Kyanite - Properties and Indian Occurrences

25-July-2020

Kyanite which is an aluminium silicate with chemical formula Al₂SiO₅, belong to the Sillimanite group of minerals comprising Sillimanite, Kyanite, Andalusite, Dumortierite and Topaz. Kyanite is an important raw material for high alumina refractories. Especially, Raw Kyanite is extensively used for making high alumina insulation Refractory Bricks.

Apart from refractory industry kyanite particularly its blue variety, is also used as gem stone. The kyanite gem stone is believed to possess certain metaphysical properties with its ability to keep the mind calm and anxiety under control. The name Kyanite was derived from the Greek word Kyanos which means blue.

Mineralogy of Kyanite

Raw Kyanite (Lumps)

Kyanite is found as subhedral and tabular to elongated, thin, bladed crystals having blue or light-green colour in the form of crystalline aggregates in schists, gneisses, granite pegmatite and occasionally in eclogites. The crystal system is Triclinic; optically kyanite is colourless and feebly pleochroic from pale-blue to colourless with one set of perfect cleavage, first and second order interference colour (yellow, grey and blue). The distinguishing features of kyanite are its higher refractive index than those of Sillimanite and Andalusite while birefringence is lower. The oblique extinction angle up to 32^O together with the biaxial interference, negative optic sign, and large optic axial angle are also distinctive for kyanite. Its hardness varies from 4 to 7 (Moh’s scale) and specific gravity is around 3.6 - 3.7.

Indian Occurrences

In the Indian subcontinent very good gem quality kyanite is found Nepal.

Kyanite is formed at medium temperatures and high pressures in a regionally metamorphosed sequence of rocks and is found associated with minerals like - muscovite, quartz, garnet, staurolite and rutile. Kyanite is also found as detrital mineral. For the Use of Kyanite in Refractory Industry the Directorate General of Technical Development (DGTD) has recommended the following specification:

	Grade-I	Grade-II	Grade-III
Al2O3 (min)	58%	54%	46-48%
Fe2O3 (max)	1.5%	1.5%	2%
PCE (min)	37 (Orton)	37 (Orton)

  

Suggested Article: Use of Kyanite as Refractory Raw Material

Recoverable reserves of medium to high grade kyanite in India and the current trend of production-utilization causes serious concern because of dwindling availability of this mineral in India. During 1960’s the hard, massive, lumpy variety of kyanite with Alumina content more than 61% and Iron content around 0.8% of Lapsa Buru mines in Kharswan (Bihar) was the largest deposit in the world. Today the source has dried up. Only poorer quality is now available which cannot be used as such. Deposits of kyanite available in a few other places some of which are being mined and supplied at present are -

Chemical Compositions of Indian Kyanite deposits

	SiO2	Al2O3	Fe2O3	TiO2	CaO	MgO	Na2O + K2O	LOI
Lapsa Buru (Bihar)	34.8	61.1	0.5 -1.3	--	0.2 -0.3	0.2	--	0.5
Singhbhum (Jharkhand)	46.5	45.97	0.5	1.5	--	--	--	1.1
Kudineerkati (Karnataka)	40.32	58.15	1.7	Tr.	0.52	--	--	1.32
Sulia (Karnataka)	32.8	61	1.85	2.7	--	--	--	1.65
Purulia (West Bengal)	38.8	46.65	2	Tr.	--	--	--	1.65
Khammam (A.P.)	51.92	27	11.25	--	--	--	--	--

Use of Kyanite as Refractory Raw Material

21-July-2020

Kyanite is one of the Alumino-Silicate group of minerals comprising Sillimanite, Kyanite, Andalusite, Dulmortierite, Topaz and Mullite all with similar chemical composition but different physical behavior and hence, uses. Kyanite, raw as well as calcined, have separate applications as refractory raw materials because of their distinct characters. 

Raw Kyanite

Raw Kyanite (Refractory Grade)

To be suitable for refractory raw material, kyanite should have very negligible amount of impurities which include free silica, alkali, iron oxides, calcium and magnesium. Kyanite, Sillimanite and Andalusite all these three minerals convert to Mullite and silica when they are calcined to temperatures between 1250^OC and 1500^OC. The inversion kyanite to mullite and silica (glass) begins at the periphery of grains and this rate of conversion depends the following:

>> Particle size

>> Firing temperature

>> Soaking time

>> Impurities present.

The heating of raw kyanite is accompanied with its volume expansion and a decrease in its specific gravity from 3.6 to 3.06, which takes place over a small range of temperature around up to 1350^OC. 
Because of this property raw kyanite - 

• Raw kyanite is extensively used for making high alumina insulation bricks, insulating mortars and castables.
• Raw kyanite fines (pulverized) can be added to refractory mixes in different proportions as a measure to control or compensate the overall shrinkage due to other raw materials. 

Calcined Kyanite

Calcined Kyanite (lumps) to be used in making Refractories

In order to make it a volume stable refractory material, kyanite is pre-calcined at 1420^OC to mullite and Cristobalite before use. Sometimes the lumps are very hard to crush after calcination. To avoid this, after calcination the kyanite lumps are, sometimes, quenched in water to make them crumble easily. Thereafter, it is ground, graded into various fractions as per requirement. These grains of calcined kyanite being volume stable are used with other raw materials for making refractory bricks and castables. The various refractory properties are:

P.C.E - above 1785^OC or 35+ (Orton)

RUL (refractoriness under load) - 1750^OC

Porosity = 24 - 25%

Properly calcined kyanite is a very good refractory raw material because of its high alumina percentage and low iron contents. As compared to other refractory raw materials kyanite can be sometimes, very handy for boosting alumina content and other refractory properties of the product at the same time maintaining its cost effectiveness.

Refractories made from calcined kyanite possess:

• Higher modulus of rupture and creep resistance
• Lower co-efficient of expansion
• High thermal shock resistance
• Increased durability, about three times that of ordinary bricks
• Superior resistance to salt attacks and chemical corrosion
• Very low co-efficient of spalling

Related Article: Kyanite - Properties and Indian Occurrences