Alumina: I "cannot coexist" with sodium!
High-temperature alumina is made from industrial alumina hydrate as the main raw material through calcination for dehydration and crystal phase transformation. Since industrial alumina hydrate is prepared in an alkaline medium, high-temperature alumina inevitably contains a certain amount of impurities.
Hazards of sodium oxide
Among them, the content of sodium oxide impurities is the main basis for grading high-temperature alumina products, and to a large extent affects the physical and chemical indicators, product quality and application performance of its downstream products. For example:
When high-temperature alumina is used as wear-resistant ceramics, the content of Na₂O directly affects the compressive strength and electrical insulation of alumina products. Generally, the higher the Na₂O content, the higher the electrical conductivity, and the worse the electrical insulation performance of alumina ceramics and wear-resistant products. At the same time, it also leads to a lower conversion rate of α-Al₂O₃, which fails to meet the quality requirement that the α-Al₂O₃ content in high-temperature alumina is more than 95%, resulting in deformation and cracking of alumina ceramics and wear-resistant products.
In electronic ceramics, the presence of Na₂O not only affects the density of electronic ceramics, but also leads to the formation of β-Al₂O₃ with certain conductivity due to the combination of Na₂O and Al₂O₃, thereby affecting their electrical properties.
In electronic ceramics, the presence of Na₂O not only affects the density of electronic ceramics, but also causes Na₂O to combine with Al₂O₃ to form β-Al₂O₃ with certain conductivity, thereby influencing their electrical properties.
Where do sodium impurities come from?
The Na₂O in high-temperature alumina comes from the raw material alumina hydrate. Na₂O in alumina hydrate is one of the main pollutant impurities formed during the alumina production process. The Na₂O carried in alumina hydrate exists in three (or four) forms.
Ⅰ. The mother liquor alkali belongs to water-soluble alkali and exists in the form of attached alkali. Na₂O can be removed by water washing; the encapsulated alkali is difficult to wash off and can only be partially removed.
Ⅱ. Lattice alkali is insoluble in water and can only become water-soluble alkali when the lattice structure rearranges.
Ⅲ. Combined alkali is insoluble in water, acid, or alkali, and does not decompose even at high temperatures.
Factors affecting the content of sodium oxide in its raw material—aluminum hydroxide
Since the sodium impurities in alumina mainly come from its precursor, aluminum hydroxide, what are the factors that affect the content of sodium impurities in aluminum hydroxide?
There are many factors affecting the content of sodium oxide in aluminum hydroxide. During the seeded precipitation operation, the concentration of the original decomposition solution, decomposition temperature, decomposition time, etc., all have a direct impact on it. However, in production practice, the following factors also significantly affect its sodium oxide content:
(1) Initial temperature of decomposition in the seeded precipitation process
In the decomposition process, various cooling measures are usually adopted to improve the output rate of aluminum hydroxide, which leads to an increase in the content of unwashable alkali in the product. Various studies have shown that the main factor affecting the insoluble sodium oxide in the decomposition products is the initial decomposition temperature. The lower the initial temperature, the higher the content of insoluble sodium oxide in the product.
(2) Particle size of aluminum hydroxide products
During the seed precipitation process of the Bayer method, materials often undergo periodic refinement. Meanwhile, the circulation volume of seed precipitation materials is large, and when the refinement cycle comes, it often lasts for a relatively long time. When the materials are refined, the amount of mother liquor entrained by aluminum hydroxide on the flat disk increases, leading to more unwashable alkali. The materials bind very tightly to each other, resulting in poor air permeability and filtration performance of the materials, which can easily cause cross-zone contamination and affect the sodium oxide content of the products.
(3) Solid content in the original decomposition solution
There are many factors affecting the solid content in the decomposition tank, such as imbalance between production and washing, low efficiency of the seed filter, commissioning or isolation of the decomposition tank, and fluctuations in decomposition rate. These factors often cause large fluctuations in the solid content of the final tank. When the solid content decreases, the material becomes too dilute, and the flat disk is prone to cross-zone contamination, which affects product quality and leads to an increase in the sodium oxide content in aluminum hydroxide.
How to remove sodium?
The currently common sodium removal processes mainly include the following:
The first method is to reduce the content of crystalline alkali and intercrystalline alkali by means such as extending the decomposition time during the decomposition process of aluminum hydroxide, and to minimize the content of attached alkali through multiple washings.
The second method is to reasonably select mineralizers. The main function of mineralizers is to increase the lattice defects of alumina, forming cation and anion vacancies, thereby helping to accelerate crystallization and effectively reducing the temperature of α-phase transformation. At the same time, mineralizers can also react with sodium oxide in alumina to form volatile sodium compounds. Therefore, mineralizers are widely used in the calcination process of high-temperature alumina. Commonly used mineralizers include aluminum fluoride, boric acid, magnesium oxide, and ammonium nitrate, etc.
The third method is to pretreat the raw materials: introducing acid through a wet process to carry out acid-base neutralization for sodium removal, and then washing off the salts generated after the reaction with hot water. The sodium removal effect varies depending on the existing forms of sodium oxide in the raw materials, and the sodium content can be reduced to below 0.05% at the lowest. This method can significantly improve the product quality.