Other Electrolytic Processes

As mentioned above, the electrolysis of NaCl to from CI2 and NaOH is the largest application of membranes in electrolytic cells. There are also other brine electrolyzes of commercial importance. NaBr brine is electrolyzed to form Br2-(Another method of Br2 formation is to treat bromide brines with CI2 derived from electrolysis.) Electrolysis of KCl brines is the preferred process for making KOH. Because K ions are less hydrated than Na ions, the membrane is more effective at blocking the backdiffusion of KOH, which allows production of KOH in concentrations as high as 47%. 

2NaCl02-tNa0Cl-tH2S04 2Cl02-i-NaCl+Na2S04-i-H20 

This process has the advantage of no unwanted byproducts. Moreover, the yield of CIO2 can exceed 90%. 

Another precursor for CIO2 is sodium chlorate NaCl03. The pulp and paper industry consumes 95% of global chlorate production. The NaCl03 can be re- 

All of these reactions produce some unwanted byproducts, which can be avoided by splitting NaClOs in a 3-compartment cell to chloric acid, HQO3, which can be stored and shipped safely [39]. The NaC103 is fed between two cation-exchange membranes, and H ions from the anolyte replace Na ions that migrate to the catholyte to form NaOH coproduct The HCIO3 can be cata-lytically converted to CIO2 by the reaction 

---

In this method, each gas is produced in a separate compartment so they have high purity. In this process, deuterium oxide, D20, is electrolyzed more slowly so the water becomes enriched in the heavier isotope. The other electrolytic process that produces hydrogen is the electrolysis of a solution of sodium chloride. 

Most metals found in nature exist as ores in combination with other elements such as oxygen and sulfur. Electrolysis can be used to separate metals from their ores and remove impurities from the metals. The general process is known as electrorefining. Other electrolytic processes are used to obtain a number of important chemicals. 

First operated in 1908 at the Osterreichische Chemische Werke, Weissenstein, Austria, and then by Degussa, Germany. This process, as well as the other electrolytic processes, was made obsolete by the invention of the AO process. 

The usual operating voltage of a fluorine cell is 8.5-10.5 V at 1-2 kAm-2. The high voltage is due to - in comparison to other electrolytic processes - relatively low electrolyte conductivity, long current paths, and especially due to the high anodic overpotential mentioned. The specific energy consumption so amounts to about 14000 kWh/t F2. This is about four times the theoretical value. 

Chlorine is produced predominantly by an electrolysis process in diaphragm, mercury, or membrane cells. In each process, a salt solution, most often sodium chloride, is electrolyzed by the action of direct electric current, which converts chloride ions to elemental chlorine. A small amount is produced by other electrolytic processes or nonelectrolytically. 

Other Chlorine Production Processes. Although electrolytic production of CI2 and NaOH from NaCl accounts for most of the chlorine produced, other commercial processes for chlorine are also in operation. 

Betts Electrolytic Process. The Betts process starts with lead bullion, which may carry tin, silver, gold, bismuth, copper, antimony, arsenic, selenium, teUurium, and other impurities, but should contain at least 90% lead (6,7). If more than 0.01% tin is present, it is usually removed from the bullion first by means of a tin-drossing operation (see Tin AND TIN ALLOYS, detinning). The lead bullion is cast as plates or anodes, and numerous anodes are set in parallel in each electrolytic ceU. Between the anodes, thin sheets of pure lead are hung from conductor bars to form the cathodes. Several ceUs are connected in series. 

Air pollution problems and labor costs have led to the closing of older pyrometaHurgical plants, and to increased electrolytic production. On a worldwide basis, 77% of total 2inc production in 1985 was by the electrolytic process (4). In electrolytic 2inc plants, the calcined material is dissolved in aqueous sulfuric acid, usually spent electrolyte from the electrolytic cells. Residual soHds are generally separated from the leach solution by decantation and the clarified solution is then treated with 2inc dust to remove cadmium and other impurities. 

Oxidative surface treatment processes can be gaseous, ie, air, carbon dioxide, and ozone Hquid, ie, sodium hypochlorite, and nitric acid or electrolytic with the fiber serving as the anode within an electrolytic bath containing sodium carbonate, nitric acid, ammonium nitrate, ammonium sulfate, or other electrolyte. Examples of electrolytic processes are described in the patent Hterature (39,40)... 

The vacuum melting process can upgrade chromium at a modest cost the other purification processes are very expensive. Thus iodide chromium is about 100 times as expensive as the electrolytic chromium and, therefore, is used only for laboratory purposes or special biomedical appHcations. 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

The products of this electrolysis have a variety of uses. Chlorine is used to purify drinking water large quantities of it are consumed in making plastics such as polyvinyl chloride (PVC). Hydrogen, prepared in this and many other industrial processes, is used chiefly in the synthesis of ammonia (Chapter 12). Sodium hydroxide (lye), obtained on evaporation of the electrolyte, is used in processing pulp and paper, in the purification of aluminum ore, in the manufacture of glass and textiles, and for many other purposes. 

Use of low-temperature molten systems for electrolytic processes related with tantalum and niobium and other rare refractory metals seems to hold a promise for future industrial use, and is currently of great concern to researchers. The electrochemical behavior of tantalum, niobium and titanium in low-temperature carbamide-hilide melts has been investigated by Tumanova et al. [572]. Electrodeposition of tantalum and niobium from room/ambient temperature chloroaluminate molten systems has been studied by Cheek et al. [573],... 

Fuel cells essentially reverse the electrolytic process. Two separated platinum electrodes immersed in an electrolyte generate a voltage when hydrogen is passed over one and oxygen over the other (forming H30+ and OH-, respectively). Ruthenium complexes are used as catalysts for the electrolytic breakdown of water using solar energy (section 1.8.1). 

As noted earlier, the kinetics of electrochemical processes are inflnenced by the microstractnre of the electrolyte in the electrode boundary layer. This zone is populated by a large number of species, including the solvent, reactants, intermediates, ions, inhibitors, promoters, and imparities. The way in which these species interact with each other is poorly understood. Major improvements in the performance of batteries, electrodeposition systems, and electroorganic synthesis cells, as well as other electrochemical processes, conld be achieved through a detailed understanding of boundaiy layer stracture. 

Concentration gradients in the electrolyte layer next to the electrode surface will develop or change as a result of the primary electrode reaction. Therefore, the current associated with these changes is faradaic, although it is also transient and falls to zero when adjustment of the concentration profile is complete. Unlike other transient processes, these processes, can be described in a quantitative way (Sections 11.2 and 11.3). The transition times of such processes as a rule are longer than 1 s. 

Among electrolytic processes used to produce materials, we customarily distinguish those in which electrodes are reacting that is, where the metal or other electrode material is involved in the reaction (Chapter 16) from those with nonconsumable electrodes (Chapter 15). A very important industrial process with nonconsumable electrodes is the electrolysis of sodium chloride solution (brine) producing chlorine at the anode and sodium hydroxide NaOH (caustic soda) in the catholyte via the overall reaction... 

Dynamic techniques are those in which electrolytic processes occur at the electrodes and therefore a finite current is passed through the electrochemical cell. Thig discussion will be limited to controlled-potential techniques, namely voltammetry and ampero-metry. While other dynamic electrochemical techniques have been developed, these two are by far the most commonly used for bioelectroanalytical studies. 

The two electrolytic processes, one with the metallic sulfide and the other with the metal itself, are industrial processes. The first one allows simultaneous recovery of metal and sulfur, and second one allows purification of impure metal. 

---