Current status of production and use of magnesium-calcium refractory materials in our country

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The current status of production and use of magnesio-calcareous refractory materials in China. Magnesio-calcareous refractory materials are a type of alkaline refractory material; they have been used as lining materials for steelmaking converters since the early 1960s, contributing to the development of China’s converter-based steelmaking industry. By the end of the 1980s, due to the widespread use of magnesia-carbon bricks, the consumption of magnesia-calcium refractory materials declined sharply and they were almost phased out, with only a few manufacturers still using them. Since the beginning of this century, with the rapid development of stainless steel and clean steel production in our country, various magnesium-calcium-based refractory materials have regained attention due to their unique performance characteristics, lower costs, and environmental advantages. Currently, it has been widely applied in refining equipment for high-grade steels such as stainless steel and clean steel, and is developing at a rapid pace. The number of manufacturers is increasing continuously, with more product varieties and higher production volumes. Product quality is improving, application areas are expanding, and the performance of these products is becoming better and better. Production and Use of Magnesio-Calcareous Refractory Materials At present, the magnesio-calcareous refractory materials produced and used in China include various fired magnesio-calcareous bricks, unfired magnesio-calcareous (carbon) bricks, as well as magnesio-calcareous tundish coatings and dry ramming materials. 1 Production and use of fired magnesia-calcium bricks At present, there are 3 main enterprises in China that produce fired magnesia-calcium bricks, located in Liaoning Province, Shanxi Province, and Zhejiang Province respectively. Additionally, a new production line for firing magnesium-calcium bricks built by a company in Henan Province is in trial production. Apart from the differences in the raw materials used, the production processes of various manufacturers are essentially the same; they basically follow the production processes used for manufacturing fired oil-impregnated magnesia dolomite bricks in the past. Paraffin is used as the binder, hot mixing and hot molding are employed, and firing takes place in a tunnel kiln; there are no significant changes in the firing procedures. The only difference lies in the method of handling the bricks after firing. The original production process for fired oil-impregnated magnesia dolomite bricks involved impregnating the fired bricks with asphalt under vacuum conditions, with the aim of reducing their porosity and thereby enhancing their strength and slag resistance. The current production process involves subjecting the fired magnesia-calcium bricks to a wax impregnation treatment at atmospheric pressure. The purpose of this is to seal the bricks and isolate them from external moisture, thereby preventing hydration. 　　In terms of production equipment, most manufacturers now use heating mixers for mixing materials. Instead of heating the granular materials separately before mixing, they heat them during the mixing process, which simplifies the production process. 　　In terms of raw materials, a factory in Liaoning Province uses synthetic magnesium-calcium sand that has been calcined in high-temperature vertical kilns, while manufacturers in Shanxi, Zhejiang, and Henan use synthetic magnesium-calcium sand that has been calcined in high-temperature tunnel kilns. 　　The physical and chemical properties of the calcined magnesia-calcium bricks produced by a certain factory are shown in Table 1. Table 1 Physical and chemical properties of fired magnesia-calcium bricks. Chemical composition/%, Volume density /g·cm-3, Compressive strength at room temperature/MPa, Apparent porosity/%: MgO, CaO, SiO2, Fe2O3, Al2O3 – 75.57, 19.85, 1.15, 0.74, 0.51, 3.05; Density: 118; Porosity: 5.0%. Fired magnesia-calcium bricks are primarily used as linings in AOD furnaces for stainless steel production, replacing the traditional magnesia-chromium bricks. 　　At present, Taiyuan Iron and Steel Company, China’s largest manufacturer of refractory materials for stainless steel production, uses fired magnesia-calcium bricks as lining materials in its AOD furnaces for stainless steel smelting. As of August 2005, the average service life of these AOD furnaces was 163 cycles, with the highest individual service life reaching 189 cycles. Some large and medium-sized stainless steel manufacturers such as Baosteel Group’s Shanghai No.1 Steel, Shanghai No.5 Steel, Shanghai Krupp, Ningbo Baoxin, and Zhangjiagang POSCO, as well as the majority of private stainless steel production enterprises in regions like Jiangsu, Zhejiang, and Guangdong, use sintered magnesia-calcium bricks as lining materials for their AOD furnaces. Mg-Ca bricks fired at high temperatures are now widely used in AOD furnaces for stainless steel smelting in China. 　　