Extraction of Ca(OH)2 from alkaline industrial wastes for use as absorbent of CO2 from air in negative emission technologies (NETs)

Calcium hydroxide could play a crucial role in absorbing CO 2 from the atmosphere through various active and passive direct air capture systems, helping achieve negative emissions. Additionally, Ca(OH) 2 can be directly applied to soil and water for enhanced weathering-based CO 2 removal. Significant amounts of calcium exist in industrial by-products and wastes, such as ashes, slags, and mine tailings—materials often landfilled or discarded. However, these materials often contain heavy metals that pose environmental risks due to potential leaching into the biosphere. Efficient extraction of calcium from such wastes would not only provide a useful resource but also reduce hazardous residues requiring disposal. Once extracted, calcium can be precipitated as Ca(OH) 2 through pH adjustment. Despite its potential, research on this process remains limited, as most studies have focused on direct mineral carbonation using concentrated CO 2 streams. This study investigates calcium extraction and Ca(OH) 2 precipitation from three industrial by-products: steel slag, iron-sponge process waste, and mine tailings. Calcium was leached using either HCl or NH 4 Cl, followed by two-stage pH adjustment with NH 4 OH and NaOH to separate impurities and precipitate Ca(OH) 2 , respectively. ICP-OES and XRD analysis showed that NH 4 Cl exhibited higher extraction selectivity, achieving 90–100% Ca(OH) 2 precipitation. In contrast, HCl extracted more calcium but had lower precipitation efficiency (71–83%) due to co-dissolution of impurities, causing some Ca-bearing species to precipitate with the heavy metals during the first precipitation stage. Although HCl produced more slaked lime (wet Ca(OH) 2 ), NH 4 Cl demonstrated strong calcium selectivity and minimal impurity leaching, making it competitive for treating mine tailings and iron sponge slag. Although residues from leaching iron-sponge slag with HCl are safe for disposal, smaller and more manageable solid residues from the intermediate stages may provide a practical alternative for treating harmful elements like sulfur, vanadium, and chromium, as they are not entirely leached in most cases.