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Encyclopedia Britannica - Main :: PYR-RAY |
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RARE EARTHS , in chemistry, the name given to a group of oxides of certain metals which occur in close association in some very rare minerals. Although these metals resemble each other in their chemical relationships, it is convenient to subdivide them into three groups: the cerium, terbium and ytterbium groups. The first includes scandium (Sc, 441.1), yttrium (Y, 89.o), lanthanum (La, 139o), cerium (Ce, 140.25), praseodymium (Pr, 140.6), neodymium (Nd, 144.3), and samarium (Sa, 150.4); the second includes europium (Eu, 152.0), gadolinium (Gd. 157.3), and terbium (Tb, 159.2); and the third includes dysprosium (Dy, 162.5), holmium (Ho, ?) erbium (Er, 167.4), thulium (Tm, 168.5), ytterbium or neoytterbium (Yb, 172.0), and lutecium (Lu, 174.0); the letters and numbers in the brackets are the symbols and atomic weights (inter-national). Although very rare, a large number of minerals contain these metals; they are chiefly found in Scandinavia, parts of the Urals, America and Australia, generally associated with Archean and eruptive rocks, and more rarely with sedimentary deposits. They are usually silicates, but many complex tantalates, niobates, phosphates, uranates and fluorides occur. The chief
mineral
mineral
The chemistry of this group may be regarded as,beginning with Cronstedt's description of the mineral cerite from Bastnaes in 1751, and the incorrect analyses published by T. O. Bergman and Don Fausto d'Elhuyar in 1784. Ten years later Gadolin investigated the mineral subsequently named gadolinite, which had been found at Ytterby in 1788 by Arrhenius. This discovery of a new earth was confirmed by A. G. Ekeberg in 1799, who named the base yttria. Cerite was examined simultaneously by Klaproth in Germany and by Berzelius
9TO yttria into two new bases which he called "erbia" and "terbia," and a true yttria, but in 186o N. J. Berlin denied the existence of Mosander's " erbia," and gave this name to his "terbia." The new erbia has itself proved to be a mixture. Marignac in 1878 separated an ytterbia which was split by Nilson in 1899 into scandia (the metal of which proved to be identical with Mendeleeff's predicted eka- boron
oxide
original
Ceria Ceria Lanthana 1 Lanthana Didymia Samaria Samaria Europia Yttria 1 I I Yttria Erbia Terbia (Mosander) (Mosander) Terbia Erbia (Delafontaine) (Berlin) Terbia Gaholinia Ytterbia Thulia Soret's X Erbia Holmia Scandia Ytterbia Holmia Dysprosia Neoytterbia Lutecia Methods of Separation.The small proportions in which the rare earths occur in the mineral kingdom and the general inter-mixture of several of them renders their efficient separation a matter of much difficulty, which is increased by their striking chemical resemblances. While it is impossible to treat the separations in detail, a general indication of the procedure may be given. The first step is to separate the rare earths from the other components of the mineral. For this purpose the mineral is evaporated with sulphuric or hydrochloric acid, or fused with potassium bisulphate, and the residue extracted with water. The solution of chlorides or sulphates thus obtained is treated with sulphuretted hydrogen, to remove copper, bismuth and molybdenum, and the filtrate, after the ferrous iron has been oxidized with chlorine, is precipitated with oxalic acid. The oxalates (and also thorium oxalate) may be converted into oxides by direct heating, into nitrates by dissolving in nitric acid, or into hydroxides by boiling with potash solution. The thorium may be removed by treating the nitrate solution with hydrogen peroxide, and warming, whereupon it separates as thorium peroxide. The next step consists in neutralizing the nitric acid solution and then saturating with potassium sulphate. Double
formula
To separate the individual metals many different methods have been proposed; these, however, depend on two principles, one, on the different basicities of the metals, the other, on the different solubilities of their salts. Bahr and Bunsen worked out a process of the first type, which utilized the fractional decomposition of the nitrates into oxides on heating. The mixed oxalates are converted into nitrates, which are then mixed with an alkali nitrate to lower the melting-point, and the mixture fused. The nitrates decompose in order of the basicities of the metals, and after a short fusion the residue is extracted with boiling water, and the basic salt which separates when the solution is cooled is filtered off. This'contains the most negative metal; and the filtrate, after evaporation and a repetition of the fusion and extraction, may be caused to yield the other oxides. A second method, based on the same principle, consists in the fractional precipitation by some base, such as ammonia, soda, potash, aniline, &c. The neutral nitrates are dissolved in water, and the base added in such a quantity to precipitate the oxides only partially and very slowly. Obviously the first deposit contains the least basic oxide
Many processes depending upon the different solubilities of certain salts have been devised. They consist in forming the desired salt and fractionally crystallizing. The mother liquor is concentrated and crystallized, the crystals being added to the filtrate from a re-crystallization of the first deposit. These operations are repeated after the manner shown in the following scheme ; the letter C denotes crystals, the M.L mother liquor, whilst a bracket means mixing before re-crystallization.Original
M!L I I l I M!L M!L C C MIL C MIL End of Article: RARE EARTHS If you wish, you can link directly to this article.
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