Guide to Electrochemical Technology for Synthesis, Separation, and Pollution Control

Guide To Electrochemical Technology For Synthesis, Separation, and Pollution Control

72 Ward Road ! Lancaster, NY 14086-9779 Tel: 716-684-0513, 800-657-9176 ! Fax: 716-684-0511 www.electrosynthesis.com

Guide to Electrochemical

Technology for Synthesis, Separation and Pollution Control

Prepared for Electrosynthesis Company, Inc. by Professor Derek Pletcher, University of Southampton

Chemical manufacturers and users are daily faced with decisions associated with the need to improve chemical processes (e.g., increase selectivity, separate difficult mixtures, decrease energy consumption, recover the value of chemicals in waste streams, minimize the discharge of a toxic by-product, etc). Sometimes appropriate technology is available, but often it is necessary to evaluate whether an R&D program is likely to provide an economic and timely solution to the needs of the company. A number of technologies may warrant consideration and possible approaches may include electrolysis. Commonly, however, research directors, plant managers, and other technical support providers have relatively little knowledge of and/or experience with electrochemical technology. This Guide seeks to show that modern electrochemical technology can offer the preferred solution to a range of problems, and several illustrative examples are described. On the other hand, electrochemistry is not the answer for all situations; therefore the Guide also discusses those factors that should be considered to determine whether electrolysis is a viable option.

72 Ward Road ! Lancaster, NY 14086-9779 ! Tel: 716-684-0513, 800-657-9176; Fax: 716-684-0511 www.electrosynthesis.com

Table of Contents

WHAT IS ELECTROLYSIS? ...... 3 APPLICATIONS OF ELECTROCHEMICAL TECHNOLOGY...... 4 WHY CONSIDER ELECTROLYSIS NOW? ...... 6 WILL ELECTROCHEMICAL TECHNOLOGY SOLVE YOUR PROBLEM? ...... 7 EXAMPLES OF ELECTROLYTIC PROCESSES ...... 8

2 WHAT IS ELECTROLYSIS? each mole of chlorine formed, two moles of sodium hydroxide must also be produced. Furthermore, it may be seen Electrolysis is a unit process in which that the rate of chemical change is chemical change results from electron proportional to the cell current and the transfer reactions across the total amount of product may be electrode/solution interfaces. A voltage calculated from the charge passed via applied between the two electrodes in Faraday’s Law. an electrolytic cell drives these reactions. A schematic of an electrolytic cell for the Overall, the process objective (the manufacture of Cl2 and NaOH is shown conversion of sodium chloride to in Figure 1. chlorine and sodium hydroxide) is achieved through two selective electrode reactions and a selective e- transport process through the membrane. In common with all cation processes, a chlor-alkali plant will need Cl2 exchange NaOH + H2 membrane several other unit processes to prepare the feedstock for the cell and recover the

+ products. In a chlor-alkali plant, the - Na - e e brine for the cell will be treated to

- - remove hardness and transition ions, ANODE Cl " ½Cl2 ½ H2 + OH # H2O CATHODE and then heated; the products are converted into the appropriate form for NaCl sale or downstream use.

Figure 1. Schematic representation of an electrolysis cell for the The technology for the chlor-alkali manufacture of chlorine and sodium hydroxide. process, including the design of the cell and the development of high- The feedstock is aqueous NaCl brine, performance cell components and the fed to the anolyte compartment between other unit processes in a plant, has been the anode and a cation permeable described,1 while a further series of membrane. Chloride is oxidized to books cover the fundamental chlorine gas at the anode surface and electrochemistry, electrochemical water is reduced to hydrogen gas and engineering and membrane hydroxide ion at the cathode. In order to chemistry.2,3,4 maintain charge balance, migration of ions must occur, and the membrane is 1 D. Pletcher and F. C. Walsh, Industrial designed to allow the passage of only Electrochemistry, Chapman and Hall, London 1990. Na+ from anolyte to catholyte. As a 2 D. Pletcher, A First Course in Electrode Processes, The result, sodium hydroxide is formed in Electrochemical Consultancy, Romsey, 1991. 3 F. C. Walsh, A First Course in Electrochemical the catholyte compartment. Also, in Engineering, The Electrochemical Consultancy, order to preserve charge balance, the Romsey, 1993. rate of transfer of electrons at the two 4 T. A. Davis, J.D. Genders and D. Pletcher, A First Course in Ion Permeable Membranes, The electrodes must be equal – therefore for Electrochemical Consultancy, Romsey, 1997. 3 APPLICATIONS OF different sites. The chlor-alkali ELECTROCHEMICAL industry is a major user of electric TECHNOLOGY power, consuming ~ 125 x 109

