<p>Steel slag (SS) has a high annual output and low utilization rate. Since SS mainly contains Ca, Fe, Si, Al, etc., in this work, formic acid (FA) leaching method is used to leach Ca²⁺ and Fe³⁺ from SS. The effects of leaching time, temperature, FA concentration, and liquid–solid ratio on the leaching efficiency of Ca²⁺ and Fe³⁺ are investigated. The leaching mechanism and leaching kinetics of FA on SS are analyzed. The results show that the optimal leaching conditions are as follows: time = 6&#xa0;h, FA concentration = 20%, T = 60&#xa0;°C, liquid–solid ratio = 10. The leaching rates of Ca²⁺ and Fe³⁺ are 97% and 54.61%, respectively. Shrinking core model (SCM) analysis shows that for Ca²⁺ leaching, the kinetics of the reaction is controlled by mixing model in the fast reaction processes at 25 and 40&#xa0;°C and controlled by the chemical reaction at 60&#xa0;°C. In the slow reaction processes, the leaching of Ca²⁺ is controlled by the mixing model. The leaching process of Fe³⁺ shows similar kinetic model. Subsequently, nano-CaCO₃ is prepared by wet carbonization, and the purity and average particle size are 96.82% and 70.50&#xa0;nm. The value-added utilization of SS is effectively achieved by a simple leaching and wet carbonization technique.</p> Graphical Abstract <p></p>