Introduction to Rare Earths
Rare Earths, the group of elements with atomic numbers ranging from 57 through 71. The rare earths are neither earths nor, as a group, particularly rare. They are metals, and are known to occur in considerable quantities in at least 200 minerals. The least abundant member of the group is promethi-um, which occurs only as a fission product in nuclear reactors. These elements were called rare because at the time of their discovery they were believed to be scarce. The name earths was used because the oxides in which these elements are found resemble limestone, magnesia, and similar substances that were once called earths.
Rare earths are more properly called rare-earth elements, or rare-earth metals. They are also referred to as the lanthanide series, after lanthanum, the first member of the group. Individual members of the group are called lanthanides. The lanthanide series occurs between barium and hafnium in the Periodic Table. All members of the series have chemical properties similar to those of lanthanum.
General Properties
In general, the rare earths are malleable and ductile—they can be hammered into shape and drawn into wire. Their freshly cut surfaces are silvery white, but the metals promptly tarnish and become gray. The rare earths are good reducing agents; that is, in chemical reactions they readily lose electrons to atoms of other elements. They can be magnetized, and are easily dissolved in dilute acids. The rare earths are always found combined with other elements.
Uses of Rare Earths
The rare-earth metals have their greatest use in the glass, ceramic, electronics, and petroleum industries. Various compounds or mixtures of the rare earths are used as abrasives in polishing lenses, mirrors, and other glass products. Other compounds are used in coloring or decoloring glass, making glass impervious to ultraviolet light, and improving the quality of optical glass. Certain rare-earth oxides have long been used in making ceramic glass. Still other compounds are important in the manufacture of certain solid-state devices and used in the red phosphors on the screen of color television tubes. Mixtures of rare-earth chlorides are used as catalysts in petroleum refining.
Misch metal is composed of a mixture of a number of rare earths. An alloy of misch metal and iron ignites when scratched and is used as a flint in cigarette lighters. The alloy is also used in gas lighters, in miners' safety lamps, and in tracer bullets. Misch metal is also used to increase the heat resistance of aluminum and magnesium, and to improve the rolling properties of steel. Oxides and fluorides of the rare earths are used to make the cores of carbon-arc electrodes, which produce a brilliant white light and are used in motion-picture lighting equipment.
Sources and Extraction
The minerals bastnaesite and monazite are the chief sources of the rare earths. Much of the rare-earth production in the United States comes from bastnaesite reserves in southern California, the world's largest single-known source of rare-earth elements. Monazite is found in greatest abundance in Australia, Brazil, China, India, Malaysia, and South Africa.
The extraction of rare earths from the minerals in which they are found is a complicated process. First, the minerals are ground to powder and then dissolved in acid. Next, a chemical is added to the solution to cause impurities to settle out. The rare-earth solution is then drawn off, and the rare earths are separated from one another—or groups of rare earths are separated—by the ion: exchange process.
In the ion-exchange process, solutions are filtered through minerals called zeolites, or through synthetic resins that act as zeolites. Zeolites exchange ions (atoms carrying an electrical charge); that is, zeolite ions are added to the solution and rare-earth ions pass into the zeolites. The zeolites are then treated with chemicals that selectively remove various kinds or groups of rare earths.
The Lanthanide Series
Lanthanum, is extensively used for carbon-arc electrodes. It was discovered in 1839 by Carl Mosander.
Symbol: La. Atomic number: 57. Atomic weight: 138.9055. Melting point: 1,688 F. (920 C). Boiling point: 6,249 F. (3,454 C). Specific gravity: 6.15. Valence: 3.
Cerium, is the most abundant of the rare earths. Cerium is used as a component of glass and as an abrasive in polishing glass surfaces. Cerium compounds serve as catalysts in the refining of petroleum. The element was discovered in 1803 by J. J. Berzelius and W. Hisinger working together, and also by M. H. Klaproth.
Symbol: Ce. Atomic number: 58. Atomic weight: 140.12. Melting point: 1,468 F. (798 C). Boiling point: 6,199 F. (3,426 C). Specific gravity: 6.66. Valence: 3 or 4.
Praseodymium, was first separated from didymium (a mixture of rare-earth elements consisting chiefly of praseodymium and neodymium) in 1885 by Baron Carl Auer von Welsbach. The existence of didymium was first reported in 1841 by Carl Mosander, who believed it to be a single element. Praseodymium is used to color glass and glazes. Glass colored with didymium is used in welder's goggles.
Symbol: Pr. Atomic number: 59. Atomic weight: 140.9077. Melting point: 1,708 F. (931 C). Boiling point: 6,354 F. (3,512 C). Specific gravity: 6.77. Valence: 3 or 4.
Neodymium, is used for the same purposes as praseodymium. It was first separated from didymium in 1885 by Baron Carl Auer von Welsbach.
Symbol: Nd. Atomic number: 60. Atomic weight: 144.24. Melting point: 1,850 F. (1,010 C). Boiling point: 5,554 F. (3,068 C). Specific gravity: 7.0. Valence: 3.
Promethium, is produced only in nuclear reactors. It was identified in 1945 by J. A. Marinsky and L. E. Glendenin, at Oak Ridge, Tennessee.
