Electrolysis aluminum production

Fluorides are used in many materials processes, and can poison the environment when they are discarded. Examples are ciyolite (sodium aluminofluo-ride, Na,AlF ) used to dissolve aluminum oxides for electrolysis, and hydrofluoric acid (HE) used in etching lamp bulbs and semi-conducting circuits. Today lamp bulbs are etched much less than they used to be to reduce fluoride disposal not much has been done to reduce the amount of cryolite for aluminum production. 

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 ... 

Electrolytic aluminum production is the most important process in both volume and significance. World production is about 15 megatons per year, consuming about 240 billion kilowatthours of electrical energy. Aluminum oxide (alumina), AI2O3, is subjected to electrolysis at a temperature of 950°C to this end it is dissolved in molten cryolite NujAlFg, with which it forms a eutectic melting at about 940°C. Carbon anodes that are anodically oxidized to CO2 in the process are employed. The overall electrolysis reaction can be written as... 

Q <33 Research the extraction of aluminum by the electrolysis of bauxite. Write a report on your findings. Include a description of the electrolytic cell and how it operates. Indicate where aluminum is produced in Canada. Also include any environmental concerns associated with aluminum production by electrolysis. 

The chlor-alkali process,34 in which seawater is electrolyzed to produce Cl2 and NaOH, is the second most important commercial electrolysis, behind production of aluminum. 

The study of molten haloaluminates was initiated because these melts are interesting from a scientific point of view. Practically, these melts are used for aluminum electroplating, as electrolytes in low temperatures batteries (see below) and, likewise, aluminum production by electrolysis of these melts has been tried (see Section II.A.7 Aluminum Chloride Electrolysis ). 

Anode effect — refers to the sudden increase in voltage and decrease in current that happens when a gas film forms on the anode during electrolysis in melted salts. It is of special importance in -> aluminum production (-> Hall-Herault process). 

See also -> aluminum production, -> Baizer-Mon-santo process, -> chlor-alkali production, -> electrochemical cell, fluidized bed electrodes, -> H-cell, -> Swiss-role cell, -> three-dimensional electrodes, -> two-phase electrolysis. 

Dec. 6, 1863, Thompson, Ohio, USA - Dec. 27, 1914, Daytona Beach, Florida, USA) Hall constructed his own chemistry laboratory in his parents home after he graduated from Oberlin College, Ohio in 1885. In 1886, Hall -and independently Heroult in France - discovered an effective method for the production of aluminum based on the -> electrolysis of dissolved aluminum in molten cryolite. This method is called Hall-Heroult process (see also -> aluminum production). 

Hall - he elaborated the effective technology of production of aluminum by electrolysis (see - aluminum production, - Hall-Heroult process). 

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... 

The use of large amounts of electrical energy in electrolysis makes production of aluminum from ores an expensive metallurgy. Methods for recycling used A1 use less than 10% of the energy required to make new metal from bauxite by the Hall-Heroult process. 

Aluminum production Until the late nineteenth century, aluminum metal was more precious than gold because no one knew how to purify it in large quantities. In 1886, 22-year-old Charles Martin Hall (1863-1914) developed a process to produce aluminum by electrolysis. He used heat from a blacksmith forge, electricity from homemade batteries, and his mother s iron skillets as electrodes. At almost the same time, one of Le Chateliers students, Paul L. T. Heroult (1863-1914), also 22 years old, discovered the same process. Today, it is called the Hall-Heroult process and is illustrated in Figure 20.22. 

Electrolytic gas evolution is a significant and complicated phenomenon in most electrochemical processes and devices. In the Hall process for aluminum production, for example, bubbles evolved on the downward-facing carbon anodes stir the bath and resist the current, both of which directly affect the heat balance and the cell voltage. Bubbles appear as a result of primary electrode reactions in chlorine and water electrolysis, and as the result of side reactions in the charging of lead-acid batteries and some metal electrowinning. Stirring of the electrolyte by gas evolution is an important phenomenon in chlorate production. Electrolytically evolved bubbles have also been used in mineral flotation. Relatively few major electrochemical processes do not evolve gas. 

Fluorine Hydrofluoric acid (HF) is a by-product of aluminum production by electrolysis cind the synthesis of chlorofluorocarbons (CFCs). Although in water HF is a weak acid, it s extremely toxic. HF poisoning causes deep ulceration or scarring of body tissue with a delayed effect, slowly replacing calcium in bodily tissue (flesh and bones). There s no pain at first because it s a weak acid, but when the calcium in the bones starts to be replaced by the more electronegative fluorine it feels similar to growing pains, and may be too late. Merely five-minute iexposure to 10,000 ppm concentration in the air is fatal to humans. 

FIGURE 10.8 Aluminum production by electrolysis. At the cathode, aluminum ions are reduced to aluminum metal. The anode reaction is production of oxygen gas, which reacts with the carbon anodes. (The cell reaction is 2 AI Oj + 3 C... 

Insufficient pre-orientation in the mesophase stage, as a result of low aromaticity of the respective residue, or through too rapid a heating to temperatures in excess of 500 °C, leads to isotropic cokes, which are principally used for the manufacture of anodes for aluminum production by electrolysis. 

The different cell technologies for aluminum production depend on the nature of the carbon anodes and the current load. However, the electrolyte composition and the operation of the electrolysis is very similar for all technologies. Information about innovations and performance data related to aluminum electrolysis has traditionally been very open. Figure 3 shows a schematic drawing of a modem prebaked cell. 

Electrolysis in Molten Cryolite - Basis for Modern Aluminum Production 3 7.2.4.1 Time for Change... 

Electrolysis has great economic impact. Applications of electrolytic cells are electroplating of metallic surfaces, rechargeable batteries, aluminum production, and purification of metals. 

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