Bayer-Hall process

Fig. 9. Energy consumption in the Bayer-Hall process of winning of aluminium as a function of development.

Aluminum. Aluminum [7429-90-5] Al, is produced worldwide by the Bayer-Hall-Heroult process. This process involves the electrolysis of alumina [1344-28-1]y Al203, dissolved in molten cryolite [15096-52-3]y Na3AIF(5 (see Aluminum and aluminum alloys). The electrolytic cells or pots operate... 

Jarrett N., Future Developments in the Bayer-Hall-Heroult Process, A. R. Burkin, ed., Wiley, New York, 1987. 

Derivation From bauxite by Bayer process and subsequent electrolytic reduction by Hall process. There are several processes for obtaining ultrapure aluminum (1) electrolytic (three-layer), (2) zone refining, and (3) chemical refining. Impurities as low as 0.2 ppm are possible. 

The processing of bauxite aluminium ore (hydrated alumina oxides) exemplifies these processes. In the Bayer process hot concentrated sodium hydroxide dissolves alumina which is subsequently separated and crystallized as aluminium hydroxide (hydrometallurgy). This is calcined to anhydrous alumina (pyrometallurgy) before being reduced in the Hall process (electrometallurgy) wherein alumina serves as the electrode from which elemental aluminium is deposited on the cathode. 

Kvande H., Light Metals 1991, E. Rooy, ed., TMS, Warrendale, PA, 1991, p. 421. Jarrett N., Future Developments in the Bayer-Hall-Heroult Process, A. R. Burkin, ed., WUey, New York, 1987. 

In electrometallurgy, electrolysis is used to produce metals from their compounds. For example, the Hall process is an electrometallurgical process crucial to aluminum production. The main source of aluminum is bauxite, AI2O3 n H2O. A hydrometallurgi-cal process, the Bayer process, separates bauxite from the irou aud silicon dioxide with which it is usually found. In this process, the bauxite is heated in a concentrated aqueous NaOH solution under high pressure. The aluminum oxide dissolves, leaving the other oxides behind in solid form. 

The first production of aluminum was by the chemical reduction of aluminum chloride with sodium. The electrolytic process, based on the fused salt electrolysis of alumina dissolved in cryolite, was independently developed in 1886 by C. M. Hall in America and P. L. Heroult in France. Soon afterwards a chemical process for producing pure alumina from bauxite, the commercial source of aluminum, was developed by Bayer and this led to the commercial production of aluminum by a combination of the Bayer and the Hall-Heroult processes. At present this is the main method which supplies all the world s needs in primary aluminum. However, a few other processes also have been developed for the production of the metal. On account of problems still waiting to be solved none of these alternative methods has seen commercial exploitation. 

Hall-Heroult An electrolytic process for making aluminum metal from alumina, invented in 1886 independently by C. M. Hall in the United States, and P. L. Heroult in France. The alumina, made by the Bayer process, is dissolved infused cryolite, Na3AlF6, and electrolyzed at approximately 1,000°C. Because of the large requirement for electricity, the process is operated only where hydroelectric power is available. The cryolite was originally obtained from a deposit in Greenland but is now made synthetically from alumina, hydrofluoric acid, and sodium hydroxide ... 

A few years later, an Austrian chemist, Karl Joseph Bayer, refined Hall s process, and it is now called the HaU-Heroult or Bayer process, which is the method used today for obtaining aluminum at very reasonable prices... 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

The principal method used in producing aluminum metal involves three major steps refining of bauxite by the Bayer process to produce alumina, electrolytic reduction of alumina by the Hall-Heroult process to produce aluminum, and casting of aluminum into ingots (Browning 1969 Dinman 1983 IARC 1984). 

Aluminum 32.9 9.6 Bayer process and Hall cell 50% bauxite ore... 

Good descriptions of the production of aluminum can be found in the literature (Grjotheim etal. [7], Grjotheim and Welch [8], Grjotheim and Kvande [9], Burkin [10], and Peterson and Miller [11]). Referring to Fig. 2 [12], the first step in the production of aluminum from its ore ( bauxite ) is the selective leaching of the aluminum content (present as oxides/hy dr oxides of aluminum) into hot concentrated NaOH solution to form sodium aluminate in solution. After solution purification, very pure aluminum hydroxide is precipitated from the cooled, diluted solution by addition of seed particles to nucleate the precipitation. After solid-liquid separation the alumina is dried and calcined. These operations are the heart of the Bayer process and the alumina produced is shipped to a smelter where the alumina, dissolved in a molten salt electrolyte, is electrolyt-ically reduced to liquid aluminum in Hall- Heroult cells. This liquid aluminum,... 

Nearly all A1 is obtained from bauxite, which contains 40- 60% alumina. Approximately 90% is refined to alumina by the Bayer process, after which the alumina is reduced to A1 metal by electrolysis in the Hall-Heroult process. In the year 2000, worldwide A1 production was -24x10 metric tons, while in 2001 the US production was -2.6x10 metric tons (USGS... 

Alternative Processes for Aluminum Production. In spite of its industrial dominance, the Hall-Hfiroult process has several inherent disadvantages. The most serious is the large capital investment requited resulting from the multiplicity of units (250 —1000 cells in a typical plant), the cost of the Bayer alumina-purification plant, and the cost of the carbon—anode plant (or paste plant for Soderberg anodes). Additionally, HaU-Hfiroult cells requite expensive electrical power rather than thermal energy, most producing countries must import alumina or bauxite, and petroleum coke for anodes is in limited supply. 

Ceramic powders SiC tor abrasives Nanosized TiQs tor sunscreen Bioactive glasses tor bone reconstruction Bayer process AI2Q3 tor the production of Al using Hall-H4roult cells... 

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. 

Electrowinning of Aluminum. Aluminum, the most abundant metal in the Earth s crust, did not become readily available commercially until the development of the Hall-Heroult process. This process involves electrolysis of dry aluminum oxide (alumina) dissolved in cryolite (sodium aluminum hexafluoride). Additional calcium fluoride is used to lower the melting point of the cryolite. The process runs at about 960 degrees Celsius and uses carbon electrodes. The alumina for the Hall-Heroult process is obtained from an ore called bauxite, an impure aluminum oxide with varying amounts of compounds such as iron oxide and silica. The preparation of pure alumina follows the Bayer process The alumina is extracted from the bauxite as a solution in sodium hydroxide (caustic soda), reprecipitated by acidification, filtered, and dried. The electrolysis cell has a carbon coating at the bottom that forms a cathode. 

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