Kyanite and porcelain manufacture
Kyanite is an aluminum silicate mineral (Al₂SiO₅) known for its unique properties and applications, particularly in high-temperature environments. It is primarily sourced from metamorphosed high-alumina shales, found in significant deposits in regions such as the southern Appalachian Piedmont and India. Kyanite's crystalline form, which can range in color from white to blue, is among several polymorphic minerals that also include sillimanite and andalusite, which form under varying temperature and pressure conditions.
In porcelain manufacture, kyanite is valued for its ability to enhance the strength and stability of porcelain products, including automotive spark plug insulators. When heated, kyanite transforms into mullite and silica glass, a process that increases volume and helps mechanically stabilize furnace linings. This property makes kyanite particularly advantageous in the production of refractories used in steel and glass manufacturing, as it competes with synthetic alternatives. Additionally, kyanite is utilized in high-strength ceramic applications, where its heat resistance and structural integrity are critical. The historical and cultural significance of kyanite, including beliefs in its healing properties, adds to its multifaceted relevance in various industries.
Kyanite and porcelain manufacture
Where Found
Because metamorphosed high-alumina shales are common in the mountain belts of the world, kyanite group minerals are widely distributed. However, concentrations of the minerals in reasonably large size are required for economic production. Major kyanite are found in the southern Appalachian Piedmont and in India. Sillimanite has been mined in India, Australia, and South Africa. Large deposits of commercial-grade andalusite occur in France, South Africa, and North Carolina.
Primary Uses
Kyanite minerals are used in high-temperature metallurgical processes. They are also used in high-strength porcelain manufacture.
Technical Definition
Kyanite is an aluminum mineral, Al2SiO5, also written Al2O3·SiO2. Two other minerals, sillimanite and andalusite, have identical composition but crystallize in different forms determined by the temperature and pressure at the time of crystallization. The three minerals are polymorphs (different forms) of Al2SiO5 and constitute the kyanite, or sillimanite, group of minerals.
Description, Distribution, and Forms
Kyanite crystallizes as blade-shaped crystals with vitreous luster and white to blue color. Sillimanite is most commonly finely fibrous and brown in color. Andalusite occurs as elongate, cigar-shaped crystals in a variety of colors. Kyanite-group minerals occur most commonly in metamorphosed high-alumina shales. Relatively high pressures and temperatures produce kyanite, intermediate pressures and high temperatures produce sillimanite, and low temperatures and pressures produce andalusite.
History
Kyanite has been mined in many parts of the world. In the past, it was treasured for its blue color. Some traditions indicate kyanite has healing powers.
Obtaining Kyanite
Kyanite minerals require varying amounts of preparation before use. Massive aggregates of kyanite and sillimanite that occur in India have been sawed or carved to desired shapes, but kyanite group resources in Europe and North America normally require separation of the minerals from associated quartz, micas, and other minerals, resulting in a granular product. The granules, which do not adhere to one another, are mixed with various materials, usually including fireclay and water, to produce a moldable product that can be used as mortar between refractory bricks or molded into bricks or other useful shapes.
As a high-temperature furnace lined with “green” (unfired) superduty refractory bricks is heated, the kyanite group minerals in the green brick and mortar convert to mullite. Uniquely, the volume of mullite and silica glass resulting from the conversion of kyanite to mullite is about 18 percent greater than the original volume of kyanite. The volume increase occurs at about the same temperature that other materials are shrinking in volume, and this phenomenon tends to mechanically stabilize the furnace lining. Therefore, there is a significant advantage to including raw kyanite in the green products.
Uses of Kyanite
The kyanite group minerals are used as superduty refractories in high-temperature metallurgical processes, especially steel production, and in high-strength porcelain products, typically automobile spark plug insulators. On heating to about 1,400° Celsius, the kyanite group minerals alter to mullite (3Al2O3·2SiO2) plus silica glass. Mullite remains stable and strong to 1,810° Celsius. The kyanite group minerals are therefore very desirable as refractories in steel and glass furnace linings and as materials for kiln furniture (product supports) in high-temperature ceramic manufacture.
Kyanite group minerals compete economically with synthetic mullite refractories. Synthetic mullite is produced by heating or fusing an appropriate mixture of high alumina and siliceous materials. Near Americus, Georgia, naturally occurring mixtures of and kaolin—and at Niagara, New York, alumina and glass-grade silica sand—are used to produce synthetic mullite.
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Bressan, David. "The Origin of Geological Terms: Kyanite." Forbes, 9 Jan. 2017, www.forbes.com/sites/davidbressan/2017/01/09/the-origin-of-geological-terms-kyanite/. Accessed 23 Dec. 2024.
Lu, Zeng, Jun Yan, and Jiabin Li. "The Fate of Kyanite in Ultrahigh-Pressure Metasedimentary Rocks, Southwestern Tianshan, NW China." Lithos, vol. 458-459, Nov. 2023, doi.org/10.1016/j.lithos.2023.107356. Accessed 23 Dec. 2024.