Aluminum electrolysis cell

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. ... 

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. 

Fig. 1 Modem piebaked aluminum electrolysis cells at Sunndal, Nwway...

Fig. 3 Schematic cross section of a prebaked aluminum electrolysis cell...

Brisson PI, Soucy G, Fafard M, Dionne M. The effect of sodium on the carbon lining of the aluminum electrolysis cell - A review. Can Metall Q. 2005 44(2) 265-80. 

To withstand the highly corrosive conditions in aluminum electrolysis cell, the inert anode materials must have excellent oxidation resistance. Among the materials under consideration, Ni-Fe-based alloys and their composite materials have shown a promise for industry application that has received great attention Unfortunately, Ni-Fe alloys under oxidizing conditions develop layered corrosion scales containing Fe O and/or other metal oxides, which may either provide some protection to further oxidation or result in more corrosion/erosion on the alloy anodes during aluminum electrolysis... 

Qingyu Li et al, Laboratory Test and Industrial Application of an Ambient Temperature Cured TiB2 Cathode Coating for Aluminum Electrolysis Cells, Light Metals, (2004), 327-331. 

The annual production of aluminum in the United States in 2000 was 3.6 Mt (1 Mt = 1.0 X 109 kg). What mass of carbon, lost from the anode of the electrolysis cell, was required to produce this amount of aluminum by the Hall process ... 

Figure 21-4 shows a schematic representation of an electrolysis cell for aluminum production. An external electrical potential drives electrons into a graphite cathode, where Al ions are reduced to A1 metal ... 

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. 

The question of which of the two metals, aluminum and sodium, is more noble in the electrolyte can be discussed in terms of reaction (86). At the temperature of electrolysis cell, the equilibrium reaction (86) is displaced to the left except... 

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. 

In Chapter 8, we found that certain decomposition reactions were carried out by use of electricity. The production of a chemical reaction by means of electric current is called electrolysis. The very most reactive metals and fluorine are usually produced in their elemental forms in electrolysis cells. The production of aluminum is a prime example. 

Electrolysis is used in a wide variety of ways. Three examples follow (1) Electrolysis cells are used to produce very active elements in their elemental form. The aluminum industry is based on the electrolytic reduction of aluminum oxide, for example. (2) Electrolysis may be used to electroplate objects. A thin layer of metal, such as silver, can be deposited on other metals, such as steel, by electrodeposition (Eig. 14-2). (3) Electrolysis is also used to purify metals, such as copper. Copper is thus made suitable to conduct electricity. The anode is made out of the impure material the cathode is made from a thin piece of pure copper. Under carefully controlled conditions, copper goes into solution at the anode, but less active metals, notably silver and gold, fall to the bottom of the container. The copper ion deposits on the cathode, but more active metals stay in solution. Thus very pure copper is produced. The pure copper turns out to be less expensive than the impure copper, which is not too surprising when you think about it. (Which would you expect to be more expensive, pure copper or a copper-silver-gold mixture )... 

As a consequence, electrolysis of solutions containing aluminum alkyl must be conducted in the absence of oxygen and water in a sealed apparatus under an atmosphere of inert gas. For technical electrolysis cells, nitrogen is generally used, due to its lower cost. The charging and removal of electrodes must be conducted via a lock system, which prevents the admission of air and moisture. Furthermore, all pieces which are to be brought into the electrolytic cell must be completely dried. 

At about the same time, electrolytes containing aluminum alkyls were utilized by YAW (Bonn) for the production of ultra pure aluminum [142]. Fig. 18 shows the closed electrolysis cell used which already contains several hundred liters of bath. 

An electrolysis cell with a content of 80 L for eight mounting frames (140 X 260 mm) was developed by Siemens (Erlangen) as a pilot plant for aluminum electrodeposition [65]. 

After coating, the rack is lifted out of the cell, while the now aluminum-plated pieces are rinsed with solvent of the electrolyte system (4). Via the liquid floodgate to the right (6), the carriers with the coated pieces leave the electrolysis cell. 

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... 

Alumina electrolysis cells consist of a rectangular steel shell lined with a 25-35 cm layer of baked and rammed dense carbon, which provides both chemical resistance and the cathode contact with the electrolyte via steel bus bars imbedded in the carbon. Normal lining life is 4—6 years, after which it is replaced as large preformed slabs. Once a reduction pot has been started the bulk of the cathode current to the carbon lining is via the pool of newly formed molten aluminum in the bottom of the cell (Fig. 12.2). 

Figure 22-1 (a) Schematic drawing of a cell for producing aluminum by electrolysis of a melt of AI2O3 in Na3[AlFg]. The molten aluminum collects in the container, which acts as the cathode, (b) Casting molten aluminum. Electrolytic cells used in the Hall-Heroult process appear in the background. 

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. 

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