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Aluminas

Discussion Questions

  • What are natural occurring aluminum oxides?
  • How is bauxite mined and processed?
  • What are some of the applications of aluminum oxides?
  • How does aluminum oxide protect aluminum from corrosion?

Aluminas

Aside from silicates, aluminas are the most abundant mineral of the earth crust. Thus, it is important for chemical engineers to know some chemistry about aluminas, because they are found and used in so many different places and technologies. Furthermore, aluminum ions often replace silicon ions in silicates forming aluminosilicates, which is discussed in the next page.

From the discussion on this page, you will be introduced to various forms of alumina, their structures, and properties so that when you encounter them, you can associate their properties with their chemical identities (compositions) and structures.

What are natural occurring aluminum oxides?

The most common ore is bauxite, which is aluminum oxide, Al2O3, mixed with oxides of silicon, iron, and other elements and varying small percentages of clay and other silicates. Physically, bauxite can be as hard as rock or as soft as mud, and its color may be red, white, buff, pink, yellow or any combination of these. The picture shows the mining of bauxite at Gove in Australia Bauxite is the product of extreme chemical weathering of aluminum-rich rocks. Australia produces the largest amount of alumina because she has a large body of bauxite. Jamica, South Africa and some other countries also have a good reserve.

 

Figure 1: One of the world's largest bauxite mines in Weipa, Australia.  Image used with permission from Wikipedia.

Aluminum oxides often coexist with silicates. Natural aluminum oxide minerals include

  • Corundum, Al2O3
  • Spinel, MgAl2O4
  • Hercynite, FeAl2O4
  • Galaxite, MnAl2O4
  • Gibbsite, Al(OH)3
  • Diaspore, AlO(OH)
  • Boehmite, AlO(OH)

Bauxites are mainly used for producing pure alumina, which is the feed stock for aluminum metal production, raw material for ceramics, and other applications.

Corundum

There are a few forms of aluminum oxide, and corundum being the most common. The structure of corundum can be viewed as a hexagonal close packed array of oxygen atoms with 2/3 of the octahedral sites occupied by Al3+ ions. Thus, the Al3+ ions are bonded to 6 oxygen in a distorted octahedron. Each such octahedron share a face with one on the upper and one on the lower layers. The distortion is caused by repulsion between Al3+ ions in octahedra sharing the faces.

Corundum is a dense (specific gravity of 3.97), hard (9 on the Mohs' scale, next only to diamond), high melting (melting point 2288 K), and insoluble in water. Crystals of corundum are usually prismatic or barrel-shaped bounded by steep pyramids. A massive grey granular corundum powder is called emery.

Colored corundum are called ruby (deep red due to presence of Cr3+ ions) and sapphire (blue, pink, yellow or green due to various degrees of Fe2+ or 3+, and Ti4+). The color may be modified by heating or irradiation. Some ruby crystals are shown here exposed in a piece of bauxite ore. Grey corundum or emery are used as abrasive, for example, emery paper (sand paper) and ruby and sapphire are for gemstones. They do have technical applications, for example, the first LASER was produced using a ruby crystal.

How is bauxite mined and processed?

Pure aluminas are used for pottery, ceramics, refractories, catalyst supports, and for the production of aluminum by the Hall process. Thus, bauxite and other aluminum containing minerals such as kaolinite (\(Al_4Si_4O_{10}(OH)_8\)) must be mined and processed to produce pure alumina. Australia produces about $3 billion worth of alumina a year from six Australian refineries. These refineries use the Bayer Process to extract aluminum hydroxide from the bauxite using hot caustic liquor. Aluminum oxide, Al2O3, is a typical amphoteric oxide, which dissolves in a strong acid and a strong base.

\[Al_2O_3 + 6H^+ \rightarrow 2 Al^{3+} + 3 H_2O \tag{1}\]

\[Al_2O_3 + 6 OH^- + 3 H_2O \rightarrow 2 Al(OH)_6^{3-} \tag{2}\]

After separation of the solids residue, the clear liquor is cooled. Depending on the pH of the solution, the aluminate ion Al(OH)63- bears various amounts of charge due to these reactions:

\[Al(OH)_6^{3-} + H6+ \rightarrow Al(OH)_5^{2-}(H_2O) \tag{3}\]

\[Al(OH)_5^{2} \cdot (H_2O) + H^+ = Al(OH)_4 \cdot (H_2O)_2 \tag{4}\]

\[Al(OH)_4 \cdot (H_2O)_2 + H^+ \rightarrow \underset{\text{a precipitate}}{Al(OH)_3(H_2O)_3} \tag{5} \]

In a neutral solution, the compound, Al(OH)3(H2O)3 or Al(OH)3 if water is ignored, forms a gelatinous precipitate. Under controlled manner, the liquor crystallizes to give particles of gibbsite of the desired chemical purity and physical characteristics. A lot of research and development has gone into this crystallization process alone.

The hydroxide ions of gibbsite form two layers similar to layers of closest packed spheres with Al3+ ions filling in some of the octahedral sites. The crystal structure of gibbsite consists of stacked double layers. It is expected that the hydroxide ions form extensive intra- and inter-layer hydrogen bonds.

