In this study, several fly ash (FA)-based composite coagulants, leached by hydrochloric acid, were prepared to treat coal washing wastewater. The concentrations of Al3+ and Fe2+/Fe3+ in the leachates and coagulants were analyzed, and optimal experimental conditions, including coagulant dosage and initial pH, were determined using various analytical techniques (scanning electron microscopy, energy dispersive spectrometry. X-ray diffraction (XRD). X-ray fluorescence (XRF), particle-size analysis, zeta potential, pH and conductivity measurements). A suspended solids (SS) and chemical oxygen demand (COD) removal efficiency from the effluent treated by one of the coagulants reached 99.61% and 96.48%, respectively, at dosages of 10g l(-1) (initial pH of 9, adjusted by CaO). This indicates that the coagulant was an effective agent for coal washing wastewater treatment, and that the leached Al3+ and Fe3+ and introduced Ca2+ may have improved the coagulation process. Analysis of the dry sludge composition and slurry particle size distribution of the coal washing wastewater showed that charged colloidal particles and the fine particle distribution in the coal washing wastewater make the wastewater treatment a difficult process. Results from this study could provide a novel approach for the treatment of coal washing wastewater and coal fly ash utilization. (C) 2011 Elsevier B.V. All rights reserved.