Study of performance and mechanism of treating high concentration stimulated fluoride-containing wastewater with Al-rich refining slag
Al-rich refining slag with CaO and Al2O3 as main components was used to treat the high concentration simulated fluoride-containing wastewater.The influences of temperature,pH value,dosage,and fluoride ions concentration on the fluoride ions removal efficiency were investigated in this study.Besides,the removal mechanism was clarified through measuring the change of total hardness in stimulated wastewater,in combination with XRD,SEM,and XPS analysis.The results show that the total CaO content of the Al-rich refining slag in this study exceeds 50％and its main mineral phase is Ca12 Al14 O33(C12 A7).The slightly acid pH value and high reaction temperature(60-80 ℃)are beneficial for the removal of fluoride ions.The removal ratio of fluoride ions in 500 mg/L and 1 000 mg/L simulated fluoride-containing wastewater can reach over 99％at pH of 7,temperature of 70 ℃,reaction time of 20 min and dosage of 15 g/L,and the concentration of fluoride ions in the effluent can be less than 10 mg/L,which indicates that Al-rich refining slag has a better treatment effect than the common precipitation agent such as lime and calcium chloride.The removal mechanism of fluoride ions from wastewater by aluminum rich refining slag mainly includes chemical precipitation and chemical bonding.Ca2+released from the hydrolysis of C12A7 in slag can react with fluoride ions in stimulated wastewater to form CaF2 precipitation;in addition,coordination bonds can form between Al(OH)-4 generated by the first step hydrolysis reaction of C12 A7 and fluoride ions in stimulated wastewater,resulting in the removal of fluoride ions.Moreover,Al(OH)3 formed by the second hydrolysis reaction of C12 A7 may play a flocculation role for fluoride ions removal.Therefore,it can be concluded that fluoride ions removal ratio is closely associated with the hydrolysis extent of C12A7,that is,closely associated with the total hardness.
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