Symbol: Pm. Atomic number: 61. Atomic weight of the most stable known isotope: approximately 145. Melting point: about 1,976 F. (1,080 C). Boiling point: 4,460 F. (2,460 C). Specific gravity: 7.22. Valence: 3.
Samarium, was discovered in 1879 by Lecoq de Boisbaudran. It is used in infrared-absorbing glass, as an absorber of neutrons in nuclear reactors, and as an additive to crystals for lasers.
Symbol: Sm. Atomic number: 62. Atomic weight: 150.36. Melting point: 1,962 F. (1,072 C). Boiling point: 3,232 F. (1,778 C). Specific gravity: 7.54. Valence: 2 or 3.
Europium, the most chemically reactive of the rare earths, was discovered and prepared as an oxide in 1896 by Eugene Demarcay, who isolated it in relatively pure form in 1901. Europium oxide is used with yttrium to form the red phosphors of color-television tubes.
Symbol: Eu. Atomic number: 63. Atomic weight: 151.96. Melting point: 1,512 F. (822 C). Boiling point: 2,907 F. (1,597 C). Specific gravity: 5.25. Valence: 2 or 3.
Gadolinium, was discovered in 1880 by Jean Charles Galissard de Marignac. It is used in metallic alloys to improve resistance to heat and oxidation.
Symbol: Gd. Atomic number: 64. Atomic weight: 157.25. Melting point: 2,392 F. (1,311 C.). Boiling point: about 5,851 F. (3,233 C). Specific gravity: 7.90. Valence: 3.
Terbium, was discovered in 1843 by Carl Mosander. It is added to chemicals used in solid-state devices. Sodium terbium borate is used in laser crystals.
Symbol: Tb. Atomic number: 65. Atomic weight: 158.9254. Melting point: 2,480 F. (1,360 C). Boiling point: 5,653 F. (3,123 C). Specific gravity: 8.23. Valence: 3 or 4.
Dysprosium, was discovered in 1886 by Lecoq de Boisbaudran. It is used in nuclear reactors to absorb neutrons.
Symbol: Dy. Atomic number: 66. Atomic weight: 162.50. Melting point: 2,564.6 F. (1,407 C). Boiling point: 4,712 F. (2,600 C). Specific gravity: 8.55. Valence: 3.
Holmium, was discovered in 1878 by J. L. Soret and M. Delafontain and, independently, by P. T. Cleve in 1879.
Symbol: Ho. Atomic number: 67. Atomic weight: 164.9304. Melting point: 2,678 F. (1,470 C). Boiling point: 4,928 F. (2,720 C). Specific gravity: 8.78. Valence: 3.
Erbium, was discovered in 1843 by Carl Mosander. When added to metals, it improves their workability. Erbium oxide is used to give glass and glazes a pink color.
Symbol: Er. Atomic number: 68. Atomic weight: 167.26. Melting point: 2,772 F. (1,522 C). Boiling point: 5,185 F. (2,863 C). Specific gravity: 9.05. Valence: 3.
Thulium, was discovered in 1879 by P. T. Cleve.
Symbol: Tm. Atomic number: 69. Atomic weight: 168.9342. Melting point: 2,813 F. (1,545 C). Boiling point: 3,537 F. (1,947 C). Specific gravity: 9.31. Valence: 2 or 3.
Ytterbium, was discovered in 1878 by J. Marignac. However, it was not isolated and distinguished from another new element—lutetium—until 1907, by G. Ur-bain and Baron Carl Auer von Welsbach (independently).
Symbol: Yb. Atomic number: 70. Atomic weight: 173.04. Melting point: 1,515 F. (824 C). Boiling point: 2,179 F. (1,193 C). Specific gravity: 6.97. Valence: 2 or 3.
Lutetium, was discovered in 1907 by G. Urbain, and, independently, by Baron Carl Auer von Welsbach.
Symbol: Lu. Atomic number: 71. Atomic weight: 174.967. Melting point: 3,013 F. (1,656 C). Boiling point: 5,999 F. (3,315 C). Specific gravity: 9.84. Valence: 3.
Related Elements
Scandium and yttrium, although not members of the lanthanide series, are usually classified with the rare earths because they are found in association with them and have similar chemical properties.
Scandium, was discovered by L. F. Nilson in 1879.
Symbol: Sc. Atomic number: 21. Atomic weight: 44.9559. Melting point: 2,802 F. (1,539 C). Boiling point: 5,130 F. (2,832 C). Specific gravity: 2.99. Valence: 3.
Yttrium, in oxide form, was discovered by J. Gadolin in 1794. In 1843 Carl Mosander showed that the oxide identified by Gadolin could be separated into the oxides of three distinct elements—erbium and terbium as well as yttrium. Yttrium makes certain alloys stronger and is used with europium oxide to form the red phosphors in color-television tubes.
Symbol: Y. Atomic number: 39. Atomic weight: 88.9059. Melting point: 2,773 F. (1,523 C). Boiling point: 6,039 F. (3,337 C). Specific gravity: 4.46. Valence: 3.