Extraction of Aluminium

7 Key excerpts on "Extraction of Aluminium"

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  eBook - PDF

  Handbook of Chemical Technology and Pollution Control

  • Martin B. B. Hocking(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  In addition, to minimize shipping costs it is usual practice to process bauxite, the crude ore, at the mine site to produce purified alumina for shipment to smelters. This means that a further factor for the siting of aluminum smelters is that they provide easy shipping access to permit economical alumina delivery from the mine. Thus, the production of aluminum from bauxite logically can be con-sidered 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. 12.2. ALUMINA FROM BAUXITE: THE BAYER PROCESS Bauxite, the principal ore used for aluminum smelting, is named after Les Baux, Provence, the village where the first deposits were discovered. Australia is now the world's largest producer of bauxite. Bauxite contains hydrated alumina equivalent to as much as 40-60% A1 2 0 3 with much of the other siliceous materials leached out over time. However it still contains 10-30% iron oxide, silica, and other impurities, making it unsuitable for direct elec-trolysis. The first commercial-scale recovery of alumina from bauxite was practiced by Henri Deville, but by 1900 this was largely replaced by the more economical process devised by Bayer in Austria, based on caustic extraction. Alumina recovery from bauxite by extraction with sodium hydroxide, now frequently referred to as the Bayer process, relies on the amphoterism of aluminum for its success (Fig. 12.1). Details of the particular alumina extrac-tion procedure required depend on the particular form of hydrated alumina which occurs in the bauxite being processed. Preliminary to any chemical processing the coarse ore is mechanically re-duced to a finely divided form and stirred with the requisite concentration of 12.2. ALUMINA FROM BAUXITE: THE BAYER PROCESS 359 Bauxite — -Make-up NaOH or—-J Na 2 C0 3 Superheat m steam Wash water Water -p .

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  eBook - ePub

  Handbook of Chemical Technology and Pollution Control

  • Martin B. B. Hocking(Author)
  • 2006(Publication Date)
  • Academic Press

    (Publisher)

  Continued growth in the production of aluminum is unlikely to be hampered by a shortage of mineral since the earth’s crust consists of about 8% aluminum, chiefly as aluminosilicates. Even though aluminum is the most abundant metallic crustal element, bauxite ores suitable for aluminum recovery only occur in more limited areas where natural leaching processes have concentrated the aluminum-containing minerals. Since the free metal is chemically reactive it is never found in nature in this form.

  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.

  TABLE 12.2

  Annual Production of Aluminum by the World’s Major Producers, in Thousands of Metric Tonnes and Percent of Worlda

  1945	1965	1970	1980	1990	2000
  Mass	%	Mass	%	Mass	Mass	%	Mass	Mass	%
  Canada	196	22.1	762	11.3	964	1,068	6.9	1,567	2,373	9.7
  France	37.2	4.2	340	5.1	380	432	2.8	325	441	1.8
  Japan	5.4	6.1	323	4.8	733	1,091	7.1	51	7	<0.1
  Norway	7.0	0.8	277	4.1	530	651	4.2	867	1,026	4.2
  U.S.A.	449	50.6	2,498	37.1	3,607	4,654	30.3	4,121	3,668	15.0
  U.S.S.R.	86.3	9.7	1,279	19.0	1,098	1,787	11.6	–	3,245b	13.3
  W. Germany	20.0	2.3	234c	3.5	308c	731c	4.8	720c	644d	2.6
  Other	37.3	4.2	1,014	15.1	2,046	4,954	32.3	–	13,004	53.3
  World	887.0	100.0	6,727	100.0	69,666	15,368	100.0	21,846	24,400	100.0

  a List includes all countries whose annual primary aluminum production exceeded 400,000 tonnes in 1980. Compiled and calculated from data of Minerals Yearbooks [2 ].

  b Russia after perestroika.

  c Totals for Germany after partition.

  d Germany after reunification.

  12.2 ALUMINA FROM BAUXITE: THE BAYER PROCESS

  Bauxite, the principal ore used for aluminum smelting, is named after Les Baux, Provence, the village where the first deposits were discovered. Bauxite contains hydrated alumina equivalent to as much as 40–60% Al2 O3

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  Industrial Chemistry of Oxides for Emerging Applications

  • Lech Pawlowski, Philippe Blanchart(Authors)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  Figure 3.1 ), currently known reserves will be exhausted in the next 25 years. Finally, for countries such as Jamaica, Guyana, Guinea and Surinam, the production of bauxite contributes more than 10% of GDP [3.5].

  The bauxite ores are generally found close to the earth's surface (see Figure 3.2 ) and the mines are opencast.

  Figure 3.2

  (a), (b) The depth profiles of bauxite deposit in India [3.7] (reproduced with permission of Elsevier).

  3.3 Methods of Obtaining Alumina

  Three methods will be described in this chapter:

  • extraction of alumina from bauxite by the Bayer process
  • the methods of very pure alumina powder synthesis
  • recovery of alumina from coal fly ash.