kWh/year, > 1% of the total

electricity generated. Not The present commercial applications of surprisingly, energy consumption electrochemical technology are many is a major consideration in process and diverse. Figure 2 is taken from a operation. Also, the scale of the recent EPRI report, Electrolytic Processes, industry warrants the 5 Present and Future Prospects, which development of highly optimized reviewed applications under the chapter cell designs and high-performance headings: anodes, cathodes and separators. A modern cell house will be fully ! The Chlor-Alkali Industry automated and will operate ! Metal Extraction continuously without maintenance ! The Manufacture of Inorganics for several years. ! The Manufacture of Organics ! The world production of ! The Recycle of Chemicals and potassium permanganate, KMnO4 , Materials is only ~ 20,000 tons/year at a few ! Separation and Purification sites. Energy consumption is a less ! Pulp and Paper important consideration in the ! Water and Effluent Treatment process economics, and the ! Atmosphere Control and electrolysis plant is more Improvement traditional, without components ! Destruction of Toxic Waste specifically developed for this ! Soil and Groundwater process. Remediation ! Electrolysis is used for the on-site manufacture of pure arsine, AsH3, for the electronics industry. It While all this technology is based on the avoids storage of highly toxic gas since the cells may be switched same fundamental principles, its on/off as the gas is required. practical manifestations may be quite Highly automated and safe units different with, for example, cell are operated on a very small scale. configurations, electrode materials and

sizes, electrolytes and separators, which A similar story could be told with are each designed to meet the particular respect to the manufacture of organic demands of the application. Even within chemicals or with respect to the a particular heading, the processes may technology for process stream recycle be quite different. For example, consider and environmental protection. Thus, the manufacture of inorganic chemicals: electrolysis is used for the manufacture

of the polymer intermediate, ! The world production of Cl2 is ~ 45 adiponitrile, at several sites with a total x 106 tons/year at some 700 world production of ~ 300,000 tons/year. It is also used for the 5 EPRI Report, Electrolytic Processes, Present and Future production of a number of fine Prospects, Report Number TR-107022, December 1997. 4 Manufacture of Inorganic Chemicals: e.g., Cl2/NaOH, Soil Remediation: in situ treatment of soils to remove - - - KOH, ClO3 , ClO2 , BrO3 , H2, O2, F2, O3, H2O2, MnO2, toxic materials. - 2- 2- 2- Cu2O, MnO4 , Cr2O7 , S2O8 , N2O5, NH2OH, SnO3 , 4+ Ce , and AsH3. Atmosphere Control: devices for removing toxic substances or improving the O2 content of atmospheres. Synthesis of Organic Compounds: including monomers, fine chemicals, and pharmaceutical and Metal Finishing: e.g., electroplating, anodizing, agrochemical intermediates. electropainting, and surface modification.

Extraction of Metals: e.g., Al, Na, K, Mg, Li, Cu, Zn, and Manufacture of Electronic Components: e.g., printed Ga. circuit boards, deposition of metal contacts, and semiconductor layers. Recycle of Chemicals and Process Streams: e.g., metal recovery and refining, recycle of redox reagents, Metals Fabrication: e.g., electrochemical machining, salt splitting, and electrodialysis for stream concentration. electroforming, and electrogrinding.

Chemicals Purification and Separation: e.g., the Corrosion Control: e.g., anodic and cathodic protection refining of metals, the separation of organics from and sacrificial anodes. electrolytes, and purification of amino acids by electrodialysis. Batteries and Fuel Cells: e.g., batteries for electronic devices, household and portable devices, and fuel cells Water and Effluent Treatment: e.g., salt removal; ClO−, for vehicles and on-site power generation. H2O2 and O3 treatment; removal of metal ions to < 1 ppm; and removal of organics, nitrate and radioactive ions. Sensors: for monitoring atmospheres, warning of toxic hazards, optimization of engine/furnace performance, Total Destruction of Toxic Materials: e.g., destruction and monitoring water quality, process streams, medical of contaminated waste from nuclear industry, and applications, etc. polychlorinated biphenyls.

