Rotary kilns are used throughout various industries from mining to chemical production and even waste management. And while they are capable of processing a broad range of materials, one essential aspect of any rotary kiln operation is preparing the material – whatever it may be – for processing in the kiln.
Rotary kilns require their charge to fall within a specific set of physical parameters (unique to each material and process goal) for optimal processing in the kiln. Although some materials may naturally fall within those specifications, in most cases, some sort of pretreatment of the material is necessary to prepare it for optimal thermal treatment in the kiln.
Here’s a look at why material pretreatment is important when working with rotary kilns, and some of the techniques available to prepare material for kiln processing.
Why Pretreatment is Important
Despite how incredibly diverse they are, rotary kilns cannot (or often should not) process material in just any shape or form. In addition to being inefficient, feeding unprepared material to a kiln can have several unintended consequences, ranging from an incomplete reaction or a failure to achieve the intended reaction, to unnecessarily long retention times and excessive energy costs.
By bringing material into the predefined range of characteristics ideal for the goals at hand, process efficiency can be maximized, ensuring that product exits the kiln in the condition intended, without wasting energy or spoiling material.
Pretreatment Objectives
The physical specifications required for processing material in a rotary kiln are dependent on the material and process goals, but typically center around moisture content, particle size distribution, and where applicable, creating a homogeneous mixture of disparate components.
In some cases, pretreatment may be required in order to successfully complete the physical change or chemical reaction. In others, pretreatment may not technically be required, but will make the process more efficient or effective in some way.
Pre-Drying Material
In many cases, it is desirable to dry material before it goes into the kiln. The high temperatures of the rotary kiln are capable of drying material, but not necessarily in an efficient manner; since material is not showered through the products of combustion in a rotary kiln (like it is in a rotary dryer), the heat transfer is much lower, making it more difficult (and more expensive) to drive off excess water. This heat transfer rate is even lower in an indirect-fired rotary kiln.
Depending on how much moisture needs to be removed from the feedstock, using the kiln to remove it could result in the need for a longer kiln, which increases the overall cost of the unit. A higher air flow velocity could also be employed instead of a longer kiln, but this would increase the amount of fines carried out via entrainment in the air flow. A greater air flow velocity also typically requires a larger off-gas treatment system.
Managing Moisture Content With a Rotary Dryer
If the material requires only a small amount of surface moisture to be removed, it may be economically feasible for moisture reduction to be carried out in the kiln, but in most cases, it is more economically practical to incorporate a rotary dryer prior to the kiln (often referred to as a pre-dryer in this setting).
The rotary dryer efficiently dries the material, leaving the kiln the sole job of carrying out the intended reaction.
Adjusting Particle Size
Particle size is a critical aspect of effectively processing material in a rotary kiln. Material may require either particle size enlargement (agglomeration) or size reduction.
Agglomeration (Particle Size Enlargement)
Particles that are too fine can become entrained in the process gas flow and carried out through the exhaust gas system (in the case of direct-fired kilns). This results in a high rate of reprocessing, and can significantly reduce overall process efficiency.
An alternative approach to processing fine materials is to use an indirect kiln (sometimes called a calciner). However, since indirect kilns are externally heated and the material is heated by contact with the shell, this type of kiln is less efficient and is typically only appropriate in settings with special circumstances.
Instead, producers may opt to agglomerate fines into larger particles. Various approaches to agglomeration exist, with each offering its own benefits depending on the process goals.
One common approach is to pelletize material via wet granulation, or agitation agglomeration. The round pellets produced by this technique are advantageous for many reasons. A common example of this can be found in the steel-making industry where iron ore goes through a “balling” step to produce large round pellets. These pellets promote optimal processing in the blast furnace, as they are capable of withstanding the charge weight of the furnace and promote optimal air flow through the bed.
Size Reduction
Particles that are too large can also be a problem. If particles are too large, the process quickly becomes inefficient, as heating the material through to its core becomes difficult, resulting in unprocessed material, longer retention times, and higher energy costs.
In such cases, particle size reduction becomes a critical step in ensuring operational efficiency. Depending on the amount of reduction necessary, various comminution options may be suitable.
Particle size must ultimately strike a balance between being small enough for effective heat transfer, while minimizing potential entrainment.
Producing a Homogeneous Mixture
In some settings, it is desirable to mix the kiln feedstock with an additive. This is commonly seen with operations employing fluxing and reducing agents, which are used to improve certain aspects of the product, or enhance the efficiency of the process.
When an additive is included, it must be thoroughly mixed with the feedstock into a homogeneous mixture in order to achieve uniform results in the product exiting the kiln.
Depending on the goals of mixing (particle size, density, etc.), a pugmill mixer or pin mixer may be appropriate.
Rotary Kiln Testing
The specific parameters that will best suit a given process are not always obvious. In many cases, testing such as that conducted in the FEECO Innovation Center is necessary to determine the optimal processing conditions and feed characteristics for reaching a specific objective as efficiently as possible.
Rotary kiln testing is typically carried out first in a batch-scale kiln, followed by a pilot-scale kiln. Through testing, producers can identify the combination of conditions necessary to produce a product with the desired specifications. This not only includes feed characteristics such as particle size and moisture content, but also process data such as temperature profiles, retention time, gas velocity, and more.
A unique advantage to the FEECO Innovation Center is that both drying and various types of agglomeration can also be tested on-site, so producers have everything they need in one place.
Conclusion
While rotary kilns are capable of processing a wide range of materials, they still require feed characteristics to be optimized in order to effectively and efficiently process material, with moisture content and particle size of the feed playing the most crucial roles. The optimal processing conditions required are often determined through testing programs.
FEECO provides the world’s most reputable rotary kilns, each customized around the specific process and material goals of the project at hand. Optimal process conditions can be gathered through batch- and pilot-scale testing in our Innovation Center, where advanced thermal treatment processes are developed every day. For more information on our custom kilns or testing services, contact us today!