In the report to the U.S. Congress in 2017, the U.S. DOE that in REEs are found throughout the earth’s crust but most often occur in low concentrations. They are not found in isolated form but in a variety of minerals where, in most cases, they exist in concentrations too small for economical extraction. The REEs are: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The rare earths are also often considered to include the metals scandium (Sc) and yttrium (Y).
In 2011, there were over 400 REE production projects in various stages of implementation worldwide. More recently, market conditions have resulted in many of these projects being cancelled or put on hold. Pending successful project development, some projects could be in production by 2022, including two in the U.S. Should these projects result in additional REE production, some supply and price stabilization may result. A number of other projects focusing on co-production of REEs and other valuable minerals are being pursued (for example, the Eco Ridge mine and Nechalacho projects, both in Canada). The average REE concentration is low (0.15 and 1.3 percent, respectively) in these two projects, but the coproduction opportunities and the composition of the REEs—favoring REEs in high demand for the foreseeable future —support continued development. Co-production, as would occur if coal, alumina, and other minor constituents having commercial value were produced, is often cited as a far more secure path toward developing sources of REEs (Streetwise Reports, 2013).
Global production of LREEs is expected to increase from 122,000 MT in 2011 to 346,000 MT in 2022, and production of HREEs is expected to grow from 13,000 MT in 2012 to 46,000 MT in 2022. A supply surplus of many LREEs and a shortage of HREEs are expected. HREEs accounted for 9.8 percent of total REE production in 2011 (Visiongain, 2012). Adequate supply is only a part of the solution to any REE supply shortfall. Additional processing, refining, and manufacturing capacity is necessary to meet growing demand (Humphries, 2013).
Future Demand
Multiple research and industry sources have projected that global demand for REEs will increase over the next 10 years, with an ample supply of LREEs but a limited supply of HREEs being available (Visiongain, 2012) (Kingsnorth, 2011) (Hatch, Gareth P). Demand is expected to be driven by growth in the technology and clean energy markets. In particular, growth in green technologies such as wind turbines, which use permanent magnets, is expected to push REE consumption higher (Visiongain, 2012). Individual REEs have variable supply and demand profiles and are found in varying proportions at mining sites. Heavy REEs, in particular, are found in smaller quantities, are more difficult to extract, and require more intensive processing for purification. As a result, they are more expensive from a value-chain perspective.
Opportunities and Challenges Associated with Producing REEs from Domestic Coal and Coal Byproduct Streams
Coal and coal byproducts are a potential source of REEs, particularly HREEs. While the concentration of total REEs in coal and coal byproducts is generally much lower relative to deposits that currently serve as commercial supply sources of REEs, the concentration of HREEs may be comparable to that found in current commercial sources and can be greater than that found in major sources of LREEs. However, the challenges associated with extraction from such feedstocks are significant. The mineralogy and distribution of the REEs in coal and coal byproducts will pose challenges for any recovery and separation concept, including those involving physical, chemical, and other more novel separation technologies. Limited data developed during this study were used to identify promising paths that can be applied to different source streams within the coal value chain to concentrate the REEs.
Numerous scientific investigations have identified coal as containing potentially significant quantities of REEs. Both coal processing refuse and ash residues have received attention for the economic value of the REEs they might contain (Coal Ash: A Resource for Rare Earth and Strategic Elements, 2013). Thus, the potential opportunities for recovering REEs from coal and coal byproducts are twofold: REEs are present in materials that are already being produced. Certain coal and coal byproducts have elevated concentrations of HREEs—which are lowest in supply, rank high in criticality and price, and are projected to increase in demand—making them potentially attractive targets for REE recovery despite their low composite REE concentrations.
The REEs found in coal often exhibit a compositional distribution of those REEs that would command a higher basket price than would be found for more conventional and well-recognized ore bodies. Basket price can be used as a rough tool to compare the compositional distribution of REE-bearing ores based on the current relative value of each element or oxide as a pure—nearly 100 percent pure—commodity. DOE/NETL performed an assessment of the amounts of REEs that are likely to be found in coal deposits and the mineral matter—particularly the clays—associated with these deposits. The initial resource estimates indicate that 6 million MT of REEs could potentially be produced from recoverable coal reserves in select western state coal basins in Montana, Wyoming, Colorado, Utah, New Mexico, and Arizona.
Similar estimates indicate that 4.9 million MT are potentially available from among the coal deposits found in Pennsylvania, West Virginia, Kentucky, and Virginia. These estimates represent total resources, with REE contents in associated mineral matter over a cutoff grade of 500 parts per million, and are based on estimates of recoverable coal reserves in these regions found elsewhere (Administration U. E., 2012). The actual amounts that could be recovered would be a function of mining practices and the economics of recovery from operations actively producing, processing, and utilizing newly mined coal. However, given (as previously cited on page 7 of this report) the forecast REE demand in the hundreds of thousands of tonnes per year, this technically recoverable resource of over 10 million tonnes in two of the U.S. coal-producing regions is substantial. In addition, there is potential additional tonnage in coal ash and coal mine refuse/reject.
