Study On The Preparation,microstructure And Properties Of Calcium Hexaaluminate Porous Insulation Material By Indirect Liquid Phase Method

With the goal of"carbon peak"and"carbon neutrality",the energy saving and emission reduction of industrial kilns has been paid more and more attention.The application of porous thermal insulation materials in the insulation layer of kilns can effectively reduce heat loss and realize energy saving and emission reduction.Currently,lightweight thermal insulation materials have defects such as high sintering temperature,long production cycle and mismatched thermodynamic properties due to the limitation of preparation methods,which limit their wide application in industry.Consequently,the preparation of lightweight and high-strength porous insulation materials at low temperature and high efficiency has become a key problem that needs to be solved.In this study,we proposed an innovative indirect liquid-phase method to prepare CA₆ porous insulation materials with excellent comprehensive performance for industrial kilns.Firstly,CA₆ was prepared by in situ sintering process using light calcium carbonate,heavy calcium carbonate,active calcium carbonate,calcium hydroxide and quicklime as calcium sources,as well as aluminum hydroxide,white corundum and industrial alumina as aluminum sources,respectively.The effects of salt type,addition amount and sintering temperature on the structure and properties of the products were systematically investigated,and the formulations and preparation processes were optimized by using Ca Cl₂ and Ca Cl₂-KCl as dielectric salts and the indirect liquid-phase method for the preparation of CA₆ porous insulation materials.Based on the above results,the matching law of thermal insulation performance and mechanical strength of CA₆ porous thermal insulation materials was investigated.The results of the composition,structure and properties of CA₆ materials showed that the sample prepared from light calcium carbonate as the calcium source and aluminum hydroxide as the aluminum source,sintered at 1500°C and held for 3 h,has excellent properties,whose flexural and compressive strengths were 7.20 MPa and 75MPa,respectively,and whose bulk density and apparent porosity were 53.01%and1.71 g·cm^-1,respectively.The preparation of calcium hexaaluminate porous insulation materials using Ca Cl₂ as a dielectric salt by indirect liquid-phase method shows that Ca Cl₂ can react with Al₂O₃ at high temperature to form Ca O and gas-phase Al Cl₃,which can act as a pore-forming and supplementary calcium source and also promote Al₂O₃ crystalline transformation.The liquid-phase environment accelerates the synthesis of CA₆ and enables its complete synthesis at 1400°C.The products obtained by this method do not require additional salt washing treatment,and the specimens with 15 wt%Ca Cl₂have excellent overall performance with compressive strength,thermal conductivity,bulk density and apparent porosity of 7.14 MPa,0.439 W·m^-1·K^-1/1000°C,1.47g·cm^-1 and 62.12%,respectively.The study with Ca Cl₂-KCl as the dielectric salt showed that the direct volatilization of KCl at high temperature,together with the partial volatilization of Ca Cl₂ and the consumption of the reaction,resulted in very little residual dielectric salt in the resulting samples,while the system further reduced the preparation temperature of the CA₆ porous insulation to 1300°C.In addition,the sample with 20wt%Ca Cl₂-KCl exhibited excellent overall performance at 1300°C,with bulk density and apparent porosity of 1.38 g·cm^-1 and 68.28%,respectively,and the compressive strength of 6.76 MPa at a low thermal conductivity of 0.322 W·m^-1·K^-1/1000°C,which is better than the sample made with Ca Cl₂ alone.In addition,the samples with 20 wt%Ca Cl₂-KCl at 1300°C exhibited excellent overall performance.The compressive strength,thermal conductivity,bulk density and apparent porosity were 6.76 MPa,0.322 W·m^-1·K^-1/1000°C,1.38 g·cm^-1 and68.28%,respectively.The study on the matching law of thermal insulation performance and mechanical strength of CA₆ porous thermal insulation material shows that the thermal and mechanical properties show a good fit with the general model and the Gibson-Ashby-Rice model,respectively,and exhibit a linear negative correlation between compressive strength and porosity,and a linear negative correlation between thermal conductivity and porosity.In addition,the expressions of the relationship between mechanical and thermal properties in different structural models were obtained,and it was deduced that when the porosity was 70%～72%,the specimens with excellent thermal and mechanical properties could be obtained,and then the prediction of thermal and mechanical properties of calcium hexaaluminate porous insulation materials could be realized.