CO₂ sequestration utilizing basic-oxygen furnace slag: Controlling factors, reaction mechanisms and V-Cr concerns

Basic-oxygen furnace slag (BOF-slag) contains >35% CaO, a potential component for CO₂ sequestration. In this study, slag-water-CO₂ reaction experiments were conducted with the longest reaction duration extending to 96 hr under high CO₂ pressures of 100-300 kg/cm² to optimize BOF-slag carbonation conditions, to address carbonation mechanisms, and to evaluate the extents of V and Cr release from slag carbonation. The slag carbonation degree generally reached the maximum values after 24 hr slag-water-CO₂ reaction and was controlled by slag particle size and reaction temperature. The maximum carbonation degree of 71% was produced from the experiment using fine slag of ≤0.5 mm under 100°C and a CO₂ pressure of 250 kg/cm² with a water/slag ratio of 5. Vanadium release from the slag to water was significantly enhanced (generally >2 orders) by slag carbonation. In contrast, slag carbonation did not promote chromium release until the reaction duration exceeded 24 hr. However, the water chromium content was generally at least an order lower than the vanadium concentration, which decreased when the reaction duration exceeded 24 hr. Therefore, long reaction durations of 48-96 hr are proposed to reduce environmental impacts while keeping high carbonation degrees. Mineral textures and water compositions indicated that Mg-wüstite, in addition to CaO-containing minerals, can also be carbonated. Consequently, the conventional expression that only considered carbonation of the CaO-containing minerals undervalued the CO₂ sequestration capability of the BOF-slag by ~20%. Therefore, the BOF-slag is a better CO₂ storage medium than that previously recognized.