Analysing the performance and influencing factors of magnesia-calcium bricks

This paper is an introduction to the performance of magnesia calcium bricks, including high temperature resistance, slag resistance, spalling resistance, wear resistance, molten steel cleaning performance and hydration resistance. MgO/CaO ratio effects, The chemical purity and density of magnesia-calcium bricks have been the subject of analysis and discussion with regard to performance. At the same time, it is recommended to produce "special" magnesia-calcium bricks that are suitable for the user's conditions of use, in order to improve their effectiveness.

Key words: magnesia-calcium brick; performance; factors of influence

Magnesia-calcium bricks, with MgO and CaO as the main chemical components, are high quality alkaline composite refractory materials, includes dolomite bricks and magnesia-dolomite bricks. Magnesia-Calcium bricks are widely used in refining equipment such as AOD furnaces due to their many excellent performance characteristics, there is also an increasing demand for various magnesia-calcium bricks. To improve the production and use of magnesia-calcium bricks in the future, in order for this high quality refractory material to better serve the steel industry and other high temperature industries in my country, the performance and influencing factors of magnesia-calcium bricks are being analysed.

Performance and control factors of magnesia-calcium bricks

Magnesia-calcium bricks are mainly used as lining materials for refining equipment such as AOD furnaces, VOD furnaces and LF furnaces in the steel industry. During use, it is subjected to various destructive effects such as high temperature melting loss, chemical erosion and penetration of slag, severe erosion and wear of slag, molten steel and air flow, thermal shock caused by rapid temperature changes, and hydration due to water absorption. This study provides a qualitative analysis of the high temperature resistance, slag resistance, spalling resistance, high temperature wear resistance, molten steel purification performance, hydration resistance and influencing factors of magnesia-calcium bricks.

Resistant to high temperatures

The high temperature resistance of magnesia-calcium bricks means that the magnesia-calcium bricks will not melt or soften and deform under high temperature working conditions, and maintain good high temperature stability and mechanical strength. Magnesia-calcium bricks are used in refining equipment such as AOD furnaces, VOD furnaces and LF furnaces, which have high working temperatures and frequent temperature changes. For example, the temperature during the oxidation period of the AOD furnace is over 1700°C, sometimes reaching around 1750°C, and the temperature in the eye area is even higher. Such a harsh high-temperature working environment requires magnesia-calcium bricks with excellent high-temperature resistance to meet production requirements. 

The main minerals in magnesia-calcium bricks, MgO and CaO, are both high temperature minerals. The melting point of MgO is 2800°C and the melting point of CaO is 2570°C. MgO and CaO do not form binary composite minerals at high temperatures and their lowest eutectic point is 2370°C. MgO and CaO also have good high temperature stability. Therefore, MgO and CaO give magnesia-calcium bricks excellent high temperature resistance.

However, a small amount of impurities in magnesia-calcium bricks can have a greater negative effect on the high temperature performance of magnesia-calcium bricks. The effect of impurities on the high temperature performance of magnesia-calcium bricks depends on the type and quantity of impurities. The more types of impurities and the higher the content of certain impurities, the greater the effect on the high temperature properties of magnesia-calcium bricks. The effect of impurities on the high temperature properties of magnesia-calcium bricks is actually the effect on the high temperature properties of the main minerals MgO and CaO. Impurities react with CaO or MgO to form certain low melting point minerals which form a liquid phase at high temperatures, reducing the high temperature resistance of magnesia-calcium bricks.

The main impurities in magnesia-calcium bricks are Fe₂O₃, SiO₂ and Al₂O₃. These three major impurities have little effect on the high temperature performance of MgO. This is because the minerals produced by their reaction with MgO have higher melting points. For example, MgO reacts with SiO₂ to form 2MgO-SiO₂ (forsterite) with a melting point of 1890°C; and reacts with Al₂O₃ to form MgO-Al₂O₃ (magnesia-aluminium spinel) with a melting point of 2135°C. MgO can solid dissolve a large amount of FeO without producing a liquid phase, and also has a strong absorption capacity for Fe₂O₃. The iron oxide content in magnesia-calcium bricks is generally less than 1.5%. It therefore has little effect on the high temperature properties of MgO.

