Are High Alumina Bricks More Expensive or Mullite Bricks More Expensive?

Although high-alumina bricks and mullite bricks are mainly made of alumina, there are still differences between the two. High-alumina bricks are made of calcined bauxite, and mullite bricks are made of sintered or fused mullite. Not the same, please inquire about specific prices.

High-alumina bricks are different according to the content of aluminum oxide (Al2O3). Currently, there are four grades commonly used: 55-60% (level III), 65-70% (level II), and 75% (level I) ), 80% (special). High-alumina products are a kind of neutral refractory material, which is made of bauxite or other materials with high alumina content and fired. It has high thermal stability, the refractory temperature is 1700°C, and the upper limit use temperature is 1300-1400°C.

The production process of various grades of high alumina bricks is roughly the same as that of multi-clinker clay refractory bricks. The clinker calcined at high temperature is first coarsely crushed by a jaw crusher, then finely crushed, sieved into the required particle size, and then goes through the processes of batching, mixing, trapping, wet grinding, molding, drying, and firing.

The production process of sintered mullite bricks is similar to that of high alumina refractory bricks. Synthetic mullite clinker is used as granular material, and the fine powder is synthetic mullite clinker, or white corundum, quartz powder and pure clay are used to prepare mixed fine powder equivalent to mullite. The granular material and fine powder are mixed in proportion, and the clinker is fired at high temperature; the large particles of clinker and clinker are recommended not to exceed 2-3 mm, and there is quite a lot of fine powder less than 0.088 mm. In this way, it is possible to fire at 1580-1600°C or extend the holding time to around 1500°C.

The ingredients can be determined according to the Al2O3 content determined by the conditions of use. The current common ingredient methods are:
1. Synthetic mullite (sintered or fused) as aggregate + synthetic mullite fine powder;
2. Synthetic mullite (sintered or fused) as aggregate + synthetic mullite fine powder + Al2O3 fine powder + high-purity clay powder;
3. Synthetic mullite (sintered or fused) and fused white corundum as aggregate + synthetic mullite fine powder + Al2O3 fine powder + high-purity clay powder.

The particle size ratio should be formulated according to the ingredient principle of “big at both ends and small in the middle”. Using sulfurous acid pulp waste liquid or poly aluminum chloride or polyphosphate as the binder, after kneading evenly, high-pressure molding, firing in the high-temperature kiln, firing temperature depends on the content of Al2O3 in refractory bricks, generally at Between 1600 and 1700°C. Mullite bricks usually contain impurity oxides such as TiO2, Fe2O3, CaO, MgO, K2O, and Na2O. The content of impurities in mullite produced from natural raw materials is higher than that of synthetic raw materials.

Properties of mullite bricks
It has high refractoriness, high load softening temperature, low high-temperature creep rate, good thermal shock resistance, slag erosion resistance, high shear modulus, etc. Mullite refractory bricks are widely used, mainly used in high-temperature hot blast stoves, large blast furnace hearths and bottoms, steelmaking electric furnace roofs, ceramic industrial kiln linings, kiln furniture, and glass kiln cement kiln linings, etc. Sintered mullite bricks produced from sintered synthetic mullite have good corrosion resistance and impact resistance and have achieved satisfactory application results within a certain range.

Using the principle of fused mullite, the fused mullite brick produced by the sintering method is mainly used in glass kilns. This kind of brick has high mechanical strength and load deformation temperature, high density, small porosity, high crystal phase content, and no abnormal phenomenon on the thermal expansion curve. However, when such substances are in contact with a large number of alkali substances at high temperatures, mullite is unstable and easy to decompose, which reduces the stability of this type of refractory material to a certain extent. In addition, the final properties of fused mullite bricks produced from fused mullite raw materials of different purity will also vary; even fused mullite with the same chemical purity (such as high-purity products) has different melting processes, The resulting refractory bricks also have different properties.

Generally speaking, the high-temperature flexural strength of sintered mullite bricks is much higher than that of fused mullite bricks, which is due to the interlacing of some small but well-developed crystal networks in the former structure. The latter has good volume stability and excellent creep resistance due to larger crystals. Moreover, fused mullite bricks have better corrosion resistance than sintered mullite bricks of the same composition. This is because when corroded by alkali substances at high temperatures, mullite will generate corundum and nephrite liquid phases, and the nephrite liquid phase has low viscosity, which is easy to lose and cause brick damage. Due to the relatively high porosity of sintered mullite, the above reaction can be carried out inside the brick body, and the destructive power is much greater; and because of the high density of fused mullite brick, the above reaction can only be carried out on the surface of the refractory brick, destroying relatively weaker.

Mullite bricks should not be in contact with alkaline substances when the temperature is above 1450°C, otherwise, the mullite will decompose. In a reducing atmosphere above 1370°C, mullite will also decompose, and part of SiO2 will become gaseous SiO2 and leave the brick body. When the temperature is higher than 1650°C, mullite will decompose even in a lower oxygen partial pressure than in a reducing atmosphere. It can be seen that the combustion temperature directly affects the firing of mullite bricks.

Are high alumina bricks more expensive or mullite bricks more expensive? Usually, mullite bricks are more expensive than high alumina bricks, please contact us for specific prices.