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, 89-0), lanthanum (La, 139-0), 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 ( international). 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 species are monazite, a phosphate of the cerium metals, containing thorium (this mineral supplies, the ceria and thoria employed in making incandescent gas mantles) ; cerite, a hydrated silicate of calcium and the cerium metals; gadolinite, a silicate of beryllium, iron, and the yttrium metals; samarksite, a niobate and tantalate of both the cerium and yttrium metals, with uranium, iron, calcium, etc.; and keilhauite, a titanosilicate of yttrium, iron, calcium and aluminium; other species are fergusonite, orthite, aeschynite, euxenite and thorianite.
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 1 799, who named the base yttria. Cerite was examined simultaneously by Klaproth in Germany and by Berzelius and Hisinger in Sweden; and a new base was discovered in 1803 which the Swedish chemists named ceria. Both these oxides have proved to be mixtures. In 1839 Mosander separated " ceria " into true ceria and an earth which he termed lanthana (Gr. \av6&.i>tii> , to lie hidden), and in 1841 he showed that his lanthana contained another base, which he called didymia (Gr. St5iyx, twins). This didymia was separated in 1879 by Lecoq de Boisbaudran into a new base, samaria, and a residual didymia which was shown by Auer von Welsbach in 1885 to consist of a mixture of two bases, praseodidymia and neodidymia; moreover, samaria was split by Demarcay in 1900 into true samaria and a new base named europia. In 1843 Mosander also split yttria into two new bases which he called " erbia " and " terbia," and a true yttria, but in 1860 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 1879 into scandia (the metal of which proved to be identical with Mendeleeff's predicted eka-boron)and a new ytterbia, which, in turn, was separated by Urbain in 1907 into neoytterbia and lutecia (C. A. von Welsbach proposed for these elements the names aldebarianum and cassiopeium). Berlin's erbia was also examined by Soret in 1878 and by Cleve in 1879; the new base then isolated, Soret's X or Cleve's holmia, was split by Lecoq de Boisbaudran in 1886 into a true holmia and a new oxide dysprosia. The same erbia also yielded another base, thulia, to Cleve, in 1879, in addition to true erbia. The original erbia of Mosander was confirmed by M. A. Delafontaine in 1878 and renamed terbia; this base was split by Marignac in 1886 into gadolinia and true terbia. These relations are schematically shown below; the true earths are in italics, mixtures in Roman.
Ceria I Ceria Lanthana I Lanthana Didymia Samaria _L Didymia Samaria Europia Praseodidymia Yttria Neodidymia Yttria Erbia (Mosander)
ladolini( Terbia Gadolinia Ytterbia Thulia Scandia Ytterbia I Soret's X Holmia I Erbia Holmia Dysprosia Neoytterbia Lutecia Methods of Separation. The small proportions in which the rare earths occur in the mineral kingdom and the general intermixture 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 salts of the general formula R 2 (S04) 3 . 3K 2 S0 4 are formed, of which those of the cerium group are practically insoluble, of the terbium group soluble, and of the ytterbium group very soluble. The sulphates thus obtained may be reconverted into oxalates or oxides and the saturation with potassium sulphate repeated.
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, etc. 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, which by re-solution as nitrate and re-precipitation yields a purer product. To the filtrate from the first precipitate more of the base is added, and the second less basic oxide is thrown down. By repeating the process all the bases can be obtained more or less pure.
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 recrystallization of the first deposit.
