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| Fig. 1: Coal fly ash in the UK. (Source: Wikimedia Commons) |
Rare Earth Elements (REEs) are a group of 17 elements with important uses for electronics, defense, and industrial processes. In particular, REEs are used to create strong magnets for direct drive wind turbines and electric vehicle motors, meaning an increasingly electrified economy will demand significantly more REEs. [1] Rare earth elements are not technically rare, but they rarely appear at high enough concentrations to make extraction worthwhile. [2] Traditionally, REEs are mined from open pit ore deposits.
In 2022, the United States consumed 9,300 metric tons (MT) of rare earth compounds and metals. [3] 74% of this supply came from China. Global REE consumption was 300,000 MT in 2022, with 70% supplied from China.
Future demand is expected to be impacted by the green energy movement, although specific estimates vary. The International Energy Agency (IEA) reports that REE demand could be 3-7 times higher in 2040 compared to 2022, which would require a supply of 900,000-2,100,000 MT. [4] This could include a ten-fold increase in demand specifically for wind turbines and EVs. [5] A report from the University of Pennsylvania provides a conservative estimated demand of 450,000 tons/year in the coming decades. [1]
In recent years, the US has placed growing emphasis on the need to ensure a domestic supply of critical minerals including REE. [2] One reason for this is the value of REEs in military equipment, with China beginning to restrict certain exports. [6]
From the 1960s until the mid-1980s, the US was the dominant supplier of REEs via California's Mountain Pass mine. [1] However, there are significant environmental costs associated with this form of mining, and China has since come to dominate the market. The US has thus encouraged the development of secondary (e.g., recycled) and unconventional critical mineral sources, one of the most prominent of which is REEs extracted from coal byproducts. [2] Most notably, REEs have been identified in sufficient volumes in the coal ash produced as a byproduct of coal combustion. [2] Of this coal ash, fly ash (see Fig. 1) allows for the most feasible processing and is the focus of numerous studies.
In 2017, the US Department of Energy reported on potential REE extraction from coal, focusing on coal plants reject materials and recovery from coal ash streams. This study estimated that approximately 11 million MT of REEs could be recovered from coal reserves in the US, including 6 million MT from western states and 4.9 million MT from coal deposits in Pennsylvania, West Virginia, Kentucky, and Virginia. [7] These estimates require mineral matter to have a cutoff grade of at least 500 parts per million. Although these numbers are partly derived from the assumption that REEs could be extracted from coarse coal reject, the following analysis focuses on fly ash since it is already marketable and available at large scale as well as less expensive to process due to its small particle size. [2]
Not all coal contains uniform amounts of REEs, and not all REEs are of equal value. In a study of fly ash from coals in the Appalachian Basin, the Illinois Basin, and the Powder River Basin in Wyoming, researchers found that the volume of REEs ranged from a mean of 336.9 ppm in Wyoming to 590.6 ppm in the Appalachian Basin. [8] In addition, heavy rare earth elements (HREEs) such as yttrium, erbium, and terbium tend to be in lower supply and are thus of higher value. These are found more abundantly in some coal stocks. [8] As per 2013, prices per kilogram of different REEs ranged from $8 to $5000. [8] Critical REEs are considered to include neodymium, europium, terbium, dysprosium, yttrium, and erbium. [8]
The 2016 study by Taggart et al. found REE measurements in samples from coal basins covering 70-80% of domestic production. Although the US DOE report from 2017 focused on REE concentrations of at least 500 parts per million, the more recent DOE cutoff is set at 300 ppm. [2] As a rough estimate of REE concentration in US fly ash, the Taggart et al. mean value of 443.57 ppm is used. [8] In addition, the mean value of extractable REE was 173.13 ppm and the mean percentage of critical REEs was 36.33%. This provides an approximated value of 62.898 ppm critical REEs. Since there are no reliable sources providing the amount of fly ash produced in the US, this is instead estimated based on coal weight. The EIA Annual Coal Report states 594.2 million short tons of coal were produced in the US in 2022. [9] Coal ash is estimated to be 10% of coal by weight, and fly ash is estimated to be 80% of this resultant ash, meaning fly ash is 8% of coal by weight. [10] This provides a value of 43.12 million metric tons of fly ash produced in 2022. It is further estimated that 50% of this fly ash was not used or sold, leaving 21.56 million tons. Estimated REE availability from fly ash in 2022 is provided in Table 1.
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| Table 1: Estimates of potential annual REE production under different production scenarios. [8,9] |
According to the IEA, the least efficient coal plants should be phased out by 2030, with remaining coal plants retrofitted by 2040 and the vast majority of electricity coming from alternative sources by 2050. [5] Although coal combustion has been declining in recent years, if it is conservatively assumed that US coal plants continue to operate at 2022 levels from 2024 through 2050 and produce REEs from fly ash, total REE production could be as shown in Table 2.