Figure 2: Some industrial applications of electrochemistry. chemicals (total productions 1,000 - With such a variety of applications, it is 10,000 tons/year) as well as some impossible to make generalizations that pharmaceutical and agrochemical will stand detailed examinations. intermediates on a smaller scale. Quite However, there are certainly several different approaches are available for myths about electrolytic processes that process stream recycle. These will should be refuted. For example: include electrolysis, electrolytic salt Myth #1: Electrolytic processes are splitting, electrodialysis, and salt very large consumers of electrical splitting with a special bipolar energy. This is only true for very large- membrane, and the choice will depend scale processes such as Cl2/NaOH and on the nature of the problem to be Al production. In the manufacture of tackled and the specification for the fine and specialty chemicals, the cost of product streams. The same technology electric power is a minor part of the can provide routes for the separation process cost. More commonly, the and purification of products. In effluent economics are dominated by the cost of treatment and water purification, the the feedstock, and the process will be objective may be the removal of salts, designed to give the highest product heavy or transition metals, toxic selectivity. organics or inorganics, or a reduction of chemical oxygen demand (COD).

5 Myth #2: Electrolysis cells are < $0.01 compared to $0.03 - $3.00 unreliable and expensive. Modern for common redox reagents. electrolysis plants can be fully sealed, ! Conditions for a wide range of automated, and computer-controlled highly selective transformations and operate continuously for many are known. months, even years. Moreover, cells ! Methods are available for chiral need not be expensive, particularly if synthesis. precious metal electrodes and ! Suitable electrolytic cells are perfluorinated membranes can be available off-the-shelf, and these avoided. may be combined with other necessary unit processes to Myth #3: Electrolysis cells are difficult construct fully integrated and to obtain. No! A number of companies, compact production systems, including Electrosynthesis Company, which may be batch or Inc., now market electrolytic cells, and continuous. Also, electrolytic there are also many companies which processes are easily computer- supply electrode materials and coatings, controlled. membranes and separators, etc. ! A wide range of modern cell Moreover, the understanding of cell components is now available, design has matured significantly during providing answers to problems the last 20 years, and implementing a that previously limited process new cell design to meet the needs of a performance. particular process is much simpler. Electrolysis should be selected as the

WHY CONSIDER preferred method because it has ELECTROLYSIS NOW? advantages over competitive chemical routes. As you evaluate conventional processing methods, we suggest that There are limitations on electrochemical you should be including consideration technology, and there are unique of electrolysis for the following reasons: solutions that overcome many of these limitations. For example, in a direct ! Electrolysis can be a selective, electrolysis, the maximum rate of convenient and cost-effective chemical change per unit area of technology for synthesis, electrode (the current density) is separations, and pollution proportional to the concentration of control. reactant in solution. An economically ! Electrodes bring about chemical viable current density often requires a change without the use of toxic high solubility of reactant. Since many reagents and usually in reactants are often relatively insoluble in conditions close to ambient aqueous electrolytes (the preferred temperature and pressure. medium for commercial electrolysis), it ! Electrons are cheap reagents. has been necessary to develop strategies The cost of a mole of electrons is to minimize this limitation. One widely successful approach is indirect

