The Rare Earths: Their Occurrence, Chemistry, and Technology

by Stanley Isaac Levy

[423] Ber. 1880, 13, 1439.

The investigation of scandium, which occurs only in extremely small quantities in the minerals employed by Nilson and Cleve, and was therefore believed to be exceedingly rare, was not continued until 1908, when Sir William Crookes[424] made a systematic investigation of a large number of minerals in order to find a convenient source of the element. He showed that scandium is present in many rare earth minerals, and selected as the most suitable for the extraction of the element a complex mineral named Wiikite, some specimens of which he found to contain over 1 per cent. of scandia (see [p. 70]). The mineral was decomposed by fusion with potassium hydrogen sulphate, and scandia extracted from the rare earths by the nitrate fusion. The separation effected on these lines was very thorough, Crookes considering a specimen of scandia unsatisfactory if it showed any trace of the dominant ytterbium line, 3694·344, on an over-exposed plate, or if it gave an atomic weight for the element higher than 44·1.

[424] Phil. Trans. 1908, A, 209, 15.

A systematic investigation of the common rocks and minerals for scandium was carried out by Eberhard in 1908, as a result of which processes for the extraction of the oxide from wolframite were worked out by R. J. Meyer (see [pp. 3], [131]). Wolframite is a tungstate of iron and manganese, containing, in addition to other oxides, small quantities of the rare earths, of which considerable proportions are found to be scandia. The mineral is fused with soda in the usual way, and the rare earths concentrated by the oxalate method. Scandium is precipitated as the fluoride by addition of sodium silicofluoride to the boiling acid solution, and purified by precipitation as the double ammonium tartrate.[425]

[425] Meyer and Goldenberg, Chem. News, 1912, 106, 13.

Whilst the researches of Crookes and Eberhard have shown how widely distributed the element really is, the minerals which they found richest in scandium still contained extremely small quantities of the oxide. The discovery of the mineral Thortveitite (see [p. 44]), which contains about 37 per cent. of scandia, is therefore of the greatest scientific interest, and will doubtless allow of a very searching examination of the properties of this interesting element.

Whilst the low atomic weights of scandium and yttrium place them, to some extent, apart from the other rare earth elements, the latter element at least is so closely allied in properties to the other members of the group that yttria is one of the typical oxides of the family. Scandium and its compounds, however, present many peculiarities of behaviour when compared with the typical members, on the grounds of which Urbain[426] has contended that scandia should not be classed among the rare earths at all. Whilst this contention is perhaps rather extreme, especially in view of the fact that in nature scandia always occurs with other yttria oxides, it must be admitted that in many respects the element is anomalous. The oxide is the weakest base of the whole group, yet the oxalate is comparatively readily soluble in mineral acids (compare [p. 132]), and the potassium double sulphate is almost insoluble in potassium sulphate. The sulphate is altogether exceptional in that it is very easily soluble in water, and crystallises out with 6 molecules of water of crystallisation. The fluoride and the carbonate both dissolve readily in excess of precipitant, whilst sodium thiosulphate precipitates a basic salt from neutral solutions.

[426] Chem. News, 1905, 90, 319.

Meyer has pointed out the close resemblance between beryllium and scandium. The oxide and salts are colourless; the latter have a peculiar sweet astringent taste, and readily yield basic salts.

The hydroxide, Sc(OH)₃, is thrown down by alkalies as a bulky white gelatinous mass; the oxide is a white powder, less readily soluble in dilute acids than most of the rare earths. The fluoride is important on account of its insolubility in mineral acids, which exceeds that of all the other rare earth fluorides, and approaches that of thorium. It is thrown down from neutral or acid solutions by addition of hydrofluoric acid or a soluble fluoride; if the solution be boiled, a soluble silicofluoride will also precipitate scandium fluoride, though no precipitate is obtained in the cold. This behaviour is due to the ease with which the silicofluoride is hydrolysed at high temperatures, according to the equation: