Removal Of Trace Iron And Micro-Spheroidizing Of Alumina

To satisfy the application requirements of alumina powders in areas of luminescent materials, abrasive materials and fine ceramics, the research about how to control the purity, morphology, particle size distribution and dispersion of alumina powders carried out. The main research results are shown as follows:(1) Based on the principle of solid-liquid extraction, a feasible iron-purification technique for industrial aluminum hydroxide was developed. Through low-temperature calcination, ball milling and solid-liquid extraction, the content of iron impurity in the low-quality industrial aluminum hydroxide with large particle size was declined from about 60ppm to 3ppm, meeting the requirements of luminescent materials. Further research indicated that the iron impurity can be released from the crystal structure by selecting suitable calcination temperature and fully exposed on the surface of particles by the ball mill. Then, the corresponding iron impurity was transferred into the extracted liquid by choosing suitable extraction solvents, and the pure aluminum hydroxide was obtained by solid-liquid separation. At last, the theoretical calculation about the loss of raw materials showed that this technique is economically feasible.(2) Aiming at obtaining spherical alumina to satisfy the application of them in the field of abrasive materials and fine ceramics, the preparation and the formation conditions of spherical aluminum hydroxide gel in water system and nonwater system were studied. The theoretical support for using aluminum isopropoxide as raw materials for hydrolysis was presented and the hydrolysis system is the key factor for the preparation of aluminum hydroxide gel with various morphologies was found. Furthermore, the formation mechanism of aluminum hydroxide gel with various morphologies was discussed and the Hamaker constant, molecular structure and polarity of the solvent was found to be the internal factors to obtain aluminum hydroxide gel with various morphologies.(3) The dispersion behavior of spherical alumina during the preparation was further studied. Based on the investigation on the formation mechanism and the characteristics of agglomeration in various stages, effective prevention methods were presented. At the stage of the formation of wet gel, choosing a smaller Hamaker constant of the solvent system to reduce the attractive potential energy between particles is effective to prevent agglomeration of the particle. At the stage of the evaporation of wet gel, removing the adsorbed water and changing the structure o functional groups on the surface by washing with ethanol are efficient to prevent agglomeration between the gel particles through the Al-O-Al bond. At the stage of the calcinations of dry gel, using the intermittent calcinations at constant temperature can break down the uninterrupted strong force that is the main reason for the formation of sintering neck between the gel particles.(4) Highly dispersed spherical a-alumina powder was prepared in octanol-acetonitrile system, and the factors influencing morphology, particle size and crystal structure of spherical alumina, such as reactant concentration, hydrolysis temperature, water amount, aging temperature, aging time, evaporation methods, calcinations temperature and calcinations methods, etc were discussed. Under a certain condition, the particle size can be controlled by tuning the reactant concentration. The micro-size alumina gel with nice sphericity can be obtained by controlling hydrolysis temperature, aging temperature and time. Highly dispersed alumina microspheres with different crystalline forms were obtained through choosing the optimum evaporation and calcination method in the different temperature.(5) The preliminary applications of alumina with different characteristics were studied. YAG was prepared from low-iron aluminum hydroxide and its luminescence properties were researched. The suspension technology for microspherical alumina was developed. The optimum range of pH of microspheres alumina suspensions was obtained by analysis suspension system zeta potential. Considering the characteristics of alumina microsphere suspension, a suitable dispersing agent was selected, and the influence of the dispersant amount on the stability of suspension system was studied.