Introduction to refractory materials for RH furnaces

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The RH vacuum degassing furnace was initially used only as a degassing device, and at that time, clay bricks and high-alumina bricks were primarily used for the refractory lining. Now, the functions of RH furnaces have been extended to oxygen blowing and powder injection, and the requirements for the refractory lining materials have become more stringent; therefore, advanced refractory materials are used. In particular, with the increase in the production of high-grade special steels, there is a strong push to adopt methods that increase the gas flow rate and introduce large amounts of gas in order to achieve stable production and rapid processing of ultra-low carbon steels. Increasing the circulation rate leads to increased wear of the refractory lining ； Increasing the amount of cold air blown in caused high-temperature peeling ； The increased amount of slag drawn into the ladle further exacerbates structural spalling and erosion; all these factors contribute to accelerated degradation of the lining material. Therefore, in current RH/RH-OB linings, directly bonded magnesia-chromia bricks are the mainstream, while semi-bonded or re-bonded magnesia-chromia bricks are used around the oxygen injection ports in these linings, with some magnesia-carbon bricks also being utilized. The refractory materials used in the top and upper tanks of RH units, since they do not come into direct contact with molten steel and slag, generally suffer less damage compared to those in the lower sections. The middle section suffers damage to its refractory lining due to erosion by molten steel and slag, or as a result of thermal spalling. The refractory lining of the immersion tube in the lower tank constitutes the highly corrosive area of the RH unit; it often determines the service life of the RH furnace. Therefore, high-temperature fired directly bonded magnesia-chromia bricks should be used for the lining of the lower tank. The most severely damaged part in the lower section of the furnace is the circulation tube; the structure of the lining limits its thickness, and the complex-shaped refractory materials require two heating cycles, which is why no single refractory material can have a sufficient service life. Furthermore, in RH-OB furnaces, the OB process also has a significant impact on the use of refractory materials. When the upper gun method is employed, the refractories are subjected to erosion by the oxygen blown in, the oxides formed from iron elements in the molten steel, and the high-temperature reaction gases. In particular, the formation of oxides rapidly erodes the working surface of the refractories; therefore, MgO-Cr2O3 bricks with a high Cr2O3 content are required to ensure a longer service life. On the other hand, bricks with a lower Cr2O3 content perform better overall when exposed to high-temperature gases. 1. Magnesia-chromium bricks for RH furnaces: High-temperature fired magnesia-chromium bricks, which are types of refractory materials such as directly bonded, rebonded, and semi-rebonded magnesia-chromium bricks, have been widely used in the linings of refining furnaces due to their strong resistance to erosion by low-alkalinity slags. There are many different varieties of magnesia-chromium bricks, and there are significant differences in their production processes, microstructures, and properties. Mg-Cr bricks can be classified according to their Cr2O3 content into Mg-Cr bricks (with a Cr2O3 content of 5–20%), Cr-Mg bricks (with a Cr2O3 content of 20–35%), and Cr bricks (with a Cr2O3 content greater than 35%). Based on the production process, they can be divided into sintered bricks and cast bricks, among others. Due to the great variety of magnesia-chromia bricks, literature has provided classifications and summaries thereof: (1) silicate-bonded magnesia-chromia bricks (commonly fired magnesia-chromia bricks). This type of brick is made from chromite with a high content of impurities (SiO2 and CaO), along with magnesia used in brick manufacturing, and is fired at around 1550°C. Its structural characteristics are that the refractory grains are bonded together by silicates, resulting in a high porosity; it has poor resistance to slag erosion and low volumetric stability at high temperatures. (2) Pre-reacted magnesia-chromia bricks. Magnesia (lightly calcined magnesium powder) is ground together with chromite and pressed into a green body, which is then fired in a furnace; bricks are subsequently produced using synthetic magnesia-chromite as raw material, resulting in \"pre-reacted magnesia-chromium bricks\". Pre-reacted magnesia-chromia bricks are an improved version of silicate-bonded magnesia-chromia bricks. (3) Direct bonding with magnesia-chromia bricks. Directly bonded magnesia-chromia refractory materials are made from chromite ore with low impurity content and relatively pure magnesia, with a firing temperature of over 1700°C. Its structural feature is that the refractory grains are in direct contact with one another; the degree of direct bonding between periclase (solid solution) – periclase (solid solution) and periclase (solid solution) – spinel (solid solution) within the bricks is high. As a result, it exhibits better high-temperature performance, greater resistance to slag erosion, and better high-temperature volume stability compared to ordinary magnesia-chromia