Electrolysis aluminum

Nonmineralized SGA flows freely, and is often known as sandy alumina because it easily covers the cryoflte bath of aluminum electrolysis cells (see Aluminum compounds, introduction). Properties typical of a sandy SGA are shown in Table 1. Aluminum smelting technology in the United States is primarily based upon sandy alumina. Older European smelting technology, however, is based upon a poor flowing, low bulk density, highly mineralized SGA called floury alumina, composed principally of a-Al O. ... 

K. Grjotheim, C. Krohn, M. Malinovsky and J. Thonstad, Aluminum Electrolysis The Chemistry of the Hall-Heroult Process, Aluminum-Verlag GmbH, Dtisseldorf, West Germany, 1977. 

Removal of fluorine from offgases of aluminum electrolysis... 

Grjotheim, K., Krohn, C., Malinovsky, M., Matiasovsky, K., and Thonstad, J. (1982) Aluminum Electrolysis, 2nd edn, Aluminium-Verlag, Dusseldorf. 

Probably the most common solid electrode is platinum, although it dissolves anodically in some melts, for example in halides. The choice of gold and silver [86] is also frequently made. Graphite is very often used because it is cheap and can be obtained in a wide range of sizes and qualities. These electrodes can be used over long periods of time, and they have a wide electrochemical stability, both anodic and cathodic. Vanadium and molybdenum are also used in appropriate systems. Studies for the use of some inert anodes made of semiconducting ceramics have been made, especially for aluminum electrolysis [87],... 

Another type of reaction is the oxidation of complex ions, for example in the case of aluminum electrolysis. These reactions are rather complicated and occur in several steps. During the first step, the discharge of oxygen ions takes place the oxygen atoms formed are adsorbed on the surface of the carbon anode and molecules of C02 are then obtained. These molecules of C02 can react with the anodic carbon, and a certain proportion of CO may appear. All these gases form bubbles which escape. Usually, the anodic processes have a high overvoltage. 

For this reason the consumable anodes must be replaced periodically. The cathode consists of a molten aluminum layer on the bottom of the cell, and the anode-cathode distance is 4-5 cm. Alumina is periodically added to the cell in the proportion that it is consumed by electrolysis. The electrode processes during aluminum electrolysis are very complex [141] and a proper understanding of these processes is important because of the economic implications energy and carbon consumption, cell control, pollution of the environment, etc. 

According to the preceding treatment, aluminum electrolysis takes place with an anodic overvoltage of about 0.5-0.6 V. The difference between the polarization potential A and the reversible anode potential is the anodic overvoltage ... 

The principal reason for the loss in current efficiency (CE) in aluminum electrolysis is the metal reoxidation by the anode gas, according to reaction (87). 

The theoretical energy requirement for the electrochemical reaction in aluminum electrolysis is 6.34 kWh/kg Al (at a current efficiency of 100%). The rest of the energy output is in the form of heat loss from the cell body it varies largely from 5.5 to 9.0 kWh/kg Al depending on the type of cell. 

Table 11 Cell Reactions in Aluminum Electrolysis When Using Inert and Carbon Anodes and the Corresponding EMF and Anodic Overvoltage (T ) Data at 1000°C...

Figure 22 Bipolar cell for aluminum electrolysis due to Alusuisse [236], The vertical bipolar electrodes consist of 3, an anode layer, a ceramic oxide 4, an intermediate conducting layer 5, a cathode layer, e.g., TiB2. 

Refs. [i] Grjotheim K, Krohn C, Malinovsky M, Matiaskovsky K, Thon-stad J (1982) Aluminum electrolysis - Fundamentals of the Hall-Herault process. CRC Press, Boca Raton [ii] Rao DB (1979) NASA SP-428 257-288 [Hi] Palmear IJ (1993) In Downs AJ (ed) The chemistry of aluminium, gallium, indium, and thallium. Kluwer Academic, London... 

Refs. [1] Thonstad J, Fellner P (2001) Aluminum electrolysis. Fundamental of the Hall-Heroult process, 3rd edn. Aluminum, Dilsseldorf [ii] Keller R, Rolseth S, Fhonstad J (1997) Electrochim Acta 42 1809... 

