As a major source of industrial waste around the world, fly ash, a byproduct of coal generated power, presents a significant waste management challenge to the energy industry.
Fly ash is host to a variety of contaminants that if not properly managed, can pose serious risks to both people and the environment.
As with many other materials, fly ash reuse and recovery is being examined for a number of applications in an effort to reach a more sustainable future, with process and product development testing services being a critical element to success.
Why Fly Ash is a Problem
According to the American Coal Ash Association, in 2014, 50,422,238 short tons of fly ash were produced in the United States alone. It’s worth noting that while fly ash is predominantly a byproduct of coal power production, it can also be produced from other power sources, such as wood and biomass fuels, though to a lesser degree.
Laced with heavy metals such as arsenic, mercury, and lead, the proper management of fly ash is of critical concern to avoid risks to human health, wildlife, and the environment. Improper management of fly ash has the potential to cause contamination of groundwater, soil, and even air.
Furthermore, fly ash has often been traditionally stored in containment ponds that can devastate surrounding areas in the event of a failure. Such an incident occured in Tennessee in 2008 when a containment pond failed at TVA’s Kingston Fossil Plant, spilling 5.4 million cubic yards of coal ash slurry into the surrounding area. The breach is said to have left upwards of five feet of the muck over 250 – 400 acres of rural land. This thrust coal ash management under a nationwide spotlight, with regulations around the ash increasing.
In addition to the serious risks it can present, fly ash can incur substantial waste management costs and can also represent a long-term liability for energy companies.These issues combined have seen a growing body of research around more sustainable, alternative methods of disposal of the waste, or ideally, reuse and/or recovery.
Potential Reuse and Recovery Applications for Fly Ash
A wide range of opportunity exists for the beneficial reuse and/or recovery of materials from fly ash, with some of the most common endeavors listed below.
Alumina Recovery from Fly Ash
One of the primary targets for recovery from fly ash has been alumina; most fly ash contains between 25 – 30% aluminum oxide (Al2O3)¹. Some sources of fly ash in China have been shown to contain as much as 45% alumina.² In addition to its use in producing aluminum metal, alumina is used throughout a host of industries as a catalyst, pigment, abrasive, and a long list of other materials.
Despite ongoing research, bauxite is currently the only commercially viable source of alumina. Researchers have been eager to find an alternative, as the Bayer process yields staggering amounts of red mud – another toxic industrial waste that poses a number of problems.
With demand for alumina on the rise, the ability to recover alumina from fly ash would not only prevent this valuable material from going to waste, but it would also help to alleviate at least part of the waste management burden, along with the associated risks fly ash can present if not handled properly. In addition, it would reduce the pressure on mined sources of alumina (bauxite), and could help to reduce the production of red mud.
One new study has taken alumina recovery research a step forward with some promising results; by combining a CaCl2 calcination step with H2SO4 leaching, researchers were able to obtain greater than a 95% recovery of alumina from the fly ash. In comparison, the study found direct acid leaching could only recover about 2 – 4.75% alumina. The study found that the calcination of fly ash at specific parameters significantly increased the leachability of the alumina from the fly ash compared to direct acid leaching.
Various approaches involving leaching and calcination for the removal of alumina from fly ash have been studied, but challenges to commercial viability still exist, with research ongoing.
Other Recovery Options
In addition to alumina, fly ash is also being studied as a potential alternative source of materials that are seeing increased demand. Among the potential materials to recover are:³
- Aluminum
- Iron
- Titanium
- Silicon
- Vanadium
- Gallium
- Germanium
- Selenium
- Lithium
- Molybdenum
- Uranium
- Gold
- Silver
- Platinum
- Rare Earth Elements (REEs)
- Magnetic Materials
- Cenospheres
- Unburned Carbon
Fly Ash Reuse in Adsorbents
Fly ash has also been increasingly studied for its use as an adsorbent. Adsorbents are widely used in applications to remove targeted components (often contaminants) from liquids and gases.
In combination with other factors, the porous structure of fly ash makes it a good candidate as a potential adsorbent. Fly ash is highly attractive as an adsorbent because it is low cost, widely available, and its reuse would solve the associated industry and environmental challenges it presents.
One study tested bagasse fly ash as an adsorbent for a variety of materials and found it to be effective for both inorganic and organic adsorbates.4 Organic and inorganic contaminants are a major industry challenge in treating wastewater, with the industry on the search for an effective, cost efficient adsorbent.
Another recent study found fly ash to be an effective adsorbent for removing blue dye 25 from aqueous solutions.
Fly Ash Reuse in Agriculture
Fly ash has also been widely researched for applications in agriculture, namely as a soil amendment.
The properties of fly ash have shown promising potential for improving a number of soil characteristics, ultimately bolstering crop production efforts. These benefits include:5
- Improvement of acidic soils with a potential to serve as a substitute for traditional lime products
- Correction of sodicity in soils with some ash potentially capable of serving as a gypsum substitute
- Increased nutrient availability as a result of improved soil pH
- Potential for decreased likelihood of nutrient loss
- Potential supply of additional nutrients including Boron, Molybdenum, Phosphorus, Potassium, Sulfur, Calcium, Copper, Magnesium, and Zinc.
