Uranium minerals uranyl carbonates

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 ... 

Calcium Uranyl Carbonate, CaUO2(CO3)2.20H.jO, occurs in nature as liebigite (see p. 273). Voglite is a similar mineral containing copper. Uranothallite is a double carbonate of calcium and uranium, Ca2U(CO3)3.10H3O (also see p. 278). 

Uranium minerals may be obtained in solution, in a suitable condition for estimation, by the following process. The ore is dissolved in aqua regia, or, if necessary, fused with alkali bisulphate and extracted mth hot hydrochloric acid. After evaporation to drjmess, the residue is taken up with dilute hydrochloric acid, and the solution saturated with hydrogeir sulphide in order to remove any copper, lead, bismuth, arsenic, antimony, or any other metal yielding an insoluble sulphide. The filtrate is concentrated and treated with ammonium carbonate, which precipitates the carbonates of the alkaline earths, iron, and most of the rare earths. The filtrate is neutralised by hydrochloric acid, evaporated to dryness, and the residue ignited to drive off ammonium salts, and then redissolved in dilute acid. The remaining rare earths, and particularly thorium, are next precipitated by the addition of oxalic acid. The filtrate, which contains the uranium in the uranyl condition, may now be precipitated by any of the methods described above. 

The aquatic transport of uranium as carbonate complexes is reflected in the formation of the uranyl carbonate minerals Rutherfordine, U02(C03), Leibigite, Ca2[U02(C03)3]-10-llH20, and Andersonite, Na2Ca[U02(C03)3]-6H20. Both natural and synthetic UO2CO3 have similar structures which contain sheets of planar ions. The uranyl groups have the normal trans... 

Uranyl carbonate minerals are common in the altered zones of uranium deposits, and tend to be amongst the most soluble of the uranyl minerals. Currently, there are 22 known stmctures of uranyl carbonates, of which 12 correspond to minerals (Table 3). In slightly alkaline to alkaline groundwater the uranyl tricarbonate cluster, with formula [(U02)(C03)3]" is the dominant species, and uranyl minerals that form from such solutions contain the uranyl tricarbonate group as an isolated cluster. 

More then 150 minerals containing uranium are known. Typically, however, uranium ores contain only about 0.1% uranium. In the commercial production of uranium metal, the ore is crushed, concentrated, roasted, and in most cases leached with sulfuric acid in the presence of an oxidizing agent such as manganese dioxide or chlorate ions to convert all of the uranium to uranyl sulfato complexes. Carbonate leaching is used to extract uranium from ores containing minerals such as calcite. The recovery of uranium... 

To summarize uranium complexing in hydrothermal solutions, the predominant species will depend on the concentration of complexing anions, which is, in turn, dependent on temperature and pH. The activity of fluoride in many uranium mineralizing systems appears to be significant, as is indicated by the abundance of fluorite and other fluoride-containing gangue minerals. In these systems uranyl fluoride complexes would predominate in acid to neutral solutions. At low temperatures carbonate complexes predominate in alkaline solutions, but, as temperature increases, carbonate complexes become less important. Phosphate complexes may be important in nearneutral solutions in which as little as 0.1 ppm phosphate is present. As temperature increases, hydroxide complexes become more important. At temperatures of 300°C and above hydroxide complexes may be the only soluble uranium species. 

The order of presentation of the uranium minerals will follow chemical groups. The U minerals are discussed first, followed by the niobates, tantalates and titanates. These two groups include the primary reduced minerals. The uranyl minerals are considered in the order hydrated oxides, silicates, phosphates and arsenates, vanadates, molybdates, sulphates, carbonates, and selenates and tellurates. Each section includes an evaluation of the known crystal chemistry and its effect on chemical variability and occurrence of mineral species. 

The uranyl carbonate minerals are found as coatings on a variety of other uranium minerals and usually occur in deposits located in arid climates or on the walls of mine adits, where they form as efflorescences. The minerals are all very soluble in water, and evidence in many localities indicates very recent deposition from water migrating away from primary deposits. Most uranium carbonates show low radioactivity, which... 

The known uranium(VI) carbonate soHds have empirical formulas, 1102(003), M2U02(C03)2, and M4U02(C03)3. The soHd of composition 1102(003) is a well-known mineral, mtherfordine, and its stmcture has been determined from crystals of both the natural mineral and synthetic samples. Rutherfordine is a layered soHd in which the local coordination environment of the uranyl ion consists of a hexagonal bipyramidal arrangement of oxygen atoms with the uranyl units perpendicular to the orthorhombic plane. Each uranium atom forms six equatorial bonds with the oxygen atoms of four carbonate ligands, two in a bidentate manner and two in a monodentate manner. 

A large number of studies of uranium sorption have been carried out in support of the nuclear waste disposal programs and the uranium mill tailings program (UMTRA). Park et al. (1992) and Prasad et al. (1997) describe studies of sorption of uranyl ion by corrensite, the clay mineral lining many fractures in the fractured Culebra Dolomite member of the Rustler Formation above the WIPP in SE New Mexico. The studies were carried out in dilute and concentrated NaCl (0.1-3 M) solutions in the presence of Ca, carbon-... 

Uranium solubility is increased even more in the nitrate microcosms, and one possible explanation is the conversion of acetate to CO2, coupled with nitrate reduction, which would give higher dissolved carbonate concentrations in the nitrate microcosms. However, PHREEQE modelling showed that the higher CO concentration would not greatly affect uranium speciation in solution and is therefore unlikely to account for the enhanced solubility. Alternatively, as nitrate is reduced to ammonium (NH/), which promotes cation exchange, this could lead to displacement of U02 from surface complexes, which are the predominant uranyl species on mineral surfaces. " ... 

Eh are usually less than 10 M because of the extremely low solubilities of these solids. In the U(V) oxidation state, uranium occurs as the UOJ ion which forms relatively weak complexes (Grenthe et al. 1992). This species is only found at intermediate oxidation potentials and low pH s and is unstable relative to U(IV) and U(VI). In oxidized surface- and groundwater-uranium is transported as highly soluble uranyl ion (UOf ) and its complexes, the most important of which are the carbonate complexes. The thermodynamic properties of these minerals and aqueous species must be known if we are to understand the reactions that may control U concentrations in natural waters. 

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