Stabilization of basic oxygen furnace slag by hot-stage carbonation treatment

Treatment and disposal of Basic Oxygen Furnace (BOF) slag, a residue of the steel production process characterized by high basicity and propensity for heavy metal leaching, is a costly burden on metallurgical plants; a sustainable valorization route is desired. The stabilization of BOF slag utilizing hot-stage carbonation treatment was investigated; this approach envisions carbonation during the hot-to-cold pathway followed by the material after the molten slag is poured and solidified. Three experimental methodologies were employed: (i) in situ thermogravimetric analyzer (TGA) carbonation was used to assess carbonation reaction kinetics and thermodynamic equilibrium at high temperatures; (ii) pressurized basket reactor carbonation was used to assess the effects of pressurization, steam addition and slag particle size; and (iii) atmospheric furnace carbonation was used to assess the effect of carbonation on the mineralogy, basicity and heavy metal leaching properties of the slag. Free lime was found to be the primary mineral participating in direct carbonation of BOF slag. Initial carbonation kinetics were comparable at temperatures ranging from 500 to 800 °C, but higher temperatures aided in solid state diffusion of CO2 into the unreacted particle core, thus increasing overall CO2 uptake. The optimum carbonation temperature of both BOF slag and pure lime lies just below the transition temperature between carbonation stability and carbonate decomposition: 830–850 °C and 750–770 °C at 1 atm and 0.2 atm CO2 partial pressures, respectively. Pressurization and steam addition contribute marginally to CO2 uptake. CO2 uptake progressively decreases with increasing particle size, but basicity reduction is similar independent of particle size. The solubility of some heavy metals reduced after carbonation (barium, cobalt and nickel), but vanadium and chromium leaching increased.