Factors affecting the carbon capture performance of steel slag and green liquor dregs in direct aqueous carbonation

Industrial alkaline side streams are attractive candidates for carbon capture via carbonation. Slags from steel manufacturing and dregs from the pulp and paper sector are abundant at industrial sites, offering the potential to offset the carbon dioxide emissions of these sectors. For this process to be techno-economically feasible it is crucial to minimize the energy expense while maximizing the carbonation efficiency. In this work, we studied the carbonation capacity of direct reduced iron slag, ladle furnace slag, electric arc furnace slag and green liquor dregs. Direct aqueous carbonation was conducted with a liquid to solid ratio of 20 L/kg and a gas mixture with 15% carbon dioxide and 85% nitrogen. The impact of three operating parameters was investigated: temperature, particle size and stirring of the mixtures prior to carbonation. It was found that for all the materials higher temperature significantly decreased the carbon dioxide uptake, due to the lower solubility of both carbon dioxide and calcium hydroxide in water. Porosity analysis showed that the steel slags have very low porosity on the micropore scale. Thus, the particle size also had a critical impact in the performance, while the effect was less prominent for green liquor dregs. Similarly, stirring the mixtures prior to carbonation allowed better wetting of the materials and lowering of particle size, which also had a positive effect. This study elucidates the impact of key operating parameters in direct aqueous carbonation with two types of industrial side-streams, and highlights the mechanisms through which they affect the carbonation process. • Temperature and particle size influence the carbon dioxide absorption capacity. • Particle size and stirring are beneficial for steel slags with low microporosity. • Increased temperature worsened carbonation due to the reduced solubility.