A New Spin on Electrowinning
By Miousse, Danielle
INNOVATIVE TECHNOLOCY EXPANDS METALS RECOVERY POSSIBILITIES
Rising regukitorv 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 wast ewaier discharge to the sewer or stream.
One way to reduce discharge of metals into the waste stream and obtain valuable metals for recvcling 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.
The Global lonix RE electrowinning system uses a rotating electrode to Improve metal yields and allow economic metals recovery from solutions with lower metals concentrations.
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 waste water treatment. It is a unique process that enables users in any indusiry that deals with metals eitliei 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 cadmiumand chromium-containing solutions will be tested soon.
ROTATING ELECTRODES
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.
Steps in the Ionix RE process include (1) feeding of solution into the reactor; (2) deposition of metal onto the cathode; (3) movement of treated solution to the filter unit; (4) filtration of any metal powder, flakes or suspended solids; (5) routing of treated solution to the wastewater system or back to the unit for continued treatment.
In operation, the solution to be treated is fed into the reactor where it will he 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 yieds can be improved by controlling the diameter/ heigh t ratio of thecathodc, anode-to-calhode distance, caihode rotation speed, applied current density, overall reaction time and pump-controlled circulation rate. Soluiion pH, temperature and conductivity also impact recovery rates.
Depending on solution concentration and other parameters, metal is recovered as either sheets (such as the copper in this photo) or powder.
We use a small, 4-incb diameter system to test and opiimize tinprocess for new customers or for solution types never tested before. The flow rate of the test system is 420ml/min (-0.1 gpm), and the reactor volume is 2 I (-0.5 gal). Our system also uses DSA anodes to prevent their dissolut ion in very aggressive media such as chloridebased solutions. The cathode is made of stainless steel.
Global lonix is currently producing two industrial units, using cathodes either IO 01 20 inches in diameter. Flow rates are 8 1/mhi (~2 gpm) and W 1/min (-8.5 gpm) respectively. Anodes and cathodes are made of the same material as the lab system. Extraction rales have proven to be linear extrapolations related to the surface area of the cathode limn the lab un it to industrial units’ sizes, lu other words, the extracnon rate of the 20-inch diameter indus trial system is approximately K)Ox the one for our laboratory size system.
Depending on metal concentralion and other operating parameters, recovered metals can l)e 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 hciwcrn toil and powder deposition depends on the metal to be deposited and the composition of the process solution.
The concentration oi 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 reciculated 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 lonix RE technology until now include Wans, sulfamate and eleclroless nickel solutions, copper-based solutions covering the entire pi I range and slightly acidic zinc: solutions. Extraction rates of 4 Io six Ib/hr have been observed on the 20-inch industrial system, while the 10- inch industrial unit IKIS achieved extraction rates of 0.5-0.75 lb/ hr.
BENEFITS
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 thai 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 llexible enough thai it can be combined with existing technologies cither to concen trate the solution before treatment or to achieve/crowastewatcrdischargc. 11 can be adapted to changes in flow rates and metal concent rat ions bvvarving cathode area and the treatment lime or method, and it can work in batch or continuous mode depending on the concentration of metals in the solution.
Because of t hellish meial recovery rales associated with the rotating electrode process, the system provides an excellent return on investment. In most (uses ROI is less than IH months-extremely rare with environmental technologies. Since the amount of metal reaching the normal wastewater ireatmeni svstcm is dramatically reduced, users will also enjoy cosi savings in energy consumption, chemical consumption and oilsite sludge disposal.
COPPER APPLICATION
A good example of industrial application of our company’s RE electn winning technology is at a manufacturer of printed circuit hoards (PdBs). The user had three different copper effluents to lreai:
* Alternative oxide bleed, 55 gal/day with metal concentralion of 40,000 ppm
* Regular micro-etch bleed, 160 gal/day with copper concentration of If)1OOO ppm
* A 10% sulfuric acid solution from if)ii exchange resin regeneration, 55 gal/day with 10,000 ppm of copper.
The customer’s objectives were lo bring ihe copper concent ration of the two bleeding solutions down to less than 75 ppm, t h us enabling on-siie 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- silte but rather sent off-siie for disposition.
Tests in our l\aboratorv proved application feasibility and measured copper removal efficiency (extraction rate). Solutions were first Heated separately, ilien lhe two bleeding solutions were mixed together for another test.
Test results were iiniloimly 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/h r (~4-o.fi Ibs/hr) of metal using a 20- inch diameter industrial unit. Process verification using lhe larger svstein dirl in fact result in an overall extraction rate of 3 kg/ hr (-6.5 Ibs/hr), confirming data extrapolated from tests with the lab unit.
Based on the cnrrem price of recovered copper and reduction in chemical consumption and disposal cost, this application achieved annual savings of more than $22,000. Return on investment required less than 12 months based on 15hr ‘day. five day/ week running lime required to treat the three effluents. A shorter ROI could he obtained if ot her copperbased process effluents available at the plant can be added to the treatable solutions.
NICKEL APPLICATION
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 Walts baths. Total treatment volume is approximately 736,000 gal/ year.
Nickel extraction rate in laboratory tests was approximately 0.4 g/miri, which extrapolated to to 2.5 kg/hr (-5.5 lb/hr) on the 20- imh industrial system. The optimi/ed process requires close control of solution pll and boric acid concentration. In fact, pH will decrease duringplau’ng, so sodium Indroxideaddition is necessarvin order in plate successfully.
Application of the rotating electrode electrowinning process transformed ihr S62.250 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 roialingelecirode clectnm inning always depend on the user’s goalswhether the want metal recovery, solution regeneration and reuse, or environmental impact reduction-as well as on the volume ot solution to be treated and the metal concentration in the soluiion. The lethnology allows treatment of eithier segregated water (if the objectives are either metal recovervor reuse of treated solution) or mixed water (if the goal is only to reduce metal discharge to the wastewater treatment svstem or, ideally, to the sewer).
LEARN MORE
APPROACHING ZERO DISCHARGE IN SURFACE FINISHING
This article describes a new electrowinning technology that can help platers achieve zero-discharge operation. For a more in-depth look at technologies that are bringing us closer to zero discharge, read Approaching Zero Discharge in Surface Finishing.
Find the link to this article online at mvw.pfontine.com/anicles/ 050702.html.
For more information on rotating-electrode electrowinning systems from Global lonix. Bo u chervil Ie. Quebec, phone 450-641-8537 or go to www.ptOnline, com.
www.pronllne.com
By Danielle Miousse, Ph.D.
Global Ionix Inc.
Copyright Gardner Publications, Inc. May 2007
(c) 2007 Products Finishing. Provided by ProQuest Information and Learning. All rights Reserved.