The ongoing pursuit of a more sustainable, circular economy is bringing about a rise in the number of potential alternative materials producers are looking to convert into limestone soil amendments (among other things).
Industrial wastes and process byproducts are increasingly being explored for use in agricultural liming. Limestone’s already-diverse response to pelletizing has made testing an essential component in developing a process that yields a product with the desired parameters, but as producers look to bring new sources of the material to market, the need for limestone process development testing is even more critical.
The following covers some of the novel sources of limestone being explored for use as a soil amendment, and why testing is so essential with these materials.
Producers Looking to Pelletize Novel Sources of Limestone on the Rise
As a soil amendment, limestone has traditionally been supplied through the mining of natural limestone sources, but as the world begins to look at process byproducts and industrial wastes as raw materials instead of wastes to be disposed of, many producers are finding that their material could serve as an alternative to natural limestone.
Some of the most commonly explored materials include:
- Precipitated calcium carbonate (both from sugar beet processing and wastewater treatment)
- Filter cake and filter cake dust
- Baghouse dust
- Synthetic limestone
Pelletized precipitated calcium carbonate (from sugar beet processing) produced in the FEECO Innovation Center
The Choice to Pelletize Limestone Soil Amendments
Many producers looking to reuse their process byproduct as a limestone soil amendment wonder if pelletizing is really necessary. The answer is no: powdered limestone is still considered an acceptable agricultural product. However, the constantly rising expectations of growers, combined with the benefits associated with a pelletized product far outweigh the initial investment in a pelletizing plant.
By pelletizing their material, producers expand their offering, creating a more desirable product.
Further, pelletized limestone is much cleaner and easier to handle, transport, apply, and store. It also gives producers greater control over particle characteristics, allowing them to tailor their products to the demands of specific markets. It is also more easily blended with other soil amendments and fertilizer granules, a common practice in the agriculture industry.
For more information on the benefits producers can gain from choosing to pelletize their product, see our article, Five Ways Limestone Quarries Can Benefit From Pelletizing.
Why Pelletizing Limestone Sources Can Be Challenging
Despite the fact that limestone is frequently pelletized for use as a soil amendment, producers often find themselves having to “reinvent the wheel” due to the natural variation that occurs across deposits.
Each limestone deposit has its own unique physical and chemical composition, which in turn means each one responds differently to the pelletizing process, necessitating thorough process development work to reveal the process conditions needed to produce product to the desired specification.
As many looking to reuse their source of limestone are discovering, each source brings its own unique challenges to the table. Some of the key factors that influence how each source will respond to pelletizing are outlined here.
Clay Content
Clay is often utilized as a binding agent in the pelletizing process. As such, limestone sources accompanied by a clay constituent essentially come with a built-in binder, making them more readily receptive to pelletizing.
It’s important to note that too much clay, however, can also be a problem, because it can make pelletizing a challenge by encouraging “mud” formation. Too high of a clay content is also undesirable because it can reduce the calcium content of the pellets (and therefore the liming effectiveness) and may reduce water flow in soil and encourage compaction.
Silica content
In contrast, silica is challenging to agglomerate, as particles do not readily bind together. Sources of limestone accompanied by silica are therefore more challenging to agglomerate and may require a higher quantity of added binder to achieve the desired pellet quality.
An especially high silica content could even make pelletizing a specific source of limestone economically infeasible due to the high requirement of binder. As with clay, a high silica content can also affect the product’s effectiveness in adjusting soil pH.
Calcite vs. Dolomite
The amount of calcite, which is predominantly made up of calcium carbonate, vs. dolomite, made up predominantly of magnesium carbonate, also influences how the material responds to pelletizing, with higher magnesium contents tending to be more “sticky” than others.
Particle Size Distribution (PSD)
Different sources of limestone are produced with varying particle size distributions. Particle size distribution (PSD) is a significant factor in how well (or not well) a material will pelletize. As such, some sources may require pretreatment such as crushing or grinding in order to bring the limestone into a PSD more amenable to pelletizing.
Moisture Content
Much like with particle size distribution, moisture content also varies across sources. Each source of limestone further has a unique range of moisture in which it will best agglomerate. Here again, pretreatment such as drying or back mixing (where recycle is mixed in with the feedstock entering the process to reduce the overall moisture content) may be necessary if the moisture content of the specific source of limestone is too high.
Drying pretreatment, often carried out in a rotary dryer, is especially common in precipitated calcium carbonate (spent lime) from the sugar beet industry, as the material comes from the refinery with a high moisture content.
Abrasiveness or Corrosiveness
Different sources of limestone can also exhibit varying levels of potential to cause corrosion or abrasion. While this does not necessarily change how the material pelletizes, it does impact how the equipment will need to be designed to best withstand constant contact with the material. The need for added protection against corrosion or abrasion is often revealed through testing.
Limestone Pelletizing Process Development
The variation that exists across limestone sources, combined with the increasing number of novel sources of the material, make comprehensive process development essential to engineering a successful production line.
FEECO recommends all limestone producers, whether they are working with natural limestone, synthetic limestone, a process byproduct or otherwise, begin their journey in the FEECO Innovation Center.
Backed by unmatched experience and extensive testing capabilities, the Innovation Center is the industry headquarters for limestone soil amendment process development. The facility is equipped for testing every aspect of converting wastes and process byproducts into a market-ready product, including:
- Drying
- Pelletizing (Drum, disc pelletizer, pin mixer, or combination)
In addition to testing batch-scale equipment, the Innovation Center offers continuous pilot-scale testing incorporating drying, screening, and recycle.
Testing in the Innovation Center confirms feasibility of pelletizing the specific source of limestone and also reveals the process conditions necessary to pelletize the product to market-ready specifications on a commercial-scale basis.
Data gathered during testing is then used to engineer the production line and commercial-scale limestone pelletizing equipment.
Conclusion
As a circular economy becomes the industry standard, more and more producers will be looking at how to transform their process wastes and byproducts, many of which hold potential for use as an alternative soil amendment to natural limestone.
As producers are discovering, pelletizing their material offers the most marketable product, but the variation that exists across sources can make pelletizing a challenge. To overcome this, producers can take advantage of testing facilities such as the FEECO Innovation Center, where limestone soil amendment production lines are regularly developed and optimized.
For more information on testing in the FEECO Innovation Center, or custom limestone pelletizing equipment, contact us today!