There is some confusion which can arise from the term magnesia (MgO) refractory. The American Society for Testing and Materials (ASTM) defines magnesia as ‘a dead-burned refractory material consisting predominantly of crystalline magnesium oxide’, but that is not the terminology used at every place. The terms magnesite, magnesia and MgO are sometimes used inter-changeably to indicate MgO. In addition, older studies have used the term ‘periclase’ for MgO, even though this is the name of the naturally occurring mineral of that composition. Over time different raw materials have been the source for the MgO used in refractories. These changes have been driven by technology, economics, availability, and the expectation of continuously improving refractory properties. Today, majority of the magnesia refractories contain sintered and / or fused grain, both being a synthetic product.
Magnesia-carbon (MgO-C) refractory is important and widely used refractory in the steelmaking. It is characterized by a reduced slag infiltration depth and a high thermal shock resistance due to its carbon (C) content. Applications of this refractory not only include wear linings in basic oxygen furnace (BOF), electric arc furnace (EAF), and steel ladle, but also functional products such as purging and taphole bricks. The success of the use of basic MgO-C refractory in the steelmaking is based on its good properties of resistance to erosion, corrosion, and thermal shock.
The development of higher-purity magnesia grains and a well engineered microstructure have led to dramatic improvements in lining life of the BOFs. Resin-bonded magnesia–graphite refractories are composed of high-purity magnesia filler grains of different sizes, antioxidant metal powder, and natural graphite flakes which are bonded by a phenolic resin.
Carbon is incorporated by natural graphites and its impurities are important since they influence its oxidation. To minimize oxidation, antioxidants are added (non-oxide powders) which react with oxygen to form gasses which move in opposite direction to the entry of oxygen, retarding the diffusion of oxygen into the refractory. Moreover, bond material is needed to moisturize ceramic grains and graphite to press them properly. For this reason, resins and / or derivatives of the pitch are used. Although the bond contributes with carbon, which protects the refractory before the attack of the iron oxide and calcium oxide, the resins and / or derivatives of the pitch burn easily and the protection appears to be transient.
Since, at least, the 1870s pressed basic raw materials, in particular burned dolomite, mixed with pitch have been available to line vessels for use in different processes of the steelmaking. The arrival of the BOF in the late 1950s and early 60s was accompanied with developments in tar bonded dolomite, magnesium- dolomite and magnesia bricks. High strength burned magnesia brick was introduced in the late 1960s and by the 1970s BOFs were being lined with pitch bonded magnesia brick and also burned and pitch impregnated magnesia brick. The latter became the standard for the charge pad and other high wear areas and this was the beginning of zoned linings. Pitch bearing brick needed some manufacturing fine tuning in order to facilitate their installation at the user’s site as excess pitch covered the brick, bonding them together while in transit to the installation site, made them slippery to handle, and modified their dimensions. During this time period, the expected lining life of BOF was in the range of 200 heats to 500 heats.
By the late 1970s, the need for better refractories became acute as a result of moving from ingot casting to continuous casting which increased the maximum temperature of the liquid steel in the BOF from the 1,500 deg C-1,600 deg C range to 1,650 deg C or 1,700 deg C or even higher. In addition, changes in steelmaking processes added stresses to different steelmaking vessels further increasing refractory wear.
MgO-C bricks were developed for use on the hot spots of EAFs and began to be applied to BOFs in the late 1970s. This entailed the replacement of pitch with resin and allowed for a dramatic increase in the carbon level as a result of graphite additions. Subsequently, antioxidants were added to the MgO-C brick to protect the carbon from oxidation.
Since MgO-C bricks were found to have higher durability than dolomite-carbon bricks which were mainly used for the BOFs then, they have gradually increased their applications as BOF refractories. The use of MgO-C refractories in the high wear areas of the BOF led to a considerable reduction in refractory consumption and these bricks were soon being incorporated into other areas as well until the whole BOF was lined with the MgO-C bricks. Although refractories are an important component of the improvement in BOF lining life, other changes such as better temperature control, development and improved control of slag chemistry, implementation of slag splashing technology, and improved refractory repair (guniting) mixes, etc. have all contributed to the ever increasing BOF lining life. Today, MgO-C bricks are widely used as steelmaking refractories, not only for BOFs and EAFs but also for vacuum degassing units and steel ladles, among other equipment.
Raw materials for MgO-C bricks
The main raw materials used in the production of MgO-C bricks are magnesia aggregate, graphite, resins, and other additives.
Magnesia aggregate – Magnesia is the main constituent of MgO-C bricks which contains around 80 % or more of the total batch. Three different types of magnesia raw materials are used to produce MgO-C brick. These are (i) sea water magnesia produced by firing Mg(OH)2 (magnesium hydro-oxide), extracted from sea water, (ii) sintered magnesia produced from natural magnesite (MgCO3) by sintering, and (iii) fused magnesia produced by melting of magnesia. The purity, crystal grain size, crystal grain boundary flux lime / silica ratio, and other properties of the magnesia aggregate affect the corrosion resistance of MgO-C bricks. Hence, it is necessary to select appropriate magnesia aggregate types to suit specific lining areas of the BOF.
