Extraction of Radium Salts from Uranium, Ores
|Pitchblende was at first the most important source of radium, and the separation was carried out in France, Germany, and Austria. The Joachimsthal deposits, however, are reported to be almost worked out, and in recent years America has begun to treat carnotite ores on an extensive scale for the production of radium. |
Various modes of treatment are employed. The original method was to roast the ore with sodium carbonate, wash with warm water, and then with dilute sulphuric acid, thus obtaining an insoluble residue containing both the radium and the polonium and leaving the uranium in solution. By boiling with sodium carbonate solution, the sulphate residue was then converted into carbonates, which could be attacked by hydrochloric acid and submitted to an analytical separation. The radium was ultimately obtained as chloride along with the barium.
By the Ulzer-Sommer process the ore is first treated with concentrated sulphuric acid for some weeks at normal temperature, or for a few hours at boiling temperature, or fused with acid sulphates. The residue is well washed and fused with sodium hydroxide or carbonate, or boiled under pressure with a concentrated solution of these. After further washing, the residue is boiled with dilute sulphuric acid, the radium and barium thus being obtained as sulphates.
For American pitchblende ores, especially where the uraninite is intimately associated with iron pyrites, a chlorination process has been suggested.
For the treatment of carnotite ores the transformation of the mineral into partly soluble vanadates and uranates by the action of sodium hydroxide and carbonate has been proposed. The residue, after washing, may be extracted with dilute hydrochloric acid, forming radium, vanadyl, and uranyl chlorides. Sodium carbonate precipitates radium carbonate and part of the sodium vanadate. By the addition of sulphuric acid, radium sulphate is formed and vanadyl sulphate goes into solution.
Ebler and Bender suggested the reduction of sulphates to sulphide by calcium carbide or hydride, or preferably a mixture of the two. The process is similar to the alumino-thermic method, the reaction being started with a fuse. The reaction product is cooled, powdered, rapidly dissolved in hot dilute hydrochloric acid, and hydrogen sulphide expelled by boiling. Lead sulphide and siliceous matter remain undissolved.
Soddy suggested the reduction of sulphates to sulphides by coal gas. For the treatment of carnotite ores, solution in boiling concentrated sulphuric acid, above 78 per cent., is apparently a promising method. From a good ore at least 90 per cent, of the radium is said to be extracted, and from a poor one about 80 per cent. After the addition of a large volume of water the insoluble residue is subjected to differential sedimentation. The fine sediment contains 87 per cent, of the radium at 20-28 times the concentration in the ore. Fusion with sodium hydrogen sulphate may be employed instead. A further concentration of the radium may be effected by solution in boiling concentrated sulphuric acid again and reprecipitation with water containing a trace of barium chloride, or by fusion with sodium carbonate and subsequent solution in hydrochloric acid.
Hot 38 per cent, nitric acid will dissolve most of the uranium and radium in carnotite. The solution may then be neutralised with caustic soda and the radium precipitated along with barium as the sulphate, reduced to sulphide by carbon in graphite crucibles at 800° C., and dissolved in hydrochloric acid.
Carnotite may also be attacked by boiling concentrated sodium carbonate solution, the barium and radium being ultimately transformed into sulphates.
For treatment of ores poor in radium Ebler and van Rhyn proposed heating to sintering temperature, 800°-1000° C., with sodium or calcium chloride and calcium carbonate for five to six hours in a muffle, and then powdering and extracting the cooled mass with dilute hydrochloric acid containing sulphuric acid and barium chloride. The crude sulphates then contain the radium in a much more concentrated form.
In the treatment of the Australian Olary ores, which contain very little radium and consist mainly of silicotitanates of iron and the rare earths, the ore is first crushed, concentrated magnetically, and fused with sodium hydrogen sulphate. When crushed and agitated with water, coarse inactive particles are separated from a radioactive "slime." The latter is treated with sulphuric acid, washed, converted into carbonates, dissolved in hydrochloric acid, and reprecipitated as sulphates. These are fused with sodium carbonate in graphite pots and the product digested with hot water. After the metallic lead has been picked out the residue is heated with hydrochloric acid and the solution evaporated to dryness. By treating with hot dilute acid insoluble silica may be removed, and by saturation with hydrochloric acid gas radium and barium chloride may be precipitated.
Purification of Radium Salts
|The method first employed by Mme. Curie for the separation of radium from the barium with which the crude salt is always associated was the fractional crystallisation of the chlorides, first from water and then from hydrochloric acid, the radium chloride being less soluble than the barium chloride. Giesel recommended the fractional crystallisation of the bromides. Scholl has made a careful study of the two systems, RaCl2:BaCl2 and RaBr2 - BaBr2, and concludes that fractional crystallisation of the bromides is the more efficient method. The ratios of radium bromide to barium bromide separating under different conditions have also been determined. Apparently this is independent of the concentration of hydrobromic acid. Fractional separation of the hydroxides has also been proposed, but this is said to be less efficient than the bromide method. |
Ebler and Bender have suggested the fractional adsorption of radium and barium salts by a colloid such as silica, or preferably colloidal hydrated manganese dioxide, and subsequent desorption by hydrochloric acid.
Separation of Metallic Radium
|By the electrolysis of a methyl alcoholic solution of radium bromide with an amalgamated zinc cathode and a silver anode Coehn obtained, in 1904, a strongly radioactive product which was probably an amalgam of radium. |
Radium amalgam also appears to be formed when sodium amalgam is shaken with a solution of a radium salt.
In 1910 Mme. Curie and Debierne isolated the metal by Guntz's method for barium. They prepared the amalgam electrolytically from about 0.1 grm. radium chloride and 10 grm. mercury and then distilled off the mercury from an iron boat in a quartz tube in an atmosphere of rigorously purified hydrogen. Distillation was complete at 700° C.
Ebler stated that he had prepared metallic radium by the decomposition by heat of the azide, Ra(N3)2, which he found to be a very stable compound. Herschfinkel, however, doubted the possibility of formation of radium azide, suggesting that it would be decomposed by the radiations from the radium, although, according to Ebler, the corresponding barium salt is not decomposed by radium rays.