Industrial electrolytic processes consume about 6% of the total electrical generating capacity of the United States, and represent the principal or only method for producing several important products. This section provides a brief summary of some important industrial applications.
Synthesis of Inorganic Chemicals
Electrolytic production of chlorine and sodium hydroxide, introduced earlier in this chapter, represents the largest electrolytic industry. The process produces chlorine, sodium hydroxide, and hydrogen from a salt solution. Production takes place at 60–95 °C and 0.1–1 MPa. Two types of cells dominated this industry for many years: the diaphragm cell described at the beginning of this chapter and a mercury cell. The mercury cell permitted operation at higher current densities and resulted in products of higher purity relative to the diaphragm cell, with similar energy requirements when the energy required to concentrate the dilute NaOH from the diaphragm cell is included. A combination of new technological developments and environmental concerns has led to the development of a third type of cell, a membrane cell, that takes advantage of a cation-exchange membrane, improved electrodes with reduced overpotentials, and corrosion-resistant polymers for cell construction to produce higher purity products than the diaphragm cell at lower energy consumption rates than the mercury cell. Therefore, most new chlor-alkali cells are of the membrane type.
Although at a much lower production scale, diaphragm and membrane cells are also used industrially to produce chlorine and hydrogen from hydrochloric acid. Hydrochloric acid is a by-product of several important nonelectrochemical industrial processes. Serious environmental concerns associated with handling and disposing of HCl can be avoided by converting it to useful products through electrolysis.
A number of other inorganic chemicals are produced by electrolysis at a smaller scale than chlorine production. For example, fluorine gas and other strong oxidizing agents such as KMnO4, H2O2, and Na2S2O8 (sodium persulfate) can be produced electrochemically. Recently, reagents such as hydrogen peroxide (H2O2) have been produced in situ by electrolysis at the quantity needed for optimal use. Another example of inorganic chemical production by electrolysis is high quality MnO2 for battery applications. In addition to these and other contemporary products, there are many other products that can be made electrochemically, but are not currently manufactured that way because of cost. For example, water electrolysis can be used to produce hydrogen and oxygen gas at high purity; however, except for some specialized applications, other methods of producing hydrogen and oxygen are currently more economical.
Electrowinning of Metals
Electrowinning is the production of metals from ores by electrodeposition from a melt or solution. The most important industrial electrowinning process is the production of aluminum using the Hall–Héroult process. In fact, more electrical power is consumed in aluminum production than in any other electrolytic process. The key innovation, made simultaneously and independently in 1886 by Hall in the United States and Héroult in France, was the discovery that alumina (Al2O3) is soluble in cryolite (sodium hexafluoroaluminate) at about 1000 °C, resulting in a conductive solution. The overall reaction for the production of aluminum is
The precise details of the chemistry are not fully understood; consequently, it is difficult to write a complete set of electrochemical reactions. However, the cathodic reaction is clearly the reduction of aluminum. Molten aluminum is denser than the cryolite solution and falls to the bottom of the crucible where it forms the cell cathode; it is periodically siphoned off as the desired product as shown in Figure 14.11. The carbon anode is consumed in the reaction and is lowered gradually into the cell at a rate of about 2 cm per day to maintain the desired cell gap. The other principal reactant, alumina, is added periodically to the melt through a hopper. Typical faradaic efficiency is near 90%, but energy efficiency is low, on the order of 25%.
Figure 14.11 Hall–Héroult process for the production of aluminum.
Other reactive metals produced by electrowinning from a molten salt include lithium, magnesium, and sodium, where chloride-based salts are typically used.
