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

Effects of Compacting Pressure on Sintering and other Properties of Refractory Bricks

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We assume that the reader is already aware with the concept of ‘Sintering’, types of sintering and also the effects of sintering on refractories. In this article we will discuss on the effects of compacting pressure also called forming pressure, on sintering and various other properties of refractory bricks.
It has been established much before by Budnikov and Blyumen that sintering processes and reactions in the solid-state are interrelated and proceed with on the phase boundaries, as in a heterogeneous system. The basis of sintering, according to their broad definition, is the capacity of the solid phase to recrystallize, which, in turn, is related to the physiochemical nature of the crystal. Pressure is said to be an important factor in accelerating reactions in solid state and in facilitating sintering at relatively low temperatures in a refractory brick.

Precautions must be taken to eliminate any pressure variation during compaction of the refractory shape. The main deleterious effect of variation in compacting pressure is the corresponding differences in greenbulk density resulting into non-uniform shrinkage after firing and some sort of distortion of warping is inevitable. The frictional force between the die wall and the powder is directly proportional to the radial stress at the wall. During a uniaxial pressing, the applied stress is in the axial direction and is parallel to the die (mould) wall. For a given axial stress the resultant radial stress depends on the fluidity of the powder under compaction. For example both the radial and axial stresses are equal when a liquid is compacted. However, when a non-elastic and incompressible solid is under axial compaction, there should not be any radial stress. Thus, it is desirable to decrease the powder fluidity in order to minimize the radial and frictional stresses or the density and stress gradients in the refractory brick.
There is no doubt that the forming pressure affects the firing behavior of the refractory materials. Such effects may be due to:
>> Decrease in pore size and better particle contact,
>> Strain energy added due to plastic flow,
>> Strain energy added due to particle interlocking, or
>> Fracture of particles at contact points.
In general increasing pressure enhances the Green Density, decreases Shrinkage, and often increases the Fired Density of refractory bricks. Higher compacting pressure (compaction) may cause plastic flow, increased strain energy, or particle fracture, which causes further increase in bulk density in refractory bricks. The effect of these variations on firing properties of a refractory brick depend on the firing time and temperature, and the nature of the refractory aggregates or refractory raw materials used, but in general decreased pore size due to compaction or particle fracture leads to increased density at lower firing temperature in a refractory brick.        

Ideal Grain Size Distribution of Refractory Raw Materials Mixture for making Magnesia Carbon Bricks

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Mag Carbon bricks - representative image
Mag Carbon Bricks
Magnesia carbon refractory bricks (MgO-C) or Carbon containing Magnesite refractories have been extensively used by steel makers for the secondary treatment of steel in basic oxygen furnaces, electric arc furnaces, and ladle furnaces. Mag Carbon refractory bricks are widely used in slag lines of BOF (Basic Oxygen Furnace) because of their superior wear resistance. The service life of Magnesia Carbon refractories used in BOFs have been pushed quite significantly (largely due to slag splashing and gunning improvements) even as the service conditions have become more severe due to the increased operating temperature required for continuous casting and the need to produce cleaner steel.
Mag Carbon bricks are made of high purity magnesia, high quality graphite, antioxidants and some additives with a suitable binder (bonding agent). Selection of raw materials, their grading and grain size distribution (Granulometry) and composition together have ultimate role in the development of various physical properties, microstructure and ultimately thermo-mechanical properties of Mag Carbon refractory bricks (MgO-C). Various different types of MgO (Magnesite) grains provide different levels of corrosion resistance.
From the literature and plant applications it has been established that Magnesia-carbon bricks having 3 mm particle size show better wear resistance as well as a few other characteristics as compared to the bricks with 5 mm size grains.
Graphite, Anti-Oxidants (Additives) and Binders used in the composition of Magnesia Carbon Bricks
The graphite flakes used in these bricks impart -
=> High thermal conductivity
=> Good thermal shock resistance
=> Low thermal expansion
=> Non-wettability by liquid slag
=> Low corrosion rates by slags
Graphite contents of typical bricks range from 4 - 35% natural flake graphite. Since oxygen affinity of carbon is very high so different kinds of antioxidant minerals are used (in fines or superfines) in order to protect refractory material against chemical corrosion. The REDOX reactions in magnesia carbon can be reduced by selection of high purity magnesite, large crystal size and use of graphite with low impurities. Slag corrosion resistance of MgO-C refractories can be improved by use of magnesite grains with less reactivity i.e. fused magnesite grains of high Bulk Density (BD) and high purity.
The above are some of the reasons which explain how selection of various raw materials can affect the performance of magnesia-carbon bricks. More on this aspect and the compositions of Magnesia-carbon refractory bricks will be discussed in a separate post:

Here, our topic is Granulometry i.e. overall grading and the grain size distribution, suitable for the best performance of MgO-C bricks. Grading and the grain size distribution are important as these are directly related with the following properties of Magnesia-carbon bricks:
=> Porosity
=> Mechanical strength
=> Spalling resistance
=> Microstructure and phase development
=> Wear resistance    
From the experience of various trials and performances it has been found that 0 - 4 mm grading is the best for MgO-C refractory bricks for all general applications and also for different shapes like Tap Hole Blocks, Sleeves, etc. (except Slide Gate refractories which will be different).
Bonding agents or binders used in Mag Carbon bricks and other carbon refractory products are immiscible with graphite and other refractory raw materials. At room temperature, they rely on binder to cure. Generally these binders or bonding agents are resin, asphalt or an organic matter and can be divided into three types: phenolic resin, modified asphalt, petroleum cracking by-product category.
The grain size distribution (granulometry) of the press mixture (powder) for MgO-C bricks with different Graphite percentages as they should be are given in the following table: - representative image

Green BD (Bulk Density), Burned BD and Moisture Content of Green Mixture (Powder) of Alumina Refractory Bricks

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Before going into large scale production of any product, based on their R&D work and the results obtained from pilot scale production laboratory has to fix certain parameters (recipe of production for that product) and provide it to the production department which the latter will follow in order to meet the required specifications and quality of that product. Two such important parameters required for manufacturing any type of refractory bricks are the ‘Moisture content’ of the green mixture (powder) and ‘Green BD’. Here our focus will be on the practical aspects of these two parameters of refractory bricks particularly those containing various percentages of alumina (Al2O3%). 

The amount of moisture or water present in the green mixture (powder) of alumina containing refractory bricks help in green binding and also add plasticity. But beyond a certain percentage, water or moisture present in the green mixture of refractory bricks would be harmful leading to an increase the rejection percentage mainly due to -

(i) Increase in porosity and
(ii) Development of radial cracks in refractory bricks.

What should be the maximum percent of moisture, green BDs corresponding to the different values of burned BDs of refractory bricks having different Alumina content (Al2O3%) etc. are given in the following table:

Refractory Bricks (Al2O3%)
Burnt BD
Green BD
Factor (Gr.BD/Br.BD)