Register |  Login | 
Login to Portal
X
 
 
Home
 
A NOTE ON RARE EARTH ELEMENTS
 
Rare earth elements or rare earth metals are a group of seventeen elements in the Periodic Table including Scandium, Yttrium and 15 Lanthanoids with Z ranging continuously from 57 to 71 ( La – Lanthanum, Ce-Cerium, Pr- Praseodymium, Nd-Neodymium, Pm- Prometheum, Sm-Samarium, Eu – Europium, Gd – Gadolinium, Tb-Terbium, Dy-Dysprosium, Ho-Homium, Er- Erbium, Tm- Thulium, Yb-Ytterbium and Lu- Lutetium) . Scandium and yttrium are considered rare earths since they tend to occur in the same ore deposits as the lanthanoids and exhibit similar chemical properties.
The term "rare earth" arises from the minerals from which they were first isolated, which were uncommon oxide-type minerals (earths) found in Gadolinite extracted from one mine in the village of Ytterby, Sweden. However, with the exception of the highly-unstable prometheum, rare earth elements are found in relatively high concentrations in the earth’s crust with cerium being the 25th most abundant element in the earth's crust at 68 parts per million.
 
 
Rare earth elements are used in many modern technological devices, including superconductors, samarium-cobalt and neodymium-iron-boron high-flux rare-earth magnets, electronic polishers, refining catalysts and hybrid car components. Rare earth ions are used as the active ions in luminescent materials used in optioelectronics applications, most notably the Nd-YAG laser. Erbium-doped fibre amplifiers are significant devices in optical-fibre communication systems. Phosphorus with rare earth dopants are also widely used in cathode ray tube technology such as television sets. The earliest color television CRTs had a poor-quality red; europium as a phosphor dopant made good red phosphors possible. Yttrium iron garnet (YIG) spheres have been useful as tunable microwave resonators. Rare earth oxides are mixed with Tungsten to improve its high temperature properties for welding, replacing thorium which was mildly hazardous to work with. Many of these are essential ingredients in mobile phones, video game machines, computers and even green technologies. Tiny amounts of rare earths dysprosium or terbium might soon be used in electric cars as these let batteries work at high temperatures.
 
 
Rare earth elements are used in many modern technological devices, including superconductors, samarium-cobalt and neodymium-iron-boron high-flux rare-earth magnets, electronic polishers, refining catalysts and hybrid car components. Rare earth ions are used as the active ions in luminescent materials used in optioelectronics applications, most notably the Nd-YAG laser. Erbium-doped fibre amplifiers are significant devices in optical-fibre communication systems. Phosphorus with rare earth dopants are also widely used in cathode ray tube technology such as television sets. The earliest color television CRTs had a poor-quality red; europium as a phosphor dopant made good red phosphors possible. Yttrium iron garnet (YIG) spheres have been useful as tunable microwave resonators. Rare earth oxides are mixed with Tungsten to improve its high temperature properties for welding, replacing thorium which was mildly hazardous to work with. Many of these are essential ingredients in mobile phones, video game machines, computers and even green technologies. Tiny amounts of rare earths dysprosium or terbium might soon be used in electric cars as these let batteries work at high temperatures.
A few sites are under development outside of China, the most significant of which are the Nolans Project in Central Australia, the remote Hoidas lake project in northern Canada and the Mt. Weld project in Australia. The Hoidas Lake project has the potential to supply about 10% of the $1 billion of REE consumption that occurs in North America every year.
 
