A novel process for recovering rare earth from weathered black earth
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I. INTRODUCTION
THE Mianning (MN) rare-earth (RE) deposit, the second largest in China, is characterized by its richness in light RE elements[1] (up to 90 pct of total RE) finely disseminated in the lump of bastnasite (CeFCO3). There has been some success recovering RE from bastnasite with conventional mineral processing techniques such as flotation, electrostatic, magnetic and gravity separation at a commercial scale.[2] However, about 20 pct of the ore deposit is in the form of weathered-black-earth slime with the sizes ranging from submicron to approximately 10 mm.[3] The major RE elements in the weathered-black-earth slime are enriched as colloidal rare-earth-oxide (REO) precipitates on noncrystalline iron and manganese oxides. Because of the colloidal and well-dispersed nature of RE in the slime (3 to 8 pct), this portion of rare earth can not be recovered with existing physical separation methods and is therefore discarded as tailings in the industrial operations. The estimated economic value of this fraction accounts for $10 million/year. Extensive research work has been conducted in our group in an attempt to develop an effective and economical process to recover RE from weathered-black-earth slime. A direct hot-acid leaching process (5 pct HCl at 75 8C) was initially tested.[1] In this process, a large amount of Fe (up to 10 g/ L) was leached out in the leachate, which presented a challenge for the subsequent separation of RE from Fe. As a result, not only was RE recovery low (only about 52 pct), but also the purity of product was not sufficiently high (less than 90 pct RE) to meet the product specifications. The known corrosive nature of HCl adds another challenge to its commercial application. Alternatively, sodium chloride roasting followed by a hot-water-leaching process was investigated.[1] With this process, the concentration of Fe in the leachate was reduced, but RE recovery was not improved R. CHI and G. ZHU, Associate Professors, are with the Institute of Nuclear Energy Technology, Tsinghua University, Beijing, 102201, P.R. China. Z. ZHOU, Postdoctoral Fellow, and Z. XU, Associate Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted March 16, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
(54 pct). In this case, a high Mn concentration (10 g/L) in the leachate was responsible for the depressed RE recovery. It is clear from our experiments that the interference of nonRE metals, such as Mn and Fe, with RE recovery has to be minimized in order to recover RE at an acceptable efficiency. In this article, a novel process, based on the recovery of Mn by reduction leaching followed by an ammonium chloride roasting and RE dissolution in hot water, is described. A schematic diagram of the process flowsheet is given in Figure 1. Using this process, RE recovery greater than 65 pct at a grade higher than 90 pct REO was obtained, with Mn and Pb being recovered as by-products. II. EXPERIMENTAL A. Sample Two ore sam
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