Extraction of Ca(OH)2 from industrial by-products for the use in a passive-DAC system for negative emissions

Abstract

Industrial alkaline by-products, originating from the steel and mining industry are produced in large amounts in Sweden and globally. These materials may contain significant fractions of MgO and CaO, in addition to other elements, e.g. Fe, Al, and Si. In a recently proposed passive-DAC system, hydrated CaO (Ca(OH)2) can be utilized to capture CO2 from the atmosphere leading to negative emissions. In this work an experimental study on the Ca(OH)2 extraction from steel converter slag (LD-Slag), blast furnace slag and mine tailings is conducted due to a high CaO content of 32-43% in these materials. An experimental set-up is proposed in which a chemical solvent extracts the Ca-ions from the by-products followed by a two-stage pH adjustment to separate impurities and precipitate the final slaked lime. Two different solvents, HCl and NH4Cl, are compared in their extraction behavior for the three different industrial waste products. A preliminary literature study provided knowledge about the parameters and methods used in this study. Liquid filtrates and solid residues were analyzed using ICP-OES and XRD respectively to obtain data for the different ion and phase compositions. It was found that HCl extracted calcium more effectively compared to NH4Cl. However, the HCl solvent did not selectively extract calcium which results in the additional leaching of impurities, such as iron, magnesium, aluminum and vanadium. The pH-swing separated the non-desired ions but also led to a loss of calcium for the HCl case. Therefore only 71-83% of the extracted calcium could be used to produce Ca(OH)2 compared to 90-100% for the NH4Cl case. The total yield of slaked lime was more substantial for the HCl case in all three by-products. However, the results for NH4Cl are competitive for the mine tailing and blast furnace slag, which makes this solvent interesting with respect to its high selectivity and no impurity leaching. In all cases a pure Ca(OH)2 was obtained which validates the method. The results indicate that impurities are effectively separated and could provide an additional value if utilized as a side stream. Further research should address the regeneration of the extraction chemicals to enhance the economical feasibility of the process. In addition the utilization of the slaked lime in the passive-DAC system would require continuing investigation of the necessary properties.