Kinetics of chlorination and microstructural changes of xenotime by carbon tetrachloride
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I. INTRODUCTION
2REPO4 ⫹ 2CCl4 → 2RECl3 ⫹ P2O5 ⫹ COCl2 ⫹ CO2 [1]
RARE earths elements are more abundant in the earth’s crust than their name suggests, occurring in many mineral deposits. However, rare earths do not occur in minerals as individual compounds; therefore, among 200 minerals, only three minerals are commercially available (bastnasite, monazite, and xenotime).[1] Whereas bastnasite and monazite have a higher proportion of the cerium subgroup of the lanthanide elements, xenotime contains approximately 60 pct of the yttrium subgroup. The search for high purity individual elements provides the possibility of synthesis of new materials suitable for many fields such as optics, electronics, alloys, and catalysis.[1,2] Traditional methods of decomposition of rare earth minerals (alkaline or acid processes) into soluble forms in water or acid solutions involve several operations that can introduce impurities and, thus, result in a more laborious separation/purification operations. The use of chlorination in extractive metallurgy is an alternative route for obtaining intermediate compounds such as metallic chlorides, considered basic feedstock in developing new processes and materials.[3] A number of investigations on the chlorination of rare earth compounds have been reported concerned mainly with the preparation and isolation of anhydrous lanthanide chlorides. In addition, direct chlorination makes it possible to realize the mutual separation of rare earths using chemical vapor transport (CVT) by forming volatile complexes with KCl and AlCl3.[4] Despite these investigations, few studies have been devoted to the investigation of kinetic, mechanism, and structural changes between rare earths minerals and gaseous chlorinating agents. In this work, we investigated the reaction between xenotime (REPO4) and carbon tetrachloride as well as the microstructural changes occurring during the chlorination. The overall chlorination reaction can be expressed as EVANDRO B. AUGUSTO, Undergraduate Student, and HERENILTON P. OLIVEIRA, Assistant Professor, are with the Department of Chemistry, FFCLRP, University of Sa˜o Paulo, CEP 14040-901, Ribeira˜o Preto, SP, Brazil. [email protected] Manuscript submitted August 15, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
Experiments were performed in order to determine the effects of chlorination temperature, particle size of ore, and chlorine partial pressure. Moreover, analytical techniques such as scanning electronic microscopy (SEM), energy dispersive X-ray spectrometry (EDX), and powder X-ray diffraction (PXRD) were used. II. EXPERIMENTAL SECTION A. Materials and Sample Preparation The xenotime ore (specific surface area ⫽ 0.84 m2/g) used in this work was grounded and classified into various size fractions from 60 to 325 mesh, and with no more any previous treatment. The composition of the xenotime obtained by EDX (at. pct) is Y 26.0 pct, P 30.8 pct, Dy 24.6 pct, Er 7.6 pct, and Yb 11.0 pct. The powder X-ray diffractogram of the ore revealed mainly the presence of yttrium
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