Sintered magnesia, also called dead burned magnesite, is produced by heating, sintering, a magnesium (Mg) bearing mineral or a synthetic compound to drive off volatile gasses. When the resultant product is further beneficiated by melting in an EAF, it becomes fused magnesia. The quantity and chemistry of the impurities present affect the high temperature grain properties. Several studies discuss the properties of magnesia grain, the changes which have taken place in its production over time and the effect of impurities at high temperatures. However, it is important to note that there has been a continuous improvement in magnesia grain quality. This has resulted in the availability of bigger grains with higher density and the continuous lowering of the silica (SiO2) and other impurities which lead to low melting point phases at high temperature. The decrease in silica can be observed in the changes in lime (CaO) to silica ratios from 2.6, in the early days, to around 4 with improvements in technology and the present level of 10 or higher. Use of these improved grains has contributed to the production of bricks with ever better physical properties and slag resistance. Tab 1 gives typical specification of magnesia aggregates.
|Tab1 Typical specification of magnesia aggregates|
|Bulk specific gravity||3.46||3.4||3.54||3.2|
|Radius of periclase||Micro-meters||200 minimum||20-40||50 minimum||20-60|
Several studies have been carried out on the effect of magnesia aggregates on the corrosion resistance of MgO-C brick. For superior corrosion and abrasion resistance of the final MgO-C bricks the magnesia aggregate is required to have several characteristics such as (i) large periclase crystal grain to reduce the extent of the grain boundary, (ii) high lime / silica ratio and small content of B2O3 (boric oxide), and (iii) high purity and minimum impurity of magnesia. Because of these reasons fused magnesia grains with high purity and large grain size show better corrosion resistance than sintered magnesia.
In MgO-C brick, carbon plays a very important role by providing non-wetting nature to the refractory. Graphite is used as the carbon source since it shows the highest oxidation resistance among different commercial sources of carbon. Carbon gets oxidized in oxidizing environment which results a porous structure with very poor strength. So, resistance against oxidation is very important for the carbon source. Due to the flaky nature of graphite, it imparts higher thermal conductivity and lower thermal expansion, resulting in very high thermal shock resistance. Along with the increase of the graphite content, the compressibility increases during pressing and results in a decrease in the porosity. Fine graphite particles are more effective to improve the corrosion resistance of the refractory bricks.
The strength of MgO-C bricks particularly during heat treatment has also been reduced by the bigger particle size of graphite. Purity of graphite is also an important factor. Impurities react with MgO and form low melting phase which results in lower corrosion resistance and also lower hot strength. The role of graphite in MgO-C bricks can be summarized as (i) it covers the spaces in between magnesia grains and fills the porous brick structure, (ii) it prevents the slag penetration into the brick due to the high wetting angle between slag and graphite, (iii) at higher temperature, magnesia is reduced to pure magnesium by carbon and the vapourized magnesium comes to the surface of the brick and reoxidized to magnesia and the formation of this dense layer of MgO and CO at the slag- brick interface prevents further penetration of oxygen into the brick, (iv) it improves the thermo – mechanical properties and spalling resistance of the bricks because of its high thermal conductivity and low thermal expansion with the size of graphite has also a great role in improving the abrasion, corrosion, and oxidation resistance of MgO-C bricks, and (v) carbon reduces Fe2O3 (ferric oxide) to FeO (ferrous oxide) and further reduction of FeO produces metallic iron (Fe), enriches the production of steel and it helps since slag containing Fe2O3 has higher corrosive action than the slag containing FeO.
Natural flake graphite is normally used for MgO-C bricks. Graphite in MgO-C bricks is not easily wetted by slag and suppresses slag infiltration. It also prevents spalling with its high thermal conductivity and low coefficient of thermal expansion. In addition, CaO and SiO2 contained in the ash of graphite migrate to the boundaries between graphite and magnesia aggregate at high temperature and form low-melting point compounds. The low-melting point compounds are considered to form liquid phases, reducing the hot modulus of rupture of MgO-C bricks and facilitating the dissolution of the magnesia aggregate into the slag. High-purity graphite is frequently used as a component of MgO-C bricks for the lining areas of BOF where high corrosion resistance is needed (Fig 1a).
Fig 1 Magnesia-carbon brick characteristics
Resins – Because of flaky and non-wetting characteristics of graphite, it is very difficult to produce a dense brick without a strong binder. In the early days, pitch was used as binder for MgO-C brick. But during operation, pitch releases large quantities of volatile matters, which are very toxic due to their high content of poly-crystalline aromatic compound (PAC) like benzoalpha-pyrenes. Also to use pitch hot pressing of the mixture is necessary.
