Comparative evaluation of physical and chemical beneficiation routes for rare earth element recovery from coal bottom ash: a case study from Soma Power Plant, Türkiye

The recovery of rare earth elements (REEs) from coal combustion by-products has attracted increasing attention due to their strategic importance and supply constraints. This study evaluates the recovery potential of REEs from bottom ash (BA) produced by the Soma Thermal Power Plant through an integrated physical – chemical beneficiation route. Characterization indicated that REEs are mainly hosted in aluminosilicate glass and mullite phases, showing limited liberation. Physical separation methods, including jigging, shaking table, centrifugal (MGS and Knelson), and magnetic separation, achieved only minor enrichment because of the low density and encapsulation of REE-bearing phases. To overcome these limitations, sequential alkaline and acid leaching were employed. Alkaline pretreatment using 5 mol/L NaOH at 90°C for 4 h partially decomposed the matrix and improved REE accessibility. Subsequent HCl leaching (5 mol/L, 90°C, 3 h) resulted in the highest ∑REE recovery (~85%) for the −0.106 mm fraction, while 4 mol/L HCl and −0.3 mm conditions provided a more selective and stable extraction. The combined alkaline – acid leaching process demonstrated a synergistic enhancement in REE mobilization. Overall, sequential chemical leaching offers an effective and scalable approach for REE recovery from coal combustion residues, supporting resource efficiency and waste valorization.