Mechanistic Insights into Rare Earth Element Dissolution from Sart (Manisa) Black Sand via Alkali Roasting–Acid Leaching

High Resolution Image Download MS PowerPoint Slide The recovery of rare earth elements (REEs) from REE-bearing heavy mineral tailings of the Manisa–Sart (Türkiye) placer deposit─with an REE-bearing phase consistent with a monazite-group phosphate host, supported by SEM–EDS microanalysis and bulk geochemical evidence─was investigated via an integrated alkali roasting–reductive acid leaching route. The effects of roasting temperature, NaOH stoichiometry, acid type and concentration, reducing agent addition, and the heating method (conventional vs microwave) on REE extraction efficiency were systematically evaluated. Roasting at 600 °C with a 1:1.5 ore-to-NaOH ratio (w/w) provided the most effective structural activation of the REE-bearing matrix, while 4 M hydrochloric acid was identified as the most effective leaching medium; sulfuric acid yielded substantially lower recoveries due to double-sulfate precipitation. The addition of hydrogen peroxide (H 2 O 2 ) enhanced cerium extraction approximately 4-fold by reductively converting insoluble Ce(IV) oxide to soluble Ce(III) species. Under optimized conditions (4 M HCl + H 2 O 2, 90 °C, 120 min), neodymium (Nd) and cerium (Ce) extraction efficiencies reached 90.7% and 50.0%, respectively. Dissolution kinetics were analyzed using the Shrinking Core Model (SCM) over a temperature range of 60–90 °C. Both the surface chemical reaction and product layer diffusion control models provided statistically comparable fits ( R 2 ≈ 0.944–0.958), and Arrhenius analysis yielded apparent activation energies of 25.79 and 42.95 kJ/mol, respectively, indicating a surface reaction-dominant mixed-control dissolution mechanism. Microwave-assisted roasting (30 min) achieved a Nd extraction of 80.1%, exceeding conventional furnace roasting (2 h) by 5.3 percentage points. This improvement is consistent with an enhanced structural activation under microwave-assisted heating. Titanium dissolution was limited to 31.5% under all conditions, suggesting a natural process selectivity against titanium under the investigated conditions. These findings provide a mechanistic framework and establish energy-conscious operational parameters for the recovery of REEs from secondary placer resources.