Rare Earth Elements- A Review

What are the Rare Earth Elements?

Rare-earth elements (REE) are variously mentioned to as ‘rare-earth metals (REM)’, ‘rare earths (RE)’, ‘rare- earth oxides (REO)’, and total rare-earth oxides (TREO). During recent times there has been much disagreement about the real number of elements comprised in the REE group. The International Union of Pure and Applied Chemistry has defined the REE as a group of seventeen chemically similar metallic elements that comprise the fifteen lanthanide elements (lanthanum to lutetium), scandium (Sc), and yttrium (Y) [10]. The latter two elements are classified as REE because of their similar physical and chemical properties to the lanthanides, and they are commonly related with these elements in many ore deposits. Chemically, yttrium resembles the lanthanide metals more closely than its neighbour scandium in the periodic table. The fifteen lanthanide elements have been further subdivided into the: light-rare earth elements (LREE) — lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), and europium (Eu); and heavy-rare-earth elements (HREE) gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) [11]. In some classifications the first seven lanthanide elements (La to Gd) are grouped as light REE and the rest (Tb–Eu) as heavy REE [12]. Crystal chemistry of monazite and Journal of Ecology & Natural Resources

xenotime (major REE-bearing phosphates) structures shows that monazite preferentially incorporates REE from La to Gd, whereas xenotime tends to incorporate REE from Tb to Lu [12]. Despite its low atomic weight, yttrium (and scandium) is included with the HREE subgroup because its occurrence, ionic radius, and behavioural properties are closer to the HREE than to the LREE [13]. The LREE are generally more abundant, and less valuable than the HREE. Furthermore, REE with even atomic numbers are more abundant than their neighbours of odd atomic numbers because of their greater relative stabilities of atomic nuclei. Other classifications further divide the REE into Middle REE (MREE) comprising Sm, Eu, Gd, Tb, and Dy, with the remainder of the elements from Ho to Lu, referred to as the HREE [14]. The rare earth elements (REE) are a group of seventeen metallic elements - the fifteen lanthanides, with atomic numbers 57 (lanthanum, La) to 71 (lutetium, Lu), together with yttrium (Y, atomic number 39) and scandium (Sc, atomic number 21). All have similar chemical properties. The lower atomic weight elements lanthanum to samarium (Sm), with atomic numbers 57 to 62, are referred to as the light rare earth elements (LREE); while europium (Eu) to lutetium, with atomic numbers 63 to 71, are the heavy rare earth elements (HREE). (The dividing line drawn between LREE and HREE can vary somewhat, and the term ‘mid REE’ is also now sometimes used.) Yttrium, although it has a lower atomic weight, is grouped with the HREE because of its chemical similarity [15]. Scandium’s properties are different enough from those of the other REE that most of the scientific and general literature excludes it and focuses on the lanthanides and yttrium – we have followed that convention here.

Where are Rare Earth Elements Found?

In looking for concentrations of any element that might be of economic interest, there are several factors to consider: the minerals in which they occur, and how easily the element can be extracted; the types of rock in which these minerals are found; the most promising geological settings in which to explore for these rocks; and the particular locations of mineral deposits which are capable of being mined and processed economically [16].

In which Minerals do Rare Earth Elements Occur?

Over 200 rare earth minerals have been described. However, in many cases there are no concentrations of these minerals that are economic to mine and in others there is no known method to extract the REE from the mineral economically. Minerals tend to be rich in either LREE or HREE, but generally include most of the REE in some quantity, if only at minor or trace levels. The proportions of the different REE within minerals vary between deposits. The name of a REE mineral often indicates, by use of the chemical symbol, the most abundant element. For example, in bastnäsite-(Ce), cerium is present in the largest quantity (it also contains significant amounts of lanthanum, neodymium and praseodymium) [16]. Bastnäsite- (Ce) is the most common but bastnäsite-(La) and bastnäsite-(Nd) also occur. (Chemical formulae are often written in an ‘idealised form’, with the mixture of REE represented by the dominant element. So, bastnäsite- (Ce) is given as CeCO3F.) The most important rare earth minerals in ore deposits are bastnäsite and monazite-(Ce) (CePO4). Each contains about 70% rare earth oxides by weight. Other significant ones are the carbonates parisite-(Ce) and synchysite-(Ce) (often found intergrown with bastnäsite), and the phosphate xenotime-(Y). The complex silicates allanite-(Ce), eudialyite and steenstrupine also contain significant amounts of REE, as do the carbonate ancylite- (Ce) and the phosphate churchite-(Y). Minerals that contain much smaller amounts of rare earth elements can act as important sources (Figure 2). For example, the oxide loparite-(Ce), mined in Russia for its niobium, also contains about 1% rare earth oxide by weight – enough to make REE worthwhile extracting. Fluorapatite (Ca5(PO4)3F) can sometimes also be a source of rare earths. If mined as a source of phosphorus for fertiliser, it is possible to extract REE as a by-product.

