Occurrence and recovery potential of rare earth elements in Finnish peat and biomass combustion fly ash

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Occurrence and recovery potential of rare earth elements in Finnish peat and biomass combustion fly ash © 2019 Elsevier B.V. Accepted version (Final draft) Perämäki, Siiri; Tiihonen, Antti; Väisänen, Ari Perämäki, S., Tiihonen, A., & Väisänen, A. (2019). Occurrence and recovery potential of rare earth elements in Finnish peat and biomass combustion fly ash. Journal of Geochemical Exploration, 201, 71-78. https://doi.org/10.1016/j.gexplo.2019.03.002 2019 Accepted Manuscript Occurrence and recovery potential of rare earth elements in Finnish peat and biomass combustion fly ash Siiri E. Perämäki, Antti J. Tiihonen, Ari O. Väisänen PII: S0375-6742(18)30699-X DOI: https://doi.org/10.1016/j.gexplo.2019.03.002 Reference: GEXPLO 6280 To appear in: Journal of Geochemical Exploration Received date: 3 December 2018 Revised date: 15 February 2019 Accepted date: 2 March 2019 Please cite this article as: S.E. Perämäki, A.J. Tiihonen and A.O. Väisänen, Occurrence and recovery potential of rare earth elements in Finnish peat and biomass combustion fly ash, Journal of Geochemical Exploration, https://doi.org/10.1016/j.gexplo.2019.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT 1 Occurrence and recovery potential of rare earth elements in Finnish peat and biomass combustion fly ash Siiri E. Perämäkia*, Antti J. Tiihonenb, and Ari O. Väisänena aDepartment of Chemistry, Renewable Natural Resources and Chemistry of Living Environment, University of Jyväskylä, P.O. Box 35, FI-40014, Jyväskylä, Finland *Corresponding author. E-mail address: [email protected], tel. +358048053495 bDepartment of Chemistry, Structural and Synthetic Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014, Jyväskylä, Finland ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2 Abstract Rare earth elements (REEs) are highly important in the modern society as a result of their wide use in various fields of industry. Their high supply risk and increase in demand has led to classification as critical materials, and consequently, new resources for REEs are being prospected widely. Coal fly ash has been suggested as a possible secondary resource for REEs, but very little information is available regarding REE occurrence in biomass or peat derived fly ash. In this paper, fly ash samples from commercial power plants using peat and biomass as fuel were studied for REEs. The average concentration of REEs was 530 ppm, with up to 920 ppm in one fly ash. Five out of seven fly ashes were identified promising for profitable REE recovery according to outlook coefficients of >0.7 and critical REE content of >30%. Four fly ashes were found to exhibit medium REE enriched distributions, whereas three fly ashes displayed light REE enriched distributions. These enriched distribution patterns, as well as the observed Gd, Ce, Eu, La, and Y anomalies in the patterns, are proposed to originate mainly from the granitic bedrock at the fuel source. Additionally, positive correlation between REEs and iron was found, indicating REE association with iron minerals. Keywords: rare earth element; fly ash; peat; biomass; occurrence; recovery ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 3 1 Introduction REEs are a group of chemically similar elements including lanthanoids, i.e. elements from lanthanum to lutetium, and commonly also yttrium and scandium. They can be classified into following groups: light rare earth elements (LREE: La, Ce, Pr, Nd and Sm), medium (MREE: Eu, Gd, Tb, Dy and Y) and heavy (HREE: Ho, Er, Tm, Yb and Lu) [1]. REEs are present in the earth’s crust in moderate concentrations varying from 0.2 ppm of Tm to 70 ppm of Ce [2], with their total concentration ranging from 100 to 220 ppm [3]. Due to their unique chemical and physical properties, REEs