Resin is found to be the best binder for MgO-C refractories because of its properties such as (i) it contains high quantity of fixed carbon which gives strong bonding property, (ii) it has a high chemical affinity towards graphite and magnesia grain, (iii) it possesses high dry strength because of its thermosetting nature, (iv) it produces less hazardous gas than tar / pitch, (v) at curing temperature (around 200 deg C), it polymerizes which gives isotropic interlocking structure, and (vi) cold crushing strength (CCS) increases with the increase of resin content.
Phenolic resin is normally used as binder for magnesia aggregate, graphite, and other raw materials in MgO-C bricks. The phenolic resin (i) has high affinity for and is kneaded well with graphite and magnesia aggregate, (ii) is high in fixed carbon and forms strong carbon bonds, and (iii) is less harmful to environmental health than tar / pitch. Phenolic resin comes in two types namely (i) the thermosetting ‘resol’ type and thermoplastic ‘novolac’ type. The type to be used is determined by considering the manufacturing process, manufacturing equipment, and other conditions.
The desired viscosity of resin is to be around 8,000 cps (centipoise), which ensures proper mixing of the other raw materials. Viscosity of resin is quite sensitive to temperature which increases with decrease in temperature. In winter, viscosity of resin increases and this causes low dispersion of ingredients in the mixer machine, whereas in summer due to high temperature, viscosity decreases which gives less strength in the green body and creates lamination. Powder novolac resin is normally used to overcome this type of difficulty. Compressibility during pressing improves with the increase in resin content and consequently the CCS of the tempered bricks increase. The resol type resin is best as binder among different resin types. Because of its lower viscosity and lower content of volatile species the bricks containing resol has the lowest porosity after heating at high temperature.
Other additives – Carbon components contained in MgO-C bricks are oxidized by oxygen and carbon di-oxide in the environment or by iron oxide in the slag. Antioxidants such as metals are added mainly to suppress this oxidation.
The main issues of using MgO-C bricks are the lower oxidation resistance as well as the poor mechanical strength of graphite which causes loosening of the constituents at high temperature leading to reduced wear resistance as well as resistance to chemical corrosion. The oxidations of carbon in MgO-C refractories happen in two ways namely (i) direct oxidation, and (ii) indirect oxidation. Direct oxidation occurs below 1,400 deg C and carbon is oxidized directly by atmospheric oxygen. Indirect oxidation occurs above 1,400 deg C and carbon is oxidized by the oxygen from MgO or slag. The resulting magnesium (Mg) vapour oxidizes again above 1,500 deg C and generates MgO which is called the secondary oxide phase or the dense layer. This dense layer gives rise to resistance to further oxidation. The reactions involved are (i) 2C(s) + O2(g) = 2CO(g), (ii) C(s) + MgO(s) = Mg(g) + CO(g), and (iii) 2Mg(g) + O2(g) = 2MgO(s). Hence, to prevent oxidation of carbon, different antioxidants such as magnesium, aluminum (Al), silicon (Si), boron carbide (B4C) are used in MgO-C refractories. Because of lower cost and higher effective protection, aluminum and silicon antioxidants are mostly used.
During the operation, aluminum metal reacts with oxygen and forms alumina (Al2O3) in the form of a fine layer. At the melting temperature of aluminum (660 deg C), liquid aluminum breaks the layer of alumina and reacts with the surrounding carbon to form aluminum carbide as shown in equations (i) 2Al(s) + 3O2(g) = Al2O3(s), (ii) 4Al(l) + 3C(s) = Al4C3(s), and 2Al (l) + 3CO(g) = Al2O3(s) + 3C(s). At a temperature higher than 1,000 deg C, Al4C3 reacts with CO to form Al2O3. Alumina directly reacts with surrounding magnesia and forms MgO-Al2O3 spinel as shown in equations (i) Al4C3(s) + 3CO(g) = 2Al2O3(s) + 9C(s), and (ii) MgO(s) + Al2O3(s) = MgO.Al2O3(s). In case of silicon antioxidant, the reaction is Si(s) + C(s) = SiC(s).
Boron carbide performs much better oxidation resistance than aluminum. Boron carbide reacts with carbon mono-oxide (CO) to form boric oxide (B2O3). Then this boric oxide reacts with MgO and produces a liquid phase compound MgO.B2O3 as shown by equations B4C(s) + 6CO(g) = B2O3(l) + 7C(s), and B2O3(l) + 3MgO(s) = 3MgO.B2O3(s). In this way boron carbide protects the carbon of MgO-C bricks and improves the lining life.
The steps in the production process of MgO-C bricks are described here. First, the raw materials are graded into coarse, medium, and fine sizes and are classified as necessary. Next, they are mixed and kneaded with a binder in pre-determined blend proportions by particle size. The kneaded mixture is press formed into bricks. Fig 2a shows typical flowsheet of production process.
