Synthesis And Application Of Lanthanum Aluminate

Magnesite-alumina refractory is applied to the clinkering zone and transition zone of cement kiln.When it is used in the clinkering zone,it often has the defects of poor performance of hanging kiln skin and decreased erosion-resistance,which limits its wide use in the clinkering zone.Lanthanum aluminate has the characteristics of high melting point,good chemical stability and small thermal expansion coefficient.It is stable in structure at high temperature and difficult to be eroded by slag.In order to improve the performance of magnesia-alumina refractory,lanthanum aluminate can be used instead of magnesia-alumina spinel to develop new magnesia refractory.Therefore,this thesis mainly studies the effects of aluminum-containing raw materials,heat treatment temperature,lanthanum-alumina ratio on the synthesis of lanthanum aluminate.The reaction mechanism of lanthanum aluminate synthesis from lanthanum oxide and magnesium aluminate spinel is investigated.The pre-synthesized lanthanum aluminate was introduced into magnesia refractories to study its effect on the properties of magnesia refractories.The performance of magnesia refractory and lanthanum aluminate of synthesis are studied by XRD,SEM and so on.The conclusions are as follow:（1）Lanthanum aluminate can be synthesized by increasing heat treatment temperature in the case of lanthanum alumina equimolar ratio using alumina,aluminum hydroxide and magnesia-alumina spinel as raw materials.Lanthanum aluminate crystal be synthesized by using magnesia-alumina spinel as raw materials forms a direct combination structure of lanthanum aluminate and magnesium oxide.Compared with alumina and aluminum hydroxide as the aluminum source,the sample has large grain size and high density.（2）With the decrease molar ratio of lanthanum to aluminum,the synthesis rate of lanthanum aluminate decreases gradually,When magnesia-alumina spinel is used as raw material.Lanthanum magnesium hexaaluminate generates when molar ratio of lanthanum to aluminum<1:6.The newly formed plate-like Lanthanum magnesium hexaaluminate fills the pores and becomes the reinforcing phase of the sample.It is beneficial to the densification of lanthanum aluminate by reducing the particle size of spinel,but it has no obvious effect on the synthesis rate of lanthanum aluminate.Increasing the heat treatment temperature has a more obvious effect on the synthesis rate of lanthanum aluminate than prolonging the holding time.（3）At 800℃-1000℃magnesia-alumina spinel and lanthanum oxide mainly have material diffusion transmission,without chemical reaction.Lanthanum oxide and spinel at contact have’captured aluminum’to form lanthanum aluminate at 1000℃-1300℃.This process is controlled by chemical reaction.The synthesis rate of lanthanum aluminate changes with temperature as Y₂=0.1024T-101.16.At 1300℃-1600℃,lanthanum oxide needs to diffuse to the surface of spinel through the newly formed lanthanum aluminate layer to continue to react to form lanthanum aluminate.Therefore,the reaction rate is controlled by chemical reaction and diffusion in this stage.The effect of temperature on the synthesis rate of lanthanum aluminate can be expressed by Y₃=0.1982T-225.29.The synthesis reaction of lanthanum aluminate tends to end at 1600℃-1700℃.This process is mainly ion diffusion,promote the growth of the neck of lanthanum aluminate and sintering densification.Finally,a dense composite structure of lanthanum aluminate and magnesium oxide is formed.（4）The pre-synthesis of lanthanum aluminate and the introduction of magnesia refractory in the form of powder can promote the sintering densification of magnesia refractory and improve the bending strength and kiln skin bonding strength of magnesia refractory.When introduced in the form of particles,the dense multiphase structure improves the corrosion resistance of magnesia refractories.The above properties are better than those of the currently used iron aluminum spinel brick,indicating that lanthanum aluminate can be used in magnesia materials.