the environmental footprint of rare earth products

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Behind the scenes of clean energy – the environmental footprint of rare earth products Praneet Singh Arshi, Ehsan Vahidi, and Fu Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03484 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Sustainable Chemistry & Engineering

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Behind the scenes of clean energy – the environmental footprint of rare earth products Praneet S. Arshi 1,2, Ehsan Vahidi 2, 3 and Fu Zhao 1,2,3,*

1

School of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47907, United States

2

Ecological Sciences and Engineering Interdisciplinary Graduate Program, Purdue University, Young Hall, Room B-40, 155 South Grant Street, West Lafayette, IN 47907, United States 3

Environmental & Ecological Engineering, Purdue University, 500 Central Dr, West Lafayette, IN 47907, United States

ACSPhone: Paragon Plus Environment *F. Zhao. Email: [email protected]; 765-494-6637


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Abstract Critical to the functionality of energy efficient lighting, off-shore wind turbines, and electric vehicles are rare earth (RE) containing phosphors and magnets. With an increase in the market penetration of these clean energy technologies, demand for RE containing components is expected to grow. However, the production of rare earth elements (REEs) has an adverse impact on the environment. Existing literature provides some information on the environmental impacts but often fails to give a detailed production pathway that can be modeled without preexisting knowledge of life cycle analysis (LCA) or a dedicated LCA software. In this study, life cycle inventories were compiled based on representative production pathways in China using facility level energy/material data. Phosphors and magnets using REEs from monazite/bastnasite deposits in Bayan Obo as well as ion adsorption clays from China’s southern provinces are covered. Analysis of inventory data shows that electricity requirement and emissions to water have the highest contributions to the impact categories of global warming, acidification and eutrophication. An interconnected Excel database system is also developed to help researchers and decision makers identify environmental hotspots and develop improvements in the production pathways.

Keywords Rare earth elements, phosphors, magnets, life cycle analysis, ecoSpold

ACS Paragon Plus Environment

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Introduction Rare earth elements (REEs) are a group of 15 lanthanides and two group 3 elements – scandium and yttrium1. Among REEs, lanthanum to europium (atomic number, Z: 57 to 63) are classified as light rare earth elements (LREEs), while gadolinium to lutetium (Z: 64 to 71) are considered heavy rare earth elements (HREEs).2 Both scandium (Z: 21) and yttrium (Z: 39) are grouped with HREEs.2 Rare earth elements, contrary to the name, are not actually rare; they are abundant in Earth’s crust.3 However, REEs exhibit similar properties and usually occur in mineral deposits in low concentrations, making them difficult to extract as individual metals.3,4 There are more than 200 known rare earth containing minerals5 ; however, monazite, bastnasite, and rare earth (RE) containing clays are the primary economically viable production sources.6 Bastnasite is a REE-carbonate-fluorine mineral and is the primary ore mineral at the Bayan Obo mines in China, the world’s largest REE deposits.7 Light REEs are primarily extracted from this mineral.7 Monazite on the other hand is a REE-thorium-phosphate mineral which is also used to extract light REEs. RE containing clay deposits on the other hand have a unique REE composition which is rich in valuable heavy REEs.7 The global RE production is currently dominated by China. In 2015, China accounted for roughly 85% of the global rare earth oxide (REO) production.8 Along with their expertise in REO extraction from respective ores, China also specializes in downstream processes such as production of rare-earth metals, magnets, and phosphors.8 The extraction and purification of RE ores require material and energy intensive processes along with radioactive thorium emissions (for monazite ores); China has faced severe environmental consequences due to its loose regulations and high production output.3,5,9 The Chinese government has acted on these environmental issues by



