NH₄Cl selective leaching of basic oxygen furnace slag: Optimization study using response surface methodology

The basic oxygen furnace slag (SL) was selectively leached by NH₄Cl. Response surface methodology and the central composite design (CCD) were employed in determining the optimal condition. The process parameters such as leaching duration, NH₄Cl concentration, liquid solid ratio (LSR), and leaching temperature were chosen as independent variables in CCD. The quadratic model was developed for process optimization and statistical experimental designs. It is found that the leaching duration, NH₄Cl concentration and liquid solid ratio are significant to the extraction of Ca²⁺ and Mg²⁺. The optimal conditions with setting maximum extraction of calcium and magnesium cations from SL are as following: 68 min (X₁), 1.8 mol/L NH₄Cl (X₂), LSR 12 mL/g (X₃), 53°C (X₄), with the predicted Ca²⁺ and Mg²⁺ concentration reaching 13.3 and 3.2 g/L, respectively. The proposed simplified leaching mechanism indicates a two-stage leaching. The prone leachable MgFe₂O₄ and Ca₂Fe₂O₅ are extracted in the fast stage, while the extraction of CaO occurs throughout slow stage.