Extraction of Calcium from Steel Slag via Low-Temperature Chlorination Roasting and Water Leaching for the Preparation of CaCO3

Abstract This paper proposes a novel coupled process of “low-temperature chlorination roasting with NH4Cl─water leaching for calcium separation─carbonation precipitation” to achieve selective recovery and utilization of calcium from steel slag. Thermodynamic calculations indicate the presence of two reaction pathways in the system: solid–solid chlorination and gas–solid chlorination mediated by HCl. Among these, the gas-phase chlorination reaction exhibits a stronger thermodynamic spontaneity. Phase analysis of the raw steel slag shows that calcium is primarily found in the phases of Ca2SiO4, Ca2Fe2O5, and Al14Ca12O33, with a positive correlation to the contents of silicon, iron, and aluminum. The roasting temperature and holding time have been identified as the core factors affecting the calcium chlorination rate. Under optimal conditions (temperature of 300 °C, holding time of 40 min, mass ratio of NH4Cl to steel slag of 2:1, and steel slag particle size of 75–106 μm), the calcium chlorination rate can reach 92.47%. Kinetic analysis based on the unreacted core shrinking model confirms that this chlorination process is controlled by the diffusion of the solid product layer. The resulting calcium-containing leachate, after carbonation precipitation, yields well-formed, uniformly shaped rhombohedral calcite particles.