(275a) Economic Assessment and Scale-up of an Eco-Friendly Continuous Bioleaching System for Recovery of Rare Earth Elements from End-of-Life Materials

We have previously developed an eco-friendly strategy to recover rare earth elements (REE) from recyclable materials that relies on microbial production of organic acids to solubilize and release metals from a solid matrix. Microbially mediated rare earth recovery processes offer an economical and green alternative that can be applied to low-value feedstocks for extraction of the valuable metals. This presentation will focus on our efforts to assess the economic benefits of this approach, seek ways to reduce costs, and to scale the process.

We have shown previously that REE could be leached from recyclable materials using microbially produced organic acids under laboratory bench-scale conditions (Reed, et. al., Hydrometallurgy (2016), 166:34-40). We have also conducted techno-economic analyses for bioleaching REE from various feedstocks (Thompson, et. al., in Review). These analyses demonstrated that the profitability of this process was sensitive to the leach pulp density which directly correlated to the size (cost) of the batch bioreactor necessary to produce organic acids. Therefore, to provide organic acids in quantities and at rates useful for commercial bioleaching, we developed a continuous bioreactor process. Furthermore, for low grade feedstocks leaching approaches with low capital and operating costs we examined ways to minimize material processing such as heap leaching.

Gluconic acid has been shown to be effective for bioleaching of rare earth elements (REE) from recyclable materials and the Gluconobacter oxydans (NRRL B58) strain is an industrially proven bacterium for efficient production of gluconic acid from glucose. Conditions for producing an optimal lixiviant (exhibiting the lowest pH) with Gluconobacter were determined in a batch reactor system while varying temperature, stirring speed, aeration, glucose concentration, and other media components. At incubation conditions a lixiviant solution was produced with pH 2.14 and the measured gluconic acid concentration was 233 mM.

With this lixiviant we were able to leach 49% and 11% of the total REE from two industrially relevant feedstocks, fluidized catalytically cracking (FCC) catalyst and retorted phosphor powders, respectively, that were contacted with the lixiviant for one day at a 1.5% solid/liquid (m/v) ratio. Increased densities had lower leaching percentages but higher total REE recovery for the same lixiviant. Initially we applied lixiviant to feedstocks in shaking tubes or flasks; however by leaching in a stirred reactor we were able to increase REE recovery efficiency by 13-16%. Furthermore, under the optimal conditions Gluconobacter was able to sustain continuous organic acid production for over 100 hours at dilution rates ranging from 0.05 hr^-1 to 0.38 hr^-1. The effluent produced at the lowest dilution rate (with pH 2.20, 173 mM gluconic acid) performed similarly to the lixiviant produced in the batch system with respect to REE leaching from the feedstocks.

We also assembled and tested a simulated bench-scale heap leaching system. Heap leaching can be advantageous compared to reactor leaching because not only are a tank and associated plumbing obviated but agitation of the solid-liquid mixture and associated energy costs are not required. We tested the system with FCC catalyst and 5× recirculation of the lixiviant, and found that REE recovery was almost as efficient to the batch system using a similar density and contact time.

Together our results demonstrated that microorganisms that produce organic acids can induce effective leaching of REE from solid waste materials, and we have been able to scale the process (1000-fold) for continuous production of organic acid, and improved leaching scale (250-fold) to further decrease potential leaching costs.