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reducing domestic production, introducing export quotas, and shutting down smaller, high polluting production facilities.10 REEs have become critically important in electronic, magnetic, and catalytic applications with their unique physical and chemical properties.11 In particular, REEs have found niche applications in clean energy technologies such as fluorescent lights, LED light bulbs, and wind turbine magnets.2 As the REE production process degrades the surrounding environment, its life cycle impacts must be thoroughly studied. One can evaluate the environmental performance of the production process by the widely used methodology of life cycle analysis (LCA).12 In recent years, a few LCA studies have been published across the spectrum evaluating the environmental impacts of the RE production process. The popular LCA database Ecoinvent (version 2.0 and subsequent versions)13 provides some information on the production of five REOs (lanthanum, cerium, neodymium, praseodymium and samarium-europium-gadolinium mixture) from bastnasite at the Bayan Obo mines as a combined unit processes.14,15 However, one should note that the actual ore composition at Bayan Obo is a mixture of bastnasite and monazite. In addition, in the Ecoinvent dataset the process emissions are based on the Mountain Pass facility. Furthermore, the process data has not been updated since 2004 which creates questions about the reliability of the results. Zaimes et al.3 determined the environmental footprint and resource intensity of REO production process from Bayan Obo mine and concluded that mining along with extraction and roasting have the largest contribution to the overall impact if a mass based allocation system to distribute the impact between multi-product processes was considered. Despite the mining process not being unique to REEs, its contribution to the overall environmental footprint was high. Also, the production of heavy REOs consumed 20 times more primary energy than steel per unit mass.3


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A recent article by Vahidi et al.11 looked at the production of mixed REOs using ion-adsorption clays. Rich in HREEs, ion-adsorption clays showed similar environmental impact in the impact categories of global warming potential (GWP) and cumulative energy demand, but significantly differed in eutrophication and acidification. Lee and Wen16 take the REE impact assessment process a step further by considering the solvent extraction and metal refining process. The environmental impacts of producing REEs from Bayan Obo mine, bastnasite deposit in Sichuan, and kaolin clays in seven southern provinces were quantified. For each unit process, lower and upper bounds were provided for every input/output material or energy flow based on information extracted from literature, industry reports, government emission standards, and industry survey. Opting for mass allocation in co-production processes, Lee and Wen concluded that the environmental impacts across major impact categories were concentrated in the oxide and metal refining process at Bayan Obo and Sichuan. The impacts using bastnasite in Sichuan were lower in absolute terms for eutrophication, acidification, global warming, and human toxicity. For insitu leaching in the southern provinces the impacts were distributed evenly between leaching and refining. It is important to note that even though a range is provided with low/high impact scenarios quantified, the material/energy flows in the unit process inventory are likely not independent. That is, in certain situations when a low value for one material input is taken, it causes an increased consumption for another material/energy input. For example, for in situ leaching one study reports that 6.7 ton ammonium sulfate and 0.2 ton sulfuric acid are consumed per ton REOs produced while the other reports 5.2 ton and 0.5 ton respectively.17,18 Moreover, Lee and Wen use rare earth industry emissions standard to approximate actual process emissions without considering process characteristics.



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Moving to REE products, one of the only LCA papers is by Sprecher et al.10, which analyzed the entire production route of neodymium-iron-boron (NdFeB) magnets using REEs obtained from the Bayan Obo mine and also looked at two hypothetical recycling processes for the magnets. The production of 1 kg of generic NdFeB magnets show that for a high-tech production process, most of the impacts are attributed to its energy use. With emission control and efficiency measures, the sensitivity analysis showed a decrease in human toxicity impact category by an order of magnitude. It was also concluded that recycling of magnets had a significantly lower impact compared to virgin production method with manual dismantling providing the maximum environmental benefit by drastically reducing the amount of wasted neodymium. It should be noted that in this study Ecoinvent processes are modified when modeling solvent extraction and RE metal production process is modeled by modifying Hall-Héroult process for aluminum production. All the studies mentioned have variations in their functional units and system boundaries. Some set the system boundary before the solvent extraction process to avoid allocation, while some consider the individual REO production or other downstream processes but have other limitations. The variations in the RE deposits, system boundary, and the limitations of these studies is summarized in the supporting information. As the use of REEs continues to increase, there is a need for an open source environmental assessment tool for researchers and decision makers. Despite having the LCA methodology, only a limited number of studies have evaluated the potential environmental impacts of RE metals and its products; possibly due to information deficiency and lack of understanding of Chinese data. This paper builds on existing literature and Chinese facility reports to evaluate the life cycle assessment of four rare earth phosphors along with neodymium-iron-boron (Nd-Fe-B) magnets, both of which are critical to the clean energy industry. Detailed material and energy flow data for each process step is compiled to create an


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interlinked Excel based model that allow users to analyze different production pathways/scenarios and to add new processes as data becomes available.

