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Showing posts with label Refractories. Show all posts
Showing posts with label Refractories. Show all posts

Sillimanite - Properties, Mineralogy and Occurrence in India

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Sillimanite belongs to the rich Alumino - Silicate group of minerals and is a polymorph of Kyanite and Andalusite represented by the chemical formula Al2SiO5. According to Wikipedia to this mineral has been named after American chemist Benjamin Silliman (1779 - 1864). Sillimanite is a very important refractory raw material especially, for high alumina refractories which are extensively used in Iron and Steel, Petrochemical, Electrical, Cement, Zinc and Glass industries.

Mineralogy of Sillimanite

Sillimanite is found as brown, grayish, pale green or white orthorhombic mineral with long slender, needle shaped crystals. Optically, it is subhedral, elongated, colourless and non-pleochroic (or colourless to pale brownish yellow) in plane polarized light. Sillimanite has low r.i., one set cleavage, and straight extinction in grains which are parallel to elongation. In pure form it has specific gravity 3.23 and Moh’s hardness 6 - 7.5.

Kyanite - Properties, Mineralogy and Indian Occurrences

Indian Occurrence of Sillimanite

Sillimanite - Properties, Mineralogy and Occurrence in India
Sillimanite occurs in fibrous masses mainly in the form of bundles and is characteristic of the innermost zone of contact metamorphosed rocks. Because of this nature of its occurrence this mineral is also known as Fibrolite.

Massive Sillimanite deposits of the Nongstoin state in the Khasi Hills of Assam (Meghalaya) in India commonly known as Assam Sillimanite, was the world famous for its quality and quantity. The host rocks were Cordierite - Biotite - Quartz - Muscovite - Gneiss or a Sillimanite - Quartz - Schist with intrusions of Granite. Now it is not available since the reserves have been depleted. The only other important occurrence is around Pipra in Madhya Pradesh (India) popularly known as Rewa Sillimanite and in Bhandara district of Maharashtra (India). The disadvantage of Rewa sillimanite is that it slightly sensitive to thermal shocks. An authorized supplier of Rewa / Bhandara sillimanite is M/s Pavri Kyanite Mines, Nagpur, Maharashtra. An alternate source is available today from Beach Sand (a by-product of Indian Rare Earths Limited, a Government of India Undertaking under its Department of Atomic Energy). One has to fulfill certain formalities with Indian Rare Earths Limited in order to procure sillimanite sand from them. The beach sand deposits are the potential sources of sillimanite along with rutile, ilmanite, monazite, garnet, zircon and quartz. The Chavara (Kerala, India), Manavalakurichi (Tamil Nadu, India) and Chatrapur (Orissa, India) are among the richest deposits in the world. Indian beach sand deposits contain about 9% rutile, 6% garnet, 3% sillimanite and 1% monazite and zircon. After magnetic removal of Monazite, sillimanite, garnet, zircon and rutile are separated from one another by gravity tabling, electrostatic and flotation techniques. 

The following table shows the Chemical Composition of Sillimanite of two different sources:

Constituents (%)

Rewa / Bhandara

Indian Rare Earths

(Beach Sand)









TiO2 / ZrO2









2.16 (TiO2)









0.60 (ZrO2)

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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. 

Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories



What is Gel Bond?

The principle behind this bonding is the formation of a ‘gel’ from a ‘sol’ which surrounds the refractory Aggregates through a network skeleton which, with further heating, develops strength & ultimately goes through sintering to form ceramic bonding. Actually the mechanism is thixotropy, which lies in the fact that some substances, when agitated (under mechanical force), pass from the state of a ‘gel’ to that of a colloidal dispersion ‘sol’ and goes back to the state of a ‘gel’ again when the mechanical forces stop. The phenomenon of thixotropy is based on the theory of dispersion & subsequent flocculation of ultrafine powders. Various sols used in the bonding process e.g. Silica, Alumina, Zircon, and Titania. The incorporation of gel bond in place of conventional binders (High Alumina Refractory Cement) has made it possible to improve the high temperature properties of castable refractories considerably mainly because of the absence of low-melting phases (CA, CA2, C12A7, C2AS, C4AF) and impurities.

