Fixation of carbon dioxide by producing carbonates from minerals and steelmaking slags: Dissertation

Capture and storage of carbon dioxide (CO₂) is internationally considered to be one of the main options for reducing atmospheric emissions of CO₂. In Finland, no suitable geological formations are known to exist for storing captured CO₂. However, fixing CO₂ as solid carbonates using silicate-based materials is an interesting alternative. The magnesium silicate deposits in Eastern Finland alone could be sufficient for storing 10 Mt CO₂ each year during a period of 200-300 years. Finnish steelmaking slags could also be carbonated, but the amounts produced provide a much smaller potential for CO₂ storage (0.5 Mt CO₂ per year) than magnesium silicates provide. The aim of this thesis was to study the possibility of reducing CO₂ emissions by producing calcium and magnesium carbonates from silicate materials for the long-term storage of CO₂ using multi-step processes. The production of carbonates from steelmaking slags and serpentinite, a magnesium silicate ore available from a metal-mining site, was studied both experimentally and theoretically. On the basis of the results, process concepts were developed and evaluated. Finally, the stability of synthetic calcium and magnesium carbonates as a medium for CO₂ storage was assessed. Experiments with aqueous extraction and precipitation processes showed that magnesium and calcium can easily be extracted from steelmaking slags and natural silicate minerals using acids. Natural minerals seem to demand stronger acids for extraction than slags. Relatively pure calcium carbonate (80-90% calcite) was produced at room temperature and a CO₂ pressure of 1 bar by adding sodium hydroxide to acetate solutions made from slag. Similarly, serpentinite was successfully converted into 93-100% pure hydromagnesite (a magnesium carbonate), using nitric acid or hydrochloric acid for the dissolution of serpentinite and sodium hydroxide for precipitation. The conversion of raw material to carbonate ranged from 60-90%. Although the results show that pure carbonates can be produced from industrial by-products and mining residues, the process concept suggested requires the recycling of large amounts of sodium hydroxide and acid, as well as low-grade heat for solvent evaporation. The methods suggested for recovering the spent chemicals were found to be expensive and cause more CO₂ emissions than the amount of CO₂ stored.