2.1.1. Bauxite Ore Treatment by the Baeyer Method

As of today, about 90–95% of the world’s aluminium hydroxide is extracted using the Baeyer process, which was proposed in 1887 ^[15]. The Baeyer method is a hydrochemical method of obtaining alumina from bauxites. Large pieces of bauxite, supplied from the mines, are first crushed, and then wet-ground in ball mills. Bauxite, caustic alkali and recycled liquor are fed to ball mills. Sometimes, a little lime is added to bauxite to facilitate their breakability. The pulp from the mills is collected in collectors, where the remaining amount of the recycled liquor with a concentration of Na₂O = 300 g/l is fed. Then, it is heated there by separation steam to 90–100 °C and soaked for 4–8 h while stirring for preliminary bauxite desiliconization, i.e., there is a transfer of the most part of the active silica from bauxite to the liquor. The mixture of the ground bauxite and the recycled liquor (raw pulp) is sent to one of the main operations. This process is called the leaching or cooking of bauxite, which is carried out in autoclaves at a temperature of 230–240 °C. The purpose of this operation is to dissolve the aluminium oxide contained in bauxite, avoiding the transfer of other components of bauxite (silica, iron oxides, etc.) to the liquor. The silica is removed by subsequent slow heating when Na₂Si(OH)₆ precipitates. The remaining pure liquor of NaAl(OH)₄ is cooled, diluted with water and neutralized with carbon dioxide. As a result, aluminium trihydrate Al(OH)₃ (gibbsite) is selectively deposited from the liquor without the residues of the dissolved silica.

2.1.2. Sintering Technique

The technique of sintering ore with lime or soda involves mixing high-silica finely ground ore (nepheline and others) with soda and limestone and sintering in rotating furnaces at 1250−1300 °C. The obtained mass is leached with an aqueous alkaline solution. The solution of sodium aluminate, obtained in this way, is separated from the sludge, then it is released from SiO₂, precipitating it in an autoclave at a pressure of about 0.6 MPa and then with lime under atmospheric pressure, and the aluminate is decomposed with gaseous CO₂ along with the formation of Al(OH)₃.

Both described methods are multistage. They include both the main technological stages of production and auxiliary operations related to waste disposal and to the repeated return of mother liquors to the circulating cycle ^[16]. The main product when using the abovementioned methods of ore processing is gibbsite (hydrargillite).

2.2.1. Aluminate Technology with Alkali Treatment

(a)

Dissolution of gibbsite in alkali, accompanied by the formation of sodium aluminate:

(b)

Reprecipitation with acid:

A continuous process based on the reaction of HNO₃ with NaAlO₂ was described by the authors of ^[18]. The principle is as follows: In the first reactor, HNO₃ and NaAlO₂ are mixed at a temperature of 30 °C to 75 °C. Then, the resulting suspension is sent to the second reactor where it is converted into pseudoboehmite. The suspension fraction is recycled in the first reactor with a ratio of 0.1 to 3 of slurry volume per volume of mixing (NaAlO₂ + HNO₃). Pseudoboehmite is then removed from the second reactor. After drying, it has a specific surface area in the range of 200 m²/g to 300 m²/g.

This method is the most common for producing aluminium gel for catalysis. Precipitation is carried out from alkaline solutions (aluminates) with acids (sulfuric, nitric, hydrochloric) or acidic solutions of salts. Various structural and texture characteristics of the resulting hydroxide are determined by the pH, temperature and nature of the anion. The crystallisation rate of pseudoboehmite is determined largely by the temperature of deposition, and bayerite is primarily determined by the pH.

To increase the dissolution velocity, gibbsite is preliminary ground to the particle size of 10 µm and/or the temperature of the reacting mixture is raised.

There are various techniques of precipitation—with a variable and constant value of pH, two-stage (cold and hot precipitation), etc. For example, cold precipitation from the sodium aluminate solution with the sulphuric acid solution is carried out at 20–25 °C and pH = 9.3–9.5. Hot precipitation is performed at 90–95 °C and pH = 9.3–9.5. Then, both modifications of aluminium oxide are mixed, and a precipitate consisting of pseudoboehmite and boehmite is obtained, which can be rinsed and filtered very well. Granules of active aluminium oxide of high mechanical strength can be obtained using this technique.

The drawback of this method is the high cost of removing sodium due to the difficulty of rinsing the gel. The aluminium hydroxide precipitate is filtered, rinsed on a filter press and moulded into granules, which are later dried and calcined at 670–820 K to obtain η- or γ-Al₂O₃.