Rare earth elements

Abstract

The rare earth elements (REE) consist of 17 elements Sc, Y and the lanthanoids La – Lu. Their magnetic and luminescent properties make them essential components of digital and low carbon technologies. They have risen to prominence because production from mining through to manufacturing is dominated by China, making REE the epitome of critical raw materials. The crustal abundance of light REE about the same as copper and even the less abundant heavy REE are much more abundant than precious metal such as gold. Most working REE mines are in carbonatite-related deposits, usually where initial mantle derived and magmatic enrichments of light REE have been further upgraded by metamorphic, hydrothermal or weathering processes. Other mines produce loparite from nepheline syenite and monazite from mineral sand placer deposits. Ion adsorption clays, in which the REE are adsorbed to clay particle surfaces and released by leaching, are the most important sources of heavy REE. REE are usually mined by conventional open pit methods, except for the ion adsorption clays in China that are now usually mined by in-situ leaching. Owing to its smaller cation size, Sc concentrates in different minerals, such as clinopyroxene and is produced as a by-product of uranium deposits, nickel laterites, and from aegirine at Bayan Obo, China. Process mineralogy is a prime concern for REE ores. Many deposits have intricately intergrown minerals; often minerals that have not been extracted commercially. After upgrading the ore minerals, cracking (dissolution) of the ore and separating the individual REE from each other are expensive, chemical-intensive processes. The environmental record of many REE mining and processing sites has been poor, and REE mining accordingly gained a bad reputation. It is important that all REE exploration projects and mines adopt best practice, thereby encouraging the continued use of REE.