Abstract: Fly ash can be used to prepare composite flocculant polyaluminum ferric chloride (PAFC),as it contains aluminum and iron.But the existence of aluminum-silicon in fly ash is in the form of complex vitreous mullite phase,which is hardly soluble in acidic solutions.Therefore,it is necessary to sinter fly ash to destroy SiO₂—Al₂O₃ bonds so as to improve its solubility in acid.Proper amount of red mud was added to fly ash to adjust  the ratio of aluminum and iron.Finally,PAFC flocculant could be prepared with modified fly ash and hydrochloric acid as main raw materials.Experimental results showed that the optimum activation conditions of fly ash were as follows:mass ratio fly ash to red mud was 0.3,sintering temperature was 750 ℃,and sintering time was 2 h;the optimum aluminum and iron leaching conditions were as follows:concentration of hydrochloric acid was 7 mol/L,liquid-to-solid ratio was 3.5 mL/g,reaction temperature was 85 ℃,and reaction time was 2.0 h.Under those conditions the leaching rate of aluminum and iron reached 90.5% and PAFC flocculant could be obtained after aging the as-prepared solution for 18 h.Flocculation tests showed that PAFC is superior to either PFC or PAC.

Key words: polyaluminum ferric chloride, fly ash, flocculant, red mud