Bauxite Hall-Heroult process

Many metals are extracted from their compounds, as found in ores, by electrolytic processes. By far the most important is the Hall-Heroult process, invented in 1886, for producing aluminium from alumina, itself refined from bauxite ore. Alumina is dissolved in molten cryolite, Na3Alp6, and electrolysed, using carbon anodes and the aluminium itself as cathode. While various details are being steadily improved, the basic process is still the same today. 

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. 

Cowles An electrothermal process for making aluminum alloys. A mixture of bauxite, charcoal, and the metal forming the alloy (usually copper), was heated in an electric furnace and the molten alloy tapped from the base. The process cannot be used for making aluminum alone because in the absence of the other metal the product would be aluminum carbide. Invented by the Cowles brothers and operated in Cleveland, OH in 1884 and later in Stoke-on-Trent, England. The electrical efficiency was poor and the process was superseded by the Hall-Heroult process. 

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). 

The industrial manufacture of aluminum is ba.sed on the Hall-Heroult process developed in 1886. In this process aluminum oxide (see Section 3.2.4.2 for the production of aluminum oxide from bauxite) is dissolved in a cryolite (Na3AlF5) melt and electrolyzed at 940 to 980°C with direct current. Molten metallic aluminum is deposited at the carbon electrode (cladding of the bottom) and taken off as a liquid. Oxygen is formed at the anode, also of carbon (presintered or Soederberg-electrode), with which it reacts forming carbon dioxide and carbon monoxide. 

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. 

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... 

In the modern version of the Hall-Heroult process, aluminum metal is obtained by electrolysis of aluminum oxide, which is refined from bauxite ore (AI2O3 2H2O). The aluminum oxide is dissolved at 1000°C in molten synthetic cryolite (NaaAlFe), another aluminum compound. The cell is lined with graphite, which forms the cathode for the reaction, as shown in Figure 20.22. Another set of graphite rods is immersed in the molten solution as an anode. The following half-reaction occurs at the cathode. 

From the time of its discovery in 1825 until near the turn of the century, aluminum was made hy reducing AlCl with a more active metal (potassium or sodium) hut only at a very high cost. Even though there was commercial production of aluminum hy 1854, aluminum was considered to be a precious metal—as gold and platinum are today— and one of its early uses was for jewelry. Napoleon 111 saw the possibilities of aluminmn for mihtary use, however, and commissioned studies on improving its production. The French had a ready source of aluminum-containing ore, bauxite, named for the French town of Les Baux. In 1886, a 23-year-old Frenchman, Paul Heroult, conceived the electrochemical method that is still in use today. In an interesting coincidence, an American, Charles Hall, who was 22 at the time, announced his invention of the identical process in the same year. Hence, the commercial process is now known as the Hall—Heroult process. 

The equation for the Hall-Heroult process to convert bauxite to pure A1 is... 

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. 

Aluminum is the most abundant metal in Earth s crust. It is a relatively reactive metal therefore, in nature, it is found as its oxide in an ore called bauxite. Aluminum is now very useful commercially, but it was not imtil 1886 that a process to obtain pure aluminum metal was discovered. Charles M. Hall (from the United States) and Paul Heroult (from France) simultaneously, but independently, determined a practical method for producing aluminum it is an electrolytic process called the Hall-Heroult process. 

Aluminum is the third most abundant element in the Earth s crust. The aluminum industry is huge because aluminum metal has so many uses in consumer products. Examples are aluminum foil and beverage cans. It is manufactured by the Hall-Heroult process by electroplating it from a molten mixture of two aluminum ores, bauxite, and cryolite. 

This process, which involves electrolytic reduction of AlaOa dissolved in molten cryolite (3NaF. AlF3), has been improved in later years and is known now as the Hall-Heroult method. Detailed descriptions of this and of some other current methodsof prepn are given Refs 5, 11,12,15, 2Q, As the source of Al Oj, the abundant mineral bauxite, AlaO,r 2HaO, is usually used... 

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,... 

With the rapid acceptance of the Hall-Heroult electrolytic method of aluminum production, facilities using this process have tended to be constructed in areas with abundant, low-cost electric power (Table 12.2). In addition, to minimize shipping costs it is usual to process bauxite ore to purified alumina at the mine site for shipment to smelters (Table 12.2). Thus, aluminum smelters are sited so as to provide easy shipping access for economical alumina delivery from the mine. The production of aluminum from bauxite logically can be considered in two steps the first, production of high-purity alumina from the working of natural bauxite deposits, and the second, electrolytic reduction of alumina to the metal. 

The metal is produced on a massive scale by the Hall-Heroult method in which aluminum oxide, a nonelectrolyte, is dissolved in molten cryolite and electrolyzed in a large cell. The bauxite contains iron oxide and other impurities, which would contaminate the product, so the bauxite is dissolved in hot alkali, the impurities are removed by filtration, and the pure aluminum oxide then precipitated by acidification. In the cell, molten aluminum is tapped off from the base and carbon dioxide evolved at the graphite anodes, which are consumed in the process. The aluminum atom is much bigger than boron (the first member of group 13) and its ionization potential is not particularly high. Consequently aluminum forms positive AT ions. However, aluminum also has nonmetallic chemical properties. Thus, it is amphoteric and also forms a number of covalently bonded compounds. 

Hall-Heroult electrolytic process (Charles Martin Hall and Paul Heroult) The industrial production of aluminum from bauxite ore made aluminum widely available. Prior to the electrolytic process, aluminum was a precious metal with a value about equal to that of silver. 

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