Preferential Recovery of Rare-Earth Elements from Coal Fly Ash Using a Recyclable Ionic Liquid

Area of Science:

• Materials Science
• Environmental Science
• Chemical Engineering

Background:

• Global geopolitical tensions highlight the critical need for secure rare-earth element (REE) supply chains.
• Coal fly ash (CFA) presents a potential domestic source of REEs, but conventional extraction methods are inefficient and environmentally taxing.
• Existing REE recovery techniques from CFA are energy-intensive and lack selectivity, leading to indiscriminate leaching of elements.

Purpose of the Study:

• To develop a novel, efficient, and selective process for extracting rare-earth elements (REEs) from coal fly ash (CFA).
• To investigate the use of a specific ionic liquid, betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]), for preferential REE extraction.
• To evaluate the impact of pretreatment and process conditions on REE leaching efficiency and selectivity.

Main Methods:

• Utilized the thermomorphic properties of the ionic liquid [Hbet][Tf2N] with water for REE extraction from CFA.
• Employed a proton-exchange mechanism for leaching REEs upon heating the IL-water mixture.
• Separated REEs by cooling the mixture, followed by mild-acid stripping to recover elements and regenerate the ionic liquid.
• Investigated the effects of alkaline pretreatment and additional betaine on leaching efficiency and separation.

Main Results:

• The [Hbet][Tf2N] ionic liquid demonstrated preferential extraction of REEs over bulk elements from various CFAs.
• Alkaline pretreatment significantly enhanced REE leaching from Class-F CFAs.
• The process showed high extraction efficiency for scandium across different CFA types.
• Class-C CFA exhibited higher leaching but lower selectivity compared to Class-F CFA.

Conclusions:

• The developed ionic liquid-based process offers an efficient and selective method for recovering REEs from coal fly ash.
• This approach provides a promising pathway for sustainable REE sourcing, particularly for critical elements like scandium.
• The thermomorphic behavior of [Hbet][Tf2N] and optimized conditions enable effective REE valorization from industrial byproducts.