Quantification of the CO₂ mineralization potential of ironmaking and steelmaking slags under direct gas-solid reactions in flue gas

The potential for slag to reduce CO₂ emissions in the iron and steelmaking industry through CO₂ mineralization has long been recognized. Direct, gas-solid CO₂ mineralization has major benefits of simplicity, cost, and CO₂ accounting but has been historically plagued by ‘slow kinetics’. To determine the cause of the slow kinetics, 22 crystalline minerals and 13 amorphous compounds common to slag were synthesized and reacted with CO₂ in an incubator held at 30 °C, a relative humidity of 90%, and a molar CO₂ concentration of 5% and 20%. It was found that diffusivity through the product layer varies by ˜8 orders of magnitude between minerals, with several minerals displaying complete passivation after only a few nanometers of mineralization. Such unreactive materials can effectively occlude mineralization of more reactive minerals (‘mineral locking’). Quantitative theories were developed to determine the influence of mineral locking and diffusivity variability on the bulk CO₂ mineralization rate. By reducing the size ratio of slag particle to the internal mineral grains the effects of mineral locking can be removed. Such alteration can be achieved by grinding or by generating larger mineral grains via a slow solidification of molten slag. As grinding is ultimately a necessary activity of direct, gas-solid CO₂ mineralization, the mineral-specific grinding energy for 39 common slag compounds was calculated and used to determine the CO₂ emissions associated with grinding. These data, along with a modification to the Shrinking Core Model, were used to determine the rate of CO₂ mineralization and the net CO₂ mineralization of blast furnace, basic oxygen furnace, and electric arc furnace slag. Results indicate that direct, gas-solid CO₂ mineralization can be achieved in 1 h at very high net CO₂ mineralization efficiencies, especially when renewable energy is the power source and when slag has been slowly solidified. Globally, this method could provide gigatonnes of CO₂ emissions reduction by the end of the century.