The three main impurities in magnesia-calcium bricks have very different effects on the high temperature properties of CaO. Of these, SiO₂ reacts with CaO to form 3CaO-SiO₂ (tricalcium silicate) or 2CaO-SiO₂ (dicalcium silicate) with melting points of 1900°C and 2130°C respectively. Both are high melting point minerals and have little effect on the high temperature properties of CaO. However, Fe₂O₃ and Al₂O₃ react with CaO to form the low-melting minerals 2CaO-Fe₂O₃ (dicalcium ferrite) and 4CaO-Al₂O₃-Fe₂O₃ (tetracalcium aluminoferrite), and so on, with melting points of 1436°C and 1415°C respectively. The formation of these two low melting point minerals has a greater negative impact on the high temperature properties of CaO, ultimately resulting in a reduction in the high temperature resistance of magnesia-calcium bricks.

From the above analysis, it can be seen that the main harmful impurities in magnesia-calcium bricks are Fe₂O₃ and Al₂O₃. Compared with Fe₂O₃ and Al₂O₃, SiO₂ is less detrimental to the high temperature performance of magnesia-calcium bricks and can be considered a minor impurity. In order to improve the high-temperature performance of magnesia-calcium bricks, synthetic magnesium-calcium refractory raw materials with high purity, high density and high MgO content should be selected for the production of magnesia-calcium bricks. Provided other requirements are met, the MgO content of magnesia-calcium bricks should be increased as much as possible. This is because MgO has a higher melting point than CaO, better volume stability at high temperatures, and the minerals formed by reaction with impurities have a higher melting point. Therefore, increasing the MgO content can improve the high temperature resistance of magnesia-calcium bricks. On the other hand, the damage caused by impurities to the high-temperature properties of magnesia-calcium bricks is mainly due to the reaction with CaO to form low-melting substances. Therefore, increasing the MgO content and reducing the CaO content can reduce the damage caused by impurities to the high temperature properties of magnesia-calcium bricks. Obviously, the high temperature resistance of MgO-rich magnesia-dolomite bricks is better than that of dolomite bricks.

Performance against slag formation

The slag resistance performance of magnesia-calcium bricks refers to the resistance of magnesia-calcium bricks to chemical erosion and penetration by slag when in contact with slag during use. It is one of the most important performance characteristics of magnesia-calcium bricks. Chemical corrosion of magnesia-calcium bricks by slag is the chemical reaction between certain components in the slag and certain components in the magnesia-calcium bricks at high temperatures. A low melt (liquid phase) is produced, causing the working surface of the magnesia-calcium brick to melt and be lost in the slag. The penetration of slag into magnesia-calcium bricks is that the high temperature liquid slag penetrates into the interior of the magnesia-calcium bricks through pores and cracks, forming a metamorphic layer of a certain thickness, and then the metamorphic layer falls off, causing damage to the working surface of the magnesia-calcium bricks.

Chemical erosion and penetration of slag is one of the main causes of furnace lining damage in various refining furnaces. Therefore, improving the slag resistance of magnesia-calcium bricks is crucial to extend the service life of the refining furnace lining. The slag-resistant performance of magnesia-calcium bricks depends on its chemical composition, organisational structure, alkalinity and temperature of the slag.

The main minerals MgO and CaO of magnesia-calcium bricks show different slag resistance properties. MgO is chemically inert to slag, even if a certain degree of reaction occurs, low melting point minerals will not be generated. MgO can dissolve a large amount of FeO without producing a liquid phase. MgO also has strong resistance to SiO₂ in the slag. Therefore, MgO has strong resistance to chemical attack by slag. However, MgO has poor resistance to slag penetration. In fact, high-temperature slag easily penetrates into the interior of MgO (periclase) to form a metamorphic layer.

CaO is more reactive with slag and more easily corroded by slag containing high FeO than MgO. CaO has poor resistance to chemical attack by slag. However, it reacts with SiO₂ in the slag to generate high melting point minerals 2CaO·SiO₂ or 3CaO·SiO₂, which increases the viscosity of the slag and inhibits the penetration of the slag into the interior of the brick. Therefore, CaO has stronger resistance to slag penetration than MgO.

The higher the purity of magnesia-calcium bricks, the lower the reactivity with slag, the lower the production of low-melting materials and the higher the ability to resist chemical erosion of slag. The denser the organisational structure of magnesia-calcium bricks, the more difficult it is for slag to penetrate into the interior of the bricks and the greater the ability to resist slag penetration.

Increasing the MgO content, purity and density of magnesia-calcium bricks can improve the ability of magnesia-calcium bricks to resist slag erosion; increasing the CaO content and density can improve the ability of magnesia-calcium bricks to resist slag penetration. In practical applications, the most suitable MgO/CaO ratio, purity and density of magnesia-calcium bricks should be determined based on the operating conditions of the refining equipment. The magnesia-calcium bricks have good resistance to chemical corrosion and slag penetration to achieve better operating results. Practice has shown that the overall slag resistance performance of MgO-rich magnesia-dolomite bricks is better than that of dolomite bricks. Therefore, high-purity and high-density magnesia-dolomite bricks fired at high temperatures are mostly used in the slag line of the refining furnace.