  3.3.1 Bayer Process

  The Bayer process was invented in 1887 by the Austrian engineer Carl Josef Bayer. Since then, the method has been applied without major changes all around the world.

  3.3.1.1 Chemical Backgrounds

  The principle of the Bayer process is based on dissolution of bauxite in a hot solution of sodium hydroxide at temperatures ranging from 140°C (gibbsite) to 240°C (böhmite). The sodium hydroxide delivers hydroxide ions:

  3.1

  At high temperatures, the sodium hydroxide dissolves solid aluminum oxide contained in the ores, forming liquid sodium aluminate:

  3.2

  This reaction can alternatively be written as an action of hydroxide ions on gibbsite:

  3.3

  Most other components of bauxite do not dissolve and remain solid. An exception is silica, which may dissolve too. Sometimes lime (CaO) is added to precipitate silica as calcium silicate. The dissolution reaction needs fresh aluminum hydroxide, which is delivered from the later part of the alumina production processes. The formed solution is clarified by filtering off solid impurities including Fe2 O3 , SiO2 and TiO2 . The mixture of solid impurities is called red mud and the color is due to the presence of iron oxide. The next step is cooling of the solution of sodium aluminate and precipitation of aluminum hydroxide. The precipitation is encouraged by seeding with high-purity crystals of Al(OH)3

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  Electrochemistry for Technologists

  Electrical Engineering Division

  • G. R. Palin, N. Hiller(Authors)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 7 Extraction and Refining of Metals 7.1 Introduction Electrometallurgy is the application of electricity in the extrac-tion of metals from their ores, and in the refining of metals. In arc furnaces and similar units the electricity is merely used to produce the high temperatures required for the extraction process. These systems will not be considered here. In many cases, however, the process is electrochemical. Metals can be refined by the dissolu-tion of the impure metal at the anode of a suitable electrolytic cell, and the deposition of the pure metal at the cathode. The extraction of metals requires the production of a water soluble compound from which the metal is deposited in an electrolytic cell, or the production of a salt which can be fused to provide the electrolyte of a cell from which the metal is deposited electro-lytically. Electrochemical processes may be the only suitable means of extraction and refining, or they may be alternatives to thermo-chemical and other processes. In a number of extractions the electrochemical process is only one stage in the sequence, and many metals are refined electrochemically after extraction by other means. Only the electrochemical processes will be discussed here. 7.2 Electrolysis of Fused Salts Cells for the electrolysis of fused salts are usually designed so that the applied electrical energy maintains the required tempera-ture in the cell, as well as decomposing the electrolyte. This means 181 182 EXTRACTION AND REFINING OF METALS that voltages several times greater than the decomposition voltage must be used, and energy efficiencies are low. The temperatures used are such that in most cases the liberated metal is in liquid form. The Alkali Metals Metallic lithium and sodium are produced exclusively by electrolysis of fused salts. Potassium metal is also produced in this way, but non-electrolytic processes are also used.

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  eBook - ePub

  Handbook of Aluminum

  Volume 2: Alloy Production and Materials Manufacturing

  • George E. Totten, D. Scott MacKenzie, George E. Totten, D. Scott MacKenzie(Authors)
  • 2003(Publication Date)
  • CRC Press

    (Publisher)

  •  Oxides in general have high melting points. Therefore, for the electrowinning of aluminum from its oxide, a suitable low melting point electrolyte must be found in which the oxide is appreciably soluble.

  •  In the manufacture of aluminum, there are two main stages. The first embraces the production of pure Al2 O3 from bauxite, and the second is the reduction of this Al2 O3 to the metal in a bath of fused cryolite (Fig. 6 ).

  •  In the electrowinning of aluminum from oxide melts, the carbon anodes are quantitatively consumed. •  The production of alumina from bauxite is the largest pressure leaching operation in the world.

  •  Similarly, the production of aluminum by the molten salt electrolysis of alumina in cryolite is the most important industrial application of molten salt electrowinning and is the largest electrolytic industry in the world.

  •  The electrolytic step in aluminum production is the most energy intensive operation (Table 1 ).

  Figure 2  (Top) Paul Héroult (1863–1914) and (bottom) Charles Martin Hall (1863–1914) invented simultaneously and independently the electrolytic process for reduction of Al2 O3 .

  Figure 3  Karl Josef Bayer (1871–1908) invented the process for producing Al2 O3 from bauxite.

  •  Material handling is a major cost factor in aluminum production. For each ton of aluminum produced, more than 3 tons of materials are transported to and from potlines. •  The electrolytic process produces aluminum with purity as great as 99.5–99.8%. Higher purity (99.99%) is obtained by an electrolytic refining process.

