The main purpose of kaolin calcination:
1. Remove organic carbon and other impurity minerals to increase whiteness;
2. Remove the moisture and hydroxyl groups of kaolin to increase the void volume and chemical reactivity of the calcined product, improve the physical and chemical properties, meet various application requirements, and lay the foundation for further development of new application fields.
Main factors affecting the calcination of kaolin:
1. Feed granularity
The argument about the granularity of Kaolin kiln has a long history. It is mainly related to the process of using calcining first and then stripping or stripping first and then calcining.
If no wet stripping is carried out before calcination, the minimum particle size of Kaolin dry grinding is -10 μm ≥ 90 %. When the particle size is large, the specific surface area of the particle is small, the contact area with oxygen is small, which is not conducive to the rapid combustion of carbon. The temperature of calcination is long, and the carbon inside the particle can not be fully in contact with oxygen, and it is difficult to oxidize and remove. The product will appear black heart phenomenon. When flaking after calcining, the whiteness of the product is greatly affected due to the existence of the sandwich.
If the flake(-10 μm ≥ 90 %) is calcined first, the powder particles are prone to agglomeration when calcined at high temperatures.
Many scholars have done a large number of experiments in this regard, and finally reached a consensus. First, the film was stripped and then entered the kiln. After calcined, the film was disassembled twice. At present, most of the high-quality coal burning Kaolin uses dry and wet combination, first grinding and then burning the production process.
2. Calcination temperature
Due to different calcination temperatures, chemical reactions and crystal structure conversion, the physical and chemical properties and application fields of 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 the products. It is concluded that the optimum calcination temperature of calcined Kaolin applied to papermaking coating is approximately within the range of 950-1000 °C, and the activity and whiteness of the products are satisfactory requirements.
The preparation of molecular sieves and active soils is used in Kaolin, which is used as functional fillers in the plastics and rubber industries. The calcination temperature is controlled in the range of 850-950 °C.
The calcined Kaolin can be used as refractory clinker and ceramic material.
It is worth mentioning that the optimal calcination temperature will also be different for Kaolin raw materials from different origins.
3. Calcination atmosphere
The calcination atmosphere mainly refers to whether the oxidation or reduction atmosphere is maintained in the furnace.
Although the oxidation atmosphere is conducive to the oxidation of organic matter, it is not conducive to the effect of calcination on stained impurities such as Fe2O3 and TiO2. The reduction atmosphere can be maintained in the kiln by adding reducing agents, controlling the amount of O2 entering the kiln, or introducing appropriate amount of CO2 into the kiln.
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 adequate the reaction, the higher the whiteness of the product.
In actual production, the best combination of calcination time, production efficiency and product performance should be sought. The coal calcined Kaolin is generally about 15 % due to the removal of hydroxyl groups and Carbonaceous materials, and the amount of burning loss is related to raw materials. The relative content of the main chemical components SiO2 and Al2O3 increased, and the content of other components did not change much.
5. Calcination equipment
The calcination equipment is also critical to the production of Kaolin. In large countries such as the United States and the United Kingdom, large-scale dynamic vertical kilns and flame-proof rotary kilns are generally used to produce calcined Kaolin. The raw materials are generally selected soft Kaolin, and the whiteness of the products is 85 % -95 %.