Reduction of CO2 emissions by mineral carbonation : steelmaking slags as rawmaterial with a pure calcium carbonate end product

Mineral carbonation is one of the options that can contribute to the reduction of carbon dioxide emissions for climate change mitigation purposes. Steel manufacturing, which is one of the biggest industrial sources of CO2 emissions, could benefit from this option by utilizing its own by-products, i.e., steelmaking slags, to combine with CO2. Additional benefits would be achieved if the end product was a pure and marketable calcium carbonate. The utilization of CaCO3 derived from steelmaking slag to replace some of the synthetic CaCO3 used in industry would simultaneously save virgin material resources, reduce carbon dioxide emissions, and provide a new utilization option for the low-value slag. The objective of the research reported in this thesis was to investigate the possibility of reducing CO2 emissions by utilizing steelmaking slags as raw material for CO2 mineralization. The target was that the end product of the CO2 mineralization would be a pure CaCO3 suitable for utilization. The suitability of the acetic acid process route suggested by Kakizawa et al. (2001) was tested first. Experiments showed that acetic acid dissolves calcium efficiently from steelmaking slags. A weak acetic acid solution was able to dissolve lime selectively from the steel converter slag, forming a clearly alkaline solution. Nonetheless, efficient CaCO3 precipitation required the addition of NaOH to prevent the regeneration of acid during carbonation, as well as to increase the alkalinity of the solution in those cases where a stronger acetic acid solution was used for the dissolution step. Preliminary process calculations revealed that chemical costs make the process clearly too expensive for CO2 sequestration. Because the production of pure CaCO3 from steelmaking slags with acetic acid was not feasible, various other solvents were tested. Three ammonium salt solutions (CH3COONH4, NH4NO3, and NH4Cl) dissolved calcium selectively from the steel converter slag and almost as efficiently as acids. The precipitated calcium carbonates were very pure. It was calculated that the production of pure calcium carbonate from steel converter slag by using ammonium salts as a solvent has clearly negative CO2 emissions. Since it was found that the method is not suitable for just any waste/by product materials that contain calcium, but should be applicable for at least those that contain free CaO, this method has potential only for minor-scale, yet significant, CO2 storage. A clear advantage is, however, that it uses a low value by-product as its raw material and has economic potential.