Ultrasound-assisted leaching of rare earth elements from phosphogypsum by citric acid

Abstract

The increasing demand for rare earth elements (REEs), together with environmental concerns about gigantic phosphogypsum (PG) stockpiles worldwide, has attracted interest in a sustainable recovery approach from secondary sources. Conventional REE extraction from PG relies mostly on strong inorganic acids, which are highly corrosive, poorly selective, and have considerable environmental impact. This study investigates the combination use of a mostly green, selective organic acid (citric acid) as a lixiviant and ultrasound-assisted leaching (UAL) as a process intensification strategy for REE recovery from PG, to overcome environmental challenges and enhance leaching efficiency (LE) and kinetics. Leaching experiments were conducted under a sequential one-factor-at-a-time (OFAT) optimization method. Silent leaching (SL) was evaluated first, and then ultrasonic irradiation was applied under different amplitudes to achieve the highest LE of UAL. Dissolved REE concentrations were quantified by inductively coupled plasma mass spectrometry (ICP-MS), and solid residues were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) to explain morphological and structural changes. The results show that citric acid is an effective leaching agent for total rare earth elements (TREE) extraction from PG since it has a strong chelating affinity toward trivalent REE ions under leading SL conditions (citric acid concentration = 2 M, temperature = 70 °C, liquid-to-solid ratio = 15 mL g⁻¹, and leaching time = 120 min). The introduction of ultrasonic irradiation further enhanced leaching kinetics and increased the LE of TREE by approximately 15.3% at an ultrasonic power of 55 W. Excessive ultrasonic power reduced efficiency due to particle agglomeration and acoustic shielding effects. Comparative experiments confirmed that citric acid significantly outperformed acetic acid under both SL and UAL conditions. Overall, the study demonstrates that ultrasound-assisted citric acid leaching is an effective and environmentally sustainable method for REE recovery from PG that supports secondary resource valorisation and circular-economy development