Applying hybrid coagulants and polyacrylamide flocculants in the treatment of high-phosphorus hematite flotation wastewater (HHFW): Optimization through response surface methodology

High-phosphorus hematite flotation wastewater (HHFW) is difficult to treat, because it possesses the properties of high concentration suspended solid (SS) and organic pollutant, high alkalinity and strong color. This study was conducted to investigate the treatment of HHFW using coagulation/flocculation process with the mixture of ferric chloride (FC) and polyaluminum chloride (PAC) as coagulants and polyacrylamide (PAM) as flocculants. Results show that a hybrid coagulant (B4), the mixture of FC/PAC in the Fe/Al molar ratio of 3:7, was found to be the most efficient in the coagulation process. PAM flocculants, such as cationic (PAM-C), non-ionic (PAM-N) and anionic (PAM-A), will improve the turbidity removal efficiency. The dosage of the coagulant B4 and the PAM flocculants was optimized using two statistic tools, central composite design (CCD) and response surface methodology (RSM). According to the analysis of variances (ANOVA), three quadratic models with high regression coefficient (R²) were obtained. Through RSM optimization, a minimum residual turbidity of 0.8 NTU was achieved when B4 and PAM-N were in the dosage of 356 mg L^-1 and 5.4 mg L^-1, respectively. The concentration of iron, aluminum and COD in the purified HHFW were 0.055 mg L^-1, 0.184 mg L^-1 and 33.92 mg L^-1, respectively. This study demonstrated that FC/PAC hybrid coagulant, coupling with PAM-N flocculant, has a great potential and economically feasible to the treatment of industrial wastewater containing high turbidity and high alkalinity.