 
Due to the phenomenon known as lanthanide contraction, yttrium, which is trivalent, is of similar ionic size todysprosium and its lanthanide neighbors. Due to the relatively gradual decrease in ionic size with increasing atomic number, the rare earth elements have always been difficult to separate. Even with eons of geological time, geochemical separation of the lanthanides has only rarely progressed much farther than a broad separation between light versus heavy lanthanides, otherwise known as the cerium and yttrium earths. Rare earth minerals, as found, usually are dominated by one group or the other, depending upon which size-range best fits the structural lattice. Thus, among the anhydrous rare earth phosphates, it is the tetragonal mineral xenotime that incorporates yttrium and the yttrium earths, whereas the monoclinic monazite phase incorporates cerium and the cerium earths preferentially. The smaller size of the yttrium group allows it a greater solid solubility in the rock-forming minerals that comprise the earth's mantle, and thus yttrium and the yttrium earths show less enrichment in the earth's crust, relative to chondritic abundance, than does cerium and the cerium earths. This has economic consequences: large ore bodies of the cerium earths are known around the world, and are being actively exploited. Corresponding ore bodies for yttrium tend to be rarer, smaller, and less concentrated. Most of the current supply of yttrium originates in the "ion adsorption clay" ores of Southern China. Some versions of these provide concentrates containing about 65% yttrium oxide, with the heavy lanthanides being present in ratios reflecting the Oddo-Harkins rule: even-numbered heavy lanthanides at abundances of about 5% each, and odd-numbered lanthanides at abundances of about 1% each. Similar compositions are found in xenotime or gadolinite.
 
 
Rare earth deposits in India are of two major types: endogenic and exogenic. The Endogenic types include some carbonatites, pegmatitic rocks (Chhotonagpur), metamorphic-metasomatic veins, the Exogenic types comprise coastal or beach placer, inland placer and offshore placer. The endogenic deposits do not appear to be very much attractive from exploitational point of view. Mainly beach placers are mined in India at present. Monazite is the principal ore mineral for REE in India, although xenotime holds out some prospect for the future. Of India's estimated reserve of 5 million tonnes of monazite, 70-75 % occurs in beach placer and the rest in the inland and offshore varieties. Monazite-content of beach sands may be upto 11 wt%. ThO2 ranges between 8-10.5%. Average S REO 60%. Inland placers contain either monazite or xenotime as the principal REE-bearing mineral. Of late, work on inland placer has started for xenotime. Factors controlling placer formation are: (1) provenance, (2) physico-chemical properties of the minerals in the placer, (3) physico-chemical ambience, the source rocks/earlier deposits are exposed to, and (4) physical process of concentration. In the development of India's beach placer deposits, granites, granitic pegmatites, migmatites, gneisses, charnockites, leptynites and khondalites provided the necessary source and the tropical climate with heavy rainfall and strong wave action was especially conducive to the concentration of the placer-minerals in suitable locales.
 
Map courtesy AMD, Hyderabad
 
The first non-strategic value addition activities of IREL in tonnage quantities was concerned with production of composite rare earth chloride, oxide and fluoride to start and later separation of 99.9% pure oxide of individual rare earths like Ce, La, Nd and Pr by multi-stage solvent extraction and fractional precipitation techniques. Oxides of this metal in higher purities are also prepared by RED in kilo gramme quantities using ion exchange technology.
Besides chemical processing of monazite both zircon and ilmenite were found worth value addition from commercial angle.
A dry grindin mill working on the principal of self attrition was commissioned by Chavara in the year 1970 to grind the as separation zircon sand to about 4.5m size (called zirflour) for its application in the ceramic industries. Much later, a wet mill with silica as grinding media was commissioned at Chavara to introduce yet another value added material called micro-zir having mesh size in the range of 1 to 3 mm finding specialized application as opacifier. In addition to such physical value addition, the MK unit had set up a small chemical plant to produce zircon frit, zirconium chloride, etc. The plant, however, is limited in size and meant primarily for making supply of zircon firt to Nuclear Fuel Complex, Hyderabad. In yet another effort on value addition to zircon, a pilot plant (capacity-3.5 TPA) was set up OSCOM to produce a whole range of zirconia stabilized with CaO, MgO and rare earths.
The most talked about value addition activity of IREL is setting up of a Chemical plant at OSCOM consisting of a Synthetic Rutile Production unit-an Acid Regeneration Unit. The SR facility is equipped with two roasters, two calciners, sixteen digestors for carrying out reduction of ilmenite, leaching of reduced ilmenite with concentrated hydrochloric acid. The leached liquor is treated in the AR unit to regenerate 20% grade HCl for its recycle and reject iron as fine iron oxide powder. The SR unit was stopped in 1997 as it was not financially viable. The company now intends to use the roasters and calciners for the production of partially value added materials like reduced and metallized ilmenite.
Contributed by: Central Geological Laboratory
CHQ, GSI
 

 
 
Feedback | Contact Us | FAQ | Whats New | Document Search | RTI | CGPB | IGCP