Kyanite

Aluminum Silicate
Al₂SiO₅

Kyanite in the form of mullite is widely used in the manufacture of glass, burner tips, spark plugs, heating elements and high voltage electrical insulations and in the ceramic industry.

The History Says

The name Kyanite is derived from the Greek kyanos meaning "blue" as the finest specimens display colors similar to Blue Sapphire. Like Diamonds, Kyanite has perfect cleavage in one direction, a fact that gem cutters take into consideration when faceting this unusual gemstone. Kyanite is commonly cut into Baguette, Octagon, Oval and Pear shapes.

The Present Scenario

India is the largest producer of kyanite in the world. The USA, the UK and Japan depend heavily on imports from India. The USA has developed considerable domestic production of synthetic mullite and provides about 22,000 tonnes of annual production equivalent to 80% of the total requirements.

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KYANITE is an important mineral used for the production of superduty refractory material mullite (3Al₂O₃.2SiO₂), used in various metallurgical and glass industries, except steel metallurgy as mullite refractory bricks are corroded by iron slag and are not highly resistant to metallic oxides.

Kyanite has the same chemical composition (Al₂O₃.SiO₂) as andalusite and sillimanite but it is distinguished from those two minerals by its physical properties.

Kyanite is easily distinguished from sillimanite or andalusite by its tabular, long bladed, acicular form and by bluish colour; and slightly lower hardness than sillimanite and andalusite. Another important property is its double hardness. Its specific gravity is 3 to 3.7.

Hardness	Associated Minerals	Chemical Composition	Colour	characteristics	Luster	Field Indicators
4.5 when scratched parallel to the long axis of the crystal and approximately 6.5 when scratched perpendicular to or across the long axis.	biotite staurolite garnets quartz andalusite sillimanite	Al2O3...62.92 % SiO2.....37.08 %	Blue White Green Yellow Pink	Sometimes intergrown with staurolite	Vitreous	crystal habit, color, luster and unusual hardness

Properties

Kyanite has a high melting-point and excellent refractory properties. When heated to about 1350ºC it converts into mullite and free silica. The reaction takes place as follows:
3Al₂O₃.SiO₂ = 2 SiO₂ + SiO₂

The conversion takes place with considerable increase in volume (which is about 20%), hence it is necessary to calcine kyanite before use. It is in the form of mullite that these alumino-sillicate minerals like kyanite, andalusite sillimanite, topaz and dumortierite are used as refractory materials. Mullite imparts highly desirable refractory properties of great strength and is stable upto 1810ºC.

The outstanding advantages of refractory bricks made from kyanite are:

• Higher modulus of rupture
• Lower co-efficient of expansion
• Increased firing range and temperature
• Increased durability, i.e., about three times that of ordinary bricks
• Negligible change in volume after prolonged heating
• More resistance to salt attacks
• Very low co-efficient of spalling

Andalusite converts into mullite at temperatures between 1380º and 1410ºC. The conversion takes place at the most with slight expansion. Hence it can be used as quarried and requires no preliminary calcination. Topaz and dumortierite readily dissociate giving off fluorine and boron and then convert into mullite.

Kyanite, sillimanite and andalusite yield 88% mullite and 12% free silica, probably as cristobalite. Dumortierite and topaz yield over 95% mullite. Commercial deposits of andalusite, topaz and dumortierite have not been reported so far in the country, except in a minor quantity as accessory minerals in kyanite deposits.

For refractory purpose, kyanite should contain a negligible quantity of impurities like iron oxide, free silica, oxides of calcium and manganese and alkalies. These impurities considerably reduce the softening point of mullite and thus affect the refractory properties. The presence of corundum in the mineral kyanite is advantageous as it reduces the percentage of free silica when calcined.

Indian kyanite is much valued, as it exhibits a low thermal expansion and produces hard grog with a high constancy in volume. Massive kyanite is preferred to coarse and bladed varieties because they yield a fragile and porous grog which is not desirable. Glass and electric porcelain manufacturers prefer kyanite containing less than 0.2% Fe.

Industrial Applications

Kyanite in the form of mullite is widely used in the manufacture of glass, burner tips, spark plugs, heating elements and high voltage electrical insulations and in the ceramic industry. In metallurgical furnaces, mullite refractory is used in sections where high melting point and resistance to spalling, slagging and chemical reactions are important. Mullite refractory particularly finds applications in melting high copper-brass and bronze, copper-nickel alloys and gold refining and in many direct-electric furnaces. Wherever the temperature is unusually high, as in furnace roof, and where slagging is severe, as in pouring areas and tap holes, mullite bricks can be used with great advantage in place of the usual fireclay bricks, which have a lower initial cost but must be replaced more frequently. Pouring ladles and electric-arc furnaces often require mullite refractory exclusively. Under similar conditions in glass tanks mullite can be used with advantage.

For minor uses the ceramic industry has found mullite bricks to be particularly useful for tunnel-kiln car tops, muffle and hearth tiles, kiln slabs, setters, saggers and other kiln furniture, in frit furnaces and in the fire-box area of enamelling furnaces. Mulllite mixed with silicon carbide is used for washes on saggers, plates and muffles. Mixed with graphite it is used as mould work for ferrous casting, coating ingot steels and slag and cinder pots. Andalusite and dumortierite have been used in the manufacture of spark plugs and pyrometer tubes; topaz as a substitute for fluorite as a slag thinner in steel manufacture; and kyanite as a glass-batch constituent.

The high cost of mullite, from twice to several times that of standard refractory raw materials, has restricted its use to a relatively few refractory units. The trend toward higher temperatures in metallurgical processes, together with rising labour costs, is expected to increase the use of mullite refractories. Withe lower prices, their use would probably be much greater.

World Resources

Kyanite can be found in a wide variety of locations around the world including:

• Brazil
• India
• Kenya
• Mozambique
• Nepal
• Russia
• Serbia
• Switzerland
• Tibet
• USA in the states of North Carolina and Georgia

India is the largest producer of kyanite in the world. The USA, the UK and Japan depend heavily on imports from India. The USA has developed considerable domestic production of synthetic mullite and provides about 22,000 tonnes of annual production equivalent to 80% of the total requirements.

Next to India, some African countries are reported to possess kyanite deposits. Small production of kyanite has been reported from Kenya, East Africa. Andalusite production is reported from Transvaal, Republic of South Africa.

In the USA, kyanite occurs disseminated in quartz vein. It is being worked on Baker Mountain, near Cullen, Prince Edward county, Virginia and on Willes' Mountain, near Dillwyn, Buckingham county. The rock mined is beneficiated to obtain kyanite of 94% purity. Some deposits of this type are also located in North Carolina and Georgia.

Bulgarian geologists, it is reported, have discovered extensive deposit of kyanite in the Rhodoper mountain.