Phosphogypsum is a by-product of the phosphates industry. Produced in significant quantities, the material is considered a waste in the US due to radioactivity, where the primary means of management is storage in “stacks.” But around the world, phosphogypsum is increasingly being utilized in beneficial reuse applications, with research around the topic growing.
How Phosphogypsum is Produced
Phosphorus is a mineral critical to all life on Earth. As a requirement of all biological beings, it is a cornerstone of nutrition for plants, animals, and people. As such, it is widely used in fertilizers and animal feed products.
This valuable and irreplaceable nutrient is derived from phosphate rock. Mined phosphate rock is first converted into phosphoric acid, which serves as the basis of phosphoric fertilizer and animal feed products (among other things). This is most frequently carried out via the wet process, in which dried phosphate rock is combined with sulfuric acid in a reaction yielding phosphoric acid and calcium sulfate, or phosphogypsum.
For every ton of phosphoric acid produced, around 5 tons of phosphogypsum are produced, making this a substantial source of industrial waste, especially given in the context that phosphatic fertilizer production is constantly on the rise in an effort to feed the exploding world population.
This, in combination with changing attitudes toward materials previously deemed “wastes,” and the increasing cost of phosphogypsum waste management, has the industry looking for a better, more sustainable approach to the prolific by-product. Unfortunately, the reuse of phosphogypsum is not as simple as some other forms of synthetic gypsum.
Synthetic Gypsum
Synthetic gypsum, namely FGD (flue gas desulfurization) gypsum – a by-product of coal-fired energy production – is already widely accepted as a replacement for natural gypsum in many settings, including use as a soil amendment, in glass manufacturing, cement products, and more. This not only provides a readily available, cost-effective alternative to mined sources of gypsum, but it also helps to keep these industrial by-products out of landfills.
While phosphogypsum is also considered a form of synthetic gypsum, it presents a unique challenge that has, in many cases, served as a barrier to widespread reuse: it is host to naturally radioactive components. Naturally-occurring radionuclides in the phosphate rock associate with the phosphogypsum during the wet process. This has earned phosphogypsum the EPA’s “Technologically Enhanced Naturally Occurring Radioactive Material,” or TENORM classification.
The US EPA currently bans the use of phosphogypsum, except in cases where the material does not exceed an average radium-226 concentration of less than 10 picocuries per gram, and will be used as a soil amendment, according to the Florida Industrial and Phosphate Research Institute.
As a result, domestically produced phosphogypsum is stored in “stacks” all over the country – massive containment ponds where phosphogypsum is indefinitely piled. The International Atomic Energy Agency (IAEA) estimates that each year, 40 million tons of the waste are added to stacks in the US, and about 90 million tons annually outside of the US. At this rate, the IAEA estimates that the amount of stacked phosphogypsum will more than double by the year 2040.
Phosphogypsum stack in Florida
Image credit: Harvey Henkelmann
The Problem with Stacks
While phosphogypsum stacks have not drawn nearly the attention that some other industrial by-product catastrophes have (think coal ash or red mud), they are still far from ideal and are certainly not without issue. The two overarching issues associated with phosphogypsum stacks are the environmental risks they can present, as well as their cost to the industry.
Environmental Challenges with Phosphogypsum Stacks
The composition of phosphogypsum varies from source to source, but the waste often contains a number of undesirable components such as heavy metals that can be detrimental to the surrounding area if not carefully managed. Furthermore, stacks often serve as a storage area for acidic process water as well.
Florida’s booming phosphates industry has dotted the land in massive piles of the by-product – land that is also subject to assault by hurricane, as was the case in 2004 when a hurricane created a hole in a phosphogypsum containment pond, allowing its contents to spill out into the nearby area.
The state has also been host to more than one sinkhole beneath phosphogypsum stacks, where the effluent has drained down into the ground below.
In addition to contamination from the effluent itself, experts also worry about the potential for radon gas produced by the material to contaminate the air in surrounding areas.
Waste Management Costs
While stacking was traditionally the most cost-effective route to phosphogypsum disposal, it is now a substantial expense for the industry. This is especially true in an age where regulations on waste materials and their disposal continues to tighten.
