Extractive Metallurgy of Rare Earths

Extractive metallurgy of rare earths F. Habashi* A short account is given on the extraction of rare earths from monazite sand, bastnasite ore, and phosphate rock of igneous origin. This includes mineral beneficiation, leaching methods, fractional crystallisation [of historical interest], ion exchange, solvent extraction, precipitation from solution, and reduction to metals. On donne un bref compte-rendu de l’extraction des terres rares a` partir de sable monazite´, de minerai de bastnae´site et de roche phosphateé d’origine igneé. Ceci inclut l’enrichissement du mine´ral, les me´thodes de lixiviation, la cristallisation fractionnelle (d’inte´reˆt historique), l’ećhange d’ion, l’extraction par solvant, la prećipitation a` partir de solution et la re´duction en me´taux. Keywords: Carl Auer, Monazite, Bastnasite, Phosphate rock, Beneficiation, Leaching, Fractional crystallisation, Ion exchange, Solvent extraction, Precipitation from solution, Reduction This paper is part of a special issue on Advances in Rare Earths: From Mine to Materials Historical introduction for gas mantle declined later when the Edison lamp was invented at the beginning of the twentieth century. Although most of the rare earth metals were discovered The second industrial application of rare earths came in Sweden, the industrialisation started in the Austrian when Carl Auer recalled the preparation of metallic cerium Empire. This was due to the fact that the Swedish by his colleagues at Bunsen’s laboratory and the sparking chemist Jons Fridrik Bahr (1815–1875) from Uppsala of the metal when scratched or drawn over a rough surface. went to Heidelberg in Germany in 1855 to analyse some He conceived that this sparking action may be utilised for rare earth minerals by the spectroscope discovered ignition purposes. However, cerium would be a too costly recently by chemistry professor Robert Bunsen (1811– material. He, therefore, prepared a cerium–iron alloy and 1899) and his colleague physicist Gustav Kirchhoff found out that when the alloy contained 30%Fe it gave the (1824–1887). Few years later Carl Auer (1858–1929) best sparking effect suitable for ignition of gases. The ‘Auer from Vienna went also to Heidelberg to study under lighter’ soon became as famous as his incandescent mantle Bunsen. He was assigned the task to separate the rare when it was produced on large scale in 1903. Production earths from these minerals. On his return home, Carl reached 1 000 000 kg annually in 1930; it served to prepare Auer took with him the remaining minerals to continue 500 million flints which replaced six billion boxes of his studies at the University of Vienna.1,2 matches. This work was conducted at his new factory at While in Heidelberg, Carl Auer had noted the re- Treibach in Carinthia which he took possession in 1900 markable light-emitting powers of the rare earth oxides and which is today a successful metallurgical plant known when they were inserted into the flame of a Bunsen as Treibacher Chemische Werke in Austria. burner. In Vienna he put this observation into practice by inventing the gas mantle, a stocking made of cotton Rare earths elements thread and soaked in a solution of the earth metal salts. After the organic matter was burned off, a skeleton of Originally, the term rare earths was only used for the the metal oxide was left. This emitted light when heated oxides, R2O3, which are similar to one other in their in a laboratory Bunsen burner. In 1887, he started a chemical and physical properties and are therefore factory at Atzgersdorf, a suburb of Vienna, to separate difficult to separate. Within the rare earth group, the the rare earths salts necessary for preparing the soaking elements scandium, yttrium, and lanthanum differ in their solution for manufacturing the gas mantles for lighting atomic structure from the elements cerium to lutetium. the streets in Vienna. He also sold the solutions to The term lanthanides was abbreviated Ln by Chemical customers in Germany, England, and USA. The marked Abstracts. Scandium occupies a special position with respect to this classification and its other properties, and therefore does not belong to either of these groups. The rare earth elements always occur in nature in association Department of Mining, Metallurgical, and Materials Engineering, Laval with each other. The isolation of groups of rare earth University, Quebec City, G1V 0A6, Canada elements or of individual elements requires costly separa- *Corresponding author, email [email protected] tion and fractionation processes owing to the great ß 2013 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute Received 28 November 2012; accepted 23 April 2013 224 DOI 10.1179/1879139513Y.0000000081 Canadian Metallurgical Quarterly 2013 VOL 52 NO 3 Habashi Extractive metallurgy of rare earths 1 Separation of monazite and other valuable minerals from monazite sand by physical methods similarity of the chemical and physical