Magnesia Alumina Series Brick Introduction
Magnesia aluminum brick is a spinel-type (RO· R2O3) basic refractory material containing about 85% MgO and 5%~10% Al2O3. The crystal phase composition of the magnesia aluminum brick is dominated by periclase, and the magnesia-alumina spinel is the matrix (magnesium-aluminum spinel replaces the calcium-magnesium olivine in the magnesia brick to become a binder of periclase). The spinel aggregates to form a crystal network after crystallization, thereby making refractory brick structure compact. Since the coarse aggregate particles are not used in the production, the entire fire brick structure is uniform and the pores are dispersed.
MgO has a certain mutual solubility with MgAl2O4. The eutectic point is 1995 °C, the solid solution containing periclase contains Al2O3: 18%, and the solid solution based on MgAl2O4 contains MgO: 39%. When the temperature drops, the mutual solubility decreases. At 1700 °C, the periclase is about 3% solid solution of Al2O3; to 1500 °C, both MgO and MgAl2O4 completely fall off.
Magnesia Alumina Series Brick Classification
Magnesia aluminum refractory bricks are divided into two types, one is MA as the binding phase, the MA is formed in the firing, fire brick is dispersed between the large particles of the magnesia; the other is the pre-synthesized MA particles, which are added to the ingredients.
(1) MA combined magnesium aluminum brick
Magnesia alumina brick with magnesia alumina spinel as the main binding phase has an important effect on the microstructure and properties of Al2O3. When the content of Al2O3 is 15%-20%, continuous magnesia alumina spinel is formed in the refractory brick, When the content of Al2O3 is less than 10%, MA is distributed in an island shape between the magnesia.
The content of CaO and SiO2 in the refractory raw materials should be strictly controlled. Since CaO is a strong flux, industrial alumina is generally used as a source of Al2O3 at home and abroad.
The highest firing temperature can reach 1610 °C. In order to prevent the formation of FeO-MgO solid solution, the iron oxide can form MF, which can promote the sintering of the refractory product without significantly reducing the fire resistance. Therefore, it should be fired in a weak oxidizing atmosphere.
(2) Pre-synthesized MA magnesium aluminum brick
The synthesis reaction mechanism of MA is ion diffusion, which can be carried out at a relatively low temperature (1050 to 1300 ° C). As the temperature increases, the reaction is intensified. At about 1500 ° C, the synthesis of MA is quite complete, forming a sufficiently complete MA crystal, but to obtain a highly densified MA composition, sintering at a higher temperature is required.
Magnesia aluminum fire bricks include ordinary magnesia alumina bricks, magnesia alumina spinel bricks, and modified magnesia aluminum spinel bricks. During the firing of magnesium-aluminum bricks, MgO and Al2O3 react at high temperatures to form high-temperature mineral phases of magnesium-aluminum spinel (MgO·Al2O3). Magnesia aluminum spinel has a small thermal expansion coefficient and the same thermal expansion, which can reduce the thermal stress caused by temperature changes. Magnesia aluminum bricks have good thermal shock resistance and other thermomechanical properties.
1. Ordinary magnesium aluminum brick
Ordinary magnesia-aluminum bricks are made of ordinary sintered magnesia and high-alumina bauxite clinker. The granular material of the bricks is made of sintered magnesia, and the powder is made of mixed powder of sintered magnesia and high alumina bauxite clinker. Magnesium aluminate spinel minerals are formed in the matrix of the brick during the firing process. It is used in steel furnaces and electric furnaces, glass kiln, refractory products, fired tunnel kiln, shuttle kiln and other kiln.
2.Magnesia alumina spinel brick
Magnesia alumina spinel bricks are also called periclite-spinel bricks. Which is based on high purity fused magnesia or high-purity two-step calcined magnesia and high-purity pre-calcined magnesia alumina spinel, using precision compounding, high-pressure forming and high-temperature firing production process. Compared with magnesia chrome bricks, this kind of magnesia alumina composite brick not only eliminates the harm of hexavalent chromium, but also has good corrosion resistance, oxidation and reduction resistance, heat-resistant peeling property and high temperature volume stability. Which is a large and medium-sized cement. The most suitable chromium-free refractory material for the rotary kiln transition zone. It has also been used in high-temperature equipment such as lime kiln, glass kiln and refining equipment outside the furnace. The physical and chemical indicators of magnesia alumina spinel brick produced by a domestic refractory material factory are: MgO 82.90%, Al2O3 13.76%, SiO2 1.60%, Fe2O3 0.80%, apparent porosity 16.68%, bulk density 2.97g/cm3, normal temperature withstand voltage The strength is 54.4 MPa, and the flexural strength at 1400 ℃ is 6.0 MPa.
3.Modified magnesium aluminum spinel brick
Magnesia aluminum spinel fire brick is very successful in the transition zone of cement rotary kiln, but which is easy to produce tissue embrittlement and structural spalling in the firing zone, which is difficult to hang the kiln skin, the alkali-resistant steam and cement clinker have poor liquid permeability. And the ability to resist mechanical stress caused by deformation of the kiln body is limited, and the application in the firing zone is limited. To this end, researchers have developed modified magnesia aluminum spinel bricks suitable for the firing zone of cement rotary kiln.
Magnesia Alumina Series Brick Manufacturing Process
The production process of magnesia alumina refractory bricks is roughly the same as that of calcined magnesia bricks, except that a certain proportion of industrial alumina or extra-grade bauxite clinker is added to the ingredients. Industrial alumina has a lower impurity content than high alumina bauxite. The addition of natural bauxite clinker to the ingredients can improve the plasticity of the mud. Under the same conditions, the bulk density of the fire bricks is higher. Industrial alumina is generally added in an amount of 5% to 10%. Usually, it is finely ground with magnesia in a certain proportion, and then added as a fine powder, which is favorable for forming a uniform distribution of a new crystalline phase of magnesium aluminate spinel in the matrix of the product. There are also production methods using pre-synthesized magnesia alumina spinel for batching bricks. The critical particle size during the batching is beneficial to improve the thermal shock resistance of the refractory product, but it is not conducive to the density and strength of the product, and generally adopts 3 mm. The particle size is generally 3-1mm and 1-0.088mm and the ratio of 3-2mm coarse particles to less than 0.088mm fine powder should be controlled to improve the thermal shock resistance of the product. The firing temperature of the magnesia alumina refractory brick is determined according to the purity of the raw refractory material, generally 30-50 ℃ higher than the firing temperature of the magnesia refractory brick, and the firing temperature of the high-purity magnesia alumina brick is 1750-1800 ℃.
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