery is possible, but it requires close control of pH and therefore is less frequently performed than, for example, cadmium or copper. Also, Altmayer suggests that nickel recovery is hampered by the absence of inexpensive suitable inert anodes that do not give off chlorine gas and disintegrate (ref. 482).
Solutions such as electroless plating solutions containing chelated metals, reducing agents and stabilizers are more difficult for the direct application of electrolytic recovery. However, there was one survey respondent that indicated they were successfully electrowinning nickel from a spent electroless solution (PS 188). Another shop (PS 164) is in the process of starting up a unit for the same purpose. One vendor (ref. 349) indicated that these baths can be processed by electrowinning after undergoing pretreatment (e.g., selective ion exchange) to break the metal-chelate bond. Another reference suggests that reducing and oxidizing agents can be combined to neutralize their effects; e.g., a printed circuit board shop can mix spent micro-etch and electroless copper baths and with proper pH adjustment create a solution that can be treated by electrowinning (ref. I3 file). Another reference indicates that electroless copper can be processed using electrowinning, but that anode life will be short (ref. 99).
Fluoborate solutions (e.g., tin, tin-lead) are not commonly treated using electrowinning due to their attack upon anode materials including iridium oxide coated titanium and niobium. However, one source (ref. 287) indicates that titania ceramic anodes coated with iridium can provide a successful application. This material and its application have been recently commercialized (ref. Kinetico file).
Certain corrosive solutions (e.g., certain etchants) may also pose problems for electrowinning because metal that is plated on the cathode may be etched off as quickly as it is plated (ref. 348). One reference suggests that increasing the current density will partially overcome the etching action of ammonomical etches when electrowinning copper from these solutions, but that complete removal is difficult to achieve (ref. 99).