Comprehensive treatment of waste salts from electrolytic manganese metal industry and recovery of manganese, magnesium, and ammonium.

A large amount of water-soluble waste salts is produced from traditional electrolytic manganese metal industry (EMMWS), and landfill treatment of the EMMWS increases the risk of pollution of the surrounding water and soil. The EMMWS mainly comprises sulphates with highly active components that can be recycled as secondary resources. Herein, a simple and efficient process comprising four-stage precipitation and separation, two-stage crystallisation, molten salt treatment, and water-leaching is proposed for comprehensive treatment of the EMMWS, as well as the recovery of manganese, magnesium, and ammonium, systematically investing the feasibility of this strategy. The results indicate that 95.3% Mn in the saturated EMMWS solution containing NH3 can be selectively oxidised to Mn oxides by O2(g), and the residual Mn can be thoroughly removed to 0.31 mg/L with (NH4)2S2O8 oxidation. 92.78% Mg in the solution after Mn removal can be precipitated as 4MgCO3·Mg(OH)2·4H2O with NH4HCO3 and NH3·H2O, and the 98.81% of residual Mg can be removed as struvite with concentrated H3PO4. (NH4)2SO4 product can be obtained by crystallisation after Mn and Mg separation. 97.49% Mn in the Mn oxides can be converted into MnSO4 via (NH4)2SO4 molten salt treatment, and a light pink MnSO4·H2O product is obtained with water-leaching and crystallisation operations. Following the proposed process, the Mg, Mn, and NH4+/NH3 in EMMWS are recycled as 4MgCO3·Mg(OH)2·4H2O, struvite, MnSO4·H2O, and (NH4)2SO4 products. All the products obtained meet the relevant national standards. Economic analysis shows that a profit of US$282.45 might be achieved for each ton of EMMWS treated. This process is of great significance for the effective treatment of EMMWS and the recovery of manganese, magnesium, and ammonium resources.