Praseodymium REE Collection rare earth metals elements

Praseodymium

Double, double, toil and trouble was the compound didymium which was discovered by chemist Carl Mosander in 1841. Fire burn and cauldron bubble was the scene in the rare-earth chemist's laboratories for the next 44 years before the elemental twins, praseodymium and neodymium, were finally separated.

Applications

  • Welder and glass blower goggles made with praseodymium and neodymium oxides protect the eyes from yellow flare and UV light.
  • Vibrant yellow ceramic tiles and dinnerware, popular in the Mediterranean region, are the result of combining praseodymium and zirconium oxides.
  • Praseodymium oxide is a catalyst to make the most widely used plastic, polyethylene, for soda bottles, bubble wrap, food plastic wrap, sandwich bags and milk cartons.

Interesting Facts

  • In 1841, Carl Mosander mistakenly thought that didymium oxide, a natural mixture of praseodymium and neodymium, was a new element. His discovery was even given the symbol Di in Mendeleev's first edition of the periodic table in 1869.
  • The primary use of praseodymium is to combine it with neodymium magnets to increase their availability to supply growing demand for high-tech applications.
  • Within a magnetic field, Praseodymium mixed with nickel PrNi5 cools to within one thousandth of a degree of absolute zero, -273.15 °C, the point where every molecule stops moving.

Discovery

Praseodymium and neodymium were discovered at the same time. Many chemists in the world believed didymium was a mixture of elements, but were unable to figure out how to separate them. So finally, when a chemist announced he had accomplished the separation in front of the Vienna Academy of Sciences on June 18, 1885, many were skeptical (Weeks and Leicester, 1968, p. 685). That chemist was Baron Carl Auer von Welsbach, who was studying in Heidelberg under the direction of German chemist Robert Bunsen. Auer von Welsbach noted "Only Bunsen, to whom I first showed the discovery, recognized immediately that a splitting of didymium had actually been accomplished. This acknowledgement from Bunsen, who had, as is known, published very beautiful and comprehensive researches on didymium, showed how unselfishly this great investigator used to judge the researches of younger men" (Feldhaus, 1928). To separate didymium, Auer von Welsbach used multiple fractionations of ammonium didymium nitrate. His discovery resulted in two new elements, which he named praseodymium and neodymium (Auer von Welsbach, 1885 [2 refs.]). The less abundant new earth was praseodymium, from the Greek prasios didumous, meaning green twin (Hedrick, unpublished).

Definition

Praseodymium is a silvery-yellow metallic metal that oxidizes moderately in air forming a green oxide coating. The metal is soft, malleable and ductile. It has a hexagonal structure, a density of 6.782 gm/cm3, a melting point of 931 °C, and a boiling point of 3520 °C. Praseodymium oxide, or praseodymia, occurs as a sesquioxide with the formula Pr2O3 and as 6 other oxide phases. The sesquioxide Pr2O3 is a yellow powder with a body centered cubic structure, a specific gravity of 6.9 gm/cm3, a melting point of 2183 °C, a boiling point of 3760 °C, and a formula weight of 329.813. The stable form in air at ambient temperatures is Pr6O11, a black powder with a monoclinic structure, a specific gravity of 6.34 gm/cm3, a melting point of 2500 °C, a boiling point of 4200 °C, and a formula weight of 1021.43 (Eyring, 1979; Ferro, 2011).

Preparation of Metal

Praseodymium metal is typically prepared by calciothermic reduction of the trihalide, typically transparent green colored crystals of PrF3, in a Ta crucible. Praseodymium metal has a low melting point and high boiling point, similar to Ce, Pr, and Nd. To prepare the PrF3, a mixture of anhydrous hydrofluoric acid and 60% argon is streamed over Pr2O3 at 700 °C for 16 hours in a platinum-lined Inconel furnace tube. This produces a praseodymium fluoride with approximately 300 ppm oxygen as an impurity. In a second purification step the oxygen content is lowered to less than 20 ppm by heating the fluoride to about 50 °C above its melting point in a platinum crucible within a graphite cell. The PrF3 is placed in a Ta crucible, reduced with a 15% excess of the theoretical amount of calcium metal required, and heated in an induction vacuum furnace under an inert Ar atmosphere to a temperature above the highest melting reductant or product. (Beaudry and Gschneidner, Jr., 1978). The Ca metal combines with the F to form CaF2 and the remaining product is a high-purity praseodymium metal.

Source

Large resources of praseodymium are contained in LREE-enriched minerals. Praseodymium occurs in the Earth's crust at an average concentration of 8 parts per million. The primary source of praseodymium is from carbonatites and the LREE-mineral bastnäsite. Bastnäsite deposits in China and the United States constitute the largest percentage of the world's rare-earth economic resources. Praseodymium is also a major constituent in the LREE-mineral monazite which constitutes the second largest segment of rare-earth resources. Monazite deposits are located in Australia, Brazil, China, India, Malaysia, South Africa, Sri Lanka, Thailand, and the United States in paleoplacer and recent placer deposits, sedimentary deposits, veins, pegmatites, carbonatites, and alkaline complexes (Hedrick, 2010). Praseodymium sourced from the LREE-mineral loparite is recovered from a large alkali igneous intrusion in Russia (Hedrick, Sinha, and Kosynkin, 1997).

