The rotary kiln is an advanced thermal processing device used in an extensive number of applications, with new uses constantly being developed.
As a cornerstone technique in engineering the raw materials and products our constantly evolving society depends on, the advanced thermal processing carried out in rotary kilns is an integral component of modern industrial processing.
As the industry’s leading provider of custom rotary kilns, we’re answering the most frequently asked questions we receive around these diverse thermal processing units.
The diverse nature of rotary kilns makes them a vessel for accomplishing just about any objective associated with thermal processing. Most commonly, rotary kilns are used to carry out the following reactions:
It’s important to note that each process listed above is a broad thermal processing technique, covering an array of applications. These specific applications often have their own name within the industry, or may facilitate a subset of reactions. For example, in the extraction of lithium ore from spodumene , calcination is used to cause decrepitation, or the shattering of the crystal structure, in order to convert alpha spodumene to beta spodumene.
Similarly, in the thermal treatment of kaolin clay , calcination is employed to remove free moisture, cause dehydroxylation, or the removal of chemically bound moisture, and other reactions.
Rotary kilns employ high temperatures and a controlled atmosphere to facilitate chemical reactions or phase changes in a wide array of materials.
More specifically, rotary kilns process material at a predetermined temperature for a predetermined amount of time (referred to as residence or retention time) based on the unique temperature profile of the material to be processed. By controlling temperature and retention time, rotary kilns can initiate and carry out chemical reactions or phase changes in a controlled setting.
Rotary kilns are a large, rotating drum that can be either of the direct or indirect configuration. In the direct configuration, the kiln can be designed for either co-current (parallel) or counter-current air flow. As solids pass through the drum, the heating medium increases their temperature. The constant rotation of the drum creates a tumbling action that redistributes the bed of material for even heat transfer throughout the bed. Tumbling flights and other internals can be added to further optimize processing.
In a direct-fired kiln, the material is in direct contact with the products of combustion, with the heat passing through the kiln’s interior. Conversely, in an indirect-fired rotary kiln, the processing environment is sealed off, and the rotating drum is externally heated in order to prevent contact between the material and any products of combustion. Instead, the material is heated through contact with the drum shell.
As a result, there are some differences in design between these two types of rotary kilns, such as the use of a heating shroud/furnace (for indirect), refractory (in the direct kiln), and the materials of construction, among other things. Indirect kilns are also commonly referred to as calciners, though the term is not always technically correct.
Direct and indirect rotary kilns are equally important in the thermal processing world, with each suited to different applications.
Rotary kilns can operate on a variety of different fuel types depending on whether they are of the direct or indirect configuration.
Indirect-fired kilns typically employ one of the following:
Direct-fired kilns can operate on the same fuel types, with the exception of electricity.
Rotary kiln design is a complex undertaking, as advanced thermal processing techniques and chemical engineering principles come into play. This is especially true considering that many kiln applications are new, and must be developed from scratch.
The design process may differ depending on how much is known about the material and its physical and chemical behavior under heat. Most often, the design process begins with a thermal and chemical analysis of the material, followed by batch rotary kiln testing .
Material is tested in either a batch indirect or direct kiln to gather initial process data points. Testing continues, advancing to a pilot-scale test kiln to scale up the process and refine process and material variables to produce a product with the desired characteristics.
This data is then used to scale up the process and design a commercial-scale kiln tailored to the needs of the specific application.
Rotary kilns can be designed for handling a broad range of capacities, from small, batch-scale units processing anywhere from 50 to 200 lb/hr, to commercial-scale units processing material in the range of 200 lb/hr to 20 TPH.
Large commodity kilns such as those used in the cement industry can process up to 50 TPH, but these size kilns are less common for many of the lower-capacity processes in use today.
A rotary kiln manufacturer will typically require the following data in designing a commercial-scale unit:
In some cases, not all data is known, in which case testing would be used to assess the material and the intended process.
While there is some overlap between rotary drum dryers and kilns, the key difference lies in the intent: is the processing intended to simply dry the material, or is some sort of chemical reaction or phase change required?
In most cases, drying will be conducted at much lower temperatures than those necessary for carrying out a reaction, so rotary dryers are often generally considered lower-temperature devices.
Rotary kilns can be constructed from a variety of different materials:
Selection of the proper material is based on the material characteristics (i.e., abrasiveness and corrosiveness), as well as the temperatures employed and whether the unit will be of the direct or indirect design. Since direct kilns employ refractory, they are typically constructed of carbon steel. Indirect kilns, however, which cannot use refractory as it would add another layer for heat to pass through, do not use refractory and therefore must be able to withstand greater temperatures and hence, are constructed from a more heat-resistant alloy.
Rotary kilns are not a stand-alone device; a complete kiln system made up of many supporting systems and components. This typically consists of:
In some cases, pretreatment equipment for pre-heating, drying, or even agglomerating the feedstock may also be necessary.
The temperature(s) at which a rotary kiln operates is specific to the reaction requirements of the material being processed and therefore differs in every setting. In general, however, rotary kilns can process material at temperatures ranging from 800 to 3000°F (430 to 1650°C).
The industrial internet of things (IIOT) is continuously advancing deeper into industrial processes, and rotary kilns are no exception.
Programmable logic controllers (PLCs), motor control centers (MCCs), and data collection systems can all be integrated into the rotary kiln system for improved data collection, process control, and advanced reporting.
Residence time, also known as retention time, is the amount of time in which the material is processed in the kiln. As with temperature, the residence time is determined solely on the requirements of the intended reaction.
The rotary kiln is an essential thermal processing device. As its use continues to spread to an increasing number of applications and industries, questions around everything from the rotary kiln’s capabilities to its operation arise. As the leading provider of custom rotary kilns, FEECO can answer all of your rotary kiln questions.
FEECO rotary kilns are robust and engineered around their precise application for optimal processing. Batch and pilot kiln testing is available in our Innovation Center to assist in commercial-scale kiln design and process scale-up, and we also offer a comprehensive line of parts and service support to keep your kiln running reliably for years to come. For more information on our rotary kilns, contact us today!
About the Authors . . .
Alex Ebben is a Process Sales Engineer and thermal processing expert.
Carrie Carlson is a technical writer and visual designer.
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