Study on Kinetic Mechanism of Bastnaesite Concentrates Decomposition Using Calcium Hydroxide
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e of the major rare earth minerals, bastnaesite (REFCO3) contributes toward 70 pct of the total rare earth elements (REEs) in the world.[1–3] This fact is mainly due to the development of the world’s two largest rare earth mines: the Mountain Pass mine in California, United States, and the Bayan Obo mine in Inner Mongolia, China.[4,5] It should be noted that the Bayan Obo deposit is a mixed rare earth ore with monazite as another chief mineral.[6] In addition, the Sichuan deposit in China is also a common bastnaesite source.[6,7] Researchers around the world have focused on developing various technological processes for decomposing bastnaesite. Concentrates of high grade can be first obtained via mineral processing, and various kinds of metallurgical methods, mainly including roasting, leaching, and solvent extraction, are subsequently employed to treat the bastnaesite concentrates.[8–10] Bastnaesite concentrates are always roasted to decompose the fluorocarbonate before being leached with strong acids.[7,11] The principal industrial process in China is now composed of oxidation roasting,
PENG CEN, WENYUAN WU, and XUE BIAN are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] Manuscript submitted October 17, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
hydrochloric acid (HCl) leaching, caustic conversion, and solvent extraction.[12] This process has been widely used, but it suffers from many problems, such as complex flows, high consumption of reagents and water, and the emission of fluoride-containing effluents and exhaust. The oxidation roasting–hydrochloric acid leaching method followed by double salt precipitation was also employed in the Mountain Pass production process before the mine was closed in 2015.[10,13] In some processes, sulfuric acid (H2SO4) was selected to leach the roasted products to enhance rare earth dissolution and decrease acid consumption.[14,15] Another common pretreatment technology for bastnaesite concentrates is caustic decomposition.[6,8] Rare earth fluorides can be converted to hydroxides through caustic soda (NaOH) digestion, which is followed by leaching of the hydroxides in HCl, double salt precipitation separation, and solvent extraction. This method is mature and clean, but it has never been adopted in industrial production due to its complicated operation and high cost. Actually, alkali liquid decomposition is generally applied in the treatment of Baotou mixed rare earth concentrates in China to facilitate the cracking of monazite.[16] In addition to these roasting, cracking and leaching processes, a high-temperature chlorination method is also commonly employed in the treatment of metallic minerals.[8] The process for the direct chlorination of rare earth ores at high temperatures was primarily performed in a special electrically heated furnace by Th. Goldschmidt A.G. in Germany.[17] The content of fluorine in the rare earth chloride products was significantly higher
than that in the products from hydrometallurgical proc
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