The surface waters contain suspended solids from the erosion of the borders. These solids may be removed through a coagulation step. The authors propose the use of power plant residues instead of classic coagulants. The extracts from acidic slag and flying ashes may be used for the removal of suspended solids from the residual wastewaters resulted in the mineral ores rinsing process. The researches were focused on: - establishing the possibility of using acid slag extracts (sulfuric and nhydrochloric) from Baia Mare and Neferal together with flying ashes as coagulants comparatively to the classic aluminum sulfate coagulant; - establishing the optimal reaction parameters (mainly pH) for these acid slag extracts.