Preparation,Structure And Properties Of Composite Magnesia Based On Magnesite Of High Calcium

Magnesia is the most important raw material for magnesia-based refractories.It mainly Prepared by magnesite and other solid ores.However,in recent years,due to the predatory mining of magnesite,there is an increasing shortage of high-grade magnesia.In fact,there are many cryptocrystalline high-purity magnesite in Tibet,but due to the free calcium oxide of the magnesite which is easy to hydrate during transportation and storage,the magnesite in Tibet has not been completely utilized.In addition,dense magnesia has poor thermal shock resistance and good wettability with slag,which makes the magnesia-based refractories poor thermal shock resistance and slag penetration,and shortens the service life of the refractories.It is an effective way to improve the properties and the service life of magnesia-based refractories by introducing additives to magnesia,eliminating free calcium oxide,and using new phase to improve the thermal shock stability and slag penetration resistance of magnesia.The purpose of this paper is to prepare dense magnesia composite raw materials from Tibetan lightly burnt magnesia containing free CaO.The minerals composition and microstructure of the composite raw materials are adjusted by introducing ZrO₂,Zr SiO₄ and Al₂O₃-ZrO₂ in order to improve the thermal shock resistance,hydration resistance and slag penetration resistance of the free CaO-containing magnesia raw materials.The following conclusions can be drawn:1.The hydration resistance and thermal shock resistance of composite material were significantly improved by introducing m-ZrO₂,but the slag penetration resistance of composite material gradually decreased with the increase of m-ZrO₂content.The addition of m-ZrO₂ eliminates free CaO and reduces the open porosity of the specimen resulting in a significant increase in hydration resistance.As the ZrO₂ content increases,the intergranular bonding increases and crack deflection and ZrO₂ fracture through the grain occurs.The introduction of m-ZrO₂ reduces the grain size of the specimens,while more Ca ZrO₃ is generated at the grain boundaries during the erosion process,resulting in wider magnesite grain boundaries and increased slag penetration.2.The addition of Zr SiO₄ significantly improves the hydration resistance and thermal shock resistance of composite material.The slag penetration resistance of MgO-Zr SiO₄ samples show a trend of decreasing first and then increasing with the increase of Zr SiO₄.With 1 wt%Zr SiO₄ was added,the relative density and strength of sample were reduced due to the formation of Ca ZrO₃ with volume expansion;With the further increase of Zr SiO₄ content,Ca ZrO₃ is transformed into c-ZrO₂ phase,and the relative density of the sample increases significantly.In addition,the formation of Ca ZrO₃and c-ZrO₂ in the sample eliminates the free CaO,and the grain size of periclase increases,caused the hydration resistance of the sample gradually improve;At the same time,the thermal expansion coefficient of the sample shows a downward trend,and the deflection of crack caused the thermal shock resistance of the sample continues to improve.When 1wt%Zr SiO₄ is added,the porosity of the sample increase and the slag penetration resistance decrease significantly;With the further increase of Zr SiO₄,the relative density and periclase grain size of the sample increase,and a small amount of Ca ZrO₃ phase is formed between the penetrated layer and the original layer,and the slag penetration resistance of the sample increases significantly.3.Adding Al₂O₃-ZrO₂ composite powder with different composition can improve the hydration resistance and thermal shock resistance of magnesia composite.The composite powder with Al₂O₃ lower than 2wt%will reduce the penetrated process of slag into the sample.The formation of c-ZrO₂ and Ca ZrO₃ eliminated the free CaO in the in the sample,and the formation of Mg Al₂O₄ promotes the grain growth and densification of the sample,and the hydration resistance of the sample is significantly improved.The toughening phases of Mg Al₂O₄ and c-ZrO₂ in the sample with Al₂O₃-ZrO₂,which improves the grain bonding strength and strength of the sample.The crack deflection in the sample occurs with the transgranular fracture of c-ZrO₂ and Ca ZrO₃,consumes the crack propagation energy and improves the thermal shock resistance of the sample.When Al₂O₃-ZrO₂ composite powder with 2wt%Al₂O₃ is added,the relative density of the sample is significantly improved,the grain size of periclase is increased,and the slag penetration resistance of the sample is improved;By further increasing the content of Al₂O₃ in the composite powder,more Mg Al₂O₄ is formed at the grain boundary of the sample,which is easy to accelerate the penetration process of slag.4.MgO-ZrO₂ composite material with 3-1mm was used to replace pure magnesia aggregates with equal particle size to prepare MgO-C refractory,the thermal shock resistance of MgO-C refractories is effectively improved.Due to the mismatch of thermal expansion coefficient between MgO-ZrO₂ composite material and small aggregate of pure magnesia（1-0mm）,there is a gap between them after 1400℃heat treatment,which reduces the strength of MgO-C refractory.Compared with pure magnesia aggregate,MgO-ZrO₂composite material has good thermal shock resistance,low coefficient of thermal expansion and less fracture under thermal shock,so that the thermal shock resistance of MgO-C refractories is effectively improved.