Simultaneous immobilization of NH4+ and Mn2+ from electrolytic manganese residue using phosphate and magnesium sources

Abstract

Immobilization of contaminants from electrolytic manganese residue (EMR) is essential for the safe stacking and reuse of EMR. This study provides experiment results for the simultaneous immobilization of NH₄⁺ and Mn²⁺ from EMR using Na₃PO₄·12H₂O and MgSO₄·7H₂O (PS) agents, as well as Na₃PO₄·12H₂O and MgO (PO) agents. The optimum reaction conditions, characteristics of immobilization, mechanism and the economy of alternative chemicals were determined and are discussed. The results indicated that the immobilization efficiencies of NH₄⁺ and Mn²⁺ were 92.4% and 99.9% respectively under the following conditions: a MgSO₄·7H₂O : Na₃PO₄·12H₂O : EMR mass ratio of 0.113 : 0.175 : 1, a CaO : EMR mass ratio of 0.03 : 1 and a reaction time of 1 h using PS agents. The concentration of NH₄⁺ in the leach liquor reduced from 1264 to 98 mg L⁻¹ after immobilization. The concentration of heavy metal ions decreased sharply in the leach liquor and met the Integrated Wastewater Discharge Standard of China (GB8978-1996). The characteristics of immobilization indicated that NH₄⁺ was immobilized to form NH₄MgPO₄·6H₂O and that Mn²⁺ was immobilized to form Mn₅(PO₄)₂(OH)₄, Mn₃(PO₄)₂·3H₂O and Mn(OH)₂. An economic evaluation showed that using PS agents had lower associated cost than using PO agents.