Preparation Of High Temperature And Low Sodium Alumina Powder Material

High temperature and low sodium alumina belongs to chemical alumina,Low sodium alumina mainly refers to the alumina with the mass fraction of sodium oxide less than 0.1%.It can be used to prepare high-grade electrical insulators.When the particle size(D(0.5)<3 ?m)and crystalline shape(w[?]?95%)can meet certain requirements,it can also be used to prepare high-grade refractory and wear-resistant ceramic materials.It is difficult for the existing process to satisfy both the mass production with low cost and the superior comprehensive performance of the products.In this paper,two kinds of preparation processes of high temperature and low sodium alumina with simple process,low cost and conventional equipment have been developed,and the high temperature and low sodium alumina meets the requirements of the comprehensive indicators.In this paper,two-stage method and direct calcination method were used to prepare high temperature and low sodium alumina.Mainly using specific surface measuring instrument,scanning electron microscope,DSC thermogravimetric analysis,XRD phase analysis,ICP atomic spectrum analysis and other analytical methods to study the changes of alumina structure and phase transformation law,the influence of additives on the crystallization mode of alumina,the mechanism of sodium removal agent,and the influence of different ball milling conditions on particle size.The results are as follows:(1)Two-stage method for preparing high temperature and low sodium alumina powders was developed.The process includes five steps:pre-roasting,sodium removal by washing,fining particles by ball milling,phase transition through calcination and secondary ball milling.The mass fraction of sodium oxide is less than 0.1%,the average particle size is 1?3?m,and the mass fraction of alpha phase is more than 99%in the prepared sample.(2)During the pre-roasting process,Heating up to 500? at the heating rate of 3?/min,the specific surface area of activated alumina obtained by roasting is the largest,reaching 176.15 m2/g.At this time,the mass fraction of Na2O can be reduced from 0.275%to 0.131%by washing.Adding 4 times excess glacial acetic acid and washing,the mass fraction of Na2O can be reduced to the lowest,reaching 0.074%.(3)During the roasting transformation process,the lowest phase transition temperature of industrial aluminum hydroxide from transition alumina to alpha alumina is 1150?,and the a phase content increases with the calcination temperature.The transformation rate curve is a parabola approximately,and the peak is in the range of 1100?1150?.the transformation rate is about 0.6%/? in this range of temperature,and the mass fraction of a phase corresponding to the temperature is 40%?45%.The mass fraction of a phase can reach 96%at 1400?.The addition of additives can effectively inhibit grain growth and promote phase transition.0.05%of nano ?-Al2O3 seeds and basic magnesium carbonate were added respectively.The original grain size could be reduced to about 600 nm and 500 nm from about 1 ?m,and the a phase was increased from 96.00%to over 99.00%.(4)A process for preparing high temperature and low sodium alumina powder by direct roasting was developed.The process included adding mineralizer and high temperature roasting,sodium removal by dilute pickling deeply and fining particles by ball milling.The mass fraction of sodium oxide is less than 0.05%,the average particle size is less than 7 ?m and the mass fraction of alpha phase is more than 96%in the prepared sample.(5)In the process of high temperature roasting,the sodium removal ability of mineralizer increased gradually with the roasting temperature.When roasting,temperature is 1400?,adding 1%H3BO3,NH4Cl and NH4F mineralizer can reduce the mass fraction of Na2O from 0.32%to 0.082%,0.070%,0.053%,and the mass fraction of alpha phase of three kinds of alumina is more than 95%.The powder is further washed with dilute hydrochloric acid with 0.012%.The mass fractions of sodium oxide in the samples correspondingly are 0.049%,0.044%and 0.043%respectively,reaching ultra-low sodium level.(6)Mineralizers can promote grain growth to some extent,The original grain size of roasted products with 1%H3BO3,NH4Cl and NH4F mineralizers is about 3 ?m,1 ?m and 5?m in turn.The average particle size is 3.870 ?m,2.216 ?m and 6.094 ?m respectively after ball milling.The particle size distribution curves of three kinds of alumina powders are close to normal distribution,and the particle size distribution is good.(7)Aluminum oxide powder prepared by the two processes can reach the level of high temperature and low sodium alumina.The purity of the powder prepared by the two-stage method is relatively pure,and no other impurities are introduced,the mass fraction of sodium oxide is less than 1%,and the average particle size is relatively fine.The mass fraction of sodium oxide in alumina powder prepared by direct calcination is less than 0.05%,reaching the ultra-low sodium level,but there are some shortcomings in purity and particle size compared with the two-stage method.The two processes are simple and easy to expand the experiment so that they can be extended to industrial production.