Aluminium oxide
Aluminium oxide, also known as alumina, is a naturally occurring chemical compound with the formula Al₂O₃. It exists in several forms, with alumina being the most prevalent and often referenced simply as aluminum oxide, although this encompasses three distinct compounds of aluminum and oxygen. The primary commercial source of alumina is bauxite, a natural ore found in tropical and subtropical regions. Aluminium oxide has diverse applications across various industries, including pharmaceuticals, where it is used in treatments for kidney disorders, and in consumer products like toothpaste and food additives.
The Bayer process, developed by chemist Carl Josef Bayer in the late 19th century, is the main method for extracting alumina from bauxite. This multi-step process involves milling, purifying, and treating the ore to yield alumina, which appears as a fine white powder. Alumina exhibits notable properties such as chemical resistance, high hardness, and thermal stability, making it suitable for applications in ceramics, electronics, and even superconductors. Although generally considered safe, alumina can cause irritation or lung issues with high or chronic exposure. Importantly, around 90% of alumina is used in the production of pure aluminum, an energy-intensive process that is increasingly seeking sustainable methods to reduce its environmental impact.
On this Page
Subject Terms
Aluminium oxide
Aluminum oxide, or aluminium oxide, is a naturally occurring chemical compound that comes in several forms, with the most common form, alumina, having the chemical formula Al2O3. Alumina is sometimes referred to simply as aluminum oxide, although this is something of a misnomer, as the term aluminum oxide technically covers three distinct compounds of aluminum and oxygen that are differentiated by varying amounts of their constituent base elements.
For commercial and industrial usage, most alumina is extracted from a natural ore known as bauxite, which is commonly found in tropical and subtropical topsoil. The substance has a broad range of industrial, commercial, and pharmaceutical applications. It is also used in medicine, particularly for patients who require treatment for kidney disorders, and in some consumer products. Alumina can also be smelted to produce pure aluminum through a technique commonly known as primary production.
Background
Alumina is the most common compound in the aluminum oxide family, a group that includes two other substances: aluminum(I) oxide, which has the chemical formula Al2O, and aluminum (II) oxide, which has the chemical formula AlO. Aluminum(II) oxide is also called aluminum monoxide, while alumina is referred to by several other names, depending on its intended application and the industry in which it is used. These alternate names include aloxide, aloxite, and alundum. Alumina is also known as aluminum(III) oxide or aluminum trioxide.
Aluminum(I) oxide has an extremely narrow range of uses, owing to its instability in its solid form at normal temperatures. Given this property, it most commonly presents as a gas and usually occurs as a by-product of the vaporization of alumina. Aluminum(II) oxide, or aluminum monoxide, is similarly rare and also occurs as a by-product of aluminum expenditure. For example, traces of aluminum monoxide have been found in the upper atmosphere following the detonation of aluminum-coated explosives.
The main method of producing commercial-grade alumina was first performed by the Austrian chemist Carl Josef Bayer (1847–1904) in 1887. Also known as the Bayer process, this technique is used to extract alumina from bauxite ore. Bayer went on to patent his technique, and it remains fundamental in the modern-day production of the compound.
The Bayer technique involves eight process stages: milling, desilication, digestion, clarification (or settling), precipitation, evaporation, classification, and calcination. During these stages, bauxite ore is cleaned and ground into finer particles to increase its overall surface area. Silica impurities are then removed before the crudely refined bauxite is exposed to a hot, caustic sodium hydroxide solution. Solid bauxite remnants are then separated from the resultant aluminate liquid with the help of chemical additives. Following these steps, the alumina is crystallized, heated, cooled, washed to remove impurities, and then roasted at very high temperatures to produce the final, solid-form product.
Alternate methods are also used to produce alumina from metallic ores, but these methods account for only a relatively small proportion of overall production. Using the Bayer technique, it is possible to generate one metric ton of alumina from two to three tons of bauxite ore.
Overview
Alumina exists in eight different subforms, with these respective forms being denoted by the symbols α, χ, η, δ, κ, θ, γ, and ρ. Of these subforms, α-alumina offers the highest level of thermodynamic stability. In its solid form, alumina appears as a fine white powder with a strong resemblance to table salt.
Key properties of alumina include strong overall resistance to chemical attacks, abrasion, electricity, and thermal shock. It also displays high levels of hardness, compression strength, dielectric strength, and heat conductivity. Bauxite, the raw material used to produce alumina, is also widely available at stable prices; this facilitates the continued manufacture and steady supply of the compound.
Alumina is used for many different commercial and industrial applications, and is a common adsorbent, catalyst, and desiccating agent. Specific applications include uses in spark plug insulators, metallic paint, dental cement, and rocket fuel. It is also found in ceramics, flame inhibitors, electrical insulation, and magnetism-based recording media. More recently, alumina has also been adopted for use as a tunnel barrier in several kinds of superconductors, including electron transistors and quantum interference apparatuses. Its resistance properties also make it useful in some radiation protection applications.
Consumer products that contain alumina include toothpaste, where it serves as an abrasive agent, and certain food additives, where it acts as a dispersing agent. Alumina is also found in many different pharmaceutical preparations, including several classes of stimulants and analgesics as well as other drugs including sildenafil (Viagra) along with some antidepressants and first-generation antihistamines. Numerous studies have been conducted to ensure the compound's safety for such applications, and while alumina has not been identified as a carcinogen, it is associated with irritation of the eyes and upper respiratory tract, as well as potentially serious conditions affecting the lungs. However, these effects only occur as the result of external or internal exposure to very high amounts of the compound, or from chronic exposure to dust containing particulate alumina.
Alumina is also used in hemodialysis, a medical treatment that removes waste, impurities, and excesses from bodily fluids including blood and urine. Hemodialysis is performed on patients with serious kidney disorders, and is usually reserved for cases in which the patient's kidneys are functioning at 10 to 15 percent of normal capacity.
About 90 percent of the global supply of alumina is used to produce pure aluminum, which is achieved through a process known as primary production. This technique involves the smelting of refined alumina using a combination of carbon and electrical current to separate the compound's aluminum constituents from its oxygen constituents. It is an energy-intensive process with a significant ecological impact, and as such, aluminum manufacturers are increasingly adopting cleaner techniques, including the use of renewable and more efficient forms of energy.
Bibliography
"Alumina (Aluminum Oxide) – The Different Types of Commercially Available Grades." AZo Materials, www.azom.com/article.aspx?ArticleID=1389. Accessed 25 June 2017.
"Alumina Powder." Reade International Corporation, www.reade.com/products/alumina-powder-al2o3. Accessed 25 June 2017.
"Alumina Refining." Aluminum Association, www.aluminum.org/industries/production/alumina-refining. Accessed 25 June 2017.
"Aluminum Oxide." Drugs.com, www.drugs.com/inactive/aluminum-oxide-40.html. Accessed 25 June 2017.
Dohmeier, Carsten, Dagmar Loos, and Hansgeorg Schnökel. "Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions." Angewantde Chemie International (edition in English), vol. 35, no. 2 (1996): pp. 129–149.
"Primary Production." Norsk Hydro ASA, www.hydro.com/en/about-aluminium/Aluminium-life-cycle/Primary-production/. Accessed 25 June 2017.
"Refining Process." International Aluminum Institute, bauxite.world-aluminium.org/refining/process/. Accessed 25 June 2017.
Tyte, D.C. "Red Band System of Aluminum Monoxide." Nature, vol. 202, 25 Apr. 1964, pp. 383–84.