Separation of Thorium.
—The separation of a crude thorium product from the acid solution obtained after decomposition of the mineral can be effected in two ways, both of which are based on the fact that thoria is less basic than the oxides of the cerium and yttrium metals. In the first, the rare earth elements, including thorium, are precipitated as oxalates by the addition of oxalic acid to the acid solution. These are again taken into solution by the action of hydrochloric acid on the hydroxides, obtained by prolonged digestion of the oxalates with sodium hydroxide; the acid solution is then treated carefully with sodium hydroxide, or pure powdered magnesia, until about one-sixth of the bases has been precipitated, the liquid being constantly stirred. Thorium hydroxide being very weakly basic is precipitated before the other hydroxides, and the precipitate obtained, after one or two repetitions, contains most of the thorium originally present in the monazite. In the second process, thorium is partially separated from the other metals by adding gradually to the solution obtained after the treatment of the mineral with sulphuric acid, the quantity of magnesia calculated to precipitate a suitable fraction of the earths, with constant stirring; this throws down a mixture of phosphates containing almost all the thorium and some of the other elements. The slimy phosphate precipitate is dissolved in hydrochloric acid, and the earths precipitated as oxalates; the precipitate must be washed thoroughly in order to remove phosphoric acid. It will be seen that these two methods differ only in that in the first the phosphoric acid is removed before the precipitation of thorium, whereas in the second the thorium is precipitated as phosphate, and this transformed into oxalate.
Quite recently, methods have been proposed by which the thorium can be separated in a fairly pure condition from the acid solution obtained from the sulphuric acid treatment. Rosenheim, Meyer and Koppel[499] protect the use of hydrofluosilicic acid (H₂SiF₆), and its salts, for this purpose. The sodium salt, added to the hot acid liquid, produces a quantitative separation of thorium silicofluoride; the precipitate is washed by decantation, and treated with sulphuric acid, the thorium sulphate being then purified directly by the sulphate method described below. A second method proposes to make use of the insolubility of thorium hypophosphate, ThP₂O₆,11H₂O, which was found by Kaufmann in 1899 to be insoluble in water, and in acids and alkalies. This method has already been in use for some years for analytical work;[500] it appears to be readily susceptible of adaptation for the technical extraction,[501] the sodium hypophosphate, Na₂H₂P₂O₆,6H₂O required as the precipitating agent being obtainable in large quantities by the electrolytic oxidation of copper phosphide, employed as the anode in an electrolytic cell.[502] This method also gives a thorium compound sufficiently free from other earths to be subjected at once to the refining process; the hypophosphate has in fact been suggested as a very suitable compound for the impregnation of artificial silk mantles directly. The thorium nitrate of commerce, however, is still prepared almost entirely from the crude product obtained by one or other of the two methods of fractional precipitation first described, so that it becomes necessary to outline the method generally employed for separating from this a compound pure enough to be suitable for the final refining process.
[499] D. R. P. 214886, October, 1909.
[500] Rosenheim, Chem. Zeitg. 1912, 36, 821; also Koss, ibid. 686
[501] Wirth, Zeitsch. angew. Chem. 1912, 25, 1678.
[502] Rosenheim and Pinsker, Ber. 1910, 43, 2003.
The crude oxalate or hydroxide is thoroughly digested with a concentrated solution of sodium carbonate. The carbonates of the cerium elements are much less soluble in sodium carbonate solution than is thorium carbonate. After thorough digestion the liquid is filtered from the undissolved carbonates. The thorium is reprecipitated from the filtrate, either as oxalate, by the addition of hydrochloric acid (if the crude material was in the form of oxalate), or as hydroxide, by the addition of sodium hydroxide. The process is again repeated, and a final digestion is then made with ammonium carbonate; addition of an alkali to the clear filtrate now gives thorium hydroxide sufficiently pure to be used for the last refining.
Purification of the Thorium Compounds.
—The object of this last stage is to remove from the thorium compound small quantities of cerium and yttrium salts which cannot be separated by the carbonate method. The chief process is the sulphate crystallisation, the principles underlying which have been thoroughly examined in the patient researches of Koppel and Holtkamp.[503] Since the process is based on the solubilities of the various thorium sulphate hydrates, it is necessary to consider these in some detail.