The calcination temperature of special grade bauxite is higher than that of grade I bauxite, the main reasons are as follows:

I. Influence of chemical composition

The ALO content of super bauxite is usually above 85%, while the ALO content of grade I bauxite is between 75% and 85%. The high content of Al₂O₃ means that super-grade bauxite is more difficult to sinter at high temperature, and higher temperature is needed to promote its chemical reaction and crystal transformation. In addition, the impurity content in super-grade bauxite is lower, such as FeO content less than 1.4%, while the FeO content of grade I bauxite is less than 3.0%. Impurities will form liquid phase during calcination, which will promote sintering. However, there are few impurities in super-grade bauxite and the amount of liquid phase generated is small, so higher temperature is needed to realize sintering.

II. Influence of mineral composition

In the process of calcination, super bauxite mainly produces minerals such as mullite and corundum, and the formation and stability of these minerals need higher temperature. For example, the formation temperature of mullite is higher, and the Al₂O₃/SiO₂ ratio in super-grade bauxite is higher, which is close to the composition area of mullite, and the secondary mullite reaction is more obvious, which requires higher temperature to complete this process.

III. Requirements of sintering process

The sintering process of super bauxite requires higher temperature to realize densification. Above 1400℃, the secondary mullite reaction is basically completed, and the liquid phase begins to promote the dissolution and precipitation of grains, leading to densification. Super bauxite needs higher temperature (1600℃~1700℃) to achieve full sintering because of less impurities and high Al₂O₃ content.

IV. Requirements for product quality

Super-grade bauxite is mainly used to produce high-purity refractories, such as high-alumina bricks and corundum bricks, which need higher refractoriness and mechanical properties. Therefore, the calcination temperature of super-grade bauxite is higher to ensure that it can form a denser and more stable structure at high temperature and meet the needs of high-end application fields.

To sum up, the high Al₂O₃ content, low impurity content and the requirement for high-quality products of super-grade bauxite determine that it needs higher calcination temperature to realize full sintering and performance optimization.