Preparation and application of high purity alumina

Crystallization pyrolysis method

Aluminum ammonium sulfate crystallization pyrolysis method

Ammonium aluminum sulfate method is widely used, and its product has good activity and uniform particle size. The refined aluminum sulfate is crystallized in the ammonium sulfate system, and the ammonium aluminum sulfate is purified during multiple recrystallization. Finally, the purified ammonium aluminum sulfate is pyrolyzed to produce alumina powder. Wang Shouping et al. [1] prepared high-purity alumina by segmented pyrolysis of ammonium aluminum sulfate, and the obtained product was high-purity spherical alumina with good dispersion and particle size of 200-300nm, which was conducive to subsequent production and utilization. The process is simple, low cost, high quality powder and easy to large-scale industrial production, but the process is prone to thermal dissolution, for the removal of potassium, calcium, halogen and other impurities difficult.

In addition, ammonia and sulfur oxides produced in the process of thermal decomposition cause environmental pollution, which is the main factor limiting its development. In this regard, Yin Yongquan et al. [2] absorbed, neutralized and concentrated the waste gas generated by the pyrolysis of ammonium aluminum sulfate to produce ammonium sulfate, which can realize the economic and environmental protection of alumina production.

Crystallization pyrolysis of ammonium aluminum carbonate

Research on perfection and improvement of aluminum ammonium sulfate method has been the focus of people for many years. The improved crystallization pyrolysis method of ammonium aluminum carbonate based on ammonium aluminum sulfate method has controlled air pollution to a certain extent and is widely used in current industrial production. The purified ammonium aluminum sulfate reacts with ammonium bicarbonate to convert it into ammonium aluminum carbonate, avoiding sulfur oxide gas generated during subsequent pyrolysis. Compared with the ammonium aluminum sulfate method, the product powder size distribution of ammonium aluminum carbonate method is not uniform, agglomeration phenomenon occurs, and the production cost is higher. Lin Yuanhua et al. [3] showed that reactant concentration, solution pH, additives and other factors would affect product purity and powder quality. All parameters and reaction conditions should be strictly controlled in the production process.

hydrolysis

Aluminum alcohol hydrolysis method

Aluminum alcohol hydrolysis method in the catalyst system, organic alcohol and metal aluminum reaction to generate aluminum alcohol solution, and then hydrolysis, high purity aluminum hydroxide obtained after sintering to get high purity aluminum oxide. In view of this method, the technical keys to ensure the quality of alumina are as follows: 1. Two, further distillation of aluminum alcohol purification, remove high melting point impurities; Three, aluminum alcohol drying, sintering conditions and powder preparation device to control the particle size of the powder. Fu Gaofeng [4] et al prepared ultrafine high-purity alumina by using aluminum sheets with aluminum content greater than 99.9% and analytical pure propanol. They needed constant temperature water bath heating, adding isopropanol and mercury chloride as catalysts, and finally prepared the product by distillation, hydrolysis, aging, drying and calcination of the generated aluminum isopropanol. The method is environmentally friendly and has high purity for the obtained alumina. However, the high purity of organic alcohol precursors leads to high production cost, complex process and difficult purity control.

Direct activation hydrolysis method

Mercury chloride activation hydrolysis method, the use of mercury chloride aluminum sheet or aluminum chips activated, and then placed in 1% concentration of aluminum sulfate solution hydrolyzed, the alumina hydroxide sol obtained after high temperature drying gel, further high temperature processing to prepare alumina powder. The key point of this process is the activation hydrolysis of aluminum. It is necessary to ensure that the raw material is aluminum flakes of high purity. If the hydrolysis is not sufficient, there will be elemental aluminum left in the product. The production cycle of high purity aluminum activation hydrolysis method is short, and the obtained alumina powder can improve the transmittance and corrosion resistance of transparent ceramics. However, the purity of raw aluminum metal is high, and strict process control is needed to ensure the purity of products. The harsh hydrolysis conditions and environmental problems limit the application of this method in laboratory.

2.3 Improvement of bayer method

The improved Bayer process can precipitate alumina hydroxide from sodium metaluminate solution after several desilication and iron removal, and then calcinate at high temperature to obtain high purity alumina with desired crystal shape. The solution purification, impurity removal and crystallization process will affect the purity and powder quality of the final product, which is the key of this method. Tang Haihong et al. [5] explored the use of barium aluminate for deep purification of sodium aluminate solution in order to avoid aluminum loss in solution purification process.

In addition, attention should be paid to the removal of sodium, adding mineralizer during roasting or adding sodium remover during hydrothermal phase conversion of aluminum hydroxide are effective means of sodium removal. In the process of crystallization, susaturation of solution, crystallization temperature, seed size and other parameters directly affect the purity and powder quality of alumina. By accurately controlling the reaction conditions, the crystallization process can proceed slowly, avoid the formation of abnormal crystal nuclei, and help to reduce the inclusion of Na, Si and other impurities. The process has low cost and wide raw material sources, but the process is complicated, the roasting temperature is high, the product yield is low, the purity is relatively low.

2.4 Sol-gel method

Sol-gel method is an important method to prepare high purity alumina at low temperature. The high purity aluminum salt was hydrolyzed in high purity inorganic salt or organic acid to produce hydrate precursor hyalosol, and then the gel was polymerized and gelated to get transparent gel, which was further heated to high purity alumina. Xu Sankui et al. [6] prepared sol by using high-purity aluminum nitrate to form hydrate precursor system under ammonia water system. In the gelation stage, high-purity alumina crystal seed was added, and aluminum trifluoride was used as additive to prepare high-purity nano-alumina at a lower temperature.

2.5 Hydrothermal synthesis method

The purity and granularity of the hydrothermal synthesis method for product control is easy, using aqueous solution as the reaction system, through the heating pressure increase the solubility of precursor, high purity aluminum and water vapor direct hydrolysis of aluminum hydroxide, in temperature and precipitation in the process of alumina, avoid aluminum hydroxide calcined to produce hard together, then through the working procedure such as drying, calcination can produce alumina powder.

first dissolved NaOH in ammonia water at room temperature, then slowly added aluminum chloride to prepare milky white flocculent precursor, and then prepared diaspore nano-powder by hydrothermal synthesis method by adjusting the pH value of the solution. The disadvantages of this method are that the equipment investment is large, high temperature and high pressure have certain dangers, the efficiency of alumina hydroxide conversion is low, usually need to add seed to reduce the phase transition temperature.

precipitation