Further, the REEs are typically associated with clays (small particles of clay trapped in the organic phase or larger partings and roof or floor material rich in clay minerals). These materials can be sources of both alumina (as the clay mineral can be described as hydrous aluminosilicates) and other valuable trace metals. As a result, opportunities to recover REEs from coal and coal byproducts appear possible, but require more information and technology development to create pathways toward both improved economics and environmental footprint. New processing systems must incorporate steps to recover, separate, and/or refine REEs without generating additional unwanted products or negative environmental impacts. These advances may also facilitate recovery of REEs from large volumes of fly and bottom ash as well as other spent and low-concentration sources. Moreover, the co-production of REEs with other useful materials available in coal and coal-byproduct sources offers the potential to further enhance the economic viability of REE recovery from coal and coal byproducts.
Figure 4 provides a conceptual schematic of the coal value chain as it relates to opportunities to recover REEs. The concentrations of REEs stemming from the production, processing, and utilization processes shown in the figure below vary greatly depending on specific source. Data generated to date through this project suggest that the most significant opportunities are associated with mining byproducts (pit cleanings and water resources), as well as preparation refuse. In Figure 4, these categories are shown in the “production” and “processing” portions of the diagram, respectively.
Analysis of REEs in Coal and Coal Byproducts
The presence of REEs in coal and coal byproducts has been known for many years (Bragg et al., 2014). Additional sampling and analysis within this (DOE’s) study were designed to better characterize the presence of REEs in coal resources at active coal mines and preparation plants (the latter comprising the “processing” portion of Figure 4), in aqueous effluents from coal mining and preparation, and in combustion byproducts (the “utilization” portion of Figure 4). Data were also gathered for coal-derived refuse materials such as fine coal refuse, coarse refuse, impounded fly ash and bottom ash, and other related coal refuse. The complete results of the fieldwork are available at https://edx.netl.doe.gov/ree/ . This repository contains data from a number of contributing organizations. More than 29,000 individual REE elemental concentrations were determined from more than 1,820 grab samples. The samples were taken at 26 coal preparation plants, 18 coal combustion units (including pulverized coal-fired units, fluidized bed units, and industrial stokers), and a number of coal mines. Core samples from state geological surveys in six states are included. In addition, a series of separation tests were run.
The most significant results are summarized in the “Analysis of REEs in Coal and Coal Byproducts” and the “Technology Assessment” sections of this report. DOE/NETL has prepared several scientific reports on this topic for public release, including publication of peer-reviewed results.
REE Sampling and Analysis Effort
The study samples were collected from: (1) state geological surveys, (2) companies operating active production and/or processing plants, (3) companies operating a variety of utilization facilities, and (4) companies owning disposal sites (active and inactive). The sampling methodology, which followed ASTM method D6357 (ASTM International) is detailed in Appendix A. REE Resource Evaluation and Estimation
The resource estimates presented below were derived by statistical analysis of U.S. Geological Survey (USGS) data (1997), (Bragg, et al.), and augmented by data taken during this study. This effort focused on one sub-region in the eastern U.S. and one in the western U.S. A methodology was developed using samples from both eastern and western states to assess REE content, both in the coal-bearing, organic portion of the deposit and in likely REE-bearing minerals associated with the coal. The assumptions made and methodology employed in developing the REE resource estimates are outlined below:
 Coal and coal byproducts are a potential source of REEs, but the concentration of REEs is generally lower than that present in commercial deposits that currently supply REEs.
 Coal and coal byproducts contain HREEs—which are least prevalent, rank high in criticality and price, and are projected to increase in demand.
Estimates made during this study indicate that, at a minimum REE concentration of 500 parts per million (ppm) in coal mineral matter, 6 million MT of REEs could be recovered from within the boundaries of the known coal reserves in select western state coal basins in Montana, Wyoming, Colorado, Utah, New Mexico, and Arizona. Similar calculations show that 4.9 million MT are available from within the boundaries of the coal deposits found in Pennsylvania, West Virginia, Kentucky, and Virginia. These estimates are based on total recoverable coal resources (Administration U. E., 2012). The estimate assumed that essentially all of the REEs are associated with the clay minerals with only minor amounts ion-exchanged into the organic phase. Some of the clays may be finely divided and trapped within the organic portion that is preferentially separated as “clean coal” during the preparation process. Because of the limited distribution and range of samples utilized in the study, the REE estimates are believed to be conservative estimates of the possible REE resources available.