Slag alkalinity (CaO+MgO/SiO₂) and temperature have an important influence on the slag resistance of magnesia-calcium bricks. Magnesia-calcium bricks have poor resistance to acidic slag and are easily corroded by acidic slag. However, it is highly resistant to highly alkaline slag and, within a certain range of alkalinity, the resistance of magnesia-calcium bricks to slag increases as the alkalinity of the slag increases (the erosive capacity of magnesia-calcium bricks to slag decreases).  The higher the temperature of the slag, the greater the reactivity between the slag and the magnesia-calcium bricks and the more severe the chemical corrosion of the magnesia-calcium bricks. The higher the temperature of the slag, the lower its viscosity and the easier it is to penetrate inside the magnesia-calcium bricks and form a metamorphic layer. Therefore, during the refining process, by increasing the alkalinity of the slag and controlling the temperature of the slag, the erosion and penetration of the magnesia-calcium bricks by the slag can be effectively reduced, and the service life of the magnesia-calcium bricks can be further improved. 

Anti-peeling performance

During the use of magnesia-calcium bricks, the working end of the magnesia-calcium bricks will crack and flake due to destructive effects such as changes in furnace temperature and slag penetration. Especially in VOD kilns, the spalling and damage of magnesia-calcium bricks is more serious. Therefore, improving the spalling resistance of magnesia-calcium bricks is also one of the main ways to increase the service life of magnesia-calcium brick furnace linings.

Depending on the cause, spalling of magnesia-calcium bricks can be divided into thermal spalling and structural spalling. Thermal spalling is the spalling of the working surface of magnesia-calcium bricks caused by periodic rapid changes in kiln temperature during use. Refining equipment such as AOD kilns, VOD kilns and LF kilns for magnesia-calcium bricks all operate intermittently and the kiln temperature changes frequently and over a wide range. For example, when refining stainless steel in an AOD furnace, the furnace temperature drops to about 1300°C before molten steel is added, the furnace temperature can reach a maximum of 1750°C during the oxidation and decarburisation phases, and the furnace temperature is about 1650°C during the reduction phase. In addition, the furnace temperature decreases with each addition of charge. Throughout the refining process, such frequent and large changes in furnace temperature will inevitably cause thermal stress in the working end of the magnesia-calcium bricks. Under the effect of thermal stress, cracks will form. As the cracks grow and expand, the working end of the magnesia-calcium brick will crack and then peel.

Structural spalling occurs when high-temperature furnace slag penetrates through pores and cracks into the interior of magnesia-calcium bricks, forming a metamorphic layer of a certain thickness at the working end of the brick. When the kiln temperature changes rapidly, due to the difference in thermal expansion properties between the minerals in the metamorphic layer and between the metamorphic layer and the original brick layer, cracks will occur in different directions within the metamorphic layer and between the metamorphic layer and the original brick layer. As the cracks continue to grow and expand, the metamorphic layer will continue to peel away.

Compared to other magnesia refractory products (such as magnesia bricks, magnesia chromium bricks, etc.), magnesia calcium bricks have better anti-flaking properties. This is because the CaO in magnesia-calcium bricks has greater creep properties at high temperatures. It can buffer the thermal stress generated within the brick due to rapid temperature changes, inhibit the formation and propagation of cracks within the brick, and improve the ability of magnesia-calcium bricks to resist thermal spalling. On the other hand, CaO in magnesia-calcium bricks reacts with SiO₂ in the slag to form high melting point minerals, which increases the viscosity of the slag. This makes it more difficult for the slag to penetrate into the interior of the brick, thins the metamorphic layer and improves the anti-spalling performance of the magnesia-calcium brick. Therefore, increasing the CaO content of magnesia-calcium bricks, especially increasing the CaO content in the matrix and making it uniformly distributed in the matrix, can significantly improve the anti-flaking performance of magnesia-calcium bricks. 

The pores in magnesia-calcium bricks can buffer the thermal stress in magnesia-calcium bricks, prevent the formation and further expansion of cracks, and help to improve the thermal spalling resistance of magnesia-calcium bricks. However, the pores in the magnesia-calcium brick, especially the through-holes, make it easier for the slag to penetrate into the interior of the brick, forming a thicker metamorphic layer which accelerates the structural spalling of the magnesia-calcium brick. Therefore, the pores in the magnesia-calcium brick have a dual effect on its anti-flaking performance. In practical applications, various factors should be fully considered to determine the porosity of the magnesia-calcium brick in order to achieve better application results. The porosity of fired magnesia-dolomite bricks used in AOD kilns is preferably controlled at 10% to 12%.

