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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.
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
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.
is an important mineral used for the production of superduty
refractory material mullite (3Al2
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 (Al2
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.
|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.
% SiO2.....37.08 %
||Blue White Green Yellow Pink
||Sometimes intergrown with
||crystal habit, color, luster
and unusual hardness
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:
= 2 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
modulus of rupture
co-efficient of expansion
firing range and temperature
durability, i.e., about three times that of ordinary bricks
change in volume after prolonged heating
resistance to salt attacks
low co-efficient of spalling
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.
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.
Kyanite can be found in a wide variety of locations around the world
in the states of North Carolina and Georgia
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
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.
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