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

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

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 23-Sept-2020 - 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. - 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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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

Gunning and Spraying - differences in these two methods of tundish wear lining, refractory lining repair and maintenance

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In this article we will try to understand the basics of Gunning and Spraying, two frequently used methods for enhancing the refractory lining life of a furnace or refractory lining repair and maintenance. Also, differences between Gunning and Spraying, preferential situations for either of these two methods in EAF, BOF, melting furnaces, steel ladles, tundish lining, etc.
There are instances when steel plants are found switching over from one method to other for refractory lining repair and maintenance depending upon their perceived and actual benefits obtained. But well-documented published data of such experiences, which can be of immense help for others, are either scanty or sparsely available. In the article A Comparative Evaluation of Different Types of Tundish Lining Refractories based on actual experiences in Steel Plants it has been tried to put together some such experiences made by others and were presented in some recently held different seminars and conferences on Refractories. 


Industry Guru - image of Gunning Maching for Gunnable Refractories
The gunning repair is a well-proven procedure by which refractory material can be applied quickly and cheaply. Initially these were alumino-silicate based and later converted to basic type magnesite based to assist with metallurgical practice.
Gunning or gunnable refractories are used for the hot repair of ladles and melting furnaces as well as relining or cold repair of the back lining. There are two basic methods: dry and wet gunning. With dry gunning the material is discharged from the machine with a maximum of 5% moisture and then fed to the gunning nozzle by an air stream where the required water, typically 5–10%, is added. With wet gunning the gunning material is moistened with water in a mixer and then pumped through a hose by means of an eccentric screw or a piston pump. At the end of the line the material is dispersed with compressed air and, if necessary, an additional liquid bonding/hardening agent can be added (shotcreting). Wet gunning has the problem that the machine or the gunning hose may get clogged by already moistened refractory material, particularly when not in continuous use; hence the process has to be carefully controlled. Also, the equipment requires more intensive cleaning and is not considered an efficient operation for applications less than 400 kg. With dry gunning, blockages in the conveying hose can be blown free by compressed air only. For optimum gunning and refractory performance the gunning material is essentially the same as the original refractory (plus binder material), but with a size distribution of 4 mm maximum. The applied thickness is typically in the range 10–30 mm and a vessel can normally be used again after 3–5 minutes.
Mixing of Water and Gunnable Refractory Materials
Three essential requirements for a good gunning repair of refractory lining are optimal moistening, homogeneous mixing of the gunning material with water, and a high quality gunning machine that guarantees even conveying. With pre-moistening a share of the gunning water is added some meters away from the gunning nozzle and relies on the turbulence within the conveyance for premixing. This approach, however, is very susceptible to operational problems. For instance adding too much water in advance may lead to clogging, particularly with quick binding systems where hardening starts inside the hose. Too much water reduces refractory quality and hence refractory lifetime. Also, the water can dissociate in the liquid steel to hydrogen, which can be detrimental to some steel grades (micro-cracks). With standard mixing heads (for dry gunning) the water is jetted through radial borings and, in order to compensate for inadequate initial moistening, the operator often works with a surplus of water so that the dust is minimized. This, however, often leads to use of incorrect water/ cement ratio, resulting in reduced refractory durability. Improvements can be achieved by use of pressure increasing pumps so that the water jet becomes sharper and more readily reaches the centre of the nozzle cross-section. Optimum nozzle-vessel distance is quite varied; between 40 and 1,000 mm depending on application.
Standard gunning refractories are magnesia, alumina or silica based refractory materials normally a monolithic applied on the areas that encounter severe wear out such as trunnions, scrap impact area and the slag line. These materials differ in price so the optimum choice depends on many factors such as steel grade, slag composition and the vessel used. A shooter type of gun is used for the gunning process to encounter hostile environment of the process. Consumption of gunned refractory is also extremely varied, ranging from 0.2-2 kg per ton steel depending on conditions of the vessel and environment.
Also read: Nomenclatures and Terminologies of Monolithic Refractory Products based on their setting or installation methods  


The principles, procedure and differences between Gunning and Spraying can be understood by the flow diagram below: - Gunning vs Spraying of refractories depicted through flow diagram
Gunning materials or gunnable refractories, refractory castables are transported through flexible hoses to the installation position, where the materials are wetted and projected through a handheld nozzle at the target area. Dry gunning allows operating farther away from the feed station, though wet gunning or shotcreting offers a faster output rate.
Since homogeneous mixing is possible in spraying (before the product is applied), the incorporation of special chemical additives can help to improve thermal stability properties of the lining and also impart good flexibility. - Refractory Spraying machins image

The most commonly faced problems of using a gunning machine or in the process of gunning are -
  • Dust formation during gunning
  • High rebound losses leading to wastage and high consumption of material
  • Difficulties in applying variable thickness leading to metal penetration and insufficient permanent refractory lives, and
  • Difficult deskulling.
Whereas when the material is applied through spraying it has the following benefit -
  • No dust formation during application
  • No rebound loss hence minimal loss of material
  • The lining thickness was better controllable, thus increasing permanent refractory lives
  • Deskulling was better.