Effect of novel surfactant emulsified collector and its chain length on enhanced flotation performance of coarse low rank coal: Experimental and molecular dynamics insights

The flotation separation of low-rank coarse coal remains a significant challenge in mineral processing owing to its highly oxygenated functionalities. Surfactant-based emulsified collectors have emerged as a promising approach for enhancing flotation performance. The current study proposes a novel emulsified collector by combining non-polar kerosene with mixed surfactants: nonionic Span 80 and cationic quaternary ammonium compounds (DTAC, TTAC, CTAC) under different alkyl chain lengths. The effectiveness and underlying mechanisms of these collectors were systematically evaluated through micro-flotation experiments, complemented by multi-scale characterization techniques, including XPS, FTIR, droplet size analysis, and molecular dynamics (MD) simulations. The experimental results demonstrate that the addition of conventional non-polar collectors is ineffective for the coarsest LRC fraction (1.00–0.75 mm), achieving a recovery of only 38.48 %, whereas the incorporation of cationic emulsified collectors results in a substantial enhancement in flotation performance. Longer-chain based surfactants (TTAC, CTAC) promoted pronounced, size–dependent recovery disparities (>90 % for the coarsest fraction); however, they also induce excessive emulsification, which leads to a marked depression in the recovery of intermediate particle sizes. Conversely, use of moderate-chain DTAC maintains optimal droplet size balance, robustly improving recovery across all coarse fractions (about 78 % for 1.00–0.75 mm). MD simulations confirm that the cationic headgroups effectively penetrate the hydration film, and anchor onto oxygen-rich sites via strong electrostatic interactions, thereby transforming the LRC surface from hydrophilic to markedly hydrophobic. In conclusion, optimizing surfactant chain length is crucial for achieving a balance between emulsion stability and effective coarse particle attachment. These findings provide theoretical insights and practical guidance for the rational design of advanced, high-efficiency flotation collectors tailored for the beneficiation of challenging, oxidized low-rank coals.