Carbon capture and storage serves as the main technology for mitigating carbon emissions. Separation based methods, such as absorption, adsorption, and membrane separation, are the most utilised separation technologies available. Mineral carbonation, however, is one of few technologies that allows to capture and storage CO2. The permanent and safe trapping of CO2 by mineral carbonation is possible due to the formation of thermodynamically stable magnesium and calcium carbonates. Mineral carbonation can be divided into direct and indirect carbonation. In direct carbonation, minerals react directly with CO2 in a single step. Indirect carbonation, on the other hand, features the extraction of reactive components (basic metal oxides) from the mineral matrix using an appropriate solvent and their subsequent reaction with carbon dioxide to form carbonate precipitates. Direct mineral carbonation normally requires high CO2-partial pressures, high reaction temperatures, and long reaction times; however, indirect carbonation only requires the presence of reactive components such as calcium and magnesium oxides in the feedstock material for their subsequent reaction with CO2-gas. Also, the process can be carried out at atmospheric conditions.
Steelmaking is an energy-intensive process that consumes ca. 19 GJ per tonne of crude steel (considered in the BF-BOF processes) and emits ca. 1.9 tonne of CO2 per tonne of steel. These figures make the steel industry one of the largest contributors of CO2 emissions. Yet, the world production of crude steel is predicted to continue to grow. During the production of liquid steel in a basic oxygen furnace (BOF), around 100 – 150 kg of slag per tonne of crude steel are produced, which depends on the hot metal quality and operational conditions. Disposal and long-term storage of such waste volumes result in major costs and liabilities for the industry. However, the high calcium content of steelmaking slags makes them suitable raw materials for CO2 capture.
The selective extraction of calcium from the slag prior to carbonation allows the formation of a pure and marketable precipitated calcium carbonate (PCC) with different crystalline polymorphs, i.e. aragonite, vaterite, calcite, and/or with different crystal habits, i.e. rhombohedral, hexagonal prism, scalenohedral, cubic and prismatic.
In our recently published work, we have investigated the effect of different CO2-gas fow rates and temperatures to produce high-quality PCC with different morphologies and particle sizes. It was observed that Ca conversion was below 15 wt.% in the absence of alkaline reagents as alkalinity is reduced when H2CO3 is formed under a constant gas fow rate. The addition of NaOH, however, allowed to significantly increase the Ca-conversion yield, while modifying the morphology of calcite particles from scalenohedral-rhombohedral- to rhombohedral shape by increasing the CO2-gas flow rate. During carbonation with addition of amine-based organic solvents, the addition of PEI limited the Ca-conversion, whereas with the addition of MEA and AEEA showed the highest Ca-conversion (ca. 80%). A combination of vaterite and aragonite particles were produced at 30 °C, while the formation of pure aragonite prevailed when the temperature was increased up to 70 °C. It was noticed that only with the addition of AEEA, the particle size could be modified from large to small aggregates by increasing the CO2-gas flow rate. The size of PCC particles formed during carbonation with addition of MEA did not show a significant difference by changing the CO2-gas flow rate nor by changing the temperature. The final PCC resulted with a 99.9 % of purity with a consumption of 2 g of solvent/g PCC.
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Some additional references:
C. Yu, C. Huang, C. Tan, A Review of CO2 Capture by Absorption and Adsorption, Aerosol Air Qual. Res. (2012) 745–769. doi:10.4209/aaqr.2012.05.0132.
M.H. Ibrahim, M.H. El-naas, A. Benamor, S.S. Al-sobhi, Z. Zhang, Carbon Mineralization by Reaction with, Processes. 7 (2019) 1–21. doi:10.3390/pr7020115.
E.R. Bobicki, Q. Liu, Z. Xu, H. Zeng, Carbon capture and storage using alkaline industrial wastes, Prog. Energy Combust. Sci. 38 (2012) 302–320. doi:10.1016/j.pecs.2011.11.002.
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