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Magnesia Alumina Spinel Brick
Magnesium-aluminum spinel has a series of excellent properties such as high melting point, good thermal shock stability, and resistance to slag erosion. It has been widely used in refractory materials and other industrial fields.
At present, the most common method for preparing magnesium-aluminum spinel is solid-phase reaction synthesis. That is, oxides, chloride oxides or carbonates are used as raw materials, and the raw materials are mixed and pressed into billets and then reacted at high temperature (greater than 1400°C) to prepare spinel. The electric melting method is another commonly used method for synthesizing spinel in industry, which also has disadvantages such as high energy consumption. Although wet chemical methods such as sol-gel method and hydrothermal method can synthesize spinel at a lower temperature, their operation process is complicated, the equipment required is expensive, and the cost is too high to meet the needs of large-scale industrial production.
Magnesium-aluminum spinel bricks produced with magnesia sand and magnesium-aluminum spinel sand as raw materials are usually called magnesium-aluminum (periclite-spinel bricks). The magnesia sand raw material used to produce periclase spinel bricks is required to have as low an impurity content as possible (especially CaO).
Domestic sintered magnesia MS95, MS97, MS97.5, and DMS97 are widely used. Spinel sand is used as granular material, and magnesia sand is used as fine powder and part of the granular material. According to the mixing, forming, and firing process of high-grade magnesia-alumina bricks, products with good high-temperature performance and high thermal shock stability can be produced.
Magnesia-alumina spinel bricks are often used in cement rotary kilns, glass kiln lattices, mixed iron furnaces, and refractory kilns with large temperature changes.
Magnesium Aluminum Chrome Spinel Brick
The magnesium aluminum chrome spinel brick system is the periclase/magnesium aluminum chrome spinel system: MgO-R2O3 (Cr2O3, Al2O3) system. In order to facilitate the comparison of the production of magnesium aluminum chrome products, Cr2O3 is introduced into MgO in the form of chromite, and the manufacturing process often accompanies the appearance of sesquioxides such as Cr2O3, Al2O3, and Fe2O3.
First of all, from the perspective of high temperature resistance and slag resistance, the MgO-MgO·Cr2O3 system has advantages in the MgO-R2O3 system. In addition to its high eutectic temperature (2350℃), it also lies in the low solubility of Cr2O3 in the silicate liquid phase.
As we all know, lower solubility should have stronger crystallization ability. It can effectively reduce the intercrystalline interface energy, so that the silicate liquid phase tends to move into the intergranular gap in an isolated state. It is easy to achieve direct bonding between periclase grains or through magnesium chrome spinel bridges, which can effectively improve high temperature strength and inhibit slag penetration, and improve slag resistance. However, Cr2O3 is highly volatile and has poor stability at high temperatures, especially under vacuum conditions.
Secondly, from the perspective of thermal shock resistance, the MgO-MgO·Al2O3 system is more superior. Because the solid solution-peptization effect of MgO·Al2O3 or Al2O3 in MgO at high temperature is much weaker than that of MgO·Cr2O3 or Cr2O3, especially MgO·Fe2O3 or Fe2O3, and the high-temperature vapor pressure of MgO·Al2O3 above 1600℃ is also lower than that of MgO·Cr2O3. Therefore, the MgO-MgO·Al2O3 system material is more stable when the temperature fluctuates and has good thermal shock resistance.
When the MgO/R2O3 ratio is constant, the MgO-MgO·Al2O3 and MgO-MgO·Cr2O3 systems are compared. It can be inferred that in the molten MgO-MgO·Al2O3 system material, there are fewer precipitated spinels in the periclase grains, and more spinels between the grains. MgO-MgO·Cr2O3 system materials have more intracrystalline precipitated spinel and less intercrystalline spinel. Obviously, changing the proportion of each component in R2O3 can adjust the distribution of spinel phase and change the microstructure.
Magnesium Iron Aluminum Spinel Brick
Magnesia-alumina spinel bricks are mainly used in cement kiln firing zones. Traditional refractory materials used in cement kiln firing zones are mainly magnesia-chrome refractory materials. The highly toxic hexavalent chromium that can be dissolved in water in magnesia-chrome bricks after use will cause serious environmental pollution. Finding chromium-free alternative products for cement kilns has been recognized by experts. Magnesia-alumina spinel bricks were proposed by RHI in the 1990s. It is a mixture of magnesia sand and pre-synthesized alumina spinel, and fired at high temperature under certain processes. It has now been widely used in the lining of cement rotary kilns to replace magnesia-chrome bricks.
The microstructural characteristics of magnesia-alumina spinel are relatively complex, mainly a combination of magnesia sand microstructure and alumina spinel microstructure. The interface between magnesia sand and alumina spinel is closely combined through the mutual diffusion of Mg2+, Al3+, and Fe2+ ions, realizing direct bonding of materials.
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