The energy and transportation sectors are major contributors to CO2 emissions and other environmental impacts. Wind power (WP) and electric vehicles (EVs) are considered potential solutions, offering advantages over traditional technologies. While these technologies promise reduced CO2 emissions and improved air quality, they rely on metals with limited availability and production capacity, often produced as companion metals in a few countries. The extraction process is energy and water-intensive, resulting in high CO2 emissions and other environmental issues. Neodymium (Nd) and Dysprosium (Dy), crucial for WP and EVs, are produced alongside other rare earth elements (REEs). Increased demand for Nd and Dy may lead to overproduction of companion REEs, impacting the technologies' sustainability. The supply-demand balance problem is further complicated by the fact that increased use of certain technologies might decrease demand for other metals, creating imbalances. For example, the shift from NiMH to Li-ion batteries in EVs could reduce the demand for some REEs. Similarly, the rise of LED technology may impact demand for other metals. Life cycle analysis (LCA) models and energy system models (ESMs) often fail to fully capture the environmental impacts of these naturally coexisting metals. Mitigation potential is overestimated unless manufacturing emissions are explicitly considered. Existing studies have addressed the impact of metals co-production on criticality scores and supply-demand balance, but none have quantified the impacts of naturally coexisting metals on the sustainability of emerging technologies utilizing the same materials. This paper aims to analyze the global implications of REE use in WP and EVs, considering the supply of naturally coexisting REEs and their associated environmental impacts.

Several studies have examined the impacts of metals co-production on metal criticality by incorporating co-production fractions into supply risk assessments. Others have analyzed supply-demand balance in general or for specific technologies like wind power. However, none quantified the supply-demand balance associated with naturally coexisting metals and their impacts on energy, water, CO2 emissions, and radioactive materials, ultimately affecting the sustainability of emerging technologies. This study addresses this gap by quantifying the impacts of naturally coexisting metals on the sustainability of WP and EVs and assessing potential solutions to the supply-demand balance problem.

This research utilized a global dynamic material flow-stock model to analyze the implications of REE utilization in WP and EVs on the supply of other coexisting REEs. The model incorporated various scenarios for REE demand and supply. Two demand scenarios were used: a low-demand scenario (Low D SC) based on changing material content (MC) in technologies, and a high-demand scenario (High D SC) with constant MC. REE demand across all applications was based on a single scenario. Three supply scenarios were considered: supply from all global deposits (without secondary sources), supply from all deposits (including secondary sources), and supply from Dy-rich deposits. Two secondary source scenarios (RSC1 and RSC2) addressed the varying levels of Dy and other REE recycling. Three scenarios for energy, water, and CO2 emissions were used, reflecting low, high, and average intensities. The model estimated annual and cumulative demand and supply for Nd and Dy in WP and EVs, considering their use in all applications. It also projected the potential supply of other REEs resulting from the demand for Nd and Dy. The model assessed energy, water, and CO2 emissions associated with REE production and oversupply. The analysis also evaluated the production of thorium and uranium, which are often co-produced with REEs, and their radioactive implications. Several mitigation options were explored: reducing WP and EVs shares, lowering REE content in technologies, limiting supply to rich deposits, enhancing recycling, using alternative sources (coal, bauxite, phosphorus), and increasing demand for other co-produced metals. A dynamic material flow-stock model was used, incorporating IEA scenarios for WP and EV development, along with parameters for market share, material content, lifetime, and recycling rates. The model calculated REE demand in WP and EVs based on these scenarios and accounted for the varying material content of the technologies over time. Supply was estimated from primary sources (all deposits or Dy-rich deposits) and secondary sources (recycling scenarios). Energy, water, and CO2 emission intensities were based on data from Chinese deposits, with adjustments for production from other regions. The model calculated the oversupply of coexisting REEs, and associated energy, water, and CO2 emissions. Alternative REE supply sources (coal, bauxite, phosphate gypsum) were also evaluated, along with their impact on supply-demand balance.

The analysis revealed a substantial increase in REE demand for WP and EVs. This led to significant oversupply of other coexisting REEs, particularly due to Dy demand. Cumulative energy, water, and CO2 emissions associated with this oversupply were several times higher than those directly related to Nd and Dy production. The CO2 emissions linked to REE demand and oversupply represented a substantial portion (10-29%) of the CO2 emissions reduction expected from EV use in IEA scenarios. Dy demand, specifically, drove the highest oversupply of REEs. Annual ThO2 and U3O8 production associated with Dy demand was substantial, raising concerns about radioactive waste management. Several mitigation strategies were assessed. Reducing REE demand through changes in material content reduced cumulative energy, water, and CO2 emissions by around 39%. Recycling strategies offered reductions of approximately 35%. Limiting supply to Dy-rich deposits significantly reduced the environmental impacts of oversupplied REEs while simultaneously increasing the impacts associated with Dy and Nd demand. The combination of these mitigation options led to significant reductions in overall emissions. Analysis of alternative REE sources like coal, bauxite, and phosphate gypsum showed varying ratios of Dy production to other REE production, revealing some potential benefits and drawbacks compared to traditional deposits. Lastly, the study included an analysis of the impacts of EVs on other metals such as lead. It found that higher recycling rates and EV adoption decreased the demand for primary lead production, potentially affecting the supply of lead's companion metals.

The study's findings underscore the importance of considering coexisting minerals when evaluating the sustainability of clean energy technologies. The substantial environmental impacts of REE oversupply highlight the need for a holistic approach. Mitigation strategies involving reduced demand, increased recycling, and targeted sourcing from rich deposits demonstrated significant potential for reducing environmental impacts. The complex interplay between demand, supply, and co-production requires integrated assessment models to accurately evaluate the sustainability implications of energy transitions. The results caution against overly optimistic assessments of emissions reductions from clean technologies without fully accounting for the indirect impacts of material sourcing and processing.

The paper concludes that the sustainability of WP and EVs is strongly influenced by the co-production of REEs. Significant oversupply of companion REEs, driven mainly by Dy demand, leads to substantial environmental impacts. Mitigation strategies, including changes to material content, increased recycling, and sourcing from rich deposits, show promise in reducing these impacts. Future research should further investigate life-cycle impacts of REE extraction from diverse sources and explore technological innovations to improve resource efficiency and reduce the reliance on critical metals.

The analysis relies on specific scenarios for future energy demands and technological advancements which may differ from real-world developments. The data used for energy, water, and CO2 emission intensities are largely based on Chinese production, and may not fully capture the variability in environmental performance across different REE deposits globally. The allocation method for co-produced materials may influence the results, and there is a need for further research to refine this aspect. The model does not consider all potential social and economic factors associated with REE production and utilization.