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

Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories

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5-Jan-2009

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:

  • 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)

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.

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


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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