Research on selective leaching of calcium from steelmaking slag with NH₄Cl as leaching solution

Utilizing calcium containing steelmaking slag (SS) to mineralize and solidify CO 2 is expected to achieve dual benefits of carbon reduction and solid waste utilization. The key factor affecting the carbon sequestration efficiency of indirect mineralization method is to achieve the efficient selective leaching of Ca from SS. In this paper, the effects of various factors on the Ca leaching process and the leaching mechanism were investigated. The orthogonal experiment results show that the concentration of NH 4 Cl solution and the solid–liquid ratio are the main controlling factors affecting the Ca leaching behavior. Under the optimized conditions of NH 4 Cl concentration of 2 mol/l, reaction time of 90 min, reaction temperature of 70°C, solid–liquid ratio of 1:35 and SS particle size less than 80 μm, the leaching rate of Ca reaches 55.9%. The phase changes during the leaching process of SS indicate that the main phases involved in the leaching reaction are tricalcium silicate (C 3 S) and dicalcium silicate (C 2 S). Besides, it is found that a small amount of monticellite (CaMgSiO 4 ) and calcium magnesium rhodonite (Ca 3 Mg(SiO 4 ) 2 ) are dissolved, while dicalcium ferrite (C 2 F) and RO phase are difficult to dissolve. It is proved that the NH 4 Cl solution exhibited better selective leaching ability for Ca in SS.