A New Spin on Electrowinning
Rising regulatory pressure is forcing metals-emitting industries, such as plating operations, to find new alternatives to reduce, recycle or value their wastes with the optimum final objective of attaining zero wastewater discharge to the sewer or stream.
One way to reduce discharge of metals into the waste stream and obtain valuable metals for recycling or sale is electrowinning. Also called electro-refining or electro-extraction, this is electrodeposition of metals from their ores that have been put in solution or liquefied. It is an important technology, allowing purification of metals including copper, cyanide cadmium, cyanide zinc, brass, tin-lead, gold, silver, and nickel from plating wastes.
Electrowinning is the oldest industrial electrolytic process. It involves application of electrical current between two electrodes. Under the correct conditions, the dissolved metal present in the solution will plate onto the cathode. Cathodes are usually flat plates made of stainless steel. Because metal deposition rates are related to available surface area, maintaining properly working cathodes is important. A relatively high concentration of metal in solution also is needed for conventional electrowinning processes to operate efficiently.
Electrowinning can recover 90 to 95% of the available metals, but it is known to operate efficiently only at high metal concentrations. This is especially true when flat plate cathodes are used. Therefore, the process is typically used on dragout recovery solutions or combined with ion exchange. Ion exchange processes a large volume of dilute rinse water and concentrates the metals. The regenerant flow is pumped to the recovery cell.
Current density has a substantial impact on metal deposition rate. Increasing the current density will increase the deposition rate to a maximum value where the metal will deposit faster than ions can diffuse through the electrolyte. Excessive hydrogen evolution at the cathode and oxygen evolution at the anode will decrease current efficiency as well as deposit quality. Innovations in the design of electrowinning devices include extending that usable current range by increasing the cathode surface area or reducing the diffusion barrier using agitation or heating.
Global Ionix is a manufacturer of metals recovery equipment to treat process or effluent solutions. Our Ionix RE process works like conventional electrowinning to catch metals in solution in continuous flow or batch mode before they reach the conventional wastewater treatment. It is a unique process that enables users in any industry that deals with metals either already in solution or that can be easily dissolved to recover valuable metals in their pure form and reduces waste disposal costs. The system has already been successfully used on electroless nickel, zinc and iron solutions, and cadmium- and chromium-containing solutions will be tested soon.
Our patented process is based on the use of rotating electrodes to allow recovery of metals in lower concentrations and enable economical recovery of a wider variety of metals from solution. The rotating electrode works by creating turbulence, thus increasing mass transport of ions in solution and effectively reducing the diffusion barrier—the electrochemical resistance to plate-out.
The system uses a rotating, cathodically polarized cylinder. Cathode rotation results in a very uniform deposit without dendrites and nodules. Metal deposits in a powder or foil form, which is then detached from the cathode surface using high-pressure water spray nozzles.
In operation, the solution to be treated is fed into the reactor where it will be electrodeposited onto the cathode. The treated solution containing the metal powder, flakes or any suspended solids is sent to the filtration unit. The final treated solution is either routed to the regular wastewater system or sent back to the RE unit to continue the treatment depending on the concentration of the solution. Metal yields can be improved by controlling the diameter/height ratio of the cathode, anode-to-cathode distance, cathode rotation speed, applied current density, overall reaction time and pump-controlled circulation rate. Solution pH, temperature and conductivity also impact recovery rates.
We use a small, 4-inch diameter system to test and optimize the process for new customers or for solution types never tested before. The flow rate of the test system is 420 ml/min (~0.1 gpm), and the reactor volume is 2 l (~0.5 gal). Our system also uses DSA anodes to prevent their dissolution in very aggressive media such as chloride-based solutions. The cathode is made of stainless steel.
Global Ionix is currently producing two industrial units, using cathodes either 10 or 20 inches in diameter. Flow rates are 8 l/min (~2 gpm) and 32 l/min (~8.5 gpm) respectively. Anodes and cathodes are made of the same material as the lab system. Extraction rates have proven to be linear extrapolations related to the surface area of the cathode from the lab unit to the industrial units’ sizes. In other words, the extraction rate of the 20-inch diameter industrial system is approximately 100× the one for our laboratory-size system.
Depending on metal concentration and other operating parameters, recovered metals can be in the form of foil or powder. High metals concentration in the process solution will definitely give a foil, while lower concentrations will result in a powder. The breaking point between foil and powder deposition depends on the metal to be deposited and the composition of the process solution.
