Three stages of bubble alumina ball formation process

Three stages of bubble alumina ball formation process

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

Blown bubble alumina ball is divided into 3 stages:

The first is the melting stage, which increases the temperature and reduces the viscosity of the melt;

The second is the blowing stage;

The third is the cooling and solidification stage.

The formation mechanism of bubble alumina ball is a volume shrinkage change process of rapid cooling of alumina high-temperature melt after spraying. This process is that the alumina melt is blown into countless small droplets under the blowing action of high-pressure air. These droplets are in the air, falling in a parabolic path. During this movement, the droplet first forms a small sphere under the action of surface tension and centrifugal force. When the small ball is quenched, the surface is instantly solidified, while the inside of the ball is still in a liquid state. Under the action of centrifugal force, the melt produces a large volume shrinkage, so that the melt quickly and uniformly solidifies in the hollow. on the outer shell of the sphere, thus completing the whole process of forming the hollow sphere.

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

When blowing alumina hollow spheres, the temperature of the alumina melt fluctuates between 2200 and 2300 °C. Alumina in the high temperature liquid state has a greater thermal expansion rate. The linear expansion coefficient of high temperature liquid is generally 3 times that of solid state.

There are two main aspects of the gas partial pressure inside the high-temperature melt: one is that in the process of electrofusion, when adding powdered industrial alumina, part of the air brought in during operation, there is still part of the gas after electrofusion Residual in the high-temperature melt, honeycomb pores are formed during the cooling process; the second is that in the electro-melting process, various oxides, especially low-melting oxides, gradually increase the volatilization rate during the electro-melting process. The volatilization of this melt can increase the gas partial pressure inside the liquid. For example, alumina begins to volatilize above 1750 °C, and the volatilization increases above 2300 °C.