Currently, there are three main processes for producing alumina worldwide: the Bayer process, the sintering process, and the Bayer-sintering combined process. The Bayer process holds a dominant position, accounting for over 95% of global alumina production. In my country, the proportion of alumina produced using the Bayer process is also over 90%, and this proportion is gradually increasing.
1.1 Bayer Process Flow
The basic process of alumina production using the Bayer process includes raw ore slurry preparation, high-pressure leaching, dilution of the leached ore slurry and separation and washing of red mud, seed crystal decomposition, separation and washing of aluminum hydroxide, calcination of aluminum hydroxide, mother liquor evaporation, and causticization with sodium carbonate monohydrate.
After crushing, bauxite is ground with lime and seed evaporation mother liquor (circulating mother liquor) to form a raw ore slurry. Then, Al2O3 is leached from the ore at high temperature, yielding a leached ore slurry composed of sodium aluminate solution and insoluble residue (red mud). The slurry is diluted with red mud washing liquid, and then the sodium aluminate solution and red mud are separated in a settling tank. The red mud is washed and discharged to a red mud stockpile. The purified sodium aluminate solution is decomposed by adding aluminum hydroxide seed crystals, precipitating aluminum hydroxide. After separation from the mother liquor, the aluminum hydroxide is washed and calcined to obtain the finished alumina. The mother liquor and washing liquid are evaporated and concentrated and returned to dissolve the next batch of bauxite. During the evaporation of the mother liquor, a certain amount of Na₂CO₃·H₂O crystallizes and precipitates from the mother liquor. This is separated and causticized with Ca(OH)₂ to form NaOH solution, which is then sent to the wet mill for batching along with the evaporated mother liquor.
1.2 Soda-lime Sintering Process Flow
Before the Bayer process, alumina was mainly obtained through sintering. However, due to the poor quality of the finished alumina produced by sintering, the complex process, and the high heat consumption, the sintering method was gradually phased out after the advent of the Bayer process.
my country's bauxite is of the monohydrate gibbsite type, with a relatively low grade. Therefore, in the early stages, alumina production was mainly achieved using the soda-lime sintering method or the Bayer-sintering combined process. With technological improvements, the Bayer process has gradually become dominant.
The soda-lime sintering process for alumina production mainly includes raw material preparation, sintering, clinker leaching, desilication, carbonation decomposition, calcination, and evaporation of the decomposition mother liquor.
Raw material preparation stage: The bauxite raw meal consists of bauxite, limestone (or lime), new soda ash (used to compensate for alkali losses in the process), recycled mother liquor, and other recycled materials.
Sintering stage: High-temperature sintering of the raw meal to produce clinker mainly containing sodium aluminate, sodium ferrite, and calcium orthosilicate.
Clinker leaching: Transferring sodium aluminate from the clinker into a solution, separating and washing insoluble residues (red mud).
Desilication: Separating insoluble compounds from the silica entering the solution to produce a high-silica-index sodium aluminate concentrate.
Carbonation decomposition: Sodium aluminate solution is decomposed with CO2. The precipitated aluminum hydroxide is separated from the sodium carbonate mother liquor, and the aluminum hydroxide is washed. A portion of the solution undergoes seed crystal decomposition to obtain a caustic alkali solution required for certain process conditions.
Calcination: Aluminum hydroxide is calcined into alumina.
Evaporation of decomposition mother liquor: Excess water is removed during evaporation. The recycled mother liquor obtained after evaporation is used to prepare raw material slurry.
1.3 Bayer-Sintering Combined Process Flow
Besides the Bayer process and the sintering process, the Bayer-sintering combined process has also played an important role in alumina production in my country. Depending on the chemical composition, mineral composition, and other conditions of bauxite, the combined process is further divided into three basic processes: parallel, series, and hybrid. Among them, Chinese alumina enterprises mainly use the hybrid process.
The hybrid process is a method that combines the series and parallel processes. The hybrid process requires the aluminum-silicon ratio of the red mud clinker to be no less than 2.3. A hybrid process plant can be considered as a combination of a series process plant and a sintering process plant. By producing a portion of aluminum hydroxide through carbon fractionation, the production capacity of the Bayer process and the sintering process can be flexibly adjusted.