Advantages of Gel Bond

Several advantages of the gel-bond compositions compared to LCC & ULCC as have been reported are:

Also Read: The Functions of Silica Fume (Microsilica) and other ultra fine additives in the formulations of Refractory Castables

  • Less mixing time since gel bond formulations do not require other minor additives or deflocculants like the cement containing castables.
  • Shorter drying time and so reduced drying flaws. This is because water is not added or required for mixing.
  • Better refractoriness because of the absence low melting phases like- anorthite or gehlinite.
  • Colloidal silica being more viscous than the water, help to maintain more separation of refractory particles which, in turn, provide better thermal shock resistance.
  • Better chemical resistance.
  • Because of the various superior properties of gel bond castables / pumpables as described above, they yield longer campaign life, less downtime and so reduce cost of furnace operation.
  • Longer shelf life since there is no hydratable phase as in LCC, ULCC.

Applications of Gel Bond Castables / Pumpables (Gel Bond Monolithic Refractories)

Because of Gel bond Castables / Pumpables (Gel bond monolithic refractories) have been found to give better results in terms of both conveniences of applications as well as properties in almost all type of industries:

  • in cement industries - high temperature rotary kiln burning zone, rotary kiln incinerators lining
  • in glass industries - outside the Glass Melting Tank furnace and sidewalls and roofs
  • in Blast furnace trough - because of the better flowability these can be more conveniently installed by a pump with reduced installation time
  • in Torpedo and other transfer Ladles then Tundish back-up lining, Electric furnace Deltas and Runners
  • in secondary operations like - Reheating furnace hearth, roof. The installation of colloidal silica bonded castables / pumpables has shown significant improvement especially in reheating furnace roof areas, both during installation and drying (which has been found to take about 60% less time than the conventional ramming mixes & plastics)
Related Post: Benefits of using Steel Fibers and Organic Fibers in  Refractory Castables and other Monolithic refractories

Colloidal Silica / Silica Sol

Colloidal Silica or Sol or Silica Sol are the different names, consists of a stable dispersion amorphous silica particles. To achieve this, the silica particles must be small enough such that they are largely unaffected by gravity. Therefore, silica particle sizes are usually of the order of less than 100 nanometers. Initially colloidal silica was used in refractories for the purpose of coating in various applications like ingot casting, investment casting etc. It was during late 80’s when for the first time colloidal silica started to be used as bonding agent in monolithic refractories. During late 80’s refractories based on colloidal silica became available in the market in ramming, gunning and castable formulations. The development of gel bond refractories with colloidal silica as the bonding agent has been a major breakthrough in refractory technology. Since the type of colloidal silica used in refractories is available commercially, it became easy for many to take advantage of this technology. In lieu of conventional binders, colloidal silica can be used as bonding agent in all type of monolithic refractories such as castables, ramming and gunning mixes. Its use in castables has given rise to the convenience of refractory applications by pumping, thus providing a considerable advantage over conventional binders. Another big advantage is that unlike calcium aluminate cement bonded refractories; these refractories do not require following specific temperature parameters for drying and hence reduce drying flaws, installation time. Colloidal silica bonded castables / pumpables not only perform better and reduce costs of furnace operation, but also eliminate work place hazards for workers. The nano sized particles of colloidal silica, due to their higher viscosity consistency, maintain uniform inter-particulate distances resulting in increase of the permeability of the mix and hence provide smooth and speedy drying as well as improved reactivity also increasing the castable sinterability, promoting mullite formation. Colloidal silica bonded castables / pumpables can be extensively used in blast furnace cast house refractories (Alumina - Silicon Carbide - Graphite formulations) as well as for all other applications as mentioned above.