Serpentinite Mine Tailings Carbonation with Carbon Dioxide from Electric Calcination

Paris agreement is binding its parties to limit global warming. To achieve the goal, carbon dioxide (CO₂) emissions should be reduced. Thus, there is a need for carbon capture and storage (CCS) in Finland and abroad. In CCS, CO₂ is captured and stored with a suitable method. A suitable method in Finland could be mineralization of CO₂ in which CO₂ is bound as stable carbonates using minerals or solid industrial wastes as raw materials for the reaction.

In this thesis CO₂ mineralization route developed at Åbo Akademi University, called the ÅA route, was studied. The ÅA route includes thermal extraction, dissolution, precipitation, and carbonation steps. The first steps aim for production of magnesium hydroxide which is carbonated into magnesium carbonate. Additionally, the recycling of the extraction reagent, ammonium
sulfate, is performed. In the carbonation step, heat is released, and it could be utilized in the process. In addition, calcination of pulp mill lime mud was studied as a source of biogenic CO₂ to be bound in the carbonation.
In the experimental part, the aim was to demonstrate the mineralization of CO₂ on a kilogram scale by following the ÅA route. As raw materials, serpentinite mine tailings from Hitura were used. The CO₂ source for the carbonation was intended to be the off-gas from electric calcination of lime mud. However, in the carbonation characterization tests no carbonation occurred in 6 h at 480 °C and total pressure of 20 bar (99 vol% CO₂ and 1 vol% water). Experiments with the
simulated off-gas were not performed. The samples of the characterization tests contained magnesium oxide which indicates that the amount of water vapor might need to be increased at least to 5 vol% in the following experiments. In addition, the fraction of CO₂ bound from the input gas could be measured. The samples also included magnesium sulfate which may have been formed due to impurities of the magnesium hydroxide intermediate.
In the extraction experiments it was observed that the heated mixture of ammonium sulfate and mine tailings adheres on steel and Kanthal APM (iron-chromium-aluminum alloy) and may cause clogging. The formed magnesium compound in this step was analyzed as efremovite which may form boussingaultite in the ambient air. Boussingaultite was detected in the magnesium
hydroxide intermediate in addition to magnesium hydroxide and hexahydrite. More laboratory experiments are recommended to be performed to study the reaction routes, increase the yield, and to minimize the consumption of ammonium sulfate.
Up to 24 wt% of magnesium in the mine tailings was extracted. This is three times the yield previously achieved with the Hitura mine tailings. Instead, only 4 wt% of the magnesium in the mine tailings was obtained into the magnesium hydroxide intermediate. There can be inaccuracy on the results due to the insufficient amount of elemental analyses. Material efficiency could be improved by using solid-liquid separation methods of an industrial scale process.
In the laboratory experiments, the total specific energy consumption of the CO₂ mineralization process was estimated as 293 000 kWh/kg of theoretically bound CO₂ while the theoretical total specific energy consumption was approximately 5.6 kWh/kg of theoretically bound CO₂. The energy consumption was mostly due to the crystallization of ammonium sulfate. An experimental basis for the specific energy consumption was thus established, showing the room for improvements. For instance, the utilization of heat from the carbonation step could decrease the
total specific energy consumption. Despite the challenges, this CO₂ mineralization method could be developed into a feasible option for the mitigation against the global warming.