A two-step phosphorus–sulfur wet-process phosphoric acid route enabling simultaneous rare earth leaching and co-production of high-whiteness hemihydrate gypsum whiskers

Abstract

The conventional wet-process phosphoric acid production results in most associated rare earth elements (REEs) being retained in phosphogypsum, rendering their recovery uneconomical. To address this challenge, this study proposes a novel two-step phosphorus-sulfur wet process. In the first step, phosphate concentrate was leached with phosphoric acid to simultaneously dissolve phosphorus and REEs. In the second step, sulfuric acid was added for selective calcium precipitation, enabling phosphoric acid recovery and the production of high-whiteness hemihydrate gypsum whiskers. Characterizations and molecular dynamics simulations were conducted to elucidate REE migration mechanisms. Experimental results indicated that the leaching rates of phosphorus and REEs reached 97.68% and 90.25%, respectively, in the leaching stage. Fluorapatite lattice dissociation occurred during this process, synchronously releasing REEs embedded in the lattice. In the decalcification stage, the phosphoric acid yield reached 98.57%, gypsum whiteness reached 98.4%, and 30.72% of REEs were lost via co-precipitation. The overall phosphorus utilization efficiency and total rare earth element (ΣREE) recovery rate of the process were 96.28% and 62.53%, respectively. Simulations based on simplified aqueous systems verified that all crystallographic planes of hemihydrate gypsum can adsorb Y³⁺, suggesting that surface adsorption may contribute to REE loss under certain conditions.