Advancements and prospects for emulsion collectors in low-rank/oxidized coal flotation separation

Emulsion, including oil-in-water (O/W) and water-in-oil (W/O) emulsions, has garnered significant attention for coal flotation separation due to its potential to enhancing dynamic processes. However, with the consumption of high-quality coal resources and the implementation of the worldwide carbon reduction strategy, large-scale separation of low-quality coal (low-rank/oxidized coal) resources have become an important choice to ensure energy supplement and green development of coal industry. This review introduces the background and importance of emulsion collector technology in low-rank/oxidized coal flotation and analyzes its fundamental principles, including collector-particle-bubble interactions involving multi-scale interfacial mechanics. Then, the review discusses research progress in various emulsion collectors, such as traditional coarse emulsion, microemulsion, nanoemulsion, high-internal phase emulsion and Pickering emulsion, from high/low energy emulsification selection, emulsion formulation optimization and oil-water interfacial behavior aspects. Specifically, the emulsion collectors can effectively enhance the dispersibility of traditional oily collectors and decrease their dosages, as well as improve the attachment efficiency and hydrophobicity of coal surface, thereby enhancing the flotation separation effects. The complicated physicochemical enhancing mechanisms between emulsion collectors, coal particles and flotation bubbles are systematically described and warrant further in-depth investigation. This review concludes with a summary of the prospects of emulsion technology in improving low-rank coal flotation efficiency, reducing agent consumption and mitigating environmental impacts. It proposes future research directions, including enhanced emulsion structure-activity relationship analysis, targeted emulsifiers design, flotation dynamic process optimization, and large-scale application, to promote commercialization and sustainable development.