Ultrasound-assisted leaching of calcium ions from fly ash to mineralize CO₂ and fire extinguishing characteristics of products

The use of fly ash slurry for mineralization to capture CO₂, along with utilizing the mineralized slurry to prevent spontaneous combustion in goaf area, can achieve the dual benefits of pollution reduction and carbon reduction, as well as disaster management. Addressing the current issues in the CO₂ mineralization process of fly ash, such as slow leaching rate of calcium ions and low mineralization capacity, proposing a new approach to use ultrasound to accelerate the leaching of calcium ions from fly ash and enhance its CO₂ mineralization efficiency. Using ion chromatography to study the leaching patterns of calcium ions in low-calcium fly ash and high-calcium fly ash slurries, and employing low-temperature N₂ adsorption and SEM-EDS techniques to analyze the changes in structure of fly ash particles under the action of ultrasound, the paper evaluates the enhancement effect of ultrasound on CO₂ mineralization of fly ash using a self-built CO₂ adsorption and mineralization reaction experimental system. Moreover, the principle of ultrasound-enhanced leaching of calcium ions from fly ash was explored. The experimental results show that the leaching rate of Ca²⁺ in low-calcium and high-calcium fly ash slurries is very slow under static conditions, reaching leaching equilibrium only after 30 days. After ultrasound-enhanced treatment, the concentration of Ca²⁺ in both types of fly ash slurries can reach the leaching level of 30 days under static conditions within 30 minutes, indicating that ultrasound significantly enhances the leaching rate of Ca²⁺ in fly ash slurries. At the same time, after ultrasound-enhanced leaching treatment, the specific surface area of low-calcium and high-calcium fly ash increased by 23.71% and 184.71%, respectively, indicating that the cavitation effect of ultrasound can disrupt the aggregation and adhesion between fly ash particles, leading to particle refinement and further development of pore structure. This promotes mass transfer processes such as Ca²⁺ leaching from fly ash into water. Mineralization test results show that after ultrasound modification, the mineralization amount of low-calcium and high-calcium fly ash increased by 410% and 22%, respectively, with mineralized carbon sequestration amounts reaching 7.66 g/kg and 82.02 g/kg, respectively, indicating that ultrasound significantly enhances the CO₂ mineralization effect of fly ash. Based on this, utilizing the mineralized CO₂ product of fly ash slurry for preventing and controlling coal spontaneous combustion. Compared to treating coal samples with original fly ash, the heat release of coal samples treated with mineralized products decreased by 3.32% and 14.24%, respectively, indicating that mineralized CO₂ products have superior coal spontaneous combustion inhibition characteristics.