Comparative assessment of six alkaline industrial residues for CO2 storage via indirect mineral carbonation

Indirect mineral carbonation of alkaline industrial residues offers a dual benefit: permanent storage of CO2 and production of useful products, namely neutralized residues and precipitated calcium carbonate. This study systematically evaluates the process for residues from six different industries, enabling the identification of general trends and material-specific features. A two-step process using aqueous ammonium nitrate was employed, where calcium was first extracted by dissolving the feedstocks, and then precipitated as calcium carbonate by bubbling CO2 into the filtered solution. While the carbon uptake varied depending on feedstock mineralogy (50–300 kg of CO2 per ton), the developed models effectively described calcium extraction across all materials, based on the particle size and a dimensionless operating parameter — the calcium capacity ratio. Impurity release emerged as a critical factor, compromising the use of certain feedstocks: sulfates reduced precipitated calcium carbonate purity, copper gave it a blue tint, and alkaline impurities like KOH increased solution pH potentially hindering recyclability of the ammonium nitrate solution. Among the tested residues, steel slags and paper sludge incineration ash proved most promising, yielding high purity (99.5%) vaterite with precipitation efficiencies around 90%. Process performance maps were developed to identify optimal operating conditions while considering the environmental impact of solvent losses in the filtered solids. This study establishes a systemic framework for assessing feedstock suitability for indirect mineral carbonation and highlights the importance of future research into the effects of impurity accumulation during solvent recycling.