Abstract:

Hydrochloric or nitric acids are used in common rare earth element processing steps such as bastnaesite leaching and mixed rare earth carbonate dissolution. Likewise, bases such as NaOH and NH₄OH are often used, including for pH control during solvent extraction. Due to the relatively high costs of these acids and bases, they can constitute the majority of operational expenses in rare earth element extraction and refining projects if not recycled within the process. Bipolar membrane electrodialysis is an energy-efficient acid/base regeneration technique that can also contribute to zero-liquid discharge operations. Recently, new bipolar membranes (BPMs) have been developed that catalyze water dissociation at the interfacial layer of the BPM. In one approach, sub-nanometer planar catalysts such as graphene oxide are deposited at the interfacial layer of the BPM to intensify the local electric field, thereby accelerating water dissociation through the second Wien effect and other interactions between water oxidized functional groups on the catalyst surface. Here, we describe experiments using electrodialysis stacks incorporating BPMs with interfacial layer catalysts operated in salt splitting mode to produce acid and base. Results indicate 15.4% or more reduction in specific energy consumption for catalyst-modified BPMs versus the state of the art. The catalyst-modified BPMs performance improvement is particularly marked at current densities greater than or equal to 1000 A/m². Acid concentrations above 1.4 mol H⁺/L and base concentrations above 2.1 mol OH^-/L were obtained. Membrane performance stability was assessed in trials exceeding 63 h. Overall, a new generation of BPMs show promise for substantially lowering the costs of bipolar electrodialysis, prompting technoeconomic reassessments of rare earth element extraction and refining processes.

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Dr. Han Zong