Hall-Herault process

Hall process (Hall-Herault process) The production of aluminum by the electrolysis of aluminum oxide dissolved in molten cryolite, haloalkane An alkane with a halogen substituent. 

Grjotheim, K., Krohn, C., Malinovsky, M., Matiaskovsky, K., and Thonstad, J., Aluminium Electrolysis—Fundamentals of the Hall-Herault process, CRC Press, Boca Raton, FL, 1982, 17. Palmear, I. J., in The Chemistry of Aluminium, Gallium, Indium, and Thallium, Downs, A. J., Ed., Blackie, London, 1993, 87. 

Invented by H. Y. Castner and operated by the Aluminium Company at Oldbury, England in 1888, in order to supply sodium for the manufacture of aluminum. It was abandoned soon afterward when the Hall-Herault process for aluminum was developed. 

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... 

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). 

The production of aluminiumis based on the electrolysis of molten alumina AI2O3 using the Hall-Herault process. Alumina is extracted in the Bayer process form the bauxite ore which contains 20 to 30 wt% Al. In 2002 the main producers of the ore are Australia (28%), Guinea (20%), Brazil (14%), Jamaica (7%), India (4%), and Guyana (3%) [1.9]. 

Carbon anodes are necessary to undertake the Hall-Herault process with prebaked anodes (Fig. 4.1). 

Aluminum is present in most rocks and is the most abundant element in the earth s crust (eight percent by weight.) However, its isolation is very difficult and expensive to accomplish by purely chemical means, as evidenced by the high E° (-1.66 v) of the A13+/A1 couple. For the same reason, aluminum cannot be isolated by electrolysis of aqueous solutions of its compounds, since the water would be electrolyzed preferentially. And if you have ever tried to melt a rock, you will appreciate the difficulty of electrolyzing a molten aluminum ore Aluminum was in fact considered an exotic and costly metal until 1886, when Charles Hall (U.S.A) and Paul Herault (France) independently developed a practical electrolytic reduction process. 

The classical process for winning aluminum is one developed by Hall and by Herault in 1886 and 1888. Aluminum oxide (AI2O3) of high purity is mixed with cryolite (NajAlFg) in the ratio of 1 10. The mixture melts at approximately 950 °C. The preparation of the pure aluminum oxide, which is usually from bauxite, is the first step in winning aluminum. This process was developed by K.J. Bayer in 1892. The bauxite is treated with concentrated sodium hydroxide and reacted with the aluminum minerals to get sodium aluminate. After aluminum hydroxide precipitates from the aluminate solution, it is then calcinated at 1200-1300 °C. 

Although mineralogically abundant, aluminum was quite expensive when it had to be produced by purely chemical means. Nowadays electrolytically-produced aluminum allows the container for a "TV dinner" to be treated as a throwaway item. The almost simultaneous but independent reporting of the cryolite-alumina electrolytic process in 1886 by C.M. Hall in the U.S.A. and by P. Herault in France is an amazing coincidence. An excellent account of the events that led Hall to his discovery is available (16). 

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