Electrolysis commercial processes

Aluminum metal is produced from aluminum oxide by electrolysis using the Hall-Heroult process, whose story is detailed in our Chemical Milestones Box. The melting point of AI2 O3 is too high (2015 °C) and its electrical conductivity too low to make direct electrolysis commercially viable. Instead, AI2 O3 is mixed with cryolite (Na3 AlFfi) containing about 10% CaF2. This mixture has a melting point of 1000 °C, still a high temperature but not prohibitively so. Aluminum forms several complex ions with fluoride and oxide, so the molten mixture... 

In the above process, finely divided iron oxide combined with sodium oxide and silica or alumina is used as the catalyst. The reaction is favored (as per Le Chatelier s principle) by high pressure and low temperature. However, a temperature of 500 to 550°C is employed to enhance the reaction rate and prevent catalyst deactivation. Although at 200°C and 250 atm the equihbrium may yield up to 90% ammonia, the product yield is too slow. The sources of hydrogen in commercial processes include natural gas, refinery gas, water gas, coal gas, water (electrolysis) and fuel oil, and the nitrogen source is liquefied air. 

Lithium metal is produced commercially by electrolysis of a fused eutectic mixture of hthium chloride-potassium chloride (45% LiCl) at 400 to 450°C. The eutectic mixture melts at 352°C in comparison to the pure LiCl melting at 606°C. Also, the eutectic melt is a superior electrolyte to LiCl melt. (Landolt, P.E. and C. A. Hampel. 1968. Lithium. In Encyclopedia of Chemical Elements.C. A. Hampel, Ed. Reinhold Book Corp. New York.) Electrolysis is carried out using graphite anodes and steel cathodes. Any sodium impurity in hthium chloride may be removed by vaporizing sodium under vacuum at elevated temperatures. All commercial processes nowadays are based on electrolytic recovery of the metal. Chemical reduction processes do not yield high purity-grade metal. Lithium can be stored indefinitely under airtight conditions. It usually is stored under mineral oil in metal drums. 

Alkali metals are produced commercially by reduction of their chloride salts, although the exact procedure differs for each element. Both lithium metal and sodium metal are produced by electrolysis, a process in which an electric current is passed through the molten salt. The details of the process won t be discussed until Sections 18.11 and 18.12, but the fundamental idea is simply to use electrical energy to break down an ionic compound into its elements. A high reaction temperature is necessary to keep the salt liquid. 

These studies identified the hybrid sulphur process (HyS), the sulphur-iodine (S-I) process and the high temperature steam electrolysis (HTSE) process as the main contenders for medium-term commercial water-splitting hydrogen production. 

Electrochemical fluorination 168,169> is a commercial process for perfluorina-tion of aliphatic compounds. The reaction is performed in liquid hydrogen fluoride -potassium fluoride at a nickel anode. The mechanism is not known free fluorine cannot be detected during electrolysis, so it seems probable that fluorination is a direct electrochemical reaction. Theoretically, hydrogen fluoride-potassium fluoride should be a very oxidation-resistant SSE, and it might well be that the mechanism is analogous to that proposed for anodic acetamidation of aliphatic compounds in acetonitrile-tetrabutylammonium hexafluorophosphate 44 K... 

Other interesting electrolytic systems have been developed based on electrolysis of triethylsulfonium bromide in acetonitrile with a lead cathode H8,ii9)j and electrolyses of aqueous solutions of sodium tetra-alkylboron compounds 245,337) Altogether, the breadth and depth of the research effort that has been devoted to the various electrolytic methods for the synthesis of tetraalkyllead compounds is remarkable. However, the successful development of a commercial process has not been accomplished, and indeed, it is probably less likely today than it was several years ago. 

The first commercial process for sodium production used the electrolysis of sodium hydroxide, but it had the disadvantage that the current efficiency was less than 50%. At present this process is the only way of producing metallic sodium. The cell reaction is... 

Hydrochloric acid is a waste by-product from the chlorination of many organic compounds. In certain cases, it would be desirable to recover the chlorine from HC1 by electrolysis of the acid. Sodium sulfate is also a common by-product of many commercial processes. This material can be converted to caustic soda and sulfuric acid by electrolysis of Na2S0i, as shown in Figure 4. 

