Uranium fractional crystallization

Uranium mineral first is digested with hot nitric acid. AH uranium and radium compounds dissolve in the acid. The solution is filtered to separate insoluble residues. The acid extract is then treated with sulfate ions to separate radium sulfate, which is co-precipitated with the sulfates of barium, strontium, calcium, and lead. The precipitate is boiled in an aqueous solution of sodium chloride or sodium hydroxide to form water-soluble salts. The solution is filtered and the residue containing radium is washed with boiling water. This residue also contains sulfates of other alkahne earth metals. The sohd sulfate mixture of radium and other alkahne earth metals is fused with sodium carbonate to convert these metals into carbonates. Treatment with hydrochloric acid converts radium and other carbonates into chlorides, all of which are water-soluble. Radium is separated from this solution as its chloride salt by fractional crystallization. Much of the barium, chemically similar to radium, is removed at this stage. Final separation is carried out by treating radium chloride with hydrobromic acid and isolating the bromide by fractional crystallization. 

Finely-ground monazite is treated with a 45% NaOH solution and heated at 138°C to open the ore. This converts thorium, uranium, and the rare earths to their water-insoluble oxides. The insoluble residues are filtered, dissolved in 37% HCl, and heated at 80°C. The oxides are converted into their soluble chlorides. The pH of the solution is adjusted to 5.8 with NaOH. Thorium and uranium are precipitated along with small quantities of rare earths. The precipitate is washed and dissolved in concentrated nitric acid. Thorium and uranium are separated from the rare earths by solvent extraction using an aqueous solution of tributyl phosphate. The two metals are separated from the organic phase by fractional crystallization or reduction. 

Protactinium (of mass number 231) is found in nature iu all uranium ores, since it is a long-lived member of the uranium series. It occurs in such ores to the extent of about part per million parts of uranium. An efficient method for the separation of protactinium is by a carrier technique using zirconium phosphate which, when precipitated from strongly acid solutions, coprecipitates protactinium nearly quantitatively. Then the protactinium is separated from the carrier by fractional crystallization of zirconium oxychloride. 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

The purification of uranium by fractional crystallization involves repeated recrystallizations of U02(N03)2 from water or nitric acid, three usually being sufficient. The composition of the crystalline product may range from U02(N03)v2H20 to U02(N03)2 6H20 depending upon the temperature, acidity and uranyl nitrate concentration of the solution.198... 

Fractional crystallization (or differential crystallization) is a process whereby two chemically compounds that form crystals with slightly different solubilities in some solvent (e.g., water) can be separated by a "tree-like" process. One should remember the herculean work by Marie Curie3, who by fractional crystallization isolated 0.1 g of intensely radioactive RaCl2 from 1 ton of pitchblende (a black mixture of many other salts, mainly oxides of uranium, lead, thorium, and rare earth elements). 

Radium is isolated in the processing of uranium ores after coprecipitation with barium sulfate, it can be obtained by fractional crystallization of a soluble salt. 

Derivation Uranium ores (pitchblende and camo-tite). The method used for isolating radium is similar to that developed by Mme. Curie and involves coprecipitation with barium and lead, chemical separation with hydrochloric acid, and further purification by repeated fractional crystallization. The metal is separated from its salts by electrolysis and subsequent distillation in hydrogen. Dry salts are stored in sealed glass tubes, opened regularly by experienced workers to relieve pressure. The tubes are kept in lead containers. 

The method of treatment consisted in effecting a concentration of some of the constituents of the residues and observing the radioactivity of the various portions into which the material was divided. It was observed that if barium was concentrated the radioactivity of that portion increased rapidly. From a ton of residues there may be prepared 10-20 kilograms of crude sulfate whose activity is about 60 times that of uranium. The Curies then converted the sulfates to chlorides and subjected the material to the process of fractional crystallization. After a number of crystallizations there was obtained in the most insoluble portion a fraction, of a gram of radium chloride which was a million times as active as uranium, One ton of pitchblende is said to contain 0.37 gram of radium, 0.00004 gram of polonium,1 and a small amount of aetinium. 