are essential in permanent magnets, lamp phosphors, catalysts, rechargeable batteries, and in numerous other high technology applications [4–6]. REEs have recently become increasingly critical due to the vast number of applications, uncertain market availability and low recycling rates [4–7]. While China was producing 81 % of the world’s REEs in 2017 [8], the supply risk has been high for many years especially in western countries. Moreover, most currently mined REE deposits have high concentrations of the abundant REEs and low concentrations of the less abundant REEs, leading to oversupply of the abundant REEs, most prominently Ce. Hence, the European Union has classified REEs among critical materials in their reports [7,9,10] and the U.S. Department of Energy has included Nd, Eu, Tb, Dy and Y in their Critical Materials Strategy in 2011 [11]. Due to these concerns, new deposits and secondary sources of REEs are being prospected widely [12]. REE recovery from coal fly ash has been a subject of interest for more than 20 years, since coal beds with REE contents of 0.2-0.3 wt% in ash were found in Russian Far East [12]. Coal fly ash is the most widely produced fly ash in the world at 750 million tons annually [13], with an average REE concentration of 404 ppm [12]. REEs and their recovery potential in coal fly ash have consequently been studied extensively [1,14-26]. However, EU’s 20 % ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 4 target of renewable energy by 2020 [27] favors the use of biomass as a CO2 emission neutral fuel alternative to coal [28]. In spite of the significant production of biomass fly ash, estimated at 480 million tons annually [28], only few studies have focused on biomass fly ash as a potential REE resource. This is likely caused by the previously reported low concentrations: typically between 10-100 ppm in biomass based ashes [29,30]. The same applies to peat and peat ash, with REE concentrations in Russian Siberian peat ashes ranging from 35 to 340 ppm (incl. La, Ce, Sm, Eu, Tb, Yb and Lu) [31]. However, high concentrations of REEs in peat ashes from mires at rapakivi granite areas in Finland have been reported, with an average REE concentration of 1 300 ppm (incl. Y, La, Ce, Pr, Nd, and Sm) and individual REE concentrations as high as 3 500 ppm for Y [32]. The total concentration of REEs in fly ash can be used to evaluate their potential for economic recovery. A cut-off grade for profitable utilization in coal ash has been suggested by Seredin [33] at ≥1000 ppm of rare earth oxides (REO), and ≥800-900 ppm in coal seams with thickness of >5 m. However, the total amount of REO in fly ash should not be the only consideration, as there are significant differences in the use and availability of individual REEs. For evaluation of REE deposit quality, REEs can be divided into critical (Nd, Eu, Tb, Dy, Er, Y), uncritical (La, Pr, Sm, Gd) and excessive (Ce, Ho, Tm, Yb, Lu) according to their production and consumption quantities, as introduced by Seredin and Dai [12]. The ratio of critical REEs to excessive REEs is called the outlook coefficient (Coutl.), which can be calculated as: (1) Promising fly ashes have high outlook coefficients (>0.7), since they contain high concentrations of critical REEs and low concentrations of excessive REEs. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 5 In order to gain knowledge about REE enrichment in fly ashes, it is useful to normalize REE concentrations to some relevant frame of reference, for example to the upper continental crust (UCC) [34]. From these appropriately normalized distribution patterns, REE rich fly ashes may be observed to be enriched with either light (L-type), medium (M-type) or heavy (H-type) REEs [12], of which the latter two are most suitable for recovery of REEs, by containing more of the least abundant REEs. Anomalous behavior is occasionally observed