Bayer process for treating low-silicon bauxite, especially gibbsite bauxite, has the advantages of simple process, convenient operation and high product quality, and its economic effect is far from that of other methods. At present, more than 90% of alumina and aluminum hydroxide produced in the world are produced by Bayer process.
Bayer process includes two main processes, namely two patents proposed by Bayer. One is that he found that as long as aluminum hydroxide is added as a seed and stirred continuously until the molar ratio of sodium oxide to aluminum oxide increases to 6, most of the aluminum hydroxide solution has been precipitated. At room temperature, aluminum oxide in sodium aluminate solution with the molar ratio of sodium oxide to aluminum oxide of 1.8 can slowly precipitate into aluminum hydroxide. When heated, the substances in alumina water and bauxite can also be dissolved, that is, the seed mother liquor can dissolve bauxite. Using these two processes alternately, bauxite can be treated in batches, from which pure aluminum hydroxide products can be obtained, which constitutes the so-called Bayer process cycle.
When heated to a certain temperature, the caustic solution dissolves alumina in bauxite;
al2o 3·H2O+2 NaOH+(3-n)H2O→2 naal(OH)4
The obtained sodium aluminate solution is decomposed under the conditions of dilution and cooling to separate out aluminum hydroxide;
Aluminum hydroxide 4 === aluminum hydroxide 3+ sodium hydroxide
The former process is called dissolution and the latter process is called decomposition. The decomposition mother liquor containing caustic soda returns to dissolve out new bauxite.
The first process of Bayer process is to crush bauxite ore into particles with a diameter of about 30 mm with a pulverizer, and then wash away impurities such as clay on the surface of the particles with water. These washed particles are mixed with the recovered Bayer process residual liquid with sodium hydroxide concentration of 30%-40%, and a suspension with solid particle size below 300 microns is formed by ball milling. With the decrease of particle size, the specific surface area of bauxite increases greatly, which is helpful to accelerate the subsequent chemical reaction. The suspension formed by bauxite and high-concentration sodium hydroxide solution enters the reaction kettle again. By increasing the temperature and pressure, aluminum hydroxide in bauxite reacts with sodium hydroxide to generate soluble sodium aluminate (NaAl(OH)4), which is called dissolution. The equation is as follows: [1].
According to the composition of bauxite, the temperature and pressure of Al2O3+2 NaOH+3 H2O → 2 NaAl(OH)4 reactor were determined. For bauxite with more gibbsite, the reaction can be carried out at 150 degree atmospheric pressure, while for bauxite with more diaspore and boehmite, the reaction needs to be carried out under pressure, and the common conditions are 200 ~ 250 degrees and 30 ~ 40 atmospheric pressure. When reacting with sodium hydroxide, various oxides of iron, calcium oxide and titanium dioxide contained in bauxite basically do not react with sodium hydroxide, forming solid precipitates, which remain at the bottom of the reactor, and they will be filtered out, and the formed filter residue is red, which is called red mud, while silica impurities contained in bauxite will react with sodium hydroxide to generate sodium silicate, and sodium silicate is also soluble in water.
SiO _ 2+2 NaOH → 2Na2SiO3+H2O Bayer process In order to remove sodium silicate, the solution is slowly heated to make silica, alumina and sodium hydroxide generate sodalite-structured hydrated sodium aluminosilicate, which is then precipitated and filtered out, leaving only sodium aluminate in the supernatant.
The hot solution enters the cooling device and is gradually cooled after being diluted with water. Sodium aluminate will hydrolyze to produce aluminum hydroxide. At this time, adding pure alumina powder will precipitate white aluminum hydroxide solid.
NaAl(OH)4 → Al(OH)3+NaOH Some manufacturers improved this step by introducing excess carbon dioxide to help produce aluminum hydroxide.
After NaAl(OH)4+CO2→ Al(OH)3+NaHCO3 filters out the generated aluminum hydroxide, the remaining high-concentration sodium hydroxide solution will be recycled to treat another batch of bauxite and dissolve out aluminum hydroxide. The generated aluminum hydroxide can be decomposed into aluminum oxide by calcination at 1000℃;
The specific calcination temperature of 2Al(OH)3 → Al2O3+3 H2O depends on the required crystal form and particle size of alumina. The produced alumina can then be electrolyzed by Hall-Arrow method to produce metallic aluminum.
Bayer process is mainly divided into three stages: dissolution, decomposition and calcination, which mainly consists of crushing and wet grinding, bauxite dissolution, red mud separation and washing, sodium aluminate solution seeded decomposition, aluminum decomposition mother liquor evaporation, sodium carbonate alkali recovery in alumina production, aluminum hydroxide calcination and other processes.
To produce 1t alumina by Bayer process generally requires 1.7 ~ 3.4t ore, 60 ~ 150kg NaOH and 200 ~ 350 kw·H electricity, with a total energy consumption of 7.4~32.6GJ. According to the composition of the products of silicon minerals in bauxite (sodium aluminosilicate hydrate Na2O Al2O3 1.7SiO2 H2O), for every dissolved 1 part of SiO2, 1 part of Al2O3 and 0.6 part of Na2O will be lost. Industrial production requires that the total recovery rate of alumina must be above 80%. If the mechanical loss of alumina during treatment is calculated as 3% ~ 5%, the theoretical dissolution rate of alumina should be greater than 83% ~ 85%. Therefore, Bayer process is suitable for treating diaspore bauxite or diaspore bauxite with Al-Si ratio greater than 6 ~ 7. For gibbsite bauxite, only active silica content should be considered. For medium-grade bauxite, Bayer-sintering combined process is more economical.
1855, the French chemist Louis le Chatterley first proposed a method of heating the Na2CO3 mixture of bauxite and sodium carbonate to 1200℃ to generate sodium aluminate, and then introducing carbon dioxide into the sodium aluminate solution to generate aluminum hydroxide. In the1880s, Russian fiber industry needed a lot of alumina as mordant. Carl Joseph Bayer, a German chemist working in St. Petersburg, put forward Bayer method and applied for a patent. The most important improvements are as follows: firstly, it is found that aluminum hydroxide will slowly precipitate from dilute alkali solution as long as aluminum hydroxide seeds are added; Second, the residual lye can be recovered, and the new bauxite can be reprocessed by increasing the concentration to realize continuous production. Bayer process soon replaced Le Chatelet process and was used together with Hall-Arrow process, which greatly increased the output of aluminum. The economic benefit of Bayer process is determined by several points. First, the proportion of gibbsite in bauxite, the more gibbsite, the less energy consumption; The second is the ratio of aluminum to silicon in bauxite. Bayer process converts silica into hydrated sodium aluminosilicate, and alumina and sodium hydroxide are lost in this process. With the gradual shortage of high-alumina and high-silicon bauxite, the loss of alumina and sodium hydroxide in this process is also increasing. Some researchers and companies put forward an improved scheme of Bayer process combined with sintering process. In addition, Bayer process will make some sodium hydroxide enter the red mud, which will bring strong corrosiveness to the red mud, and its PH value will be as high as112, which will bring serious environmental problems, such as the 20 10 dyke of the red mud storage yard of Hungarian Aika Aluminum Factory, which will lead to tragedy.