Obviously the fractions contain salts which increase in solubility as one passes from the left to right, and with sufficient care and patience this method permits a complete separation. The salts which have been used include the sulphates, nitrates, chromates, formates, oxalates and malonates. R. J. Meyer (Zeit. anorg. Chem., 1904, 41, p. 97) separates the cerium earths by forming the double potassium carbonates, e.g. KiCei(COa)i . 12H 2 O, which are soluble in potassium carbonate solution, being precipitated in the order lanthanum, praseodymium, cerium and neodymium on diluting the solution; C. A. von Welsbach (Chem. News, 1907, 95, p. 196; 1908, 98, pp. 223, 297) separates the metals of the ytterbium group by converting the basic nitrates into double ammonium oxalates and fractionating; C. James (ibid., 1907, 95, p. 181; 1908, 97, pp. 61, 205) formed the oxalates of the yttrium earths and dissolved them in dilute ammonia saturated with ammonium carbonate; by boiling this solution the earths are precipitated in the order yttrium, holmium and dysprosium, and erbium; he also fractionally crystallized the bromates (see, e.g. Jour. Amer. Chem. Soc., 1910, 32, p. 517, for thulium). Complex organic reagents are also employed. Neish (Jour. Amer. Chem. Soc., 1904, 26, p. 780) used meta-nitrobenzoic acid ; O. Holmberg separates neodymium, praseodymium and lanthanum (and also thorium) with meta-nitrobenzene sulphonic acid, and has investigated many other organic salts (see Abs. J. C. S., 1907, ii. p. 90), whilst H. Erdmann and F. Wirth (Ann., 1908, 361, p. 180) employ the 1-8 naphthol sulphonates.
In order to determine whether any chosen method for separating these earths is really effective, it is necessary to analyse the fractions. For this purpose two processes are available. We may convert the salt into the oxalate from which the oxide is obtained by heating. A weighed quantity of the oxide is now taken and converted into sulphate by evaporating with dilute sulphuric acid. The sulphate is gently dried until the weight is constant, and from this weight the equivalent of the earth can be calculated. When repeated fractionation is attended by no change in the equivalent we may conclude that only one element is present. This process, however, is only rough, for the elements with which we are dealing have very close equivalents. A more exact method employs the spectra spark, arc, phosphorescence and absorption ; the evidence, however, cannot in all cases be accepted as conclusive, but when taken in conjunction with chemical tests it is the most valuable method.
Chemical Relations. The rare earth metals were at first regarded as divalent, but determinations of the specific heats of cerium by Mendeleeff and Hillebrand and of lanthanum and didymium by Hillebrand pointed to their trivalency; and this view now has general acceptance. They are comparatively reactive: they burn in air to form oxides of the type MezOj; combine directly with hydrogen at 2oo-3oo to form hydrides of the formula MHz or MH; nitrides of the formula MN are formed by passing nitrogen over the oxides mixed with magnesium; whilst carbides of the type MC are obtained in the electrolytic reduction of the oxides with carbon. In addition to the oxides MjOs, several, e.g. cerium, terbium and neodymium, form oxides of the formula MOj. The sesquioxides are bases which form salts and increase in basicity in the order Sc, Yb, Tm, Er, Ho, Tb, Gd, Sm, Y, Ce, Nd, Pr, La; the latter hissing with water like quicklime.
The placing of these elements in the periodic table has attracted much attention on account of the many difficulties which it presented. The simplest plan of regarding them all as trivalent and placing them in the third group is met by the fact that there is not room for them. Another scheme scatters them in the order of their atomic weights in the last four groups of the system, but grave objections have been urged against this plan. A third device places them in one group as a bridge between barium and tantalum. This was suggested by Benedick in 1904 (Zeit. anorg. Chem., 1904, 39, p. 41), and adopted in Werner's table of 1905 (Ber. 38, p. 914), whilst in 1902 Brauner (ibid. 32, p. 1 8) placed the group as a bridge on a plane perpendicular to the planes containing the other elements, thus expanding the table into a three-dimensional figure. The question has also been considered by Sir William Crookes (Jour. Chem. Soc., 1888, 53, p. 487; 1889, 55, pp. 257 et seq.), whose inquiries led him to a new conception of the chemical elements.
REFERENCES. For the general chemistry see R. Bohm, Seltene Erden (1905); Abegg, Handbuch der anoreanischen Chemie (1906), vol. iii. (article by R. T. Meyer) ; H. Moissan, TraM de chimie minerals (1904), vol. iii. (article by G. Urbain) ; Roscoe and Schorlemmer, Treatise on Chemistry (1908), vol. ii.; P. E. Browning, Introduction to the Rarer Elements (1909); see also A. W. Stewart, Recent Advances in Physical and Inorganic Chemistry (1909). For the rare earth minerals see J. Schilling, Das Vorkommen der seltenen Erden im Mineralreiche (1904).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)