Study On The Preparation Of Ultrafine Spherical Alumina By Precipitation

With extensively excellent properties such as high rigidity, good abrasion-resistance, high heat-resistance and high corrosion-resistance, etc, alumina is widely applied in fields of structural and functional materials. Ultrafine spherical alumina has huge market demand and wide applied foreground in abrasive and catalyzed materials. The investigation of spherical forming rules of ultrafine alumina powders has great practical significance as well as academic refered meaning for morphology research of other ultrafine powders.and its preparation technics have academic instructed significance for the controlled and forming rules of spherical appearance. So it had important practical and academic meanings in preparing ultrafine spherical alumina particles by low-cost methods.Preparation technics and spherical principles of precursor aluminum hydroxide were investigated in this paper by homogeneous precipitation method using aluminum salt and urea as raw material. The calcined methods of alumina were also discussed. The effect of various technological conditions and surfactant concentration on morphology,size and dispersion of particles were further discussed. The optimum conditions:the molar ratio of aluminum sulphate versus aluminium chloride is 2,[Al3+]=0.05mol/L,[urea]=0.6mol/L,reaction temperature is 90℃,heat-preservation time is 60min,the precursor was washed by alcohol for several times, well dispersed precursor AlOOH was then gained with average particle size of 7μm.After adding more than ten kinds of surfactants into solution, we found that non-ionic surfactants have better effects than ionic surfactants. Particles' dispersion have intimated relations with HLB values of surfactants. Non-ionic surfactants like PEG and H10 have best effect in enhancing dispersion and improving spherical appearance. The optimum concentration of PEG is 0.18wt% while after adding 0.35wt% H10, precursor with spherical degree of 1.06 and particle size of 0.58μm were achieved. By comparing dispersion effect of several mixed surfactant systems, we have firstly discovered H15-sodium stearate was the best. Monodisperse precursor with spherical degree of 1.1 and particle size of 0.36um were obtained when the concentration of H15 and sodium stearate was 0.75wt% and 0.14wt% respectively. Analyses from Thermogravimetric (TG) and Fourier transform infrared spectroscopy (FTIR) curves, PEG can only form weak layer on the surface of particles, however H15-sodium stearate can form a strong organic layer by chemical bonding with-OH group in particle. The decrease of adsorbing water inside particles is the direct reason for the improvement of dispersion. Inorganic salt NaCl can further increase the dispersion of particles. While [NaCl]=0.05M, monodispersed precursor with spherical degree of 1.07 and particle size of 0.38μm were gained. Further increase [NaCl] to 0.1M, particle size decrease to 0.15μm with spherical degree of 1.02.The dehydration process of precursor and the heat transformation of alumina were analyzed by DSC-TG and XRD. Precursor A1OOH will transform to amorphous Al2O3,γ-Al2O3 and at last toα-Al2O3 when temperature surpass 1000℃. The agglomeration of particles may be the restricted factor in preparation of monodispersed alumina particles during calcined process. NaCl can effectively prevent particle agglomeration in high temperature. The optimum [NaCl] is 0.05M, monodisperse amorphous Al2O3 particles with spherical degree of 1.08 and particle size of 0.4μm can be gained at 600℃, monodispersed sphericalγ-Al2O3 particles with particle size of 0.5μm can be achieved at 800℃,α-Al2O3 particles with less agglomeration can be obtained at 1000℃.