fig4
![Alkali pretreatment effects on acid leaching recovery of rare earth elements from coal waste of the Western Kentucky No. 13 and Fire Clay seams](https://image.oaes.cc/020683d7-f854-43cd-b83e-bde768784680/4853.fig.4.jpg)
Figure 4. Acid leaching recoveries of TREEs, HREEs, and LREEs from the (A-C) Western Kentucky No. 13 and (D-F) Fire Clay coal wastes pretreated with NaOH solutions of different concentrations. TREEs: Total rare earth elements; HREEs: heavy rare earth elements; LREEs: light rare earth elements.