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The microstructure of the MgO-CaO-Fe₂O₃ alkaline mixture plays a crucial role in its performance during electric furnace melting. To ensure optimal results, the levels of SiO₂ and Al₂O₃ should be minimized, as they can negatively impact the material’s stability. Additionally, the proportions of CaO and Fe₂O₃ must be carefully controlled to maintain a certain amount of free CaO, which helps enhance the material’s resistance to SiO₂ erosion in the electric slag environment. A common approach is to blend high-iron, high-calcium magnesia with specific chemical compositions into a well-structured powder mixture, following strict particle size specifications. This method is straightforward and offers good sintering properties, but it may still lead to matrix erosion by slag during use. To address this issue, increasing the MgO content or incorporating higher-purity magnesia are effective strategies. Other methods also exist, but their selection depends on application conditions and cost considerations.
Among these, using a combination of high-iron, high-calcium magnesia with high-purity magnesia proves to be efficient, cost-effective, and highly effective. When used as hot feed, the fine content (below 0.074 mm) must also be controlled to optimize performance. The exact formula should be determined based on factors such as usage conditions, production capacity, and product pricing. Several typical technical properties of MgO-CaO-Fe₂O₃ alkaline mixtures are summarized for reference.
During operation, the MgO-CaO-Fe₂O₃ mixture typically experiences minimal mechanical stress in AC electric furnaces, mainly undergoing cold refilling or hot repair. In contrast, DC electric furnaces and iron alloy furnaces may require more intensive maintenance, including beating, although repairs are not always possible in DC furnaces.
Due to the presence of a moderate amount of the low-melting 2CaO·Fe₂O₃ phase, the mixture can be sintered effectively, forming a ceramic-like structure that enhances the working surface’s strength and resistance to mechanical impact during scrap loading. As the molten pool expands, the partial pressure of oxygen decreases, leading to the decomposition of 2CaO·Fe₂O₃:
**2CaO·Fe₂O₃ → 4CaO + 4FeO + O₂**
With the extremely low oxygen partial pressure in the molten steel, FeO diffuses into the periclase lattice, forming a solid solution:
**xMgO + 4FeO → (x+4)(MgFe)O**
This process increases the size of the crystal grains and strengthens the direct bonding between them, thereby improving the refractory’s thermal stability. The absorption of FeO also helps control sintering shrinkage, keeping it within acceptable limits.
When the mixture comes into contact with slag, the primary attack comes from SiOâ‚‚ and FeO. Key reactions include:
**2CaO + SiO₂ → 2CaO·SiO₂**
**FeO + (MgFe)O → (MgFe)O**
The formation of the high-melting-point 2CaO·SiO₂ phase prevents further slag penetration. Meanwhile, reaction (4) increases Fe²⺠concentration in the periclase solid solution, promoting crystal growth and densification of the working layer. Although this reduces the melting point slightly, the reaction is limited to a thin surface layer. Only when Fe²⺠concentration becomes very high does the material begin to melt into the slag. Since both reactions occur at the working surface, the resulting high-melting-phase products make the slag less aggressive toward the mixture.
In practical applications, the alkaline compound used in the early stages of the 150t UHP AC electric furnace at Tianjin Steel Pipe Company was imported from Aomag. It had an average furnace life of 260 cycles, with a maximum of 305. To achieve local production, Liaoning Magnesium Mine Refractory Co., Ltd. began using its own alkaline mixture in 1993, and it has performed exceptionally well ever since. The average furnace life reached 282 cycles, with a peak of 332, surpassing the imported material. The same mixture is now used by China International Iron & Steel Products Co., Ltd., where the furnace bottom life exceeds 700 cycles, and the slope life goes beyond 200.
In conclusion, the production of MgO-CaO-Fe₂O₃ alkaline mixtures is simple, and the material’s performance is excellent due to the presence of an appropriate amount of 2CaO·Fe₂O₃, which allows good sintering at medium temperatures. During use, it transforms into a high-melting phase, offering outstanding overall performance. It has proven highly effective in AC, DC, and iron alloy furnace bottoms, delivering significant operational benefits.