Carbon negative backfill mining in coal mines for carbon neutralization: Chemical carbon fixation performances with mineralized gangue

Safe, efficient, and low-carbon coal mining is vital, especially for China, where coal remains the main energy source. Minimizing rockburst risks and ecological damage, as well as developing low, zero, and carbon negative mining, become the main task of the coal industry. However, their realization is hindered by the increasing accumulation of by-products of coal mining and utilization, such as abundant gangue, fly ash, coal-based solid waste, and CO₂. To mitigate these problems, the current study focuses on carbon negative backfill mining (CNBM), proposing two CNBM approaches: (i) physical carbon fixation with high-porosity gangue and (ii) chemical carbon fixation with mineralized gangue. To this end, a sealed carbon fixation stirred autoclave was designed for experiments on solid waste gangue, and results were analyzed to reveal the effects of stirring rate, reaction temperature, CO₂ pressure, and solid-liquid ratio on carbon fixation performance. The gangue under study had carbon fixation potential, with a theoretical maximum carbon fixation capacity of 10.17 g/kg. Carbon fixation capacity and carbonation degree were positively related to stirring rate and pressure, being negatively related to temperature and solid-liquid ratio. Noteworthy that CO₂ pressure, which had the highest correlation with carbon fixation capacity, was classified into a group of “smooth-influencing factors”. In contrast, temperature and solid-liquid ratio were considered “abrupt-influencing factors”, which should be finely adjusted to avoid sharp deterioration of carbon fixation capacity. Finally, the main challenges faced by CNBM were summarized, and potential research directions for backfill mining under carbon fixation were discussed, including CO₂ migration and fixation mechanism, collaboration between filling body and CO₂, surface activation of coal-based solid waste, CO₂-related accelerated mineralization approaches, as well as safe and efficient CO₂ transport approaches.