Ca_(2)Mg_(2)Al_(28)O_(46)对水泥窑过渡带用镁质耐火材料结构和性能的影响

Effect of Ca_(2)Mg_(2)Al_(28)O_(46) Addition on Structure and Properties of Magnesia Refractories for Cement Kiln Transition Zone

水泥回转窑过渡带由于难于结窑皮和所用耐火材料热导率较高,窑壳外表面温度高,散热损失大。本工作以氢氧化镁、碳酸钙和煅烧α-氧化铝为原料合成出Ca_(2)Mg_(2)Al_(28)O_(46),以此为骨料和烧结镁砂制备出方镁石–尖晶石–铝酸钙耐火材料。借助扫描电子显微镜、X-射线衍射仪等手段研究Ca_(2)Mg_(2)Al_(28)O_(46)的添加和烧成温度对镁质耐火材料结构和性能的影响。结果表明:1) Ca_(2)Mg_(2)Al_(28)O_(46)引入到镁质耐火材料中,因烧成过程方镁石存在使Ca_(2)Mg_(2)Al_(28)O_(46)发生分解,方镁石–尖晶石–铝酸盐三元体系在高温下形成液相,借助于液相辅助Kirkendall效应形成了孔洞实现了轻量化,并且随着Ca_(2)Mg_(2)Al_(28)O_(46)加入量增加,材料显气孔率升高,体积密度、耐压强度和荷重软化温度降低;相同Ca_(2)Mg_(2)Al_(28)O_(46)加入量,降低Ca_(2)Mg_(2)Al_(28)O_(46)粒度可使上述提及性能提高。2)随着烧成温度升高,材料的显气孔率降低、体积密度增加,但耐压强度并未升高,荷重软化温度保持在1 540℃以上较高的水平,当加入10%(质量分数)、0.50~1.00 mmCa_(2)Mg_(2)Al_(28)O_(46)颗粒,材料在1 700℃烧成时荷重软化温度可达1 603℃。3) 1 350℃侵蚀实验结果表明,引入10%粒度为0.088~0.500 mm Ca_(2)Mg_(2)Al_(28)O_(46)颗粒的样品,在1 650℃烧成具有最好的抗水泥物料侵蚀性能。