The reason why sintered magnesia-calcium bricks can replace magnesia-chromium bricks and become the main lining material for AOD furnaces is that these magnesia-calcium bricks possess many excellent functional properties; they have the ability to purify molten steel. The free CaO present in these bricks can adsorb non-metallic inclusions such as , , Al2O3, and SiO2 from the molten steel, thereby purifying it. The ability of magnesia-calcium bricks to resist high-alkalinity slag (R greater than 1.5) makes them superior as refractory materials compared to magnesia-chromium bricks; steelmakers can carry out smelting operations in conditions with high-alkalinity slag, thereby extending the service life of the furnace lining. At the same time, it can also improve the desulfurization effect. In particular, magnesia-calcium bricks containing a certain amount of ZrO2 can match, or even exceed, the various performance characteristics of magnesia-chromium bricks. On the other hand, the cost of using magnesium-calcium bricks is lower. The main raw material for producing magnesia-calcium bricks is natural dolomite, which is abundant in supply, easy to use, and inexpensive. Chromium ore, the raw material used for producing magnesia-chromium bricks, is scarce in China, so it has to be imported from abroad at high prices. Therefore, the production cost of magnesia-calcium bricks is lower than that of magnesia-chromium bricks. Thirdly, the use of magnesia-chromia bricks results in the generation of hexavalent chromium, which causes environmental pollution, whereas this problem does not arise with the use of fired magnesia-calcium bricks. It is precisely because of these advantages that fired magnesia-calcium bricks are gradually replacing magnesia-chromium bricks in many application areas. 　　In the coming years, as a large number of stainless steel expansion and new construction projects such as those carried out by Taiyuan Iron and Steel, Baosteel, Jiuquan Iron and Steel, and Zhangjiagang POSCO are completed and put into operation, China’s stainless steel production capacity will continue to grow, and the demand for sintered magnesium-calcium bricks will also increase accordingly. 2 Production and Application of Non-firing Magnesium-Calcium (Carbon) Bricks At present, the production of non-firing magnesium-calcium (carbon) brick refractory materials in China uses various types of magnesium-calcium sands, electrofused magnesia, and flake graphite as primary raw materials, with anhydrous resin serving as the binder. The composition of the brick material typically uses magnesia-calcium sand as the aggregate, with electrically fused magnesia and graphite as fine powders. The electrofused magnesia powder and graphite should be pre-mixed before blending. To reduce the viscosity of the binder and facilitate mixing, it is best to use the binder at a temperature of around 50°C; in cold seasons, the binder must be heated for refractory materials. The shaped unburned bricks need to be heat-treated at a temperature of 200–250°C to cure the binder and improve the strength of the bricks. Then, the surface is treated with organic substances to prevent hydration. 　　The manufacturers of non-combustible magnesium-calcium (carbon) bricks in our country are mainly located in provinces such as Liaoning, Shandong, Hubei, Jiangsu, and Zhejiang. The physical and chemical properties of some non-fire-resistant magnesia-calcium (carbon) bricks are shown in Table 2. Table 2: Physical and chemical properties of unfired magnesium-calcium (carbon) bricks. Chemical composition/%, Volume density/g·cm-3, Compressive strength at room temperature/MPa, Apparent porosity/%, Loss on ignition. MgO, CaO, C: 62.63, 26.39, 7.17, 2.95; 63.5–73.5, 5.1, 4.86; 62.61, 30.11, 2.97; 80.9, 7.1, 55.96; 30.18, 7.02, 2.96; 52, 2.92. Unfired magnesium-calcium (carbon) bricks are primarily used as refractory materials in the linings of various steel refining ladles and in the roof panels of AOD furnaces. 　　A 100t LF—VD refining ladle at a steel plant in the south is used for refining cord steel, and domestically produced magnesia-calcium bricks with a CaO content of 33%–35% are used on the part of the ladle wall away from the slag line. The service life is over 80 cycles, with a maximum of up to 94 cycles. The average service life of similar imported bricks from abroad is 81 uses, with a maximum service life of 84 uses. 　　A steel company in the north used non-burning magnesia-calcium-carbon bricks for the linings of its 225t steel ladles; the average service life of these refractory materials was 116.8 cycles, which is 37.5 cycles more than when aluminum-magnesium-carbon bricks were used. This approach also reduced the amount of impurities in the molten steel as well as their quantity, and decreased the size of those impurities, thereby contributing to an improvement in the quality of the molten steel. A steel pipe company achieved satisfactory results by using non-burning magnesia-calcium-carbon bricks at the slag line of its 150t LF—VD steel refining ladle. In a stainless steel company in East China, non-burning magnesia-calcium bricks and non-burning magnesia-calcium carbon bricks (with a carbon content of 3%–5%) are used as the working lining in 120t VOD furnaces, and the results have been satisfactory. Another steel company also achieved good results by using non-burning magnesia-calcium-carbon bricks at the slag line of its 90t ASEA—SKF refining ladle. In the newly launched stainless steel production line of a steel company in the west, non-burning magnesia-calcium bricks are used for the furnace lid of the 120t AOD furnace and the lining of the steel refining ladle. Some of them use domestically produced bricks, which have achieved good performance and are expected to replace imported bricks. A steel plant in Shandong uses 90t LF ladles for the refining of cord steel, and non-burning magnesia-calcium bricks as well as non-burning magnesia-calcium carbon bricks are also being tested as lining materials. 