Introduction
Alumina (Al₂O₃) is one of the most important oxide raw materials used in the refractory industry. Its extensive application is mainly associated with its high refractoriness, chemical stability, hardness, wear resistance, and resistance to corrosion and erosion at elevated temperatures.
Alumina exhibits several polymorphic crystal structures. The most important phases encountered in alumina production and refractory processing are α-Al₂O₃, γ-Al₂O together with several other transitional alumina structures. Among these phases, α-Al₂O₃ (corundum) is the thermodynamically stable phase at high temperature and is the most important crystalline phase for refractory applications.
The fundamental Al₂O₃ structure consists of Al³⁺ cations and O²⁻ anions. In α-Al₂O₃, oxygen ions form a nearly close-packed arrangement, while Al³⁺ ions occupy approximately two-thirds of the available octahedral sites. This produces a highly stable three-dimensional crystal structure.
The strong Al–O bonding and stable corundum structure are responsible for many of the important properties of alumina, including:
- High melting temperature
- High hardness
- High-temperature stability
- High chemical stability
- Good resistance to erosion
- Good resistance to many slags
- High mechanical strength at elevated temperatures
The stability of α-Al₂O₃ is one of the main reasons why high-purity alumina is such an important refractory raw material.
Forms of Alumina Used in Refractories
Several types of alumina-containing raw materials are used in refractory production. Most alumina used in industry is produced from bauxite through the Bayer process, in which the alumina contained in bauxite is extracted by digestion with sodium hydroxide and subsequently precipitated as aluminum hydroxide, known as Bayer hydrate. Bayer hydrate is predominantly present in the form of gibbsite and is converted into alumina through thermal treatment (calcination).
Approximately 90% of calcined alumina is used for aluminum production as metallurgical-grade alumina, while the remaining 10% is used for non-metallurgical and chemical applications. The different types of alumina are briefly described below.
Metallurgical Alumina
Metallurgical-grade alumina, also known as metal-grade alumina or smelter-grade alumina (SGA), is the term used for alumina intended for the production of primary aluminum metal. Historically, this type of alumina was produced from aluminum hydroxide using rotary kilns. However, today it is generally produced using fluid-flash calciners or fluidized-bed calciners. In the fluid-flash process, aluminum hydroxide is fed into a counter-current stream of hot air. The hot air is generated by the combustion of fuel, typically oil or natural gas.
The first stage of the process involves the removal of free moisture, followed by the removal of chemically bound water present in the structure of aluminum hydroxide. These dehydration processes occur over a temperature range of approximately 180–۶۰۰ °C. The resulting dehydrated alumina is primarily present as a form of transition (active) alumina. Its specific surface area gradually decreases as the temperature increases toward approximately 1000 °C. Further calcination at higher temperatures promotes the formation of α-alumina (corundum), which is the most thermodynamically stable form of alumina. In this type of alumina, the α-alumina content is typically in the range of approximately 20–۵۰%, while the remainder consists of transition alumina phases, commonly dominated by γ-alumina.
Calcined Alumina
Calcined alumina is produced by calcining aluminum hydroxide, generally obtained through the Bayer process. It is mainly composed of α-Al₂O₃ after appropriate calcination and is widely used as a fine or intermediate component in refractory formulations. Its main functions include increasing the alumina content, improving matrix packing, and providing a source of corundum during firing.
Reactive Alumina
Reactive alumina is generally produced by crushing and grinding calcined alumina. The calcination of this type of alumina is typically carried out at relatively high temperatures, generally around 1600 °C or higher, resulting in the conversion of the alumina grains predominantly into the α-Al₂O₃ phase. In addition, the Na₂O content of reactive alumina is carefully controlled and kept as low as possible.
Reactive alumina is used in applications where exceptional mechanical strength, wear resistance, high-temperature resistance, surface finish, and chemical stability are required. In particular, it is selected when the high-temperature mechanical behavior of the refractory or ceramic body is a critical performance requirement. Because of its high purity and controlled chemistry, reactive alumina exhibits a well-defined sintering behavior. Furthermore, the low level of impurities and consequently the limited formation of a glassy phase in bodies containing reactive alumina can reduce creep and deformation at elevated temperatures.
Tabular Alumina
Tabular alumina is a type of high-density, high-strength α-alumina with a melting point of approximately 2050 °C. It is produced by sintering crushed agglomerates of calcined alumina at temperatures of up to approximately 1900 °C in a shaft kiln. Tabular alumina was first produced in 1934. Its name is derived from the characteristic tablet-shaped corundum crystals that develop during the sintering process. Tabular alumina is produced in the form of spherical particles, which, after crushing and screening, are available in a wide range of particle sizes, from approximately 12.7 mm to 325 mesh.
The microstructure of tabular alumina consists predominantly of platelet-shaped corundum crystals, typically ranging from approximately 50 μm to more than 400 μm in size. Tabular alumina contains approximately 5 vol.% closed porosity, with pore sizes generally below 10 μm and an average pore size of approximately 0.71 μm. The presence of these closed pores reduces its bulk density compared with white fused alumina (WFA) and brown fused alumina (BFA). However, the open porosity of tabular alumina is considerably lower, typically around 1.5%.
The principal impurities in tabular alumina are Na₂O, originating mainly from the Bayer process, and iron-containing impurities, which can be introduced during the grinding process.
Fused Alumina
Fused alumina was first produced in the late 1990s by melting alumina using carbon electrodes. Depending on the type and concentration of impurities present, fused alumina is available in several forms, including brown, white, pink, and black fused alumina.
Similar to tabular alumina, the raw material used for the production of white fused alumina (WFA) is Bayer-process calcined alumina, whereas bauxite is used as the principal raw material for the production of brown fused alumina (BFA). White fused alumina is produced by melting calcined alumina in an electric arc furnace (EAF). After melting and solidification, the material is crushed, ground, and screened into the required particle sizes.
The crystals in white fused alumina are generally larger than those in tabular alumina, while its total porosity is typically below 1%. The pores are generally larger than those in tabular alumina, with pore sizes reported in the range of approximately 31–۴۷ μm. Overall, both the open and closed porosity of fused alumina are lower than those of tabular alumina. In fused alumina, differences in the cooling rate between the outer and inner regions of the particles result in variations in crystal size. The relatively rapid cooling of the outermost regions promotes the development of larger corundum crystals, whereas the slower cooling of the interior can result in the formation of smaller corundum crystals.
Bubble Alumina
consists of hollow spherical alumina particles and is therefore referred to as bubble alumina. This type of alumina is produced by melting high-purity raw materials in an electric arc furnace at approximately 2000 °C. The resulting molten alumina is then atomized using compressed air, and hollow spherical particles are formed as a result of surface-tension effects during solidification.
Because of its hollow particle structure, bubble alumina has a low bulk density and very low thermal conductivity, making it particularly suitable for the production of lightweight insulating refractories. Bubble alumina has a melting point of approximately 2100 °C and is chemically inert, which makes it suitable for high-temperature insulation applications where low thermal conductivity, low density, and chemical stability are required.
