Electrowinning is different from other recovery technologies (e.g., evaporation, ion exchange) in that an elemental metal is recovered rather than a metal bearing solution. The recovered metal is usually not pure enough to be used as anode material in plating processes. More often, it is sold as scrap metal.
Electrowinning is particularly applicable for removing metal from solutions containing a moderate to high concentration of metal ions (>3,000 mg/l). Below 1,000 to 2,000 mg/l of metal, the conventional electrowinning process becomes very inefficient. Therefore, it is not thought of as a "compliance" technology, i.e. a technology that will meet wastewater discharge standards. Rather its benefit is in recovering valuable metals that would otherwise be converted to metal hydroxide sludge by the wastewater treatment system.
High surface area (HSA) electrowinning, developed during the 1970s and commercialized in the 1980s with the reticulate cathode design, extends the applicability of this technology to low concentration solutions and in some cases HAS electrowinning may serve as a compliance technology for specific wastestreams. HAS units employing reticulate cathodes are designed as flow though tanks where the electrolyte passes through a series of cathodes. Each reticulate cathode is made up of thread-like material that is woven into a sheet and given a metalized surface. During use, the ions plate onto the surface of the cathode (up to 5 to 10 lbs./ft2). The cathodes are subsequently removed and sold as scrap.
Commercial units employ a variety of strategies designed at increasing plating efficiency at the relatively low metal concentrations found in typical electrolytes available for electrowinning. This is usually accomplished by design innovations that focus on causing motion of the electrolyte across the surface of the cathode or increasing the surface area of the cathode (e.g., HAS). Solution movement reduces the effect of concentration polarization, a condition where the thin film of electrolyte surrounding the cathode is depleted of metal ions. A high cathode surface area permits efficient operation at low metal concentrations.
Electrowinning is applied to a wide variety of chemical solutions found in the PWB industry. Metals that are most commonly recovered by electrowinning are copper, gold, and silver. For practical purposes, the degree to which a metal can be recovered by electrowinning depends on its position in the electromotive series. In general, 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.
ApplicationsIon Exchange Cation Spent Regenerant. Ion exchange is employed in a variety of configurations in PWB shops, ranging from closed-loop treatment of single rinse systems to a major component (particularly in small shops) of the waste treatment system handling combined rinse streams from a majority of the wet processes. The metal-rich spent cation regenerant is an ideal and logical candidate for electrowinning. For most PWB ion exchange installations, sulfuric acid is the cation regenerant of choice which is a particularly favorable electrowinning electrolyte.
The ion exchange-electrowinning combination is most efficient with copper-bearing rinses that may be combined from several sources or accomplished at the location of a single process. Although ion exchange may also be employed on tin-lead and gold rinse systems, cation resins primarily employed to remove lead or gold are not usually regenerated on-site due to the difficulty of the cycle or the expense of the required regenerant.
Drag-out Tanks. Since the efficiency of electrowinning falls off as the concentration of metal falls, electrowinning of rinse water is, in general, less efficient that that of ion-exchange regenerant where ion exchange serves to concentrate metal. Nevertheless, electrowinning is quite effective at greatly reducing the introduction of metal into flowing rinses ultimately treated by the shops main waste treatment system when employed on drag-out rinses.
Drag-out tanks are rinse tanks, initially filled with water. Parts are first rinsed in this tank, then proceed to the flowing rinse system. In the most efficient configuration, a drag-out rinse is placed after a heated process tank, and the contents of the drag-out tank are returned (recovered) to the process tank as evaporative loss make-up. The level of the drag-out tank is made-up with fresh water, thereby maintaining the overall concentration of the drag-out tank below that of the process tank. This simple arrangement is quite effective at reducing the mass of metal entering the flowing rinses, but the efficiency of the drag-out tank is a function of the temperature (evaporation rate) of the process fluid. Cool process fluids create little evaporative headroom and little drag-out fluid can be returned.
A major source of copper-bearing drag-out in the PWB shop is the copper electroplating tanks. Today, the overwhelming electrolyte of choice for PWB copper electroplating is copper sulfate. This bath is generally maintained at 80°F or below making it less than an ideal candidate for conventional drag-out recovery rinsing. Some shops have opted for a closed loop ion-exchange/electrowinning system to handle the flowing rinses of the copper electroplating process. A second effective alternative is to employ a drag-out rinse tank connected to an electrowinner, through which the drag-out tank solution is continuously circulated. In this configuration, the metal concentration of the drag-out tank is maintained at a low level (determined by the introduction rate due to drag-in, the metal removal rate of the electrowinner, particularly at low concentration levels which can be greatly enhanced by the use of high-surface-area cathodes), thereby reducing the drag-out of metal to the flowing rinse. A properly sized electrowinning unit can maintain the drag-out tank metal concentration well below 100 mg/L, compared to the 14-25 g/L copper concentration contained in the process fluid. The effect being, with the drag-out and electrowinning configuration, the introduction of copper into flowing rinses will be reduced by two orders of magnitude when compared to a standard flowing or counterflowing rinse system.
In the copper sulfate example, the drag-out tank gradually accumulates sulfuric acid which is not an unfavorable environment for PWBs and therefore, only rarely will the tank need to be dumped. When applied to other plating or preparation processes, the build-up of constituents (the electrowinning is only removing metal) from the process fluid may require more frequent dumping of the drag-out tanks, lowering the overall efficiency of the system.
