What is the difference between magnesia brick and high alumina brick?

Magnesia bricks and high alumina bricks are two different materials and refractory products of different properties, which can also be used in high temperature kilns, but many customers have made a difficult choice when choosing, and cannot distinguish the characteristics and uses of the two types of bricks. .

Magnesia bricks belong to MgO products and belong to alkaline refractory bricks. Why do you say that? This is because magnesium oxide is an alkaline oxide, has the properties of alkaline oxides, and belongs to a cementitious material. White or off-white powder, odorless, tasteless, non-toxic, is a typical alkaline earth metal oxide, chemical formula MgO. The melting point is 2852°C, the boiling point is 3600°C, and the density is 3.58g/cm3 (25°C).

The raw materials for the production of magnesia bricks mainly rely on magnesite. The production process of magnesia bricks is to mix dead-burned magnesia (particles below 5 mm to fine powder) with an appropriate amount of sulfite pulp waste liquid or caustic magnesia as a binder Compression molding under a press. The composition of magnesia brick is: metallurgical magnesia, sulfite pulp waste liquid, caustic magnesia. Metallurgical magnesia is MgO decomposed from magnesite above 1600 ℃, its composition is: MgO: 92-93.5%, Fe2O3: 1.5-3%, AL2O3: 0.5-3%, CaO: 1-2.5%, SiO2 : 2-2.5%.

Disadvantages of magnesia bricks: The refractoriness of magnesia bricks can reach more than 2000 ℃, and its softening temperature under load varies greatly with the melting point of the cemented phase and the number of liquid phases produced at high temperatures. Generally, the softening starting temperature of magnesia bricks under load is between 1520 and 1600 °C, while high-purity magnesia bricks can reach 1800 °C. The starting temperature of magnesia brick softening under load is not much different from the collapse temperature. The linear expansion rate of magnesia bricks at 20~1000℃ is generally 1.2%~1.4%, and it is approximately linear.

When the liquid phase appears in the brick at high temperature, it will shrink suddenly. The thermal conductivity of magnesia bricks is relatively high, second only to carbon bricks and silicon carbide bricks in refractory products, and it decreases with the increase of temperature. The thermal shock resistance of magnesia bricks is poor, and improving the purity of magnesia bricks can appropriately improve the thermal shock resistance. The ability of magnesia bricks to resist acid slag is very poor, so they cannot be directly contacted with silica bricks when used, and neutral bricks are generally used to separate them. The electrical conductivity of magnesia brick at room temperature is very low, but it cannot be ignored at high temperature (such as 1500 ℃). If it is used in the bottom of electric furnace, attention should be paid. The hydration resistance of magnesia bricks is poor, and it is easy to hydrate when exposed to water, and cracks occur, reducing its strength. Therefore, pay attention to moisture, rain and snow during storage and transportation.

High-alumina bricks belong to AL2O3-SiO2 products, which are also called aluminum-silicon products. In other words, they can also be called neutral refractory bricks. From my point of view, the main oxides of high-alumina bricks are AL2O3 and SiO2. Phase, and AL2O3 belongs to R2O3 trivalent oxide, SiO2 belongs to RO2 tetravalent oxide, the combination of the two is to be able to produce liquid phase sintering reaction at high temperature to form mullite The main raw material of aluminum brick is kaolinite AL2O·2SiO2·2H2O, the melting point is 1750~1787℃, and the density is 2.58~2.67g/cm3. Therefore, under the condition that the raw material has a high temperature sintering liquid phase reaction, on the one hand, it can form mullite faster. With a high temperature melting point, the load softening temperature of high alumina bricks has a great relationship with the original taste.

The softening temperature of the lowest grade high alumina brick under load is 1450℃. Why is high alumina brick a neutral product? Because AL2O3 is a typical amphoteric oxide, it is alkaline in the case of strong acid, and the oxide that is acidic in the case of strong alkali can act with both acid and alkali. Both produce salt and water. Therefore, high alumina bricks are neutral refractory products, but there is also a point that AL2O3 is more than 70% products.

Magnesia bricks and high-alumina bricks are two commonly used products for masonry and repair in our kilns. Before choosing to use, we must analyze and confirm the characteristics of the bricks and their properties to avoid losses caused by wrong selection.