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Three Key Parameters to Improve the Service Life of High Purity Magnesia Bricks
High-purity magnesia bricks are structural materials for high-temperature furnace linings, and their use temperature can reach 2200℃. However, when used under such conditions (such as the ultra-high temperature environment in carbon black reactors), the purity and performance of the casting sand of high-purity magnesia bricks need to be carefully balanced. We know that refractory materials (including magnesia refractories) must not only be resistant to high temperatures and corrosion/erosion but also must be as stable as possible during use. It is generally believed that the purity of magnesia sand and the porosity and periclase grain size in physical properties are key parameters to improve the service life of high-purity magnesia bricks.
The MgO content in magnesia is an important indicator of magnesia quality. However, it is not enough to use the MgO content as an evaluation criterion. The relative content of impurities, especially those that are easy to form melts under high-temperature conditions, is also important. Among the impurity components, B2O3 and SiO2 are the first, followed by Al2O3, Fe2O3, MnO and CaO. Their influence depends not only on their content, but also on the CaO/SiO2 ratio to evaluate the influence of each component on the quality of magnesia. It is usually desirable to have a high CaO/SiO2 ratio in magnesia in order to obtain the best high-temperature performance.
Magnesia refractories used in ultra-high temperature environments require high purity: because high-purity magnesia refractories have high high-temperature strength. Experiments have shown that the high-temperature flexural strength of magnesia refractories tends to decrease slowly with the MgO content from 90% to 98%. However, when the MgO content exceeds 98%, it suddenly increases, and when the MgO content increases to more than 99%, its high-temperature strength can be significantly improved in this case. The influence of impurity types has become insignificant because in this case, the impurities are isolated at the junction (corner) of periclase grains. Therefore, the MgO-MgO direct bonding structure is well developed, the high temperature strength is high, the material has high wear resistance, and strong corrosion/erosion resistance.
Density (porosity)
The relative density μ of refractory materials and the porosity ε have the following relationship:
ε=1-μ
This shows that materials with low porosity have high density, so porosity is a measure of material density. They have an impact on the erosion of materials. The criteria for evaluating porosity are:
(1) The volume of pores (density, i.e., total porosity ε).
(2) The type of pores (closed pores are more favorable than open pores.
(3) Pore size and shape (specific surface area).
Periclite grain size
The erosion of periclase grains starts with the foreign components (substances) at the interface. Therefore, the periclase grain size increases, and the specific surface area is correspondingly reduced. It can be seen that increasing the periclase grain size reduces the tendency of magnesia refractory materials to be eroded, indicating that coarse-grained sintered magnesia has the advantage of high erosion resistance.
Electrofusion is the fundamental method for producing large-grained magnesia. Compared with sintered magnesia, fused magnesia has the following advantages:
(1) Good densification (melted magnesia with the shell material removed has almost no open pores).
(2) Large periclase grains (especially when the block is properly cooled).
For sintered magnesia, the following technical measures can be taken to increase the periclase grain size:
(1) Early technical measures include:
1) Adding Cr2O3 to increase the mobility of the periclase lattice to increase the grain size and promote densification.
2) Cultivating large grains with TiO2: and Fe2O3 through the transition liquid phase.
3) Adding oxides and calcining at the highest temperature in the vertical kiln to achieve large grains. In actual production, in order to promote the growth of periclase grains, a small amount (less than 0.5%) of Cr2O3, Fe2O3, especially ZrO3, and oxides and salts of titanium, vanadium, manganese, aluminum, and copper are often used. Rare earth elements promote grain growth, but the mechanism is different.
(2) Using natural microcrystalline magnesite as raw material and sintering at very high temperatures in a vertical kiln, high-quality sintered magnesia with periclase grain sizes ranging from 50μm to 200μm can be obtained.
Through the above analysis, it can be considered that the special high-purity magnesia refractory products produced by high-quality magnesia with a purity of 98.5%-99% MgO, a bulk density greater than 3.35g/cm3, and a periclase grain size of 50μm to 200μm can adapt to the operating conditions of the ultra-high temperature (2100℃) area in the carbon black reactor. For the high temperature (1725-1925℃) area, 97.5%~98% MgO magnesia refractory products are used for lining, and partition masonry is carried out to obtain better economic/technical effects. Under the condition of continuous operation of the carbon black reactor, it is proved that its service life is sufficient. However, when the furnace operating conditions are difficult to ensure continuous operation, the discontinuous damage of the furnace lining caused by spalling is an important reason why its service life is not very ideal.
