Synthesis Of Ca₂Mg₂Al₂₈O₄₆ And Enhancing Gas Corrosion Resistance Of CA₆ Based Refractories

Coal catalytic gasification technology is the third-generation coal gasification technology,and it is one of the most effective process ways for producing coal-based synthetic natural gas.The internal working conditions of the fluidized bed catalytic gasifier require the refractory lining to maintain volume stability under high-temperature reducing atmosphere（CO,H₂）,resist the corrosion of alkaline coal ash,alkali vapor and water vapor,and withstand the erosion of high-speed gas-solid two phase flow.The chromic-containing materials used in current coal gasifiers cannot match the requirements of lining materials of coal catalytic gasifiers,moreover,Cr⁶⁺pollution may occur in the process of using.Therefore,the development of lining materials of fluidized bed catalytic gasifiers with chromium-free,volume stability under reducing atmosphere,high structural strength,and excellent resistance to alkali vapor and vapor steam corrosion is of great significance for ensuring energy security and reducing environmental pressure in China.In order to solve problems above,the resistance to alkali corrosion of chromic-containing materials and four refractory raw materials were firstly investigated in this paper.The resistances to alkali salt and alkali vapor corrosion of the material,with calcium hexaaluminate(Ca Al₁₂O₁₉,short for CA₆)and magnesium aluminum spinel（Mg Al₂O₄,short for MA）as the main raw materials was also investigated by simulating the working conditions in gasifier.Secondly,the synthesis and growth mechanism of Ca₂Mg₂Al₂₈O₄₆(short for C₂M₂A₁₄),as the matrix reinforcing phase of CA₆ material,was investigated by solid phase sintering method.The effects of pre-sintering process,type of raw materials,type and content of additives on the microstructure,sintering properties,thermal and mechanical properties of C₂M₂A₁₄ were systematically investigated.Then,the changes in the composition and microstructure of C₂M₂A₁₄ after alkali and water vapor corrosion were studied,which is used to reveal the anti-erosion mechanism of C₂M₂A₁₄.Finally,the enhancement mechanism of the in-situ synthesis of C₂M₂A₁₄ ternary phase on the mechanical properties,alkali and water vapor corrosion resistance of CA₆ materials were systematically studied.On the basis of the work mentioned above,the main conclusions can be made as follows:1.The formation of low density compounds due to the reaction between alkali and components such as Cr₂O₃ and Al₂O₃ was responsible for the structure destruction of the chromic-containing material;CA₆ and MA were good materials of lining materials of coal catalytic gasifier,and the plate-like C₂M₂A₁₄ ternary phase in the matrix increases the mechanical strength of the refractories,it also played a positive role in improving the resistance of alkali salt and alkali vapor corrosion.2.Grain boundary diffusion was the dominated densification mechanism of C₂M₂A₁₄ at the end of sintering;grain boundary migration and lattice diffusion were the main control mechanisms of grain growth.CA₆ crystal provided a template for the formation of C₂M₂A₁₄.Mg²⁺entered the CA₆ lattice and formed spinel intermediate layer by filling the captions vacancies in the magnetoplumbite structure,then C₂M₂A₁₄was generated with the increasing of Mg²⁺ion concentration,this process followed the two-dimensional nucleation theory.Two-dimensional nucleation growth was the main growth mechanism of C₂M₂A₁₄ crystal;the screw dislocations also provided a step source for crystal growth under low saturation conditions.3.The pre-heating process eliminatd the adverse effects of intermediate reactions on the densification of C₂M₂A₁₄.The sintering properties and mechanical strength of C₂M₂A₁₄ materials were significantly improved due to the introduction of MA as a source of magnesium,and micron and sub-micron intragranular pores were introduced in materials,which caused by the acceleration of diffusion and mass transfer.Compared with CA₆,the linear expansion coefficient of C₂M₂A₁₄ was slightly larger,however,due to the increase in the concentration of lattice point defects and the complexity of the crystal structure,the degree of scattering among phonons were enhanced,resulting in an increase in thermal resistance.The thermal conductivity of C₂M₂A₁₄ is 1.49 W·m^-1·K^-1 at 800℃,which is 46.1%lower than that of CA₆.4.The boron-rich liquid phase increased the solid-liquid surface tension among the grains,and accelerated the flow mass transfer and atomic diffusion at high temperature,so the addition of H₃BO₃ promoted the sintering of C₂M₂A₁₄.Preferential solid solution of Ti⁴⁺in the CA₆ lattice and its non-equivalent substitution of Al³⁺inhibited the solid solution process of Mg²⁺and the formation of C₂M₂A₁₄,so the introduction of Ti O₂ was harmful to the densification of C₂M₂A₁₄.The Mn²⁺entered CA₆ lattice by replacing Mg²⁺,the lattice distortion caused by the solid solution effect promoted the grain growth of the C₂M₂A₁₄,and drived the crystals morphology to develop into equiaxed.The apparent porosity decreased dramatically from 32.5%to8.4%after heating at 1700°C when 3 wt%Mn O was added,but the zener pinning effect caused by Mg_1-xMn_xAl₂O₄ generated by the excessive Mn O addition prevented grain boundary from moving,which to a certain extent inhibited the discharge of the porosity,in turn,was harmful to the densification of the material.5.C₂M₂A₁₄ can maintain the stability of phase composition and crystal structure under alkali and water vapor environment.However,the continuous scouring of high-temperature water vapor caused the precipitation of Si O₂ in the silicon-rich liquid phase,then dissolved in water vapor and produced volatile gas phase,resulting in the special microscopic features of concave holes and terraced stripes appeared on the surface of some grains.6.The optimized pore structure was available for in-situ C₂M₂A₁₄ phase generation,and the bridging structure of plate-like C₂M₂A₁₄ significantly improved the mechanical strength and thermal shock resistance of CA₆ refractories.The permeation resistance of potassium or water vapor was increased due to the lower porosity and average pore size as well as the higher ratio of micro pores.At the same time,good bonding of aggregate-matrix and high mechanical strength also inhibited the“alkaline cracking”phenomenon and the formation of continuous cracks during vapor corrosion,gave excellent resistance to alkali vapor and water vapor corrosion.