for Ce, Eu, La, Gd, and Y in the otherwise fairly smooth UCC normalized REE distributions patterns of coals and coal fly ashes [34]. This is due to Ce and Eu being redox-sensitive elements, differentiating from the other REEs by occurring at oxidation states +4 and +2, respectively. Anomalous behavior of La, Gd, and Y, on the other hand, is based on the slight differences in the stabilities of REE complexes [34]. In a negative anomaly, the studied material is depleted with respect to a certain element when compared to the other REEs, and vice versa in a positive anomaly. REE anomalies develop under specific conditions, and the observation of these anomalies can give insights into the geochemical history and sedimentsource region where the fuel originates. This is the first paper to investigate REE concentrations and distribution patterns in peat and biomass fly ash from commercial power plants in Finland. REE content in fly ashes is evaluated using outlook coefficients, content of critical REEs, and enrichment compared to the UCC. REE distribution patterns and anomalies are assessed for information about the geochemistry of the region where the fuel originates. Correlation of REEs with other elements as well as fly ash mineralogy is studied to gain knowledge about REE associations with certain minerals. Ashes being produced from energy production in Finland alone during 2012 were 1.0 million tons [35], presenting a potentially important secondary resource for REEs. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 6 2 Materials and methods 2.1 Samples and reagents Seven fly ash samples were obtained from two combined heat and power (CHP) plants located in central Finland. Power plant 1 utilizes fluidized bed combustion (FBC) and power plant 2 circulating fluidized bed (CFB) technology. The samples were collected from the first rows of electrostatic precipitators. The CHP plants used a mixture of peat and biomass as a fuel, of which composition is presented in Table 1. Standard reference material (SRM) 1633c coal fly ash from the National Institute of Standards and Technology (NIST) was used to verify sample treatment and analytical procedure. Coal fly ash SRM was considered to be a suitable alternative in monitoring the sample treatment and analytical procedure, since no peat and biomass fly ash SRM was available. High-purity water was used throughout the work, with resistivity of 18.5 MΩ·cm, produced with a Purelab Ultra water purification system supplied by Elga (Buckinghamshire, U.K.). Table 1 Fly ash samples’ collection dates, fuel compositions and power plants (1: FBC, 2: CFB) Sample ID Date collected (month/year) Fuel composition (peat:bio %) Power plant A 5/2010 30:70 1 B 3/2010 50:50 1 C 2/2012 50:50 2 D 1/2010 65:35 1 E 2/2010 70:30 1 F 3/2012 80:20 1 G 5/2010 100:0 2 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 7 2.2 Microwave digestion A dried subsample of 250 mg was weighed into a Teflon microwave vessel, into which 7.0 ml of nitric acid (Sigma-Aldrich, puriss p.a., 65-68 wt%), 1.0 ml of hydrochloric acid (Sigma-Aldrich, puriss p.a., 37-39 wt%), and 1.5 ml of hydrofluoric acid (Merck, EMSURE ISO, 40 wt%) was added. The sample was mixed with the acids by swirling the vessel. The sample vessel was closed and placed in a microwave oven Mars 6 iWave, supplied by CEM (North Carolina, U.S.A.). The temperature was ramped with microwave power of 290-1800 W during 20 minutes to 200 °C, in which the temperature was held for 15 minutes. After cooling, the vessel was opened and 10 ml of 5 % (m/v) boric acid (Sigma- Aldrich, purity >99.99%) water solution was added. The vessel was closed and placed in the microwave oven. The temperature was ramped with microwave power of 290-1800 W to 170 °C during 20 minutes, in which it was held for 15 minutes. After cooling, the vessel was opened and the sample was filtered (Whatman no. 42 filter paper) into a plastic 50 ml volumetric flask. The microwave vessel was rinsed three times with 5-10 ml of ultrapure water, which was poured onto the filter paper. The sample was finally diluted to 50 ml volume with ultrapure water. 