Alumina

Irsalan Mohamed Asif - 1333309

[DATE]

Background

Alumina is one of the most cost effective and widely used material out of the engineering ceramics. The raw materials are readily materials are readily available and reasonably priced, from which it is made up. It takes form in nature as a white or nearly colourless substance and can serve as the raw material for a broad range of advanced ceramic products.

Composition

Alumina is composed of Aluminium and oxygen with the chemical formula Al2O3. It is the most common occurring of several aluminium oxides and specifically identified as Aluminium (III) oxide. Its most common name is Alumina but is also referred to as aloxide, aloxite or alundum depending on particular forms or applications. In Nature it is commonly found in its polymorphic crystalline state labelled α- Al2O3. Polymorphism refers to the ability of alumina to exist in more than one form or crystal structure. In its α- Al2O3 structure it compromises of the mineral corundum which is transparent but varieties of which can form ruby and sapphire. Corundum is the most naturally occurring form of crystalline Alumina but rubies are sapphires are gem forms, which owe their colour to impurities. However, it is from the more abundant ores of bauxite, cryolite and clays from which the material is commercially extracted and purified.

Crystal structure and bonding

In the most common and stable crystalline form of alumina it has a trigonal Bravais crystal lattice structure (1). The oxygen ions forming a hexagonal close–packed structure and Aluminium ions filling about two thirds of the octahedral spaces. Hexagonal sites are the corner atoms in the cell

while the octahedral sites are present between the two layers of vertical stacking (Figure 1).

Alumina possess strong ionic interatomic bonding forming a fixed lattice, giving rise to its desirable material characteristics (1). This is formed because the oxygen atoms on either end of Al2O3 each have a double covalent bond with an aluminium atom. The central Oxygen atom has one covalent bond with each aluminium neighbour again having two covalent bonds (1,2). Each Aluminium has three covalent bonds, two with one oxygen atom and one with the other oxygen atom. There are some disagreements on whether Alumina is ionic or covalent as most ionic elements have an electronegative difference of 2.0 or above however, alumina has an electronegative difference of 1.83. However, recent measurements into the bonding of alumina have concluded that it has 57% ionic bonds and 43% covalent bonds thus, still classifying it as an ionic material

Key Properties (2)

Figure 1 : illustrating the crystal structure of Alumina.

Hard, wear and corrosion resistant – The hardness of corundum the most occurring crystalline form of Alumina is such that it makes it suitable for use as a component in cutting tools. It is also responsible for resistance of metallic Aluminium to weathering. Metallic Aluminium is very reactive with atmospheric oxygen, and a thin passive layer of Alumina is formed on any expose Aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising.

Resists strong acid and alkali attack – Alumina is amphoteric which means that it displays some of the properties of both acids and bases. It will react with some bases and some acids.

Good thermal conductivity – Alumina is an electrical insulator but has a high thermal conductivity for a ceramic material.

Good electrical insulation – The high volume resistivity and dielectric strength make alumina an excellent electrical insulator.

High strength and stiffness High compression strength Resistant to abrasion Available in purity ranges from 94%, an easy metalisable composition, to 99.8% for the most

demanding high temperature applications.

Manufacturing route (2)

Alumina is mainly found in bauxite, the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite, boehmite and diaspore along with impurities. However, Bauxite only contains 30 -54 % alumina and two to three tonnes of bauxite are required to produce one tonne of Alumina(2). Purification occurs via Bayer process.

1.Milling: The Bauxite is first washed and crushed, reducing the particle size and increasing the available surface area for the digestion stage. Lime is added to make it into a slurry.

2.Desilication: Bauxites that have a high level of Silica (SiO2) go through a process to remove this impurity, because Silica can cause problems with the quality of the final product.

3.Digestion: A NaOH solution is used to dissolve the aluminium-bearing minerals in the bauxite to form a sodium aluminate supersaturated solution(2). Conditions within the digester are set

according to the properties of the bauxite ore. Ores with a high gibbsite content can be processed at 140°C, while böhmitic bauxites require temperatures between 200 and 280°C. The pressure is not important for the process but is at 3.5 MPa. The slurry is then cooled in a series of flash tanks to around 106°C at atmospheric pressure. Although higher temperatures are often advantageous, a major disadvantage is the possibility of other oxides other dissolving into the NaOH.

4.Clarification/Settling: The first stage of clarification is to separate the solids (bauxite residue) from the pregnant liquor (sodium aluminate remains in solution) via sedimentation. Flocculants are added to assist the sedimentation process(2). Further clarification is done using security filters to ensure that the final product is not contaminated with impurities.

Al(OH)3 + Na+ + OH- → Al(OH)4- + Na+

AlO(OH) + Na+ + OH- + H2O → Al(OH)4- + Na+

sodium aluminate supersaturated solution reactions

5.Precipitation: In this stage, the alumina is recovered by crystallisation from the pregnant liquor, which is supersaturated in sodium aluminate(2). Crystallisation is driven by progressive cooling of the pregnant liquor, resulting in small crystals of aluminium trihydroxite (Al(OH)3,), which then grow and agglomerate to form larger crystals

6.Classification: The crystals formed in precipitation are classified into size ranges. This is normally done using cyclones or gravity classification tanks. The coarse size crystals are destined for calcination after being separated. The fine crystals, after being washed to remove organic impurities, are returned to the precipitation stage to be agglomerated.

8.Calcination: The Separation is fed into Rotary kilns (Calciners) where they are roasted at temperatures of up to 1100°C to drive off free moisture and chemically-connected water, producing alumina solids. Calcination reaction :

2Al(OH)3 → Al2O3 + 3H2O

Alumina, a white powder, is the product of this step and the final product of the Bayer Process.

Applications of Alumina (3)

As a medical implant, Alumina is a highly inert material and resistant to most corrosive environment even the dynamic environment which is the human body. It is unreactive thus eliciting little response from surrounding tissues and can remain unchanged after many years in use. However, the body does recognise Alumina as a foreign material and will cover it with a fibrous tissue where possible.

Articulating surfaces in joint replacements: Due to its ability to be polished to a high surface finish and its excellent wear resistance, Alumina is often used for wear surfaces in joint replacement prosthesis. Examples of these are Femoral heads for hip replacements and wear plates in knee replacemnets.

Dental applications: Dense Alumina has been used for tooth replacements specifically for replacements of teeth. In many cases single crystal Alumina is used since poly crystalline Alumina can be fractured while inserting the implant into the dental root. Typically, single crystal Alumina is the core of the implant around which polycrystalline Alumina is fused.

Bone Spacers: Alumina with more than 30% porosity can be used as bone spacers to replace sections of bone that have been removed due to cancer or trauma. The porous nature of the implant allows for bone cell infiltration of the implant, eventually resulting in new bone formation. Alumina can also be used for maxillofacial region in replacing the jaw bone or parts of it.

Cochlear implants: Some parts of the cochlear implant such as the antennae have been made with Alumina because of the high stiffness and good thermal conductivity it displays.

References

Figure 1 : http://sist.fnal.gov/archive/97-topics/papers/JosePerez/paper.html . Accessed on 15/04/2014

1. Paglia, G. (2004). "Determination of the Structure of γ-Alumina using Empirical and First Principles Calculations Combined with Supporting Experiments" . Curtin University of Technology, Perth. 2.Gitzen, W.H (2006) Alumina as a Ceramic Material, Westerville: John Wiley & sons.

3. Ratner, B.D; Hoffman, A.S; Schoen, F.J; Lemons, J.E (2012) Biomaterials Science: An Introduction to Materials in Medicine, 3rd edn., Oxford: Elsevier.