Copper and zinc are the principal metals recovered by electrowinning from aqueous solutions. The hydrometallurgical process used to do this includes acid leaching followed by extraction and then electrowinning. Historically, most copper has been made by smelting, a competing process. Electrowinning is performed in lined concrete tanks into which alternate rows of anodes and cathodes are placed. The spacing between electrodes is about 5 cm. The operating current density for copper ranges from 150 to 1500 A·m−2, although maximum values of 350–400 A·m−2 are more common. The cathodic reaction is the reduction of the metal, which is plated onto the cathode. The anodic reaction is oxygen evolution on, for example, Pb electrodes. Electrolyte temperatures range from 40 to 60 °C, cell voltages from 1.9 to 2.5 V, and current efficiencies from 80 to 95%. Air sparging, electrolyte circulation, or ultrasonic agitation can be used to increase mass transport and, consequently, the maximum current density. The purity of the copper produced by electrowinning can be quite high (99.999%) and is typically ready for market. In contrast to copper, most zinc is now produced by electrowinning, where the process used is similar to that used for copper. The cell voltage for zinc electrowinning, however, is somewhat higher at 3.3 V.
ILLUSTRATION 14.10
How much carbon is consumed for each kg of Al produced? Assume a faradaic efficiency of 100%.
Estimate time before carbon electrode must be replaced. The carbon electrodes are 1.5 m × 0.7 m × 0.7 m with a bulk density of 1500 kg·m−3. Assume a rate production of 1500 kg aluminum per day.
Electrorefining
In contrast to electrowinning, the purpose of electrorefining is to purify rather than to recover the metal. Aspects of copper electrorefining were used to illustrate several concepts in Chapter 4—you may want to review those parts. Metallic copper, often from a smelting process (approximately 99.5% Cu), is used as the anode. During the refining process, the copper anode is dissolved and copper is plated at the cathode. Any impurities that are more noble than copper stay with the anode and are not dissolved. Impurities that are more active than copper dissolve with the copper into the electrolyte. These active impurities, however, remain in the electrolyte and do not plate out with the copper at the cathode; they are later precipitated out or otherwise removed or recovered from the electrolyte. The net result is the electrodeposition of high-purity copper at the cathode (e.g., 99.999%).
A variety of metals can be purified by electrorefining. For example, nickel, cobalt, lead, and tin can all be refined electrochemically in aqueous solution. Active metals such as aluminum can also be purified in this manner with use of a molten salt electrolyte. The cell voltage for electrorefining tends to be lower than that used for electrowinning as the equilibrium potential is essentially zero for the electrodes of nearly the same composition. Current densities are also modest in order to maintain high purity product and avoid anode passivity (where applicable). As a result, the operating cell voltage for copper electrorefining is only about 0.25 V.
Electrosynthesis of Organic Compounds
A large number and variety of organic reactions can be carried out electrochemically. In fact, organic electrochemistry is considered to be a mature branch of organic synthesis, and most organic reactions that involve electron transfer can be performed by electrochemistry. Types of reactions include oxidation and reduction of functional groups, cleavage, substitutions (e.g., halogenation), additions (e.g., hydrogenation), coupling (e.g., dimerization), and rearrangement. See Further Reading at the end of this chapter for examples of specific reactions.
Organic electrosynthesis reactions may be performed directly or indirectly. Direct synthesis reactions are heterogeneous reactions that take place directly on the surface of the electrode. In most cases, the electrochemical reaction forms a reactive intermediate or radical that undergoes further reaction in close proximity to the electrode surface to produce the desired product. Indirect electrosynthesis reactions take place via a mediator, which in turn reacts homogeneously in solution with the organic reactant to produce the desired product. The mediator is regenerated electrochemically once it has reacted to affect the desired synthesis. Therefore, there are no waste or disposal concerns since the mediator is recycled and not consumed. Most mediators (catalysts) for indirect synthesis are inorganic redox couples such as the following:
Reductions: Sn4+/Sn2+, Cr3+/Cr2+, Ti4+/Ti3+, Zn2+/Zn, Na+/NaHg
Oxidations: Ce3+/Ce4+, Cr3+/Cr6+, Mn2+/Mn3+, Mn2+/Mn4+, Ni (OH)2/NiOOH, I−/I2,Br−/Br2,Cl−/ClO−
Indirect reactions may be advantageous when they can be used to replace organic reactions that have a high overpotential and sluggish kinetics or that tend to passivate the electrodes. Indirect reactions are also favored when the redox catalysts can provide enhanced selectivity. The catalyst regeneration and the chemical reaction steps can take place in the same reactor (in-cell) or in different reactors (ex-cell). An ex-cell strategy, made possible through the use of indirect reactions, permits separate optimization of the catalyst and organic reactions. Multiphase reactions are also possible with the catalyst regeneration in the aqueous phase and the organic reaction in a separate organic phase. Use of multiple phases can facilitate product separation and enhance the commercial viability of a process. As always, however, there are trade-offs between the simplicity of a direct process and the enhanced flexibility of an indirect process that must be considered carefully in the design process.