In addition to occupying significant areas of land, stacks are a costly expense for the industry, incurring costs around $125/ton P2O5.¹ This does not figure in long-term liability or costs associated with environmental cleanup in the event of a catastrophe, nor does it account for the especially costly endeavor of permanently closing a stack at the end of its useful life.
The potential to create a saleable product from phosphogypsum would be a huge economic win for the industry, creating a profit stream in place of waste management costs – tens of billions in profits, according to a paper given by John Wing at the 2016 AIChE Clearwater Phosphate Conference.1
In addition to all of this, the creation of phosphogypsum “landfills” is by no means a sustainable practice. Just as with regular landfills, potential for environmental harm, combined with an increased need for raw materials and a growing focus on sustainability, has many wondering how these piles of “waste” can be turned into an opportunity.
Uses for Phosphogypsum
While use in the US is limited, other countries are already putting this industrial by-product to good use. Outside the borders of the US, phosphogypsum is widely used in a number of applications (in India, phosphogypsum has been redefined as a co-product, further encouraging its reuse). Like many industrial byproducts, its use is frequently seen in the agriculture and construction industries…
Use as a Soil Amendment
Gypsum is a widely recognized soil conditioner, offering a number of benefits to soil and doubly serving as a source of calcium and sulfur. Gypsum is especially beneficial to clay soils and is also used to treat sodic soils.
As a source of gypsum, phosphogypsum can and does substitute natural gypsum as a soil amendment in many countries, including Australia, Brazil, and Spain, to name a few. Studies have even found that the application of phosphogypsum can increase crop yields in many cases.
There is disagreement among groups regarding the risks phosphogypsum can present in terms of radioactivity. While the EPA allows very limited use, the IAEA cites several studies that showed no significant uptake of radioactivity by crops, or buildup of radioactivity in soils in most cases.
Use in Construction and Building Materials
Phosphogypsum has also been studied for use in construction and building materials. Use in such applications is always attractive for industrial by-products, as it provides an outlet for mass utilization, which is often a key requirement of waste reuse. Furthermore, as the world continues to grow and develop, the need for construction materials continues to be a reliable outlet.
Phosphogypsum has been studied for use in cement, bricks, plaster, and more, often with added benefits over virgin materials. When used as a replacement for limestone, for example, phosphogypsum has proven to offer a better roadbed.
However, in some cases, depending on the application and the source material (as well as other materials going into the mix), radiation exposure can be possible. As such, it is recognized that the use of phosphogypsum in such materials must be carefully considered, taking measures such as diluting the phosphogypsum content to a safe level.
Other phosphogypsum uses such as conversion to sulfur, use as a landfill cover, or use in the production of ammonium sulfate (among other applications) are also being studied.
Processing Phosphogypsum
As an industry in its infancy, the processing paths to reuse have yet to be clearly defined. In some cases, phosphogypsum may be used as-is for a given application, where in other cases it may require drying or other processing steps before it can be reused for the intended purpose.
When used as a soil amendment, it is likely to be granulated into a more easily-handled, dust-free form, similar to the processing of natural or synthetic gypsum for use as a soil amendment.
Further complicating matters is that phosphogypsum can vary considerably from one source to the next, with different composition and varying physical characteristics. This necessitates thorough testing of phosphogypsum to confirm feasibility and gather the data necessary for scale up around the unique source of phosphogypsum to be processed and its intended application.
Conclusion
Phosphogypsum is a large-scale waste in the phosphates industry, incurring substantial management costs every year. And while there are some challenges to its beneficial reuse, successful reuse cases around the world, along with the opportunity to mitigate the risks associated with stacking and turn a profit in place of costs has many looking at how to bridge the gap to widespread reuse. Phosphogypsum holds significant potential for reuse as a soil amendment, in construction materials, and other applications, but applications must be carefully examined.
FEECO is a pioneer in processing wastes into value-added products. Our testing and tolling facility, The Innovation Center, offers comprehensive testing for developing products from industrial byproducts – and the processes to make them a reality. Data gathered in the Innovation Center is then used by FEECO engineers to design and manufacture custom process equipment – including rotary dryers and complete pelletizing/granulation lines. To see if we can help you with your phosphogypsum reuse idea, contact us today!