Abstract: The gasification slag after carbon separation is difficult to realize effective utilization because of its high content of water and the presence of a small amount of residual carbon. To address these problems, a mineralization based on indirect carbonation to sequester CO₂ and recycle calcium extraction to prepare nano-calcium carbonate process is proposed. The gasification slag after carbon separation mainly consists of CaO, Al₂O₃, Fe₂O₃, MgO, as well as some non-metallic components such as SiO₂. Most of the metal components exist in amorphous form. After preliminary screening of acidic leaching agents, it was found that hydrochloric acid can effectively destroy the structure of gasification slag and dissolve the metal components in gasification slag. In this paper, the effects of leaching agent type, concentration, reaction time, temperature and liquid-solid ratio on the leaching rate of calcium from decarbonized gasification slag were investigated in detail. The results showed that the highest calcium leaching rate of 98.79% was achieved under the leaching conditions of 2 mol/L HCl, liquid-to-solid ratio of 20 mL/g, reaction temperature of 50 ℃ and reaction time of 90 min. Meanwhile, the effects of CO₂ flow, temperature and time on the carbonation efficiency and precipitated calcium carbonate (PCC) crystal structure were investigated. In the carbonation stage, the main factor affecting carbonation efficiency is CO₂ flow. This is because excessive CO₂ will cause carbonic acid to form in the solution and partially dissolve the precipitated CaCO₃, resulting in a sharp decrease in carbonation efficiency. And the carbonation efficiency gradually increases with the increase of reaction temperature. Generally speaking, increasing the temperature is beneficial for chemical reactions. However, owing to the exothermic nature of the carbonation reaction, the positive promotion effect of high temperature on the reaction process is weakened, and the solubility of CO₂ in water is reduced, resulting in a slow decrease in carbonation efficiency. The effect of reaction time on carbonation process has the same trend as the change in reaction temperature. The highest carbonation efficiency could reach 99.59% by optimizing the carbonation reaction conditions. In addition, the reaction temperature and time significantly affected the calcium carbonate crystal structure and micromorphology. The formation of calcium carbonate crystals mainly goes through three stages. In the first stage, as the reaction time prolongs, disordered amorphous calcium carbonate rapidly dehydrates to form ordered calcium carbonate crystal structure. At high supersaturation, vaterite begins to nucleate and undergoes spherical growth through nucleation at the growth front. Gradually, the solubility of amorphous calcium carbonate gradually decreases, and vaterite continues to grow into polycrystalline spheres composed of roughly equal sized crystals. In the second stage, vaterite is formed under equilibrium conditions, and its crystal size almost no longer increases, leaving part of the remaining amorphous calcium carbonate dissolution and crystallization process. In the third stage, vaterite begins to decompose and forms calcite or aragonite through dissolution-recrystallization process. Experiments result have shown that vaterite and calcite are formed at low temperatures, and aragonite is formed when heated to a certain temperature. As the reaction time increases, the particle size of calcium carbonate gradually increases. Therefore, lowering the reaction temperature and time is more favorable to the formation of vaterite type calcium carbonate.