Synthesis of High-Purity and Stable Vaterite Via Leaching–Carbonation of Basic Oxygen Furnace Slag

Against the backdrop of the limited industrial application and low added value of precipitated calcium carbonate produced through the indirect carbonation of basic oxygen furnace slag (BOFS), this study proposed a simple approach (extracting Ca2+ from BOFS using ammonium chloride (NH4Cl) and subsequently optimizing the aqueous carbonation process) for synthesizing high-purity vaterite with greater added value. The influencing factors in the enhancement of Ca2+ leaching efficiency and maximization of carbonation to sequester CO2 were studied. Within a very short time (10 min), a Ca2+ leaching ratio of 60.3% was achieved, while a 99.5% carbonation ratio was attained by adding NH4OH to produce high-purity (98%) homogeneous vaterite. The alkaline pH (∼10.6) of the solution was refined using ammonia to achieve a higher CO32–/Ca2+ ratio and solution supersaturation for the stimulation of vaterite nucleation. The adsorption of NH4+ and metal cations from BOFS on the surface of vaterite acted as a crystalline control agent to inhibit the transformation of vaterite into calcite. In addition, the carbonation filtrate can be recycled at least six times with a sustained extraction-carbonation efficiency of over 60% while maintaining the vaterite crystalline integrity. The solid residue was mainly composed of silica gel and inert reducible oxide (RO) phase, which can be used as a potential supplementary cementitious material due to its pozzolanic properties. Findings from this study provide new insights into the valorization of BOFS to mitigate CO2 emissions and promote the sustainable industrial application of the BOFS indirect carbonation process.