Indirect mineral carbonation of blast furnace slag with(NH_4)_2SO_4 as a recyclable extractant

Large quantities of CO₂ and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO₂ emission reduction and comprehensive utilisation of the solid waste. In this study, a recyclable extractant, (NH₄)₂SO₄, was used to extract calcium and magnesium from blast furnace slag (main phases of gehlenite and akermanite) by using low-temperature roasting to fix CO₂ through aqueous carbonation. The process parameters and efficiency of the roasting extraction, mineralisation, and Al recovery were investigated in detail. The results showed that the extractions of Ca, Mg, and Al can reach almost 100% at an (NH₄)₂SO₄-to-slag mass ratio of 3:1 and at 370 °C in 1 h. Adjusting the pH value of the leaching solution of the roasted slag to 5.5 with the NH₃ released during the roasting resulted in 99% Al precipitation, while co-precipitation of Mg was lower than 2%. The Mg-rich leachate after the depletion of Al and the leaching residue (main phases of CaSO₄ and SiO₂) were carbonated using (NH₄)₂CO₃ and NH₄HCO₃ solutions, respectively, under mild conditions. Approximately 99% of Ca and 89% of Mg in the blast furnace slag were converted into CaCO₃ and (NH₄)₂Mg(CO₃)₂•4H₂O, respectively. The latter can be selectively decomposed to magnesium carbonate at 100–200 °C to recover the NH₃ for reuse. In the present route, the total CO₂ sequestration capacity per tonne of blast furnace slag reached up to 316 kg, and 313 kg of Al-rich precipitate, 1000 kg of carbonated product containing CaCO₃ and SiO₂, and 304 kg of carbonated product containing calcium carbonate and magnesium carbonate were recovered simultaneously. These products can be used, respectively, as raw materials for the production of electrolytic aluminium, cement, and light magnesium carbonate to replace natural resources.