Preparation of ultra-clean coal by NaOH leaching with alkali liquor purification and regeneration

This study used flotation cleaned coal (FCC) with an initial ash content of 2.87% as the feedstock to achieve deep deashing via NaOH alkaline leaching, while simultaneously investigating the purification and recycling of Si/Al-containing spent alkali liquor. During the leaching process, the effects of alkali concentration, leaching time, temperature, and liquid-to-solid ratio on deashing behavior were systematically evaluated. Increasing temperature promotes deashing; however, excessively high alkali concentration or prolonged reaction time induces secondary precipitation of aluminosilicate phases, thereby inhibiting further deep deashing. Under the recommended conditions (80 min, 140°C, 170 g·L−1 Na2O, L/S = 4:1), the ash content of FCC was reduced from 2.87% to 1.08%, corresponding to a deashing efficiency of 62.37%. For spent alkali liquor purification, the synergistic use of CaO and MgO significantly reduced the concentrations of SiO2 and Al2O3 to 1.035 and 0.163 g·L−1, respectively, corresponding to removal efficiencies of 45.15% and 89.73%. Recycling tests demonstrate that the regenerated alkali solution can still produce ultra-clean coal with an ash content below 1.2%, confirming the recyclability and stability of the system. This work provides a fundamental basis for deep deashing of flotation cleaned coal and resource recycling in alkaline leaching systems.