Solution-chemistry analysis of ammonium bicarbonate consumption in rare-earth-element precipitation
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9/11/03
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Solution-Chemistry Analysis of Ammonium Bicarbonate Consumption in Rare-Earth-Element Precipitation R. CHI, Y. HU, G. ZHU, S. XU, Z. ZHOU, and Z. XU A solution-chemistry analysis is applied to estimating the consumption of ammonium bicarbonate in the recovery of rare-earth (RE) elements from leachates of weathered clays. The theoretical analysis shows that a two-step process is needed for recovering RE from the leachates of the weathered clays by precipitation using ammonium bicarbonate. The first step is a precipitation at solution pH 5 to remove impurities such as Fe and Al. The second step is to precipitate RE by adjusting the solution pH above 8. The consumption of ammonium bicarbonate was found to depend on the concentration of RE elements and impurities in the leachates. The total amount of ammonium bicarbonate consumption for the entire process was determined experimentally, and the results showed an excellent agreement with that calculated based on solution-chemistry analysis. The decomposition of H2CO3 was identified as one of the main causes of ammonium bicarbonate overdose, accounting for up to 41 pct in comparison to 20 pct consumption for the removal of impurities. The amount of ammonium bicarbonate required in terms of the NH4HCO3: RE2O3 (RE oxides) molar ratio was found to be 4:1 for maximal RE recovery. An overall RE recovery around 90 pct was achieved with a product purity being about 90 pct. I. INTRODUCTION
RARE-EARTH (RE) elements are widely used as essential components in permanent magnets, catalysts, special steels, superconductors, and fluorescent powders, to name a few[1] applications. According to their atomic weight and properties, the RE elements can be divided into light (La, Ce, Pr, and Nd), medium (Sm, Eu, and Gd), and heavy (Td, Dy, Ho, Er, Tm, Yb, Lu, and Y) RE elements.[1] In general, the light RE elements are extracted from bastnasite and monazite minerals, while the medium and heavy RE elements are disseminated in weathered clay minerals as absorbed ions.[2–6] These RE elements can be released into solutions by ion-exchange mechanisms using certain types of cations. The dissolved RE elements can then be recovered by solvent extraction or precipitation using either oxalic acid[7] or ammonium bicarbonate (NH4HCO3). Compared to precipitation, solvent extraction[8] requires more expensive equipment, and therefore, the capital cost is higher. For industrial operations, RE precipitation with ammonium bicarbonate is preferred, as ammonium bicarbonate is less expensive than oxalic acid and does not cause severe environmental consequences. A commonly encountered problem in RE recovery using ammonium bicarbonate precipitation is the excess consumption of ammonium bicarbonate, with a molar ratio of NH4HCO3 to RE2O3 greater than 2.5 times the stoichiometric requirements. The exact reason for this excess requirement of NH4HCO3 is most likely due to the precipitation of other released non-RE metallic species such as Fe3, Al3, Mg2 R. CHI, Professor, is with Wuhan
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