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In which types of Rock do these Minerals Occur?

REE are found principally in carbonatites. These are igneous rocks comprising more than 50% carbonate minerals, principally calcite (calcium carbonate), but in Journal of Ecology & Natural Resources

some cases magnesium-bearing carbonates (dolomite, magnesite) or iron-bearing carbonates (siderite), that crystallised from a high-temperature liquid from deep in the Earth. Carbonatites almost always contain REE, which constitute about 3,500 ppm of these rocks, so they are the most obvious place to look for REE. REE are found in high concentrations in some, but not all, alkaline igneous rocks (which make up only about 0.5% of all igneous rocks). They are characterised by their high contents of alkali metals. There are also some secondary deposits of REE. These are essentially produced by weathering of primary sources and their subsequent concentration by physical or chemical means. ‘Placer’ deposits are produced by physical concentration of rare earth and other heavy minerals from weathering; ‘laterite’ deposits result from enrichment of REE by in situ chemical alteration. There is one known example of a further type of secondary deposit, known as ion-adsorption clays, in China [16]. The REE in these deposits were released by weathering of REE-rich granites and subsequently adsorbed by clay minerals. There are also some rare earth mineral concentrations of economic interest in granite pegmatites and some hydrothermal (hot water) vein systems. Researchers have recently found elevated concentrations of REE in deep sea mud in the Pacific Ocean, which may also constitute a potential resource [17, 18].

In what kinds of Geological Environment are these Rocks Found?

The carbonatites and alkaline igneous rocks are characteristically found in the interiors of tectonic plates, that is, away from the active plate margins where volcanic activity is at its greatest. They are commonly associated with the major rift systems, such as the East African and Baikal rifts and the Rhine Graben. The carbonatites are mostly confined to continental areas, while the alkaline rocks also occur over much of the world’s oceanic intraplate areas on volcanic islands. Although more than two thirds of carbonatites that have been dated are Phanerozoic in age (less than 500 million years), they are overwhelmingly concentrated in areas comprising older Precambrian rocks [19].

What are the Environmental Effects of Rare Earth Production?

The association of the radioactive elements thorium and uranium with REE deposits is one of the main environmental problems. The heavy REE in particular tend to concentrate in the same geological environments as thorium. A few percent of thorium by weight is common in monazite formed in granitic rocks, and in the mineral sands derived from them. These marine and alluvial placer deposits used to be the most important source of REE during the 1950s and 1960s. However, they have generally fallen from favour because of their radioactivity. Other deposits, such as those associated with carbonatite rocks, have much lower levels of thorium. In common with most mining and processing, REE production uses a great deal of energy. Where the energy is derived from fossil fuels, this will add to carbon emissions (in the absence of widespread Carbon Capture and Storage) – especially if much of it comes from coal fired power stations, as currently in China. Other environmental problems can arise from the extraction and processing of REE, including pollution from the chemicals used. These are often the result of inadequate environmental regulation or enforcement [20].

Analytical Procedure

Trace and rare earth elements concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) using an Element X-series 2 at National Oceanography Centre Southampton (NOCS). Solutions for measurement by ICP-MS were diluted to appropriate total concentrations with a 3% nitric acid solution containing an internal spike with In-Re (5 μg kg-

1) and Be (20 μg kg-1) [21]. Artificial element standards were produced at 2, 5, 10, 25, 50, 75, 100, 125, 150, 175 and 200 μg kg-1 and used together with the internal spikes to calibrate the instrument and monitor sample drift corrections. All standard calibration curves display less than 3.5% analytical error with excellent linearity. International rock standards BHVO2, BIR1 and JB3, were run in triplicate as unknown to monitor accuracy of calibration with excellent reproducibility between triplicates. All analysis for trace and rare earth elements, Mg, Al, Fe and Mn are accurate and fall within the range of published recommended values. Sodium was measured in excess by 6% and 3% in BHVO2 and BIR1 respectively, whereas Ca and Ti concentrations were 3% below recommended range in JB3. Measurement reproducibility on international standards was also checked for discrepancies between standards prepared with and without ammonium oxalates to check for matrix effects from the oxalates. Standard dilution to daughter solutions were prepared by including ammonium oxalate matching the final concentration of (NH4)2C2O4 in samples analysed by the ICP-MS [22]. Excellent reproducibility of results, showing less than 5% difference between standards with and without oxalates, indicates the absence of any matrix effects.