Powder spheroidization methods include physical methods and chemical methods. According to different material aggregation methods, the methods for preparing spherical alumina can be systematically divided into three categories: gas phase method, liquid phase method and solid phase method.
1. Ball milling method Ball milling method is the most common method for preparing ultrafine alumina powder. Usually, the rotation or vibration of the ball mill is used. The raw materials are impacted, ball milled and stirred by the abrasive, and the large particle size powder is refined into ultrafine powder. body. The size of the obtained spherical alumina powder particles mainly depends on the particle state of the raw materials and the preparation process. Advantages: simple operation, low cost and high output. Disadvantages: The surface of the prepared spherical powder particles is relatively rough, resulting in an increase in the specific surface area and increased powder activity, which is prone to inter-particle agglomeration, and is not suitable for the preparation of spherical powder particles with high quality requirements.
2. Homogeneous precipitation method The precipitation process in a homogeneous solution is the formation of crystal nuclei, then aggregates and grows, and finally precipitates from the solution. It is usually in a non-equilibrium state, but if the concentration of the precipitant in the homogeneous solution can be reduce, or even slowly generate, then a large number of tiny crystal nuclei will be uniformly generated, and the final fine precipitate particles will be evenly dispersed in the entire solution, and will maintain a state of equilibrium for a long time, which This method of obtaining precipitation is called homogeneous precipitation. For the homogeneous precipitation method, if the size of the precipitated particles obtained is within the range of colloidal particles, then this method is also called the sol-gel method. Advantages: The homogeneous precipitation method is relatively mild, with high spherical rate, average particle size of 400nm-10μm, low purity and good dispersion. Disadvantages: In order to obtain spherical powder, aluminum sulfate must usually be used as raw material, so harmful sulfides will be produced during the calcination stage. After sintering, there is agglomeration and many channels.
3. Sol-gel method The sol-gel method is to use alkoxide or inorganic salt to undergo hydrolysis or polymerization to form a precursor sol, then wash with alcohol, age and finally calcinate to obtain alumina powder. This method must be accurately controlled System pH and reactant concentration. Advantages: good uniformity, high chemical purity, etc. Disadvantages: The preparation process is more complicated and the cost is higher.
4. Sol-emulsion-gel method This method was developed on the basis of the sol-gel method. In the early days, the sol-gel method was mostly used to prepare alumina sol, and more was to study the structure of the obtained colloid. Gradually Therefore, this method has become a common method for preparing ultrafine powders. In order to obtain spherical powder particles, people use the interfacial tension between the oil phase and the water phase to create tiny spherical droplets, so that the formation of sol particles and gelation are all controlled. Confinement is carried out in tiny droplets, and finally spherical precipitated particles are obtained. In the sol-emulsion-gel method, in order to form the emulsion, a large amount of organic solvents and surfactants are used, the separation process of the spherical powder in the emulsion is very tedious, and it is not easy to maintain the powder during the drying and calcination stages spherical.
5. Dropping ball method The dropping ball method is to drop the alumina sol into the oil layer (usually using paraffin, mineral oil, etc.), and form spherical sol particles by the effect of surface tension, and then the sol particles are gelled in the ammonia solution. Finally, the gel particles are dried and calcined to form spherical alumina. This method is a further improvement on the process of the sol-emulsion-gel method. The emulsion technology is applied to the aging stage of the sol, and the oil phase is kept still, eliminating the need for separation of powder and oily reagents.
