Detailed introduction to the raw material for producing corundum: bauxite

So far, the production of corundum mainly uses two types of raw materials: one is to directly melt or sinter natural bauxite into brown corundum, alumina corundum, sub white corundum, etc; Another method is to use industrial alumina for electric melting or sintering into white corundum, dense corundum, plate-like corundum, sintered corundum, etc. More than 95% of industrial alumina is extracted from bauxite. It can be seen that bauxite is an important raw material for producing corundum. Aluminum oxide can also be extracted using alum and garnet.

Bauxite, also known as high alumina bauxite or bauxite, is mainly composed of monohydrate alumina (Al2O3 · H2O) and trihydrate alumina (Al2O3 · 3H2O) minerals. According to the different structures of boehmite, it can be divided into boehmite (or α - Al2O3 · H2O boehmite) and boehmite (or γ - Al2O3 · H2O boehmite, boehmite). The so-called bauxite is not a mineral name, but a mixture of monohydrate hard alumina, monohydrate soft alumina, and trihydrate alumina. Its main chemical component is Al2O3, with a general content of 40% to 80%. The chemical composition of natural bauxite varies greatly. In addition to Al2O3, there are also minerals such as SiO2, TiO2, Fe2O3, CaO, MgO, K2O, Na2O, etc. These chemical components make up the following minerals: trihydrate alumina, monohydrate alumina, as well as silicate minerals (i.e. silicate, rhodochrosite, kyanite, with chemical formulas of (Al2O3 · SiO2), kaolinite (Al2O3 · SiO2 · H2O), rutile (TiO2), as well as dickite and iron containing minerals. Bauxite mainly has oolitic, bean shaped, detrital, and cryptocrystalline (mud like) structures, as well as earthy and dense blocky structures. All colors ranging from white to ochre are generally red for those with high iron content and gray white for those with low iron content. Due to different bonding materials, the color changes greatly, with the advantage of reddish brown spots. The appearance characteristics of bauxite are relatively complex, but there are also patterns to follow. Generally, those of the same level have similar characteristics.

China's bauxite is mainly distributed in provinces and regions such as Henan, Shanxi, Guangxi, Guizhou, and Shandong. The main minerals are boehmite and kaolinite, followed by boehmite, trihydrate aluminate, rutile, diopside, and iron containing minerals. Its characteristics are high Al2O3 content, high SiO2 content, and low Fe2O3 content. Therefore, Al2O3/SiO2 is relatively low, mostly ranging from 4 to 7. Except for Pingguo bauxite in Guangxi, most of the high-quality bauxite with Fe2O3 content below 5% and Al2O3/SiO2 ≥ 10 is also rare. For ore types, the vast majority are of the boehmite kaolinite type, commonly known as high silica low iron boehmite bauxite, or high alumina high silica low iron boehmite kaolinite bauxite. The bauxite in Pingguo area, Guangxi can be referred to as high-speed railway boehmite kaolinite type bauxite. A series of bauxite with different Al2O3 contents are composed of two minerals, namely boehmite and kaolinite, in different proportions. Only Fujian and Guangdong have a small amount of trihydrate alumina type, but Al2O3/SiO2 is relatively low. Guangxi and Shandong also have a considerable amount of high sulfur bauxite.

Abroad, it is mostly of the trihydrate alumina type, but in Europe it is mostly of the monohydrate alumina type, while in Greece it is of the monohydrate hard alumina monohydrate soft alumina type. From a chemical composition perspective, most bauxite mines abroad have lower SiO2 content, higher Al2O3/SiO2 content, and generally higher Fe2O3 content.

Al2O3 is the main component in bauxite, and the higher its content, the better. Currently, the production of industrial alumina requires an aluminum silicon ratio of not less than 3.0~3.5. Other oxides are considered impurities, and SiO2 is a harmful impurity in alkaline production of alumina. High SiO2 content in bauxite will reduce the production efficiency of smelting brown corundum or producing industrial alumina, and increase the impurity content. Bauxite contains high levels of Fe2O3, although most of it can be removed, it has a certain impact on product quality. If the content of TiO2 is high, it is also detrimental to product quality, while if the content of K2O and Na2O is high, it has a greater impact on product quality.

Whether it is smelting brown corundum or producing industrial alumina, there are strict requirements for the quality of bauxite. The classification of bauxite according to chemical composition and usage in China's GB/T3497-1983 is shown in the table:

The bauxite used for smelting brown corundum must undergo calcination. The calcined high alumina clinker not only improves the grade and reduces the power consumption of corundum smelting, but also stabilizes the furnace condition and improves production efficiency. A series of physical and chemical changes occur during the heating process of bauxite, and studying it is not only of great significance for the production of alumina, smelting brown corundum, and sub white corundum, but also the basis for manufacturing high alumina refractory materials. The changes during heating of bauxite can be divided into three stages, namely decomposition stage, secondary mullite stage, and recrystallization sintering stage.

The product of high-temperature calcination of bauxite ore is called high alumina bauxite clinker, which is the main raw material for high alumina refractory materials and can also be used to smelt corundum, especially sub white corundum. The requirements for high alumina bauxite clinker are strict, not only requiring high Al2O3 content, but also minimizing the content of difficult to reduce oxides.

China is a country with abundant bauxite resources in the world, but due to the influence of natural laws of deposit formation, the distribution of high-grade ores is not concentrated, and mechanized mining is severely mixed. In addition, the natural types of ores are complex and difficult to strictly classify. In recent years, a considerable amount of experimental research has been conducted on China's bauxite from the perspective of refractory materials, and gratifying results have been achieved. In the future, further work will be carried out on synthetic raw materials to form a series of high-quality refractory raw material products with Chinese characteristics.