Magnesium salt-synergistic roasting of mixed rare earth concentrate: killing two birds with one stone for enhanced rare earth leaching and fluorine fixation

Abstract

Rare earth (RE), often referred to as the "vitamins" and "the mother of new materials" for modern industry, are crucial resources underpinning national competitiveness and industrial advancement. Bastnaesite and monazite are the primary sources ensuring a stable global supply of RE. The Bayan Obo deposit is the world's largest RE deposit and is characterized by the coexistence of bastnaesite and monazite. However, traditional methods for processing Bayan Obo mixed RE concentrate (MREC), such as the sulphating roasting and the NaOH decomposition method, pose environmental pollution concerns. This study proposes a novel method involving magnesium salt-assisted synergistic roasting of MREC to enhance RE decomposition and extraction. With this method, leaching efficiencies reached 87.66 % for rare earth oxides (REO), 93.91 % for P, and 37.65 % for F. Compared with the process without magnesium salts, REO and P extraction increased by 48 and 74 percentage points respectively, with notably reduced fluorine extraction. During roasting without additives, bastnaesite decomposes into rare earth oxides and oxyfluorides, whereas monazite remains stable. However, adding magnesium salts promotes the decomposition of monazite. MgCl₂·6H₂O initially decomposes into Mg(OH)Cl, which subsequently reacts with monazite, forming easily leachable rare earth oxychlorides and magnesium phosphate, thereby enhancing RE extraction from monazite. Simultaneously, Mg(OH)Cl interacts with bastnaesite and its decomposition products, generating easily leachable rare earth oxychlorides along with refractory rare earth oxides and MgF₂, significantly reducing fluoride-containing wastewater. Furthermore, compared to MREC, the roasted product shows significantly enhanced pore characteristics, providing increased contact area and active reaction sites for subsequent leaching, thus facilitating RE extraction. Microscopically, after roasting, the formed rare earth oxychlorides exhibit a needle-like or whisker-like morphology with a relatively loose structure. After leaching, the residue primarily consists of monazite, rare earth oxides, and MgF₂, with MgF₂ displaying a dense, smooth, and lacteal. This study provides a novel and environmentally friendly approach for efficiently recovering rare earth from mixed rare earth concentrates.