Rare-Earth Elements Extraction from Low-Alkali Desilicated Coal Fly Ash by (NH₄)₂SO₄ + H₂SO₄

Area of Science:

• Materials Science
• Metallurgical Engineering
• Environmental Science

Background:

• Coal fly ash (CFA) is a refractory waste containing valuable alumina and rare-earth elements (REEs).
• REEs in CFA are often encapsulated within a core-shell structure involving mullite and amorphous aluminosilicates.
• Efficient extraction of REEs from CFA is crucial for resource recovery and waste valorization.

Purpose of the Study:

• To develop an optimized method for enhancing rare-earth element (REE) extraction from coal fly ash (CFA).
• To investigate the effect of pre-dissolving amorphous aluminosilicates on REE leaching efficiency.
• To evaluate the leaching kinetics and mechanisms for Al and REEs from modified CFA.

Main Methods:

• Pre-treatment of CFA to dissolve amorphous aluminosilicates, forming a low-alkali mullite concentrate.
• Sulfuric acid leaching of the concentrate with ammonium sulfate addition.
• Analysis of leaching parameters: temperature, liquid-to-solid ratio, and time.
• Characterization of leached residues using XRD, XRF, SEM-EDS, and LD.

Main Results:

• Pre-treatment significantly accelerated REE extraction, achieving 70-80% recovery of Sc and other REEs within 5 hours.
• Direct sulfuric acid leaching of raw CFA yielded less than 20% REE extraction under identical conditions.
• Optimized leaching (110 °C, L/S=10, 120 min) resulted in >60% total REE extraction with <30% Al extraction.
• Leaching kinetics studies confirmed enhanced REE transfer into the soluble phase.

Conclusions:

• Dissolving amorphous aluminosilicates prior to sulfuric acid leaching is an effective strategy to enhance REE recovery from CFA.
• The developed method offers a selective approach, maximizing REE extraction while minimizing aluminum co-extraction.
• This research provides a viable pathway for the sustainable utilization of coal fly ash as a source of critical REEs.