bricks. (4) Remelted granule rebonded magnesia-chromium brick (electrolytically rebonded magnesia-chromium). By using the electrofusion method, magnesia and chromite (lightly calcined magnesium powder or periclase combined with chromite) are reacted thoroughly and uniformly to produce magnesium-chromium raw materials in the form of periclase solid solutions and spinel solid solutions with more ideal structures; bricks made from these raw materials are known as fused granule-recombined magnesium-chromium bricks. Due to the high purity of the brick-making raw materials, they all need to be fired at high temperatures of over 1750°C or even ultra-high temperatures. Its microstructural characteristics include a uniform distribution of components such as spinel, low porosity, direct contact between refractory grains, high compressive strength, good erosion resistance, and high strength at high temperatures; however, its drawback is poor thermal shock stability. (5) Semi-recombined magnesia-chromia brick. Magnesium-chromium bricks made with artificially synthesized materials as the particles, and those using chromite ore and magnesia as fine powders, should both be referred to as semi-recombined magnesium-chromium bricks. In China, magnesium-chromium bricks made using electrically fused magnesium-chromium materials as particles, or using co-sintered materials as fine powder or a mixture of chromite ore and magnesia powder as fine powder, are all referred to as semi-recombined magnesium-chromium bricks. Its firing temperature is above 1700°C, and the refractory grains within the brick are primarily bonded directly to one another; its advantages include good thermal shock resistance, as well as decent resistance to erosion and scouring. (6) Sintered magnesia-chromia brick (also known as fully synthetic magnesia-chromia). Magnesia-chromia sand synthesized through 100% sintering is used as a raw material for brick manufacturing; the magnesia-chromia bricks produced by high-temperature firing are what is known as co-sintered magnesia-chromia. Its features include good erosion resistance and good volume stability at high temperatures. (7) Non-firing magnesia-chromium brick (or chemically bonded magnesia-chromium). Chemically bonded non-firing magnesia-chromium bricks are generally made using magnesia and chromite as raw materials for brick production, with sodium polyphosphate, sodium hexametaphosphate, or water glass serving as the binder to compress the magnesia-chromium bricks. No high-temperature firing is required; it only needs to be baked at around 200°C. Since it has not been fired at high temperatures, magnesia will hydrate and cannot be stored for a long time. (8) Cast magnesium-chromium product bricks. Using magnesia and chromite as the main raw materials, along with a small amount of additives, these materials are mixed, pressed into pellets, and dried at low temperatures. The resulting pellets are then crushed into blocks, melted in an electric arc furnace, poured into molds, and annealed to produce parent bricks. These parent bricks are further processed through cutting, grinding, and other cold working methods to create products of various specific shapes. The structural feature of cast magnesium-chromium bricks is a uniform distribution of components; the refractory grains are in direct contact with each other, and silicates exist in the form of isolated islands. These bricks exhibit excellent resistance to melting erosion, penetration, and scouring by molten materials, but they have poor thermal shock stability. 2. Properties of magnesia-chromium bricks Magnesia-chromium bricks are magnesium-based refractory materials containing chromium trioxide. Chromium oxide, by solid-solving in periclase and forming low-expansion magnesia-chromium spinel, increases the wetting angle of the liquid phase on the refractory phase, enhances the degree of direct bonding, and creates an ideal periclase-magnesia-chromium spinel-microcrack composite structure. As a result, these magnesia-chromium materials possess a high load softening temperature, high strength at high temperatures, good resistance to erosion, thermal shock stability, a low thermal conductivity, and excellent suitability for forming a protective layer on the furnace lining. Magnesia-chromia bricks offer the highest performance-to-price ratio among the existing refractory materials in firing zones, which is why they are widely used as refractory materials. Magnesium-chromium bricks of different varieties exhibit varying properties due to differences in their composition, manufacturing processes, and thus in their microstructural structure. Table 1 lists the typical properties of several types of magnesia-chromia bricks. 