一、Alumina Introduction

 

Alumina, chemical symbol: Al2O3, molecular weight 102, pure alumina is a white amorphous powder, also known as alumina in the mining, ceramics and materials science, density 3.9-4.0g/cm3, melting point 2050 °C, boiling point 2980°C, insoluble in water, amphoteric oxide, soluble in inorganic acid and alkaline solution. Alumina has 9 crystalline states, mainly alpha and gamma-type variants, which can be extracted from bauxite industrially. It is the main raw material in the production of aluminum electrolysis. Alumina is a typical material for structural ceramics. It is usually applied to structural parts that are subject to mechanical stress. It is particularly useful for its high melting point, high hardness, corrosion resistance, and electrical insulation properties under harsh conditions. The alumina present in nature is called corundum (α-Al2O3) and is the product of a volcanic eruption. It is a colorless crystal in the rock, it can also be dyed with other alumina impurities (chromium oxide and iron oxide, etc.) with colored crystals, red is called rubies, and other corundum stones are called sapphires.

Industrial alumina is a thermally decomposed dehydration product of various alumina hydrates. They form a series of isomorphous crystals, some of which are in the dispersed phase and some are transitional.

The commonly used alumina structure types are as follows:

?? Alpha-alumina. Also known as corundum structure, is the most stable alumina crystal form. It is a representative structure of an M2O3 type oxide having a cubic closest packed oxygen atomic layer, and 2/3 of the octahedral coordination between oxygen atoms is filled with metal atoms. That is, α-alumina is an aluminum ion that forms an ion binding bond with an oxygen ion, and an aluminum atom is surrounded by six oxygen atoms to form an hexahedral hexacoordinated type. Because the ion radius of aluminum ions is the smallest of M2O3, it binds closely to oxygen ions and becomes the highest hardness trivalent metal oxide.

      Α-alumina is insoluble in water and acid, and it is the basic raw material for metal aluminum production. It is also used for making various refractory bricks, refractory enamels, refractory tubes, and high-temperature resistant experimental instruments; it can also be used as abrasive, flame retardant, fillers. Etc.; high-purity alpha alumina is also a raw material for the production of artificial corundum, synthetic ruby, and sapphire; it is also used in the production of board bases for modern large-scale integrated circuits.

Γ-alumina, γ-Al2O3, is an over-alumina formed during the dehydration of alumina hydrates such as gibbsite and aluminum hydroxide. When heated at 1350°C, it transforms into a dense structure of α-Al2O3, which results in extremely high firing shrinkage. Judging from the structure, it has voids, good adsorptive power to other substances, large surface area, high adsorption of water and other substances, and therefore can be used as adsorbents and desiccants. The chemical properties are more lively and easy to react with acid or alkali solution.

Beta-alumina. Strictly speaking, β-Al2O3 does not belong to alumina. The chemical composition can be approximately represented by MeO6Al2O3 and Me2OAl2O3. (In which, MeO refers to alkaline earth metal oxides such as CaO, BaO, SrO, and Me2O refers to alkali metal oxides such as Na2O, K2O, Li2O.) β-Al2O3 is only a polyaluminate compound with a very high content and has significant ion conductivity. Sex and relaxation polarization phenomenon, large dielectric loss, poor electrical insulation properties. Alumina for electrolytic aluminum production, which is usually produced in the production of alumina, is a mixture of α-Al 2 O 3 and γ-Al 2 O 3 .

Aluminum oxide used in the production of aluminum electrolysis consists mainly of α-Al2O3 and γ-Al2O3. The quality of alumina directly affects the purity of the resulting metallic aluminum and the technical and economic indicators of aluminum electrolysis production. Therefore, alumina as a raw material for aluminum electrolysis has certain requirements for its chemical purity and physical properties.

1. The chemical purity of alumina. In addition to Al2O3, alumina usually contains small amounts of impurities such as SiO2, Fe2O3, Na2O and H2O. Aluminum oxide for electrolytic aluminum must have a high chemical purity because it contains oxide impurities (eg, Fe2O3, SiO2, etc.) that are more positive than aluminum and that these elements will first precipitate at the cathode during the electrolysis process. In aluminum, aluminum is impure. The metal oxide impurities (such as Na2O) that are more negative than aluminum in the alumina react with the electrolyte and change the normal composition of the electrolyte, which is not favorable for the electrolytic operation.

    The crystal water remaining in the alumina is expressed as a burn-down. It is also a harmful impurity. HF is generated by the action of water and AlF3 in the electrolyte, causing the loss of fluoride salts and contaminating the environment. In addition, when the high-absorbing or moisture-absorbing alumina comes in contact with the high-temperature molten electrolyte, it will cause the electrolyte to splash and endanger the safety of the operator.

 

二、 Physical properties of alumina
 

    In addition to the strict requirements on the chemical composition of aluminum oxide for aluminum electrolysis production, it also requires that the alumina dissolves rapidly in the cryolite melt, the precipitation of the bottom of the electrolytic tank is less, and the electrolyte covering the electrolyte is good in the insulation, in the air No moisture absorption, less flying loss, good fluidity, ease of transportation and easy feeding of electrolyzers. All these properties depend on the physical properties of the alumina. The indicators for the physical properties of alumina are: angle of repose, α-Al2O3 content, bulk density, particle size, and specific surface area.

?? Rest angle. Refers to the natural inclination of the material on a smooth surface. Alumina with a larger angle of repose is more soluble in the electrolyte and can be well covered on the electrolyte crust during the electrolysis process, with less flying losses.

?? Al2O3 content. The content of α-Al2O3 reflects the degree of calcination of alumina. The higher the calcination degree, the more α-Al2O3 content, and the moisture absorption of alumina decreases with the increase of α-Al2O3 content. Therefore, the electrolytic alumina needs to contain a certain amount of α-Al2O3. However, the solubility of α-Al2O3 in the electrolyte is poorer than that of γ-Al2O3.

3 bulk density. The bulk density of alumina refers to the mass of material per unit volume in the natural state. Alumina, which typically has a low bulk density, facilitates dissolution in the electrolyte.

4 grain sizes. The particle size of alumina refers to the degree of thickness. The particle size of the alumina must be appropriate, and the coarseness of dissolution in the electrolyte is slow, and even precipitates.

5 specific surface area. The specific surface area of ??alumina refers to the total area of ??the sum of the outer surface area and the inner pore surface area per unit weight of material. It is an important indicator of the level of activity of a substance. Alumina with a large specific surface area has good solubility in the electrolyte and has a high activity, but it is easily hygroscopic.

According to the physical properties of alumina, Al2O3 is usually divided into three types: sand, flour, and intermediate. These three types of Al2O3 have large differences in physical properties.

    The sand-like Al2O3 has a small bulk density, a large specific surface area, a slightly smaller angle of repose, contains a smaller amount of α-Al2O3, coarse and more uniform, and high strength. Flour-like alumina has a larger bulk density, a small specific surface area, contains more α-Al2O3, more fine-grained particles, and poor strength. The physical properties of the intermediate alumina are somewhere in between.