Fig 2 Production process for MgO-C bricks
Uniaxial forming with an oil press or a friction press is normally used as the pressing equipment. The magnesia aggregate and graphite in MgO-C bricks show orientability depending on the forming direction of the press. The strength and thermal conductivity of MgO-C bricks show anisotropy (Fig 2b). It is hence important to consider the forming direction of bricks when laying the bricks. A cold isostatic press (CIP) with small anisotropy is also used for the production of large refractory products such as bottom blowing tuyere bricks and taphole bricks.
Formed bricks are dried to remove moisture and other volatile components, processed, and coated as needed, visually inspected for cracks, chips, and other defects, and dispatched after removing defective bricks.
Wear mechanisms of MgO-C bricks
The basic wear mechanisms of MgO-C bricks are roughly classified into (i) corrosion, (ii) oxidation, (iii) MgO-carbon reaction, (iv) spalling, and (v) abrasion.
Corrosion – The dissolution and elution phenomena of magnesia aggregate by slag can be divided into (i) dissolution and elution of the magnesia aggregate by intrusion of the silica and lime components into the periclase grain boundaries in the magnesia aggregate, and (ii) dissolution of the periclase by diffusion of the ferrous oxide component into periclase crystals (melting point reduction by formation of MgO-FeO complete solid solution). These phenomena proceed at the same time. In any case, the dissolution and elution phenomena of the magnesia aggregate into the slag highly affect the wear mechanism of MgO-C bricks. This is supported by the fact that high-purity raw materials and electro-fused magnesia with few grain boundaries are applied to badly damaged areas, that the MgO content of the slag during blowing is intentionally increased, and that the wear rate of bricks is reduced by coating with the slag whose MgO content is adjusted.
Oxidation – The carbon contained in the MgO-C bricks plays the role of suppressing the penetration of slag components into the bricks, but it also has the drawback of being oxidized. The carbon oxidation phenomena can be divided into three types namely (i) liquid phase oxidation, (ii) gas phase oxidation, and (iii) oxidation of carbon by MgO (MgO-carbon reaction). Liquid-phase oxidation is mainly caused by iron oxides in the slag. The iron oxide concentration in the slag has a big influence on the wear rate of MgO-C bricks. As expressed by the reaction equation of FeO(s) + C(s) = Fe(s) + CO(g), this phenomenon gasifies the carbon comprising the matrix of the brick and induces the structural embrittlement of the brick. Fig 3a shows an example of liquid phase oxidation. Highly brilliant Fe precipitates are confirmed in the void layer below the working surface or immediately below the void layer.
Fig 3 Wear mechanisms of MgO-C bricks
Gas phase oxidation is the phenomenon by which the carbon in the brick matrix burns. It is caused by oxygen and carbon di-oxide (CO2) in the environment. Normally, in BOFs, gas-phase oxidation is likely to become a problem in the BOF cone which is not adequately protected with slag and is easily exposed to air. A common remedy is the preliminary addition of active metal powder or similar material to the brick mixture.
MgO-carbon reaction – The MgO-carbon reaction is a phenomenon likened to the wear mechanism of MgO-C bricks. The oxidation reaction of carbon in MgO (MgO-C reaction) is given by the equation MgO(s) + C(s) = Mg(g) + CO(g). Whether this reaction proceeds to the right depends on the temperature, partial pressures of magnesium and carbon mono-oxide. The reaction is controlled by the dissipation rate of Mg(g) and CO(g) from the working surface of the lining. In the equilibrium state, where each partial pressure is 0.1 MPa, the above reaction starts at 1,850 deg C. If either or both of Mg(g) and CO(g) fall below 0.1 MPa, the reaction proceeds from the left to the right. In a refractory, which can be regarded as an open system, formed Mg(g) diffuses and the partial pressure of magnesium in the refractory decreases considerably. As a result, the above reaction occurs at a considerably low temperature and causes the structural embrittlement of the refractory.
Spalling – Spalling damage is classified into thermal spalling and mechanical spalling. Fig 3b shows the relationship between the graphite content and spalling resistance. Normally, higher the content of graphite with high thermal conductivity, the smaller the temperature gradient in the thickness direction of the refractory lining becomes. That is, the thermal expansion difference in the refractory lining decreases and the spalling resistance improves. BOFs are normally lined with MgO-C bricks with a graphite content of 15 % to 20 %. Given the large effect of equipment availability, MgO-C bricks with higher graphite contents are frequently used in intermittently operating EAFs, for example.
Mechanical spalling is caused by the thermal stress produced when the refractory lining thermally expands under restraint conditions. Mechanical spalling is likely to occur in the BOF lining after a relatively few heats. Normally, the BOF lining continuously peels off in the circumferential direction. If mechanical spalling occurs, the stress concentrations in the BOF lining are mitigated by measures such as providing expansion allowance joints, changing brick allocations, and adjusting the number of joints.
