Image 1: Phosphogypsum stacks in Florida (Source: US Environmental Protection Agency, 2018)

Phosphate rock mining

Phosphate rock is predominantly mined and processed to obtain phosphorus – a key ingredient used to enhance plant productivity (Chen & Graedel, 2015). Currently, there are two predominant mining approaches for phosphate rock: open-pit mining, and underground hard-rock mining (Steiner et al., 2015). Following extraction, the phosphate rock is then processed to produce phosphoric acid, which is then converted to phosphate fertilisers (Liang et al., 2017). Here, phosphate rock undergoes wet chemical processing, which uses sulfuric acid to first digest phosphate minerals (Liang et al., 2018).

Fig. 1: The two main processes (thermal and wet process) used to process phosphate, as well as the end products of phosphate rock extraction and processing (Source: Tayibi et al., 2009)

Fig. 2: Wet process of phosphoric acid production, illustrating the chemical agents used and the outputs generated through wet processing (Source: Abdelouahed & Reguigui, 2011)

As shown Fig. 2, the wet process of phosphate rock generates two products: phosphoric acid and phosphogypsum (principally CaSO4.2H2O), with the latter being a waste product. Phosphogypsum, while mostly made up of calcium sulfate dihydrate, contains several impurities such as phosphoric acid, phosphates, fluorides and organic matter, and is usually in the form of a grey, damp, powder or silt (Saadaoui et al., 2017). Additionally, phosphogypsum also contains the bulk of naturally-occuring uranium, thorium, radium, and heavy metals (US Environmental Protection Agency, 2019). While the characteristics of phosphogypsum is heavily dependent on the phosphate ore composition and quality, wet processing has been found to result in the selective separation and concentration of naturally-occurring radium in phosphogypsum (Sahu et al., 2014). To elaborate, about 80% or radium is concentrated in phosphogypsum, while almost 86% and 70% of uranium and thorium respectively are concentrated in phosphoric acid instead (Tayibi et al., 2009). This has therefore raised concerns about the leaching of radioactive elements in disposal sites, as well as the release of radon gas into the atmosphere.

Fig. 3: Flowchart illustrating phosphate rock processing byproducts, secondary processes of these byproducts and typical treatment protocols, with phosphogypsum presented to be containing radioactive rare earth elements (REEs) (Source: Chen & Graedel, 2015)

Global phosphogypsum production is estimated to be around 280 million tonnes per year, with 28% of it disposed into water bodies, and 58% of it stored in tailing ponds (Turner et al., 2022). The disposal of phosphogypsum in water bodies or storage in ponds or leaps are often done without purification (Rashad, 2017), thereby resulting in extensive contamination. Currently, phosphogypsum is managed using wet stacking, in which filtered phosphogypsum is mixed with water and pumped into settling ponds, and the solid residue is then placed into stacks (Turner et al., 2022). These phosphogypsum stacks pose multiple environmental risks, as they are often not watertight nor covered with any inert material (Tayibi et al., 2009). The percolation of water often induces edge outflows that consist of toxic wastewater and leachates, introducing heavy metals and radionuclides into the waters and sediments of estuarine systems (Guerrero et al., 2019). This is exemplified in the outflow of leachates in the phosphogypsum stacks of Huevela, a region in southwestern Spain, which has since led to the deep pollution of underlying salt-marsh sediments (Guerrero et al., 2019). 

The open storage of these phosphogypsum stacks has also been found to contain significant levels of radioactivity due to high radon concentrations in the waste stacks. This has been posited to lead to an increase in radon inhalation rates and consequently, atmospheric radon concentrations (López-Coto et al., 2014). As discussed in a previous blog, radon exposure and inhalation has been found to lead to lung cancer, with radon itself being responsible for half of the total effective dose received by the population.

