Highlights

• Counterintuitive LCA findings: recycling NiMH batteries for rare earths can report ~4–9× higher CO₂ emissions per kg of REE than primary extraction from ores—but this reflects allocation choices, not environmental inferiority.
• The comparison is not apples-to-apples: recycling systems recover multiple metals (Ni, Co, REEs), use different functional units, and vary by geography, energy mix, and system boundaries.
• The review calls for harmonized LCAs with transparent data, consistent functional units, and proper co-product credits to fairly assess recycling’s true climate benefits versus concentrated primary mining.

In a new open-access literature review in Metals, Daniel Sánchez Piloto and Denise Crocce Romano Espinosa, Department of Chemical Engineering, Polytechnic School, University of São Paulo) with Amilton Barbosa Botelho Junior, Department of Chemical Engineering, Norwegian University of Science and Technology, NTNU evaluate the climate impacts of producing rare earth oxides (REO) from primary ores (bastnäsite and monazite) versus recovering rare earths from end-of-life nickel–metal hydride (NiMH) batteries. Their headline result is counterintuitive: the reviewed recycling Life Cycle Assessments (LCAs) imply higher reported greenhouse-gas emissions for recycling when normalized narrowly to rare earth output—on the order of ~85–179 kg CO₂-eq per tonne of recovered REE mixture versus ~13–22 kg CO₂-eq per kg of REO reported in selected primary-extraction LCAs—leading the authors to describe recycling as ~4–9× higher under certain simplifications.

Crucially, the authors of this paper caution that this “REE-only” comparison is not cleanly comparable because recycling systems are multifunctional (recovering Ni and Co alongside REEs), employ different functional units, and vary by energy mix, geography, system boundaries, and allocation choices—so the result should not be read as “recycling is environmentally worse.”

Study Approach and Methods

This is a structured literature review. The authors screened studies from major databases (Scopus, ScienceDirect, Web of Science) and focused on Life Cycle Assessment (LCA) results reporting climate impacts (GWP, CO₂-eq). For primary routes, they selected LCAs reporting emissions per kg of mixed REO from bastnäsite/monazite pathways. For NiMH recycling, they prioritized industrial-scale studies, supplementing with lab-scale work where necessary. They also highlight a key gap: no harmonized, direct industrial-to-industrial LCA comparison of primary REO production versus NiMH REE recovery was identified.

Key Findings

• Primary extraction emissions are dominated by chemical processing and separation, including energy- and reagent-intensive steps (notably solvent extraction chains).
• Recycling can appear higher-carbon when impacts are allocated primarily to REEs, because NiMH recycling typically targets multiple co-products (especially Ni and Co). Treating recycling as if it only produces REEs can over-assign burdens.
• Several recycling LCAs were not designed to isolate process-specific emissions for REE recovery, requiring approximate allocations from whole-system results.

Limitations That Matter

The authors emphasize that differences in functional units, system boundaries (cradle-to-gate vs cradle-to-grave), LCI databases, regional electricity mixes, and allocation procedures materially constrain comparability. The reported ranges are best viewed as directional magnitudes under specific modeled contexts, not a definitive verdict on the inherent sustainability of recycling.

REEx Implications and What Should Follow

The review challenges simplistic narratives: recycling is not automatically lower-carbon under today’s industrial configurations—especially when framed narrowly around rare earth output alone. Yet recycling remains strategically valuable because it diversifies supply, reduces dependence on concentrated mining geographies, and may deliver larger net benefits when co-product credits and avoided primary production are properly modeled.

Next steps should include harmonized LCAs with consistent functional units, transparent industrial datasets, and scenarios reflecting cleaner grids and evolving “green solvent extraction” and improved separation routes.

Citation: Sánchez Piloto, D.; Espinosa, D.C.R.; Botelho Junior, A.B. Environmental Impact of Extraction of Rare Earth Elements from Primary Sources and NiMH Batteries: A Literature Review. Metals 2026, 16(3), 254. https://doi.org/10.3390/met16030254 (opens in a new tab)