What Is Mullite Refractory?

Introduction to mullite:
Mullite is a refractory raw material mainly composed of 3Al2O3.2SiO2 crystalline phase. Mullite is divided into natural mullite and synthetic mullite. There are few natural mullites, which are generally synthesized artificially.

The chemical composition of mullite is 71.8% Al2O3 and 228.2% SiO. The mineral structure is orthorhombic system, and the crystals are in long column, needle and chain arrangement. The needle mullite is interspersed in the products to form a solid skeleton. Mullite is divided into three types: α- Mullite, equivalent to pure 3Al2O3 · 2SiO2, abbreviated as 3:2 type; β- Mullite, with excess Al2O3 in solid solution, slightly expanded in size, called 2:1 for short; γ- Mullite, with a small amount of TiO2 and Fe2O3 in solid solution. Mullite is chemically stable and insoluble in HF. Its density is 3.03g/cm3, Mohs hardness is 6-7, melting point is 1870 ℃, thermal conductivity (1000 ℃) is 13.8W/(m · K), and linear expansion coefficient (20-1000 ℃) is 5.3 × 10-6 ℃, elastic modulus 1.47 × 1010Pa.

Mullite has good high-temperature mechanical and thermal properties, so synthetic mullite and its products have the advantages of high density and purity, high high-temperature structural strength, low high-temperature creep rate, small thermal expansion rate, strong chemical resistance, and good thermal shock resistance.
The key index to evaluate the quality level of mullite is the phase composition and density of mullite.

Common forms of mullite:
Generally, mullite materials can be directly synthesized from kaolinite, sillimanite group minerals, aluminum hydroxide or aluminum oxide and silicon dioxide. The clay material reacts with alumina or sillimanite group minerals and industrial alumina to form primary and secondary mullite under heating condition F. The primary mullite is formed within the range of 1000~1200 ℃. The temperature is further increased and the crystallization is increased. The formation of secondary mullite usually ends at 1650 ℃. In order to produce dense mullite products, two-step sintering method is commonly used.

Mullite has two crystal forms: acicular and prismatic. The glass phase and the chemical composition phase of the material are reinforced by the acicular mullite. At the same time, the fire resistance of the acicular mullite material is higher than that of the prismatic mullite material. Kaolinite is rapidly heated to above 1400 ℃ to form acicular mullite. Otherwise, it will form prismatic mullite when it is slowly heated to a lower temperature. There are also reports of tubular shape and spherical mullite. The former is speculated to be caused by the tension caused by the uncoordinated size of silica and alumina tetrahedrons, and the latter is the so-called nitrogenous mullite. The anisotropy of mullite thermal expansion makes it have good thermal stability. When mullite material is used as a feeder accessory, it can be directly replaced to the running feeder without preheating.

Synthetic method of mullite:
The methods of mullite synthesis can be divided into sintering method and electric melting method. The sintering method can be divided into dry method and wet method according to the way of raw material preparation. The dry method process is to grind the ingredients together and burn them in rotary kiln or tunnel kiln after ball pressing or billet pressing; The wet process is to grind the batch material into slurry by adding water, then press the filter to dehydrate it into mud cake, and then vacuum squeeze the mud into mud section or mud blank, and then burn it.

The electric melting method is to add the batch into the electric arc furnace, melt it in the high temperature formed by the electric arc, and then crystallize it by cooling. When using natural raw materials (such as bauxite) for batching, it is allowed to directly crush the block raw materials to particles less than 1.5mm without grinding, and then mix them with other powder raw materials in the mixer.
The sintering synthesis of mullite is generally carried out at 1650~1700 ℃. The main technological factors affecting the synthesis of mullite by sintering are the purity, fineness and calcination temperature of raw materials. The synthesis of podzolite by sintering mainly depends on the solid phase reaction between Al2O3 and SiO2. Therefore, improving the dispersion of raw materials will accelerate the process of in-phase reaction. Especially<8 μ M particles have a great effect on the formation and sintering of synthetic mullite. It can be seen that the full mixing and fine grinding of raw materials is an important process condition to promote the solid state reaction of mullite synthesis.

Mullite generally starts to form at 1200 ℃ and ends at 1650 ℃. At this time, it is in micro shape, and the crystallization develops well when the temperature exceeds 1700 ℃. It can be seen that the combustion temperature directly affects the formation and crystal development of mullite. Therefore, heating to a certain sintering temperature and prolonging a certain holding time are necessary conditions for mullite synthesis. The purity of raw materials used for mullite synthesis is very strict, and a small amount of impurities will reduce the content of mullite. In industrial production, it is inevitable to bring in various impurities, including Fe2O3, TiO2, CaO, MgO, Na2O and K2O, among which Na2O and K2O are the most harmful. They inhibit the formation of mullite, lead to the generation of a large number of silicon rich glass phases, and reduce the content of mullite. Fe2O3 will delay the process of mullitization and increase the amount of glass phase. When a small amount of TiO2 exists, some Ti ions enter the mullite character to form a solid solution, which promotes the formation of mullite and crystal growth. When the TiO2 content is too high, it still acts as a flux.

Molten mullite is prepared by melting the batch in an electric arc furnace and cooling mullite from the melt. Its crystallization process is similar to that of the phase diagram of Al2O3-SiO2 system. When Al2O3 in the batch is higher than 71.8% of the theoretical group in mullite, mullite solid solution with excess Al2O3 is formed, namely β- For mullite, corundum phase will appear only when Al2O3 > 80%. The mineral phase composition of fused mullite is generally mullite crystal and glass phase. Compared with sintered mullite, electrofused mullite has well-developed crystals, large grains, few defects, and crystal size hundreds of times that of sintered mullite, so its high temperature mechanical properties and corrosion resistance are relatively good.

The mullite refractories produced by Kerui Refractories include mullite light aggregate bricks, corundum mullite bricks, mullite castables, light mullite castables, corundum mullite castables, etc.