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Showing posts with label Refractory Raw Materials. Show all posts
Showing posts with label Refractory Raw Materials. Show all posts

Use of Sillimanite as Raw Material in Refractories

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Sillimanite as a natural and untreated mineral is a very important raw material for high alumina refractories which are extensively used in Iron and steel, Petrochemical, Electrical, Cement, Zinc and Glass industries. - Sillimanite Rocks image
Sillimanite when heated above 1545­OC converts to Mullite and the excess silica as glass, crystoballite or tridyamite. The formation of the glassy phase can be reduced by addition of a small percentage of technical or calcined pure alumina fines (like - HGRM 30 etc) which reacts with this excess silica to form mullite, which in turn help in enhancing the quality of the product.

Due to the very low expansion or contraction on heating, sillimanite need not be calcined before use. Unlike sillimanite from most of the sources in the World which are used as it is, the Rewa sillimanite (found in Madhya Pradesh, India), because of its impurities, should not be used as such. It is always better once to wash these lumps in the raw material yard itself and then after shifting to the Mill House and crushing, grinding pass through magnetic separator to eliminate the free iron impurities.

The ideal firing temperature of green refractory bricks made of sillimanite grains as a major raw material is 1450 - 1500OC, to be fired either in a batch type or a tunnel kiln. The soaking time will vary depending upon the volume, shape, setting and other constituents of the bricks (particularly raw clay used and sintering aid, if any).

Sillimanite Refractories

Sillimanite refractories are characterized of high refractoriness, very low coefficient of thermal expansion, high refractoriness under load (RUL) and mechanical strength with great resistance to thermal shock (spalling resistance), abrasion and slag corrosion. Due to their exceptionally high resistance to spalling and corrosive actions of molten glass, chemical attacks of soda, borax and other frits, they are most suitable for Glass Melting Furnace (GMT), Oil fired Furnace, Cement Rotary Kiln, Blast Furnace, Electric Arc Furnace (EAF) roofs, Hot Metal Mixer, Combustion Chambers and Metallurgical operations done in Zinc Furnace, Gold Refining Furnace etc. - Sillimanite Bricks image

Particularly in Glass industry sillimanite refractories have got many applications, such as in glass melting tank furnaces (to be discussed in detail in a separate article) in all parts open to the products of combustion like combustion chambers, flues, door pillars which may have to support heavy load at high temperatures, recuperators and such other parts which are liable to be subjected to fluctuations of temperature. Assam Sillimanite once available in good quality and quantity, even for export, were used to be cut into blocks of various sizes from solid rock at the site of deposits itself which were then sent to the user’s site for their direct use in construction of Glass Melting tank Furnace bottom. But now it is a forgotten past! However, there are quite a few suppliers in India who manufacture these GT blocks, mostly using certain percentage of sillimanite sand or even sillimanite lumps after crushing and grinding. To name a few are Maithan Ceramics Limited (MCL), Tata Refractories Limited (TRL), OCL India Limited, ORIND (quality ?) etc. So far the properties like density (BD), porosity, mechanical strength, slag corrosion resistance and consistency of performance etc. are concerned, high capacity machine pressed bricks are far too superior to those made by pneumatic ramming. Although it is a general practice to give some ‘patching/finishing’ manually before inspection - dispatch to particularly big and complicated shape refractory bricks but from the customers’ point of view it is most important that during inspection it must be ensured that except for the ‘look’ only, the refractory brick (or block) does not depend much on the ‘finishing’, if at all, done on it. Patching is a wrong practice as it is done to camouflage the flaws which could be detected by seeing the brick.

For GT blocks the machine finish of the surface is very important. It must be evenly polished or ground to ensure that the warpage is negligible. One of the main criteria for acceptance of these blocks should be that in the assembly there should not be any open joint (the specification could be from 0.2mm - 0.3mm filler gauge up to a maximum 20 - 25mm depth from the top). This is a must to avoid the penetration and subsequent crystallization of glass and alkali vapours in these joints. To meet this criterion the manufacturer should have facilities for grinding of these blocks minimum in 4 faces and in some bricks up to 6 faces and then marked accordingly. 

Related Article: Sillimanite - Mineralogy and Occurrence in India

Kyanite - Properties and Indian Occurrences

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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 - image of Kyanite lumps
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 32O 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:

Al2O3 (min)
Fe2O3 (max)
PCE (min)
37 (Orton)
37 (Orton)

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

Na2O + K2O
Lapsa Buru (Bihar)
0.5 -1.3
0.2 -0.3
Singhbhum (Jharkhand)
Kudineerkati (Karnataka)
Sulia (Karnataka)
(West Bengal)
Khammam (A.P.)

Use of Kyanite as Refractory Raw Material

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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

Industry Guru - image of 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 1250OC and 1500OC. 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 1350OC. 
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

Industry Guru - image of calcined kyanite lumps
Calcined Kyanite (lumps) to be used in making Refractories
In order to make it a volume stable refractory material, kyanite is pre-calcined at 1420OC 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 1785OC or 35+ (Orton)
RUL (refractoriness under load) - 1750OC
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