References

Ben Abdelouahed, H., & Reguigui, N. (2011). Radiotracer investigation of phosphoric acid and phosphatic fertilizers production process. Journal of Radioanalytical and Nuclear Chemistry – J RADIOANAL NUCL CHEM, 289, 103–111. https://doi.org/10.1007/s10967-011-1035-9

Chen, M., & Graedel, T. E. (2015a). The potential for mining trace elements from phosphate rock. Journal of Cleaner Production, 91, 337–346. https://doi.org/10.1016/j.jclepro.2014.12.042

Chen, M., & Graedel, T. E. (2015b). The potential for mining trace elements from phosphate rock. Journal of Cleaner Production, 91, 337–346. https://doi.org/10.1016/j.jclepro.2014.12.042

Guerrero, J. L., Gutiérrez-Álvarez, I., Mosqueda, F., Olías, M., García-Tenorio, R., & Bolívar, J. P. (2019). Pollution evaluation on the salt-marshes under the phosphogypsum stacks of Huelva due to deep leachates. Chemosphere, 230, 219–229. https://doi.org/10.1016/j.chemosphere.2019.04.212

Liang, H., Zhang, P., Jin, Z., & DePaoli, D. (2017). Rare-earth leaching from Florida phosphate rock in wet-process phosphoric acid production. Minerals & Metallurgical Processing, 34(3), 146–153. https://doi.org/10.19150/mmp.7615

Liang, H., Zhang, P., Jin, Z., & DePaoli, D. W. (2018). Rare Earth and Phosphorus Leaching from a Flotation Tailings of Florida Phosphate Rock. Minerals, 8(9), Article 9. https://doi.org/10.3390/min8090416

López-Coto, I., Mas, J. L., Vargas, A., & Bolívar, J. P. (2014). Studying radon exhalation rates variability from phosphogypsum piles in the SW of Spain. Journal of Hazardous Materials, 280, 464–471. https://doi.org/10.1016/j.jhazmat.2014.07.025

(PDF) Radiotracer investigation of phosphoric acid and phosphatic fertilizers production process. (n.d.). Retrieved March 24, 2023, from https://www.researchgate.net/publication/251415281_Radiotracer_investigation_of_phosphoric_acid_and_phosphatic_fertilizers_production_process

Rashad, A. M. (2017). Phosphogypsum as a construction material. Journal of Cleaner Production, 166, 732–743. https://doi.org/10.1016/j.jclepro.2017.08.049

Saadaoui, E., Ghazel, N., Ben Romdhane, C., & Massoudi, N. (2017). Phosphogypsum: Potential uses and problems – a review. International Journal of Environmental Studies, 74(4), 558–567. https://doi.org/10.1080/00207233.2017.1330582

Sahu, S. K., Ajmal, P. Y., Bhangare, R. C., Tiwari, M., & Pandit, G. G. (2014). Natural radioactivity assessment of a phosphate fertilizer plant area. Journal of Radiation Research and Applied Sciences, 7(1), 123–128. https://doi.org/10.1016/j.jrras.2014.01.001

Silva, L. F. O., Oliveira, M. L. S., Crissien, T. J., Santosh, M., Bolivar, J., Shao, L., Dotto, G. L., Gasparotto, J., & Schindler, M. (2022). A review on the environmental impact of phosphogypsum and potential health impacts through the release of nanoparticles. Chemosphere, 286, 131513. https://doi.org/10.1016/j.chemosphere.2021.131513

Steiner, G., Geissler, B., Watson, I., & Mew, M. C. (2015). Efficiency developments in phosphate rock mining over the last three decades. Resources, Conservation and Recycling, 105, 235–245. https://doi.org/10.1016/j.resconrec.2015.10.004

Tayibi, H., Choura, M., López, F. A., Alguacil, F. J., & López-Delgado, A. (2009). Environmental impact and management of phosphogypsum. Journal of Environmental Management, 90(8), 2377–2386. https://doi.org/10.1016/j.jenvman.2009.03.007

Turner, L. E., Dhar, A., Naeth, M. A., Chanasyk, D. S., & Nichol, C. K. (2022). Effect of soil capping depth on phosphogypsum stack revegetation. Environmental Science and Pollution Research, 29(33), 50166–50176. https://doi.org/10.1007/s11356-022-19420-7

US EPA, O. (2019, February 11). What kinds of consumer products contain radioactive materials? [Overviews and Factsheets]. https://www.epa.gov/radiation/what-kinds-consumer-products-contain-radioactive-materials