6 electrolysis, where a redox mediator (a ! The concentration of transition redox couple soluble in water) is used to metal ions from plating wash transfer the electrons between the waters (ED). electrode and the reactant. Essentially ! The conversion of waste salt the active oxidation state (for oxidation streams to acid and base (either or reduction) is continuously generated by electrolysis or bipolar at the electrode and allowed to react membranes). with the substrate away from the ! Recovery of pure NaCl from electrode, either in-cell or ex-cell. Many seawater (ED). indirect electrolyses employ multiphase ! Isolation of pure organic acids conditions. Couples used successfully from chemical streams (ED). for oxidation include Ce3+/Ce4+, ! On-site electrogeneration of ClO-, 3+ 2- 2+ 3+ Cr /Cr2O7 , Mn /Mn , and H2O2, and ozone for effluent Os(VI)/Os(VIII) couples with chiral treatment. ligands. The redox species are always recycled; hence, only small amounts of redox reagents are used and there is no WILL ELECTROCHEMICAL TECHNOLOGY SOLVE YOUR stoichiometric by-product stream from PROBLEM? the redox reagent. Other approaches include three-dimensional electrodes and the use of gas diffusion electrodes Of course, there can be no general for gaseous reagents. answer to this question. It will depend on your specific requirements and the Many kinds of electrochemical ability of both electrochemical applications are now commercial technology and competing approaches realities for process stream recycle, to meet your challenge. Clearly, for product isolation, and water and electrochemical technology to be your effluent treatment, including: chosen route, there must be a reasonable match between the capabilities of the ! Removal of heavy, transition, and technology and your technical needs. precious metals from effluent to We have already cautioned about levels << 1 ppm. generalizations and myths about ! The removal of many organics, electrolytic processes. Furthermore, we commonly to a COD < 10 ppm. would reiterate that there have been ! The decolorization or significant advances in the availability detoxification of effluent streams and performance of cells and cell by selective oxidation/reduction. components as well as in knowledge of ! Removal of salinity from water to electrode reactions and electrochemical < 200 ppm (by electrodialysis, engineering; overall, it is now easier to ED). take a process through to commercial ! The selective removal of ions – operation. The key to success in an R&D - e.g., NO3 from aqueous solutions program is to select an appropriate using special membranes (ED). approach and to identify how modern components and concepts can be

7 applied. If you or your colleagues are EXAMPLES OF ELECTROLYTIC unfamiliar with the field, we would PROCESSES strongly recommend that you talk to a suitable consultant or approach an (a) The Synthesis of an Intermediate for organization with knowledge and the Cephalosporin Antibiotic, experience in developing electrolytic Ceftibuten6 processes.

The reaction in Figure 3 was identified Below, you will find several examples of as a step in the conversion of the readily successful electrochemical processes available cephalosporin C and the third developed in recent years. We have deliberately O O O O stressed the key features O O HH HH that make them successful HO NH S HO NH S + 2e- and also tried to give you N N CH2OCOCH3 some idea of what the O - CH3COO- O COOH plants might look like. We COOH hope that you will find Figure 3. A step in the conversion of cephalosporin C to third generation cephalosporin them interesting, although antibiotics. we would not necessarily expect there to be close analogies generation cephalosporin antibiotics. between the examples and your Preliminary studies showed that the requirements. They are intended only to highest yields of the product from the illustrate the possibilities. electrochemical reduction were obtained with an aqueous phosphate buffer, pH 7 - 8, and a temperature ~ 280 K. The reduction was always accompanied by significant H2 evolution, which lowered the current efficiency. The preferred cathode material was tin, and it was found advantageous to use an extended area tin cathode fabricated from tin mesh since this gave improved mass transport of the reactant to the cathode in a flow cell. A compact and convenient pilot plant was built based on a commercial electrolyzer, the FM21-SP (marketed by ICI). It had a cathode made by welding expanded Sn mesh to both sides of a Sn plate, giving a nominal cathode area of 0.42 m2. The

6 G. D. Zappi, 12th International Forum on Electrolysis in the Chemical Industry – Clean and Efficient Processing, Sheraton Sand Key, Clearwater Beach, Florida, October 1998. 8 unit had DuPont’s stable perfluorinated drivers were high selectivity at full cation exchange membranes and conversion (reflecting the high cost of catalyst-coated titanium anodes suitable starting material) and high product for O2 evolution placed on either side of purity; both requirements were the cathode. The flow circuit was based achieved in the electrochemical process. on one 100-gallon fiberglass anolyte (0.5 M H2SO4) tank, a 30-gallon polyethylene catholyte tank, a heat exchanger, and a (b) The Production of recirculating cooler. The system used a Aminoanthraquinone7 1000 A power supply and a number of safety devices and sensors to allow For some years, Hydro Quebec has been automatic shutdown in the event of developing indirect electrolytic process malfunctions. The pilot plant processes for the manufacture of organic was operated batchwise but unattended compounds where the electrochemical during electrolysis; each batch step is the oxidation of Ce(III) to Ce(IV) converted ~ 3 kg of starting material. in aqueous methanesulfonic acid. This With current densities of 1.2 - 2.0 kA medium is advantageous because of the m-2, the batch time was 10 - 12 hours; the high solubility of the cerium salts which conversion was 99% and the chemical allows a higher current density and yield ~ 95%. The development of this gives higher rates for the reaction process from very preliminary between Ce(IV) and the organic laboratory scale to completion of the substrate. An early target was the pilot plant trials took ~ 20 man months. production of anthraquinone where the Ce(IV) was reacted with naphthalene. It is valuable to point out two further More recently, a process for the features of this process. First, it would production of aminoanthraquinone has have been preferable to carry out the been described. The route envisaged reduction with equivalent substrate was: with the cephalosporin ring structure

