Bible Encyclopedias
Rare Earths

Terbia

Gadolinia

Soret's X

Erbia

Holmia

Ceria Ceria Lanthana Lanthana Didymia Sa maria Didymia Samaria Europia Praseodidymia Neodidymia Yttria Yttria Erbia Terbia (Mosander) (Mosander) I I Terbia Erbia (Delafontaine) (Berlin) 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 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 R2(S04)3.3K 2 SO 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, &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, 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. 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 Solution C M.L C M .L C M.L

C M.L C M.L C M.L C M.L C M.L C M.L C M.L 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. K2Ce2(C03)4.12H 2 0, 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; 0. 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 Me203; combine directly with hydrogen at 200 0 -300° to form hydrides of the formula MH. 2 or MH 3; nitrides of the formula MN are formed by passing nitrogen over the oxides mixed with magnesium; whilst carbides of the type MC 2 are obtained in the electrolytic reduction of the oxides with carbon. In addition to the oxides M203, several, e.g. cerium, terbium and neodymium, form oxides of the formula M02. 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. 18) 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. 2 57 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 anorganischen Chemie (1906), vol. iii. (article by R. T. Meyer); H. Moissan, Traite de chimie minerale (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 Physical and Inorganic Chemistry (1909). For the rare earth minerals see J. Schilling, Das Vorkommen der seltenen Erden im Mineralreiche (1904).

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