Controlling Selectivity of Surface Electro-Precipitation (SEP) in the Recovery of Rare Earth Elements (REE) from Aqueous Feedstocks.

SEP is an emerging green separation technique for the recovery of REE and other valuable elements from unconventional feedstocks. Its industrial adoption requires comprehensive mechanistic knowledge of its selectivity for REE vs typical background cations to achieve the desired separation. To bridge this gap, we experimentally studied SEP of neodymium Nd in chloride, nitrate, and sulfate solutions, in the absence and presence of calcium, aluminum, iron, zinc, and cobalt. We found that SEP is nonselective in the mass-transfer regime. It becomes selective in the mixed regime, with higher purification factors for elements with larger gaps in precipitation pH. At the same potential and initial pH, the selectivity of SEP in the mixed regime is controlled by the current (OH generation rate) and background ions. In the case of Fe, it additionally depends on the catalytic activity of the SEP cathode in the production of HO. We demonstrated for the first time that the in situ production of hydrogen peroxide in SEP can be used to selectively remove Fe from a multielement solution. The reported selectivity-recovery figures make SEP highly competitive, especially when its other advantages are factored in. The results of this study can be the basis for developing a suitable SEP-based strategy for preconcentrating REE and other valuable elements from diluted secondary resources.