Enhanced removal of Mn2+ and NH4+-N in electrolytic manganese residue leachate by electrochemical and modified phosphate ore flotation tailings

The low concentrations of Mn2+ and NH4+-N in electrolytic manganese residue leachate (EMRL) have seriously hindered the development of the electrolytic manganese industry. In this study, an electrochemical process combined with H2C2O4-modified phosphate ore flotation tailings (MPOFTs) for the synergistic removal of Mn2+ and NH4+-N in EMRL was investigated. The results showed that the substances CaMg(CO3)2, CaSO4·2H2O, SiO2, CaZn(CO3)2, CaMg0.77Fe0.23(CO3)2 and (Ca5(PO4)3F) in POFTs were decomposed by H2C2O4 and that Ca2+, PO43- and Mg2+ reacted with C2O42- and PO43- to form Ca(COO)2·H2O, MgHPO4·3H2O and MgCa2(PO4)2·2H2O. The concentration of NH4+-N decreased from 74.5 mg/L to 2.89 mg/L, and Mn2+ decreased from 165 mg/L to 0.3 mg/L, when the current density was 15 mA/cm2, the dosage of MPOFTs was 5 g/L, the reaction temperature was 293 K, the reaction time was 2.5 h, and the initial pH was 9. The removal mechanism indicated that Mn2+ was removed in the form of Mn(OH)2,Mn3O4, MnOOH, Mn5(PO3)OH2(PO4)2·4H2O and Mn3(PO4)2·7H2O. NH4+-N was removed by MgNH4PO4·6H2O and Mg3(NH4)2(HPO4)4·8H2O. This study provides technical support for harmless treatment of EMRL and resource utilization of POFT.