Pelletizing is a method of agglomeration, or particle size enlargement, in which material fines are processed into pellets or granules. Pelletizing is used throughout a multitude of industries to process thousands of materials from difficult to handle powders and fines, into easy to handle pellets.
With many benefits, and a highly customizable process, pelletizing has become a staple in many industries, with new applications for this valuable process constantly on the rise.
Benefits to Pelletizing
Pelletizing a material can offer a number of benefits. Because of this, it has become a popular method of improving product performance, easing handling challenges, and even targeting desired product characteristics.
Dust Suppression
In general, any method of agglomeration improves the handling and application of a material, most notably because of the de-dusting it offers. The benefits of a granular (de-dusted) product over raw fines are many:
Improved Product Performance: Product performance can be improved in a variety of ways as a result of agglomeration. Granular soil amendments, for example are more likely to deliver targeted results over their powdered form, because they do not become windblown and are much easier to accurately apply.
Similarly, glass batch is pelletized because powdered glass otherwise gets taken up in glass furnaces, throwing off carefully formulated blends. Pelletized glass ensures formulas remain intact.
Improved Product Handling and Application: Pelletized products are much easier to handle and apply over raw material fines. Pellets are easier to feed, due to improved and more consistent flowability.
Pellets are also much less dusty. Some materials may even be pelletized prior to landfilling in order to reduce dust loss during transport and handling, as well as to avoid material becoming windblown.
Reduced Waste: A dusty material often results in excess material waste. Agglomeration avoids this issue by significantly reducing the amount of dust produced.
Compared to other methods of agglomeration, pelletizing specifically offers these benefits:
Faster Product Breakdown: Pellets created via tumble growth agglomeration offer faster product breakdown compared to granules created in a roll compactor. This is because roll compaction utilizes extreme pressure to form granules, creating a highly dense product. Pellets created via pelletizing are not as dense, and therefore are strong enough to hold up to handling and application, but can still break down as needed. This is especially valuable when working with fertilizer or soil amendment products.
Reduced Attrition: Attrition refers to the breakdown of granule edges into fines, producing excess dust. Attrition is commonly seen with granules created via compaction granulation or extrusion, because the granules produced have jagged edges that rub against each other, causing edges to break down. Pellets produced via pelletizing offer significantly reduced dust and attrition, because they are round, so there are no sharp edges to degrade.
Increased Product Formulation Capabilities: Pelletizing, particularly on a disc pelletizer, is a highly flexible process, allowing for a significant amount of product customization to occur. Additives can be included to improve product formulations, and a number of variables can be adjusted to target specific end product parameters, such as particle size distribution, crush strength, flowability, and more. In essence, pelletizing allows producers to create a granular product that suits their exact needs.
A Premium Product: For the many reasons listed here (reduced dust, reduced attrition, faster product breakdown, and customization), products created via pelletizing are considered a premium product.
Formation of Pellets through Pelletizing
Unlike pressure methods of agglomeration, such as compaction granulation or briquetting, pelletizing is considered a wet process, because moisture (in the form of a binding agent) is used to agglomerate the fines into larger particles, as opposed to the extreme pressure used in compaction or briquetting.
It’s important to note that while the terms are often used interchangeably, pelletizing is distinctly different from pelleting. Pelleting typically refers to the extrusion process, where cylindrical pellets are formed by forcing material through a dye.
Pelleting is commonly seen with wood, fuel, and feed products such as rabbit food. This method of agglomeration results in a dusty product, as the jagged cylinder edges rub against each other and break off.
Pelletizing involves tumbling material fines against each other in the presence of a binding agent. This causes the fines to become tacky and stick together. As they continue to tumble, they pick up more and more fines, a phenomenon referred to as coalescence.
Pelletizing can be carried out using a rotary drum or disc pelletizer, with disc pelletizers being the more common choice. For information on choosing between a disc pelletizer and rotary drum, see Rotary Drum or Pelletizing Disc.
A typical pelletizing operation is illustrated here.
Preparation of Raw Materials
Material is first commonly pre-conditioned. Not all pelletizing operations utilize a pre-conditioning step, but those that do tend to see many benefits, including increased capacity, reduced binder costs, and an improved end product.
Pre-conditioning is typically carried out in a pin mixer, with the pugmill mixer being an alternate choice for more arduous materials. The material feedstock is fed into the mixer at a continuous rate, along with a liquid binding agent. The intense spinning action of the pin mixer provides an intimate mixing of both the solid and liquid feeds, creating a homogenous mixture. As the material moves through the length of the mixer, small agglomerates begin to form, referred to as seed pellets.
Pelletizing
The seed pellets then exit the mixer and are fed onto the disc pelletizer. For operations that do not utilize a pre-conditioning stage, this is where the process begins, with raw feed and binder being fed onto the disc at a continuous rate.
As the seed pellets rotate on the disc, material and binder are continuously added at a specified rate for optimal agglomerate formation. The seed pellets tumble against the fines, picking up more as the disc rotates. This essentially “grows” the pellets until they have reached the desired size, at which point they exit the pelletizer via centrifugal force.
When utilizing a rotary drum, the process is largely the same, with only the drum replacing the disc in the process flow. Material and binder are fed into the drum at a continuous rate. The rotating drum is set at a slight angle to help the material move through the drum. Here again, the tumbling action within the drum causes the material to pick up more fines and grow by coalescence.
Thermal Processing
After pellets have been discharged from the disc pelletizer or rotary drum, they are carried via conveyor to a rotary dryer, where they are dried into their final form. Some operations also implement a cooling step after drying, as seen in the diagram above.
What Materials Can Be Pelletized?
Pelletizing is an incredibly diverse and flexible process, lending itself to thousands of materials; nearly any material in powder or slurry form can be transformed into a dry, granular product. Common pelletizing applications include:
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- Gypsum
- Limestone
- Coal
- Fly Ash
- EAF Dust
- Chemical Powders
- Ores
- And more…
Conclusion
Pelletizing is an invaluable process, utilized across a wide range of industries to enhance product performance, improve handling and application, and create a premium product with targeted product specifications.
FEECO has been a pioneer in pelletizing since 1951, providing the best in custom agglomeration equipment and process solutions, including material testing, product design, and process development and optimization. For more information on our pelletizing capabilities, contact us today!