How are high-quality alumina hollow spheres formed?

The formation of alumina hollow spheres is completed in a short period of time (10-30s) by blowing alumina melt under high-pressure air. The ball forming process is actually a rapid cooling and shrinking process. The melting point of alumina is 2050°C, and the temperature suitable for blowing the ball should be 200~300°C higher than the melting point, that is, it needs to be heated to 2200~2300°C. If the temperature of the melt is low and the viscosity is high, it is easy to blow into a thick-walled ball or a honeycomb hollow ball.

Blown alumina hollow spheres are divided into 3 stages:

One is the melting stage, increasing the temperature and reducing the viscosity of the melt;

The second is the blowing stage;

The third is the cooling and solidification stage.

The mechanism of the formation of alumina hollow spheres is a volume shrinkage process of rapid cooling of alumina high-temperature melt after injection. This process is that the alumina melt is blown into countless small droplets under the blowing action of high-pressure air. These small droplets fall in a parabolic path in the air. During this movement, the small droplets first form a small ball under the action of surface tension and centrifugal force. When the small ball is quenched, the surface is solidified instantaneously, while the inside of the ball is still in a liquid state. Under the action of centrifugal force, the melt shrinks greatly at the same time, so that the melt solidifies quickly and evenly in the hollow core. On the shell of the ball, the entire process of forming a hollow ball is completed.

The shrinkage cavity (shrinkage) inside the hollow sphere comes from the volume expansion of alumina during heating. Although alumina only has a linear expansion of 1.6%~1.8% when heated to 1800°C, when the oxide is heated to the melting point temperature, when the substance changes from a solid phase to a liquid phase, a volume expansion of 20%~40% will occur. . This volume expansion is related to the chemical bonds of the molecules. The larger the chemical bond in the molecule, such as the ionic bond, the larger the volume difference between the volume of the melt and the solid. On the contrary, the larger the covalent bond, the smaller the volume difference. When alumina changes from a solid phase to a liquid phase, the molar volume expands by about 23.5%. Therefore, thermal expansion and contraction are the fundamental reasons for the formation of alumina hollow spheres. This is the same reason that the production of fused cast bricks is prone to shrinkage cavities.

When blowing alumina hollow spheres, the melt temperature of alumina fluctuates between 2200 and 2300°C. Alumina in a high-temperature liquid state has a greater thermal expansion rate. The linear expansion coefficient of high temperature liquid is generally three times that of solid state.

The interior of the high-temperature liquid contains a certain partial pressure of gas, which accelerates the expansion of the high-temperature liquid and is an important factor to promote the formation of alumina hollow spheres. A large number of honeycomb pores can be seen from the fused corundum block. The apparent porosity of the granular fused corundum sand (3-5mm) is also about 10%-15%. Thus, it can be explained that there is a relatively large gas partial pressure inside the melt.