Abrasion – Among the damage of MgO-C bricks in BOFs, abrasion damage by the liquid steel is likely to occur especially in the bottom and taphole areas. These lining areas are characteristic in (i) that the slag and liquid steel coexist and flow together, (ii) that the slag coating layer is difficult to form, and (iii) that the liquid steel flow causes the dislodgement and outflow of graphite and magnesia aggregate pieces. Fig 4a shows the relationship between the wear by the liquid steel and the hot modulus of rupture (HMOR). The wear by the liquid steel is decreased as the hot modulus of rupture increases. The hot modulus of rupture can be effectively improved by structural densification or metal addition.
Fig 4 Characteristics of MgO-C refractory bricks
Characteristics of MgO-C bricks and their application to specific lining areas of BOFs
In the design of the refractory lining of the BOF, the respective lining areas differ in the damage mechanism, frequency, and quantity. Zoned lining is normally adopted for changing the thickness and quality of MgO-C bricks in different lining areas to make the overall damage balance as uniform as possible throughout the refractory lining of the BOF. Tab 2 shows the main wear mechanisms and the needed properties for the specific lining areas of the BOF.
|Tab 2 Wear mechanisms and needed properties for specific lining areas of BOF|
|Zone of BOF||Main cause of wear||Mainly required properties|
|Mouth and upper cone||Mechanical damage of skull removal, and oxidation by air||o||o||o|
|Taphole||Oxidation by air, and abrasion by liquid steel stream||o||o|
|Slag line||Corrosion by slag||o|
|Charging side||Mechanical damage by scrap charging, abrasion by hot metal stream, and thermal spalling||o|
|Trunnion side||Corrosion by slag, and abrasion by liquid steel||o||o||o|
|Lower cone||Corrosion by slag, and abrasion by liquid steel||o||o|
|Tuyere||Thermal spalling, and back attack by injected gas||o||o|
The throat and cone areas of BOF have damage problems, such as gas phase oxidation, physical impact during deslagging, and cracks. These are because of the thermal expansion of the barrel. The dislodgment of bricks is prevented by such measures as adding SiC (silicon carbide) as an antioxidant, employing anchors driven into the steel shell to secure the bricks, and using metal cases for fusion bonding.
Damage of the taphole sleeve is dominated by abrasion by liquid steel flow and is considered to be accelerated by repeated heating and cooling during operation and by gas phase oxidation. The durability of the taphole sleeve is improved by adjusting the antioxidant addition and increasing the hot modulus of rupture.
Slag corrosion is dominant in the slag line, trunnions, and steel bath areas. Improvements have been made, such as densification by changing the particle size composition and binder type, suppression of structural deterioration because of cyclic thermal loading, and use of lime containing aggregate with good slag coating properties.
The charging pad is subjected to mechanical impact when the hot metal is received from the hot metal ladle and when scrap is charged as a cold iron source. The MgO-C bricks for the charging pad area are increased in strength by decreasing the carbon content and increasing the metal addition content. Concerning spalling damage, study reports are available for improving the spalling resistance by changing the binder and flake graphite types.
Factors causing damage to the bottom tuyeres include graphite oxidation, slag corrosion, and spalling. Another factor is mechanical damage because of the back attack of the bottom blown gas and due to the flow abrasion by the liquid steel. The bottom tuyere area is made with the MgO-C bricks which have higher graphite content than that of the bricks used in the walls. These bottom tuyere MgO-C bricks also have additives made to prevent the oxidation of graphite and to improve their strength.
MgO-C bricks have high resistance to corrosion, spalling, and slag penetration thanks to the characteristics of their constituents. Furthermore, their strength and physical properties, corrosion resistance, and spalling resistance can be considerably changed by changing the particle size composition of MgO aggregate and the addition contents of flake graphite and antioxidants. In actual operation, the blend composition of MgO-C brick raw materials is finely tuned to suit the wear pattern of specific lining areas of the BOF. The zoned lining method is employed to use different refractories in different lining zones of the BOF. As a result, the life of the BOF is extended and the cost of the BOF refractory is reduced.
The bricks are cold-pressed into shape and are used in lining of the BOF and the slag lines of secondary steelmaking vessels as well as applications in other steelmaking el furnaces. The liquid slag is in contact with the refractory during the refining process, where temperatures more than 1,650 deg C are common. Since the slag is floating over the liquid steel in the furnace, local convection currents develop near the slag-refractory-steel-air intersection which leads to small-scale circulating flows that increase dissolution.
The overall wear of the lining depends on chemical and physical considerations. The chemical potential between the refractories and the steelmaking slag determines the driving force to dissolve the refractory lining. The conditions of the slag include its chemistry, degree of saturation of MgO, oxygen potential, temperature, temperature equilibrium, and wetting characteristics between the slag and refractory.
The slag can be conditioned to decrease the potential for periclase dissolution by saturating it with MgO. The refractory reaction with the slag can be decreased by decreasing its wettability. The use of large-flake graphite is a common method to achieve the low wetting characteristics. To retain the carbon to develop a ceramic bond and resist oxidation, metallic additions, known as antioxidants, are made to the refractory. Several studies have been done for developing the proper chemical composition, size, and distribution of these additives to improve their effectiveness.
