Industrial Production Of Aluminum Hydroxide
Bayer Process
It was invented in 1888 by Austrian Bayer (K.J.Bayer). The principle is to use caustic soda (NaOH) solution to warm and dissolve alumina in bauxite to obtain sodium aluminate solution. After the solution is separated from the residue (red mud), the temperature is lowered, aluminum hydroxide is added as a seed crystal, after a long time of stirring, the sodium aluminate is separated into aluminum hydroxide, washed, and calcined at a temperature of 950~1200℃. Obtained alumina products. The solution after precipitation of aluminum hydroxide is called mother liquor, which is recycled after evaporation and concentration. Due to the different crystalline structures of gibbsite, boehmite and diaspore, their solubility in caustic soda solution is very different. Therefore, different dissolution conditions, mainly different dissolution temperatures, must be provided. The gibbsite type bauxite can be dissolved at 125~140℃, and the diaspore type bauxite can be dissolved at 240~260℃ and lime (3~7%) is added. The main progress of the modern Bayer process lies in: ① large-scale equipment and continuous operation; ② automation of the production process; ③ energy saving, such as high-pressure enhanced dissolution and fluidized roasting; ④ production of sandy alumina to meet aluminum electrolysis and flue gas The need for dry purification. The process flow of the Bayer process is shown in Figure 1.
The advantages of Bayer process are simple process, low investment and low energy consumption. The lowest energy consumption per ton of alumina is only about 3×106 kcal, and the alkali consumption is generally about 100 kg (calculated as Na2CO3).
The economic effect of the Bayer process is determined by the quality of the bauxite, mainly the content of SiO2 in the ore, which is usually expressed by the ratio of aluminum to silicon in the ore, that is, the weight ratio of the content of Al2O3 to SiO2 in the ore. Because in the dissolution process of the Bayer process, SiO2 is transformed into sodalite-type hydrated sodium aluminosilicate (Na2O?Al2O3?1.7SiO2?nH2O), which is discharged with the red mud. Each kilogram of SiO2 in the ore will cause a loss of 1 kilogram of Al2O3 and 0.8 kilogram of NaOH. The lower the aluminum-silicon ratio of bauxite, the worse the economic effect of the Bayer process. Until the late 1970s, the aluminum-silicon ratio of the bauxite processed by the Bayer process was greater than 7-8. As high-grade gibbsite-type bauxite resources are gradually decreasing, how to use other types of low-grade bauxite resources and new energy-saving technologies have become important research and development directions.
Soda lime sintering method
It is suitable for processing high-silicon bauxite. The bauxite, sodium carbonate and lime are mixed in a certain proportion and sintered in a rotary kiln to form sodium aluminate (Na2O? Al2O3), sodium ferrite (Na2O? Fe2O3, original The clinker composed of calcium silicate (2CaO? SiO2) and sodium titanate (CaO? TiO2). Then use a dilute alkali solution to dissolve the sodium aluminate in the clinker. At this time, the NaOH obtained by the hydrolysis of sodium ferrite also enters the solution. If it dissolves If the conditions are properly controlled, the calcium orthosilicate will not react with the sodium aluminate solution in a large amount, but will be discharged from the red mud composed of calcium titanate, Fe2O3?H2O, etc. The sodium aluminate solution obtained by dissolving the clinker is specifically desiliconized During the process, SiO2O forms hydrated sodium aluminosilicate (called sodium silica slag) or hydrated garnet 3CaO? Al2O3? xSiO2? (6-2x) H2O precipitation (where x≈0.1), so that the solution is purified. The CO2 gas is passed through Add refined sodium aluminate solution, add seed crystals and stir to obtain aluminum hydroxide precipitate and mother liquor whose main component is sodium carbonate. Aluminum hydroxide is calcined to become alumina product. Al2O3 in hydrated garnet can be reused containing Na2CO3 The mother liquor is extracted and recovered.
