Study on leaching behavior differences of rare earth elements from coal fly ash during microwave-assisted HCl leaching

ABSTRACT

Coal fly ash (CFA) as a potential resource for rare earth elements and yttrium (REY) has attracted extensive attention over the past few years. This paper focuses on studying microwave systems for the leaching of REY from CFA. Leaching parameters including hydrochloric acid (HCl) concentration, microwave heating time, microwave heating rate, and temperature were assessed. The optimal recovery of 74.91% REY, 86.77% light rare earth elements (LREY), and 34.79% heavy rare earth elements (HREY) are obtained at the conditions of 3 M HCl, T = 220°C, heating rate 39°C/min, and leaching time 30 min. The closed microwave system produces hydrogen chloride fluid or hydrogen chloride gas that is easier to react with CFA than hydrogen ions. It was revealed by the analysis of XRD, particle size, porosity, and SEM that fracture development benefits the entrance of leaching reagents into the inner layer of CFA. A positive linear correlation was found between leaching efficiency and REY³⁺ ionic radius. The stability of the REY-O bond also remains consistent with the ionic radius, leading to behavior differences during the leaching. This work was targeted to provide a theoretical and technological foundation for microwave-assisted HCl leaching for the extraction of REY in CFA.

KEYWORDS:

• Rare earth elements
• coal fly ash
• microwave-assisted leaching
• ionic radius
• waste utilization

CrediT authorship contribution statement

Hangchao Chen: Conceptualization, Methodology, Data curation, Formal analysis, Investigation, Writing-original draft. Zhiping Wen: Methodology, Visualization. Jinhe Pan: Conceptualization, Investigation, Validation, Writing-review & editing, Resources. Lei Zhang: Investigation, Software. Xin He: Writing-review & editing. Dmitry Valeev: Writing-review & editing Changchun Zhou: Project administration, Resources, Supervision.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplemental material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/19392699.2022.2163390.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China (NSFC) [No.92062109, 51974309, 52204292 & 42002184], the National Key R&D Program of China [No.2021YFC2902601], the National Postdoctoral Program for Innovative Talents [No. BX2021362], and Postdoctoral Science Foundation of China [No.2022M723393]. Special thanks are given to Prof. Ningning Zhang & his funding from NSFC [No. 51904240] and PhD Dmitry Valeev & his Project of the State Assignment (Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences, № FMUS-2019-24). Thanks should be given to Dr. Shulan Shi for her review works.