Iron- and steelmaking processes produce a large quantities of greenhouse gas and metallurgical slag. Using Ca/Mg-rich phases in the slag to capture and bind the CO2 via mineral carbonation is a promising approach to reduction of emissions and solid wastes to be landfilled. However, Cr-bearing stainless steel slag (SSS) cannot straightforwardly be employed for carbon capture and storage (CCS) or rather carbon capture and utilization (CCU). For the dual-purpose of chromium immobilization and co-extraction of calcium and magnesium, a slag modification using added MnO is performed followed by an acid leaching treatment. Results show that the MnO content has a significant influence on the phase composition and element distribution of SSS. A Box-Behnken design (BBD) based acid leaching treatment of SSS is investigated and optimized. Second-order polynomial regression models that reveal a functional relationship between processing parameters and leaching yields of calcium and magnesium are established and verified by the analysis of variance (ANOVA). Model calculation results show a good agreement with the experimental data. The direct (linear) and cross-correlated effects of the processing parameters on the leaching yields are illustrated by three-dimensional (3D) response surfaces. The maximum leaching yields of calcium and magnesium obtained in this work are 65 % and 55 %, respectively, while for chromium the leached amounts are well below legislative limits. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.