Recovery of Rare Earth from Sedimentary Rare Earth Ore Using an Ammonium Sulfate Roasting and Leaching Process

Rare earth from a newly discovered sedimentary rare earth ore (SREO) in Guizhou Province, China, was extracted through ammonium sulfate ((NH₄)₂SO₄) activated roasting and leaching with sulfuric acid. The phase changes and reaction mechanism during the roasting process were explored through thermodynamic analysis, XRD, thermogravimetric analysis and scanning electron microscopy. When roasted at 650°C, first, the layered structure of kaolinite was destroyed by the reaction between (NH₄)₂SO₄ with the SREO. Meanwhile, it is conducive to the reaction of rare earth minerals with (NH₄)₂SO₄ to produce soluble rare earth sulfate. Second, the surface of the roasted product is loose, which makes it easier for sulfuric acid to erode into the ore. Therefore, the leaching rate of rare earth elements (REEs) improved after acid leaching. Under the optimized reaction conditions, specifically, a roasting temperature of 650°C, an (NH₄)₂SO₄ to SREO mass ratio of 2.5:1, a roasting time of 1 h, a leaching temperature of 80°C, a leaching time of 2 h, an acid concentration of 5 mol/L, and a liquid-to-solid ratio of 9:1 (mg/L), the leaching rate of REEs reached 90.05%, 18.57% higher than that of roasting without (NH₄)₂SO₄ addition.