The preparation methods, advantages and disadvantages of alumina balls
Powder spheroidization methods include physical methods and chemical methods.
According to different material aggregation modes, the preparation methods of spherical alumina can be systematically classified into three categories: gas-phase method, liquid-phase method, and solid-phase method.
1. Ball Milling Method
The ball milling method is the most common technique for preparing ultrafine alumina powder. Typically, through the rotation or vibration of a ball mill, the raw material is impacted, milled and stirred by grinding media, and large-sized powder is refined into ultrafine powder.
The particle size of the prepared spherical alumina powder mainly depends on the particle state of the raw material and the preparation process.
Advantages: Simple operation, low cost, high output.Disadvantages: The surface of the prepared spherical powder particles is relatively rough, resulting in an increased specific surface area and enhanced powder activity, which easily leads to interparticle agglomeration. Therefore, it is not suitable for preparing spherical powder particles with high-quality requirements.
2. Homogeneous Precipitation Method
Precipitation in a homogeneous solution is a process involving nucleation, subsequent aggregation and growth, and final precipitation from the solution, which is usually non-equilibrium. However, if the concentration of the precipitant in the homogeneous solution can be reduced or even generated slowly, a large number of uniform tiny nuclei will be formed. The resulting fine precipitate particles will be uniformly dispersed throughout the solution and maintain an equilibrium state for a long period of time. This precipitation method is known as the homogeneous precipitation method.
For the homogeneous precipitation method, if the size of the obtained precipitate particles falls within the range of colloidal particles, the method is also referred to as the sol-gel method.
Advantages: Mild reaction conditions, high sphericity, average particle size 400 nm–10 μm, good dispersibility.Disadvantages: Aluminum sulfate is usually required as the raw material to obtain spherical powder, so harmful sulfides are generated during the calcination stage. There is agglomeration and porous structure after sintering.
3. Sol-Gel Method
The sol-gel method involves the formation of a precursor sol via hydrolysis or polymerization of alkoxides or inorganic salts, followed by alcohol washing, aging, and final calcination to obtain alumina powder. Precise control of the system pH and reactant concentration is required in this method.
Advantages: Good uniformity, high chemical purity, etc.Disadvantages: Relatively complex preparation process and high cost.
4. Sol-Emulsion-Gel Method
This method was developed on the basis of the sol-gel method. In the early stage, the sol-gel method was mostly used to prepare alumina sol, with more focus on the organizational structure of the resulting colloid. Gradually, it became a common method for preparing ultrafine powder.
To obtain spherical powder particles, the interfacial tension between the oil phase and the aqueous phase is used to create tiny spherical droplets, so that the formation and gelation of sol particles are confined within the tiny droplets, ultimately yielding spherical precipitate particles.
In the sol-emulsion-gel method, a large amount of organic solvents and surfactants are used to form the emulsion. The separation process of the spherical powder from the emulsion is very complicated, and it is difficult to maintain the spherical shape of the powder during drying and calcination.
5. Dropping Method
The dropping method involves dropping alumina sol into an oil layer (usually paraffin oil, mineral oil, etc.), forming spherical sol particles under the action of surface tension. The sol particles are then gelated in an ammonia solution, and finally the gel particles are dried and calcined to form spherical alumina.
This method is a further process improvement of the sol-emulsion-gel method, applying emulsion technology to the aging stage of the sol while keeping the oil phase stationary, thus eliminating the separation treatment of the powder from the oily reagent.