Preparation Of Super-fine Aluminum Hydroxide And The Technique For The Improvement Of Its Thermal Stability

Super-fine aluminum hydroxide is mainly produced by seeded precipitation from sodium aluminate solution in China. There are many shortcomings in that the particle size of the products is too widely distributed and morphology of particles can not be controlled effectively during the production process of super-fine aluminum hydroxide. Aluminum hydroxide is easily decomposed when it is mixed with hot organic polymer materials. Its lower decomposition temperature limits its application performance and application field. In order to solve the problems presented in the production and application of super-fine aluminum hydroxide, preparation process of super-fine aluminum hydroxide was studied systematically and the technique for the improvement of its thermal stability was developed in this dissertation.Laser particles size analyzer, scanning electron microscopy(SEM), zeta potential analyzer, dynamic light scatter particle size analyzer, slurry dispersion stability analyzer, energy dispersive spectrometer(EDS) and other measure methods were utilized to study the precipitation process of super-fine aluminum hydroxide from sodium aluminate solutions with gel-seed, the technology for the control of particle morphology and for the enhancement of the decomposition temperature of super-fine aluminum hydroxide was developed. The main conclusions and innovations are as follows:1) The effects of the main factors such as precipitation temperature, gel-seed ratio, precipitation time and the concentration of the solution etc on the precipitation ratio and the particle size of the precipitated aluminum hydroxide from sodium aluminate solution were investigated. The action of gel-seeds in the precipitation process was studied for the first time, the possible mechanism for the precipitation of super-fine aluminum hydroxide from sodium aluminate solution with gel-seeds was proposed based on the temperature change and the analysis results of the liquors.2) According to fractal theory, the granularity distribution quality of super-fine aluminum hydroxide was associated with its fractal dimension of particle size distribution (PSD).The effects of the main process parameters, namely precipitation temperature, gel-seed amount, precipitation time and the concentration of the solution, on the PSD of super-fine aluminum hydroxide precipitated from sodium aluminate solution were investigated. It was found that the PSD of aluminum hydroxide precipitated from the liquor had the fractal characteristics of particle group, the results of PSD were in correspondent with its fractal dimension. So the fractal dimension of PSD can provide a credible criterion to the quality of the PSD.3) The influence of the impurities of sodium carbonate and sodium sulfate in sodium aluminate solution on the precipitation ratio and the PSD of super-fine aluminum hydroxide precipitation was studied, and their impact mechanism on PSD was analyzed. Both sodium carbonate and sodium sulfate in sodium aluminate solution could reduce the precipitation ratio and make the size of aluminum hydroxide precipitation coarser. The fractal dimension of PSD went up with the increase of the concentration of sodium carbonate or sodium sulfate. Sodium sulfate had greater effect on the precipitation ratio and PSD than sodium carbonate.4) Kinetics of super-fine aluminum hydroxide precipitation from sodium aluminate solution with gel-seed was studied. By studying the effects of precipitation temperature, the concentration of caustic soda, gel-seed ratio and the concentration of solution on the seeded precipitation kinetics, kinetic equation of super-fine aluminum hydroxide seeded precipitation from sodium aluminate solution was put forward. Based on the results of dynamics light scatter and SEM, the crystal growth mechanism of super-fine aluminum hydroxide from sodium aluminate solution with gel-seeds was obtained.5) The surfacial electrical property of super-fine aluminum hydroxide and the rheological behavior of its aqueous suspension were studied. It was found that both the content and the category of electrolytes affected the surfacial electrical property and surfacial potential. Highly charged ions had greater effect on the surface potential. The addition of small amount of electrolytes could make the super-fine powder disperse better by electrostatic repulsion. Proper amount of inorganic electrolytes and macromolecule surfactants in the slurry could improve its rheological behavior and reduce its viscosity. The mechanism how additives affected the rheology of the slurry was explored.6) The technology for the control of the morphology of super-fine aluminum hydroxide by seeded precipitation was developed. Super-fine aluminum hydroxide with plate shape was prepared via adding a small amount of organic additives with hydroxyl groups or carboxyl groups which acted as the crystal habit modifier agents for the control of the growth of aluminum hydroxide crystal. Spherical super-fine aluminum hydroxide was prepared with micro emulsion where aluminum hydroxide crystals could grow directionally under the mediating of organic templates. The action mechanism of organic additives on the crystal growth of aluminum hydroxide was also investigated.7) Combining with the current production process of aluminum hydroxide in China, a novel method to prepare aluminum magnesium hydroxide composite flame retardant with high thermal stability was developed with the key technologies such as adverse feeding with single flow, addition of dispersants and surface modifier additives. The difficulties namely dispersion and aggregation of super-fine powder were overcome, and the compatibility between inorganic material and organic polymer material was enhanced. The economical and feasible technology to improve the application performance of super-fine aluminum hydroxide flame retardant was provided.