- Time:Mar 21, 2023
Magnesium oxide aggregate and fine powder account for more than 80% in magnesia castables. During the storage of magnesia raw materials or the preparation and maintenance of magnesia castables, magnesium oxide will react with water to form magnesium hydroxide. The hydration reaction in magnesium castables produces a large amount of magnesium hydroxide, which leads to a series of problems, and the research on its anti-hydration has become a problem that cannot be ignored.
1. Hydration mechanism of magnesium oxide
Magnesium oxide has a NaCl-type face-centered cubic crystal structure, and magnesium ions are in the octahedral vacancies of oxygen ions. The reason for the activity of magnesium oxide crystals can be explained as follows: the surface defects of oxides can be generated by ion vacancies. In the magnesium oxide structure, O2- and Mg2+ are connected to each other through Coulomb force to maintain their state. However, the energy released by the Coulomb interaction is greater than the negative electron affinity of O2-, and when the Coulomb force interaction field is strongly disturbed, cation vacancies are generated. O2- is an unstable state that is easily converted to O- while releasing 8.9eV energy. In the magnesium oxide structure, the two nearest O2- are adjacent. Each give an electron that is transformed into O-. O- is a free radical, and its movement in the lattice will form holes, while O- will stabilize and form active site centers on the surface. Therefore, it is inferred that magnesium oxide crystals can form the above-mentioned structural defects, endow them with reactivity, and thus easily undergo hydration reactions with water or water vapor.
2. Inhibiting the hydration method of magnesium oxide
Based on the research on the hydration mechanism and kinetic influencing factors of magnesium oxide, there are many reports on methods for improving the hydration resistance of magnesium oxide. Among them, the methods to reduce the hydration of large particles of magnesia can be summarized as follows:
(1) Sintering method. Add a small amount of sintering aids to the magnesia raw material for doping sintering, such as MgF2 to promote sintering densification, or add a small amount of rare earth oxides such as NiO, TiO2, Y2O3 and CeO2 to form a solid solution with MgO, reduce the activation energy of grain growth, and benefit MgO grain growth also improves the hydration resistance. But doping may reduce the refractoriness or erosion resistance of magnesia. Yang Jialiang added 2.5% high-quality bauxite to the high-purity magnesia raw material to obtain a hydration-resistant magnesia castable aggregate material containing a periclase/spinel multiphase structure.
(2) Surface modification of magnesia-calcium sand. Magnesia-calcium sand is impregnated with phosphoric acid solution and oxalic acid solution to form a hydration-resistant protective film with a thickness of 5-10 μm on the particle surface.
The above-mentioned protective layer has the problem of heating and decomposition, and even the decomposition products have an impact on the quality of molten steel. Yin Hongfeng's research team used MgO carbothermal reduction and diffusion oxidation to deposit MgO coatings on magnesia-calcium sands. As the coating deposition temperature increased to 1600 ° C and the time was extended to 6 hours, the maximum coating thickness was 54 μm, which was significantly improved. Hydration resistance of magnesia-calcium sand.
The hydration rate of magnesia fine powder is greater than that of granules. The idea of suppressing the hydration of magnesium oxide fine powder is to reduce the dissolution rate of magnesium oxide or reduce the formation of magnesium hydroxide. For example: adding a small amount of CaCl2 to the MgO suspension, due to the same ion effect, a Cl- "separation layer" is formed on the surface of magnesium oxide (MgO H+), which hinders the reaction of OH- with it to form magnesium hydroxide. Alternatively, a chelating agent is used, which reacts with Mg2+ prior to OH-, and generates insoluble matter, which acts as a protective layer on the surface of magnesium oxide particles.
3. Solutions to hydration problems in magnesium castables
The adverse effects of magnesium hydroxide generated from hydration in magnesium castables mainly include the following four aspects. First, since the density of magnesium oxide (~3.58g/cm³) is higher than that of magnesium hydroxide (~2.4g/cm³), the hydration reaction to generate magnesium hydroxide is accompanied by volume expansion. Larger expansion will generate structural stress in the castable. When the strength of the castable in the curing stage is too low to resist this stress, cracks will be generated, the material structure will be destroyed, and the mechanical properties will decrease. Second, as the temperature increases, the reaction between magnesium oxide and water vapor inside the castable accelerates. Third, the rapid hydration of magnesium oxide reduces the fluidity of the castable, resulting in shorter construction time and faster flow attenuation. Fourth, the magnesium hydroxide produced by hydration will thermally decompose rapidly during the heating process of the castable, and the water vapor pressure will cause local stress in the material. In severe cases, the material will crack, that is, the anti-burst performance is poor. Therefore, from the perspective of reducing the harm caused by the formation of magnesium hydroxide, ways to improve the hydration resistance of castables include:
(1) Add hydration inhibitor directly to inhibit the formation of magnesium hydroxide.
(2) Selection of magnesia types. Magnesium oxide reacts much faster in water vapor than with liquid water. That is, during the heating process of the castable at 100-200 ° C, the magnesia particles in the castable continue to undergo a stronger hydration reaction, and it occurs in a localized and non-uniform form, so a stress gradient is formed in the material, which affects the material Structural damage is greater. The magnesia commonly used in the field of castables are fused magnesia, sintered magnesia and active magnesia (light burnt magnesia. The initial grain size of fused magnesia is the largest, the grain boundary is the least, and the reactivity is the lowest, so its hydration The reaction is the weakest. Compared with fused magnesia, sintered magnesia contains more impurities (CaO, SiO2, Al2O3, Fe2O3, B2O3, etc.). According to Soudier's research work, the hydration rate of sintered magnesia is mainly related to Impurities are related to CaO/SiO2. The larger the ratio in the raw material, the higher the hydration rate. The active magnesia has the highest activity and the fastest hydration rate. In actual production, in order to reduce the hydration problem, more fused magnesia is used Sand or sintered magnesia with controlled impurity content is used as the raw material of magnesia castable.
(3) Regulate the drying system of the castable to prevent the temperature from rising too fast and causing cracking due to moisture volatilization. Or use microwave heating to dry the castable, so that the material is heated from the inside, greatly shortening the drying cycle to about 1 hour; at the same time, the hardening process of the castable is accelerated.
(4) Optimize the bonding system to improve the bonding strength of materials.
(5) Control the microscopic morphology of magnesium hydroxide to improve the microstructure of the castable, and use additives to accelerate nucleation to form magnesium hydroxide with small grain size, which can fill holes in the material and improve the strength of the castable.
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