Abstract
A Comparative Study of Sulfuric Acid and Citric Acid Leaching of Rare Earth Metals from Phosphogypsum
José E. Ortiz-Castilloa,1*, Wenying Liub,1, David Dreisingerc,1, Susan Baldwind,2
1Department of Materials Engineering, The University of British Columbia, Vancouver, Canada
2Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, Canada
ajeortiz@student.ubc.ca, bwenying.liu@ubc.ca, cdavid.dreisinger@ubc.ca, dsue.baldwin@ubc.ca,
Abstract
Rare earth elements (REEs) are critical to modern technologies, including renewable energy, defense systems, and electronics. However, global REE supply remains highly concentrated. This creates supply risks, motivating interest in secondary resources such as phosphogypsum (PG), a byproduct of phosphoric acid production. This study evaluates the feasibility of recovering REEs from PG using sulfuric acid and citric acid leaching under controlled conditions. Head-grade characterization confirmed the presence of Ce, Nd, Y, and Dy, along with impurity elements including Al, Mg, and Fe. Sulfuric acid leaching experiments were conducted at acid concentrations ranging from 0.2 to 2 M and temperatures between 40 and 85 °C to assess REE extraction efficiency and selectivity. Optimal recoveries of 76.0% Ce, 73.6% Nd, and 76.9% Y were achieved at 40 °C using 1 M H₂SO₄, as confirmed by both solution chemistry and solid residue analyses. Structural and mineralogical characterization using X-ray diffraction and scanning electron microscopy revealed pronounced modification of the PG matrix, indicating that REE release is governed by gypsum dissolution–recrystallization processes. Citric acid leaching and mixed sulfuric–citric acid systems were further investigated as alternative extraction approaches. Citric acid alone achieved comparable REE recoveries with improved selectivity over impurity elements. Mixed acid systems enabled a reduction in inorganic acid consumption while maintaining the same REE extraction efficiency. This highlights the potential of organic-acid-assisted leaching as a more sustainable method for REE recovery from PG.
Keywords: phosphogypsum, organic acid leaching, rare earth extraction, citric acid, sulfuric acid leaching