Preparation process of activated alumina

① Dehydration method

Activated alumina is generally produced by heating and dehydration of aluminum hydroxide between 450 and 600 ℃. According to the different raw materials, there are mainly the following methods for synthesizing aluminum hydroxide:

Acid precipitation method. Neutralize alkaline aluminum salt solution with acidic substances. First, aluminum oxide monohydrate is precipitated from aluminate solution with acid, and then aluminum hydroxide is obtained through aging, washing, drying, and other processes, often referred to as acid precipitation method (alkali method). The nitric acid method is currently commonly used to prepare pseudo boehmite products with narrow pore size distribution and good formability. The acid precipitation method has simple process and good repeatability. The prepared pseudo boehmite has a large specific surface area, high production efficiency, and low environmental pollution, but the anions in the product are difficult to completely remove.

Alkali precipitation method. Neutralize acidic aluminum salt solutions with alkaline substances. Precipitation of monohydrate alumina from aluminum salt solution using alkali, followed by aging, washing, drying, and other processes to obtain pseudo boehmite products, is often referred to as the alkali precipitation method (acid method). This method uses aluminum chloride and ammonia water as raw materials, with stable product quality, easy impurity removal, and difficulty in generating trihydrate alumina. Under the same preparation conditions, the grains are intact and larger. Its disadvantage is that it requires high purity of raw materials and it is difficult to completely remove impurities and anions.

Alcoholic aluminum hydrolysis method. Aluminum hydroxide is hydrolyzed to produce alumina monohydrate, which is then aged, filtered, and dried to obtain high-quality pseudo boehmite, which is then dehydrated to obtain γ- Al2O3 has a very high purity, good product crystal structure, and easy control of pore structure. However, this method for producing alumina is costly and expensive. Currently, there is relatively little research and application of this method in China, while this method is commonly used abroad.

Carbonization method. Preparation of pseudo boehmite by carbonization method, using CO2 and NaAlO2 reaction to prepare γ- Al2O3 carrier. The process idea of carbonization method is to introduce CO2 into sodium aluminate (NaAlO2) solution. Due to the fact that this method can be combined with the actual situation of aluminum plants, it is an economic route to prepare sodium aluminate solution, an intermediate for producing aluminum hydroxide from bauxite in industry, through carbonization method. It can simplify the process route and equipment, and reduce environmental pollution.

② Sol gel method

This method involves boiling metal aluminum in hydrochloric acid or aluminum chloride solution to obtain a transparent and colorless aluminum sol. Then, the aluminum sol is mixed with a solution of hexamethylenetetramine, dropped into a hot oil column to form a ball, and then aged, washed, dried, and calcined to obtain alumina. Obtained by this method γ- Al2O3 small balls are characterized by low density, large pore volume, and good strength. They are easy to wash during production, and the filtration process is omitted, making it easy to achieve continuity.

③ Citric acid method

Tan Yanan et al. mixed crystalline aluminum chloride with pore forming agents such as citric acid and starch, and directly dried and calcined to produce an amorphous activated alumina. Compared with conventional methods for preparing activated alumina, this preparation method eliminates steps such as neutralization, aging, filtration, and washing, shortens the preparation cycle, and the process is easy to control. No need to prepare acid and alkali solutions, thus saving a lot of water and avoiding sewage discharge.