In the manufacture of magnesia-calcium bricks, increasing the proportion of large particles and correspondingly reducing the proportion of medium particles and fine powder can improve the spalling resistance of magnesia-calcium bricks.

By adding a suitable amount of ZrO₂ fine powder to the powder of magnesia-calcium bricks, the high melting point mineral CaZrO₃ is generated in the matrix, which can effectively prevent the occurrence of cracks and improve the toughness and anti-flaking properties of magnesia-calcium bricks.

Wear resistance

Magnesia-calcium bricks are used to line refining equipment such as AOD furnaces, VOD furnaces and LF furnaces. During the refining process, high pressure gas is continuously blown into the furnace, causing violent agitation of the molten steel and slag in the furnace. This causes severe erosion of the working surface of the magnesia-calcium bricks, resulting in continuous wear. Therefore, magnesia-calcium bricks are required to have good high temperature wear resistance.

The high temperature wear resistance of magnesia-calcium bricks is specifically reflected in the high temperature strength. The higher the high temperature strength, the better the high temperature wear resistance. The main factors affecting the high temperature strength of magnesia-calcium bricks are MgO/CaO ratio, chemical purity and density.

This is because the high-temperature strength of MgO is better than that of CaO. Therefore, increasing the MgO/CaO ratio of magnesia-calcium bricks can improve their high temperature strength. And the high-temperature strength of magnesia-calcium bricks increases as the MgO/CaO ratio increases. Therefore, the high temperature strength of magnesia-dolomite bricks is better than that of dolomite bricks.

The impurities in magnesia-calcium bricks produce a liquid phase at high temperatures, softening the magnesia-calcium bricks and causing a reduction in high temperature strength. Therefore, the higher the chemical purity of magnesia-calcium bricks, the less liquid phase is produced at high temperatures and the greater the high temperature strength. Therefore, improving the chemical purity of magnesia-calcium bricks, especially improving the purity of the matrix, can improve the high temperature strength of magnesia-calcium bricks.

The denser the organisational structure of magnesia-calcium bricks, the stronger the bond between the mineral crystals and the higher the high temperature strength.

To improve the high-temperature wear resistance of magnesia-calcium bricks, synthetic magnesia-calcium sand with a high MgO/CaO ratio, high purity and high density should be used in the production of magnesia-calcium bricks. As much high purity fused magnesia fine powder as possible should be added to the brick making powder to increase the MgO content in the matrix, and an appropriate amount of ZrO₂ fine powder should be added to strengthen the matrix of the magnesia-calcium bricks. The bricks must be formed using a high-pressure brick press, and the magnesia-calcium bricks must be fired at ultra-high temperatures to increase their density, thereby improving the high temperature strength and wear resistance of the magnesia-calcium bricks.

Performance of purifying molten steel

The outstanding performance of magnesia-calcium bricks is their purifying effect on molten steel. During use, free CaO in magnesia-calcium bricks comes into contact with molten steel. It can adsorb [S], [P] and non-metallic inclusions such as Al₂O₃ and SiO₂ in molten steel, reduce the [O] content in molten steel, etc., and promote further purification of molten steel. These have been confirmed by production and experimental studies. After the 225t ladle wall bricks (non-slag line) of Shougang Second Steelmaking Plant were changed from unburned magnesia-alumina bricks to MgO-CaO-C bricks, the [O] content and the number of inclusions in the molten steel were reduced. The particle size of inclusions is reduced and the purity of the molten steel is improved.

The chemical analysis of lime bricks after use at the contact point between the steel drum and the molten steel has been demonstrated by Naoyuki Talc and others. The main external components absorbed by lime bricks from molten steel are S, Al₂O₃ and SiO₂, etc. X-ray diffraction has shown that these constituents are present in the form of CaS, 12CaO-7Al₂O₃ and 3CaO-SiO₂. The lime bricks in the tundish in contact with the molten steel and the MgO-CaO bricks in contact with the slag line, such as the steel drum, absorb external components such as S, Al₂O₃ and SiO₂. This shows that CaO can absorb S, Al₂O₃ and SiO₂ in molten steel and has the function of purifying molten steel. Research by Li Nan and others shows that CaO in alkaline refractories plays a key role in the dephosphorisation of molten metal. When CaO reaches 25%, significant dephosphorisation effects can be achieved.