The drag-out electrowinning configuration can also be employed after tin-lead, nickel and gold plating, with good to excellent results. Recovery of gold from drag-out tanks is common for obvious economic reasons although many shops also opt for ion exchange of gold rinse water which is another effective method of gold recovery. Nickel recovery from dragout tanks following nickel electrolytic plating is less common. Nickel plating baths are generally heated to above 120°F, making conventional drag-out recovery effective. Also many shops only plate nickel on connector edges (only a small portion of the PWB is immersed and the drag-out is much lower compared to copper electroplating) and a significant percentage of PWBs may receive no nickel plating at all, making an investment in nickel drag-out recovery less attractive. On the other hand, the value of nickel is 3 to 4 times that of copper making nickel recovery more economically attractive to shops that do full panel nickel plating or otherwise generate above-average nickel drag-out.
Tin-lead plating, like copper sulfate, is generally performed at low temperature making the drag-out, electrowinning configuration attractive. The move in the industry away from tin-lead to tin-only plating and the competition from ion exchange of tin-lead flowing rinses has limited the use of electrowinning. Although the drag-out with electrowinning configuration will perform nearly as effectively with tin lead as with copper, a few factors combine to make the overall system somewhat more expensive and less attractive strategically. The tin-lead plating operation is basically the only source of lead in the rinsewater of a PWB shop (other sources may contribute minute amounts). Cation exchange essentially can remove nearly all lead from the rinses from this operation thereby preventing any lead from reaching the conventional waste treatment system. Elimination of lead from rinsewater available from ion exchange may be viewed favorably to the reduction of lead achieved by electrowinning. Also, the common electrolyte for tin-lead plating is flouboric acid which necessitates the use of expensive, precious-metals-coated anodes.
Tin is not usually recovered (as it is usually not regulated) except incidentally along with lead in ion exchange or electrowinning systems.
Spent Process Fluids. It is possible to electrowin several spent process fluids to recover metal and/or to reduce the burden on the general waste treatment system. While there are several spent process fluids found that are easily electrowinned, it is not an extremely common practice due mainly to a combination of competing technologies or treatment methods, and the cost of handling irregularly timed or one-time dumps of potential electrowinning candidates.
A large process fluid waste stream that can be electrowinned is spent micro-etchant. The currently favored micro-etchant chemistries are sulfuric-persulfate or sulfuric-peroxide. These baths contain 20-40 or more g/L of copper when spent. Both are strong oxidizing solutions and the spent bath is usually reduced with sodium meta-bisulfite or other reducing agent before electrowinning. With very high copper concentrations and the favorable sulfuric acid-based electrolyte, electrowinning proceeds at very high efficiencies, near coppers theoretical maximum plating rate of 1.19 g/amp-hour. A unit capable of delivering 500 amps can easily plate a pound of copper out of such solutions in a single hour, the proceeds from which can easily cover the cost of energy and depending on the level of automation and the amount of labor involved in the handling and preparation of the electrolyte, may also cover the operating costs.
A study at one circuit board shop involving copper recovery from microetchant demonstrated that an 80 to 90% electrowinning efficiency could be achieved while reducing metal content to 1 mg/l in the waste stream (ref. 7). Two types of cathodes were used in an air-sparged electrolytic recovery cell during a two-step operation. Flat reusable stainless-steel sheet cathodes were first used to reduce the copper concentrations of the solution from 20 grams per liter down to 500 milligrams per liter. The cathodes are removed and the copper is peeled off (up to 2 lbs. of copper per cathode). The second step employs disposable high-surface-area cathodes that collect up to 3 lbs. of copper per cathode and reduce the copper concentration of the solution below 1 mg/l. After two years, the original stainless-steel cathodes were still in use and nearly 1,000 lbs. of copper has been collected. The process reportedly reduced hazardous waste generation by more than 35 tons. Another benefit is a reported 50% reduction in the cost of operating the wastewater treatment system.
Electrowinning of micro-etchant competes with common and simple treatment methods. In the case of sulfuric-peroxide, chilling is very effective. Both sulfuric-peroxide and sulfuric-persulfate spent baths can be shipped off-site for copper recovery. This option is often attractive to small shops that produce only a hundred gallons of spent micro-etchant/year.
Another large process waste stream is spent sulfuric acid dips. These, too, are readily electrowinned. The concentration of copper in these spent baths, however, is low, usually 1 g/L or less. While electrowinning can reduce the copper concentration much lower, efficiency at this concentration is reduced and plating times per unit of copper recovered much higher.
Spent electroless copper is also an electrowinning candidate, but more effective, easier methods are available. Spent electroless copper is produced steadily by bail-out (to make room for frequent additions) and the bath itself is relatively short-lived. Most shops prefer to treat the bailout by passing it through activated foam canisters that cause copper to plate-out on the media surface. The effluent from these cartridges is nearly metal-free.
Spent gold baths are commonly electrowinned although credits for gold content can also be obtain by shipping the spent bath for off-site recovery.
Other spent electroplating fluids (tin, tin-lead, copper, nickel) are not produced in large quantities. Copper sulfate baths may last for several years and the timing of the dump is based solely on analysis. Similar bath lives are common for nickel and tin-lead making an electrowinning recovery strategy for these baths uncommon.
Restrictions. 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. Fluoboric acid electrolytes, such as tin-lead fluoborate, generally require platinized anodes, affecting the cost-effectiveness of electrowinning such solutions. Solutions containing chelated metals, reducing agents, and stabilizers are more difficult for direct application of electrowinning.
Nickel recovery using electrowinning is possible, but it requires close control of pH and is therefore performed less frequently than for metals such as copper and gold.