Further dehydration converts Al(OH)3 into diaspore and boehmite, both of which have the stoichiometry AlO(OH). Gibbsite is is converted to alumina, Al2O3 by calcination. Alumina is marketed as the feed stock to smelters for the production of aluminium metal, ceramics, catalyst supports and other applications.

What are some of the applications of aluminum oxides?

Mineralogists consider a mineral a homogeneous solid body, formed by natural process that has a regular crystal structure with a limit range of atomic compositions. Engineers are mainly interested in properties and their applications. Scientists are interested in correlate the relationship of structures and properties. Engineers deal with natural and synthetic materials alike. Aluminum oxide is a basic material for the ceramic industry. For more details regarding the properties of alumina, consult the data sheet for alumina ceramics.

Aluminas are basic materials for ceramics, and they are useful for lining containers and mass transferring pipes, especially if heat resistance is required. Intricate tools such as the 95 % alumina ceramic rotor for 20 cm rotary valve have been made of these materials. Ceramics are related to many technologies. A list of resources related to ceramics gives many companies, whose main products are made of ceramic materials. For example, aluminas are used for paint, ink, coating and filling paper, adhesives, rubber, pharmaceuticals, tiles, bricks, cooking utilities, table wares, electronic components, porcelain, pottery, dental restoration, and plastics.

Technological changes demand materials with new specific properties. Since changes take place all the time, new materials are also developed all the time. Additions of specific amounts of other oxides to aluminas produce composite materials whose properties differ from both parent materials. This type of blending is a new frontier of material engineering.

Aluminum Oxide protects Aluminum from Corrosion?

Aluminum is a very reactive metal if it is not protected by its aluminum oxide film. It is much more reactive than zinc and iron, but far less reactive than magnesium. Their oxidation reduction potentials are given below for you to compare.

Fe3+ + 3 e- = Fe(s),       Eo = -0.037 V.
Fe2+ + 2 e- = Fe(s),       Eo = -0.447 V.
Zn2+ + 2 e- = Zn(s),       Eo = -0.76 V.
Al3+ + 3 e- = Al(s),       Eo = -1.67 V.
Mg2+ + 2 e- = Mg(s),       Eo = -2.70 V.

If the oxide film is cracked under aerated water, Al3+ is formed instantly along with OH- ions. Thus, an oxide film is formed immediately, sealing it from further corrosion at the anodic site. Another important fact is that Al2+ ions will not form, and the aluminum oxide is an inert substance.

Example 1

Calculate the molar volume of aluminum and aluminum oxide.

SOLUTION
The question requires the densities of aluminum and aluminum oxides. The CRC Handbook give their densities as 2.702 and 3.97 g/cc respectively. Thus, molar volumes of Al and aluminum oxides are

Vo of Al = 26.98 / 2.702 = 9.96 cc per mol of Al
Vo of Al2O3 = (26.98 + 24.0) / 3.97 = 12.84 cc

DISCUSSION
By applying the Pilling and Bedworth model, aluminum oxide formed has a larger molar volume per Al than the metal itself. Thus, the Pilling and Bedworth model also apply to aluminum and its oxide.

Furthermore, aluminum oxide and some hydrates have different densities,

Al2O3.H2O, 3.014 g/cc.
Al2O3.3 H2O, 2.42 g/cc.

The formation of Al(OH)3 forms a protective layer. This formula suggest the formation of gibbsite, density 2.44 g/cc leading to a molar volume of 32.0 cc.

Questions

  1. What is bauxite?
  2. What are the two colored forms of corundum?
  3. What is the hardness of corundum?
  4. What is the arrangement of the oxygen atoms in corundum?
  5. What is an amphoteric oxide?
  6. What solution is used to extract aluminum from bauxite in the Bayer process?Is aluminum a more reactive metal than zinc towards oxygen?
  7. Why does aluminum not rust?

Solutions

  1. Bauxite is an ore of aluminum oxide and other metal oxide.

    Skill -
    To be able to identify the ore bauxite from the description given in this page.

  2. Red corundum is ruby and blue corundum is sapphire.

    Skill -
    Explain the relationship of various materials, even if they appear very different.

  3. 9 on the Mohs' scale, next to diamond.

    Skill -
    Explain the properties of corundum and relate them to its applications.

  4. The oxygen atoms pack as a hexagonal array.

    Skill -
    Describe the structure of corundum, an important mineral related to ruby and sapphire.

  5. An oxide that dissolves in strong acid and strong base.

    Skill -
    Explain why a concentrated basic solution is used in the Bayer process.

  6. A strong base is used to dissolve aluminum oxide.

    Skill -
    Describe the chemistry of the Bayer process for the extraction of alumina.

  7. Yes, it is.

    Skill -
    Explain the chemical property base on electromotive potentials.

  8. A thin layer of aluminum oxide protects it from oxidation.

    Discussion -
    The thin layer does not cover the luster of aluminum metal.

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