Formation of Layered Double Hydroxide By MgO Neutralization Process And Its Application to Water Treatment of Fluorine Contaminated Effluent

Abstract

Removal of fluorine in effluent with low concentration is difficult technique and an effective method has not been established. This paper describes a novel fluorine-removal process that composed the following processes, addition of Al(III) source to feed water, conditioning of the slurry with magnesium oxide (MgO), neutralizing reaction, returning the settled slurry, and gravitational settling of the solids. This process allows the formation of a mixed Mg(II)-Al(III) hydroxide precipitate, which gradually develops into Layered Double Hydroxides (LDH) compound. MgO, which serves as both the neutralizer and the Mg(II) source, contributes to both the excellent settling and dewatering characteristics of the resulting solids. The sludge returning technique, well-known as High Density Sludge (HDS) method, is employed to prolong the sludge retention time to dissolve as much MgO as possible. Repeated batch-wise fluorine-removal tests showed that the residual fluorine concentration gradually decreased according as the number of batch test increased. Optimum fluorine removal ratio reached 96 %, where initial fluorine concentration 20 mg/L decreased below the national environmental standard, 0.8 mg/L. Powder X-ray diffraction analysis indicated the presence of both LDH phase and Periclase phase in the formed solids. Considering the good settling and dewatering characteristics of solids, the proposed process served as alternative treatment technique for low concentrated fluorine-containing effluents.