3 Magnesio-calcium tundish coatings and dry ramming materials Magnesio-calcium tundish coatings and dry ramming materials, as amorphous refractory materials containing CaO, are highly favored by steel plants due to their excellent performance. They are used in continuous casting tundishes for the production of clean steel, offering a long service life and good metallurgical results. 　　Magnesium-calcium tundish coatings are prepared using magnesia and natural dolomite, as well as refractory limestone or slaked lime, to which a certain amount of binder and additives are added. Since there are no effective measures yet to completely solve the hydration problem of CaO, raw materials containing free CaO cannot be used directly in the production of magnesium-calcium type tundish coatings; instead, CaCO3 or Ca(OH)2 is generally used, which is then converted into CaO after being heated to a certain temperature. 　　The physical and chemical properties of the magnesium-calcium tundish coating are shown in Table 3. Table 3 Physicochemical properties of magnesium-calcium tundish coatings. Chemical composition/%; Volume density/g·cm-3 (110°C × 24 h); Pressure resistance at room temperature/MPa (110°C × 24 h): MgO, CaO, SiO2, Fe2O3, Al2O3 – 57.78, 18.72, 6.98, 1.12, 0.80, 2.28 respectively; 9.4. Magnesium-calcium tundish dry ramming refractory materials are made from main raw materials such as magnesia and magnesium-calcium sand, with solid resins or low-melting-point inorganic salts used as binders. Dry ramming construction is employed, in which the metal mold is heated to melt or solidify the binder, thereby forming a fixed working layer for the tundish. Compared to magnesium-based tundish coatings, dry ramming materials require a shorter baking time and are easier to use, but they are more expensive. At present, there are many steel enterprises in our country that use magnesia-calcium ladle coatings or dry ramming materials on continuous casting ladles. With the rapid development of clean steel production in our country, these two types of magnesium-calcium-based amorphous refractory materials will see even wider application. The future development of magnesium-calcium refractory materials in China 1. Developing magnesium-calcium refractory materials with high CaO content. China has richer reserves of natural dolomitic marble than of magnesite; while magnesite is used not only for producing magnesium-calcium refractory materials but also for manufacturing various important magnesium-based refractory materials such as pure magnesium, magnesium-carbon, and magnesium-aluminum based materials. Therefore, going forward, while meeting the requirements of steelmaking production, natural dolomite should be utilized as much as possible, and magnesium-calcium refractory materials with high CaO content should be developed, especially pure dolomite refractory materials, in order to conserve the relatively scarce magnesite and enable it to be used more extensively in the production of other magnesium-based refractory materials or for other purposes. 　　On the other hand, magnesio-calcium refractories with a high CaO content have a better effect on purifying molten steel than those with a low CaO content. Therefore, from the perspective of purifying molten steel, it is also necessary to make more use of natural dolomite and develop magnesium-calcium refractory materials with a high CaO content (CaO greater than 50%) for use in various refining equipment. 2 Development of high-grade magnesio-calcium refractory materials At present, the magnesio-calcium refractory materials produced in China still cannot fully meet the needs of the steelmaking industry. In areas such as the tuyeres of AOD furnaces and their surrounding regions, since domestic magnesia-calcium bricks perform relatively poorly, some steel mills still use magnesia-chromium bricks or imported magnesia-calcium bricks in these areas. Therefore, high-quality magnesia-calcium bricks suitable for critical areas such as the tuyere zone of AOD furnaces should be developed as soon as possible. The main approach is to use high-purity, high-density electrofused raw materials ； Add an appropriate amount of ZrO2 to the brick material ； Select an appropriate particle size distribution for the brick material, and control the porosity of the bricks, etc. To improve the high-temperature properties of magnesia-calcium bricks, such as their resistance to high-temperature corrosion, mechanical wear, and thermal shock stability. Furthermore, magnesium-calcium bricks with different performance characteristics must be developed according to various usage conditions, in order to create a series of such products. 