Enhancing CO₂ Mineralization Rate and Extent of Iron and Steel Slag via Grinding

<p>Roughly 10% of the CO₂ emissions from iron and steel making are attributable to the direct release of CO₂ from the thermal decomposition of carbonates to produce flux, mainly CaO, used for impurity removal. Notably, these direct emissions remain even if carbon-based steelmaking is replaced by hydrogen-based steelmaking. After removing impurities from the molten metal, this flux becomes the solid waste product called ‘slag’, a primarily Ca-silicate material. The transformation of slag back into carbonates is thermodynamically spontaneous with negative ΔG in the ambient environment, meaning that ~10% of the CO₂ emissions from iron and steel making could be negated if equipment and methods were developed to support CO₂ mineralization. However, the rate of CO₂ mineralization using slag is slowed by several environmental, geometric, and processing factors. We leverage an experimentally verified model of CO₂ mineralization to determine how to efficiently accelerate the process. Increasing the crystallinity of slag, increasing the relative humidity, and reducing the grain size of slag particles provide the greatest increase in CO₂ mineralization rate at the lowest energy penalty. Increasing the concentration of CO₂ and the temperature provide only modest increases in the CO₂ mineralization rate while incurring a substantial energy penalty. For steelmaking slags, CO₂ mineralization represents low-hanging fruit as the current reuse pathways are low value. For ironmaking slag, replacing the production of amorphous slag for the cement industry with the production of crystalline slag for CO₂ mineralization becomes financially preferable when a carbon price/tax exceeds 67.40 USD/t-CO₂.</p>