To accelerate the flocculation and sedimentation of the electronegative slimes and ultrafine tailings in Sijiaying, a fly-ash-based magnetic coagulant (FMC) was prepared by leaching fly ash in acid mine drainage. Univariate experiments, Plackett-Burman design, steepest-ascent method, and Box-Behnken design were employed to obtain the optimum levels of the factors for the maximum settled height (SH). In addition, the action mechanism of FMC was analyzed and the application effects in Sijiaying were evaluated. The results show that the neutralization and magnetic separation are the main action mechanism of FMC. Significant economic and environmental benefits will be brought by FMC through reducing the flocculant costs greatly, recycling the overflow water for beneficiation directly, and reusing the fly ash and acid mine drainage environmentally. (C) 2016 Elsevier B.V. All rights reserved.