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An introduction to MgO-C refractory bricks

An introduction to MgO-C refractory bricks
MgO-C refractory is widely used in steel making application, mainly in steel ladles , LD converters, electric arc furnaces and also in secondary steel making. It is a basic refractory with superior slag /metal corrosion and penetration resistance and excellent thermal shock properties at high temperatures. In steel ladle applications a carbon content of 8-20 wt% is used. The function of the C is to fill the porous structure, improve the slag / metal corrosion and penetration resistance due to its non-wetting character and enhancement of thermal shock resistance due to its high thermal conductivity and low thermal expansion characteristics. Again formation of a nascent dense layer of MgO at the working surface of MgO-C brick, due to oxidation of Mg (produced on reaction between MgO and C) restricts the penetration of slag / metal components and thereby further improves the penetration and corrosion resistance. But C suffers from poor oxidation resistance and may oxidise to form CO and CO2 resulting in a porous structure with poor strength and corrosion resistance. Prevention of carbon oxidation is done by using antioxidants, which reacts with incoming oxygen, gets oxidised and protects carbon, thus retaining the brick structure and properties. These antioxidants play a vital role in the MgO-C brick performance.


Again use of high amount of carbon in the refractory has many disadvantages too. Higher the carbon means higher thermal conductivity that results more amount of heat los through the refractory. Again higher the heat loss, higher will be the shell temperature of the steel vessel, resulting in higher chances of deformation of shell and reduction of ladle life. Also higher carbon increases the chances of carbon pick up in steel, which is in contradiction to steel making, a decarburization process. Furthermore use of carbon in refractory will increase the generation of CO and CO2 gases and thus may become a concern for global environment. Hence, globally the researchers and scientists are considering and working for reduction in the total amount of carbon in MgO-C brick without compromising with the final characteristics. The present work is also aimed to reduce the carbon content in the MgO-C refractory brick using nano carbon, replacing the conventionally used graphite. Nano carbon content is varied from 0.3wt% to 1.5wt% and graphite was used up to 5wt%.

Technological improvement in the manufacturing of iron and steel has changed significantly the operating practice. Increase in furnace capacity, operating temperature, hot-metal temperature and throughput are common to all units. These radical changes along with the need of improved practices for better manufacturing and application environment are demanding a new generation of refractory material with improved properties, performance and life with eco-friendliness.

Refractory, a non-metallic inorganic material, with very high melting temperature, excellent mechanical properties both at room temperature and at high temperatures and high resistance to withstand rapid temperature fluctuations, including repeated heating and cooling. They have also good corrosion and erosion resistance to molten metal, glass, slag and hot gases etc. Because of good thermal stability of refractories they are used in kilns, furnaces, boilers, incinerators and other applications in industries like iron and steel, non-ferrous metal, cement, glass, ceramics, chemicals etc.

Many of the scientific and technological inventions and developments would not have been possible without refractory materials. Manufacturing of any metal without the use of refractory is almost impossible. The ASTM C71 defines the refractories as “nonmetallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 1000 oF (538 0C)”. The type of the refractory to be used is dictated by the conditions prevailing in the application area. Generally refractories are classified into two different groups:(a) based on raw materials , the refractories are subdivided into three categories such as acidic (Zircon, fireclay and silica), basic (dolomite, Magnesite, magnesia-carbon, alumina-magnesia –carbon, chrome-Magnesite and Magnesite- chrome) and neutral (alumina, chromites, Silicon carbide , carbon and mullite) and (b) based on manufacturing process , the refractories are subdivided into two categories such as shaped refractories (available in the form of different brick shapes ,and includes the oxide and non-oxide systems) and unshaped refractories which includes mortars, castables and other monolithics).

Refractories of iron & steel industries encounter a very stringent environment. There is a heavy load of molten metal at very high temperature (>1600oC), corrosive slag attack, FeO corrosion from metal, abrasion and thermal/mechanical spalling caused by molten metal and slag, etc. Again there is always a challenge from further improvement in the metal extraction technology up-gradation, higher temperature of operation, longer service life.

Hence use of advanced refractory lining with very high corrosion and spalling resistance, excellent mechanical characteristics even at high temperature with ease of application and enhanced lining life, less down time and environmental friendliness are essentially required.

Being a major consumer of refractory, iron and steel industries control the demand and supply market of the refractory. As the production of crude steel is increasing with time, the production of refractory has also increased significantly. Besides, there has been a drastic change in the refractory technology in recent years. Strong demands are emphasized in various fields; like extended service life of the steel ladles, rationalization, improvement of working environment, energy saving and production of material with higher quality etc. Presently the expected crude steel production in the current fiscal of India is expected to be ~ 70 Million tons. The steel sector in India is growing very fast and expert are expecting a production ~ 100-120 million tons by 2020. Maintaining the pace of the steel sectors, refractory industries are also growing very rapidly.

