High-Temperature Rotary Calcining
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 ApplicationsCalcining 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.
Rotary CalcinersIndirectly 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.
The processing of hazardous, toxic, radioactive and explosive products can also be accomplished when calciners are equipped with sealing systems that prevent interchange between their internal atmosphere and local ambient conditions. In addition, the total volume of process off-gas from an indirectly heated rotary unit is very small when compared with a direct-fired unit, allowing for a less expensive and smaller off-gas system for meeting local, state and federal environmental regulations.
The indirectly heated calciner lends itself to multiple heat zones and the ability to hold a product at a constant temperature, as well as to ramp a product up to temperature at a controlled rate. The units are usually divided into two or more heating zones to provide for a required product temperature profile.
Rotary KilnsDirectly heated rotary kilns transfer heat from the burner flame envelope and the hot gases traveling down the length of the cylinder to the product being calcined (see Figure 2). The operating configuration is primarily a countercurrent flow for process kilns, with a burner located on the discharge end of the kiln providing for the hottest gases to come in contact with the hot product.
Radiation is the primary form of heat transfer, with convection secondary and conduction minimal. Rotary kilns can be used effectively when the particle size range, specific gravity and shape permit reasonable velocities through the cylinder. For direct-fired process kilns, cylinder bed loading is generally in the 10% range. Higher bed loading can lead to the material bed having an untreated core, resulting in a non-homogeneous product condition.
Whenever the temperature requirement for a product being processed in a direct-fired rotary unit starts to exceed 540°C, it becomes necessary, from a mechanical component integrity standpoint, to use a refractory-lined rotary kiln. Above the 1205°C level, the direct-fired rotary kiln, which can achieve up to 1650°C product temperatures, is the only practical rotary thermal processing unit. The product temperature ceiling for indirectly-heated high-temperature rotary units is 1205°C because of the metallic alloys and cast tube sizes that are currently available.
Versatile ProcessingWhatever the terminology, indirectly and directly heated high-temperature rotary calciners and kilns are widely used in the chemical and mineral process industries. These multipurpose units can handle many heating applications and process a variety of materials.
For more information regarding high-temperature rotary calcining, contact ALSTOM Power, Air Preheater Co., Raymond Operations, 4525 Weaver Pkwy., Warrenville, IL 60555; (630) 393-1000; fax (630) 393-1001; e-mail email@example.com ; or http://www.airpreheatercompany.com .
- Heat Treating
TYPICAL PROCESS MATERIALS
- Ceramic Compounds
- Activated Carbon
- Molybdenum Compounds
- Rare Earths
- Metallic Oxides
- Uranium Compounds