The concentration of metal in the solution also determines whether processing can be performed in batch or continuous flow mode. Diluted rinse waters can be treated in a single pass, whereas more highly concentrated solutions must be treated in batch mode and recirculated until the metal in solution is completely removed or the lower concentration limit fixed by the customer is achieved.
Solutions that have been treated successfully with the Ionix RE technology until now include Watts, sulfamate and electroless nickel solutions, copper-based solutions covering the entire pH range and slightly acidic zinc solutions. Extraction rates of 4 to six lb/hr have been observed on the 20-inch industrial system, while the 10-inch industrial unit has achieved extraction rates of 0.5–0.75 lb/hr.
Rotating electrode technology offers several benefits compared with conventional electrowinning. These include ease of operation, application flexibility, and economy.
The system can be fully automated and requires very little manpower to operate. In fact, we have found that the system can operate with a maximum of one hour per day of operator attendance. This includes inspection, changing of filter bags, solution analysis and solution transportation.
The technology is flexible enough that it can be combined with existing technologies either to concentrate the solution before treatment or to achieve zero wastewater discharge. It can be adapted to changes in flow rates and metal concentrations by varying cathode area and the treatment time or method, and it can work in batch or continuous mode depending on the concentration of metals in the solution.
Because of the high metal recovery rates associated with the rotating electrode process, the system provides an excellent return on investment. In most cases ROI is less than 18 months—extremely rare with environmental technologies. Since the amount of metal reaching the normal wastewater treatment system is dramatically reduced, users will also enjoy cost savings in energy consumption, chemical consumption and off-site sludge disposal.
A good example of industrial application of our company’s RE electrowinning technology is at a manufacturer of printed circuit boards (PCBs). The user had three different copper effluents to treat:
- Alternative oxide bleed, 55 gal/day with metal concentration of 40,000 ppm
- Regular micro-etch bleed, 160 gal/day with copper concentration of 15,000 ppm
- A 10% sulfuric acid solution from ion exchange resin regeneration, 55 gal/day with 10,000 ppm of copper.
The customer’s objectives were to bring the copper concentration of the two bleeding solutions down to less than 75 ppm, thus enabling on-site treatment of the residual solution; and to remove copper from the sulfuric acid solution and allow reuse of the acid five to 10 times. Concentrated bleeding streams were not treated on-site but rather sent off-site for disposition.
Tests in our laboratory proved application feasibility and measured copper removal efficiency (extraction rate). Solutions were first treated separately, then the two bleeding solutions were mixed together for another test.
Test results were uniformly positive, with extraction rates between 0.3 and 0.5 g/min in the test system; this extrapolates to extraction of 2 to 3 kg/hr (~4-6.5 lbs/hr) of metal using a 20-inch diameter industrial unit. Process verification using the larger system did in fact result in an overall extraction rate of 3 kg/hr (~6.5 lbs/hr), confirming data extrapolated from tests with the lab unit.
Based on the current price of recovered copper and reduction in chemical consumption and disposal cost, this application achieved annual savings of more than $228,000. Return on investment required less than 12 months based on 15 hr/day, five day/week running time required to treat the three effluents. A shorter ROI could be obtained if other copper-based process effluents available at the plant can be added to the treatable solutions.
A second industrial application is based on nickel recovery at a generic collision replacement parts manufacturer. The company treats two types of effluent, both containing about 3000 ppm of nickel from Watts baths. Total treatment volume is approximately 736,000 gal/year.
Nickel extraction rate in laboratory tests was approximately 0.4 g/min, which extrapolated to 2.5 kg/hr (~5.5 lb/hr) on the 20-inch industrial system. The optimized process requires close control of solution pH and boric acid concentration. In fact, pH will decrease during plating, so sodium hydroxide addition is necessary in order to plate successfully.
Application of the rotating electrode electrowinning process transformed the $62,250 annual expense of processing the effluent for disposal into $173,442 of revenue from the rebate of nickel recovered with the system. The result, driven by record high nickel prices, is an annual saving of $235,692.
Of course, benefits of rotating-electrode electrowinning always depend on the user’s goals—whether they want metal recovery, solution regeneration and reuse, or environmental impact reduction—as well as on the volume of solution to be treated and the metal concentration in the solution. The technology allows treatment of either segregated water (if the objectives are either metal recovery or reuse of treated solution) or mixed water (if the goal is only to reduce metal discharge to the wastewater treatment system or, ideally, to the sewer).
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