Magnesite is a natural ore composed almost entirely of MgCO3, and its theoretical chemical composition is MgO447.82%, CO252.18%.
Natural magnesite belongs to trigonal crystal or cryptocrystalline white magnesium carbonate. Magnesite can form a continuous solid solution with siderite. As the iron ore content increases, so does the density and refractive index. Magnesite contains impurities that can promote its sintering, but when the content of impurities is large, its fire resistance is strongly reduced. Specifically, the most harmful impurity to magnesite is CaCO3. During the calcination process, CaCO3-CaO + CO2 ↑ forms free CaO. In the production process, free CaO hydrates and causes cracking of the brick. On the other hand, CaO can form low melting point silicates (CMS, C3MS2) or polycrystalline C2S. Other impurities such as SiO2, Al2O3, Fe2O3, etc. will reduce the refractory properties of magnesite, so the content of impurities should be limited.
The content of MgO in magnesia has a great influence on the melting loss of magnesia particles in the molten slag and the penetration of molten slag. The magnesite with relatively low MgO content and impurities has less silicate phase in the structure, which reduces the degree of periclase separation of periclase crystals and improves the degree of direct combination of periclase and periclase in magnesite. To prevent the melting and melting loss of molten magnesia. If the content of impurities is high, more silicate phases are formed, especially the B2O3 compound 3 MgO. B2O3, 2 MgO. B2O3 has a melting point of only 1350 ° C, which will greatly reduce the fire resistance of the magnesia and the high temperature performance of MgO particles.
Experiments show that as the B2O3 content increases, the melt loss index increases. And because B2O3 exists on the grain boundary of periclase, the fire resistance of the grain boundary is reduced, and more low-melting silicate minerals are also formed, which promotes the loss of periclase to the slag.
In addition, the presence of B2O3 also promotes the reaction between MgO and C, degrades the structure of the brick, and accelerates the loss of the brick.