In tune with the changing trends in steelmaking, especially in ladle metallurgy, the high performing shaped refractories are on an increasing demand in recent years. The higher campaign lives and the variability of the newer steel making operations are decided by the availability and performance of such shaped refractories with superior high temperature mechanical strength, erosion and corrosion resistance. Initially the ladles were used only to transport the steel from steel making unit to casting bay, but now a day the refining process is also carried out in the same. Thus steel producers through the world have been putting on a continuous effort to improve the ladle life in order to increase the performance of ladles as well as reduce the specific consumption of refractories so as to have a strong grip over cost and quality of steel and also to increase the ladle availability with lesser number of ladles relining per day. Due to the above reasons, there had been a great technological evolution in ladle lining concept such as: Zonal lining concept, which deals with both selection of refractory quality and refractory lining thickness.

MgO-C bricks have dominated the slag line of ladles for at least a decade as they possess superior slag penetration resistance and excellent thermal shock resistance at elevated temperature because of the non- wetting properties of carbon(graphite) with slag, high thermal conductivity, low thermal expansion and high toughness. Increased steel production has lead both refractory manufacturers and users to resume interest on further improvement of thermo-chemical properties of MgO-C refractories. Presence of nano (size<100 nm) particles in MgO-C refractories has also improved the durability, thermal shock resistance, corrosion resistance and oxidation resistance.

Again formation of a nascent dense layer of MgO at the working surface of MgO-C brick, due to oxidation of Mg (produced on reaction between MgO and C) restricts the penetration of slag / metal components and thereby further improves the penetration and corrosion resistance. But C suffers from poor oxidation resistance and may oxidize to form CO and CO2 resulting in a porous structure with poor strength and corrosion resistance. Prevention of carbon oxidation is done by using antioxidants, which reacts with incoming oxygen, gets oxidized and protects carbon, thus retaining the brick structure and properties. These antioxidants play a vital role in the MgO-C brick performance.

Again use of high amount of carbon in the refractory has many disadvantages too. Higher the carbon means higher thermal conductivity that results more amount of heat loss through the refractory. Again higher the heat loss, higher will be the shell temperature of the steel vessel, resulting in higher chances of deformation of shell and reduction of ladle life. Also higher carbon increases the chances of carbon pick up in steel, which is in contradiction to steel making, a decarburization process. Furthermore, use of carbon in refractory will increase the generation of CO and CO2 gases and thus may become a concern for global environment. Hence, globally the researchers and scientists are considering and working for reduction in the total amount of carbon in MgO-C brick without compromising with the final characteristics. The lining of ladle depends to a greater extent on the wear rate of MgO-C refractory arising from slag penetration and structural spalling. With the development of secondary refine techniques of steel the production rate of high purified steel has increased dramatically. So it is aimed globally to reduce the carbon content in MgO-C refractory.

Use of nano carbon, having high surface area resulting in a wide distribution of carbon particles even at low percentages, the entire matrix of the brick can be covered. Again the fixed carbon which comes from the graphite and resin in conventional brick has glassy phase in nature but by introducing nano carbon the fixed carbon has graphite phase. Being graphite phase it’s mechanical strength as well as oxidation resistance is better than glassy phase. By introducing nano carbon in the MgO-C refractory the pore size could be further minimized and therefore it is possible to achieve improvement in corrosion resistance. Nano particles are dispersed in the resin, and during the curing time large number of micro cracks are developed due to shrinkage from evaporation of volatile materials. Strength further. In the existing MgO-C Refractories, high thermal conducting graphite promotes the diffusion of the thermal shock profile generated in the refractory body and result in an excellent ability to absorb the mechanical stress due to the thermal expansion and shrinkage, On the other hand, in new technology it was found that flexibility of the nano particle boundaries in the nano structured matrix and nanometer size porosity generated during the dissipation of volatile matter in the resin component by control of the carbonization process, absorbed the thermal expansion and shrinkage generated in the individual refractory particles.

* A Basic idea of MgO-C Refractory:

MgO-C brick is a composite material based on MgO and C and bonded by high carbon containing pitch and resin, with some metallic powder as anti-oxidants to protect the carbon. These MgO-C bricks are made by high pressure and are of unburned type. These are known to posses excellent resistance to thermal shock and slag corrosion at elevated temperatures. Thus these materials have found extensive applications in steel making processes especially in basic oxygen furnaces, electric arc furnaces, lining of steel ladles, etc.

MgO-C bricks have the following features:

1. High refractoriness as no low melting eutectic occurs between MgO and C.

2. Graphite, the carbon source, has very low thermal expansion; hence in the composite form of MgO-C the thermal expansion is low.

3. Graphite, having a unshared free electron, has very high thermal conductivity, which imparts high thermal conductivity in the MgO-C composite,

4. As the thermal expansion is low and the thermal conductivity is high the thermal shock resistance of MgO-C is very high.