Carbon is added to the refractories since it is not wetted by slags. Traditionally, refractory chemical corrosion resistance is improved by decreasing the size or number of pores, but this can make the refractories more sensitive to thermal shock damage. Hence, methods have been developed to incorporate carbon to fill the spaces between the magnesia, and other refractory oxide grains. The carbon then forms a film blocking the slag penetration thereby minimizing the damage it causes the brick.
The early carbon sources were tars which were replaced by pitches or resins. The carbon level so achieved can be further increased with the addition of carbon black and graphite. Pitch could be used to hold the refractory components together as in pitch bonded brick carbon is forced into the pores of the bricks as in burned pitch impregnated brick. The residual carbon contained after tempering and coking, was around 5 % for pitch bonded brick and 2.5 % or slightly higher, for burned impregnated brick. As against this, magnesia-carbon brick can have 8 % to 30 % carbon while the most common range is between 10 % and 20 %. The carbon in burned impregnated brick also affected their porosity by decreasing it from around 18 % to around 12 %, but did not detrimentally affect the physical properties of the bricks.
The graphite used in refractories is a naturally occurring flake material selected for its high temperature stability and chemical inertness. The important properties are flake size, carbon content and ash or, impurity level. A consequence of the flake shape is that during production, they preferentially align themselves leading to anisotropic brick properties. In particular, its thermal conductivity is higher in the long against short direction.
The ground work for Mag-C refractories was set, among others, by Herron and coworkers, who showed that the carbon in pitch impregnated burned magnesia brick prevented slag from penetrating the pores. Dense zone formation, thought to be a characteristic of magnesia-carbon refractories, was known before these brick were developed. Work using laboratory generated and field samples of pitch bonded and pitch impregnated magnesia and dolomite brick showed that an MgO dense zone formed at high temperature protects the carbon and thereby prevents slag penetration. Fig 1b shows a part of a ‘relatively continuous dense magnesia zone between the carbon-bearing zone and the carbon-free zone’ observed in a pitch impregnated brick fired in the laboratory using conditions which simulated the BOF environment. The dense zone was thought to be the result of the reduction of the MgO in the presence of carbon and at steelmaking temperatures, which vapourized the magnesium as a gas. There was a point at which the oxygen potential and temperature relationship was no longer stable leading to the precipitation of magnesium vapour to form a magnesia dense zone.
Carbon plays an important role in improving the thermal shock resistance and the slag corrosion resistance of the MgO-C refractory, due to its low thermal expansion coefficient and its poor wettability with slag. However, C is susceptible to oxidation, particularly in high temperatures, which is classified into direct oxidation [2C(s) + O2(g) = 2CO(g)] and indirect oxidation [C(s) + MgO(s) = CO(g) + Mg(g)]. A number of studies have shown that several factors, such as porosity, reactivity of the graphite, gas composition, and flow characteristics, can dramatically affect the direct oxidation rate of the refractory. Also, the oxidation kinetics of the MgO-C refractory has been studied with respect to different firing temperatures and holding times. Majority of these studies have used a lower temperature (1,400 deg C maximum) for investigation, in order to avoid the indirect reaction, or only studied the single carbon content of the MgO-C refractory. In general, the service temperature of the MgO-C refractory is typically higher than 1,500 deg C. Hence it is necessary to study the oxidation behaviour of the MgO-C refractory with different carbon contents at higher temperatures.
The wettability characteristics between liquid slag and refractories are an important indicator for the slag corrosion resistance of the refractory, which is characterized by the contact angle between the solid–liquid phases. In general, the refractories have superior slag corrosion resistance when they have poor wettability with liquid slag. One of the studies has suggested that the carbon in the MgO-C refractory effectively hinders the penetration of slags by repelling the slag and slowing the diffusion of Mg2+.
Another study investigated the wettability between the MgO-C substrate and the ladle furnace (LF) refining slag, where it has been found that the MgO-C substrate remained not-wetted by the liquid slag when the temperature was below 1,460 deg C. In fact, the wettability between MgO-C refractory brick and liquid slag depends on several factors, such as the carbon content, temperature, slag composition, and porosity of the MgO-C refractory bricks. However, there have been few studies to examine the effects of these factors on the slag corrosion resistance of the MgO-C refractory bricks through altering its wettability with slag.
The oxidation resistance and the wettability characteristics are two important factors regarding the service life of the MgO-C refractory bricks. However, few studies have combined the two aspects to examine the effects which the carbon content has on the performance of the MgO-C refractory bricks.
