Study On The Preparation And Performance Of Lightweight Refractories For Working Linings Of The High-temperature Zones In Cement Kiln

Lightweight refractories are a class of materials that have both the excellent serviceability of dense refractories and the low thermal conductivity of light refractories.Its successful application will not only achieve a certain degree of energy saving and emission reduction in the cement industry but will also allow the problem of high energy consumption in the high-temperature industry to be improved.A common method of preparing lightweight refractories is to introduce lightweight aggregates into the material,using the idea of"weaken aggregate,enhance matrix"to achieve lightweight.This method is effective in avoiding the problem of aggregates that are too high in performance to be fully effective,but it can leave some of the refractory’s serviceability compromised.Therefore,in this paper,starting from the in-situ lightweight of refractory materials,a suitable raw material formulation was selected based on study of the high temperature corrosion behavior of cement materials on refractory raw materials.Lightweight mullite-corundum refractories,lightweight spinel-corundum refractories,and lightweight periclase-hercynite refractories for the high-temperature zone work lining of cement rotary kilns have been prepared by combining the Kirkendall effect of pore-forming orientation with the liquid phase diffusion and the in-situ decomposition method.The influence of process conditions on the structure and properties of the lightweight refractory was studied.Through the above research work,the main conclusions are as follows:（1）Magnesia has excellent resistance to the corrosion of cementitious materials and the corrosion products are often distributed in the grain boundary of periclase.Hercynite reacts with cementitious materials to produce more product phases and the corrosion product phases can greatly influence the properties of the melt.The spinel reacts easily with cement materials to produce dodecaaluminate(Ca₁₂Al₁₄O₃₃)with low melting point.Meanwhile,the difference of expansion coefficient between spinel and corrosion product often leads to cracks in boundary layer.At the same time,the difference in expansion coefficients between the spinel phase and the corrosion product phase often leads to cracks in the boundary layer between the two.The corrosion reaction between corundum and cementitious material results in the formation of the corrosion product layer which changes the corrosion process of the corundum phase from direct to indirect.The presence of a large amount of silica oxide in the mullite phase increases the saturation solubility of alumina in the liquid phase of the cement material,making it less resistant to corrosion by the cement material.（2）The introduction of quartz particles enables the lightweight preparation of mullite-corundum refractories by in situ pore formation within the refractory through the micro-scale Kirkendall effect assisted by the high-temperature liquid phase.During the firing process of the specimen,the quartz particles undergo a phase change to form cristobalite,which is fused at high temperatures to produce a silica-rich liquid phase that continuously diffuses and reacts into the matrix under the action of capillary forces.The reacted mullite phase not only improves the thermal shock resistance of the material but also reacts further with the cementitious materials at high temperatures to produce the anorthite phase which effectively improves the resistance of the material to penetration by the cementitious materials.The pores left by the diffusion of the silica-rich liquid phase reduce the thermal conductivity of the material(1.55 W·（m·K）^-1)but the material still maintains a high compressive strength（85.0 MPa）and refractoriness under load（1636°C）.（3）Calcium carbonate@Al₂O₃ and Dolomite@Al₂O₃ particles with a core-shell structure were prepared using a granulation process.During high-temperature firing,the calcium carbonate and dolomite located in the core decompose,and the decomposed Ca²⁺and Mg²⁺diffuse and react towards the outer shell under the Kirkendall effect.Under the combined effect of in situ decomposition and the Kirkendall effect,cavities appear in the core of the particles.The introduction of Calcium carbonate@Al₂O₃ and Dolomite@Al₂O₃ particles into the spinel-corundum refractory reduces the thermal conductivity of the material(1.36 W·（m·K）^-1 and 1.79W·（m·K）^-1)and has a beneficial effect on the thermal shock resistance and resistance to penetration of cement materials was improved.The prepared lightweight spinel-corundum refractories still have high compressive strengths（119.8 MPa and 128MPa）and refractoriness under load（1700°C and 1683°C）.（4）The performance of the periclase-hercynite refractories gradually decreases as the hercynite content increases.When 3%hercynite spinel is added,the prepared periclase-hercynite spinel refractory at 1600°C can have high compressive strength（64.1 MPa）and refractoriness under load（1700°C）.While the introduction of alumina hollow spheres,Calcium carbonate@Al₂O_3,and Dolomite@Al₂O₃ particles respectively enables the lightweight of periclase-hercynite spinel refractories,the corrosion resistance of the material is impaired.The multi-layer structure created by the reaction of Dolomite@Al₂O₃ particles during the firing process disperses the thermal stress and resists further penetration of the cement material into the material during thermal shock.Therefore,when comparing the effects of the introduction of the three particles,the introduction of Dolomite@Al₂O₃ particles not only reduces the thermal conductivity of the material(2.43 W·（m·K）^-1)but also preserves the properties of the material better.