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

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

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11-July-2020
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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6-July-2020
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:   

https://www.industry.guru - 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%)
Moisture%
Burnt BD
Green BD
Factor (Gr.BD/Br.BD)
40.00
7.00
2.20
2.36
1.073
45.00
6.50
2.25
2.41
1.071
50.00
6.50
2.45
2.62
1.069
60.00
5.50
2.50
2.65
1.060
70.00
5.50
2.60
2.75
1.058
75.00
5.00
2.65
2.78
1.049
80.00
4.50
2.70
2.82
1.044










Refractory composition, production of Magnesia Carbon Refractory Bricks and their properties

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In one of our previous posts on Magnesia Carbon refractory bricks (MgO-C), we have already discussed on the selection of various raw material ingredients required for the production of MgO-C refractory bricks with an emphasis to their grading etc. In this article we bring you one MgO-C (Mag Carbon) refractory brick composition and further guidance (tips) on its production along with the tested results of some of the refractory properties of the brick made from this composition.

Refractory Composition (Formulation) for MgO-C Refractory Bricks

Item No
Raw Materials
Grading (mm)
Quantity
1
China 97FM (Fused Magnesite)
2.0 - 4.0
18 %
2
do
0.6 - 2.0
42 %
3
do
0 - 0.6
20 %
4
China 98FM or Seawater DBM 98 HD
Flour (Ball Mill Fine)
7.5 %
5
Coarse Flaky Graphite (94% LOI)
– 0.10 (> 40 %)
12.5 %
6
Al-metal Powder
3 parts
7
Si-metal Powder
1 part
8
Liq. Resin (Durez-40)
3 parts
9
Solid Resin (Powder)
0.6 parts
10
Hexamine
0.25 parts
11
MEG (Meth. ethyl glycol)
0.6 parts

Mag Carbon Refractory Bricks - Production (Tips)



=> Mixing Sequence:
(A) First co-mix item nos. 4, 5, 6 and 7 together in a Ball Mill for at least 6-8 hrs, unload and keep separate.
(B) Mix MEG with (0-0.6) DBM fraction (item 3). Then add the co-mix powder made in step (A) to it and remix.
(C) Take (2-4) & (0.6-2) DBM grading (item no 1 and 2), add hot Resin (item 8). Mix thoroughly to get perfect coating. Add mixture made in step (B) and remix the batch intensively. Now add powder Resin (item 9) then, Hexamine (item 10) and mix it finally. Keep the mixture (powder) in closed polythene bags 35-40 kg in each bag, for ageing 8-12 hrs.
=> Heat the liquid Resin to 60-65OC before mixing. Heating reduces the viscosity and thus help for better coating.
=> Don't use any mixture after it has dried up. Try to make mixture in perfectly pre-calculated quantities as reuse of such mixture (powder) even by adding more MEG is not advisable and should be avoided as far as possible.
=> While making any shaped brick by pneumatic Ramming it must be done continuously until the shape is complete.

Properties and Test Results

Magnesia Carbon Bricks - Properties
Properties
Results
MgO %
94 (min)
Fixed Carbon %
8 (min)
Green BD (gm/cc)
3.1 (min)
Coked BD (gm/cc)
2.95 (min)
AP %
(after coked after coked in reducing atmosphere at 1000OC )
14 (max)
AP % (original condition)
7 (max)
CCS (kg/cm2)
450 (min)
MOR at room temp (kg/cm2)
140 (min)
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