Extraction of rare earth elements from a South American ionic clay

Ionic clays, formed by the natural weathering of REE-bearing minerals and the adsorption of the resulting liberated REE ions onto the clay surface, are an important resource for critical rare earth ele-ments (REEs). Here, a two-step desorption process using ammonium sulfate with active pH adjustment using sulfuric acid was developed to extract REEs from a South American clay. The desorption process was optimized using response surface methodology approach and the optimum operating conditions were determined to be 0.15 mol/L ammonium sulfate, pH 3, liquid to solid ratio of 3/1, and 25 degrees C with 20 min residence time. It is shown that this ionic clay is significantly different from previously reported clays, e.g., from southern China, as it consists of three modes of REEs, including ion-exchanged REEs physically adsorbed on the clay surface, hydrolyzed REEs chemically adsorbed on the clay surface, and mineralized (non-desorbable) REEs within the clay. Mechanistic investigations through progressive acidification during desorption and adsorption isotherms show that REE desorption/adsorption occurs due to the combined action of physical ion exchange adsorption and surface complexation chemical adsorption, with their relative importance depending on the pH of the system and the amount of sulfate anions present within the solution. This work supports overall efforts to utilize ionic clays as a relatively new resource for REEs to empower the development and adoption of modern green technologies such as wind turbines and electric vehicles.(c) 2022 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.