Methodology The LCA methodology as per ISO 14040 is adopted in this study.19

Figure 1. System boundary for LCA study



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Scope and System Boundary The RE production pathway for three types of mineral deposits in China are considered due to their market dominance.8,20 Monazite and bastnasite minerals are found in the Inner Mongolia region (northern China) while RE containing ion adsorption clays have a wide distribution over southern China.20 As per standard LCA methodology, the system boundary is defined after identifying the RE deposits to be investigated.21 As shown in Figure 1, all processes from mining to rare earth metal and product manufacturing are included. However, the downstream processes such as use, disposal and recycling are not included. For each individual process, the flows are scaled to 1 kg of primary product output.

Production Pathway and Inventory Analysis REO Production - Bayan Obo deposit The Bayan Obo mines contain monazite and bastnasite minerals.20 This deposit serves as the largest REE producing mine in China22 with the ratio of bastnasite to monazite in the ore being 7:3 by mass.3 According to Chinese reports, monazite/bastnasite ores are used to produce sufficient quantities of lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide and a samarium-europium-gadolinium mix oxide.23 The production pathway to obtain REOs includes mining, beneficiation, acid roasting, leaching, followed by solvent extraction and calcination. The first step in the process is open pit mining using drilling and blasting to obtain the iron ore. The output includes a small amount (5-6% REO equivalent) of rare earths ores.10 The mining inventory obtained from Sprecher et al. builds on the Ecoinvent process of iron mining with additional environmental flows of rare earths.10 The mined ore undergoes a two-step beneficiation


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process. The first involves ball milling and magnetic separation to isolate the iron ore. The tailing, which contains monazite and bastnasite is then processed to get a 50% REO equivalent concentrate via flotation. Since iron ore beneficiation is a multi-output process (i.e. producing iron ore concentrate and tailing containing monazite and bastnasite),24 economic allocation is used to determine the environmental impact associated with concentrated REO ores. As the iron ore beneficiation uses mined iron ore as input, such an allocation also means that the environmental impacts of iron ore mining is also allocated to rare earth production with same coefficient. In the following acid roasting process, 50% REO concentrate is heated to 550ºC with 93% sulfuric acid to produce rare earth sulfates (roughly 50% RE sulfate present). The method currently employed in China uses advanced pollution control and recycles some of the sulfuric acid.25 Following acid roasting, the RE sulfates are leached and converted to 92% rare earth chlorides, and the conversion efficiency of this process is roughly 97%. 26 The mixed RE chloride (92% pure) undergoes solvent extraction (SX) process where the aqueous solution is mixed with an organic extractant such as di-(2-ethylhexyl) phosphoric acid.27,28 The separation process is carried out by taking advantage of the small difference in basicity between different rare earths. At the end of the solvent extraction process, high purity (99% pure) aqueous solutions of individual rare earth ions are generated.27,29 The inventory is based on an industry report from a processing facility in Baotou for solvent extraction process.23,30 This facility has undergone an upgrade by switching from coal to natural gas to reduce environmental impact.23 Future plans include waste water treatment to insure zero pollutants release in the water stream.30 The rare earth carbonates are then obtained by precipitation of rare earth ions in the aqueous solutions and then the precipitated RE carbonates are further calcinated at 750 – 800oC to form individual REOs using natural gas as the heat source.23



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REO Production – Ion Adsorption Clays Clay deposits from South China contain 0.04 – 0.25% concentration REOs7 with the industrial reserves a mere 3% of the total Chinese reserves.31 Despite the low concentration and reserves, the production of REEs from these deposits have increased over the past decade.31 The lower concentration clay deposits are economically viable due to their ease of mining and capacity to concentrate with weak acids along with the high demand for HREEs.7,31 Schüler et al. identified seven provinces in southern China where HREEs are economically extracted: Jiangxi (36% of total mined RE clay), Guangdong (33%), Fujian (15%), Guangxi (10%), Hunan (4%), and YunnanZhejiang (combined 2%).31 Up to 15 individual REO can be separated from these clay deposits.32 Out of the various techniques used to extract REEs from ion-adsorption clays, in-situ leaching has become dominant.11 The ion-adsorption clays are leached using ammonium sulfate and the leachate is then precipitated using ammonium bicarbonate or oxalic acid.11 After drying and calcination, 90-92% mixed REOs are produced.11 The REO mixture from the leaching process undergoes a combined solvent extraction process at a facility in Southern China. The process inventory includes conversion to RE chlorides, repeated SX using organic solvents such as P204 and P5073,32, and precipitation using oxalic acid. The high purity individual RE precipitate is further roasted to obtain separated pure oxides.3,10,32 It should be noted that compared with the solvent extraction facilities in Bayan Obo which produces only 4 REO products, the SX operation of ion-adsorption clays in Southern China produce 15 individual REOs.


