(366af) Assessing the Sustainability of Recovering Rare Earth Elements from a Toxic Wastewater Slurry: Combined Life Cycle Assessment and Technoeconomic Analysis Study

I work on rare earth element (REE) recovery from phosphogypsum, a wastewater of fertilizer production. For one part of my project, I use x-ray absorption spectroscopy to identify the structure of REEs complexed by engineered peptides. In collaboration with several other research groups, we are designing these peptides to be more selective for REE separations than conventional solvent extraction technologies. Incorporating this new selective separation, I use system analysis (life cycle assessment, technoeconomic analysis, global uncertainty/sensitivity analysis) to assess the sustainability of a landscape of novel REE recovery systems.

keywords: system analysis, sustainability assessment, life cycle assessment, technoeconomic analysis, global sensitivity/uncertainty analysis, optimization

Abstract

Phosphogypsum (PG) is a waste generated from fertilizer production consisting primarily as a slurry of gypsum, acidic water, and various heavy metals (e.g., U, Th, rare earth elements). PG is stored indefinitely in ‘stacks’, which slowly release these toxic and radioactive chemicals into the environment. Since PG is generated at hundreds of millions of tonnes per year, indefinite storage in stacks is quickly becoming untenable necessitating new treatments. Some potential new treatments generate useful products for use in road construction and agriculture, but are not economically feasible. A new treatment that recovers high-value rare earth elements (REEs) from PG could make PG remediation profitable.

REEs (consisting of lanthanides, scandium, and yttrium) are considered critical materials and are used in a variety of modern technologies including wind turbines, lasers, consumer devices, and electric vehicles. The current production of REEs is achieved by an energy and chemically intensive process (beneficiation, leaching, separation by solvent extraction, and refinement) from the mining of REE ores in primarily one geopolitical region. Investigations into a more sustainable supply of REEs from abundant secondary sources, such as toxic PG waste, is vital to securing a stable global REE supply chain. With a minimum REE content of ~0.02 wt% rare earth elements (REEs), PG has the potential to meet the annual rare earth element consumption within the United States (~9,000 tons REE/year) while also making PG remediation profitable. Developing a sustainable source of REEs from PG will enable clean and efficient transition to green energy technologies.

Currently, it is unclear how to best recover the dilute REEs from the PG waste (conventional solvent extraction is inefficient and has high environmental impact). In this work, we propose a treatment train (technological readiness level < 3) for the recovery of REEs from PG which includes a bio-inspired adsorptive separation to generate a stream of pure REEs. We assessed its financial viability and life cycle environmental impacts via life cycle assessment (LCA), techno-economic analysis (TEA), and identify targeted improvement opportunities through global uncertainty/sensitivity analysis and scenario analysis. We determined that this system can be profitable (net present value of $200 million, internal rate of return of 17%, and minimum selling price (MSP) of $48/kg REO) and shows reduced environmental impact in several ReCiPe 2016 environmental impact categories (land use, ecotoxicity, eutrophication, ionizing radiation, material resource depletion, and human toxicity) when compared to conventional REO production and PG stacking. However, the system underperforms in other impact categories (climate change, terrestrial acidification, ozone depletion, photochemical oxidant formation, particulate matter formation, and fossil resource depletion). The endpoint analysis shows that the system outperforms conventional REO production in ecosystem quality (93.0% of the impact) and resource depletion (96.2% of the impact) but underperforms in human health (213% of the impact). The life cycle environmental impacts and financial viability are primarily driven by chemical consumption in the leaching, concentration, and wastewater treatment process sections ($15/kg REO). In addition to high chemical costs, the large capital cost of the selective adsorption resin ($18/kg REO) limits profitability. Scenario analysis shows that the system is profitable at capacities larger than 100,000 kg/hr PG for PG with REE content above 0.5 wt%. The most dilute PG sources (0.02-0.1 wt% REE) are inaccessible using the current process scheme (limited by the cost of acid and subsequent neutralization) requiring further examination of new process schemes and improvements in technological performance. Overall, this study evaluates the sustainability of a first-of-its-kind REE recovery process from PG and uses these results to provide clear direction for advancing sustainable REE recovery from secondary sources.