Limestone drying, typically carried out in rotary dryers, is utilized throughout this industrial mineral’s lifecycle, from quarrying to finished product. The following explores why drying is essential for quality limestone, its role in production, key considerations, and the importance of testing and equipment design for a reliable drying process.
Why Dry Limestone?
Whether destined for use as a fertilizer, building material, or other use, quarried limestone may go through a range of different processes to prepare the material for its end use. Drying is one of the first and most critical steps, playing an integral role in allowing limestone producers to optimize their production lines, bring a salable product to market, and ensure their products meet customer expectations.
Settings Where Limestone Drying is Necessary
Depending on the end use of the limestone, drying may occur once or a few times throughout processing. The following applications are most common.
Mineral Processing
Drying is often one of the first steps after quarrying, preceded only by initial crushing and sorting in many cases. By preparing the material properly from the start, manufacturers ensure improved production at every stage.
Depending on the intended use of the product, a washing step may also be employed. Whether or not a washing step is employed, drying typically follows primary crushing as a way to improve downstream screening, sorting, and if applicable, fine grinding, all of which could otherwise be significantly hindered by a high-moisture material.
The moisture content of quarried limestone can vary significantly, ranging anywhere from 5% to 15% or higher. To meet market and downstream production requirements, limestone must typically fall within the range of 0.5% to 5% moisture content, though this, too, varies depending on the end use.
Dried, ground limestone
Benefits of Drying Limestone in the Mineral Processing Stage
Drying at this stage offers several benefits:
- Improved transportation and shipping economics
- Enhanced production line flow and reduced clogging of downstream equipment, especially screens
- More streamlined handling
- Less clumping and caking during handling and storage
- Assurance that limestone will meet market expectations for moisture content
When the resulting limestone is calcined, drying also helps to improve the efficiency of the calcination step; if moisture removal was not carried out prior to calcination, the kiln would have to be much larger to accommodate both moisture removal and calcination.
Pelletizing
Limestone is frequently pelletized into uniform granules with minimal dust. While this process can vary, the preferred approach in many cases is the combination of a pin mixer and disc pelletizer, as this setup promotes a highly refined product with optimal appearance, handling, and performance qualities.
If limestone has been stockpiled outdoors, drying may also be necessary prior to pelletizing, in order to meet the feedstock requirements necessary for pellet formation.
During the pelletizing process, a liquid binder is used to facilitate agglomeration, as well as aid in end product crush strength. As a result of this moisture addition, a drying step becomes necessary.
Drying at this stage is a finishing step used to bring the product into the moisture content required by its intended application.
Benefits of Drying Limestone After Pelletizing
While drying is a necessary part of the pelletizing process, it also offers a few benefits:
- Improved product shelf life
- Reduced potential for product caking
- More polished, refined granules (when carried out in a rotary dryer)
Limestone before and after pelletizing and drying in the FEECO Innovation Center
Limestone Dryers
Limestone drying may be carried out in different types of industrial drying equipment, with the most common being rotary dryers, followed by fluid bed dryers.
Rotary Dryers
3D Rendering of A FEECO Rotary Dryer with flights visible
Thanks to their heavy-duty build and high throughput, rotary dryers offer ideal processing capabilities in both mineral dressing and pelletizing settings when processing limestone.
Additionally, the rotating motion paired with lifting flights that create a “curtain” or cascading action of material ensures that the material is properly and evenly dried.
Rotary dryers are also favored particularly in rock drying settings because of their tolerance to variation in feedstock parameters such as moisture content, particle size distribution, and more. While feedstock should always be as uniform as possible, rotary dryers do not experience disruptions due to slight variations in material and generally produce a uniform result.
Rotary dryers also offer a high level of opportunity for customization, which can sometimes be critical in designing for specific sources of limestone. Common customizations and modifications include:
- Co-current or counter-current design
- Incorporation of a combustion chamber
- Custom inlet/feed area design
- Customized flight design and pattern
- Thicker shells for abrasion resistance
- Use of alternative materials of construction
- Special burner options that limit emissions such as NOx and CO
- Knockout chamber (discharge breeching) design
- Seal options
Fluid Bed Dryers
Fluid bed dryers use pneumatic conveying to suspend limestone in a fluidized state, exposing the entire material surface and efficiently cooling it down. A fluid bed dryer works by pushing a predetermined air flow velocity through a perforated distributor plate at the bottom of the vessel, causing the material to fluidize.
