Calcination is a thermal treatment process where a material, usually an inorganic one, is heated to a high temperature. The goal is to cause chemical dissociation or separation. This process is often used in the production of inorganic materials. It involves heating the material to a temperature below its melting point in the presence of limited air or oxygen.
What is the Process of Calcination in Kiln? 5 Key Steps Explained
1. Heating and Chemical Dissociation
The primary objective of calcination is to heat the material to a point where chemical changes occur. For example, calcium carbonate (CaCO3) undergoes calcination to form calcium oxide (CaO) and carbon dioxide (CO2). This reaction is a decomposition reaction where the calcium carbonate breaks down into its constituent components due to the heat.
2. Removal of Volatile Impurities and Moisture
Calcination is also used to remove volatile impurities and chemically bound moisture. For instance, borax (Na2[B4O5(OH)4]·8H₂O) can be calcined to remove the chemically bound water, leaving behind a more concentrated or altered form of the material.
3. Use of Rotary Kilns
Calcination is often carried out using rotary kilns, which are cylindrical furnaces that rotate to mix and heat the material evenly. These kilns can operate at temperatures ranging from 800 to 2,200°F and are used for various thermal processes including calcination, incineration, and thermal desorption.
4. Types of Rotary Kilns
There are two main types of rotary kilns: direct and indirect. Direct rotary kilns are more efficient as they allow direct heat transfer between the material and the heating medium. In these kilns, the process gas passes through the drum either in the same direction (co-currently) or in the opposite direction (counter-currently) to the material, enhancing the heat transfer and mixing.
5. Advances in Kiln Technology
Modern advancements have led to the development of precalciner dry process kilns. In these kilns, the energy-intensive process of decarbonating limestone in the raw mix is completed before the feed enters the rotary kiln. This precalcination step significantly improves the efficiency and output of the kiln, allowing for shorter and smaller-diameter rotary kilns to achieve high outputs.
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