Alkali leaching, uranium

Leaching. Treatment with suitable solvents (acids or alkalies) converts uranium contained in the ore to water-soluble species. The uranium is... 

Other factors also affect the mobility of uranium in soil. A field study performed near an active carbonate leach uranium mill showed that uranium in an alkali matrix can migrate to the groundwater (Dreesen et al. 1982). Uranium mobility may also be increased due to the formation of soluble complexes with chelating agents produced by microorganisms in the soil (Premuzie et al. 1995). 

Dilute alkali leaching of ores, principally with solutions of sodium carbonate or ammonium carbonate, is a technique of some commercial importance. In the rare metal field, however, its use is almost entirely restricted to two elements, uranium and vanadium, which form soluble complexes in carbonate solutions. The relatively inexpensive soda-ash grade of sodium carbonate is adequate for the purpose. 

Direct alkali leaching of uranium alone is used in the Eldorado plant at Beaverlodge, in Canada, where the minerals are pitchblende and uraninite. Since the uranium is partially in the tetravalent state, it is not readUy leached by sodium carbonate solutions unless in the presence of an oxidant. Potassium permanganate is very efiective for this purpose< but air or oxygen under pressure are preferred on economic grounds. The reaction is essentially as follows ... 

Leaching of uranium ores can proceed with acid or alkali. 

Leaching with alkali always takes place at high temperatures, either under pressure (5 to 6 bar, 95 to 120°C) or at atmospheric pressure (75 to 80°C). The leaching agent used is sodium carbonate, sodium hydrogen carbonate or ammonium carbonate. Uranium(Vl) oxide is converted in this process into uranyl tricarbonato-complexes ... 

The uranium(IV) is first oxidized in the alkaline medium by ambient oxygen to uranium(VI). Leaching with alkali under pressure has been carried out for a long time. 

Confirmation of this explanation is unequivocally provided by the presence in the reactor zones of at least half of the more than 30 fission products of uranium. Although soluble salts, such as tho.se of the alkali and alkaline earth metals, have been leached out, lanthanide and platinum metals remain along with traces of trapped krypton and xenon. Most decisively, the observed distribution of the various isotopes of these elements is that of fission products as opposed to the distribution normally found terrestrially. The reasons for the retention of these elements on this particular site is clearly germane to the problem of the long-term storage of nuclear wastes, and is therefore the subject of continuing study. 

Precipitation of uranium from duate. In the Dawn mill, uranium was recovered from eluate by two-stage precipitation with alkali. In the first stage, pH is increased to 3.3 to 3.6 by addition of CaO slurry. This precipitates most of the ferric iron that may have been adsorbed with the uranium and removes most of the sulfate as CaS04. Because the filter cake contains 1 to 2 percent uranium, it is returned to the leaching circuit. 

Ion-exchange can be used for the extraction of uranium from sodium carbonate leach liquors. However, the process economics are usually unfavourable since a fairly pure concentrate can be obtained by direct alkali precipitation from the carbonate solution. The leaching stage in the ion-exchange process also tends to be more expensive than acid leaching, except in special cases where limestone is present. Consequently, little or no commercial use is made of the process at present. 

To extract the uranium from the ore, the latter is first concentrated by standard metallurgical techniques and then leached in strong acid or alkali. The dissolved uranium is then recovered from the leach solution by solvent extraction or ion exchange. The product, known as yellow cake , contains in the range of 70%-90% uranium oxide, as a mixture of UO2 and UaOg. 

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