| The carbon materials used in Al2O3-SiC-C castables for blast furnace iron runner are easily oxidized at high temperature,which undermines the oxidation resistance and thermal shock resistance etc.Previous research showed that the andalusite(D59:Al2O3=59.10 wt%)as a part of the aggregate in Al2O3-SiC-C castables enhanced their oxidation resistance properties.This is probably because the Si O2-rich glass originated from the decomposed andalusite blocks the pores of castables,impeding oxygen into the center of castables,thereby improving the oxidation resistance of the refractory castables.It should be noted that the andalusites used in the above-mentioned study are of high grade.However,the different grades of andalusites have various impurity contents,which could significantly influence the amount of Si O2-rich glass generated from andalusite mullization.Nevertheless,limited reports can be found on the effect of low-grade andalusite(P57:Al2O3=57.46 wt%)on the oxidation resistance of Al2O3-SiC-C castables.In this work,two different grades of andalusite(D59:Al2O3=59.10 wt%;P57:Al2O3=57.46 wt%)were utilized to replace the brown corundum with similar-size in order to study the effect of the content of impurities in andalusites on the amount of Si O2-rich glass generated from andalusite mullitization.Based on the above research,the addition of antioxidant Si(0.0,0.5,1.0,1.5 wt%)of the castables was adjusted,and the effects of introduction of low-grade andalusite on the dosage of Si in Al2O3-SiC-C castables were investigated.And whether sufficient oxidation resistance of the refractory castables could be maintained when corundum aggregates are substituted by andalusite and the amount of antioxidant Si is reduced was explored.The results of this work are as follow:The effects of two different grades of andalusite on the oxidation resistance of the castables were investigated.The results shows that the addition of low-grade andalusite could effectively improve the oxidation resistance of the castables,compared with the samples with high-grade andalusite(D59),which is possibly due to two aspects.On one hand,impurities are conducive to promoting the transformation of andalusite(P57),in which more Si O2-rich glass are generated,which would block the entry of O2 and impede O2 into the castables,and thus contributing to improving the oxidation resistance of the refractory castables.On the other hand,higher liquid phase originating from P57-andalusite decomposition would react with Al2O3 of the matrix to generate more secondary mullite,which interpenetrates between aggregates and matrixes and thus lowers the porosity of the castables containing low-grade andalusite,consequently further enhancing the oxidation resistance of the castables.The possibility of reducing the dosage of metal Si of castables after the introduction of the low-grade andalusite was investigated.The results show that the samples with less dosage of Si(0.0?1.0 wt%)and with andalusite had thinner oxidation layer,in comparation of the samples with high dosage of Si(1.5 wt%)but without P57-andalusite,indicating that the addition of andalusite could certainly reduce the dosage of Si of the castables but not decrease the oxidation resistance of the castables.As such,by introducing the low-grade andalusite into castables,the dosage of Si can be decreased.The effects of two different grades of andalusite on the other physical properties of the castables were studied.The results show that,compared with the castables containing high-grade andalusite(D59),the addition of low-grade andalusite was benefical to sample's volume stability,high temperature strength,as well as thermal shock resistance.This is because volume expansion caused by andalusite mullitization significantly compensates for the liquid phase volume shrinkage at high temperature,which is conducive to the volume stability of the castable;then,higher amount of composite structure of the mullite and silica-rich liquid phase generated from mullitization of P57-andalusite significantly absorb the thermal stress during the thermal shock process,thereby improving the thermal shock resistance of the castables.Besides,higher liquid phase glass is formed in the samples containing P57-andalusite,in which much more generated secondary mullite would fill pores between aggregate and matrix and play a strengthening role,thereby improving the high temperature strength of castables.Our work also demonstrated that introducing the low-grade andalusite aggregates and reducing the dosage of Si are not able to reduce the oxidation resistance of castables,and the high temperature strength,as well as thermal shock resistance of castables were also not declined.The reason is that the Si O2-rich glass phase generated by the transformation of andalusite could react with alumina powder to form the secondary mullite.These mullite fills the pores between aggregate and matrix and plays as a strengthening role,and thereby improving the high temperature strength of the castables.Besides that,the composite structure of mullite and liquid phase formed by the transformation of P57-andalusite is also favorable to preventing samples from the thermal stress concentration during the thermal tests,and thus enhancing the thermal shock resistance of the castables. |
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