Introduction
Andalusite is an aluminosilicate mineral used in the manufacture of refractories and porcelain for spark plugs. It is a typical mineral formed through low-grade thermal metamorphism and is commonly found in mudstones within contact metamorphic zones. It mainly forms under conditions of a high geothermal gradient and a low pressure-to-temperature ratio.
It has a low and controllable expansion rate, making it suitable for applications that require high volume stability (e.g., precision ceramic kiln furniture).
Its resistance to thermal shock is superior to that of kyanite, allowing it to withstand frequent temperature fluctuations.
Basic Properties
Chemical composition: The main component is aluminum silicate (Al₂O₃·SiO₂). Together with kyanite and Sillimanite , it is referred to as one of the "three stone minerals." Theoretical composition includes 63.1% Al₂O₃ and 36.9% SiO₂.
Physical properties: Appears as light red columnar crystals at room temperature. Refractoriness ranges from 1810°C to 1850°C. At high temperatures (1300°C to 1500°C), it decomposes into mullite and fused silica. It exhibits a volume expansion of approximately 4% to 6%, with a lower expansion rate than kyanite but better volume stability.
Process
Beneficiation and Purification:
Impurities are removed through crushing and screening. The Fe₂O₃ and TiO₂ content is reduced by magnetic separation or flotation. After purification, the Al₂O₃ content can reach 55%–65%.
Calcination Treatment:
Raw Andalusite: Directly used in unshaped refractories, taking advantage of its low expansion characteristics to regulate volume stability.
Calcined Andalusite: After calcination at 1350–1450 °C, the phase transformation is complete. It is commonly used in the production of high-grade refractory bricks and prefabricated components.
As Refractory Materials
Unshaped refractories are used directly without firing, which helps save fuel and energy. However, their volume stability at high temperatures significantly affects their service life. In practice, when fired materials and plastic refractories are used in proportion, the mix typically contains a certain amount of clay and inorganic binders. This can cause the unshaped refractory to shrink during high-temperature heating and cooling cycles, leading to cracks and spalling, which shorten the service life of the material.
To control and reduce high-temperature contraction over long-term use, a fixed amount of Andalusite can be added to the formulation. By utilizing its expansion-stabilizing properties, this approach can eliminate the minor shrinkage of unshaped materials and extend their service life-potentially up to five years.
As Refractory Bricks
Andalusite is calcined to produce shapes that can be used in hot air furnaces, hot air towers, reheat furnaces, and other critical areas. It can also be used to manufacture various auxiliary casting and operating equipment, as well as kiln facilities, high-temperature aluminosilicate insulators, and sand mold binders.
Using Andalusite-based refractory fiber for furnace linings can save 30% to 50% energy compared to refractory clay or lightweight brick linings. In addition to reducing fuel consumption and improving stability, Andalusite refractory bricks can save more than 40% of the general consumption of refractory materials.
Raw Materials for the Production of Silica-Aluminum Alloys, Alumina, and Aluminum Metals
Due to its high Al₂O₃ content and low levels of iron, titanium, and calcium oxide impurities, Andalusite can be used in the production of silica-aluminum alloys with 60% aluminum content without requiring additional alumina. This simplifies the production process and improves overall efficiency.
Advantage
1. Low Energy Consumption
Andalusite is an anhydrous silicate mineral and does not contain water of crystallization. As a result, Andalusite raw material does not require pre-calcination and can be directly used in production. Compared with bauxite and other materials, Andalusite eliminates the need for pre-calcination, thereby reducing energy consumption during processing.
In addition, refractories made from Andalusite also save energy during use. When used in critical areas such as hot blast furnaces, soaking furnaces, and mixing furnaces in iron and steel smelting, high-alumina and unshaped refractories made from Andalusite can extend furnace service life by over 150–200 cycles, reduce material loss by 43%, save approximately 12% in energy consumption, shorten smelting time, and lower production costs.
2. Chemical Stability
Andalusite transforms into a mixture of mullite (3Al₂O₃·2SiO₂) and free SiO₂. In the Al₂O₃–SiO₂ binary system, mullite is the only chemically stable phase. Therefore, refractory products made from Andalusite exhibit excellent chemical stability.
3. Volume Stability
When used in high-temperature environments, the transformation of Andalusite into mullite is accompanied by slight volume expansion (linear expansion rate of 1%–1.5%). Compared with other materials, this volume change is relatively small, meaning that the product maintains good volume stability. This micro-expansion helps fill joints between bricks and cracks in castables, effectively blocking the penetration of molten slag and metal-resulting in excellent slag resistance.
Moreover, this volume expansion is irreversible, so the joints remain tight after cooling. When Andalusite is added to the matrix of high-alumina castables, although workability may decrease, volume stability improves significantly. If the plasticity of Andalusite is enhanced, it will be an excellent material choice for high-alumina castables requiring high volume stability.
Additionally, in the application of low-creep clay bricks for hot blast stoves, the small volume effect of mullite formed from Andalusite helps improve the overall volume stability of the product.
4. Good Creep Resistance
After being converted into mullite, Andalusite forms a needle-like mullite network microstructure, providing good creep resistance and high-temperature mechanical strength. Using high-grade pyroxene, Andalusite, and fused mullite as the main raw materials, high-performance, low-creep clay bricks can be developed for blast furnace hot blast stoves-offering excellent creep resistance, abrasion resistance, and high-temperature volume stability.
5. Strong Thermal Shock Resistance
At around 1500 °C, most of the Andalusite transforms into mullite, with the mullite phase accounting for about 80% and the glass phase around 20%. During thermal shock, the liquid glass phase absorbs thermal stress, helping prevent cracks and reducing spalling.
Application
Iron and Steel Industry
Blast furnace body, outlet bricks, intermediate ladle lining
→ Excellent thermal shock resistance; expansion helps compensate for joint shrinkage between bricks.
Cement Industry
Rotary kiln transition zone, kiln mouth guard bricks
→ Resistant to alkali slag erosion; maintains structural stability at high temperatures.
Ceramics and Refractories
Kilns, saggars, high-temperature furnace linings
→ Excellent thermal stability; reduces the risk of cracking.
Unshaped Refractory Materials
Low-cement castables, repair compounds
→ Controlled expansion rate improves overall resistance to cracking.
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