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Rare Earth Metals and Alloys The metal refining process, unlike the REO production process, is not specific to any region in China.16 Pure rare earth metals can be processed from REO using metal refining processes such as molten salt electrolysis, metallothermic reduction, and electrowinning. Out of the various techniques, molten salt electrolysis process is commonly used for the electrolytic reduction of metal compounds to pure metals.33 In this process, lithium fluoride, RE fluoride, and REO are the primary input materials.34 The first step is to make the RE fluoride using separated REO from solvent extraction process.34 Some of the produced RE fluoride is sold separately while the remaining is taken for electro-refining. In the second step, RE fluoride to lithium fluoride ratio of about 10:1 by mass and a total REO to RE fluoride ratio of 10-15:1 by mass is used to produce pure RE metal. Plant level electricity consumption data was available which was allocated between the two steps based on the output mass. The same facility uses a similar two-step process to create metal alloys as well. The first step, as previously stated, is used to create the mixed RE fluoride. The next step uses RE fluoride, respective REOs, lithium fluoride, and another alloying metal (if required; for example - iron) to produce the RE or RE-iron alloy.34

Rare Earth Phosphors For RE phosphor production process inventory is obtained from a detailed material inventory from an environmental assessment report for a facility in Jiangsu, China.35 The facility produces a combined 4000 tons of red (Y2O3: Eu3+), green (MgAl11O19 : Ce3+, Tb3+), blue (BaMgAl10 O17: Eu3+), and mixed RE phosphors annually.35 Some of the red, green and blue phosphors are sold separately while the remaining is used to produce mixed RE phosphors.35 Phosphor production requires one to two high temperature sintering processes based on the type of phosphors. Red



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phosphors require a single stage of sintering at 1350oC for 4.5 hours in the presence of a reducing agent while green and blue phosphor production requires one high temperature sintering at 1550oC for 4.5 hours followed by a high temperature reduction step at 1400oC for 3 hours in the presence of hydrogen gas. Intermediate steps involve crushing the agglomerate after each sintering step. Once the final phosphor agglomerates are formed (either after a single high temperature step or double), it is crushed again, washed and oven dried to obtain the pure powdered phosphor.35

Rare Earth Magnets The environmental impact of neodymium-iron-boron magnet is modeled based on the overall production flows obtained from two industry reports in China.36,37 The usual production method involves strip casting using neodymium (Nd), iron (Fe), and boron (B) to form NdFeB flakes. These flakes are jet-milled, magnetically aligned and then compacted. The pressed magnets are sintered, sliced and grounded to desired shape and smoothness. The last step is the application of a protective layer.38 Generally, separated Nd and Pr is not used unless required for high-end applications.10 The exact composition of the magnet changes based on the application. Operation in high temperature environments calls for the use of dysprosium in the production process.36,37

Life Cycle Impact Assessment The Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI) and International Reference Life Cycle Data System (ILCD) methods are used in this study.39,40


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Excel-based LCA Tool Given the importance of REEs in clean energy technologies, an LCA tool that is well documented, easily accessible, and open to customization while covering typical REEs production pathways is highly desirable. To meet these necessities, an Excel based model has been created to calculate the environmental impacts of each major process in the production pathway. The division of the cradle to gate process into its subsequent stages (as seen in the previous section) gives a good resolution of environmental hotspots. The environmental impacts from each unit process is mathematically calculated within the Excel file (attached as supplementary information), based on the inventory data and associated characterization factors. To make the Excel tool truly work independently from any LCA software, characterization factors are included for all impact categories from ILCD and TRACI in separate Excel files. Within the life cycle inventory of REE production, default Ecoinvent dataset is used if a good match can be found for a given material or energy flow.41 To increase the accuracy of the results, the use of surrogate is avoided as much as possible by creating new inventories. For example, barium chloride and naphthenic acid used during the solvent extraction process do not have a good match in the Ecoinvent database thus new inventories are built. If required, standard economic allocation is used for each multioutput process. The effects of market volatility are countered by including the price based allocation factors in the model. The market value42 and output concentration can be altered which is reflected in the allocation factors and in turn the environmental impact of the linked REO Excel data sheet. In an interconnected model, any change in the inventory quantity or related upstream process is immediately visible downstream. The model combines the best of an existing database with the open nature and flexibility of Excel. This makes it convenient to conduct sensitivity and scenario



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analysis. Furthermore, the Excel files are structured in ecoSpold v1 data format to facilitate data exchange.

