Alumina production method
[China Aluminum Network] So far, numerous methods have been proposed for extracting alumina from aluminum ore or other aluminum-containing raw materials. Due to technical and economic factors, some methods have been abandoned while others remain in the experimental stage. The existing methods for producing alumina can be categorized into four types: the alkali method, the acid method, the acid-alkali combination method, and the thermal method. Currently, the alkali method is widely used in large-scale industrial production.
Bauxite is the most critical aluminum resource globally, followed by alunite, nepheline, and clay. In the world's alumina industry, apart from Russia using nepheline to produce a portion of alumina, nearly all alumina worldwide is derived from bauxite.
Bauxite primarily consists of gibbsite, boehmite, or diaspore. In China, the bauxite resources suitable for alumina production are all of the diaspore type.
The alumina content in bauxite varies significantly, ranging from around 30% to over 70%. Besides alumina, the main chemical impurities in bauxite include silicon dioxide, iron oxide, and titanium oxide. Additionally, there are trace amounts of calcium and magnesium carbonates, potassium, sodium, vanadium, chromium, zinc, phosphorus, antimony, bismuth, sulfur, and other compounds along with organic matter. Notably, bismuth, though present in small quantities in bauxite, gradually accumulates in the circulating mother liquor during the alumina production process and can be effectively recovered, becoming a significant source of bismuth production.
One of the key indicators for assessing the quality of bauxite is the ratio of alumina to silica content, commonly referred to as the aluminum-to-silica ratio.
In the alkali method of alumina production, the aluminum ore is treated with an alkali (such as NaOH or Na2CO3) to convert the alumina in the ore into a sodium aluminate solution. Impurities like iron and titanium in the ore and most of the silicon form insoluble compounds. The insoluble residue (red mud) is separated from the solution, either discarded after washing or subjected to comprehensive treatment to recover useful components. The pure sodium aluminate solution can then be used to precipitate aluminum hydroxide. After separation, washing, and calcination, alumina products are obtained. The decomposed mother liquor is recycled for processing the next batch of ore. The alkali process of alumina production includes the Bayer process, the sintering process, and the combined Bayer-sintering process.
The Bayer process was developed by Austrian chemist Karl Joseph Bayer from 1889 to 1892 to extract alumina from bauxite. Over a century, the process has undergone numerous technological improvements, yet its fundamental principles remain unchanged. In honor of Bayer's significant contributions, this method has always been called the Bayer process.
The Bayer process comprises two primary stages: first, dissolving alumina from bauxite under specific conditions (a term used in the alumina industry, i.e., leaching, which applies hereafter), followed by extracting aluminum hydroxide from a supersaturated sodium aluminate solution. This constitutes Bayer's two patents. Essentially, the Bayer process extracts alumina from bauxite using hydrometallurgical methods. Silica-containing minerals lead to losses of Al2O3 and Na2O during the Bayer process alumina production.
In the Bayer process, after crushing the bauxite, it is wet-milled with lime and recycled mother liquor to create a qualified pulp. The pulp undergoes pre-desiliconization and is preheated to the dissolution temperature before being dissolved. After cooling the dissolved pulp via auto-evaporation, it enters the sedimentation and separation process for dilution and red mud (solid residue after dissolution). The secondary steam generated in the evaporation process is used for preheating the pulp. Following sedimentation and separation, the red mud is washed and stored in a red mud yard, while the separated crude liquid (sodium aluminate solution containing solid float, the same below) is sent to the leaf filter. The crude solution is termed "semen" after most of the suspended matter is removed through leaf filtration. The semen enters the decomposition process and is decomposed by seeding to yield aluminum hydroxide. After fractionation, separation, and washing of the decomposed aluminum hydroxide, part of it is returned to the seed decomposition process as a seed crystal, while the remainder is calcined to obtain an alumina product. The decomposed mother liquor separated after the seed crystal is decomposed is evaporated and returned to the dissolution process, forming a closed loop. Aluminum hydroxide is calcined to produce alumina.
The dissolution conditions required for different types of bauxite vary significantly. Gibbsite-type bauxite dissolves well at 105°C. Boehmite-type bauxite dissolves more rapidly at a dissolution temperature of 200°C. Diaspore-type bauxite must be dissolved at temperatures higher than 240°C, with typical industrial dissolution temperatures at 260°C. Dissolution time is no less than 60 minutes.
The Bayer process is used to treat bauxite with a high aluminum-to-silica ratio. The process is straightforward, the product quality is high, and its economic benefits far surpass those of other methods. When handling easily soluble gibbsite-type bauxite, the advantages are even more pronounced. Currently, over 90% of the alumina and aluminum hydroxide produced globally are manufactured using the Bayer process. Due to the unique characteristics of China's bauxite resources, approximately 50% of China's alumina is produced by the Bayer process.
The process that combines the Bayer process and the sintering process is referred to as the combined production process. The joint process can be divided into parallel joint process, serial joint process, and mixed joint process. The method chosen to produce alumina is primarily determined by the grade of the bauxite (i.e., the aluminum-to-silica ratio of the ore). Generally, from a technical and economic perspective, the ore-silica ratio is typically around 3 for the sintering method; for ores with an aluminum-to-silica ratio higher than 10, the Bayer process can be used; when the bauxite grade falls between these two, the joint process can be employed to fully leverage the respective advantages of the Bayer process and the sintering process, achieving better technical and economic indicators.
Currently, the annual global alumina production is about 55 million tons, and China's alumina production is approximately 6.8 million tons. As a major aluminum hub in China, Henan Gongyi Aluminum's output accounts for a significant portion.
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