Characterization and stability assessment of rare earth tailings under dry and paste stacking conditions

The management of rare earth element (REE) tailings presents unique challenges due to their fine-grained nature, carbonate-rich composition, and potential environmental sensitivity. As Europe develops new carbonatite and phosphate-hosted REE projects, large volumes of tailings will require safe and sustainable storage. This study integrates physicochemical characterization, triaxial testing, particle breakage analysis, and numerical modelling to evaluate the behaviour of REE tailings within a tailings storage facility. The tailings were predominantly composed of ankerite (68.8%) and calcite (24.8%), with a specific gravity of 2.99 and fine angular morphology. TCLP testing indicated low overall metal mobility, with minor release of Zn (1.27 mg/kg), Cu (0.30 mg/kg), and Ni (0.06 mg/kg). Triaxial testing demonstrated a strong confinement dependency of behaviour. Both dry and paste-stacked specimens transitioned from limited dilation at 100 kPa confinement to predominantly contractive behaviour at 300–800 kPa, with dry-stacked specimens mobilizing higher strength and stiffness than paste-stacked specimens. Critical-state analyses produced well-defined critical-state lines ( $${R}^{2}$$ = 0.95–0.98), confirming that the material can be interpreted within a critical-state framework. Particle breakage analysis of a representative specimen tested at 600 kPa yielded a modified breakage index of $${B}_{r}$$ = 20.46%, while PSD and FESEM analyses confirmed particle degradation and fabric modification following shearing. These observations indicate that particle breakage contributes to the confinement-dependent contractive response of the carbonate-rich tailings. PLAXIS 2D simulations reproduced the observed stress-deformation trends, indicating greater stress mobilization in dry-stacked deposits and greater stress redistribution in paste-stacked deposits. The study provides an integrated framework linking mineralogy, critical-state behaviour, particle breakage, and numerical modelling for the design and management of future REE tailings storage facilities.