- Improvement of other soil factors, including water holding capacity
According to the study, various studies have shown improvements in crop yield as a result of fly ash treatment, though caution needs to be exercised in order to avoid potential adverse effects or risks.
Fly ash holds significant potential in the agriculture industry for the many diverse benefits it can offer to soils.
As a dusty material, agglomeration of fly ash via pin mixer and disc pelletizer is frequently examined for processing this powdery material into a granular soil amendment product that is easy to use, handle, and apply, and is nearly dust-free.
Fly Ash Reuse in Lightweight Aggregates
Unlike most reuse opportunities for fly ash, its use in the concrete industry as a lightweight aggregate (LWA) is well established.
Fly ash has proven to offer a wide range of benefits when used as a LWA, to the extent that it has become a key material in the construction industry; the use of fly ash as a LWA can provide economic benefits such as reduced structural, shipping, and handling costs, as well as environmental benefits that come along with the utilization of a waste material.
Similar to its application in agriculture, when used in concrete, fly ash is typically pelletized using tumble growth agglomeration methods. It is then dried and sintered to produce a product that will integrate seamlessly into the concrete production process.
In addition to its use in concrete, fly ash has also been explored for other uses in construction, such as structural fill applications.
Fly Ash-Based Proppants
Perhaps the most recent inquiry into fly ash reuse has been around its potential as a proppant.
Proppants are used in the hydraulic fracturing process to “prop” and hold open rock fissures allowing natural gas or oil to flow freely out to the main wellbore for collection.
While the proppant industry has been well served by frac sand and manufactured proppants, fly ash could be a promising new tool in the fracking industry; LWP, a proppant supplier, recently secured a patent on their proppants made from the dusty waste. According to a news release, the company’s fly-ash based proppants actually exceed standards compared to many non-fly ash proppants.
Fly Ash Dust Suppression
In many cases, fly ash is coming from a storage pond and requires drying before it is ready for reuse or recovery applications. Unfortunately, fly ash drying is made challenging as a result of the dusty nature of fly ash. As such, dust suppression via agglomeration is often employed prior to drying.
Agglomeration is also frequently used when working with a dry fly ash material in order to improve its handling (along with other) characteristics. For example, although not a reuse application, many companies have taken to conditioning fly ash prior to deposit into landfill; processing fly ash in a pugmill mixer with water or a binder can de-dust the material so it does not become windblown upon deposit. This agglomeration process may also help to make the sample more stable and less likely to leach toxic components into the surrounding environment.
Similarly, if the intention is to use fly ash as a soil amendment product, agglomerating the dusty ash into a granular form will not only improve handling and transportation, but will also improve its performance as an end product.
A wide range of other techniques (or a combination thereof) may be utilized to optimize the properties of fly ash for its intended use.
Fly Ash Testing
With ongoing research and development, testing services have become a key focus for those looking to solve the fly ash dilemma. Testing centered around process and/or product development for the potential avenues listed above (or even processing for improved disposal) is critical to the success of any intended fly ash reuse or recovery application.
Various methods in both agglomeration and thermal processing hold potential for turning this problematic waste into a value-added product.
Alex Ebben, FEECO Process Sales Engineer and thermal processing expert, states: “We are seeing all sorts of interest around testing fly ash for various applications – both agglomeration and thermal. Testing is critical when it comes to fly ash, because the reuse market is not well established, and different sources of fly ash can vary a lot.”
In addition to testing out various methods of agglomeration, the FEECO Innovation Center offers batch and pilot scale testing services for drying of fly ash, as well as high temperature applications such as sintering and calcination. Testing approaches can be combined into a continuous process loop to integrate multiple testing methods as a single process.
In 2016, the American Coal Ash Association announced that for the first time in history, the amount of coal ash (a combination of fly ash and bottom ash) reuse in the United States exceeded 50%. ACAA Executive Director Thomas H. Adams, had this to say:
“It is gratifying to know that for the first time, we are using more of these valuable resources than we are throwing away,” adding, “With some help from markets and regulatory certainty, we look forward to continuing to grow these practices that conserve natural resources, make products that are more durable, and dramatically reduce the need for landfills.”
Conclusion
Fly ash is a massive stream of industrial waste that presents a number of environmental risks and waste management challenges for producers.
In an effort to improve sustainability and reduce the risks associated with fly ash, a wide variety of reuse and recovery applications are being examined, with research ongoing. Testing various process avenues to fine-tune end product characteristics and develop a recipe for process scale-up is critical to any intended reuse or recovery application.
FEECO has been helping companies to transform fly ash into value-added products for decades. With extensive experience around the dusty material, FEECO offers material testing (process and product development), as well as custom equipment engineering and complete system design. For more information on our fly ash capabilities, contact us today!
SOURCES
- Studies On Flyash As An Adsorbent For Removal Of Various Pollutants From Wastewater (PDF)
- Yunusa, Isa A. M. et al. “Application of Coal Fly Ash in Agriculture: A Strategic Perspective.” Critical Reviews in Environmental Science and Technology 42.6 (2012): 559–600. Web.