In a second form of this method, due to Haber, the impure hydroxide is dissolved in hydrochloric acid, and the acetate precipitated by addition of sodium acetate. The precipitate is filtered off and re-dissolved in acid, and the acetate again thrown down by means of sodium acetate. The precipitate is then dissolved in nitric acid, and the solution evaporated to dryness. In this form the method gives very good results, even from a comparatively crude product; but the process is, of course, considerably more expensive than the sulphate purification.
The high price of the necessary reagents, again, is a bar to the technical application of the very simple and efficient process of Wyrouboff and Verneuil. These authors suggest the precipitation of thorium peroxide from a warm dilute neutral solution by means of hydrogen peroxide, a process which is quantitative and yields a very pure product. The last traces of the cerium metals can be completely removed by a second precipitation. The cost of hydrogen peroxide is too high, however, to allow its employment on such a large scale, and the method has not, in consequence, come into general use.
The thorium nitrate obtained after purification by the sulphate method, or by the less generally employed acetate method, is usually considered sufficiently pure for technical purposes. Even now, however, it may contain traces of sulphate, of iron, of alkalies, and of cerium metals. If absolute purity is desired, the salt may be dissolved, and freed from all impurities, except the cerium compounds, by precipitation with ammonium oxalate and thorough washing; the oxalate may then be dissolved in chromic acid, and potassium chromate solution added drop by drop; the precipitated thorium chromate is nearly free from other rare earth compounds, and repetition of the process will give a pure salt. The separation from cerium metals may also be effected by the hydrogen peroxide process. If the technical processes are carefully carried out, however, a thorium nitrate of a very high degree of purity may be obtained, and the laboratory purification need only be undertaken if material is needed for very accurate quantitative work.
Preparation of Thorium Nitrate from Mantle-ash.
—Since the ordinary incandescent mantle, in use, consists only of the pure thoria and ceria, with small quantities of alumina, lime, and magnesia, which have been employed to strengthen the ‘head,’ the working-up of mantle-ash gives an easy means of obtaining the nitrates, and high prices are accordingly paid for the ash in quantity. At one period of great competition between rival manufacturers, canvassers went from house to house in many large towns buying up mantle residues, to be used for the extraction of the thorium for ‘lighting-fluid.’
For this purpose, the oxides are treated with hot concentrated sulphuric acid, the cooled residue dissolved in water, and the thorium and cerium precipitated free from compounds of aluminium, magnesium, and calcium by oxalic acid. If pure thorium nitrate, free from cerium, is required, the oxalates are added to the last precipitate from the double carbonate purification in the [treatment of monazite] (vide supra), and the ordinary processes of refinement continued; more often, however, the mixed nitrate for impregnation of the mantle-fabric is required, and this is obtained by ignition of the oxalates and solution of the oxides so obtained in nitric acid, more cerium nitrate being added if necessary.
Extraction of Cerium Nitrate.
—Since monazite is primarily a phosphate of the cerium metals, the percentage of thoria being usually quite low (vide [Monazite], [Chapter VI]), very large quantities of compounds of the cerium group of elements are annually produced in the process of extraction of thorium. There is at present a very limited demand for these compounds (vide [Chapter XXI]), no important uses having yet been found for them. In the ordinary process of extraction of the thorium, these elements remain as the sparingly soluble double carbonates, whilst the thorium double carbonate is removed in solution. From the mixed salts which contain 50-60 per cent. of the cerium compound, the cerium nitrate required for the manufacture of mantles is prepared, but the amount so used is a small fraction of the whole, and large quantities of compounds of cerium and the allied elements are available as soon as profitable uses can be found.
Three processes are in general use for the preparation of cerium nitrate from the mixed carbonates; all of these are based on the fact that cerium can become tetravalent, forming in this condition compounds which can readily be separated from those of the allied elements, which can be obtained only in the trivalent condition. When ceria is dissolved in hot nitric acid, ceric nitrate, Ce(NO₃)₄, is formed, though the action of nitric acid on cerous carbonate or oxalate gives rise to cerous nitrate. Two of the three processes are based on this reaction, and for these the mixed carbonates are dissolved in hydrochloric acid, freed from foreign elements by precipitation with oxalic acid, and the oxalates ignited to the oxides, which are then dissolved in the required quantity of nitric acid. In the first process the cerium is precipitated from this solution by merely pouring it into a large excess of very dilute nitric acid, when a yellow basic ceric nitrate is precipitated; this is washed with dilute nitric acid by decantation, dissolved in concentrated acid, and purified by a second precipitation in the same way. In the second process, separation is effected by addition to the nitric acid solution of the calculated quantity of ammonium nitrate; the solution is concentrated to incipient crystallisation, and on cooling the double ceric ammonium nitrate, Ce(NO₃)₄,2NH₄NO₃, separates. This is collected, washed with dilute nitric acid, and recrystallised until a pure salt is obtained. The double nitrate can be readily decomposed by ignition, leaving ceria, which is dissolved in nitric acid; the nitrate is obtained by evaporation.
The third method, due to Drossbach, is based on the oxidation of cerium salts in neutral solution by potassium permanganate. The mixed carbonates are dissolved in hydrochloric acid, a further quantity of the carbonates stirred in, to neutralise excess of acid, and a solution of the required quantity of potassium permanganate added. The reaction is said to proceed according to the equation: