Synergistic separation mechanisms of impurities in ionic liquid aqueous solution from the cellulose regeneration process

Ionic liquids (ILs) have shown significant potential for industrial application in the cellulose regeneration process. However, the accumulation of impurities, especially phosphorus (P) from the raw materials, in the ionic liquid (IL) aqueous solutions generated during the cellulose regeneration process, seriously affects the performance of IL and the quality of the final products. In this study, a crystallization precipitation method was employed to investigate the removal of impurities from IL aqueous solutions. The results demonstrated that the presence of IL accelerated the phase transformation of amorphous calcium phosphate to hydroxyapatite, although it prolonged the time to reach equilibrium. The highest P removal efficiency was achieved at a Ca/P molar ratio of 1.67:1, with an increase in pH further enhanced the removal efficiency. The morphology and size of crystallization precipitates were significantly influenced by the reaction time, pH, IL concentration, and temperature. By optimizing the reaction conditions, a P removal efficiency of 88.82 % was achieved in IL aqueous solutions generated during the established hundred-ton-scale cellulose regeneration demonstration production line, with the process also exhibited synergistic removal capabilities for NO3-, SO42-, and Mg2+. These findings suggested that this method effectively removed the P and other inorganic impurities from the IL aqueous solutions generated during the cellulose regeneration process.