3.1 Experimental principle
Previous efforts failed to extract REEs in coal wastes since the occurrence of REEs in silicon-aluminum of coal wastes, which are not in extractable ionic status. In this study, prior to the regular acid leaching procedure, the calcination pretreatment of coal wastes using Na2CO3 under elevated temperatures was initialized. It’s likely that the original silica-aluminate minerals may be decomposed to produce new silica-aluminate as well as silicate substances in their higher activity (Wen et al. 2020). The specific reactions are shown as follows:
Al2O3+Na2CO3=2NaAlO2+CO2↑ (1)
SiO2+Na2CO3=Na2SiO3+CO2↑ (2)
This was a so-called the activation procedure for REEs extraction from coal wastes. The consequence of this activation resulted in the dissolvable silica-aluminate at least in acid environment. Simultaneously, REEs occurring in coal wastes could be also accessible in the acid leaching. The specific reactions are likely as follows:
RE2O3+6HCl⇌RECl3+3H2O (3)
Fig. 4. shows the SEM images of coal fly ash and coal gangue before and after the activation procedure. Coal fly ash, which is from the higher temperature combustion process showed the melting trend in its regular round shape grains. The was not maintained and it showed a mixing status of melting grain in regular shape and irregular structures after the activation that was even still in a condition of the similar elevated temperatures, implying the somehow decomposition of silica-aluminate crystal structure during the activation. Coal gangue, from a natural deposition condition, showed its typical shape character in the irregular stacking structure. Under an elevated temperature calcination during of the activation, the regular melting contrastingly and clearly showed-up. Fig 5. and Fig. 6 shows comparison of XRD plots of coal fly ash and coal gangue. It was found that the original main mineral compositions of mullite and quartz in ash and kaolinite and quartz in coal gangue disappeared. Alternatively, the activation process changed them into crystals of sodium metasilicate and sodium silicate-aluminate, implying the completeness of the activation.
3.2 The REEs extraction of coal fly ash
Figure 7 shows the extraction efficiency of REEs after the pretreated calcination at 800°C and 850°C. Generally, a higher calcination temperature at 850°C seemed better to the enhanced extraction efficiency of REEs except for element Y, and as for elements La and Pr, the calcination at 800°C seemed enough for the desirable extraction efficiency. Noticeably, elements of Ce and Tb were two showing the significant different upon the change of the calcination temperature from 800oC to 850oC, while the extraction efficiency of the element Ce as very sensitive on the calcination temperature than that of the element of Tb. Fig. 8 shows the extraction efficiency of REEs upon the usage of the calcination additive of the sodium carbonate. The usage of the sodium carbonate was determined based on the theoretical molar ratio of chemical reactions, as shown in Eq (1) and (2), which is that between the actual silica-alumina mineral contents in the coal fly ash and coal gangue and the sodium carbonate. The molar ration was varied from 1:0.7 to 1:1.0 in this study. It’s found the extraction efficiency of REEs gradually improved upon the increasing usage of sodium carbonate until a molar ratio at 1:1.0 when the extraction efficiency of REEs (pH=2) achieved its highest. The aforementioned investigations were true in views of the light REEs, heavy REEs and naturally the total REEs.
Fig. 9 shows the effect of pH in the aqueous extraction environment on the extraction of REEs (Zhang et al. 1987). The decrease of pH significantly increased the extraction efficiency of REEs. Studies also indicated an aqueous extraction environment at pH=4 seemed not effective on the REEs leaching. This was not true when pH dropped to 3 and further 2, the extraction efficiency of most REEs can be achieved by about 60% and further as high as 90% (pH=2). The acid extraction seemed not functional well on the element Ce during the acid extraction, especially at pH=4 and 3 where the extraction efficiency corresponded to only 23% and 30%, respectively. This was significantly changed to 86% in the extraction efficiency when pH=2. This study confirmed the optimal pH in the aqueous environment for the maximum extent of REEs leaching should be about 2. The aforementioned investigations were true in views of the light REEs, heavy REEs and naturally the total REEs.
3.3 The REEs extraction of coal gangue
Similar results can be obtained in the investigations on the REEs extraction of coal gangue. Figure 10 shows the extraction efficiency of REEs of coal gangue after the pretreated calcination at 800°C and 850°C, implying a higher temperature at 850°C for the calcination was necessary to enhance the extraction efficiency of REEs. Except for element elements La, Pr, and additionally Nd in the coal gangue case, the calcination at 800℃ seemed enough for the desirable extraction efficiency of REEs. That the sensitive response of the extraction efficiency of the element Ce was again found on the calcination temperature, and insensitive for the Tb was similarly shown. Tb. Figure 11 shows the extraction efficiency of REEs in coal gangue upon the usage of the sodium carbonate. It was found that the extraction efficiency of REEs expectedly enhanced upon the increased usage of the sodium carbonate. A molar ratio of 1:1.0 during the calcination of the coal gangue can maintain the extraction efficiency by the later-on acid leaching achieved as high as 90-100%. The aforementioned investigations were true in views of the light REEs, heavy REEs and naturally the total REEs.
Fig. 12 shows the effect of pH in the aqueous extraction environment on the extraction of REEs in an alternative case of coal gangue. Similar to that of coal fly ash, the decreased pH of the acid leaching environment also significantly enhanced the extraction efficiency of REEs. Again for the coal gangue an aqueous extraction environment at pH=4 did not showed expectedly effective on the REEs leaching. This can be significantly changed when pH dropped to 3 and further 2, the extraction efficiency of most REEs can be achieved by about 55% and further as high as 95% (pH=2). As for the element Ce, its similar lower extraction efficiency was found during the acid extraction at pH=4 and 3, but significant higher to be over 95% in the extraction efficiency upon pH of the acid leaching environment was adjusted to be 2. The aforementioned investigations were true in views of the light REEs, heavy REEs and naturally the total REEs.