There are four main synthetic methods for sodium aluminate, including Bayer process, sintering method, combined method, and aluminum hydroxide alkaline hydrolysis method.
The detailed synthesis method is as follows
(1) Bayer method
The Bayer method uses bauxite as raw material, and the mass ratio of alumina to silica in the bauxite is greater than 8. The bauxite is crushed and ground, and then mixed with the caustic soda solution, and sent to the autoclave for high temperature and high pressure to make aluminum. The alumina in the alumina turns into sodium aluminate and is transferred to the solution. After separation, the sodium aluminate solution is obtained, and after evaporation, sodium aluminate is obtained. In recent years, the high-pressure dissolution process of lye has progressed from discontinuous to continuous, and pipelined dissolution has begun to be used in Bayer process.
(2) Sintering method
The bauxite, soda ash and limestone are finely mixed, sintered at a high temperature of 1200 ° C or higher, the clinker is leached with water, and the sodium aluminate solution after leaching is subjected to silicon removal to obtain a refined sodium aluminate solution, and evaporated to dryness. That is, the product is obtained, and the sintering method is further divided into a dry material sintering method and a wet material sintering method. At present, the wet material sintering method has basically replaced the dry material sintering method.
(3) Joint law
The joint method refers to a production method in which the Bayer method and the sintering method are used in combination. In the production, if Bayer method is the main method, the sintering method is supplemented, the high silicon aluminum bauxite is treated by Bayer method, and then the obtained red mud is treated by sintering method. This is called tandem joint method. If the low silicon bauxite ore is treated by the Bayer process, the high silicon bauxite is treated by the sintering method, and the two methods are simultaneously carried out in the production process, and then the crude liquids obtained by the two methods are combined, which is called parallel joint method. The sodium aluminate solution is evaporated to obtain the product.
(4) Aluminum hydroxide alkali solution
The crude aluminum hydroxide was added to a temperature of 50 to 80 ° C in a sodium hydroxide solution to raise the temperature to 110 ° C, and the mixture was kept for 3 hours to obtain a sodium aluminate solution, which was evaporated to dryness to obtain a product.
Summary of main synthetic method processes
(1) Bayer process uses bauxite as raw material, which requires alumina content of 40% to 70%, silica 4% to 20%, iron oxide 1% to 20%, and alumina in bauxite and The mass ratio of silicon oxide is greater than 8. The bauxite is pulverized and ball-milled to a particle size of 85% through 220 mesh, and then wet-mixed with lye, high-pressure dissolution, heated at 150 ° C for 2 to 3 h, and alumina in bauxite is transferred as sodium aluminate. The solution, while impurities such as silicon oxide, iron oxide, calcium oxide, etc. become red mud and precipitate. In order to facilitate the separation of the sodium aluminate solution from the red mud and enhance the desiliconization, the eluate can be diluted until the alumina content is reduced from 250 to 300 g/L to 100 to 120 g/L.
The sodium aluminate solution and the red mud are separated in a multi-layer settling tank, and then the sodium aluminate solution is evaporated to dryness to obtain a finished product.
(2) Sintering method The bauxite having an aluminum to silicon ratio of 3 to 6 is crushed, mixed with limestone, wet-milled in a ball mill in proportion, and a soda ash solution is added at the same time. The ground slurry is mixed and formulated in a certain proportion. The prepared slurry is sprayed into the rotary kiln at a pressure of 12 to 15 kg/cm 2 and sintered at a temperature of 1200 to 1250 ° C. The clinker after the slurry is wet-dissolved in a conical ball mill together with an adjustment liquid such as a red mud washing liquid, so that the alumina becomes a sodium aluminate solution, and impurities such as silicon oxide and iron oxide become solid red mud. The dissolved mud is separated by standing sedimentation and vacuum filtration to separate the solution and the residue. The red mud residue was washed back four times with hot water, and the washing liquid was returned to dissolve the clinker. The isolated sodium aluminate solution contains about 100 to 110 g of alumina per liter and contains 2.5 to 4 g/L of silica. The crude liquid is sent to the autoclave for direct heating with steam, and desiliconization is carried out in the presence of lime milk at 5 to 6 atm. After the solution is boiled for about 2 hours, most of the silicon is removed by insoluble sodium aluminosilicate. The slurry is filtered by a sedimentation and a leaf filter, and the silicon residue is separated and removed, and then evaporated to dryness to obtain a finished product.
Advantages and disadvantages of different methods
Bayer method is suitable for bauxite with low silicon content. The process is simpler than sintering method, with less investment, lower cost and high product quality. However, when the processed bauxite grade is low, its economic superiority is also It is reduced accordingly. The sintering method is suitable for high silicon bauxite as raw material, and the process is more complicated than the Bayer method, the investment is large, the cost is high, and the product quality is also poor, but the total recovery rate of alumina is high. The combined method has the characteristics of the above two methods, and can be used with either high silicon aluminum bauxite or low silicon aluminum bauxite.