2.3 Elemental analysis The digested samples were analyzed for REEs using an ICP-MS (inductively coupled plasma –mass spectrometer) NexIon 350D, supplied by PerkinElmer, (Massachusetts, U.S.A.) equipped with prepFAST 4DX autosampler, supplied by Elemental Scientific (Nebraska, U.S.A.). The digested fly ash samples were diluted hundredfold with prepFAST for the analysis. PerkinElmer PurePlus multi-element standard containing 10 ppm of Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, Y, Yb was used for the calibration of ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 14 sources of REEs, and in Finland where the fuel originates, 53 % of the bedrock consists of granitic rocks and 22% of migmatites [38]. Some ferns have been found to accumulate REEs, and the roots usually have higher concentrations of REEs compared to other parts of the plants [39]. Finland also has REE rich areas, in which REE appear for example in apatite (CaPO4) with monazite inclusions [(Ce,La,Nd,Th)PO4)], allanite [(Ce,Ca,Y)2(Al,Fe)3(SiO4)3(OH)], and fergusonite [(Y,Er,Ce,Fe)(Nb,Ta,Ti)O4] [40]. REE content in some of these deposits are as high as 4.2 wt% [41]. Weathering of minerals and accumulation in the peat bog due to sorption by humic matter are possible explanations for the observed REE distributions in fly ashes. 3.3 Associations with other elements and minerals Major and minor element concentrations from the elemental analysis are presented as oxides in Table 4. Silica is the major component in all but one fly ash, with concentration range of 30-53 wt%. This is consistent with the XRD analysis of fly ash samples, with quartz (SiO2) identified as the main component from the diffraction patterns, presented in Fig. 2. Silica in fly ash originates not only from the fuel, but also from the bed sand used in fluidized bed combustion. The fly ashes also contain high concentrations of oxides of calcium, aluminum, and iron in the range of 6-23 wt%, 10-14 wt%, and 4-10 wt%, respectively. Oxides of K, Na, Mg, and P are present in all fly ashes with concentrations of 1-3 wt%. In XRD analysis, plagioclase (NaAlSi3O8 - CaAl2Si2O8), anhydrite (CaSO4), orthoclase (KAlSi3O8), and calcite (CaCO3) were identified in most of the fly ash samples. Additionally, berlinite (AlPO4) was identified in fly ash F and G, enstatite (MgSiO3) in fly ash B, and maghemite (Fe2O3) in fly ash F. Minor element oxides are present in the fly ashes in ppm levels, and were not identified in XRD analysis. Major and minor elemental concentrations in the studied fly ashes are similar to those found in coal fly ashes [13] as well as wood and ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 15 woody biomass ashes [29], although biomass ashes have significant variations in elemental composition due to different biomass types. Interpretation of XRD diffraction patterns of fly ash samples presented some challenges due to peak overlap and low sample crystallinity, and some peaks (especially the ones overlapping with quartz main peak at theta value 26.5°) could not be unambiguously identified. Table 4 Major and minor element concentrations (wt%) expressed as oxides in fly ashes A-G. Fly ash samples Component A B C D E F G SiO2 30.7 33.7 47.6 46.5 53.3 43.8 30.4 Al2O3 11.9 12.5 12.4 13.0 14.1 12.5 9.9 Fe2O3 4.0 6.9 9.3 6.2 9.0 9.9 7.7 CaO 6.4 14.2 15.2 6.7 8.6 7.1 23.3 K2O 3.0 3.1 3.3 2.5 2.3 2.2 1.4 P2O5 0.93 2.7 2.8 1.1 2.0 1.4 1.1 Na2O 2.3 1.6 1.6 2.0 1.6 1.7 1.3 MgO 1.3 2.4 2.4 1.2 1.6 1.4 2.2 SO3 0.31 1.7 3.5 0.17 0.25 0.40 3.1 MnO 0.22 0.64 0.47 0.19 0.24 0.19 0.09 TiO2 0.36 0.35 0.35 0.33 0.37 0.37 0.27 BaO 0.13 0.18 0.16 0.10 0.11 0.10 0.08 SrO 0.05 0.09 0.08 0.05 0.06 0.05 0.05 ZnO 