## 3 Results and Discussion
### REEs Extraction of the Solid Residue via the Acid Leaching  
After the CFA was activated and washed, the REEs that originally occurred in the silicate aluminate bonded state in CFA can be released. REEs became their oxides through the calcination treatment and accessible through the water and alkali washing treatment, which was likely to be easily soluble in weak acid solution. Based on our previous experimental study, the pH of the leaching solution at about 2.00 could leach-out REE oxides efficiently, following the leaching reaction equation in the acid leaching condition as follows:  
\[\text{Re}_{2}\text{O}_{3}+6\text{H}^{+}\rightleftharpoons 2\text{Re}^{3+}+3\text{H}_{2}\text{O} \tag{6}\]  
Figure 6a,b show the extraction efficiency results of the single element of LREEs and HREEs, while Table 6 shows those of the total LREEs, the total HREEs, the total REEs as well as the acid consumption. In Figure 6 and Table 6, Treatment 1 represents the acid leaching of the activated CFA sample without further water and alkali washing treatment, Treatment 2 represents the acid leaching of the activated CFA sample with only the water washing, Treatment 3 represents the acid leaching of the activated CFA after both water and alkali washing. In Treatment 1, the extraction efficiency of the total REEs could be achieved by 88.81%. Among them, the extraction efficiency of HREEs and LREEs reached 100.63% and 84.07%, respectively, and only Ce and Tb were somehow lower at only 68.35% and 50.48%, respectively. This is likely due to the transition of some Tb\({}^{3+}\) to Tb\({}^{4+}\) in CFA  
Figure 5: SEM images of zeolites synthesized at different crystallization times. (**a**) Refers to the solid substance naturally precipitated at an ambient temperature; (**b**) refers to the crystallization product for 1 h; (**c**) refers to the crystallization product for 3 h; (**d**) refers to the crystallization product for 5 h.  
under an elevated calcination temperature, and the leaching-out of Tb\({}^{4+}\) was more difficult using hydrochloric acid [42]. In Treatment 2, the extraction efficiency of the total REEs was achieved by 92.23%, among which that of LREEs was slightly improved and that of HREEs was still maintained as very high. Even the extraction efficiency of Ce has been significantly improved, reaching 77.66%. In Treatment 3, the extraction efficiency of the total REEs reached 91.24%, and as expected, the LREEs and HREEs were similarly high. Comparing the three groups and the relevant results in the literature shown in Table 7, one may find a beneficial improvement in the acid leaching of REEs via the treatment of water and alkali washing procedures. The REEs extraction rate has significantly improved. The water and alkali washing jointly worked on the removal of water-soluble Na\({}_{2}\)SiO\({}_{3}\) and water-insoluble aluminosilicate in the activated CFA. Therefore, the porous structure was developed to allow the REEs within CFA to be accessible. Moreover, the water and alkali washing could reduce the silica precipitation during the follow-up of acid leaching, resulting in the significant acid saving. This has been clearly shown in Table 6. For the hydrochloric acid consumption among the three groups, the acid consumption was 3.55 mL\(\cdot\)g\({}^{-1}\) in Treatment 1, dropped to 2.69 mL\(\cdot\)g\({}^{-1}\) in Treatment 2, and significantly dropped to 0.9 mL\(\cdot\)g\({}^{-1}\) in Treatment 3, achieving a total of 74.5% in acid saving. It is clear that the Na\({}_{2}\)SiO\({}_{3}\) and aluminosilicate remaining in the activated CFA consume a large amount of acid.