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The relative quantities of ore mined and waste rock (i.e., overburden) removed to produce the rare earth elements—their rock-to-metal ratios—were calculated for 21 individual operations or regions covering nearly all mine production in 2018. The results indicate that the rock-to-metal ratios for the total rare earth elements ranged from a low of 1.6 × 101 to a high of 3.6 × 103, with operations in Brazil and Russia having the lowest ratios and ion-adsorption clays operations in China and Myanmar having the highest. For comparison, the global average rock-to-metal ratio for the total rare earth elements (9.8 × 102) fell between that of cobalt (8.6 × 102) and tungsten (1.1 × 103). Driven by their relative abundance in the ore and unit prices that were used in the economic allocation of the environmental burdens, the global rock-to-metal ratio for individual rare earth elements was lowest for cerium (2.3 × 101) and lanthanum (7.7 × 101) and highest for dysprosium (1.7 × 104), terbium (3.7 × 104), and lutetium (6.4 × 104). Like the rock-to-metal ratios for the total rare earth elements, rock-to-metal ratios for individual rare earth elements varied by roughly two orders of magnitude among the various operations examined. An alternative perspective of only accounting for the overburden that is physically removed in ion-adsorption clays in-situ operations yielded global rock-to-metal ratios that were an order of magnitude lower or less for many of the rare earth elements.

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1. Introduction

The rare earth elements (REEs)—the lanthanides and yttrium—have increasingly captured the attention of policymakers, manufacturing industries, researchers, and academics over the past decade due to their heightened supply risk (Blengini et al., 2020Nassar et al., 2020Nassar and Fortier, 2021) stemming from the concentration of their production in China (Gambogi, 2022), the near-complete import reliance of many consuming nations for them (Nassar et al., 2020), their limited end-of-life recycling (Graedel et al., 2011Jowitt et al., 2018), the “imbalance” of supply among the individual REEs (Elshkaki and Graedel, 2014), and their use in a wide range of applications that includes fast-growing clean energy technologies, as well as defense applications, for which substitution is generally limited (Alonso et al., 20122023Graedel et al., 2015). The environmental impacts of REE mining and processing have also been gaining increased attention. In addition to noting the elevated levels of radioactivity due to the presence of thorium, China´s State Council reports that the mining, dressing, smelting, and separating of REE ores has “severely damaged surface vegetation, caused soil erosion, pollution, and acidification, and reduced or even eliminated food crop output” (Information Office of the State Council, 2010). Although at a much smaller scale, these environmental concerns are not restricted to operations in China. In 2019, for example, the production of radioactive waste at a REE processing facility in Malaysia almost caused the closure of that facility (Law, 2019).

Consequently, there has been growing interest in understanding the life cycle impact of REEs. A recent review (Bailey et al., 2020) indicates that, as of 2020, there have been at least 24 life cycle inventories (LCIs) on REEs, with somewhat varying scopes. Most studies focus on one or two major carbonatite deposits, such as Bayan Obo in China, Mountain Pass in the United States, and Mt. Weld in Australia, with few examining the recovery of REEs from heavy mineral sand (HMS) or ion-adsorption clay (IAC) deposits (e.g. Deng and Kendall, 2019Schulze et al., 2017Vahidi et al., 2016Zapp et al., 20222018). This is noteworthy given the differences among these deposit types and their processing method. For example, economically viable bastnaesite-monazite bearing carbonatite deposits typically have rare earth oxide (REO) grades constituting a few percent of the ore. These deposits are predominantly composed of “light” REEs, namely La, Ce, Nd, and Pr. Moreover, REE-carbonatite mines are typically open-pit operations (with notable overburden removal campaigns) that produce few (if any) co-products. In contrast, operations that process monazite-xenotime bearing HMS are predominately focused on recovering titanium (ilmenite and rutile) and zirconium (zircon) minerals, with the rare earth minerals typically constituting a very small fraction of both the raw sands and the associated revenues. Moreover, these operations utilize dredging techniques that may not require the removal of any waste rock (i.e., overburden) to access the raw sands. IAC deposits have notably different REE distributions than bastnaesite and monazite and are the largest source of “heavy” REEs (i.e., Gd–Lu and Y) globally. IAC deposits typically have very low REO grades (generally in the range of 0.05–0.2%) (Golev et al., 2014), making their processing viable via in-situ, tank, or heap leaching in China and, more recently, in Myanmar (Burma).

One aspect of the environmental impacts of REEs that has not been fully addressed is that of Total Material Requirements (TMR). TMR analyses attempt to holistically quantify the total amount of material required to produce the product in question by including the direct, indirect, and hidden material flows associated with the production process (Watari et al., 2019). For mineral commodities, the quantity of ore mined and waste rock removed (i.e., overburden that must be removed to access the ore) is the major contributor to the TMR of mineral commodities (Kosai and Yamasue, 2019Watari et al., 2019). Assessed on a relative basis, a mineral commodity´s “rock-to-metal ratio” (RMR) is an analogous metric that can be used to evaluate these environmental burdens across commodities and provides an additional factor for product designers to consider when selecting materials. A recent study (Nassar et al., 2022) examined the RMRs for 25 mineral commodities and found significant variation across commodities, with Earth-abundant commodities like aluminum, iron, magnesium, and silicon having RMRs on the order of 101 and geologically scarce elements, like gold and rhodium, having RMRs on the order of 106. That analysis also found significant variation in RMRs—often of several orders of magnitude—among operations for individual mineral commodities, with ore grade being the largest controlling factor (Nassar et al., 2022). This is noteworthy given the correlation between ore grade and energy usage and, in turn, Global Warming Potential (Calvo et al., 2022Koppelaar and Koppelaar, 2016Magdalena et al., 2021Mudd, 2010)—a matter of special consideration for mineral commodities like the REEs that find significant use in clean energy technologies such as wind turbines and electric vehicles.

In this analysis, we utilize the same approach as the previous RMR study (Nassar et al., 2022) to provide a detailed examination of the RMRs for the REEs. Given differences in processing methods, REE distributions, and ore grades, the goal of this work is to provide a comprehensive evaluation of the RMR for the REEs across deposit types and mining methods, covering virtually all known mine production in 2018. The work therefore aims to provide not only a globally representative assessment but also intends to offer a better understanding of the variability in RMR values across different operations.

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