A solution chemistry approach to the study of rare earth element precipitation by oxalic acid
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I.
INTRODUCTION
RARE earth elements play a critical role in civilization and human life. In general, they are a critical component in superconductor materials, permanent magnets, catalysts, special steels, and fluorescent powders. Currently, the major source of medium-to-heavy rare earth (RE) elements is from weathered clay minerals formed through complex physical, chemical, and geobiological processes under natural conditions. The rare earth elements, collectively referred to as RE, are often migrated from RE-containing granite and igneous rocks to kaolinite and halloysite, and enriched to a level of economic value.[1,2,3] The clay minerals act as an inorganic ion exchange resin onto which the hydrated RE elements adsorb. Therefore, these RE elements can be released into solutions by ion exchange process using certain types of cations, which react with clays stronger than hydrated RE ions. The dissolved RE elements are then recovered by either solvent extraction[4] or precipitation using oxalic acid (H2C2O4).[5] The latter is often used in industrial operation due to process simplicity and effective recovery.[6] A common observation in RE recovery by precipitation is excess use of oxalic acid with a H2C2O4 to RE2O3 mole ratio greater than a stoichiometric requirement of 1.5. The exact reason for this excess requirement of oxalic acid is not clear, although other released non-RE metallic species, such as Fe3+, Al3+, Mg2+, and Ca2+, are considered to consume a portion of the acid added. In this article, a solution chemistry approach is applied to a phenomenological analysis of the system, which allows quantitative description of various contributions of oxalic acid consumption. With the approach developed here, an operating condition of minium oxalic acid consumption with a maximized purity of RE oxalates is derived. R. CHI is Associate Professor with the Institute of Nuclear Energy Technology, Tsinghua University, Beijing 102201, People’s Republic of China. Z. XU is Associate Professor with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted June 30, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
SOLUTION CHEMISTRY CONSIDERATION
In a typical precipitation process of RE elements by oxalic acid, three main acid consumption mechanisms need to be considered, including stoichiometric requirement, excess acid to achieve complete precipitation, and precipitation and complexation of other coexisting non-RE metallic species. The relative contributions to total acid consumption by each of these mechanisms can be described quantitatively through a solution chemistry analysis as described below. A. Equilibrium of Diprotic Oxalic Acid To describe the precipitation reaction quantitatively, it is essential to know the speciation distribution of acid precipitants. For a diprotic oxalic acid, a two-step dissociation equilibrium can be expressed by[7] → H+ 1 HC O2 H2C2O4 ← 2 4 → H+ 1 C O22 HC2O42 ← 2 4
K1 5 5.90 3 1022
[1]
K2 5 6.40 3 1025
[2]
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