Mineral carbonation of alkaline industrial residues such as steel slags has the potentialfor CO2utilisation and slag landfill volume reduction. Several studies have demonstrated the extraction of calcium from slag and carbonation to precipitated calcium carbonate (PCC). However, the fundamentals of the process are not well understood and multiple challenges associated with energyefficiency, economic viability, and PCC quality remain to be addressed. Important among the challenges are: improvement of the rate and extent of calcium extraction, reduction of impurity extraction during the dissolutionstep, solvent recycle, and control of quality parameters (such as particle size distribution and morphology) of PCC. In this study, we seek to understand the dissolution characteristics of steel slag in various strongly and weakly acidic media in order to investigate two process possibilities – indirect (two-step) carbonation, and pressure-swingcarbonation. The experimental results show that calcium and silicon from dicalcium silicate and tricalcium silicate phases undergo congruent dissolution, while brownmillerite phase is not susceptible to leaching. We do not notice any passivation effects (which has been strongly suggested in the literature) that retard calcium leaching either in strongor weakacids, including under CO2pressure. Reduction of particle size was found to increase the calcium extraction efficiency, and this could be explained on the basis ofthe influence of slag morphology on calcium selectivity. When leaching of impurities was suppressed, a comparable extent of calcium leaching was achievable for fine and coarse particle fractions of slag. Compared to the indirect carbonation route, pressure-swing carbonation seems to present some advantages; we noticed high CaCO3supersaturation and slow silica precipitation kinetics which has positive implications for the particle size distribution and product quality. Further advantages are the use of milder operating conditions, high extraction efficiency and the possibilityof producing a silica-PCCcomposite. Ionic strength and pH are identified to be primarydrivers for silica, calcite growth and their aggregation. We further extend the study to computer simulations of surface electrical properties of silica and calcite coupled with their growth kinetics to gain insights intotheir colloidal stability and silica-PCC composite formation.

Carbonation of steel slags: Combining two waste streams to create value

2019 Fall National Meeting

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