Rotary calciners and kilns effectively and efficiently calcine a variety of materials for a wide range of applications

Rotary calciners can be used for processing fine materials to temperatures of up to 1205°C.

Differences of opinion exist concerning what should be considered a calciner or a kiln. Some industries refer to all high-temperature rotary units, direct and indirect, as kilns. Others refer to both indirect and direct units as calciners. Thermal process equipment suppliers have generally settled on labeling indirect units as rotary calciners and direct units as rotary kilns.
However, the reality is that both indirect and directly heated units handle calcining applications. The basic difference is that indirectly heated units have their heat source separated from both the process material and process off-gases by the rotating cylinder. On the other hand, direct-fired units have their heat source, which consists of products of combustion in the form of a flame envelope and hot gases, in contact with the process material and process off-gases.

For the purposes of this article, the term "rotary calciners" will refer to indirectly heated units and "rotary kilns" will be used for directly heated units.

Calcining Applications
Calcining is a broad term that generally refers to thermal processing at elevated temperatures to effect a desired change in a given material. Calcination can involve oxidation, reduction, pyrolysis, removal of chemically bound water, crystal structure changes, etc.
Calcining has many applications in a variety of process industries. For example, manganese dioxide can be reduced to manganese oxide at 950°C in indirectly heated calciners under a hydrogen or methane atmosphere. Depending on the purity required for the calcined material, the higher-purity form can be used as an electrolytic grade in the manufacture of batteries while the lesser-purity form can be used as an alloying grade for the manufacture for various types of steel. Additional applications include:

The removal of free and bound water from kaolin at temperatures to 1000°C in direct-fired kilns under oxidizing conditions. The calcined kaolin features the necessary brightness and opacity for a variety of paper-making and filler processes.
The reduction of ammonium diuranate to uranium oxide at around 800°C under a combined hydrogen and steam atmosphere. The powered product is then pelletized and sintered for use in fuel rods for nuclear reactors.
The removal of free and bound water from alumina at temperatures to 1200°C in indirect-fired calciners under inert conditions. The process achieves the necessary surface area, pore volume and crush strength required for the manufacture of catalysts.
The removal of volatiles from spent activated carbon at approximately 800°C in indirect-fired calciners in an atmosphere of steam. The reactivated product is then recycled back to absorption columns used by the gold mining industry.
The heat treatment of iron oxide compounds, or hard ferrites, in direct-fired kilns at temperatures to 1300°C under oxidizing conditions. The powdered product is formed into various shapes and then sintered for use in permanent magnet applications.

Figure 1. Indirectly heated rotary calciner operation.

Rotary Calciners

Indirectly heated calciners transfer heat through the rotating cylinder wall to the product being calcined (see Figure 1). Radiation is the principal medium of heat transfer, with convection and conduction heat transfer both being low. The gas velocities within the cylinder are low since heat transfer is indirect; therefore, this type of unit permits the processing of micron-sized material, as well as fragile products, with very low levels of particulate entrainment and particle breakage. Cylinder bed loading is generally in the 10% range, with higher values generally leading to conditions resulting in a non-homogeneous product.

Rotary calciners with alloy cylinders and gas-tight rotary seals can be used for processing products at temperatures up to 1205°C, and because the heating is done indirectly, special atmospheres such as nitrogen, argon, hydrogen, carbon dioxide and oxygen can be used. With these types of process gases passing over the material bed inside the cylinder, the unit becomes a chemical processing reactor for functions such as reducing, oxidizing and heat treating.

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