3. Chromium contamination and detoxification methods Magnesium-chromium bricks are important materials used in furnaces operating under severe conditions. However, in high-temperature and alkaline environments, the chromium trioxide (Cr2O3) contained in these bricks reacts with alkali metal oxides present in the furnace gases to form hexavalent chromium compounds of the type R2CrO4. In the presence of sulfur, chlorine, and alkalis as well, solid solutions of the type R2(Cr·S)O4 can be formed. Both of these compounds are toxic water-soluble substances; whether released into the atmosphere from the kiln exhaust or remaining in the bricks removed after use, hexavalent chromium ions can dissolve into surface water or groundwater due to rainwater, posing serious hazards to humans and animals. Theoretically, the available detoxification methods are wet detoxification and fire detoxification. (1) Detoxification by fire therapy. The strongly reducing atmosphere generated by the combustion of porous carbonaceous materials is used to convert hexavalent chromium into lower-valent forms. The pyrolytic detoxification process is simple, effective, and does not pose any problems with treating the leachate. (2) Wet detoxification. The pH of the wastewater is adjusted to 2.0–3.0 using sulfuric acid, and then hexavalent chromium is converted into trivalent chromium using chemical reducing agents such as sulfur dioxide, sodium sulfite, bisulfite, dithionite, or ferrous sulfate. Trivalent chromium is subsequently removed through precipitation using hydroxides (usually lime). The wet detoxification method has acceptable effectiveness, but it involves many steps, and the wastewater generated can cause secondary pollution to the environment. Although there are wet and thermal detoxification methods, due to costs and other factors, there is still no effective solution to the chromium pollution caused by the magnesium-chromium waste bricks resulting from their use. The fundamental solution to the chromium pollution problem in industries such as cement, ceramics, and metallurgy is to use chromium-free alkaline refractory materials. As countries pay increasing attention to environmental issues, and particularly in developed countries in Europe and the United States where laws have been enacted to ban the use of magnesia-chromia bricks, the development of chromium-free refractory materials has become a topic of active research. 4. Development of chromium-free refractory materials for RH furnaces. Chromium-free refractory materials have been under research, development, and improvement since the 1980s. The development of chromium-free refractory materials focuses mainly on four typical types of materials: magnesium-zirconium, magnesium-spinel, magnesium-spinel-titanium, and magnesium-spinel-zirconium. Among them, due to the excellent high-temperature properties and chemical inertness of zirconia, zirconium-containing refractory materials have become a focus of research. There is very little research on magnesium-zirconium refractory materials in China at present; some of the existing studies focus on magnesium-zirconium castables. The research and application of magnesium-zirconium bricks in glass furnaces are well established, while research on such bricks for RH furnaces is still in its initial stages. Abroad (such as in Japan), there may be successful studies that have not been reported. Zirconium-magnesium spinel bricks are produced by adding ZrO2 (especially in fine powder form) to spinel; as a result, these bricks have a dense structure, high strength at high temperatures, and good volume stability. This gives them corrosion resistance, resistance to structural spalling, and the ability to retain materials on their surface (commonly known as maintaining a protective layer on the furnace). However, their high thermal conductivity leads to an increase in the temperature of the furnace, and research is currently being conducted to improve this aspect. Titanium-zirconium corundum refractory materials are made using A12O3-ZrO2-TiO2 and A12O3-ZrO2-TiO2-SiO2 as raw materials to form slabs; adding a certain amount of AZT or AZTS to high-alumina or aluminocarbon refractory nozzles can further modify their properties, resulting in nozzles with higher strength and excellent erosion resistance, without a decrease in thermal shock resistance, thereby extending their service life. The raw material for AZT is produced by electrofusion or sintering methods. The annealing temperature of the molten AZT has a direct impact on its mineral composition; when annealed at 1400°C, TiO2 almost entirely transforms into Al2O3·TiO2, resulting in a very low coefficient of thermal expansion. Sintered AZTS has an even lower thermal expansion coefficient than molten AZT, and its cost is also lower. AZTS is composed of m-ZrO2, mullite, Al2O3·TiO2, and corundum, and has a much lower coefficient of thermal expansion than molten AZT or AZS materials. Although the A12TiO5 composite material possesses properties such as a near-zero coefficient of thermal expansion, low thermal conductivity, a high melting point, as well as good resistance to thermal shock and thermal impact, it also has two major fatal weaknesses: first, there are significant differences in the crystal structure along various crystal axes, which leads to the formation of numerous microcracks during cooling, resulting in very low mechanical strength ； Secondly, at temperatures between 800 and 1300°C, it decomposes easily into rutile and corundum phases, losing the excellent properties of Al2TiO5; thus, this limits the widespread use of this material. Table 1 Typical properties of several types of magnesia-chromia bricks. Brick type, Chemical composition %, Apparent porosity %, Compressive strength at room temperature kg/cm2, Flexural strength at high temperature kg/cm2 (1480°C): MgO, Cr2O3, Al2O3, Fe2O3, SiO2, CaO. Fully bonded: 71.3, 58.9, 16.9, 18.8, 6.2, 13.0, 2.8, 6.0, 1.3, 2.0, 1.0, 0.8; 16.1, 13–18, 820, 600–1100, 841, 100–145 (1450°C). Semi-rebonded: 66.1, 53.4, 18.7, 27.2, 7.8, 6.8, 4.7, 10.5, 1.3, 1.3, 1.3, 0.8; 13.1, 14.3, 102, 054, 157 (1400°C). Rebonded: 64.4, 20.8, 6.5, 6.5, 1.2, 0.6, 11.9, 1080, 130