Limestone and gypsum are regularly pelletized for use as soil amendments and other products. Unbeknownst to many, however, limestone and gypsum respond so readily to agglomeration, that they are often employed as an additive in the pelletizing (agitation agglomeration) process when working with hard-to-agglomerate materials.
The addition of limestone or gypsum opens up opportunities for processors working with bulk solids that are otherwise resistant to agglomeration, allowing them to realize the many benefits pelletizing has to offer.
Here’s a look at how limestone and gypsum can be used to make pelletizing otherwise-challenging materials possible.
About Pelletizing (Agitation Agglomeration)
There are many types of agglomeration, or particle size enlargement. In this case, we’re referring specifically to agitation agglomeration, also known as non-pressure agglomeration, tumble-growth agglomeration, wet granulation, or more commonly, pelletizing.
How PELLETIZING Works:
The pelletizing process works as such: material fines are made tacky through the addition of moisture. As the fines continue to move through the agglomeration equipment, a tumbling motion initially causes the fines to clump together. Once joined together, the tumbling motion causes these clumps to roll into a more dense particle known as a seed pellet.
Material fines and liquid binder are continuously added to promote the seed pellets becoming tacky and picking up additional fines as they tumble, growing the pellets in a layering effect known as coalescence.
As such, this type of agglomeration relies on a few key elements:
Moisture
Moisture is a primary focus during the pelletizing process, as it must fall within a predetermined range for pellet formation. This range is unique to each material. Moisture is typically added through the inclusion of a liquid binder, or even simply water. The liquid binder promotes tackiness, as well as increases the bulk density of the end product.
Because this moisture must be added, materials already at or close to the required moisture range may require drying prior to pelletizing in order to accommodate the addition of the liquid binder.
Particle Size Distribution
Similar to moisture range, achieving the right particle size distribution of the material is also important for proper agglomeration, and may require some sort of size reduction step prior to agglomeration.
As with moisture, the ideal particle size distribution is unique to each material. Within that range, however, it is often desirable to have a good distribution of the particles throughout the different sizes, as this allows smaller particles to fill in the spaces between larger particles, creating a stronger agglomerate.
Motion
Motion is the catalyst that brings everything together, but not just any motion is sufficient. Motion must encourage tumbling and rolling in order to facilitate coalescence. This also implies that speed is an important factor in achieving the appropriate motion.
Typical pelletizing equipment includes agglomeration/granulation drums, disc pelletizers (pan granulators), pin mixers, and pugmill mixers. In many cases, a combination of equipment is employed to achieve the desired granule characteristics.
What makes a material difficult to agglomerate?
One would assume that if all three elements described above were present, the material would readily agglomerate. However, this is not always the case. Each material responds uniquely to pelletizing; some materials agglomerate readily, while others are more challenging and may require a bit of finessing to achieve agglomeration.
There are many reasons for which a material might respond poorly to agglomeration. Oftentimes, particle size, particle shape, or even the chemical composition (some components may react and ultimately inhibit agglomeration) are to blame. This is where limestone and gypsum come in.
How Limestone and Gypsum Improve Pelletizing
Both limestone and gypsum respond readily to wet granulation techniques. As such, by incorporating either one into the formulation, the material becomes easier to agglomerate. The specific reason why the inclusion of limestone or gypsum helps is tied to the root problem standing in the way of effective agglomeration. To put it plainly, limestone and gypsum simply have many of the qualities that tough-to-agglomerate materials don’t. This often centers around particle size, shape, and chemical composition.
Particle Size & Shape
Particles that are either coarse or too large often tend to not stick together, or may stick together, but have a lot of void space, creating a weak pellet with a low density. Limestone or gypsum, which are more finely ground, help to fill in the void spaces between coarse particles, promoting a more dense pellet with fewer void spaces.
Chemical Composition
A material’s chemical composition can also be the reason it is resistant to agglomeration. Organic materials, for example, often take a long time to absorb moisture, inhibiting agglomeration. By adding limestone or gypsum, this time is reduced and the material is more apt to agglomerate.
Inorganic materials or materials high in silica content generally struggle to stick together. By adding limestone or gypsum, the material can acquire enough tackiness to allow particles to stick together.
Determining How Much Limestone or Gypsum to Add for Agglomeration
The challenge with this approach is determining the inclusion ratio, or rather, how much limestone or gypsum is needed to promote agglomeration enough that achieving the desired end product specifications becomes possible.
Depending on the material, this may be a little, or a lot. If the material only needs a little help, an inclusion of 10% limestone or gypsum may be sufficient. If the material is very difficult to agglomerate, the inclusion rate could go all the way up to 90% or more, in which case the product may no longer be practical or economically viable.
Determining the inclusion rate of limestone or gypsum in an agglomeration process can only be done through testing in a facility such as the FEECO Innovation Center. Here batch- and pilot-scale equipment provide the opportunity to test materials on a smaller scale.
Limestone or gypsum can be successively added to the formulation in greater quantities until the desired results can be achieved in batch equipment. Testing may then move on to pilot-scale equipment to determine if the process is viable on a continuous basis.
Limestone or Gypsum?
The choice between using limestone or gypsum is typically dependent on the end use of the product. Each material has its own unique qualities that could either be a benefit or a detriment, depending on how the end product will be used.
For example, both limestone and gypsum have agronomic value, making them appropriate for use in such applications. Gypsum, however, also serves as a source of sulfur, which may be undesirable for some agronomic applications.
Some forms of gypsum can also have cementitious properties, which can be a benefit to some industrial applications.
Material availability and cost also factor into the choice between limestone and gypsum.
Conclusion
Pelletizing is a powerful tool in improving the properties of bulk solids, but not all materials respond readily to agitation agglomeration techniques. In addressing challenging materials, both limestone and gypsum serve as additives that can make agglomeration of such materials possible, and may even add a few benefits along the way. How much limestone or gypsum is required to achieve the desired results can only be determined through testing.
With extensive experience in agglomerating hundreds of materials, FEECO is the leading provider of pelletizing equipment, systems, and services, including process development, parts and service support and training. For more information on our custom equipment or testing services, contact us today!