One of the studies has examined the oxidation resistance of the MgO-C refractory bricks by comparing the bulk density, apparent porosity, cold crushing strength, and oxidation rate of the fired refractory bricks with different carbon contents. The wetting behaviour of the MgO-C refractory bricks with liquid slag has been observed insitu, and the contact angle and apparent volume have been compared. The results this study have been (i) the bulk density, apparent porosity, and cold crushing strength of the cured MgO-C refractory decreased as the carbon content increased with these properties degraded after firing, especially at higher C content, (ii) the regenerated MgO in the MgO-C refractory bricks effectively hindered the carbon oxidation in the lower carbon content range, and increased the cold crushing strength and bulk density in the fired refractory bricks to some extent, and (iii) there is liquid slag penetration into the MgO-C refractory bricks, which decreased the apparent volume during the wetting process with the penetration extent being closely related to the contact angle between the MgO-C refractory bricks and the liquid slag.
The concept of adding a metallic component to a refractory was already patented in 1935, a long time before the technology was available to produce magnesia-carbon brick. In 1983 a patent for the addition of metallic magnesium to a chemically bonded brick was issued. The idea was to increase the quantity of magnesium gas generated which should lead to a thicker dense zone. This concept was adopted in practice, and a commercial product was successfully produced. To carry out production several hurdles had to be overcome, not the least of which was how to handle metallic magnesium, a spontaneously flammable material in the presence of nitrogen, carbon di-oxide, oxygen and / or water.
With the increased use of magnesia-carbon refractories it was realized that graphite, in the 10 % and higher level, behaved in a completely different manner than the residual carbons from pitch or resins. Fig 5a shows that the higher carbon levels decreased slag penetration, increased the thermal conductivity of the brick leading to more effective cooling of the hot face, and reduced its modulus of elasticity (Young’s modulus) with the latter two improving its thermal shock resistance.
Fig 5 Characteristic properties of MgO-C bricks
For ensuring that the carbon of the brick would be available for a long time, its rate of oxidation had to be reduced. The formation of the dense zone to protect it was already known and ways to increase its speed of formation and / or its stability were being studied. To this end aluminum and / or silicon metals were added to commercial MgO-C brick, but a major problem remained, the laboratory determined dense zone could not be detected in post mortem brick. It was speculated that the technique used for brick removal from actual field installations somehow destroyed it or that slag penetration during cooling was to blame. By 1997, this problem was no longer an issue and dense zones from used brick were being analyzed.
Rymon-Lipinski provided the theoretical basis for determining the steps in which magnesium, aluminum, and silicon metals react to provide the carbon protection in a MgO-C brick. Tab 3 shows the expected reactions as a function of temperature and position within the BOF lining.
Fig 5b shows the expected dense zone morphology of a metal containing MgO-C brick. Compare this to the one shown in Fig 1b which is more of a bridging between grains. In the latter case the quantity of contained carbon is considerably lower than that found in a modern MgO-C bricks which results in the MgO grains being very close to each thereby providing a scaffold for the newly precipitated material.
If the metal added to the brick is Al then the dense zone consists of an MgAl spinel not pure MgO. Fig 6 shows the reaction products from the hot face to the cold face of a used MgO-C brick with aluminum addition. It can be seen, that in addition to the MgAl spinel, nitride, and carbide phases were also formed while the brick was in service.
Fig 6 Properties of MgO-C bricks
Although a number of studies are there with regards to the addition of magnesium, aluminum, and silicon to the MgO-C brick, other additives have also been studied. At some point, interest was shown in additions of glass, magnesium-boron (Mg-B), titanium oxide (TiO2), borides and carbides. Of these, boron carbide (B4C) has been found to be a useful addition to MgO-C. Antioxidants also improve the strength of the brick as compared to similar ones without it. Fig 6b shows the effect of aluminum on the strength of the brick after firing to different temperatures.
At the same time that the early studies on antioxidants was being carried out, MgO and carbon as raw materials and in contact with each other were studied to establish the important parameters for achieving good refractory properties which has led to increased life in actual applications.
Although MgO-C refractories have shown low wear in actual applications, the move from the laboratory to the steel plant was slow until the rate of carbon oxidation could be controlled. A number of studies have been carried out to determine its role and how to reduce its oxidation. Here the development of the early MgO-C bricks and the role of antioxidants in the dense zone formation are tracked. Their role can be summarized as described below.
At low temperatures, less than around 1,200 deg C, there is a decrease in the porosity of the brick. Above around 1,400 deg C, graphite oxidizes in the presence of MgO and both magnesium and carbon are lost from the refractory. If the refractory is above around 1,500 deg C for long periods of time and, in the presence of carbon a magnesium vapour is formed which reacts to form the MgO or an MgAl spinel dense zone at the hot face of the brick. This prevents the ingress of air or slag into the brick hence ‘resisting’ the oxidation.
The commonly added antioxidants of aluminum and silicon were used since they were low cost and could afford effective protection. Borides, in particular boron carbide, were added since they filled pores at the intermediate temperatures thereby prevented air to react with the refractory components. It is interesting to note that brick based on this technology are used with great success, not only in the BOFs, but also in majority of EAFs and steel ladles and the metal addition concepts have expanded to encompass Al2O3-SiC-C (ASC) brick, submerged entry nozzles (SENs), slide gate plates, etc.