The monomer XXVIII is copolymerized with tetralluoroethylene to give a polymer-containing pendant fluorosulfonyl groups that are then hydrolyzed and acid exchanged to produce Nafion (XXIX) The resulting polymer combines the chemical, thermal, and oxidative stability of perfluorinated polymers such as polytetra-fluoroethy lene with the properties ofahighly acidic fluorinated sulfonic acid. Nafion is used in a variety of electrochemical applications such as the synthesis of chlorine and caustic and as the conductive membrane of many modern fuel cells. It has also been used in water electrolysis and as an acid catalyst in many proprietary commercial processes. 

This section discusses these routes in the order shown. Aqueous electrolysis of HCl is a commercial process that has been practiced widely. The anhydrous route is developmental but has the potential for large reductions in operating cost. The incentive for indirect electrolysis is the reduction in cathode voltage when reducing certain metals rather than hydrogen ions. The last-named process depends on the added value for its commercial success. The chief product is a metal, with chlorine as a useful byproduct. Sodium and magnesium are the examples covered in the text. 

Electrometallurgy. A major application of electrochemical principles and techniques occurs in the manufecture of such metals as aluminum and titanium. Plentiful aluminum-containing bauxite ores exist in lai e deposits in several countries, but it yms not until electrochemical techniques were developed in the United States and France at the end of the nineteenth century that the cost of manufecturing this light metal was sufficiently reduced to make it a commercially valuable commodity. This commercial process involved the electrolysis of alumina (aluminum oxide) dissolved in fused cryolite (sodium aluminum fluoride). During the century that followed this process s discovery, many different uses for this lightweight metal ensued, from airplanes to zeppehns. 

It is now established that electrolysis can make a contribution to organic synthesis both in the laboratory and on an industrial scale. The size of the new edition of Baizer[29] (well over 1000 pages) testifies to the number and variety of reactions which have now been reported. However, the chief lesson of recent years has been that commercial processes are dependent on the design of a total system. Thus in addition to finding... 

This sketch shows the basic elements of such an electrolysis. However, in the setup as shown, liquid lithium tends to rise to the surface, where it can catch fire. In the commercial process, the products of the electrolysis are collected in separate containers, and air is not allowed to come in contact with them. 

Potassium and Potassium Compounds The principal source of potassium and potassium compounds is potassium chloride, KCl, which is obtained from underground deposits. Small amounts of this salt are used to prepare the metal. Potassium is prepared by the chemical reduction of potassium chloride rather than by electrolysis of the molten chloride, as in the preparation of sodium. In the commercial process, potassium chloride is melted with sodium metal by heating to 870°C. 

The aggressive, acidic nature of the electrolysis medium and the chemical reactivity of fluorine limit the choice of materials and determine the simple and sealed cell design based on a mild steel tank, with a volume of the order of 1 A number of cell designs are operational, and Table 3.1 provides a broad comparison of several commercial processes. 

Mantell, C. L., Tetra Alkyl Leads by Electrolysis, Commercial Plant, Electro-Organic Chemical Processing, Chemical Process Review No. 14, Noyes Development Corp., Park Ridge, N.J., 1968, pp. 165/70. 

Caustic Soda. Diaphragm cell caustic is commercially purified by the DH process or the ammonia extraction method offered by PPG and OxyTech (see Fig. 38), essentially involving Hquid—Hquid extraction to reduce the salt and sodium chlorate content (86). Thus 50% caustic comes in contact with ammonia in a countercurrent fashion at 60°C and up to 2500 kPa (25 atm) pressure, the Hquid NH absorbing salt, chlorate, carbonate, water, and some caustic. The overflow from the reactor is stripped of NH, which is then concentrated and returned to the extraction process. The product, about 62% NaOH and devoid of impurities, is stripped free of NH, which is concentrated and recirculated. MetaUic impurities can be reduced to low concentrations by electrolysis employing porous cathodes. The caustic is then freed of Fe, Ni, Pb, and Cu ions, which are deposited on the cathode. 

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