Abstract

Several metals, particularly the Rare Earth Elements (REE), are considered critical for technological development and key enablers of the sustainability agenda. Nevertheless, their general availability is limited, being mainly produced in China. Therefore, increasing effort has been observed for the REE and Rare Earth Oxides (REOs) recycling process development. One of REOs application are Solid Oxide Electrolyser Cells (SOEC), an emerging technology to produce green hydrogen (H₂) industrially, with a lifetime currently about 5 years or more, representing a significant future source of REOs. Therefore, the recycling of a SOEC Membrane Electrode Assembly (MEA), consisting of electrodes, electrolyte, contact/barrier layer, is developed as part of a research project, and the respective process is analysed in the current work. A recycling process consumes several resources to achieve its goal, and there are always unavoidable losses according to the second law of thermodynamics (2LT), contributing to a “spiral” economy rather than to a “circular” one. In this way, simulation-based Life Cycle Assessment (LCA) is applied, aiming to understand the environmental impacts (e.g., climate change, acidification) of the MEA recycling, while exergy conservation is assessed by quantifying processes irreversibilities correlated to entropy generation according to the 2LT. Software HSC Sim 10.5.3 is applied for simulating the processes in terms of mass, energy, and exergy balances, combined with software openLCA 2.4.1 for the calculation of environmental impacts using Environmental Footprint 3.1 method. In addition, the ecoinvent cut-off version 3.11 is applied as the background database (mainly for energy/utilities). The assessment focused on the leaching of electrodes and electrolyte, while the underling experimental work is not part of the current paper. First, the electrodes leaching experimental conditions are obtained from laboratory experiments (1-6 h, 40-90 °C, 10-500 g/L, 4.5 mol/L hydrochloric acid) and subsequently modelled. The leaching of the undissolved residue (electrolyte) is also evaluated (sulphuric acid, different additives, and solid/liquid ratios). For both steps, it is identified the optimal conditions under which the laboratory leaching yield is maximized, while the environmental impacts and exergy losses are minimized. For the electrodes leaching, the optimized conditions are selected (70°C, 2-3 h, 300-400 g/L), and the electricity consumption is considered a hotspot in terms of environmental impacts. Regarding the electrolyte dissolution step, at higher solid/liquid ratios, the environmental impact and exergy losses per kg of recovered metal is significantly reduced, despite a minor decrease in the leaching yields. Electricity and sulphuric acid are the hotspots of the observed results, while the considered additives have low influence on the results. The current processes have a Technology Readiness Level of 4 and the electricity consumption considered in the assessment does not yet reflect industrial scale operation. Therefore, their performance can be further enhanced through heat management. Nevertheless, scenarios to estimate heat loss are also considered. This work contributes to recycling optimization, beyond the technical perspective. In future work, the digital twin of the process will be expanded to include further recycling steps up to the obtention of final products, leading to a fully optimized and digitalized SOEC recycling process.