3 Development of dry magnesium-calcium ramming material for refining steel ladles Magnesium-calcium materials have the capability to purify molten steel, and are particularly suitable for use in various refining steel ladles (furnaces). Therefore, to meet the requirements of clean steel production, dry magnesium-calcium ramming materials for refining ladles (furnaces) and corresponding construction and baking techniques should be developed. Once developed successfully, it will be a very popular magnesium-calcium refractory material. 4 Development of magnesium-calcium castables 　　Since refractory materials containing CaO tend to hydrate, this poses significant limitations on their production and use, especially those with high CaO contents. Therefore, both domestically and internationally, research is being conducted on waterproofing technologies for CaO-containing refractory materials. The main methods involve treating the surface of particles made from CaO-containing refractory materials using carbonization and phosphoric acid impregnation, or applying an anhydrous organic material to the particle surface to form a waterproof protective layer that isolates CaO from the outside environment thereby preventing hydration. Alternatively, certain additives (such as TiO2, ZrO2, etc.) can be incorporated during the batching stage; during sintering, these additives react with CaO to form high-temperature compounds that are resistant to hydration. These research efforts have achieved certain results, but they are still in the laboratory testing stage and it is difficult to apply them to industrial production. Therefore, it is necessary to continue advancing research on the waterproofing technology for magnesium-calcium refractory materials. Once this technology is developed and applied in industrial production, magnesium-calcium-based castables can be created for use in various steel refining ladles and tundishes. 5 Development of magnesium-calcium refractory materials for cement kilns At present, many countries abroad have already begun to use magnesium-calcium bricks extensively in areas such as the firing zone and transition zone of cement rotary kilns, replacing magnesium-chromium bricks and thereby avoiding environmental pollution caused by the use of magnesium-chromium bricks. Among them, the utilization rate of various magnesia-calcium bricks in the firing zones of cement rotary kilns in the United States has reached 86%, while only a few manufacturers in China use imported magnesia-calcium bricks in their cement rotary kilns. Therefore, it is necessary to develop magnesium-calcium bricks for cement rotary kilns by taking into account our country’s national conditions, drawing on foreign experience, and making use of our abundant natural dolomite resources. “During the Eighth Five-Year Plan period, our country carried out work in this area. At that time, the Fire Resistance Research Institute of the Building Materials Research Institute collaborated with a refractory materials factory in Tianjin to use synthetic magnesio-calcium sand produced from magnesium ore from Shandong as raw material for manufacturing magnesio-calcium bricks with a CaO content of 30.21%. These bricks were tested in the firing zone of a Φ3.7m×53m cement rotary kiln at a cement factory in Liaoning, and good performance was achieved; their service life met the requirements set for the \"Eighth Five-Year Plan\" research projects. Going forward, building on previous work, development of magnesia-calcium bricks for cement rotary kilns will continue. By introducing components such as ZrO2 into magnesium-calcium materials, magnesium-calcium bricks with excellent erosion resistance, kiln lining properties, and thermal shock stability have been developed to meet the needs of cement rotary kiln production in China, and to achieve chromium-free refractory materials for cement kilns in the country at the earliest possible time. Conclusion With the rapid development of stainless steel and clean steel production in China, China’s magnesium-calcium refractory materials have entered a new stage of development. New production lines are being added continuously, while existing manufacturers are expanding their production scales and introducing new products. Some of them cooperate with foreign companies to jointly produce high-quality magnesium-calcium refractory materials. For example, Taiyuan Iron and Steel Refractory Materials Company collaborates with a foreign company to produce high-quality magnesium-calcium refractories; a production line for manufacturing magnesium-calcium sand using one-step ultra-high-temperature calcination technology has already been established in a location in southern China. This will significantly reduce the production cost of magnesia-calcium sand. It can be predicted that magnesium-calcium refractory materials will become the main type of high-quality basic refractory materials in China. They will not only meet the needs of stainless steel and clean steel production as well as other industries in the country, but can also be exported. 　 This post was last edited by laomao123 on 2009-2-28 18:51.]