NO2 2 NO O NO2 O itself, but the reduction of the substrate Ce(IV) then gives a significant amount of by- product. It was therefore worthwhile to O O NH2 O carry out a peracid oxidation to form the Na2S sulfoxide prior to cathodic reduction (a O type of decision familiar to synthetic chemists). Second, although the current - efficiency was reasonable at low Ce(III) - e " Ce(IV) conversions, it became low at full conversion. It was, however, The electrochemical regeneration of the advantageous to the downstream Ce(IV) oxidant has been carried out in processing to ensure full conversion. three different commercial electrolyzers, The low current efficiency was acceptable because the energy 7 S. Harrison, 10th International Forum on Electrolysis in consumption was not at all significant to the Chemical Industry – The Power of Electrochemistry, Sheraton Sand Key, Clearwater Beach, Florida, the process economics. The economic November 1996. 9 and in each it was possible to achieve a (c) The Manufacture of Hydriodic Acid8 current efficiency of 92 - 95% for a 70% conversion of a 1 M solution of Ce(III) at The requirement was for very high a current density of 4 kA m-2. Again, the purity 57% HI. The conventional cost of the nitronaphthalene was the chemical process employed reduction of largest factor in the process economics; iodine with red phosphorus, a difficult therefore, it was critical to optimize the technology with effluent problems. chemical yield of the reaction sequence. Some years ago, an electrochemical Two-phase conditions were identified process was described. Iodine was made for conversion of the nitronaphthalene soluble in the catholyte by complexation to nitronaphthoquinone in a chemical with HI, and the cathode reaction was yield of ~ 95% at a conversion of 99%. − - − The Diels-Alder reaction and final I3 + 2e " 3I reduction were also shown to give high chemical yield. and the anode reaction was

- + 2H2O - 4e " O2 + 4H A pilot plant was built and operated. The regeneration of Ce(IV) and the in a cell with a perfluorinated cation coupled oxidation of nitronaphthalene exchange membrane and a sulfuric acid were carried out continuously over a anolyte so that H+ migrated through the period of 750 hours, and the yield of membrane. The process, however, had nitronaphthoquinone was maintained at problems associated with shortcomings 90%. The other chemical steps were in membrane performance. Some sulfate carried out batchwise in 600 - 750 liter passed through the membrane, leading reactors, and high-quality to unacceptable sulfate contamination of aminoanthraquinone was isolated. the HI product. In addition, some iodide passed from catholyte to anolyte where A plant with an annual production of its oxidation led to insoluble iodine and 3,300 tons was designed and costed. The fouling of the membrane. A recent electrolyzer required a total anode area development overcame these problems. 2 of 120 m which could be constructed as The key change was to the anolyte, a “black box” unit, ca 1 m x 1 m x 2 m. which became iodic acid; the major Raw materials represented 70% of the anode reaction remained the evolution production costs, and the total direct of oxygen but some oxidation manufacturing cost $3,370/ton of − - − + aminoanthraquinone (assuming that the IO3 + H2O - 2e " IO4 + 2H cost of nitronaphthalene as $2,200/ton). The total equipment (electrochemical + also occurred. As a result, any iodide chemical) cost was estimated as $11 reaching the anolyte was consumed million. − - − 3IO4 + I " 4IO3