properties of their containing some thorium and small amounts of ura- compounds, which explains why the history of their nium. It is widely distributed in the Earth’s crust. It discovery has extended over such a long period. occurs in small proportions in granites. When such rocks The word ‘rare’, when used to describe this group of are weathered, grains of monazite are carried by waters, elements, originates from the fact it was thought that then deposited at the mouths of rivers, together with the these elements could only be isolated from very rare heavier constituents of the parent rock, to form black minerals. Considering their abundance in the Earth’s sands known as monazite sand. The monazite in these crust, the term rare is now inappropriate. These elements sands is usually present in rounded grains, showing that are lithophilic and are therefore concentrated in oxidic the grains have previously been rolled to and fro in compounds such as carbonates, silicates, titano-tantalo- streams of water. Monazite sands occur mainly in Brazil, niobates, and phosphates. India, Australia, and USA. Xenotime is also a lantha- The abundance of the rare earth elements taken nide phosphate but the individual lanthanides occur in a together is quite considerable. Cerium, the most common different proportion from that in monazite. It occurs rare earth, is more abundant than cobalt. Yttrium is more mainly in South East Asia associated with alluvial tin abundant than lead, whereas Lu and Tm are as abundant deposits. as Sb, Hg, Bi, and Ag. For all practical purposes Monazite sand and xenotime can be easily concen- promethium does not occur in nature. It forms only in trated from the sands by physical methods. For nuclear reactors. example, a monazite sand containing 1% monazite can be concentrated by gravity, magnetic, and electrostatic Raw material methods to a concentrate containing 85% monazite The major raw material for rare earths is monazite sand, (Fig. 1). Some physical properties of monazite and xenotime, bastnasite, and phosphate rock. xenotime concentrates are given in Table 1. A chemical analysis of the concentrates is given in Table 2. The Monazite and xenotime composition of the lanthanide fraction in monazite, in Monazite derives its name from Creek meaninc to be xenotime, and bastnasite is given in Table 3. Scandium, alone. The mineral monazite is a lanthanide phosphate although in the same group with yttrium, lanthanum, Table 1 Physical properties of lanthanide phosphate minerals Monazite Xenotime Colour Yellow to red brown Pale yellow to brownish green Specific gravity 4.9–5.54.45–4.59 Hardness (Mohs) 5 4.5 Crystal structure Monoclinic Tetragonal Canadian Metallurgical Quarterly 2013 VOL 52 NO 3 225 Habashi Extractive metallurgy of rare earths Table 2 Chemical analysis of lanthanide phosphate Ce)2(Nb,Ta,Ti)2O6 in the Kola Peninsula and also in concentrates most uranium minerals as trace substituents. Monazite Xenotime Scandium occurs in trace amounts in most rare earth concentrate/% concentrate/% minerals. In many minerals, scandium is present in a dispersed state. Wolframite and cassiterite can contain P2O5 24–29 … up to 1% scandium, so that scandium is a byproduct of Ln2O3* 55–65 52–63 the production of tungsten and tin. Uranium minerals ThO2 5–10 1–3 contain much smaller amounts of scandium, but, since U O 0.2–0.40.5–3.5 3 8 uranium is produced in relatively large quantities, SiO2 1–3 … CaO 0.2–0.8… scandium is produced in appreciable quantities also. Fe2O3 1–2 … . Phosphate rock Al2O3 0 1–0 8… . ZrO2 0 7 2–3 Tonnage wise, phosphate rock is the most important as SnO2 … 0–9 compared with the other material; about 120 millions *Ln, lanthanide. tons of rock are treated annually while only 30 000 tons of monazite and xenotime. However, no production of and the lanthanides, is not present in any of these rare earths from this source is actually conducted. minerals. Apatite is the principal constituent of phosphate rock. Monazite and xenotime are the main source of The mineral received its name from the Greek word thorium and the lanthanides; uranium is recovered as meaning I deceive when it was realised that it was frequently confused with other mineral species, includ- a byproduct. ing beryl and tourmaline, prior to the latter part of the Bastnasite eighteenth century. Phosphate rock is the main raw material for the production of phosphatic fertilisers. The most important mined rare earth deposit is at the Phosphate rock is composed mainly of: Mountain Pass Mine in California, where up to 40 000 t/a (i) Fluorapatite, Ca (PO ) F bastnæsite ore concentrate (70% REO) is produced by ore 10 4 6 2 (ii) Hydroxyapatite, Ca (PO ) (OH) beneficiation. Other important bastnæsite deposits are in 10 4 6 2 (iii) Carbonato-apatite, Ca10(PO4)6CO3 Burundi, Madagascar, and in Bayan Obo, near the town Depending on the proportion of each component, the rock of Baotou in Inner Mongolia in China.

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