Introduction Cement industry is one of the major carbon-emitting industries,which is characterized by a thermal efficiency below 54%and an energy consumption ranging from 3–4 GJ per ton of cement clinker.China annual average cement production reaches 2341 million tons,contributing to approximately 14.3%of the total CO_(2) emissions.In addition to the CO_(2) emissions resulting from the high-temperature decomposition of limestone,which serves as a primary raw material,the cement production process significantly contributes to elevated CO_(2) emissions due to its substantial energy consumption.The cement rotary kiln is a pivotal equipment in the production process,with its high energy consumption primarily attributed to subpar performance of certain refractory materials and an irrational configuration.Consequently,this leads to elevated temperatures(reaching up to 350–400℃)in both the transition zone and firing zone of the kiln shell,resulting in a substantial energy loss accounting for 8%–15%of the total heat input.Previous studies demonstrated that the implementation of light weight refractory materials could effectively mitigate heat loss from the kiln shell,thereby enhancing energy efficiency.The CA6/MA composite material C_(2)M_(2)A_(14) has the excellent high-temperature performance and chemical stability of both CA6 and MA.It is proven to be an effective ladle lining material in the non-slag line part.Incorporating a proper quantity of Fe_(2)O_(3) during the synthesis process of C_(2)M_(2)A_(14) results in a denser product under identical firing conditions.In this paper,C_(2)M_(2)A_(14) was introduced into magnesite refractories to achieve light weighting of magnesite refractories.In addition,the mechanism of achieving light weighting and the effects of addition amount and particle size on the microstructure and properties of magnesite refractories were also investigated.Methods Magnesium hydroxide,calcium carbonate and calcinedα-alumina fine powders as raw materials were mixed.Also,1%Fe_(2)O_(3) was added to the mixture according to a specific mass ratio.The mixture was then pressed into cylindrical samples with a diameter ofφ36 mm×36 mm.C_(2)M_(2)A_(14) was synthesized via firing at 1780℃for 5 h.The synthesized material was crushed and sieved to obtain fine particles with different sizes of 1–3 mm,0.088–1.000 mm,0.5–1.0 mm,0.088–0.500 mm,and≤0.088 mm.Magnesia and materials such as C_(2)M_(2)A_(14) were mixed according to the specified ratio,and then pressed into cylindrical samples with dimensions ofφ36 mm×36 mm andφ36 mm×50 mm,as well as crucible samples with the dimensions ofφ50 mm×50 mm.These samples were fired at different temperatures(i.e.,1600,1650℃and 1700℃)for 3 h.The bulk density,apparent porosity,true density,and cold crushing strength at room temperature of A sample with the size ofφ36 mm×36 mm were characterized according to national standards.The load softening temperature of a sample with the size ofφ36 mm×50 mm was measured.The pore size distribution of the original brick was determined by a fully automatic mercury porosimeter.Cement material in crucible was heated at 1350℃for 6 h to investigate its corrosion-resistance to cement material.The microstructure before and after corrosion of the crucible samples and original bricks was determined in a backscattered electron imaging mode by field corrosion scanning electron microscopy.The phase composition of the samples was characterized by X-ray diffraction with a software named Jade.Results and discussion The results show that at 1600℃,the apparent porosity of sample increases while their bulk density and cold crushing strength decrease as the amount of C_(2)M_(2)A_(14) increases.Also,the load softening temperature decreases.The apparent porosity of samples with particle sizes ranging from 0.5–1.0 mm increases from 23.2%to 32.8%with increasing C_(2)M_(2)A_(14) from 10%to 40%.The bulk density decreases from 2.70 g/cm^(3) to 2.36 g/cm^(3),which is lower than the one of dense refractory(which is also made of the same material)at 2.94 g/cm^(3),indicating a trend towards light weighting.At the same addition amount of 10%,the particle size of C_(2)M_(2)A_(14) becomes smaller,the apparent porosity of the sample decreases,and the bulk density and cold crushing strength both increase.The load softening temperature is closer.The apparent porosity of samples with different particle sizes(i.e.,0.5–1.0 mm and 0.088–0.500 mm)gradually decreases,while the bulk density increases as the firing temperature increases.However,there is no corresponding increase in cold crushing strength.The maximum cold crushing strength is 56.6 MPa,and the particle sizes both exhibit a load softening temperature of>1540℃,meeting the requirements for application in cement kiln transition zones.The presence of magnesia in the matrix during firing induces the decomposition of C_(2)M_(2)A_(14),as evidenced by XRD and SEM analysis.In the magnesite–spinel–aluminate ternary system,a liquid phase forms at elevated temperatures,facilitating a pore formation through liquid-assisted Kirkendall effect and achieving a light weighting in the samples.Furthermore,increasing the amount of C_(2)M_(2)A_(14) results in a higher apparent porosity,while leading to a decrease in bulk density,cold crushing strength,and load softening temperature.The corrosion resistance of group D and E samples with particle sizes of 0.5–1.0 mm and 0.088–0.500 mm was investigated at 1350℃by a static crucible method with the addition of 10%C_(2)M_(2)A_(14).The crucibles of D series samples exhibit a visible corrosion penetration,while those of group E samples show a negligible corrosion penetration.No adhering slag appears on the side walls in either group.The microstructural analysis reveals that different sizes of C_(2)M_(2)A_(14) particles are responsible for variations in penetration depth between the two group samples.Larger and more pores occur in group D samples,allowing cement material to penetrate through them.The introduction of C_(2)M_(2)A_(14) particles with the sizes of 0.088–0.500 mm in group E results in smaller pores resulted from the reaction between C_(2)M_(2)A_(14) particle and magnesia matrix,making them denser and effectively inhibiting the penetration by a low melting point cement phase.Conclusions 1)Introducing Ca_(2)Mg_(2)Al_(28)O_(46) was introduced into magnesium refractor during firing.Magnesite–spinel–aluminate ternary system formed a liquid phase at a high temperature.The pores formed with a liquid phase assisted the Kirkendall effect and resulted in magnesium refractory to achieve lightweight.The apparent porosity of samples increased,the bulk density,compressive strength and the refractoriness under load decreased with increasing the amount of Ca_(2)Mg_(2)Al_(28)O_(46) additions.At the same amount of Ca_(2)Mg_(2)Al_(28)O_(46) added,the performance could be improved via reducing its particle size.2)As the firing temperature increased,the apparent porosity of specimen decreased,the bulk density increased,but their compressive strength did not increase,and the refractoriness under a load maintained at a high level(>1540℃).The refractoriness under a load could reach 1603℃when 10%(in mass fraction)of Ca_(2)Mg_(2)Al_(28)O_(46) particles with the sizes of 0.50–1.00 mm were added and fired at 1700℃.3)The static crucible experiments at 1350℃showed that the specimens with the introduction of Ca_(2)Mg_(2)Al_(28)O_(46) particles with the sizes of less than 0.50 mm at 10%addition had the optimum corrosion resistance to cement materials when the specimens were fired at 1650℃.