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To date, many methods have been proposed for extracting alumina from bauxite or other aluminum-containing raw materials. Due to technical and economic reasons, some methods have been phased out, while others are still in the experimental research stage. The proposed methods for alumina production can be classified into four categories, namely the alkaline method, the acid method, the acid-alkaline combined method, and the thermal method. Currently, only the alkaline method is used for large-scale industrial production. Bauxite is the most important aluminum ore resource in the world, followed by alunite, nepheline, clay, etc. In the global alumina industry today, with the exception of Russia which uses nepheline to produce some alumina, almost all alumina in the world is produced using bauxite as raw material. Bauxite is an ore primarily composed of gibbsite, boehmite, or diaspore. To date, all the bauxite resources available in our country for alumina production are of the gibbsite-type. The alumina content in bauxite varies greatly, ranging from as low as about 30% to over 70% in higher cases. Apart from alumina, the main impurities in bauxite are silicon oxide, iron oxide, and titanium oxide. In addition, it also contains small amounts or trace amounts of carbonates of calcium and magnesium, compounds of elements such as potassium, sodium, vanadium, chromium, zinc, phosphorus, gallium, scandium, sulfur, and organic substances. Although gallium is present in small amounts in bauxite, it gradually accumulates in the circulating mother liquor during the alumina production process, allowing it to be recovered effectively and thus becoming the main source for gallium production. One of the main indicators for assessing the quality of bauxite is the ratio of alumina content to silica content in the bauxite, commonly known as the aluminosiliceous ratio. In the alkaline method for producing alumina, alkalis (NaOH or Na2CO3) are used to treat bauxite, converting the alumina in the ore into a sodium aluminate solution. Impurities such as iron and titanium in the ore, along with the majority of silicon, become insoluble compounds. The insoluble residue (red mud) is separated from the solution; after washing, it is discarded or subjected to comprehensive treatment to recover the useful components contained within it. Pure sodium aluminate solution can be used to selectively precipitate aluminum hydroxide; after separation and washing, calcination yields alumina product. The decomposed mother liquor is recycled to process another batch of ore. There are various processes for producing alumina by alkaline methods, including the Bayer process, the sintering process, and the combined Bayer-sintering process. The Bayer process is a method for extracting alumina from bauxite, invented by the Austrian chemist K·J·Bayer between 1889 and 1892. Many improvements have been made in manufacturing techniques over the past century, but the basic principles remain unchanged. To commemorate Bayer’s great contribution, this method has continued to be known as the Bayer process. The Bayer process includes two main steps. First is the dissolution of alumina from bauxite under certain conditions (a term commonly used in the alumina industry, namely leaching). The same applies here); followed by the process in which aluminum hydroxide precipitates from the supersaturated sodium aluminate solution – these are the two patents proposed by Bayer. The essence of the Bayer process is to extract alumina from bauxite using hydrometallurgical methods. In the Bayer process for alumina production, siliceous minerals cause losses of Al2O3 and Na2O. In the Bayer process, bauxite is crushed and then fed, along with lime and recycled mother liquor, into wet grinding to produce a suitable slurry. After pre-desilication, the pulp is preheated to the leaching temperature for leaching. The leached slurry is then cooled by self-evaporation before proceeding to the dilution and sedimentation separation process for the red mud (the solid residue remaining after leaching). The secondary steam generated by the self-evaporation process is used for the preliminary preheating of the slurry. After sedimentation and separation, red mud is washed and sent to the red mud storage area, while the separated coarse liquid (sodium aluminate solution containing solid suspended matter, the same hereafter) is sent to the leaf filter. The crude liquid, after having most of the suspended particles removed through leaf filtration, is called the refined liquid. The semen enters the decomposition process, where aluminum hydroxide is obtained through seed decomposition. After being separated, the aluminum hydroxide undergoes grading and separation washing; part of it is returned to the seed decomposition process as seeds, while the other part is calcined to produce alumina products. The decomposition mother liquor separated after seed crystal decomposition is returned to the leaching process through evaporation, forming a closed-loop cycle. Aluminum hydroxide is converted into aluminum oxide after calcination. The leaching conditions required for different types of bauxite vary greatly. Bauxite of the gibbsite type can be dissolved effectively at 105°C; bauxite of the boehmite type achieves a faster dissolution rate at a temperature of 200°C, whereas bauxite of the diaspore type must be dissolved at temperatures above 240°C, with a typical industrial dissolution temperature of 260°C. The dissolution time should be no less than 60 minutes. The Bayer process is used to treat bauxite with a high aluminum-to-silicon ratio; it features a simple process flow and high product quality, and its economic efficiency is far superior to that of other methods. Its advantages are even more prominent when used to process readily leachable bauxite of the gibbsite type. At present, over 90% of the alumina and aluminum hydroxide produced worldwide is manufactured using the Bayer process. Due to the particularities of China’s bauxite resources, approximately 50% of alumina in China is currently produced using the Bayer process.