Underclay sediments are a potential source of rare earth elements (REE) for use in advanced technology applications. Citric acid has been identified as an environmentally sustainable organic acid capable of leaching REEs from ionically-bound and secondary mineral fractions. An analysis of the kinetic profiles from selected citric acid leaching experiments was performed to discern the effects of experimental conditions on reaction rates. These analyses can infer potential desorption mechanisms and be used to assess extraction efficiencies for each leaching strategy. The lixiviant acidity and feedstock composition had outsized impacts on extraction efficiency. Samples enriched in secondary REE-phosphate bearing minerals (i.e. crandallite) demonstrated higher REEs extracted compared to samples with ion-exchangeable fractions. Samples at pH &lt;3 exhibited higher extraction efficiencies at 24 hours (~7%REE) compared to those with an initial pH of 4.5 to 7 (&lt; 1% REE). Under more basic conditions (pH &gt;6), the decrease in REE extraction efficiency is likely due to REE precipitation caused by the lower liquid-solid (L:S) ratios of these samples. Lower L:S ratios compromise the buffering capacity of the citric acid and make it difficult to maintain the extracted REEs in solution. The kinetic data were fit to commonly applied desorption models (zero to third order, Elovich, two-constant, and parabolic diffusion) to determine the desorption mechanisms. Samples taken at pH &lt;3 showed REE leaching behavior that best fits two parabolic diffusion equations indicating that two distinct kinetic mechanisms take place. In the first 24 hours, it appears REEs are desorbed from the outer surface of particles with a parabolic diffusion rate constant of 5.0x10-1 μg g-1 min-0.5 (r2=0.9934). After 48 hours, the rate constant is reduced by an order of magnitude to 5.7x10-2 μg g-1 min-0.5 (r2=0.9528) and implies that slower REE desorption from within the aggregates begins to dominate the kinetics. At higher pH values (pH 4.5-6), the initial parabolic rate constants were also an order of magnitude lower within the first 24-48 hours, ranging from ~4x10-2 μg g-1 min-0.5 to ~6x10-2 μg g-1 min-0.5.The decrease in kinetics and lower extraction efficiencies are likely due to the clay content and more basic pH of these samples. Overall, this study provides insight into the process optimization and kinetic mechanisms which govern REE leaching from underclay sediments.

Extraction kinetics of rare earth minerals from ion-adsorbed underclays

ACS Spring 2022

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