UTILIZATION OF STEEL SLAG FOR PERMANENT CO2 SEQUESTRATION AS CaCO3: PERFORMANCE EVALUATION OF DIRECT AND INDIRECT CARBONATION PATHWAYS

High-calcium steelmaking slag has great potential to be a low-cost, green CO₂ sorbent because of its high CaO content and favorable characteristics. Based on the available experimental data as well as from characterization studies performed on the steelmaking slag, the following conclusions can be drawn: 1. There is a large amount of CaO (43.66 wt%) in steelmaking slag and it is alkaline in nature, which makes it a good candidate for capturing CO₂ and remediating the environment. 2. Two types of carbonation methods were evaluated: direct carbonation (gas–solid reaction) and indirect carbonation (soluble calcium removed from the solid). Each method has its own advantages. 3. The length of time the CO₂ is in contact with the steelmaking slag has a dramatic impact on the efficiency of carbon capture. Most effective carbon capture occurs when CO₂ contacts the steelmaking slag between 10 and 15 minutes. 4. When the length of exposure to the steelmaking slag exceeds 15 minutes, the carbon capture efficiency decreases, due to the saturation of active sites on the steelmaking slag and the formation of a CaCO₃ layer that inhibits further CO₂ diffusion into the steelmaking slag. 5. Reducing the size of the steelmaking slag particles maximises the carbon capture efficiency because it provides a larger surface area and therefore a better gas–solid interaction. 6. Characterisation studies indicate that highly reactive phases (CaO and CaFe₂O₄) and a porous microstructure exist in the steelmaking slag and contribute to the reactivity and effective capture of carbon dioxide by the steelmaking slag. 7. Though direct carbonation is simple and inexpensive, the rate of reaction is limited by the time required for diffusion of the gases through to the steelmaking slag. 8. The indirect method of carbonation, while more complex than the direct method, makes more calcium available to react with the CO₂ and significantly improves mass transfer, achieving approximately 69–70% efficiency for CO₂ capture. 9. CaCO3 is a stable end product, providing the potential for a value-added use, whilst also substantially lowering atmospheric carbon dioxide emissions. The method also provides a sustainable means of utilising waste materials and aligns with the principles of a circular economy by converting industrial byproducts into beneficial resources.  In terms of providing an effective means of CO2 capture from steelmaking slag, the combination of direct and indirect carbonation provides greater efficiency and also improves the flexibility of the overall process.

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