  3    RAW MATERIALS

  The production of aluminum by fused salt electrolysis takes place in an electrolytic cell which consists of a carbon anode, a molten cryolite–alumina electrolyte, a pool of liquid aluminum, and a carbon-lined container to hold the metal and electrolyte. Fresh alumina is added from time to time and the metal formed is siphoned out periodically. The reactions taking place can be simplified as follows:

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  Comparative Inorganic Chemistry

  • Bernard Moody(Author)
  • 2013(Publication Date)
  • Arnold

    (Publisher)

  With a sufficiently elevated temperature, all but calcium and alu-minium should be reduced by carbon but various other factors make the extractions difficult. The equilibrium mixture must be cooled rapidly to The application of electrolysis in extraction processes 137 Table 8.4 Affinities of oxide formation compared E.C.S. order 0°C 500°C 1000°C 1500°C 2000°C 0 2 + 2Ca 2CaO Ca Ca 0 2 + 2Mg 2Mg0 Mg Na 0 2 + |AI §AI 2 0 3 AI Mg 0 2 + 4Na■ 2Na 2 0 Mn AI 0 2 + 2Mn ■ 2MnO Na Mn 0 2 + | C r · §Cr 2 0 3 Cr Zn O 0 2 + 2Zn 2Zn0 Zn CO E -Fe 0 2 + 2C0 2C0 2 co-■S-s Cr 0 2 + Sn ■ Sn0 2 Fe Ni 0 2 + Fe ■ 2FeO H 2 Sn 0 2 + 2H 2 ■ 2H 2 0 ( C -Pb 0 2 + 2Ni ■ 2Ni0 [C-> H C 0) (O (0 2 + C · C0 2 ) Ni Hg 0 2 + 2Pb 2PbO Sn* Ag [0 2 + 2C ■ 2C0] Pb 0 2 + 2Hg ■ 2HgO dec 0 2 + 4Ag ■ 2Ag 2 0 dec φ tO ^ (to the corresponding -σ ΡΈ5 oxidation states) *Data incomplete over whole temperature range avoid re-oxidation. Carbide formation may occur and there is the problem of finding suitable furnace linings. Aluminium may be used in the thermite process for the production of a number of metals. For sulphides and chlorides the direct recovery of metals may also be studied from the thermo-dynamic standpoint. Oxidation as a means of isolating some elements The isolation of metals involves reduction. There are far fewer non-metallic elements and their extraction methods vary widely. Some require reduction, some physical separation and others, oxidation. Molten sulphur is pumped up from the depths of the earth. Oxygen, nitrogen and inert gases are separated from the air, except for helium which is obtained from natural gas. Phosphorus is produced by reduction of the pentoxide, displaced from calcium phosphate by silica, using carbon in the electric furnace.

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  Alumina Ceramics

  Biomedical and Clinical Applications

  • Andrew J. Ruys, Andrew John Ruys(Authors)
  • 2018(Publication Date)
  • Woodhead Publishing

    (Publisher)

  This is of course the Bayer process which has dominated global bauxite refining for over a century, because of its low cost, efficiency, and the knowledge benefits of over a century of fine tuning the process. It has already been discussed in depth.

  3.8.2 Wet acid processing

  This process has been trialed in the 20th century using acid to extract alumina from clays [94 –96 ]. A couple of pilot plants were built in the United States, which has very limited bauxite reserves (see Table 3.1 ), one in 1945 and one in 1963. The process did not prove to be cost effective due to equipment corrosion, a limited capacity for recycling the acid, and the high pick up of iron from iron oxide dissolution. On the plus side, there was little silica dissolution.

  Table 3.1

  Alternative bauxite refining processes and associated energy cost [45 ]

  Refinery name	Production method	Ore type	Energy intensity GJ/t Al2 O3
  Zhengzhou	Combined	Diaspore	34.15
  Shanxi	Combined	Diaspore	37.28
  Guizhou	Combined	Diaspore	43.31
  Zhongzhou	Combined	Diaspore	52.17
  Pinguo	Bayer	Gibbsite/Diaspore	15.1
  Shandong	Sinter	Diaspore	40.5
  (Un-named) France	Bayer	Diaspore	13.52
  Pinjarra (Australia)	Bayer	Gibbsite	11.21
  Shennigola (Greece)	Bayer	Diaspore	14.86
  Stratford (Germany)	Bayer	Gibbsite	9.6

  3.8.3 Carbothermic furnace processing

  This process was first patented a century ago as a means of making alumina abrasives [97 ,98 ]. The technology never gained significant traction over the subsequent decades.

  3.8.4 Alkaline sinter processing

  The original bauxite refining method invented by Le Chatelier in 1855 was a soda sinter process. Bayer's innovation of 1887–92 revolutionized bauxite refining because of his low-temperature (low-energy) NaOH digestion process. However, for bauxite ores containing 8%–15% silica, conventional Bayer digestion cannot be used as the sodium consumption by the reactive silica makes the process commercially unviable.

  In China, and Russia/Eastern Europe, there is an abundance of bauxite ores that are either high in silica, high in MHA boehmite/diaspore, or both. Therefore, many refineries are compelled to use either a pure soda/lime sinter process for refining or a combined process involving a Bayer digestion followed by a soda sinter of the red mud [99 ]. Sintering is a much higher energy consumption process as shown in Table 3.1 [45

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