|Thorium is extracted from its phosphate contained in monazite concentrates. Two industrial ways of processing are commercially significant:|
- Treatment by sulphuric acid at 200°C (sulphatization)
- Treatment by alkaline at 140°C
All rare earth elements, thorium and phosphoric acid pass in the sulphuric solutions. When the solution pH reaches 1 thorium phosphate is precipitated. The precipitation is isolated and dissolved in nitric acid. Then, thorium nitrate is extracted by organic solvent from which thorium may be easily washed out as complex compounds. During the concentrate alkali uncovering metal hydroxides stay in the sediments and trisodium phosphate gets into the solution. Precipitation is separated and dissolved in hydrochloric acid. Thorium hydroxide is precipitated by decreasing the solution pH until 3.6-5. ThO2, ThCl4 and ThF4 are obtained from separated and purified thorium compounds; these are basic initial substances for thorium metal extraction by metallothermic methods or molten salts electrolysis.
Extraction of Thorium Compounds from Monazite Sand and other Minerals
|The economic value of monazite sand depends on the percentage of thoria it contains; if this falls below 5 per cent, the value of the sand is reduced, if below 4 per cent, it becomes very small. Consequently means are taken to remove foreign matter occurring with the sand - first by means of water, in imitation of tidal action, and secondly, by taking advantage of the different magnetic susceptibilities of monazite and its associated minerals. This concentration of the sand is effected prior to its exportation, and can be carried out so effectively that less than 5 per cent, of foreign matter remains in association with the monazite. |
For decomposing the sand it has been proposed to mix it with lime, coke, and a little fluorspar, and to heat the mixture in an electric furnace until phosphorus ceases to be evolved. The usual plan, however, is to decompose it with sulphuric acid. The first step, known as " breaking," consists in heating the sand to about 200° C. with twice its weight of sulphuric acid of 1.84 density until the monazite is completely decomposed. A thick whitish paste is produced, which is gradually stirred into water and the insoluble matter allowed to settle. The solution then contains the sulphates of the various metals, together with the excess of sulphuric acid and the phosphoric acid derived from the monazite. To separate the thorium from this solution advantage is usually taken of the fact that thorium phosphate is less soluble in dilute acid than the phosphates of the other metals that are present. Consequently, by partially neutralising the acid solution a precipitate is obtained rich in thorium phosphate. As a neutralising agent magnesia has numerous advantages over others that might be proposed. This fractional precipitation of the thorium phosphate must be repeated several times before a precipitate is obtained consisting almost entirely of the thorium salt, and a certain proportion of the thorium is inevitably lost in the mother-liquors which carry away the bulk of the other rare earth elements.
The subsequent operations are concerned mainly with removing phosphoric acid and the remainder of the tervalent rare earth elements from the thorium. For eliminating phosphoric acid, the crude thorium phosphate may be dissolved in hydrochloric acid and the rare earths precipitated as oxalates by the addition of oxalic acid. Numerous methods have been described for removing the other rare earths still present; these are based upon the following facts - (i) When thorium is precipitated, together with other metals, as basic carbonate by means of sodium carbonate solution, an excess of precipitant re- dissolves the thorium salt, whereas the carbonates of the cerium metals are almost insoluble in sodium carbonate and those of the yttrium metals (which are present only in very small amount) are but slightly soluble; (ii) thorium oxalate is readily soluble in ammonium oxalate solution, in which the oxalates of the accompanying rare earth elements are practically insoluble; (iii) the hydrated sulphates of thorium and the accompanying rare earth elements are comparatively sparingly soluble in water and their relative solubilities are widely different; and (iv) thorium acetate is considerably less soluble in water than the acetates of the other rare earth elements.
The sulphate crystallisation (iii) is widely used. At 15° C. the following hydrates are in equilibrium with their solutions, and their solubilities are as follow -
|Crystalline Phase.||Grams of Anhydrous Sulphate dissolved in 100 grams of Water.|
At 55° C., however, the following conditions obtain -
|Crystalline Phase.||Grams of Anhydrous Sulphate dissolved in 100 grams of Water.|
Accordingly, so far as solubilities are concerned, thorium sulphate can be separated from the accompanying sulphates much better at 15° C. than at 55° C., and for another reason it is essential for crystallisation to take place at a low temperature. The tetrahydrate Th(SO4)2.4H2O is a flocculent, unworkable precipitate; it is the stable phase at temperatures above 43° C. when aqueous solutions are used, and is stable at even lower temperatures in the presence of mineral acids.
The final purification usually leaves the thorium in the state of hydroxide or basic carbonate, which is then converted into the nitrate for use in the manufacture of incandescent mantles.
Since thorium nitrate of a high degree of purity is prepared technically on a large scale there is little or no need for the chemist to prepare his own thorium compounds from the minerals. When, however, this is desired, it is usually best to separate the total rare earths by precipitation with oxalic acid in a dilute acid solution. Monazite may, as usual, be decomposed with sulphuric acid, while thorite may be decomposed by either hydrochloric or sulphuric acid, silica removed in the usual manner, and traces of heavy metals precipitated with hydrogen sulphide. When the oxalates contain only a small percentage of the thorium salt a good method for the separation of the thorium is that devised by Wyrouboff and Verneuil. When the oxalates consist mainly of the thorium salt, conversion into sulphate and repeated recrystallisation is a useful process, and it is then advisable to have separated the rare earths as hydroxides, by precipitation with ammonia, before converting then into oxalates.
For the final purification of thorium compounds in the laboratory the following methods are available, in addition to the sulphate method - (i) precipitation of the thorium as hydrated peroxide by adding a solution of pure hydrogen peroxide to a neutral or faintly acid solution of nearly pure thorium nitrate; (ii) fractional precipitation of the thorium as chromate; (iii) precipitation as thorium oxalate by adding oxalic acid solution, drop by drop, to a boiling solution of the nitrate strongly acidified with nitric acid, until a few drops of the precipitant produce no further effect; (iv) crystallisation of thorium acetylacetone from chloroform and sublimation of the product in vacuo. For practical details of the processes the reader is referred to the literature.
Preparation of Metallic Thorium
|Thoria is an oxide which is reduced with great difficulty; and, owing to the tendency of thorium to combine with nitrogen, carbon, and other elements, it is doubtful if the metal has ever been obtained in a state of purity. |
Thorium has been prepared by heating potassium thorium chloride, 2KCl.ThCl4, with sodium in a sealed iron cylinder, and by the reduction of thoria with magnesium. When, however, thoria is reduced with silicon or aluminium a silicide or an alloy with aluminium is produced, whilst reduction with carbon in an electric furnace yields the carbide ThC2, or a mixture of this with metal. A better product has been obtained by heating thorium chloride, sublimed in a current of hydrogen, with sodium in an evacuated glass tube; and the same method has been employed by von Bolton, who has obtained the metal in the form of a shining ribbon by compressing it with copper, rolling, and then dissolving the copper in nitric acid. Von Wartenberg has prepared the metal by the electrolysis of thorium chloride dissolved in a fused mixture of potassium and sodium chlorides. Thorium has also been produced by the reduction of some of its volatile organic compounds, e.g. the acetylacetone, by sodium vapour.