The maps below (Figures 5a, 5b, and 5c) overlay the calculated resources of REEs developed for this study onto an outline of the major coal basins in the U.S. The resource estimates are provided as tons of REEs per block (a “block” being a section one kilometer (km) by one km in surface area and one meter in depth). These maps were developed based on the original USGS data accounting for the full depth of the coal deposit and including associated clay and other mineral matter that are treated as partings or benches and often discarded as refuse.
The analysis then produced data on the amount of REEs likely to be found in the full deposit (not just the portion that is predominately coal). The overlay of the REE resource coincides with the general regions from which samples were drawn. These resource maps also made use of other data related to known faults and geologic features known to correlate to REE enrichment as shown on the map. A far greater number of samples from the eastern coal regions were included in the USGS source material. The USGS Coal Quality database originated in the 1970s and was updated most recently in 1998. Some sourcing bias remains; in particular, the number of samples from the Gulf Coast lignite region was small.
Data from ten states—six in the west and four in the east—were used to develop this analysis. These data indicate that two coal regions in the U.S. contain significant quantities (10 million MT) of REEs and that some of these deposits might be candidates for REE recovery (particularly those shown in shades of red). Both the northern and central Appalachian basins (Pennsylvania, West Virginia, and Kentucky) show numerous occurrences with higher mass loadings of REEs. Similar data for the western states examined are less common, but the source material used in this analysis included far fewer data points in that part of the country. These states contain approximately 62 percent of the estimated recoverable coal reserves.
REE Extraction Potential: Characterizing Deposits
Coal formations are three-dimensional and the coal quality and concentrations of trace elements (including REEs) will vary within formations by depth and location. For example, the measured compositional characteristics for several bench samples vary across the seam by a factor of threeon a whole-sample basis. In addition, well log data were analyzed to explore the correlation between data from the logs and the known presence of REEs. A hand-held device based on X-ray fluorescence was used to screen samples in the field. These techniques and the resulting data can also support assessment of the three-dimensional nature of the REE deposits. The concentration of REEs within a coal seam is not necessarily constant from top to bottom or from one location to
another.
REE Extraction Potential: Processing
Approximately 250 coal preparation plants are processing coal in the U.S. Typically, the plants will have a feed stream, two refuse streams, and a clean coal product stream. (In some cases, preparation plants may produce a second product stream with a different set of specifications.) A series of coal washability tests was conducted in order to better understand REE partitioning potential during coal preparation. Samples covered approximately 10 percent of the preparation facilities in the U.S., chiefly in the northern, central, and southern Appalachian coal basins and in the Illinois basin. Comparable studies were also performed on a limited set of coals from western sources (Alaska, Colorado, North Dakota, Montana, and Texas).
Results have indicated that the REEs found in the clean coal product fraction, from processing coal in a preparation plant (the processing portion of Figure 4), are fully mixed into the organic phase, primarily as fine particles. This suggests that, for the fraction of REEs in raw coal that report to the clean coal product, little can be done to extract these REEs until after combustion.. The rare earth concentrations in the lowest-density fractions indicate that: (1) ash-based material remaining finely dispersed within the coal has the strongest affinity for the REEs, and/or (2) the REEs are bound within the organic matrix by adsorption with humic acid components. Data obtained during this study supports the enriched finely dispersed mineral hypothesis, the first of these alternatives. Figure 6 presents results for one set of analyses for the coarse refuse stream from one coal preparation plant. It shows the total concentrations of REEs for each of the specific gravity fractions that were used in evaluating the separation of clean coal from mineral matter and the effect of that split on the REE concentration in each fraction. The average concentration of REEs found in the material increases successively from the 1.6 x 1.8 float through the 2.1 x 2.2 float, then remains constant until a significant drop occurs with the 2.9 sink.
Additional studies were conducted to determine the mass balance around an operating coal preparation plant. The data in Table 2 represent an actual mass balance around a single preparation plant. The total flow and REE flow numbers are measured values. The REE data for this plant are typical of many of the plants evaluated in this study, with the bulk of the REE mass found in the coarse reject (which contains approximately 63 percent of the total. The Float-and-sink analysis uses a series of heavy liquids diminishing (or increasing) in density by accurately controlled stages for the purpose of dividing a sample of crushed coal into gravimetric fractions either equal-settling or equal floating at each stage. (Mines, 2014)
REEs entering the plant), typical of the various plants studied. For this plant, approximately 30 percent of the REEs are included in the clean coal product, which may be recoverable at the power plant after combustion.
For another portion of this study, a sample of the middlings was subjected to a series of grinding and separation steps to assess the potential of releasing the minerals from the coal hosting the REE minerals. The results are shown in Figure 7, and indicate that the REE-containing materials are fine grained and report to the low-gravity fractions. This conclusion is supported by mineralogical studies that were also performed. As a result, target resource streams can be identified and potential approaches to concentrating, separating, and processing the REEs from coal and coal byproducts can begin to be explored.
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