Rare earth extraction from bastnaesite concentrate by stepwise carbochlorination-chemical vapor transport-oxidation
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10/3/04
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Rare Earth Extraction from Bastnaesite Concentrate by Stepwise Carbochlorination-Chemical Vapor Transport-Oxidation LI-QING ZHANG, ZHI-CHANG WANG, SHU-XUN TONG, PENG-XIANG LEI, and WEI ZOU A stepwise carbochlorination-chemical vapor transport-oxidation process is developed for the green rare earth extraction from a bastnaesite concentrate using carbon as reductant, chlorine gas as chlorination agent, SiCl4 gas as defluorination agent, AlCl3 as vapor complex former, and (O2 H2O) mixed gas as oxidant. Between 500 °C and 800 °C, the apparent activation energy of the carbochlorination within 2 hours changed from 17 to 10 kJ/mole roughly for the initial 20 minutes and final 1.5 hours, respectively, in the absence of SiCl4, but these values reduced to 15 and 5.9 kJ/mole under 10 kPa of SiCl4 gas, while the rare earth chloride conversion for 2 hours was 43 to 81 mol pct in the absence of SiCl4 and 55 to 99 mol pct under 10 kPa of SiCl4 gas. After carbochlorination at 550 °C for 2 hours in the (Cl2 SiCl4) atmosphere for efficient rare earth extraction and thorium-free volatile by-product release, thorium was removed by chemical vapor transport at 800 °C for 0.5 hours in the (Cl2 SiCl4 AlCl3) atmosphere and alkaline earths were separated from rare earths by oxidation at 700 °C to 1000 °C in the (O2 H2O) atmosphere for 0.5 hours, followed by water leaching at room temperature. Their combination allows a clean and efficient rare earth extraction from the concentrate.
I. INTRODUCTION
RARE earth minerals always contain both rare earths and non–rare earths including more or less radioactive elements, and thus, the green metallurgy of rare earths is difficult as compared with other metals.[1] Bastnaesite is mainly a fluorocarbonate of light rare earths and contains only 0.1 to 0.3 wt pct of thorium; thus, it is an important rare earth source based on radioactivity safety considerations.[1] At present, bastnaesite concentrate is commonly treated by calcination-acid dissolution or directly by acid dissolution or alkaline roasting for rare earth extraction,[1] but these processes require complicated operations and result in a large amount of low-concentration thorium-containing slag and wastewater, which cannot meet the requirement of thorium effluent standard. The Goldschmidt process at 1000 °C to 1200 °C for carbochlorination of bastnaesite concentrate, using carbon as reductant and chlorine gas as chlorination agent,[2] has many advantages over the wet processes, but it is no longer in industrial use due to the high fluorine content in the rare earth product[2] and the thorium radioactive contamination both for rare earth chloride product and for non–rare earth volatile by-product.[3] We[4] have recently developed a stepwise carbochlorination – chemical vapor transport (SC-CVT) process for the rare earth extraction and separation from a mixed bastnaesite-monazite concentrate, which is obtained from the world’s largest rare earth deposit located at Baiyunebo in China, based on thermodynamic
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