3D Rendering of a FEECO Fluid Bed Dryer
Fluid bed dryers are smaller than rotary dryers and typically more cost-effective, offering an effective alternative, but they are not a fit in all settings. They are typically not ideal for initial ore processing, as the variation common in feedstock parameters can cause frequent process upsets in a fluid bed dryer.
Further, the volume of air required for minimum fluidization is not ideal when working with larger particles. For additional information on the comparison between rotary and fluid bed dryers, see Rotary Dryer or Fluid Bed Dryer.
Material Considerations When Drying Limestone
As with any material, limestone presents its own unique set of challenges when it comes to drying. Chief among them are limestone’s abrasive quality and its potential for clumping, both of which must be addressed during initial dryer design.
Abrasion
The extent of limestone’s abrasive quality can vary from one source to the next, but all should be addressed through the proper selection of materials of construction. Preparing for abrasion is especially important during the initial mineral processing stages, when particle sizes are larger and have the potential to inflict more damage.
Depending on the extent of abrasion, abrasion-resistant steel may be necessary for the entire drum shell, as well as flight construction.
In fluid bed dryers, abrasion-resistant steel can be employed, as well as careful regulation of air flow to reduce entrained solids abrasion.
Clumping
Limestone is also prone to clumping, particularly at higher moisture contents. As clumping progresses to buildup, it can cause a number of issues and must be minimized through careful dryer design.
Rotary dryers offer several approaches to minimize clumping and buildup:
- The incorporation of a “bald” or flightless section at the dryer inlet, which allows for some moisture reduction prior to the introduction of flights.
- The integration of knockers on the drum’s exterior to dislodge any potential buildup.
- The addition of a trommel screen or “grizzly” on the discharge end to break up any clumps that may have formed during drying.
Rotary dryer with trommel screen/grizzly (flights can also be seen)
Fluid bed dryers offer less opportunity to address buildup through customization. Instead, any clumping must be addressed prior to the fluid bed to avoid disrupting the process, though the drying chamber can be constructed of stainless steel to discourage sticking in the dryer itself.
Factors That Influence the Drying Process
Several process variables influence the drying process:
- Air flow velocity
- Air flow configuration (co-current or counter-current – rotary only)
- Temperature profiles
- Feed rate
- Retention time
- Drum slope and speed (rotary only)
- Percent fill
- Bed distributor plate configuration (fluid bed only)
- System pressure
Product Quality
Drying also contributes in part to a variety of end product characteristics, through proper management of the aforementioned process variables. Product quality parameters influenced by drying include:
- Attrition
- Bulk density
- Moisture content
- Crush strength
- Compression strength
- Flowability
Is Dryer Testing Needed for Limestone?
The many process variables that can affect limestone quality, paired with limestone’s potential for variation (and therefore unpredictability), often make testing in a facility such as the FEECO Innovation Center an essential step in developing an effective drying system for limestone.
The Innovation Center offers testing for both rotary and fluid bed dryers. Testing can be conducted for drying only, or as part of a larger pelletizing process in a continuous process loop through the use of our pilot plant.
Testing can be used to confirm initial feasibility, establish critical process data, and even help to inform on critical operating data such as baghouse efficiency, fuel consumption, and exhaust gas treatment.
Fluid bed dryer used for testing in the FEECO Innovation Center
Conclusion
Drying plays an indispensable role in limestone processing, significantly impacting the quality and efficiency of the final product. From the early stages of mineral processing to the final steps in pelletizing, the careful control of moisture content ensures that limestone products meet the rigorous demands of their specified applications. Rotary and fluid bed dryers, each with their unique advantages, provide the necessary flexibility and customization to handle the complexities of limestone drying. By addressing key material challenges like abrasion and clumping, and optimizing process variables, producers can achieve a reliable, high-quality drying operation. Testing in a specialized facility like the FEECO Innovation Center is often an invaluable step in ensuring optimal dryer performance and product quality.
FEECO is the provider of choice for limestone dryers because in addition to the industry’s most reputable dryers, we offer comprehensive solutions that extend far beyond just equipment. Our process development services, conducted in the FEECO Innovation Center, allow us to tailor the drying process to the unique needs of each limestone source, ensuring optimal performance and product quality. Additionally, FEECO provides extensive parts and service support, offering the expertise and resources necessary to keep your operation running smoothly and efficiently. With decades of experience in designing and manufacturing custom limestone dryers, FEECO is committed to delivering reliable, high-quality solutions that meet the demands of your production process from start to finish. For more information on FEECO’s extensive knowledge of limestone processing at any stage, contact us today!