Fractional crystallization. Volatile metals with much lower boiling points than uranium, such as magnesium (1103°C), zinc (906°C), and cadmium (767°C), have been extensively studied as solvents for separating constituents of irradiated metal fuel by fractional crystallization, followed by evaporation of the solvent metal from the separated fractions. For example, in liquid magnesium, the solubility of plutonium or thorium is high, but uranium is very low. A process of this type was developed at Argonne National Laboratory [P6] for concentrating plutonium in the uranium metal blanket of a breeder reactor from 1 percent to 40 percent. 

I submitted to a fractionated crystallization two kilograms of purified radium-bearing barium chloride that had been extracted from half a [metric] ton of residues of uranium oxide ore. ... 

But even if we leave the enigmatical radon family alone the situation still remains unclear. In 1900 W. Crookes observed a strange phenomenon. After fractional crystallization of a uranium compound he obtained a filtrate and a precipitate. Uranium remained in the solution but it exhibited no activity. On the contrary, the precipitate did not contain uranium but exhibited a high-intensity radioactivity. On the strength of his observations Crookes made a paradoxical conclusion that uranium was not radioactive by itself, and its radioactivity was due to some admixture which he managed to separate from uranium. As if he had ill premonitions, Crookes refrained from giving the admixture any definite name and referred to it as uranium-x (UX). Later it was found that uranium restores its activity after... 

Almost immediately after the discovery of radioactivity, Marie Sklodowska Curie and Pierre Curie began more detailed studies of the new phenomenon. Guided by their observation that some natural uranium ores, such as pitchblende, were more highly radioactive than corresponded to their uranium content (Sklodowska Curie 1898), they fractionated the ores chemically, using the intensity of radioactivity in the fractions as evidence for further radioactive substances. The result was the discovery, in June 1898, of a new radioactive element in the bismuth fraction (Curie and Curie, 1898) the Curies named it polonium in honor of Marie s homeland. A few months later, in December 1898, they were able to report the discovery of another radioactive element, this one in the barium fraction separated from pitchblende (Curie et al. 1898) they named it radium. The subsequent isolation of radium from barium was accomplished by fractional crystallization of barium chloride, with radium chloride always being enriched in the crystalline phase. It soon became possible to characterize radium spectroscopically by optical emission lines (Demar9ay 1898) and, thus, to confirm the discovery by an independent identification. By 1902, M. Curie had isolated 120 mg of pure... 

Another major turning point in the history of nuclear science came with the discovery of fission by Otto Hahn and Fritz Strassmann in December 1938 (Hahn and Strassmann 1939a, b). In several laboratories in Rome, Berlin, and Paris, a complex series of P-decay chains resulting from neutron irradiation of uranium had been investigated since 1934, and these chains had been assigned to putative transuranium elements formed by neutron capture in uranium with subsequent P" transitions increasing the atomic numbers (see Sect. 1.2.3). But then evidence appeared that known elements in the vicinity of uranium, such as radium, were produced as well. When Hahn and Strassmaim attempted to prove this by a classical fractional crystallization separation of radium from barium serving as its carrier, the radioactivity turned out to be barium, not radium hence, new and totally unexpected type of nuclear reaction had to be invoked. 

From a solution containing iron and some rare earth metals, Debierne precipitated a mixture of hydroxides. It was radioactive, an activity that could not have its origin in uranium, radium or polonium. A new element could be isolated by fractional crystallization of magnesium lanthanum nitrate. The element was named actinium after the Greek word aktinos, meaning ray . Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300°C. 

Pm is the longest-lived known isotope with a half-life of 17.7 y. Its abundance in Earth s crust, arising from spontaneous fission of uranium, is estimated at 4.5 x 10-20 ppm, a level far too low for isolation by macroscopic methods such as fractional crystallization... 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

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