Impurities Accumulation on the Surface of Alumina Hydrate Particles in Bayer Technology

The purity, structural surface, particle dimensions, particle size distribution, and the associated reactivity of chemical and surface properties are the most important and most required properties of alumina hydrate special brands. The purpose of this paper concerns the common metallic impurities accumulation on the surface of alumina hydrate particles, during the sodium aluminates decomposition in liquid phase, during the entire aluminum hydroxide crystallization stage in the Bayer technology.

Investigation of Removal Mechanism of Sapphire in Plasma Assisted Polishing

Single crystal sapphire is widely used as the material for precision equipments, due to its high hardness, chemical inertness and light transmission. However, it is difficult to obtain a scratch-free and damage-free sapphire surface with high-efficiency through traditional mechanical polishing or etching. We developed plasma assisted polishing (PAP) for the finishing of difficult-to-machine materials, such as silicon carbide, diamond, and sapphire. In this article, preliminary research results are showed about PAP applied to polishing of single crystal c-plane sapphire substrates. Combination of helium based atmospheric pressure water vapor plasma irradiation and silica abrasive polishing drastically increased removal rate of the sapphire c-plane. XPS measurements of the surfaces with and without irradiation of water vapor plasma revealed that alumina hydrate was formed by plasma irradiation at low temperature of less than 40°C. It is assumed that formation of alumina hydrate promoted the removal rate of sapphire.

Effect of αk on Aluminium Trihydroxide Crystallization from Sodium Aluminate Soluntion under Ultrasound

The ultrasound intensification crystallization process of aluminium trihydroxide from seeded sodium aluminate solution is investigated with various αk(the molecule ratio of Na2O/Al2O3). The experiment results indicate that ultrasound intensification can enhance precipitation ratio of seeded sodium aluminate solution, while large αkdoes not good to improvement of the precipitation under ultrasound. When αkis 1.45, 2.0 and 2.5, the precipitation ratio increase 2.5%, 2.0% and 0.9%, respectively under ultrasound intensification. The mechanism of enhancement on precipitation of alumina hydrate is acoustic cavitations that enhance or alter reactions process of aluminate ions, Al (OH)-4. The mass percentage of precipitation production grain size <45μm is increased by 4.3 % under ultrasound intensification with initial αkat 2.0, which is the biggest. Key words: ultrasound; sodium aluminate solution; precipitation; aluminium trihydroxide