Results and Discussion Rare Earth Oxide Production Separated pure REO is one of the major outputs of the entire production pathway and marks the system boundary for several other studies. Table 1 shows the TRACI characterized environmental impacts of a few high purity segregated REOs sourced from Bayan Obo deposits and ionadsorption clays. The environmental impacts of other REOs produced using the two sources are tabulated in the supplementary information. Table 1. Life cycle impacts of 1 kg REO common to Bayan Obo mines and ion-adsorption clays.

TRACI Acidification [kg SO2Equiv.]

Cerium Oxide Bayan South Obo China

Lanthanum Oxide Bayan South Obo China

5.50E-02 4.82E-01 6.00E-02 5.26E-01

Neodymium Oxide Bayan South Obo China

Praseodymium Oxide Bayan South Obo China

7.38E-01

6.46E+00 1.31E+00 1.15E+01

Eutrophication [kg N-Equiv.] 1.08E-02 1.99E-01 1.17E-02 2.17E-01 1.44E-01

2.67E+00 2.57E-01 4.74E+00

Global Warming Air, excl. biogenic carbon [kg CO2Equiv.]

6.36E+01 1.59E+02 1.13E+02

6.66E+00 4.75E+00 7.26E+00 5.18E+00 8.93E+01

Human Health Particulate Air 3.51E-02 5.27E-03 3.83E-02 5.75E-03 4.71E-01 [kg PM2,5-Equiv.]

7.07E-02

8.37E-01

1.26E-01

Human toxicity, cancer [CTUh]

3.29E-07 1.99E-07 3.59E-07 2.17E-07

4.42E-06

2.67E-06

7.86E-06

4.76E-06

Ozone Depletion Air [kg CFC 11-Equiv.]

1.31E-06 7.21E-07 1.43E-06 7.87E-07

1.75E-05

9.67E-06

3.12E-05

1.72E-05

Smog Air [kg O3-Equiv.]

9.88E-01 3.04E-01 1.08E+00 3.32E-01 1.32E+01

4.08E+00 2.36E+01 7.26E+00

Individual impact category specific value Low High


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Allocated results show that ion adsorption clays have a lower environmental footprint in most impact categories except acidification and eutrophication. For ion-adsorption clays, direct ammonium and sulfate emissions to water during the leaching process contribute heavily to the acidification and eutrophication potential. Furthermore, the upstream impact of producing ammonium bicarbonate and ammonium sulfate used during the leaching process have a significant environmental burden. Monazite/bastnasite minerals on the other hand use open pit mining and diesel generators, which have a significant environmental footprint in other impact categories like particulate emissions, global warming potential, and smog.

Rare Earth Metals Molten salt electrolysis process adds to the already high upstream environmental impacts of the REO production. Figure 2 shows the percentage contribution of each stage to the cradle-to-gate environmental impacts of producing pure neodymium metal using monazite/bastnasite minerals. The contribution chart of neodymium metal manufacturing shows only a small contribution from the metal refining process. Some of the processes are grouped together for the sake of readability – beneficiation includes the step of producing bastnasite/monazite containing tailing and recovering concentrated bastnasite/monazite; combined acid roasting includes acid roasting and water leaching; while combined SX includes SX and calcination to obtain pure segregated REOs. The mining stage contributes significantly to the smog air (66%) and the particulate emissions (78%) impact categories due to use of diesel powered machines and open pit mining technique. The beneficiation step accounted for 50% of human toxicity (cancer) impact categories due to its use of chromium steel balls (which has a high upstream footprint) for crushing. As a process stage, solvent extraction uses several chemicals/acids and therefore accounts for 39% of global warming



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potential, 35% of eutrophication potential, and 39% of human toxicity (non-cancer) with high contribution for other impact categories as well.

