CO₂ sequestration and CaCO₃ recovery with steel slag by a novel two-step leaching and carbonation method

The steel smelting process produces extensive CO₂ and Ca-containing steel slag (SS). Meanwhile, the low value utilization of steel slag results in the loss of Ca resources. CO₂ sequestration utilizing SS can reduce carbon emissions while achieving Ca circulation. However, conventional SS carbon sequestration methods suffer from slow reaction rates, finite Ca usage efficiency, and difficulty separating the CaCO₃ product from SS. Herein, an innovative two-step leaching (TSL) and carbonation method was presented based on the variations in leaching efficiency of activated Ca under different conditions, aiming at efficient leaching, carbon sequestration, and high-value reuse of SS. This method employed two NH₄Cl solutions in sequence for two leaching operations on SS, allowing the Ca leaching rate to be effectively increased. According to the findings, TSL could increase the activated Ca leaching rate by 26.9 % and achieve 223.15 kg CO₂/t SS sequestration compared to the conventional one-step leaching (CSL) method. If part of the CaCO₃ is recovered as a slagging agent, about 34.1 % of the exogenous Ca introduction could be saved. In addition, the CO₂ sequestration of TSL did not significantly decrease after 8 cycles. This work proposes a strategy that has the potential for recycling SS and reducing carbon emissions.