0.04 0.10 0.07 0.03 0.03 0.03 0.01 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 16 Fig. 2. XRD analysis diffraction patterns with identified sample constituent peaks. Correlation of REEs with major and minor elements, as well as peat content of the fuel, was studied by calculation of squared Pearson’s correlation coefficients (R2), which are presented in Fig. 3 for components with R2>0.3. Fig. 4 shows La and Lu, representing light and heavy REEs, concentrations plotted against Fe2O3, SiO2, Al2O3 content in fly ash and peat content of the fuel. REE concentrations correlate positively with Fe content in the samples, with R2 values 0.63-0.83, indicating association of REEs with iron minerals. A single iron mineral (maghemite) was observed in XRD analysis of one fly ash sample, but other Fe containing minerals were most likely present as well. It is likely that samples consisted of several species in such low individual quantities that they could not be identified in XRD analysis. Fig. 3. Squared Pearson’s correlation coefficients between REEs and Fe2O3, SiO2, and Al2O3 concentrations in fly ashes. The heavier REEs were also found to correlate positively with silica and aluminum oxide to some extent, with R2 values up to 0.59 and 0.35, respectively. This could indicate heavier REE associations with aluminosilicates in fly ashes. Aluminosilicates plagioclase and ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 17 orthoclase were observed in XRD analysis of most of the fly ash samples. Franus et al. [42] have reported REE in coal fly ash to correlate with oxides of Al and Si, similar to Vassilev et al. [43], who studied biomass ash and found REE to correlate with S, Ti, and phosphates in addition to oxides of Si and Al. For other major and minor elements, as well as for the peat content of the fuel, the R2 values were lower than 0.3, and associations with REEs could not be concluded. Correlation of REEs with peat content of the fuel had R2 values up to 0.2, which is not statistically significant. However, visual evaluation of data in Fig. 4 indicates that there might be some positive correlation between REE concentrations and peat content of the fuel. The low correlation coefficient is due to fly ash G with 100 % peat content, which deviates from the otherwise observed rather linear trend. Peat used as a fuel during the collection of the fly ash samples can originate from different batches and locations in Finland, which makes assessment of correlation between REEs and peat content difficult. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 18 Fig. 4. Content of Fe2O3, SiO2, and Al2O3 (wt%) and peat percentage of fuel as functions of La and Lu concentrations (ppm) in fly ashes. 4 Conclusions This paper is the first to present REE concentrations in fly ashes from commercial CHP plants using peat and biomass as fuel, and to produce new information about REE occurrence in bio-based fly ashes. The observed REE concentrations at an average of 530 ppm were significantly higher than previously reported for biomass fly ash, and were similar to those of coal fly ash. Five of the seven fly ashes were classified promising for economic recovery of REEs according to the outlook coefficients and the concentration of critical REEs. The studied fly ashes were enriched with medium or light REEs and had concentrations up to 6 times higher compared to the UCC. Positive Gd anomalies, and some Ce, Eu, La, and Y anomalies, were present in the UCC normalized REE distributions, most likely originating from Finnish bedrock of granitic rock and migmatites. REEs we found to be associated with iron, and to some extent with silicon and aluminum, suggesting REE associations with iron minerals and possibly with aluminosilicates in biomass and peat fly ash. Declaration of interest One of the funders of this work, Jyväskylä Energy Group has a patent (Patent no. FI123432B) where