Pellets, or granules, come in a variety of different sizes, strengths, and shapes. Two types of granules that are often compared are round granules (or “pellets”) and extruded pellets. Despite frequent comparison, these agglomerate types are significantly different, each offering distinct advantages and disadvantages.
The Technique: Extrusion vs. Tumble Growth
Extrusion (Pressure)
Extrusion is a form of pressure agglomeration, meaning that particles are joined together into a form via high pressure. Using an extruder (sometimes called a pellet mill), material is forced through a die at a high pressure. The exiting material is typically in the form of a cylinder. This cylinder is broken up into the desired size pellets by a rotating mechanism that shears the cylinder into pellets as it exits the machine.
The result is a highly-compacted, tube-shaped pellet. The extrusion process is often used for products such as wood stove pellets, and various animal feeds, among other things.
Extrusion is known for its simplicity and low capital investment. It requires less equipment compared to the tumble growth method, though it should be noted that die maintenance can contribute significantly to operating expenses.
Tumble Growth (Non-Pressure)
Round pellets are created through non-pressure agglomeration, often referred to as pelletizing. A variety of different types of equipment and combinations thereof are available to produce granules with the desired characteristics. This includes disc pelletizers, rotary drum agglomerators, pin mixers, and pugmill mixers (paddle mixers).
Pelletizing is similar to making a snowball; fines are tumbled in the presence of a liquid binder, causing them to become tacky and pick up additional fines as they tumble.
This process is more complex than extrusion, as many variables can be adjusted to achieve the desired end product characteristics. Additionally, since this is a wet process, a drying step is required. The pelletizing system is generally more involved and more costly than extrusion.
The Product
Extruded Pellets
Because extruded pellets are created through pressure, they are generally strong, though in less than ideal conditions, they can become weak and friable. Strength is beneficial in applications that require a strong, dense pellet that will not break down easily.
The major disadvantage to extruded pellets is that they are very dusty; when an extruded pellet is sheared off, it creates an exposed end with jagged edges. As this exposed end rubs against the other pellets, it begins to break away and degrades into dust (this is often referred to as attrition). This dust is often discarded as waste, resulting in lost product.
Additionally, dust can be problematic for those handling and using the pellets.
Round Pellets
The pellets produced in a tumble growth agglomeration operation are considered a premium product and are typically used as such.
Though it depends greatly on the type of application, there are many advantages to using a rounded pellet product; because a round pellet is created without pressure, there is less compaction and density within the pellet. This means there is much more void spaces within the pellet, making it ideal for materials that will need to soak up any moisture. In the case of an extruded pellet, the material is so compacted, that there is far less void space for moisture to be held in.
Another advantage to using round pellets is that they are less dusty, because there are no jagged edges to break off.
Conclusion
While round pellets are often compared with those produced via extrusion, the two types are significantly different, with round pellets offering a premium product, and extruded pellets being less costly.
FEECO is a leader in tumble growth agglomeration technologies, offering services such as feasibility and pilot testing, to custom equipment, and parts and service support. For more information on our agglomeration capabilities, contact us today!