Study on the flotation separation efficiency of surfactants on the coal direct liquefaction residue raffinate - Jiali Guo; Xiaoshan Pei; Wenwen Dai; Panpan Fan; Ruizhi Dou; Changfu Feng; Qingyu Liu; Jiancheng Wang; Weiren Bao; Lianping Dong; Minqiang Fan (Fuel, 2026)