Mining

Praseodymium is mined from a variety of ore minerals and deposits using various methods. Bastnäsite is mined in the United States as a primary product from a hard-rock carbonatite. The deposit is mined via bench-cut open pit methods. Ore is drilled and blasted, loaded into trucks by loaders, and hauled to the mill. At the mill the blasted ore is crushed, screened, and processed by flotation to produce a bastnäsite concentrate. In China, bastnäsite and lesser amounts of associated monazite are also mined from a carbonatite. The ore is recovered as a byproduct of iron ore mining by hard-rock open pit methods. After crushing the ore is separated from the iron ore by flotation to produce a bastnäsite concentrate and a bastnäsite-monazite concentrate (Hedrick, 1990).

Monazite is recovered from heavy-mineral sands (specific gravity >2.9) deposits in various parts of the world as a byproduct of mining zircon and titanium-minerals or tin minerals. Heavy mineral sands are recovered by surface placer methods from unconsolidated sands. Many of these deposits are mined using floating dredges which separate the heavy-mineral sands from the lighter weight fraction with an on-board wet mill through a series of wet-gravity equipment that includes screens, hydrocyclones, spirals, and cone concentrators. Consolidated or partially consolidated sand deposits that are too difficult to mine by dredging are mined by dry methods. Ore is stripped by typical earth-moving equipment with bulldozers, scrapers, and loaders or by water jet methods. Ore recovered by these methods is crushed and screened and then processed by the wet mill described above. Wet mill heavy-mineral concentrate is sent to a dry mill for processing to separate the individual heavy-minerals using a combination of scrubbing, drying, screening, electrostatic, electromagnetic, magnetic, and gravity processes. Vein monazite has been mined by hard-rock methods in South Africa and the United States (Hedrick, 2010). Loparite is mined by underground methods using room and pillar methods. Ore is drilled and blasted and removed from the mine. The ore is then processed by the same hard-rock methods as applied to bastnäsite to make a loparite concentrate.

Selected praseodymium minerals

Bastnäsite-(Ce) (Ce,La,Nd,Pr)(CO3)F
Monazite-(Ce) (Ce,La,Nd,Th)(PO4)
Loparite-(Ce) (Ce,Na,Ca,Sr,Th)(Ti,Nb,Ta,Fe+3)O3
Allanite-(Ce) (Ca,Ce)(Al2,Fe+2)(Si2O7)(SiO4)O(OH)
Parisite-(Ce) Ca(Ce,La)2(CO3)3F2
Ancylite-(Ce) SrCe(CO3)2(OH) · H2O
Britholite-(Ce) Ca2(Ca,Ce)3(SiO4,PO4)3(OH,F)
Cerite-(Ce) (Ca,Ce)9(Fe,Mg)(SiO4)3(HSiO4)(OH)3

References

Andres, Klaus, and Paul H. Schmidt, 1977, PrNi5 as a cryogenic refrigerant (filed October 20, 1975): United States Patent 4028905, June 14, 5 p.

Auer von Welsbach, Carl, 1885, Die Zerlegung des Didymus in seine Elemente [The Separation of Didymium into its Elements], Chemische Berichte, part 3, v. 18, p. 605.

Auer von Welsbach, Carl, 1885, Die Zerlegung des Didymus in seine Elemente [The Separation of Didymium into its Elements], Monatshefte fuer Chemie, v. 6, p. 477-491.

Beaudry and Bernard J. and Karl A. Gschneidner, Jr., 1978, Preparation and Basic Properties of the Rare Earth Metals: chapter 2 in Handbook on the Physics and Chemistry of Rare Earths-Volume 1:Metals, (Gschneidner, Jr. and Eyring, editors), North-Holland, New York, p. 173-232.

Eyring, LeRoy, 1979, The binary rare earth oxides: chapter 27 in Handbook on the Physics and Chemistry of Rare Earths-Volume 3:Non-Metallic Comounds I, (Gschneidner, Jr. and Eyring, editors), North-Holland, New York, p. 337-399.

Feldhaus, 1928, Zum 70 - Geburtstage von Auer von Welsbach [To 70 - Birthday of Auer von Welsbach]: Chemiker-Zeitung, September 1, v. 52, p. 22-23.

Ferro, Sergio, 2011, Physicochemical and Electrical Properties of Praseodymium Oxides: International Journal of Electrochemistry - Volume 2011, Hindawi Publishing Co., open access journal, 7 p.

Hedrick, James B., Rare earth history: unpublished manuscript, 11 p.

Hedrick, James B., 2010, Rare earths: chapter in Mineral commodity summaries 2010, U.S. Geological Survey, p. 128-129.

Hedrick, James B., 1990, Rare earths - The lanthanides, yttrium, and scandium: chapter in Minerals Yearbook 1990, U.S. Geological Survey, v. 1, p. 903-922.

Hedrick, James B., 1991, Rare earths - The lanthanides, yttrium, and scandium: chapter in Minerals Yearbook 1991, U.S. Geological Survey, v. 1, p. 1211-1237.

Weeks, Mary E., and Henry M. Leicester, 1968, Discovery of the Elements (7th ed.): Easton, Pennsylvania, Journal of Chemical Education, 896 p.

Pr Applications (Hedrick, 1991)(Gschneidner, Jr., 2011)

Electrons per shell:
2, 8, 18, 21, 8, 2
Atomic number,
Protons, Electrons:
59
Number of Neutrons:
82
Atomic Mass:
140.90765 amu
Melting Point:
931 °C
Boiling Point:
3520 °C
Density @ 293 K:
6.782 g/cm3
Crystal structure:
hexagonal
Color:
unknown