8 J. D. Genders, 10th International Forum on Electrolysis in the Chemical Industry - The Power of Electrochemistry, Sheraton Sand Key, Clearwater Beach, Florida, November 1996. 10 production of drinking water. These can before it was oxidized at the anode. be very large plants producing several Therefore, the fouling problems were thousand cubic meters of high-quality overcome. With no sulfate in the cell, it water per day from brackish feedstock cannot contaminate the HI product and tens of thousands of square meters while any iodate or periodate passing of membranes with up to 500 membrane through the membrane into the pairs in each cell unit. catholyte would react with iodide to reform iodine, leading only to a slight A further interesting development has loss in current efficiency. been the introduction of membranes with some selectivity of ion transport. Clearly, it was desirable to carry out the One such membrane is Neosepta ACS reduction of HI3 to complete conversion, (Tokuyama Corp.) which gives and this was achieved in a two-cell - 2- selectivity for NO3 with respect to SO4 . system. The major part of the Eurodia Industrie S.A. has employed conversion (I2 concentration decreased these membranes in its units for from 4.6 M to 0.4 M) was carried out in removing nitrate from drinking water the “production cell,” a conventional (nitrate in drinking water is an plate and frame cell with graphite plate increasing problem, especially in electrodes. The catholyte from this cell Europe). Eurodia Industrie has also then passed to a “polishing cell” with a sought to advance cell component three-dimensional cathode fabricated design and has incorporated optimized from graphite felt. Overall, the two-cell designs into the electrodialysis stacks process led to 57% HI with a very high for 5 commercial plants around Europe. specification including an iodine content These produce 250 - 4000 m3 of treated < 10 mM. The yield based on iodine was water per day and contain 200 - 4000 m2 close to 100%, and the overall current of membrane, usually in 2 - 4 stacks. efficiency was 90%. Note that the Typically, they reduce the nitrate level process has no unwanted effluents. A from 100 - 150 ppm to 10 - 30 ppm. pilot plant was operated for 1000 hours Based on EUR 20 electrodialysis stacks, without any significant drop in the investment cost varies between $150 performance, and a full-size plant was - 300 m-3 per day depending on plant then designed. size (500 - 4000 m3 treated water per day) and the fraction of nitrate removed (0.65 - 0.85). The total operating cost, (d) The Selective Removal of Nitrate including membrane replacement and 4,9 from Water electric power consumption, lies in the range of $0.05 - 0.10 m-3. A major development of the past twenty

years has been the introduction of large- scale electrodialysis plants for the

9 B. Gillery, 12th International Forum on Electrolysis in the Chemical Industry – Clean and Efficient Processing, Sheraton Sand Key, Clearwater Beach, Florida, October 1998. 11 (f) The Recovery of Copper from Effluent robust, and inexpensive design and in the Scottish Whiskey Industry1,10 these are sold as a complete unit. The size of a typical unit is 1 m long and A number of companies market 0.18 m in diameter. electrochemical technology for the removal of transition, precious, and heavy metal ions from Solid polymer disc effluents. Moreover, the electrolytic cells are based on Polypropylene pipe 1 m long x 0.18 m diameter quite different cell concepts, and the process objectives may either Cylindrical anode, usually expanded Ti mesh with catalytic coating for O2 evolution be the recovery of metal value or

for the effluent to meet legal Cylindrical carbon requirements for discharge. Here, felt cathode we will illustrate this technology with an example that employs the RenocellTM (formerly known as Effluent flow TM Porocell ). Figure 4. Schematic of the Renocell metal recovery cell.

Renocell is based on a three- In the Scotch whiskey industry, dimensional cathode fabricated from distillation is carried out in copper pot graphite felt with a very high surface stills, and this leads to an effluent area/unit volume. It has cylindrical stream contaminated with Cu(II). At geometry and is usually undivided; it is Invergordon Distillers, this leads to shown schematically in Figure 4. 7 m3/hour of an effluent stream known Cylindrical, carbon felt cathode is as “spent lees” containing 10 - 30 ppm of surrounded by a cylindrical mesh Cu(II). The “spent lees” contains a high anode, usually an expanded Ti mesh concentration of organics but little with a precious metal oxide coating electrolyte, and it exits the distillation catalytic for O2 evolution. The effluent unit at 85o C. Despite low conductivity, flow is directed through the cathode by the Cu(II) concentration is decreased to the solid polymer disc at the top. This < 1 ppm by a Renocell unit. The unit has flow through the cathode gives uniform 5 cells in parallel, and has a footprint of mass transport throughout the cathode 0.5 m x 0.75 m. The unit continuously surface and a reasonable mass transfer operates, removing ~ 2 kg/day of coefficient. The cell is fitted into a copper, and cartridges are replaced polypropylene pipe whose inlet/outlet every month. Cathodes are then loaded have quick-release couplings so that the with some 10 - 20 kg of copper and are cathode, designed as a cartridge, may be sent away for copper recovery. Renocell changed rapidly when it is fully loaded units operate in many countries on a with metal. The cartridges are a simple, wide range of metals including precious, transition, and heavy metals, 10 G. Sunderland at The 8th International Forum of producing exit streams that are Electrolysis in the Chemical Industry - Applied Electrochemical Technology, Lake Buena Vista, Florida, acceptable to the local environment. November 1994. 12 a) (g) The Recovery of anion cation 32% Sodium exchange exchange HX NaOH Hydroxide from a membrane membrane Waste Aqueous Sodium Sulfate Na+ Stream11 H+ OH- X-

