Effects Of Incorporation Of Andalusite Aggregates On Properties Of Al₂O₃-SiC-C Castables

The aggregates used in the Al2O3-SiC-C castables for blast furnace iron runner are mostly brown corundum particles at present.However,the production of brown corundum has been restricted because of the serious pollution during fusion processing.Andalusite is one kind of raw material with abundant reserves,and the refractory containing andalusite has excellent volume stability and thermal shock resistance.Nevertheless,little research can be found on the application of andalusite aggregates in Al2O3-SiC-C iron runner castables.In this work,brown corundum aggregates used in Al2O3-SiC-C iron runner castables were replaced by andalusite aggregates with different sizes(8-5mm,5-3mm,3-1mm)and after different temperature(1300?,1400?,1500?,1600?)preheating.Then the effects of the particle size and preheating temperature of andalusite aggregates on the volume stability,thermal shock resistance,oxidation resistance,high temperature strength and corrosion resistance of castables were investigated.The possibility of application of andalusite aggregates in the Al2O3-SiC-C iron runner castables was explored.Firstly,the linear change of the castables was studied.The results show that the linear change of the samples with different sizes(8-5mm,5-3mm,3-lmm)of andalusite aggregates increases and the sample with 5-3mm andalusite aggregate has the highest linear change,which is mainly due to the volume expansion accompanied by andalusite mullitization.When the pre-firing temperature of andalusite aggregates was increased from 1300? to 1600?,the linear change of the samples is reduced.When the pre-firing temperature of andalusite aggregates is raised,the amount of residual andalusite in the samples is decreased,resulting in the reduced volume expansion of the samples after heat treatment.The particle size and pre-firing temperature of the andalusite aggregates can be controlled to adjust the expansion ratio to offset the volume shrinkage of the castables during long-term use,thereby increasing the volume stability of the castables.Secondly,the thermal shock resistance of the castables was investigated.The results indicate that the addition of the andalusite aggregates improved the thermal shock resistance of the samples,because the composite structure of the mullite and silica-rich liquid phase formed by the conversion of andalusite has the effect of release thermal stress during the thermal shock process.When the particle size of andalusite aggregates is reduced from 8-5mm to 5-3mm,the thermal shock stability of the samples increases because of higher transformation ratio of andalusite and high amounts of mullite and silica-rich glass.However,the thermal shock stability of the samples decreases when the particle size of andalusite aggregates is reduced from 5-mm to 3-1mm.Much more silica-rich glass would be formed in the sample containing 3-mm andalusite aggregate;the glass phase would fill the pores,promote sintering and densification,consequently increasing the strength of the materials.The denser structure and higher strength may be detrimental to the thermal shock resistance.Thirdly,examined was the oxidation resistance of the castables.It is found that the oxidized layer of the samples containing andalusite was thinner than that of sample without andalusite after thermal shock,which indicate that the addition of the andalusite aggregates improved the oxidation resistance of the samples.The silica-rich liquid phase produced by the decomposition of andalusite fills the pores,which would prevent oxygen from entering the interior of the samples,reduce the oxidation of SiC and C,and thus improve the oxidation resistance of the samples.When the particle size of the andalusite aggregates decreases and the pre-firing temperature increases,the amount of silica-rich liquid phase increases and the porosity of the samples decreases,which further improves the oxidation resistance of the samples.Fourthly,the high temperature strength of the castables was studied.The results show that the high temperature strength of the sample increases after the addition of andalusite aggregates because the silica-rich glass produced by the decomposition of andalusite reacts with the AlO3 in the samples to form the secondary mullite,which is beneficial to the high temperature strength of the samples.When the particle size of andalusite aggregates decreases from 8-5mm to 3-lmm,the silica-rich glass produced by the decomposition of andalusite increases and the amount of secondary mullite is increased,which also contributes to the improved high-temperature strength of the samples.Finally,the corrosion resistance of castables was investigated.It is found in the microstructure of the sample after the corrosion tests that the andalusite particles in the vicinity of the slag were eroded and the corundum particles were not corroded,indicating that the corrosion resistance of the andalusite particles is weaker than that of the corundum particles.The results of oxidation resistance experiments show that the residual amounts of SiC and C in the samples containing andalusite aggregates is relatively higher than those in the sample without amdalusite.SiC and C have poor wettability to slag and do not react with slag,consequently reducing the slag corrosion of the castables containing andalusite aggregates.