Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2023 Apr 4;57(13):5414-5423.
doi: 10.1021/acs.est.2c09273. Epub 2023 Mar 21.

Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash

Pan Liu  1 , Simin Zhao  1 , Nan Xie  1 , Lufeng Yang  2 , Qian Wang  1 , Yinghao Wen  1 , Hailong Chen  2 , Yuanzhi Tang  1
Affiliations

Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash

Pan Liu et al. Environ Sci Technol. .

Abstract

Due to the growing demands of rare earth elements (REEs) and the vulnerability of REEs to potential supply disruption, there have been increasing interests in recovering REEs from waste streams such as coal fly ash (CFA). Meanwhile, CFA as a large industrial waste stream in the United States (U.S.) poses significant environmental and economic burdens. Recovery of REEs from CFA is a promising solution to the REE scarcity issue and also brings opportunities for CFA management. This study demonstrates a green system for REE recovery from Class F and C CFA that consists of three modules: REE leaching using citrate, REE separation and concentration using oxalate, and zeolite synthesis using secondary wastes from Modules I and II. In Module I, ∼10 and 60% REEs were leached from the Class F and C CFA samples, respectively, using citrate at pH 4. In Module II, the addition of oxalate selectively precipitated and concentrated REEs from the leachate via the formation of weddellite (CaC2O4·2H2O), while other trace metals remained in solution. In Module III, zeolite was synthesized using wastes from Modules I and II. This study is characterized by the successful recovery of REEs and upcycling of secondary wastes, which addresses both REE recovery and CFA management challenges.

Keywords: coal fly ash; rare earth elements; resource recovery; waste management; zeolite.

PubMed Disclaimer

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
XRD patterns of the raw CFA samples and their products for (a) F-1 CFA and (b) C-1 CFA. From top to bottom: (1) raw CFA samples, (2) CFA samples after REE leaching using citrate (pH 4.0, 50 mM citrate, and liquid-to-solid ratio of 200 mL/g), (3) REE-rich oxalate products after REE separation using oxalate, and (4) zeolite products after hydrothermal synthesis at 150 °C. Vertical gray shadings indicate dissolved mineral phases after leaching. Red and blue bars are powder diffraction standards: hydroxy-sodalite ([Na1.08Al2Si1.68O7.44·1.8H2O], PDF 31-1271), tobermorite ([Ca5(OH)2Si6O16·4H2O], PDF 19-1364), and weddellite ([CaC2O4·2H2O], PDF 17-0541). Q (quartz, [SiO2]), M (mullite, [Al6Si2O13]), A (anhydrite, [CaSO4]), P (periclase, [MgO]), L (lime, [CaO]), T (tricalcium aluminate, [Ca3Al2O6]), Hm (hematite, [Fe2O3]), Wh (whewellite, [CaC2O4·H2O]), and H (halite, [NaCl]).
Figure 2
Figure 2
Influence of citrate concentration on metal leaching from (a) F-1 and (b) C-1 CFA samples. Leaching condition: 4 h, pH 4, liquid-to-solid ratio of 200 mL/g, and citrate concentrations at 0 (blank), 10, 50, and 100 mM; room temperature.
Figure 3
Figure 3
Fraction of metals remained in the citrate leachate as a function of added sodium oxalate. Leaching solutions of panels (a) and (b) are from F-1 and C-1 CFA samples, respectively. After each oxalate addition, the whole system was allowed to react for 30 min.
Figure 4
Figure 4
(a) Enrichment factor of metals in oxalate products compared to their corresponding concentrations in raw CFA samples F-1 and C-1. (b) Percentage of critical REEs (Nd, Eu, Tb, Dy, Y, and Er) vs total REEs of raw CFA samples and oxalate products. Gray points in panel (b) are summarized United States (U.S.) CFA samples from Taggart et al.
Figure 5
Figure 5
SEM images of the zeolite products after hydrothermal synthesis at 150 °C for samples (a) F-1 and (b) C-1.
See this image and copyright information in PMC

References

    1. Dutta T.; Kim K.-H.; Uchimiya M.; Kwon E. E.; Jeon B.-H.; Deep A.; Yun S.-T. Global demand for rare earth resources and strategies for green mining. Environ. Res. 2016, 150, 182–190. 10.1016/j.envres.2016.05.052. - DOI - PubMed
    1. Long K. R.; Van Gosen B. S.; Foley N. K.; Cordier D.. The Principal Rare Earth Elements Deposits of the United States: A Summary of Domestic Deposits and A Global Perspective. In Non-Renewable Resource Issues; Springer, 2012; pp 131–155.
    1. Tracy B. An Overview of Rare Earth Elements and Related Issues for Congress. Congr. Res. Service 2020, 1–16.
    1. Seredin V. V.; Dai S. Coal deposits as potential alternative sources for lanthanides and yttrium. Int. J. Coal Geol. 2012, 94, 67–93. 10.1016/j.coal.2011.11.001. - DOI
    1. Franus W.; Wiatros-Motyka M. M.; Wdowin M. Coal fly ash as a resource for rare earth elements. Environ. Sci. Pollut. Res. 2015, 22, 9464–9474. 10.1007/s11356-015-4111-9. - DOI - PMC - PubMed

Publication types