There have been sporadic attempts to produce aluminum by carbothermic reduction [3, 4]. In this approach, akin to the way iron oxides are reduced to iron in the iron blast furnace, the consumption of electrical energy is avoided or at least reduced. There have also been investigations of the production of aluminum by electrolysis of aluminum compounds other than the oxide (e.g. [5]). Some of these alternative electrolytic technologies have even reached a commercial scale [6] but the only method for aluminum production in industrial use today appears to be electrolysis in Hall-Heroult cells. Consequently, the present paper is confined to these cells. The literature on these cells is large. A recent search of the web of science with the subject Hall cell and similar subjects revealed 79 titles aluminum electrolysis yielded 109 publications. This number excludes papers published in the annual Light Metals volume of the Minerals Metals and Materials Society (TMS). Light Metals contains approximately forty papers each year on Hall cells. Consequently, the authors have made no attempt at a comprehensive examination of the literature on these topics. Rather we have included... 

R. F. Boivin, A Volume-Integral Method for Calculating the Magnetic Field in Aluminum Electrolysis Cells, Report No. R95-50, CERCA, Montreal, April 11, 1995. 

More information on cryolite-based melts can be found in specialized books by Grjotheim et al. (1982) and Thonstad et al. (2001) devoted to the fundamentals of aluminum electrolysis. 

Titanium diboride exhibits a high melting point, electronic conductivity, wetability by molten aluminum, and a resistance towards chemical attack of aluminum and molten fluorides. Due to these properties, TiB2 is considered to be the most promising material for inert cathodes in aluminum electrolysis. Also zirconium diboride belongs to the category of promising constructive materials due to its favorable properties. 

The current efficiency in modern cells of aluminum electrolysis may exceed 95%. It is generally accepted that the major part of loss in current efficiency is due to the reaction between dissolved metal and electrolyte. Model studies by 0degard et al. (1988) indicates that sodium dissolves in the electrolyte in the form of free Na, while dissolved Al is predominantly present as the monovalent species ALF. Any electronic conductivity is most likely associated with the Na species, which may form trapped electrons and electrons in the conduction band. Morris (1975) ascribed the loss in current efficiency during Al production to electronic conduction. In a theoretical and experimental study. Dewing and Yoshida (1976) subsequently maintained that the electronic conductivity was too low to account for the loss in current efficiency in industrial aluminum cells. However, the existence of electronic conduction in NaF-AlF3 melts was demonstrated later by Borisoglebskii et al. (1978) also. 

One of the most important technological parameters in molten salt chemistry is surface tension, as the majority of important reactions take place at the interface of electrolytes or molten reacting media. In aluminum electrolysis, for instance, this parameter influences the penetration of the electrolyte into the carbon lining, the separation of carbon particles from the electrolyte, the coalescence of aluminum droplets and fog in the electrolyte, the dissolution rate of aluminum oxide in the electrolyte, etc. Similar is the effect of interface in aluminum recovery. 

In many cases there are only three to four main components which define the physico-chemical properties of an industrial electrolyte. Minor components play only a marginal role. e.g. in the aluminum electrolysis, the main components of the electrolyte are cryolite, NasAlFe, aluminum fluoride, and aluminum oxide. The other components, like CaP2 and Mgp2, are present in low concentrations only and do not affect substantially the properties of the electrolyte. It is thus often sufficient to investigate only the system composed of these main components. 

K. Gijotherm, C. Krohn, M. Malinovsky, K. Matiasorvsky, and 1. Thonstadt. Aluminum Electrolysis. 2nd ed. Dnsseldorf Verlag, 1982. 

Lithium is a fascinating example of an element, that was originally considered a chemical laboratory curiosity, but finally found to be an ultratrace element which in all probability is essential to humans. Moreover, it became a potent and safe drug, with specific effects mainly in the treatment of manic-depressive illness, and also a valuable versatile industrial material with a well-established broad spectrum of applications and possibilities for further developments. The importance of lithium will increase, for example by the discovery of lithium-dependent enzymes, proteins or hormones, the resolution of its biochemical mechanism in affective disorders, and progress in the battery sector, in the nuclear technology, or with the aluminum electrolysis (Schafer 1995, 2000). 

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