5. Non wettability of carbon gives similar character to MgO-C bricks and thus it prevents the penetration of slag and molten steel.

6. Better ability to absorb stress, thus keeping down the amount of discontinuous wear due to cracks.

* Main constituents of MgO-C bricks:

1. Magnesia grains are the main constituent of the system and offer very high resistance to basic slag corrosion, but suffer from poor thermal shock resistance.

2. Graphite imparts non-wetting nature to MgO-C that improves the corrosion and improves thermal shock resistance of the system but it susceptible to oxidation.

3. Antioxidants that tend to retard overall kinetics of oxidation of carbon and improves high temperature strength by the formation of carbides.

4. Binder that keeps the different components of the refractory together. Volatiles from the binder are the first to go out leaving behind carbon.

* Application of MgO-C brick:

Converters:



After the development the MgO-C brick was applied in basic oxygen furnaces. Now a day the entire lining is done by MgO-C brick. These bricks have enhanced the productivity of steel making by increasing the furnace availability. By using MgO-C bricks clean steel can be produced with less refractory consumption.

Table.1: Different MgO-C refractory in different zone of BOF.



Applicationarea

Refractories used

Top conical

Normal MgO-C brick

Tap hole sleeves,

MgO-C

Barrel

More fused magnesia (large crystal) less SWM, carbon content12-13 %

Bottomconical

Less carbon content 8-10 wt%, more SWM

Bottom

Combination ofSea watermagnesia and fused

magnesia with carbon containing 8 wt%.


However, improved operating conditions and repair technologies have significantly extended the service life of the converter linings and hence the amount of MgO-C bricks used for converter linings should decrease in future.

Electric furnaces:



Since the development of Electric furnaces in 1970’s the MgO-C bricks are applied in most of its lining areas. Now they are used mainly in hot spots and furnace bottoms, including theslag line. Now a day they have also been used for bottom blowing plugs, the sleeves of furnace-bottom tap holes and furnace bottom electrodes of DC electric furnaces.

Secondary refining furnaces:

The use of MgO-C bricks has been considered for reduced pressure operations, for example, in RH degasser where the MgO-C reaction has seemed to be a source of trouble. This reaction is more significant at lower pressure at high temperatures. However, the effect of a slag coating on bricks may eliminate the problems at hot surface. Therefore MgO-C bricks may be usable in furnaces operating under reduced pressure.

Ladles:

Refractories used for ladle lining must able to withstand the increasing severity of service conditions associated with the secondary steel making in order to produce various grades of steel with stringent specifications. The condition during the steel refining processes are aggressive, which makes the refractory materials used in steel teeming ladles susceptible to high degree of corrosion. In addition to corrosion, brittle nature of refractory materials gives limitation to their applicability.

Some of the important properties requirements of refractories used in steel ladle are:

  • High corrosion resistance to steel and slag
  • High abrasion resistance by liquid metal
  • High thermal spalling resistance
  • High hot strength and
  • Low molten steel penetration
Fig.1.1: shows the schematic view and various zones of steel ladle. The different working lining designs of the steel ladle is given in table 1.2.



Fig.1.1: Ladle lining with different refractories.



Table 2: Different working lining designs insteel ladlesin India
Area

Bottom

Metal Zone

Slag Zone

Free Board



MgO-C

MgO-C

MgO-C

MgO-C



Al2O3-MgO-C

Dolomite

70% Al2O3

70% Al2O3

Refractory

70% Al2O3

Al2O3-MgO-C

80% Al2O3

80% Al2O3

bricks

80% Al2O3

70% Al2O3

MgO-Cr2O3

Cr2O3-MgO

used

MgO-Cr2O3

80% Al2O3







80% Al2O3)

Cr2O3-MgO






For the past several years, refractories based on MgO and C had performed tremendously well in many applications such as basic oxygen furnace (BOF), electric arc furnace (EAF), varieties of vessels and ladles for secondary refining treatments as compared to bricks without carbon due to high thermal conductivity, low thermal expansion, non-wetting nature and chemical inertness to slag and high thermal shock resistance of carbon present in the system.
MgO-C refractory, one of the highest consumable refractory item in steel sector with a specific consumption as high as 3.0 kg/ton in BOF and 2.5 kg/ton in EAF for the best shop’s practice, is the top most concern for any steel manufacturer. MgO-C refractories are unfired refractory, which is manufactured by mixing magnesia aggregates & fines, graphite and other additives with liquid resin and pitch as a binder. Next the mix is uni-axially pressed using hydraulic press with a specific pressure of 2 T/cm2. The pressed bricks were then tempered at 200 – 400o C for about 12 h to facilitate the polymerization of resin into carbon and to eliminate residual water, volatile matters and phenols, thereby developing sufficient strength. The physical, thermal, thermo-mechanical and thermo –chemical properties of MgO-C refractories have improved significantly by selecting the right raw materials with respect to purity, grain size of MgO, binders, bonding systems and additives in nano range.

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