Degradation of these materials results from the interactions among these chemical, thermal and mechanical phenomena. These linings suffer corrosion and wear processes which take place mainly in the area of contact with the slag. Steelmaking slags are complex mixtures of various oxides including CaO, SiO2, Al2O3, MgO, and FeO. In addition, the compositions of the slag can vary according to the stage of the process considered.
These refractories have resulted in better performance in the steel production process. These refractories basically consist of grains (or aggregates) of magnesia (1 mm to 7 mm in size) representing around 70 % of the refractory and the matrix (or filler) composed of graphite and finer magnesia (50 micrometers to 500 micrometers). Refractory brick is complemented by bond (or cement), which is used to form a strong connection between the matrix and aggregates, and the addition of antioxidants such as Al, Si, Mg, B4C, SiC, CaB6 and ZrB2. The compositions normally contain 80 % MgO to 93 % MgO, between 7 % to 20 % graphite and the additions of antioxidants are between 2 % to 3 %.
The degree of corrosion resistance of these materials is linked to percentage, size and quality of the MgO grains among other variables. These MgO grains are to have a low level of impurities (SiO2, Fe2O3, and B2O3) and the porosity of grains is to be low. The bond and graphite containing these materials are other important factor to determine their corrosion resistance. While carbon gives higher thermal conductivity, the main drawback is that the carbon is oxidized in air atmosphere (at process temperature), generating pores in the structure of the refractory. The presence of these pores promotes the penetration of the slag and consequently decreases the protective quality of the lining.
In recent years, Nippon Steel Corporation has developed the Multi-Refining Converter (MURC) process whereby dephosphorization and decarburization are continuously conducted in a single converter. In this process, the penetration of low-basicity slag into refractories and the corrosion of the refractories by low-basicity slag exerted clearly visible effects on the MURC converter. The quality deterioration of iron ore, coke, and other steel raw materials increased the impurities ([Si], [P], [S]) in the hot metal. This situation in turn aggravates the operating severity of MURC converters, increased the corrosion rate of MgO-C bricks, and highlights the need for increasing the durability of MgO-C bricks.
Recent trends in technology
Evaluation of technology for simulating refractory corrosion in an actual BOF – Since the MgO-C bricks is used in an operating BOF, the technology to pre-simulate the opeating BOF on a laboratory scale and to evaluate the actual durability of MgO-C bricks in the BOF is very important in determining the material improvements to be made and the expected refractory cost, among other purposes. A recent study proposed the method of evaluating the corrosion resistance of MgO-C bricks by repeatedly heat treating and loading samples in order to reproduce the deterioration and corrosion of MgO-C bricks in the operating BOF during long use. MgO-C bricks were thermally loaded by repeatedly heat treating them at a temperature of 1,500 deg C or higher and at a temperatures of 500 deg C or lower. This procedure physically loosened the structure of MgO-C bricks and structurally degraded the MgO-C bricks by the MgO-C reaction (Fig 4b). The proposed method has well reproduced the structural deterioration of MgO-C bricks in an actual BOF. The simulated durability is shown to correspond well with the actual durability of MgO-C bricks.
Technology for suppressing MgO-C reaction – The structural deterioration of MgO-C bricks by the MgO-C reaction is considered to greatly contribute to the wear of the MgO-C bricks. In recent years, technology has been developed for suppressing the MgO-C reaction by changing the particle size composition of MgO-C bricks. Reducing the reaction area between the MgO aggregate and the carbon material is also effective in suppressing the MgO-C reaction. Reducing the quantity of 0.1 mm and finer particles in the magnesia aggregate is reported to suppress the structural deterioration of MgO-C bricks due to the MgO-C reaction and to improve the corrosion resistance of MgO-C bricks (Fig 4c).
Reduction of graphite content and improvement of spalling resistance – In recent years, reduction in the graphite content of MgO-C bricks has been studied from the viewpoint of suppressing the decrease in durability by eliminating the oxidation of graphite and from the viewpoint of reducing the heat loss. The graphite content reduction improves the corrosion resistance and decreases the thermal conductivity. As a result, the heat loss is reduced but the spalling resistance is also reduced. Several efforts have been made to improve the spalling resistance of MgO-C bricks. A study has reported that improved the spalling resistance of low-graphite MgO-C bricks is achieved by covering the MgO aggregate with tar pitch. Also, technology is under development for sharply reducing the carbon addition content while maintaining the spalling resistance of MgO-C bricks by adding carbon nano particles of size of a few nanometers to a few tens of nanometers.
The operating pattern and lining life of BOFs have evolved with the development of the lining refractories of the BOF. In recent years, the MgO-C bricks for BOF have technologically matured, but examples have been reported whereby the durability of MgO-C bricks has been highly improved by the different initiatives. It is necessary to continue further efforts to improve the refractory technology and achieve technology innovations to lead to the further evolution of the steelmaking processes.
Leave a Comment