Rotary Calcining

What Is a Calciner?

A calciner is a type of industrial equipment used to heat solid materials at high temperatures to promote physical or chemical change. While calcination does not melt the material, it is achieved at lower temperatures than its melting point. Calcination is commonly used to eliminate moisture content, release volatile substances and initiate chemical reactions like decomposition or phase change.

Calciners are found across several industries, including cement, mining, mineral processing, chemicals, metallurgy, energy and environmental processes.

What Is Calcination?

Calcination is a thermal process where a material is heated in a controlled environment under conditions that are usually low in oxygen and in some cases with little to no oxygen present. Combustion is not the intent, but rather to transform the material. Calcination can result in one or more of the following:

-         Dehydration of chemically bonded water or moisture

-         Decomposition of carbonates, hydrates, etc.

-         Decrease in material reactivity and/or density

-         Conditioning of the material for subsequent processing

Examples include calcination of limestone (calcium carbonate) to produce lime (calcium oxide) and carbon dioxide.

How a Calciner Works

Most calciners work in similar ways, while varying slightly in design. The typical sequence of operation includes:

Material Input

Material to be processed is fed either continuously or in batches to the calciner. The feed system is designed to maintain a constant input rate and an evenly distributed feed stream. This ensures a uniform heat exposure to the feed material, which is necessary for the desired chemical reaction to occur uniformly.

Temperature Control

The calcined material inside the calciner is subjected to high temperatures, generally between 600°C and greater than 1200°C depending upon the intended application. Heat is delivered to the calcined material through:

-         Burner fired systems

-         Combustion gas

-         Electrical heating systems

-         Recovery of waste heat from another process

Proper temperature control is required to ensure the intended chemical reaction occurs with minimal damage to the calcined material.

Chemical Reaction

When the material reaches the temperature needed for calcination, chemical reactions begin to occur as follows:

-         Evaporation of gases like CO2 and/or H2O

-         Crystal structure changes to the calcined material

-         Increases in pore volume and/or surface area

The time that the calcined material remains in the calciner is carefully controlled to obtain consistent results.

Gas Collection and Emission

Any gases produced during the calcination process are collected through exhaust systems. Gases that have been produced during the calcination process may require treatment through the use of cyclones, baghouse collectors, scrubbers or other forms of pollution control to meet environmental standards.

Cool Material Output

Once the calcination has occurred, the processed material is discharged from the calciner and then cooled prior to being stored or used as feedstock for additional processing. The material may be cooled naturally or through a separate cooling system.