Neodymium Metal Production - Monazite/Bastnasite Minerals Smog Air [kg O3-Equiv.] Resources, Fossil fuels [MJ surplus energy] Ozone Depletion Air [kg CFC 11-Equiv.] Human toxicity, non-canc. [CTUh] Human toxicity, cancer [CTUh] Human Health Particulate Air [kg PM2,5-Equiv.] Global Warming Air, incl. biogenic carbon [kg… Global Warming Air, excl. biogenic carbon [kg… Eutrophication [kg N-Equiv.] Ecotoxicity [CTUe] Acidification [kg SO2-Equiv.] 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Mining

Beneficiation

Combined Acid Roasting

Combined Solvent Extraction

Electrorefining

Figure 2. Percentage contribution of process stages for 1 kg Nd metal sourced from Bayan Obo deposits.

A comparison with the environmental impact assessment of Lee and Wen is carried out for neodymium and cerium metal production (detailed tables are included in the supporting information). Due to the difference in impact categories or the measuring units, only three impact categories were comparable. The cerium comparison reveals that the average environmental impacts are higher in Lee and Wen for most environmental impact categories while in neodymium the result is reversed. Lee and Wen use a mass-based allocation system which allocates a higher


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impact to the cerium oxide as it has a high output quantity while the economic allocation in this study gives more weightage to the neodymium output due to its higher economic value, resulting in the differences.

Rare Earth Phosphors Table 2 shows the cradle to gate environmental impact of 1 kg blue, green, red, and mixed phosphors. Each type of phosphor requires different REO inputs. Blue phosphors require europium oxide; green phosphors require terbium oxide and cerium oxide; while red phosphors require yttrium oxide and europium oxide.

Table 2. Life cycle impacts of 1 kg phosphor.

Blue Phosphor

Green Phosphor, CeO from South China

Green Phosphor, CeO from Bayan Obo

Red Phosphor

Acidification [kg SO2-Equiv.]

2.37E+00

5.62E+00

5.56E+00

5.01E+01 2.96E+01

Eutrophication [kg N-Equiv.]

9.46E-01

2.29E+00

2.26E+00

2.07E+01 1.22E+01

Global Warming Air, excl. biogenic carbon [kg CO2-Equiv.]

3.71E+01

6.82E+01

6.84E+01

5.05E+02 3.04E+02

Human Health Particulate Air [kg PM2,5-Equiv.]

4.43E-02

7.84E-02

8.26E-02

5.63E-01

3.41E-01


3.25E-06

4.48E-06

4.50E-06

2.09E-05

1.34E-05

Ozone Depletion Air [kg CFC 11Equiv.]

3.84E-06

8.61E-06

8.69E-06

7.50E-05

4.44E-05


2.56E+00

4.55E+00

4.65E+00

3.25E+01 1.96E+01

TRACI

Individual impact category specific value Low High


Mixed Phosphor


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Out of all the input REOs, only cerium oxide (CeO) can be sourced from both Bayan Obo mines and ion-adsorption clays. However, it is interesting to note that the relative impact of CeO is negligible on the overall footprint of green phosphors (1% for most impact categories) due to the low overall REO environmental footprint and hence a change in CeO source does not change the result drastically. On comparing the environmental impact figures of different phosphors, the results indicate a higher footprint of red phosphors across all the impact categories. This can be attributed to the larger amount of REOs used to produce red phosphors considering the contribution of upstream REO production has on the overall phosphor production process.

Blue Phosphor Production Smog Air [kg O3-Equiv.] Resources, Fossil fuels [MJ surplus energy] Ozone Depletion Air [kg CFC 11-Equiv.] Human toxicity, non-canc. [CTUh] Human toxicity, cancer [CTUh] Human Health Particulate Air [kg PM2,5-Equiv.] Global Warming Air, incl. biogenic carbon [kg CO2Equiv.] Global Warming Air, excl. biogenic carbon [kg CO2-Equiv.] Eutrophication [kg N-Equiv.] Ecotoxicity [CTUe] Acidification [kg SO2-Equiv.] 0% 10% 20% 30% 40% 50% 60% 70% 80% 90%100% Mixed REO Production

Solvent Extraction

Phosphor Production

Figure 3. Percentage contribution of process stages for 1 kg blue phosphors using europium oxide sourced from ion-adsorption clays.