The desulphurisation effect is best when the CaO content is between 50% and 70%. This should also be one of the main reasons why dolomite bricks are widely used on ladles in European countries.

Increasing the CaO content of magnesia-calcium bricks can improve the performance of magnesia-calcium bricks in the purification of molten steel. The performance of dolomite bricks in the purification of molten steel is better than that of magnesia-dolomite bricks. Therefore, when refining clean steel, magnesia-calcium bricks with high CaO content should be used as much as possible to achieve better purification effects. It should be noted that the purification effect of magnesia-calcium bricks on molten steel is not only related to the CaO content, but is also influenced by many other factors. Such as the refining equipment and its operating methods, the types of steel being refined, the content and types of impurities in the molten steel, etc. In actual production applications, magnesia-calcium brick grades should be reasonably selected based on specific application conditions and comprehensive consideration of various factors to achieve ideal application effects.

Anti-hydration performance

Because magnesia-calcium bricks contain free CaO, when exposed to external moisture during storage and use, they will hydrate, resulting in a loose structure of the magnesia-calcium bricks, a reduction in various performance characteristics and, in severe cases, scrapping. . Improving the hydration resistance of magnesia-calcium bricks is therefore one of the most important technical issues in the production and use of magnesia-calcium bricks.

The higher the free CaO content of magnesia-calcium bricks, the lower the hydration resistance. Therefore, the hydration resistance of magnesia-dolomite bricks is better than that of dolomite bricks. This is one of the reasons why magnesia-dolomite bricks are more commonly used than dolomite bricks. Ways of improving the hydration resistance of magnesia-calcium bricks Firstly, try to reduce the free CaO content in magnesia-calcium bricks; secondly, avoid contact between magnesia-calcium bricks and external moisture.

Reduce the CaO content of the magnesia-calcium bricks and increase the MgO content so that the MgO in the magnesia-calcium bricks forms a continuous phase and the free CaO is in a separated state and is surrounded by MgO. Add appropriate amounts of additives such as ZrO₂, TiO₂, etc. to the ingredients of the magnesia-calcium bricks to react with the free CaO to form water-resistant minerals and to encapsulate the free CaO.

High density synthetic magnesia-calcium raw materials are used. In order to meet the high temperature performance of magnesia-calcium bricks, a small amount of impurities is allowed in the synthetic magnesia-calcium raw materials. Impurities react with CaO at high temperatures to form hydration resistant minerals, which also promote sintering of the materials and increase the density of the magnesia-calcium bricks.

In the production of bricks, the raw materials are dried and the binder is fully boiled to prevent moisture from entering the bricks. Produced magnesia-calcium bricks should be sealed and packaged in time to prevent contact with external moisture.

When building the kiln lining, the magnesia-calcium bricks must not come into contact with moisture. Once the bricks have been laid, they must be fired and used in good time. During use, prolonged stoppages of the kiln should be avoided as far as possible. During the shutdown period, the kiln lining must be baked to prevent the kiln lining from cooling and absorbing moisture and causing hydration.

Analysing factors affecting magnesia-calcium brick performance

From the above analysis it can be seen that the main factors affecting the performance of magnesia-calcium bricks are: MgO/CaO ratio, chemical purity and density of the magnesia-calcium bricks. 

Conclusion

By analysing and discussing the performance of magnesia-calcium bricks, we understand the performance of magnesia-calcium bricks, the main factors affecting the performance of magnesia-calcium bricks and ways to improve the performance of magnesia-calcium bricks. This has laid the foundation for better production and use of magnesia-calcium bricks in the future. It should be noted that the performance of magnesia-calcium bricks depends on their performance and the conditions under which they are used. Good performance can only be achieved when the performance is adapted to the conditions of use. On the contrary, some magnesia-calcium bricks may have good performance but not necessarily good results. This requires magnesia-calcium brick manufacturers to first conduct detailed analysis and research on the user's application conditions (types of refining equipment, main types of steel for refining, refining operation methods, etc.) when producing magnesia-calcium bricks, and to clarify the user's purposes and requirements.Comprehensive factors from all aspects, and then determine the main physical and chemical indicators and production process parameters of magnesia-calcium bricks (such as raw material varieties, brick ratio, etc.) to produce "special" magnesia-calcium bricks with performance suitable for user conditions. . This is the only way to achieve the ideal use effect. Therefore, it is recommended to promote this production model in the production of magnesia-calcium bricks in the future. According to different manufacturers, different refining processes, different refining equipment and different application parts, we produce "special" magnesia-calcium bricks with different performance. The stronger the target and the higher the degree of specificity, the better the effect will be.