In addition, it is also required that the dissolution speed of alumina in cryolite melt is fast, the bottom of the electrolytic cell is less deep, the coating on the electrolyte has good heat preservation property, it is not hygroscopic in the air, has less flying loss and good fluidity, which is convenient for transportation and automatic feeding of electrolytic cell. All these properties depend on the physical properties of alumina.
The physical properties of alumina are characterized by repose angle, α - Al203 content, bulk density, particle size, specific surface area and wear coefficient.
1. The angle of repose. The repose angle of alumina refers to the inclination angle of natural accumulation of materials on a smooth plane. Alumina with larger repose angle is easier to dissolve in the electrolyte, and can cover the electrolyte shell well in the electrolysis process, and the flying loss is small.
2. A-Al2O3 content. The content of a-Al2O3 in alumina reflects the calcination degree of alumina. The higher the calcination degree is, the more a-Al2O3 content is. The hygroscopicity of alumina becomes worse with the increase of a-Al2O3 content. Therefore, the alumina for electrolysis should contain a certain amount of a-Al2O3. But the solubility of a-Al2O3 in electrolyte is worse than that of y-al2o3.
3. Bulk density. The bulk density of alumina refers to the weight per unit volume of material in natural state. Generally, alumina with small bulk density is favorable for dissolution in electrolyte.
4. Particle size. The particle size of alumina refers to its fineness. The particle size of alumina must be appropriate. If it is too coarse, it will dissolve slowly or even precipitate in the electrolyte; if it is too fine, it is easy to fly and lose.
5. Specific surface area. The specific surface area of alumina refers to the total surface area of the sum of the external surface area and the inner pore surface area of the material per unit weight. It is an important indicator of the activity of substances. Alumina with large specific surface area has good solubility and activity in electrolyte, but it is easy to absorb moisture.
6. Wear coefficient. The so-called wear coefficient is the percentage of the change of the particle size content in the sample after the alumina is abraded in the fluidized bed under certain conditions. Wear coefficient is a physical index of alumina strength.
According to the physical properties of alumina, alumina can be divided into three types: sand, flour and intermediate. The physical properties of these three types of alumina are quite different. Sandy alumina has smaller bulk density, larger specific surface area, slightly smaller repose angle, a small amount of a-Al2O3, more and more uniform coarse particles and higher strength. Flour like alumina has larger bulk density, smaller specific surface area, more a-Al2O3, more fine particles and poor strength. The physical properties of intermediate alumina are between the two. The physical properties of various alumina are shown in table 1-3.
