Metal recovery from iron slag via pH swing-assisted carbon mineralization with various organic ligands

Abstract

In this study, we suggest aqueous carbon mineralization of iron slag to selectively produce both solid carbonate and rare earth elements (REEs) under organic ligands such as acetate, propionate, and butyrate, which are the major derivatives of various biogenic wastes. An amorphous phase of iron slag was used as the feedstock, which was confirmed to be quartz and aluminosilicate, and its carbon mineralization was performed with a pH swing-assisted process. When conducting the aqueous carbon mineralization in a series of leaching, precipitation, and carbonation processes by elevating the pH from 3 to 10, all organic ligands exhibited higher leaching efficiencies and recovery yields for all metal species than the inorganic ligand (nitrate). The maximum leaching efficiencies of Ca, and Mg at pH ∼ 3 found in acetate were 63.55 and 97.35 wt%, respectively. The total rare earth elements (tREEs) including Y, La, Ce, Pr, and Nd recovered over 50 wt% for all ligands from the pristine iron slag; acetate, propionate, and butyrate exhibited tREEs recovery yields of 54.92, 51.10, and 53.76 wt%, respectively, while nitrate only showed 43.46 wt% in the precipitation step. These metal recovery enhancements are due to the distinct solubility of metal species and their co-precipitation behavior with organic ligands depending on the pH. Owing to the high Ca affinity of the organic ligands, structures of solid carbonates were dominantly controlled as a high-purity calcite, which contained less than 10 ppm of Fe.