Rare earth elements (REEs) are a group of 17 (including scandium (Sc) and yttrium (Y)) silvery-2 white heavy metals. Compounds containing REEs have immense applications in electronic and 3 electrical equipments, lasers, glass, semiconductors and magnetic materials. Due to their increasing importance in economic developments and potential of their supplies being at risk , they have been classified as critical elements by the U.S. Geological Survey (USGS).