ANODE Many processes CATHODE produce an effluent NaX stream, which is essentially an aqueous solution of a pure salt in high b) concentration. Even 100 C

when the stream is u r

sodium sulfate, it r 90 e n

often cannot be t

discharged, and an f 80 f i attractive option c i Ammonium Sodium e can be to convert n 70 Sulfate Hydroxide c y

the stream back to (

% Anion Exchange Membrane: Neosepta AMH acid and alkali. In 60 Cation Exchange Membrane: Nafion 902 ) principle, this can DSA - oxygen anode, Ni cathode be achieved with 50 either electrolysis or 20 25 30 35 40 45 bipolar membranes, Concentration (wt %) although with the latter approach, the Figure 6 (a) A common cell configuration for salt splitting; i.e., converting a salt to acid and concentration of base. (b) Current efficiency as a function of product concentrations for the Electrosynthesis Co/Ormiston Mining process. acid and alkali which can be - + 2H2O - 4e " O2 + 4H produced is limited to ~ 5%. One

electrolytic approach is based on a - " − three-compartment cell with both anion 2H2O + 2e H2 + 2OH and cation permeable membranes (see Figure 6). The salt stream is fed to the and the acid, HX, and base, MOH, are − + central compartment between the anion formed by migration of X and M and cation permeable membranes. through the anion and cation permeable Proton and hydroxide ions are membranes under the influence of the produced at the anode and cathode potential field between the electrodes. respectively by the reactions The limitation of this approach is the characteristics of the anion permeable membrane. Such membranes do not have a high transport number when 11 J. D. Genders and J. Thompson, US Patent (1992) contacted by a high proton 5098532. 13 concentration. Therefore, while a good sulfate. These conditions prevail in the current efficiency for the process is Canadian Midwest. possible initially, once the acid concentration in the anolyte begins to Conclusion build up, back-migration of protons occurs (i.e., protons transfer from the Electrochemical technology has become anolyte into the center compartment an important processing tool for instead of sulfate transporting in the synthesis, separations, and pollution opposite direction). Only a limited acid control. Electrochemical cells and concentration can be formed, and as sophisticated cell components are protons build up in the center readily available, as is the technical compartment, they start to pass through expertise to assist you. Many examples the cation permeable membrane as well are now well-documented commercial and this limits even the caustic soda realities, and many more are advancing concentration. through piloting toward commercial- The Electrosynthesis Company, Inc., in ization. We hope this Guide has been conjunction with Ormiston Mining in useful in presenting the importance of Saskatchewan, found a convenient considering electrochemical technology, solution to this limitation. Ammonia along with conventional processing was added continuously to the anolyte options, from the earliest stages of your tank so as to maintain the anolyte at pH product or process development 2. The overall process then becomes analysis.

4NH3 + 2Na2SO4 + 4H2O "

4NaOH + 4(NH4)2SO4 + 2H2 + O2 and in these conditions, a good current efficiency is maintained to high concentrations of both ammonium sulfate and sodium hydroxide. The process was scaled up in a cell designed by the Swedish company ElectroCell, with electrode areas of 0.4 m2. The electrodes were plates, the cathode was nickel, and the anode was an O2- evolving coated titanium. The process was operated at 2.5 kA m-2. Good performance was achieved over extended trials, and it was possible to manufacture 35% sodium hydroxide. Clearly, the process is only appropriate where cheap ammonia is available and there is also a market for ammonium

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