two of the authors, S.P. and A.V., are included as inventors. The patent is based on previous research collaboration related to the topic of this paper. Role of the funding source The fly ash samples and information about the fuel composition and combustion process were received from Jyväskylä Energy Group. The funders had no role in research design or in ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 19 the collection, analysis and interpretation of the data or in the writing of the paper. Nor did they have any role in the decision to submit the paper for publication. Acknowledgements This work was supported by Jyväskylä Energy Group and the University of Jyväskylä, Department of Chemistry. Eemeli Rytkönen is thankfully acknowledged for assistance with the digestion of fly ash samples. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 20 References [1] Blissett RS, Smalley N, Rowson NA. An investigation into six coal fly ashes from the United Kingdom and Poland to evaluate rare earth element content. Fuel 2014;119:236–9. doi:10.1016/j.fuel.2013.11.053. [2] Gupta CK, Chrishnamurthy N. Extractive Metallurgy of Rare Earths. 1st ed. Boca Raton: CRC Press; 2004. doi:10.1201/9780203413029 [3] Long KR, Van Gosen BS, Foley NK, Cordier D. The Principal Rare Earth Elements Deposits of the United States—A Summary of Domestic Deposits and a Global Perspective. U.S. Geological Survey Scientific Investigations Report 2010–5220. 2010. [4] Massari S, Ruberti M. Rare earth elements as critical raw materials: Focus on international markets and future strategies. Resour Policy 2013;38:36–43. doi:10.1016/j.resourpol.2012.07.001. [5] Alonso E, Sherman AM, Wallington TJ, Everson MP, Field FR, Roth R, et al. Evaluating Rare Earth Element Availability : a Case With Revolutionary Demand From Clean Technologies. Environ Sci Technol 2012;46:3406–14. doi:10.1021/es203518d. [6] Binnemans K, Jones PT, Blanpain B, Van Gerven T, Yang Y, Walton A, et al. Recycling of rare earths: A critical review. J Clean Prod 2013;51:1–22. doi:10.1016/j.jclepro.2012.12.037. [7] European Commission. Critical raw materials for the EU, Report of the Ad-hoc Working Group on defining critical raw materials 2010, 84 p, Brussels. doi:10.1002/eji.200839120.IL-17-Producing. [8] U.S. Geological Survey, Mineral Commodity Summaries, Rare earths. 2018. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 21 doi:10.1007/3-540-06125-8. [9] European Commission. Report on critical raw materials for the EU, Report of the Ad hoc Working Group on defining critical raw materials. 2014, 41 p, Brussels. [10] European Commission. Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions on the 2017 list of Critical Raw Materials for the EU. Communication 2017, 8 p, Brussels. [11] Critical Materials Strategy, U.S. Department of Energy, 2011, 190 p. https://www.energy.gov/sites/prod/files/DOE_CMS2011_FINAL_Full.pdf [12] Seredin V V., Dai S. Coal deposits as potential alternative sources for lanthanides and yttrium. Int J Coal Geol 2012;94:67–93. doi:10.1016/j.coal.2011.11.001. [13] Blissett RS, Rowson NA. A review of the multi-component utilisation of coal fly ash. Fuel 2012;97:1–23. doi:10.1016/j.fuel.2012.03.024. [14] Ribeiro J, Valentim B, Ward C, Flores D. Comprehensive characterization of anthracite fly ash from a thermo-electric power plant and its potential environmental impact. Int J Coal Geol 2011;86:204–12. doi:10.1016/j.coal.2011.01.010. [15] Pires M, Querol X. Characterization of Candiota (South Brazil) coal and combustion by-product. Int J Coal Geol 2004;60:57–72. doi:10.1016/j.coal.2004.04.003. [16] Querol X, Fernández-Turiel J, López-Soler A. Trace elements in coal and their behaviour during combustion in a large power station. Fuel 1995;74:331–43. doi:10.1016/0016-2361(95)93464-O. [17] Smolka-Danielowska D. Rare earth elements in fly ashes created during the coal burning process in certain coal-fired power plants operating in Poland - Upper Silesian Industrial Region. J Environ Radioact 2010;101:965–8. doi:10.1016/j.jenvrad.2010.07.001. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 22 [18] Fiket Ž, Medunić G, Furdek Turk M, Kniewald G. Rare earth elements in superhighorganic-sulfur Raša coal ash (Croatia). Int J Coal Geol 2018;194:1–10. doi:10.1016/j.coal.2018.05.002. [19] Hower JC, Dai S, Seredin VV, Zhao L, Kostova IJ, Silva LFO, et al. A Note on the Occurrence of Yttrium and Rare Earth Elements in Coal Combustion Products. Coal Combust Gasif Prod 2013;5:39–47. doi:10.4177/CCGP-D-13-00001.1. [20] Hower JC, Groppo JG, Joshi P, Dai S, Moecher DP, Johnston MN. Location of Cerium in Coal-Combustion Fly Ashes : Implications for Recovery of Lanthanides. Coal Combust Gasif Prod 2013;5:73–8. doi:10.4177/CCGP-D-13-00007.1. [21] Dai S, Xie P, Jia S, Ward CR, Hower JC, Yan X, et al. Enrichment of U-Re-V-Cr-Se and rare earth elements in the Late Permian coals of the Moxinpo Coalfield, Chongqing, China: Genetic implications from geochemical and mineralogical data. Ore Geol Rev 2017;80:1–17. doi:10.1016/j.oregeorev.2016.06.015. [22] Lin R, Howard BH, Roth EA, Bank TL, Granite EJ, Soong Y. Enrichment of rare earth elements from coal and coal by-products by physical separations. Fuel 2017;200:506– 20. doi:10.1016/j.fuel.2017.03.096. [23] Dai S, Jiang Y, Ward CR, Gu L, Seredin V V., Liu H, et al. Mineralogical and geochemical compositions of the coal in the Guanbanwusu Mine, Inner Mongolia, China: Further evidence for the existence of an Al (Ga and REE) ore deposit in the Jungar Coalfield. Int J Coal Geol 2012;98:10–40. doi:10.1016/j.coal.2012.03.003. [24] Gao J, Peng B, Fan H, Kang J. Solvent extraction kinetics of rare earth elements. Talanta 1996;43:1721–5. doi:10.1016/0039-9140(96)01954-6. [25] Pan J, Zhou C, Liu C, Tang M, Cao S, Hu T, et al. Modes of Occurrence of Rare Earth Elements in Coal Fly Ash: A Case Study. Energy and Fuels 2018;32:9738–43. doi:10.1021/acs.energyfuels.8b02052. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 23 [26] Cao S, Zhou C, Pan J, Liu C, Tang M, Ji W, et al. Study on Influence Factors of Leaching of Rare Earth Elements from Coal Fly Ash. Energy and Fuels 2018;32:8000– 5. doi:10.1021/acs.energyfuels.8b01316. [27] European Commission. Renewable Energy Road Map. Renewable energies in the 21st century: building a more sustainable future. Communication 2007, 18 p, Bryssels. doi:10.1080/13880290902938435. [28] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the composition and application of biomass ash. Part 1. Phase-mineral and chemical composition and classification. Fuel 2013;105:40–76. doi:10.1016/j.fuel.2012.09.041. [29] Thy P, Lesher CE, Jenkins BM, Michelle, Gras A, Shiraki R. Trace metal mobilization during combustion of biomass fuels. California Energy Commission Public Interest Energy Research Program Final Report 2008:128. [30] Zhang FS, Yamasaki S, Kimura K. Rare earth element content in various waste ashes and the potential risk to Japanese soils. Environ Int 2001;27:393–8. doi:10.1016/S0160-4120(01)00097-6. [31] Arbuzov SI, Maslov SG, Finkelman RB, Mezhibor AM, Ilenok SS, Blokhin MG, et al. Modes of occurrence of rare earth elements in peat from Western Siberia. J Geochemical Explor 2018;184:40–8. doi:10.1016/j.gexplo.2017.10.012. [32] Yliruokanen I., Lehto S. The occurrence of rare earth elements in some Finnish mires. Bull Geol Soc Finl 1995:27–38. doi:10.17741/bgsf/67.2.002. [33] Seredin VV. A new method for primary evaluation of the outlook for rare earth element ores. Geol Ore Depos 2010;52:428–33. doi:10.1134/S1075701510050077. [34] Dai S, Graham IT, Ward CR. A review of anomalous rare earth elements and yttrium in coal. Int J Coal Geol 2016;159:82–95. doi:10.1016/j.coal.2016.04.005. [35] Statistics Finland. Official Statistics of Finland (OSF): Energy supply and ACCEPTED MANUSCRIPT