The alkaline leaching process of arsenic-containing solid waste discharged during nonferrous metal smelting affords typical high-salinity alkaline arsenic-containing wastewater (HSAW). In this study, for the first time, Me (Ca2+ and Mg2+)-AsO43--OH--H2O and Me (Ca2+ and Mg2+)-AsO43--CO32--H2O systems are studied based on a thermodynamic equilibrium diagram and an arsenic removal experiment, proving that the removal of arsenic using single metal ions in the presence of CO32- is infeasible because of carbonate coprecipitation. Based on this observation, a new method that uses magnesium ammonium complex salts (MACSs) for HSAW treatment is proposed. Based on the thermodynamic calculations of the Mg2+-AsO43--NH4+-CO32--H2O system and the arsenic removal experiment, carbonate and arsenate can be selectively separated by the formation of magnesium ammonium arsenate (NH4MgAsO4·6H2O). In an arsenic solution containing 150-g/L Na2CO3, the arsenic removal rate and the arsenic grade of the precipitation product reach 90.16% and 27.13%, respectively, when the molar ratios of Mg2+/NH4+:As(V) are 1.8:1 and 2:1, respectively. The proposed method is successfully employed for treating a leaching solution of alkaline arsenic slag discharged during antimony smelting. The findings of this study will broaden the basic theory of HSAW treatment and lay a foundation for the resource treatment of arsenic-containing solid waste.
Keywords: High-salinity alkaline wastewater; Magnesium ammonium arsenate; Selective separation.
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