In spite of the many possibilities that exist, relatively few organic electrosynthesis reactions have been successful industrially. Even some of the early successes are no longer performed commercially. Steckhan (2012) estimated that approximately 200 reactions have been performed at the pilot scale with more than 100 commercially available. It is difficult to get a precise number because the details of many of industrial processes are often kept confidential.
The most significant industrial process is the production of adiponitrile, an intermediate in the production of Nylon®. It is the only organic electrosynthesis process where the volume of production is consistent with that of a commodity chemical (300,000 metric tons·yr−1). The reactions are shown below. The cathode reaction is the electro-hydro-dimerization of acrylonitrile, and oxygen is evolved at the anode.
(14.20)
The overall reaction is
(14.21)
These reactions occur in an undivided bipolar stack using aqueous sulfuric acid as the electrolyte. Another process, currently under development, is the electrochemical synthesis of ethylene glycol, which has the potential to become another high-volume process.
There are many potential advantages to the electrochemical synthesis of organic compounds. The inherent advantage is that electrons serve as the oxidizing and reducing agents. These electrons are, in general, inexpensive and clean relative to chemical agents. The rate of reaction is activated with potential rather than temperature. Thus, the mild conditions characteristic of electrochemical synthesis are well suited for chemicals that are heat sensitive. Also, since the current is directly proportional to the reaction rate, these reactions are inherently easier to control. Closely connected to the ability to control the process is the potential for high selectivity from electrochemical processes. Selectivity is particularly important for high-value specialty products. In spite of these advantages, the number of commercial processes is small, as are the volumes produced, as mentioned previously.
Given the advantages of organic electrosynthesis, why are there not more successful industrial processes? What are the key factors that contribute to a successful process? It turns out that energy costs and initial capital costs associated with the electrochemical cells are not typically the problem. At the risk of overgeneralizing, the factors that are often most important are the availability and cost of the reactants, the reaction yield that can be obtained, the ability to inexpensively separate the product(s) from reactant(s), the availability of a suitable, stable electrolyte, and the ability to achieve an acceptable production rate. Because of the low conductivity of organic solvents, it is necessary to add a supporting electrolyte, which must then be separated from the product downstream. Many commercially successful processes involve water soluble reactants and products and utilize sulfuric acid as the electrolyte. Alcohols and acetic acid are also used industrially with some frequency. Cosolvents can be used to enhance solubility if needed. Separation can be facilitated by phase separation where feasible.
ILLUSTRATION 14.11
Naphthoquinone can be formed from naphthalene by the following reaction, where cerium acts as the mediator.
Regeneration of the mediator takes place in a separate reactor. What is the desired reaction in the regeneration reactor? If regeneration is done in aqueous solution, what is the most likely cathodic reaction? The faradaic efficiency can be greater than 90%. What is the most likely side reaction? The standard potential of the Ce3+/Ce4+ is 1.72 V.
SOLUTION:
Since cerium is reduced in order to make naphthoquinone, it must be reoxidized in the regeneration reactor. The most likely cathodic reaction in an aqueous system is hydrogen evolution. Since the standard potential of the cerium reactions is well above the oxygen potential, it is most likely that oxygen is evolved during the oxidation process. Oxygen evolution can be reduced by choosing an electrode surface with a high O2 overpotential.
Another important factor that may easily be overlooked is the need to consider organic electrosynthesis as a design alternative early in process development. It is difficult and expensive to consider such options at an advanced stage of design. Consequently, it is important for commercial success that a company has the expertise needed to consider electrochemical options as part of their normal design process. This factor is likely to become more important in the future as society shifts to solar power as the primary energy source from which electricity can be generated directly rather than from fuels as is currently the case.
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