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Figure 3 shows the percentage contribution of various stages to the cradle-to-gate environmental impacts of the process. The mixed REO production stage includes RE carbonate production by leaching and subsequent calcination to obtain 92% mixed REO product. Considering ion-adsorption clays as the source, the figure shows the massive environmental burden of phosphor production and the mixed RE oxide production stage. With a lower amount of overall RE use in the final production step, the electricity and alumina consumption contribute the most to the overall environmental impacts of the stage across the impact categories of smog, particulate emissions, and human toxicity. As the Chinese electricity production still largely uses coal, the phosphor production step contributes a substantial amount in the human toxicity (cancer) impact category due to the electricity consumption. The mixed REO production from in-situ leaching of ion-adsorption clays however contributes over 90% to the acidification and eutrophication potential of due to the high ammonia and sulfate emissions to water during this stage.

NdFeB Magnets Table 3 shows the environmental impacts of 1 kg NdFeB magnet production modeled from the

inventory of two independent facilities assuming independent use of the two source deposits. In the table, the TDK (Tokyo Denkikagaku Kōgyō, or Tokyo Electronics and Chemicals) pathway refers to the process used by Guangdong TDK- Guangsheng Rare Earth New Materials Co. Limited. It should be noted that both magnet production pathways require dysprosium as input RE metal but monazite/bastnasite deposits do not yield commercially usable quantity of it. Hence, despite selecting two sources for the other input REEs, the required dysprosium for the magnet



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production step is only from ion-adsorption clays. Considering the two production pathways (assuming the same REE source deposit), it is clearly visible that one of the pathway has a bigger environmental footprint. The driving factor for the difference in environmental burden is electricity consumption between the two pathways. However, it would not be wise to compare the two in absolute sense as the two produce magnets of varying grades and coercivity.

Table 3. Life cycle impacts of 1 kg NdFeB magnets produced by two facilities using two REE sources. Guangdong TDKGuangsheng Rare Earth New Materials Co. Limited

TRACI

Bayan Obo

South China

Dongguan Longyue Limited Inc. Bayan Obo

South China

Acidification [kg SO2-Equiv.]

2.41E+00 5.50E+00

3.65E-01

2.78E+00

Eutrophication [kg N-Equiv.]

8.61E-01

7.87E-02

1.13E+00

Global Warming Air, excl. biogenic carbon [kg CO2Equiv.]

8.92E+01 7.53E+01 4.14E+01 3.46E+01

Human Health Particulate Air [kg PM2,5-Equiv.]

3.10E-01

9.46E-02

1.97E-01

5.13E-02


3.94E-06

3.00E-06

2.08E-06

1.55E-06

Ozone Depletion Air [kg CFC 11-Equiv.]

1.26E-05

8.33E-06

6.75E-06

4.24E-06


1.01E+01 5.14E+00 5.53E+00 2.29E+00

2.22E+00

Individual impact category specific value Low High Figure 4 looks at the stage-wise contribution for the TDK production pathway with dysprosium

obtained from ion-adsorptions clays and neodymium from monazite/bastnasite minerals. The mixed REO production step includes mining, beneficiation, roasting, and leaching to obtain the grade of mixed REOs which is used for solvent extraction to produce pure individual REOs. Figure 4 shows that the magnet production step has some contribution to the global warming potential


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and ecotoxicity footprint, which is due to the high electricity consumption. The remaining impacts are dominated by the upstream REE production. As the amount of neodymium used is much higher than dysprosium, its contribution is much higher in most impact categories. For example, the cradle to gate neodymium production (includes mixed REO production, SX, and electrorefining) accounts for 59% of the total GWP, 74% of the total smog potential, and 84% of the particulate emission. However, the dysprosium obtained from ion-adsorption clays has a much higher acidification and eutrophication potential (75% acidification, 87% eutrophication).

NdFeB Magnet Production (TDK) - Mixed REO sources Smog Air [kg O3-Equiv.] Resources, Fossil fuels [MJ surplus energy] Ozone Depletion Air [kg CFC 11-Equiv.] Human toxicity, non-canc. [CTUh] Human toxicity, cancer [CTUh] Human Health Particulate Air [kg PM2,5-Equiv.] Global Warming Air, incl. biogenic carbon [kg CO2Equiv.] Global Warming Air, excl. biogenic carbon [kg CO2-Equiv.] Eutrophication [kg N-Equiv.] Ecotoxicity [CTUe] Acidification [kg SO2-Equiv.] 0% 10% 20% 30% 40% 50% 60% 70% 80% 90%100% Mixed REO Production

Solvent Extraction

Electrorefining

Magnet Production

Figure 4. Percentage contribution of process stages for 1 kg NdFeB magnets produced by the TDK pathway using dysprosium obtained from ion-adsorptions clays and neodymium from monazite/bastnasite minerals.



