Mullite is a high-quality refractory material. It was first discovered and named on the Isle of Moore in Scotland. The composition of aluminum and silicon is a range, which can exist stably under normal temperature and pressure. Natural mullite is relatively rare and is usually prepared by heat-treating aluminum-silicon compounds. Its microstructure is columnar or granular. Because of its high refractoriness, it is usually used in high-temperature refractories. In addition to the advantages of high temperature resistance, high strength, and low thermal conductivity, it also has the advantages of excellent thermal shock stability, small high temperature creep value and good chemical corrosion resistance.
There are mainly the following 5 methods for the preparation of mullite raw materials
(1) Solid phase reaction sintering method
The preparation method by solid-phase reaction sintering method has become perfect so far, and it can be divided into the following types: mechanical activation, addition of suitable sintering aids, and addition of a certain amount of mullite seed crystals. The most common method is to add a suitable sintering aid to prepare.
Du Jing et al. used high-purity kaolin and industrial alumina lightly burned at 1000°C as raw materials, and added part of ZnO, talc and zircon sand as additives to synthesize low-cost, high-purity mullite. Its relative content is 97%, and its bulk density is 2.89g/cm3.
(2) Hydrolysis precipitation method
The hydrolytic precipitation method refers to the formation of precipitation in the solution by adding a precipitation agent, and then forming a powder that meets the requirements after heat treatment. The hydrolysis precipitation method can be divided into two types: co-precipitation method and single-phase precipitation method.
(3) Hydrothermal crystallization method
The hydrothermal crystallization method mainly uses the principle of dissolution-recrystallization. The raw material is first dissolved in a hydrothermal medium, and then the ion or molecular clusters in the solution are transferred to the growth zone with seed crystals by convection, and crystals will be precipitated after the solubility is saturated. The powder prepared by the hydrothermal crystallization method has higher particle purity and better dispersibility, and the synthesized powder generally does not need to undergo high temperature heat treatment.
Xue Rujun and others used coal-series kaolin as raw materials to synthesize ultra-fine mullite powder by hydrothermal crystallization method, and explored the effects of heat treatment temperature, hydrothermal crystallization temperature, solid-liquid ratio, alkali concentration and time on mullite. The effect of stone synthesis and its particle size.
(4) Sol-gel method
The sol-gel method prepares a gel with a three-dimensional network structure by uniformly mixing inorganic substances or metal alkoxides as precursor materials, after undergoing a chemical reaction of hydrolysis and condensation, and then polymerizing colloidal particles. Then put it in a constant temperature drying box for drying. After heat treatment, the organic components are oxidized and volatilized, and finally an inorganic non-metallic material is obtained. Generally speaking, according to the mixing degree of the gel precursor, mullite gel can be divided into single-phase mullite gel (SiO2 and Al2O3 are mixed at the molecular level) and two-phase mullite gel (SiO2 and Al2O3 is mixed at the nanometer level. The mixing degree of the mullite gel precursor has a decisive effect on the phase transition and formation temperature of mullite.
The formation process of dual-phase gel mullite is generally carried out in two ways:
1. Mullite is carried out by the reaction of amorphous SiO2 and transition state Al2O3;
2. The gel begins to form Al-Si spinel phase at around 1000°C, and then reacts with amorphous SiO2 to form mullite at 1200°C.
Zhao Huizhong and others used TEOS and analytically pure aluminum isopropoxide (ALP) as the main raw materials to prepare a nano-level mullite precursor solution, and finally prepared a particle size of 30-50nm with a specific surface area of 30-50nm. 95.81-138.91m2/g mullite material.
(5) Gas-solid reaction method
The gas-solid reaction method refers to a multi-phase reaction process in which the gas phase and the solid phase participate in the reaction at the same time in the reaction system. According to the presence or absence of catalyst, it can be divided into gas-solid phase catalytic reaction process and gas-solid phase non-catalytic reaction process. Xu Xiaohong and others used kaolin fine powder and industrial Al(OH)3 fine powder as the main raw materials, plus AlF3 and V2O5 as catalysts, and prepared mullite whiskers by a gas-solid reaction catalyzed process.
