According to different production methods, magnesia is mainly classified into the following categories:
1. Electrically fused magnesia:
Magnesium oxide obtained by refining magnesite in an electric furnace at 2750 ℃ is called fused magnesia. Generally, it has extremely strong high-temperature resistance, is not easy to be melted by alkaline substances, and is not easy to be eroded by molten metals at high temperatures. It has excellent thermal conductivity at room temperature. The high refractory carbon magnesia brick (composed of graphite and fused magnesia) made from it as a raw material is widely used in converter steelmaking and off furnace refining, resulting in shorter melting cycles, longer furnace life, and improved steel quality. Compared to ceramics fired from aluminum oxide, ceramics made from aluminum oxide exhibit better high-temperature performance. In general, compared to sintered magnesia, fused magnesia has the advantages of larger grains and higher density, and can be used as a high-quality raw material for producing magnesium based refractory materials.
Fused magnesia
Figure 1 shows the microstructure of fused magnesia. Its main crystalline phase is periclase, with a large grain size, the largest of which can reach the millimeter level. The intercrystalline silicate phase is distributed in the form of a thin film, with a film thickness of 1-10 μ Within the range of m, its composition is mostly CMS or C3MS2. The grain boundaries are thin and relatively straight. Due to its few intergranular and intragranular pores, high bulk density, and large crystal size, it has excellent high-temperature performance.
2. Reburned magnesia:
Also commonly referred to as low-grade magnesia, its production involves placing massive magnesite ore in a shaft kiln of about 20-50m3, using coke as its fuel, and using one-step calcination and manual discharge to produce a magnesia product with a MgO content of about 92% and a bulk density of about 3.10g/cm3. The magnesia produced in this way is cheap and serves as the main raw material in the production of ordinary magnesia fired bricks and amorphous refractory materials.
Reburning magnesia
Figure 2 shows the microstructure of the calcined magnesia. The main crystalline phase is periclase, with small crystal sizes, fine and flat grain boundaries, and often accompanied by silicate phases. The composition fluctuates between C3S-C2S-C3MS2-M2S. The periclase grains are mostly silicate bonded, with a large number of silicate phases and open pores between the grains, resulting in poor high-temperature performance.
3. Medium grade magnesia:
Medium grade magnesia is a magnesia selected from high-quality magnesite ores that has been lightly burned, finely ground, balled, and calcined to improve its density and purity. Its MgO content is ≥ 95%, and its bulk density is ≥ 3.15g/cm3. The bulk density is generally between 3.20 and 3.25 g/cm3, and is divided into two grades: 94 and 95. Medium grade magnesia is now an important component of the magnesia category, and as a raw material, it is widely used in fired bricks such as medium grade magnesia bricks and some magnesia based amorphous refractory materials.
Medium grade magnesia
Figure 3 shows the microstructure of medium grade magnesia. The main crystalline phase is periclase, which is mostly rounded and irregular granular. The crystal size is large, accompanied by silicate phases, and is mainly cemented by silicate phases. There are also many pores, and the composition is between C3S-C2S-C3MS2-M2S, with good performance.
4. High purity magnesia:
In order to solve the problem that large crystalline magnesite ores are not easily sintered, the "two-step calcination" process is commonly used in China to produce high-purity magnesia with MgO ≥ 97% and bulk density ≥ 3.25g/cm3. Therefore, at present, it is commonly used in China to produce high-purity magnesia by lightly burning high-quality magnesite ore in a reverberatory furnace fueled by coal gas, then finely grinding and lightly burning the magnesium powder using a Raymond machine, and then using a high-pressure ball press to dry press the ball, finally calcining it in a high-temperature shaft kiln at a temperature of 1500 ℃ to 2000 ℃. The obtained high-purity magnesia has a MgO content of 97% and a bulk density of 3.25 to 3.30 g/cm3. Currently, high-purity magnesia is divided into two brands: 97 and 98. The output of this product in China is not stable. There are few manufacturers in Liaoning Province that can produce high-purity magnesia with MgO ≥ 97.5%, C/S ≥ 2, and bulk density ≥ 3.30g/cm3.
