Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash.

With the continuous addition of oxalate, calcium oxalate is the only phase that is oversaturated and predicted to precipitate (i.e., saturation index, SI > 0), which is in line with the XRD and SEM results. In contrast, the precipitation of oxalate with REEs or other metals (e.g., Mg, Sr, Cu, and Zn) is not thermodynamically favorable (i.e., SI <0). The decrease of Ca concentration in F-1 and C-1 leachate calculated by PHREEQC ( Figure S10 ) showed a trend similar to the decreases of REEs ( Figure 3 ).

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