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

https://www.industry.guru - 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.

https://www.industry.guru - 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

Importance of Tundish Design and Flow Modifier Refractories in Steel Making | Refractory Industry Guru

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 23-Sept-2020


www.industry.guru - Steel Tundish labelled image
To transfer finished melt steel from a ladle to mould in a continuous casting process, an intermediate vessel is used which is called tundish. The role of tundish is to deliver the molten metal to the moulds evenly and at a designed throughput rate and temperature without causing contamination by inclusions. Inclusion float out, slag vortexing, till end slab volume and residual metal in tundish are a strong function of tundish hydrodynamics. Tundish design as well as flow control devices / modifiers are known to have strong influence on tundish hydrodynamics. 



www.industry.guru - images of Tundish Flow Modifier Refractories


One of the major functions of steel making tundish is to enhance inclusion floatability and thereby, produce clean steel. For the removal of inclusion through floatation, wall adhesion and agglomeration the flow patterns inside the tundish play an important role, which in turn

Melt flow in any given tundish can be favourably altered by incorporating suitable tundish flow modifiers (TFM) and/or changing the design of the tundish. The flow modifiers play an important role in promoting the floatation of nonmetallic inclusions in steel.

Now-a-days refractory makers are offering customized refractory solution. The new age tundish refractories facilitate temperature homogenization, removal of macro-inclusion, prevention of nozzle clogging etc. inside tundish. To streamline the flow and compress turbulence inside tundish various Flow Control Devices (FCD) are being used in place of traditional FCDs or tundish furniture like Dams, Weirs, Charge Pads, and Side Wall Pads etc. 


Industry Guru - Used Steel Tundish image
The next generation FCDs are popularly known as Tundish Flow Modifier (TFM), Tundish Flow Optimizer (TFO) etc. are precast refractory shapes made of Ultra Low Cement Castables (ULCC) having 85 - 90% alumina. The interior of tundish flow modifiers or flow optimizers as you say it, are designed in such a way that incoming steel gets a churning effect which results into inclusion flotation and subsequent absorption at the tundish powder level. Tundish argon diffusers are also being used to reduce inclusion in steel.

Eventually, it is tundish design from the viewpoint of metal flow and appropriate selection of refractory materials with their right positioning inside tundish that holds the key to the success of subsequent operations in steel making.


Kyanite - Properties and Indian Occurrences

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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
https://www.industry.guru/2020/07/kyanite-properties-and-indian-occurrences.html - 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:


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)

  
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
--
--
--
--
--