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| Table 2: Estimates of total REE production from 2024-2050 under different production scenarios. [8,9] |
Although there is considerable uncertainty associated with these estimates, they are far from the multi-million ton figures provided by the DOE or the anticipated future demand.. There are other opportunities along the coal production chain to process REEs such as using coarse refuse, though it is not clear that these will be cost effective or competitive with other potential end uses. This also depends on the ongoing market rates for various REEs and government subsidies.
The analysis thus far has focused on the potential volume of REEs that could be extracted from fly ash, but it has not considered the inputs, costs, and environmental burden of this process. Park et al. (2021) write that the foremost problem associated with REE production is the considerable quantities of by-products and wastewaters that are also generated, owing to the extensive use of strong acids during extraction and purification processes. [11] In addition, there is no consensus around an ecologically-friendly methodology for extracting REEs.
REE studies often consider extraction using heated nitric acid digestion, potentially combined with other substances such as hydrofluoric acid. [11] Taggart et al. used a ratio of 10 mL nitric acid to 0.1 g of fly ash, however this was chosen to optimize analysis rather than extraction. [8] Park et al. used a ratio of 5 mL of nitric acid and 2 ml of hydrofluoric acid to 0.1 g of coal ash. [11] Applying this to the volumes of unused and total fly ash produced in 2022, critical REE extraction would require between 875 billion and 1.76 trillion liters of nitric acid. Facilities would then need to dispose of massive quantities of these harmful acidic byproducts. The quantities of nitric acid needed make this an unrealistic technical process.
As for potential economic feasibility, a rough estimate of the unit price of REEs from fly ash can be calculated. The relative percentage of each critical REE in samples extracted from fly ash is drawn from Taggart et al., and 2022 prices are drawn from the USGS Mineral Commodities Summary (aside from erbium, which is unavailable and thus the 2013 price is drawn from Taggart et al.). [8,3] Based on these values, one ton of critical rare earth oxide from fly ash could be sold for roughly $126,118 (with the rare earth oxide content being slightly higher than the volume of rare earth elements). [8,3] Although greater demand could drive up prices, this would need to overcome the considerable costs associated with testing extraction processes, purchasing nitric acid and other concentrates, industrial heating equipment, transport machinery, and other materials for a relatively small volume of REEs relative to expected demand.
Electric vehicles and renewable energy are critical components of a decarbonized world, and these require far greater supplies of REEs. [5] The US is highlighting coal as a potential domestic source of REEs, and yet in addition to greenhouse gas emissions, coal mining and processing results in localized air and water pollution, the emission of particulate matter, and health problems disproportionately impacting those of poor socioeconomic status. [12] The need for rare earth elements and the capacity to extract REEs from coal byproducts creates a challenging tradeoff, whereby the materials of a green economy may create local and global environmental harms. The ongoing need for REEs could inadvertently extend the life of coal mines and coal-fired power plants. At the moment, extracting REEs from coal fly ash appears technically infeasible and economically uncertain, and the potential production is estimated far below the US DOE estimates. This will continue to pose a challenge for the US in the coming decades.
© Julia Ilhardt. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] O. Serpell, B. Paren, and W.-Y. Chu, "Rare Earth Elements: A Resource Constraint of the Energy Transition," University of Pennsylvania, May 2021.
[2] D. Bagdonas et al., "Rare Earth Element Resource Evaluation of Coal Byproducts: A Case Study From the Powder River Basin, Wyoming," Renew. Sustain. Energy Rev. 158, 112148 (2022).
[3] "Mineral Commodity Summaries 2023," U.S. Geological Survey, January 2023.
[4] "The Role of Critical Minerals in Clean Energy Transitions," International Energy Agency, March 2021.
[5] Net Zero by 2050: A Roadmap for the Global Energy Sector," International Energy Agency, May 2021.
[6] "China Controls the Supply of Crucial War Minerals," The Economist, 13 Jul 23.
[7] "Report on Rare Earth Elements from Coal and Coal Byproducts," U.S. Department of Energy, January 2017.
[8] R. K. Taggart et al., "Trends in the Rare Earth Element Content of U.S.-Based Coal Combustion Fly Ashes," Environ. Sci. Technol. 50, 5919 (2016).
[9] "Annual Coal Report 2022," U.S. Energy Information Administration, October 2023.
[10] A. Cwirzen, "Properties of SCC With Industrial By-Products as Aggregates," in Self-Compacting Concrete: Materials, Properties and Applications, ed. by R.Siddique (Woodhead Publishing, 2020).
[11] S. Park et al., "Characterization of Rare Earth Elements Present in Coal Ash by Sequential Extraction," J. Hazard. Mater. 402, 123760 (2021).
[12] M. Hendryx, K. Zullig, J. Luo, "Impacts of Coal Use on Health," Annu. Rev. Public Health 41, 397 (2020).
