Uraninite leaching

Uranium, too, is widely distributed and, since it probably crystallized late in the formation of igneous rocks, tends to be scattered in the faults of older rocks. Some concentration by leaching and subsequent re-precipitation has produced a large number of oxide minerals of which the most important are pitchblende or uraninite, U3O8, and camotite, K2(U02)2(V04)2.3H20. However, even these are usually dispersed so that typical ores contain only about 0.1% U, and many of the more readily exploited deposits are nearing exhaustion. The principal sources are Canada, Africa and countries of the former USSR. 

The separation of impurities such as calcium, barium and radium can be effected by the precipitation of their sulfates. In the case of sulfuric acid leaching of uraninite ore, the leach liquor contains radium which is removed if barium chloride is added since barium sulfate acts as a carrier for radium sulfate. 

The incorporation of anions, as for example, S04 , CO2-, etc., makes leaching possible through the formation of stable uranyl (VI) oxyanions. In sulfate leaching, an observation of the potential-pH diagram for the uranium system reveals that uranium species in solution may be in the form of cations U02+, neutral species U02(S04)2 or anions U02(S04)4-. The oxidation of uraninite, U02, in acid solutions, transforming U(IV) to U(VI), yields soluble uranyl sulfate through the reaction as shown below ... 

Alkaline leaching is carried out by using sodium carbonate solution. In this case any U(IV) present in the ore must also be oxidized to U(VI). The uranium species soluble in carbonate leach solutions in the uranyl tricarbonate ion. The formation of this ion by solubilization of a hexavalent uranium mineral such as camotite, or a tetravalent uranium mineral such as uraninite, may be represented by the following reactions ... 

Uranium is deposited widely in the Earth s crust, hence it has few ores, notably the oxides uraninite and pitchblende. The ores are leached with H2SO4 in the presence of an oxidizing agent such as NaClOs or Mn02, to oxidize all the uranium to the (+6) state as a sulfate or chloride complex. On neutralization with ammonia a precipitate of yellow cake , a yellow solid with the approximate composition (NH4)2U207 is formed. This is converted into UO3 on ignition at 300 °C. This can be purified further by conversion into uranyl nitrate, followed by solvent extraction using tributyl phosphate in kerosene as the extractant. 

Bacterial leaching of uranium ores is the subject of a subsequent section of this chapter. Here it suffices to say that the bacterial activity in such situations is that of enhancing the rate of oxidation of sulfides, through which ferric sulfate and sulfuric acid are liberated to speed up the process of solution of uraninite. 

Crouse and Brown [C5] describe pilot-plant studies on recovery of uranium and thorium from Canadian uraninite by sulfuric acid leaching followed by solvent extraction in a two-cycle amine extraction process using trioctylamine to extract uranium and di(tridecyl)amine to extract thorium. 

Acidic and basic dissolution of uraninite (UO is depicted below as an example of U(IV) leaching. Sitmlar reactions may be written for pitchblende (UaO. Uraninite and pitchblende are the two most impoitant uranium minerals. The leaching of oxidized [U(VI)] uranium minerals such as camotite, K20 2U03 V 0) 3H20, is readily achieved in both acid and basic circuits since oxidation is not required. Oxidation-leaching processes are rate limited by the oxidation step. 

The primary minerals pitchblende and uraninite predominate, and their composition approximates to UsOs or a mixture of oxides intermediate between UsOg and UO2. These oxides are easily leached by dilute sulphuric acid, in the presence of an oxidant to convert the uranium to the hexavalent condition. It is customary to regard the form of the resulting uranium in solution as being uranyl sulphate, UO2SO4. In practice, however, high proportions of the anionic complexes 1102(804)2 and U02(S04)8 are also present. Use is made of these complexes in the later stages of... 

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 ... 

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