352 In-situ Mineral Carbonation in Abandoned Coal Mines by Using CO2 Micro-Nanobubble *A. Hamanaka1, K. Itakura2, G. Deguchi3, K. Takahashi2, J. Kodama4, J. Kim2, Y. Hama2, T. Sasaoka1, H. Shimada1 1Department of Earth Resources Engineering, Kyushu University, Japan, (*Presenting author: hamanaka@mine.kyushu-u.ac.jp) 2Graduate School of Engineering, Muroran Institute of Technology, Japan 3Underground Resources Innovation Networks, Japan 4Division of Sustainable Resources Engineering, Hokkaido University, Japan ABSTRACT The practice of abandoned mined-out areas without backfilling them is prevalent in underground mining. This often leads to significant environmental and safety concerns, including widespread surface subsidence that can cause damage to infrastructure such as roads and buildings. Conversely, these abandoned sites, characterized by numerous cracks and voids, present a compelling opportunity for large-scale carbon capture, utilization, and storage (CCUS) projects. Their inherent high permeability is particularly advantageous because it allows for fluid injections without the need for excessively high pressure. However, the significant risk of CO2 leakage arises when applied to goafs (mined-out areas), which are abundant in cracks and voids. To mitigate this risk, this study explores an alternative approach: the injection of CO2 solidifying materials containing CO2 micronanobubble (MNB) water. This innovative technology stabilizes CO2 in water for extended periods, allowing the injection of quantities that exceed its normal solubility. CO2 reacts with CO2 solidifying materials, which are calcium-rich materials in the underground environment, to form stable carbonate minerals, providing a safe and permanent method for storing large volumes of CO2. This paper presents key findings from the Mikasa City CO2 underground fixation project, which investigates the in-situ mineral carbonation of an abandoned underground coal mine. The results confirm that a slurry of blast furnace slag (BFS) mixed with CO2 MNB water can be injected into the goaf at depths greater than 400 m using low injection pressure due to the high permeability of the site. Furthermore, the experimental results suggest that the mechanical properties of the solidified slurry and the amount of CO2 fixation are directly influenced by the high-pressure conditions in the underground environment. These conditions are presumed to accelerate the hydration and carbonation reactions of the solidifying materials, promoting more effective mineral carbonation. This study demonstrates the significant potential of abandoned underground mines for safe and permanent CO2 storage, paving the way for a novel and sustainable solution to CCUS challenges.
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