Analysis of Five Factors Affecting Calcination of Kaolin

The main purpose of kaolin calcination is to: (1) remove organic carbon and other impurity minerals to improve whiteness; (2) remove the moisture and hydroxyl groups of kaolin to increase the void volume and chemical reactivity of the calcined product, and improve physical and chemical properties. To meet a variety of application needs and to lay the foundation for further development of new application areas.

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

1.Feed granularity

The debate on the particle size of kaolin into the kiln has been long-standing, and it is mainly related to the process of stripping after first calcination or calcining after stripping.

If the wet stripping is not carried out before calcination in the kiln, the minimum particle size of the kaolin dry grinding is -10 μm ≥ 90%. If the particle size is large, the specific surface area of the particles is small, and the contact area with oxygen is small, which is not conducive to the rapid combustion of carbonaceous materials. The time required for constant temperature calcination is long, and the carbonaceous material inside the particles cannot be sufficiently contacted with oxygen, and it is difficult to oxidize. In addition, the product will appear black. When the film is peeled off after calcination, the whiteness of the product is greatly affected by the presence of the sandwich.

If the film is first calcined (-10 μm ≥ 90%) and calcined, the agglomeration phenomenon is likely to occur due to the powder particles being calcined at a high temperature.

Many scholars have done a lot of experiments in this area, and finally reached a consensus, first peeling the film and then entering the kiln, and then depolymerizing and stripping the film after calcination. At present, high-quality coal-fired kaolin is mostly produced by dry and wet combination, first grinding and then burning.

2. Calcination temperature

Due to the different calcination temperatures, the chemical reactions occurring and the crystal structure conversion are different, the physical and chemical properties and applicable fields of the calcined kaolin are also different.

Many scholars have done a lot of work on the influence of calcination temperature on the physical and chemical properties of products. It is concluded that the optimum calcination temperature of calcined kaolin used in paper coating grade is about 950-1000 °C, and the activity and whiteness of the product are both reached. The ideal request.

Molecular sieves and activated soils are prepared and used as kaolin for functional fillers in plastics and rubber industries. The calcination temperature is controlled within the range of 850-950 °C.

The kaolin after high-temperature calcination can be used as a refractory clinker or ceramic material.

It is worth mentioning that the optimum calcination temperature will be different for kaolin raw materials from different regions.

3. Calcination atmosphere

The calcination atmosphere mainly means maintaining an oxidation or a reducing atmosphere in the furnace.

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

4. Calcination time

The 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 as well as product performance should be sought. Coal-based calcined kaolin has a loss of amount of hydroxyl and carbon, and the loss on ignition is related to raw materials, generally around 15%. The relative content of the main chemical components SiO2 and Al2O3 increased, and the content of other components did not change much.

5. Calcining equipment

Calcination equipment is also critical to the production of kaolin. In the United States and the United Kingdom and other countries with high kaolin production, calcined kaolin is generally produced by large dynamic shaft kiln and flame-type rotary kiln. The raw materials are generally selected soft kaolin, and the product whiteness is 85%-95%.

China's high-quality soft kaolin resources are less, but the coal-seated Kaolin soil has a large amount of quality. Therefore, most of China's calcined kaolin is made from coal-based kaolin.

The main production equipment includes flame-flow rotary kiln (heated with diesel, gas or natural gas or electric), shaft kiln, tunnel kiln and shuttle kiln. The flame-insulated rotary kiln can conveniently adjust the atmosphere, the temperature is controllable, the material is evenly heated, the product has high whiteness, stable quality and easy operation, and has become the main equipment for calcination of coal-based kaolin in China.