Preparation and coagulation performance of poly-ferric-aluminum-silicate-sulfate from fly ash

Poly-ferric-aluminum-silicate-sulfate (PFASS) with various (Al + Fe)/Si molar ratios was prepared from fly ash to evaluate their coagulation efficiencies in oily wastewater, and the structure and morphology of which were characterized by infrared spectra (FTIR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Therefore, the results indicate that 16:1 (Al + Fe)/Si mole ratio is beneficial to forming of Si–O–Al and Si–O–Fe bonds, PFASS consists of NaAl(SO4)2·(H2O)6 and other crystals, the morphology of PFASS is largely influenced by (Al + Fe)/Si molar ratio, and when the (Al + Fe)/Si mole ratio is 14:1 or 12:1, a chain-net structure with three dimensions appears. Moreover, the coagulation experiment results show that the removals of turbidity and COD from oily wastewater are affected by coagulant dosage and (Al + Fe)/Si mole ratio of PFASS. Too much or too little PFASS is not conducive to the coagulant efficiency, and the optimal (Al + Fe)/Si mole ratio is 12:1 for the turbidity removal and 16:1 for COD removal. Nevertheless, when the dosages of PFASS reach the effective coagulation levels in the oily wastewater, COD are not reduced along with turbidity removal.

Research highlights
► Poly-ferric-aluminum-silicate-sulfate (PFASS) consists of NaAl(SO4)2·(H2O)6 and other crystals.
► The morphology of PFASS is largely influenced by (Al+Fe)/Si molar ratio.
► The optimal (Al+Fe)/Si mole ratio is 12:1 for the turbidity removal and 16:1 for COD removal.