Yuan Lei's Little Knowledge | Factors Affecting Calcination of Kaolin

Release time:

2024-01-29 09:34

The main purposes of kaolin calcination are:

(1) remove organic carbon and other impurity minerals to improve whiteness;

(2) remove the moisture and hydroxyl contained in kaolin to improve the void volume and chemical reaction activity of calcined products, improve physical and chemical properties, meet various application requirements, and lay the foundation for further development of new application fields.



The main factors affecting the calcination of kaolin are: feed particle size, calcination temperature, atmosphere, time and process equipment.


01 feed particle size

If there is no wet peeling before calcination in the kiln, the minimum particle size of kaolin dry grinding is-10μm ≥ 90%. The particle size is larger, the specific surface area of the particle is small, and the contact area with oxygen is small, which is not conducive to the rapid combustion of the carbon, the need for a long time of constant temperature calcination, and the carbon inside the particle can not fully contact with oxygen, it is difficult to oxidation removal, the product will appear black heart phenomenon. When peeling after calcination, the whiteness of the product is greatly affected by the presence of the sandwich.

If the first peeling (-10μm ≥ 90%) and then calcined, the powder particles are prone to agglomeration when calcined at high temperature.

After a large number of experiments, it is agreed that the first stripping and then into the kiln, calcination and then secondary depolymerization stripping. At present, most of the good coal-based combustion kaolin is a production process of combining dry and wet, grinding before burning.


02 Calcination temperature

The calcination process of kaolin is a process of continuous heating and temperature rise. Due to the loss of hydroxyl groups and phase transformation, the heating rate is definitely not a uniform ascending process. The loss of hydroxyl groups of kaolin at 530~650 ℃ is an endothermic process. The heating rate at this stage must be slower, which is conducive to the removal of surface hydroxyl groups and internal hydroxyl groups between some tetrahedral lamellae and octahedral lamellae. 950 ℃ by amorphous structure into crystalline silicon aluminum spinel phase, is an exothermic process, uniform heating, heating speed must be faster, but due to the calcination temperature should be strictly controlled, the operation should pay special attention. If the temperature is too high, the content of mullite generated in the calcined product is high, that is, the hardness of the product is increased, and the particles are agglomerated to a certain extent, resulting in a decrease in particle size.



03 Calcination atmosphere

The calcination atmosphere mainly refers to maintaining an oxidizing or reducing atmosphere in the furnace.

Although the oxidizing atmosphere is conducive to the oxidation of organic matter, it is not conducive to the effect of calcination on the dyeing impurities such as Fe2O3 and TiO2. The reducing atmosphere in the kiln can be adjusted by adding a reducing agent, controlling the amount of O2 introduced into the kiln, or introducing an appropriate amount of CO2.


04 Calcination time

Calcination time has a great influence on the activity and whiteness of kaolin. In general, the longer the calcination time, the more complete the reaction and the higher the whiteness of the product.

In actual production, the best combination of calcination time and production efficiency and product performance should be sought. Due to the removal of hydroxyl and carbon, the loss on ignition of coal-based calcined kaolin is related to the raw material, generally about 15%. The relative content of the main chemical components SiO2 and Al2O3 increased, and the content of other components changed little.


05 Calcination Equipment

Calcination equipment is also critical to the production of kaolin. The main production equipment are flame-proof rotary kiln (with diesel, gas or natural gas or electric heating), vertical kiln, tunnel kiln and shuttle kiln. Among them, the flame-proof rotary kiln can easily adjust the atmosphere, the temperature is controllable, the material is heated evenly, the product whiteness is high and the quality is stable, and the operation is simple. It has become the main equipment for calcination of coal-based kaolin in China.

Influence, calcination, kaolin, product, atmosphere, hydroxyl, time, particle size, process, due

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