Calcination is one of the most common processes used in the ceramics industry among many others. The most common application for Calcining is the decomposition of calcium carbonate to calcium oxide and carbon dioxide, in order to create cement. The product of calcination is usually referred to as “calcine,” regardless of the actual minerals undergoing thermal treatment. Calcination is carried out in furnaces or reactors (sometimes referred to as kilns or calciners) of various designs including many of the furnaces offered by CM such as kilns, box furnaces, and more sophisticated production batch furnaces like our CM 100 series.
Key to a proper calcining process is the absence or limited supply of air or oxygen applied to and other solid materials to bring about a thermal decomposition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials.
Calcining is often confused with Sintering and Annealing process and indeed there is some conceptual overlap in these. Calcining is generally used more broadly than simply removing CO2 from carbonates. It It involves decomposition of any suitable salt to form an oxide, including carbonates, nitrates, hydroxides, sulfates, and others.
Sintering of traditional ceramics, including pottery, tile, sanitary ware, dinnerware, etc., takes place in the presence of a viscous liquid, and it typically includes chemical changes as well. For instance, the clay in the original material decomposes.
Annealing of metals is used to reduce the strength and hardness and increase the ductility. It can take place in air or in neutral or reducing atmospheres, but usually is done in air. Annealing of glass is used to reduce internal stresses arising from molding and other processes. It always is done in air.
Examples of calcination processes include the following:
- decomposition of carbonate minerals, as in the calcination of limestoneto drive off carbon dioxide;
- decomposition of hydrated minerals, as in the calcination of bauxiteand gypsum, to remove crystalline water as water vapor;
- decomposition of volatile matter contained in raw petroleum coke;
- heat treatment to effect phase transformations, as in conversion of anataseto rutile or devitrification of glass materials
- removal of ammoniumions in the synthesis of zeolites.
Furnaces For Annealing, Sintering and Calcining
CM provides furnaces used extensively for all these processes. In production batch environments our Rapid Temp Series of Box and Tube furnaces has proven highly flexible and adaptable to both atmosphere and load-in requirements of most reducing processes. For more information please contact CM today.