Research On The Mechanism Of Impurity Removal In Preparation Of Boron Powder By Self-propagating High-temperature Synthesis (SHS) Metallurgy

With the advantage of high specific surface area,high activity,high heat of combustion and no pollution for using,the amorphous boron powder is widely used in rocket solid propellants,aerospace,metallurgy and new materials synthesis,and electronic high-tech fields.Although the preparation of high-activity boron powder has been achieved by SHS metallurgy method,the product is not ideal,the magnesium content reach over 13.0wt%and the insoluble in boron powder exceeds 15.0wt%,which seriously affect its using performance.On the basis of previous preliminary studies,the present paper systematically researched on the three dominant processes of preparation of boron powder by self-propagating high-temperature synthesis including the SHS process,acid leaching process and washing process.Furthermore,in order to solve the problem of high concentration of impurities in boron powder,a series of investigations about secondary reduction of amorphous boron powder were conducted.The present paper obtained the following conclusions:(1)The SHS experimental results show that with the consideration of lower magnesium content and lower hydrogen peroxide insoluble content,and also to make it easier to initiate the SHS reaction,proper SHS experimental conditions are needed:magnesium particle size-30 mesh,non-activated boron oxide,1:1 boron oxide and magnesium proportioning,unpressed sample,and the vacuum atmosphere.(2)The single factor experiments of acid leaching process show that the optimal experimental parameters are liquid-solid ratio 18:1,hydrochloric acid concentration of 3 mol/L,leaching temperature of 90℃,leaching time for 90 minutes,and leaching times for 1.(3)Washing process shows that agglomeration state of boron powder can be improved with proper washing conditions.So the proper washing parameters are washing liquid-solid ratio of 40:1,washing temperature of 80℃,and washing time for 60 minutes.(4)The optimum parameters of secondary reduction process are boron oxide and boron powder ratio of 1.5:1,reduction temperature of 800℃,soaking time of 90 minutes,and samplemaking pressure of 0 MPa.(5)Impurity phases in boron powder are crystal boron,Mg2B2O5,and MgB2 which associate with boron powders.(6)Hydrogen peroxide insoluble consists of crystal boron,MgB2,B2O,B2O3 and B6O.(7)The magnesium content of primary boron powder is 11.20wt%,which is 2.8wt%lower than the boron powder before experiments have been done and the content of hydrogen peroxide insoluble conternt is 9.06wt%,which is 6%lower than before.Secondary reduction process shows that the magnesium content can be lowered down to 3.16wt%,which is better than the standard of SB boron 90,and the hydrogen peroxide insoluble is 9.03wt%.Primary amorphous boron powder exhibits uniform size distribution,and the range of particle size is from 200nm to 2μm with irregular polyhedron shape,which is able to fulfill the low-cost and large-scale application.The particle size distribution of secondary reduction boron powder particle size ranges from dozens of nanometers to 1μm,which could meet the demands of high-end requirements.