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The environmental impact data for NdFeB magnet production using these two facility reports vary from the environmental impacts calculated by Sprecher et al. (comparison table added in the supporting information). The TDK production pathway reveals a higher environmental footprint in all comparable environmental impact categories, while the Dongguan facility environmental impacts are closer to the range of values provided by Sprecher et al. (2014). The metal reduction process data in Sprecher et al. is modeled based on the Hall-Heroult process for aluminum production in the Ecoinvent database. The range of values are based on assumptions of baseline and low-tech emissions from the best-case scenario of the Hall-Heroult process, which itself is modeled from a Norwegian aluminum smelter in the Ecoinvent database. Furthermore, Sprecher et al. use several Ecoinvent processes for the European production rather than the Chinese or global scenario, resulting in the differences. These include soda, transportation, hydrochloric acid, and ammonium bicarbonate used at various stages of the production process.

Data Uncertainty Life cycle analysis has inherent uncertainties conditioned upon the accuracy of the product or process inventory data. Effects of these uncertainties are usually taken into account if low/high range of material/energy consumption can be defined. However, actual product data of facilities along the rare earth production pathways has been extremely hard to find. In addition, as mentioned in the literature review, material/energy consumptions within a process are not independent. Low consumption of one chemical at a facility may come at the cost of higher energy consumption or higher use of another chemical. Given these constraints, only a sensitivity analysis is carried out in this study with a focus on global warming. The results are shown as tornado plots and can be found in supplementary information. For blue phosphor results show that if the cradle to gate


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electricity consumption is increased by 10%, the GWP of the phosphor increases by 3.5%. For blue phosphors using REOs from ion-adsorption clays, the GWP increases by 2.16% if the ammonium sulfate consumption is increased by 10%. Similarly, for magnets produced by TDK with dysprosium obtained from ion-adsorptions clays and neodymium from Bayan Obo mines, the change in electricity consumption by 10% only changes the GWP by 2.8%. These values suggest that over the production cycle of the RE-based products, changing one material/energy input will not result in a drastic change in the environmental impact.

Remarks

This analysis compiled the step-by-step inventory of selected RE product manufacturing and performed a cradle to gate life cycle analysis to evaluate its environmental impacts. A transparent and user editable Excel model was created that can offer support to the growing REE industry. The tool i.e. Critical Materials Life Cycle Assessment Tool (CMLCAT) version 1.0 will be periodically updated as new information/data becomes available (see supporting information for tool access). As the demand for REEs increases, there is a pressing need to find efficient and less polluting production pathways. The first step to finding an alternative is to estimate the impacts of existing steps to use as a benchmark. Results of this study as well as the Excel based tool enable researchers to model and evaluate the environmental impacts of new production and recycling methods. Although Chinese REE production pathways were considered in this investigation, these data sets have a broader international application. There are several discovered sites around the world that can be used for REE extraction, each with unique mineral composition and site peculiarity, resulting in a different production processes. The production pathway can be replicated internationally by changing the location of input materials and hence its respective environmental impact, to evaluate the overall footprint of the new process. Currently however, optimizing



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existing steps through waste water recycling, reduction in energy requirement, fuel switching, or any other viable process is essential. Results indicate that implementing waste treatment processes could drastically reduce the acidification and eutrophication potential of sulfate and ammonia emissions for ion-adsorption clays. Furthermore, a long-term approach for cleaner grid power can drastically reduce the overall GHG emissions of the process electricity consumption.

Supporting Information: Literature review summary; allocation method; life cycle impact assessment method; data documentation and format; package use, life cycle analysis results; sensitivity analysis; software user manual; copyright statement.

Acknowledgments This work is supported by the Critical Materials Institute (CMI), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the U.S. Department of Energy.

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Abstract Graphic:


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Synopsis: An open source LCA tool is developed to quantify the environmental impacts of rare earth products used in wind turbines and high efficiency lighting.


the environmental footprint of rare earth products

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