Thermodynamic Mechanism Analysis of Calcification Roasting Process of Bastnaesite Concentrates
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CONSIDERING its great reserves and high rare earth oxide (REO) content, bastnaesite (REFCO3) is one of the world’s major rare earth minerals.[1,2] It is the main resource of light rare earth elements (LREEs), and 70 pct of the total rare earth elements (REEs) in the world are extracted from bastnaesite.[3–5] Mountain Pass in the United States and Bayan Obo polymetallic, Weishan, and Sichuan deposits in China are the most important bastnaesite ores around the world.[2,6] Among these, the deposits in Mianning, Sichuan (China), are representative bastnaesite ores with 80 pct of the rare earth oxides concentrated in bastnaesite. More than 98 pct of the REEs are LREEs such as lanthanum, cerium, praseodymium, and neodymium and the main associated minerals are barite (BaSO4), calcite (CaCO3), and fluorite (CaF2), suggesting comprehensive utilization value.[7] Bastnaesite can be easily separated from gangue minerals via a series of beneficiation processes, resulting in bastnaesite concentrates. Then the concentrates are converted into rare earth products following a series of metallurgical processes mainly composed of concentrate decomposition, leaching, and solvent extraction. Various processes have been proposed and applied to industrial production of bastnaesite concentrates in China. Rare earth elements are effectively used, while other valuable elements are ignored. Moreover, the discharge of waste water and gas has caused serious resource waste and environmental pollution.[8,9] For bastnaesite, fluorine is the most important constituent in addition to RE. In recent years, some new methods have been reported in succession. These
PENG CEN, WENYUAN WU, and XUE BIAN are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] METALLURGICAL AND MATERIALS TRANSACTIONS B
methods can realize comprehensive utilization of bastnaesite resources to some extent.[10–14] Based on their strengths and weaknesses, we put forward a novel calcification roasting decomposition method for bastnaesite concentrates in our previous study.[15] Calcium hydroxide was added and fluorine in the minerals turned into calcium fluoride during oxidizing roasting. As an important resource in fluorine chemical industry, the generation of calcium fluoride can promote the economic value of bastnaesite resources. Meanwhile, it could reduce the damage to the environment by decreasing the amount of gas and water containing fluorine. In that work, the experimental roasting conditions and major mineral phase transformations were studied, but a detailed thermal decomposition mechanism was not determined. In many studies on bastnaesite decomposition, the major reactions have been listed but some of them cannot be verified thermodynamically due to the lack of some thermodynamic data.[7,8,16] Considering the difficulties obtaining pure substances of some rare earth minerals such as bastnaesite (REFCO3) and rare earth oxyfluoride (REOF), it is challenging to determine their thermodynamic parameters through experiments. I
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