Thorium speciation in synthetic anhydrite: Implications for remediation and recovery of thorium from rare-earth mine tailings

• Published in: Hydrometallurgy 2022-10, Vol.214, p.105965, Article 105965
• Main Authors: Chadirji-Martinez, Kamil, Grosvenor, Andrew P., Crawford, Andrew, Chernikov, Roman, Heredia, Emillio, Feng, Renfei, Pan, Yuanming
• Format: Article
• Language: English
• Subjects: anhydrite; REE mine tailings; synchrotron X-ray absorption spectroscopy; Th extraction; thorium
• ISSN: 0304-386X; 1879-1158
• DOI: 10.1016/j.hydromet.2022.105965

Knowledge about the capacity and mechanisms of thorium uptake in anhydrite is required for not only the safe management of radioactive rare earth element (REE) mine tailings but potential recovery/remediation of this radionuclide from these waste materials for the fourth-generation nuclear power plants. In this study, we investigated Th-doped anhydrite synthesized in both aqueous and sulfuric acid solutions, including samples grown in conditions simulating the Sulfuric Acid Roasting Method (SARM) with monazite-(Ce) ores, at a wide range of temperatures and pH values. Inductively coupled plasma mass spectrometry analyses show samples grown hydrothermally at 353–473 K and pH = 0.3–3 contain 2310–8880 mg/L Th, and 1780 mg/L Th when anhydrite was synthesized by evaporation at 551 K and pH = 0.3. Synchrotron Th LIII-edge X-ray absorption spectroscopy reveals that lattice-bound Th in anhydrite, via the substitution Th4+ + O2− ⇄ Ca2+ with an interstitial O2− ion, can account for at least 1780 mg/L Th, while Th-rich phases such as thorianite and thorite are present in those with still higher Th contents. Therefore, lattice-bound Th in anhydrite must be considered for safe management strategies and future extraction technologies for the recovery/remediation of Th from radioactive REE mine tailings. This study suggests that anhydrite is a major carrier of thorium in the tailings of rare earth element mines. Recovery of thorium from tailings not only reduces their radioactivity, but represents a valuable resource for nuclear fuel production. This study suggests anhydrite as a major host of thorium in rare-earth mine tailings. Technologies for the remediation and recovery of thorium from rare-earth mine tailings must consider anhydrite. [Display omitted] •Anhydrite is a major host of thorium in residues of rare earth extraction with sulfuric acid.•Thorium uptakes into anhydrite via substitution for calcium.•Anhydrite has a capacity of thorium uptake at least 1780 mg/L.•Sulfuric acid roasting at lower temperatures minimizes insoluble thorium-rich phases.•Thorium in anhydrite can be used for more effective recovery of this radionuclide.