Uraninite dissolution

After uraninite, coffinite (USi04-nH20) is the most important U-bearing mineral at Cigar Lake. It often occurs as a secondary alteration product of uraninite along microfractures, cracks, and boundaries of uraninite grains. Along with uraninite dissolution, coffinitization of uraninite is another process responsible for the release of U. 

This conclusion seems inconsistent with mineralogical observations indicating that uraninite is in fact present in the Tono Uranium Deposit (Section 2). A possible explanation for this apparent dichotomy is the experimental observation of Neck and Kim [28] that uraninite surfaces in contact with an aqueous phase at pH >3 may be coated with a thin layer of U02(am). Under such conditions, uraninite dissolution is effectively irreversible, and solubility is controlled by the amorphous surface layer. Additional experimental studies and observations of relevant natural systems are needed to test this hypothesis. 

As shown by Grandstaff (1976) the cations, thorium, lead and the rare earths, associated with uraninite retard the dissolution of UO2 significantly. 

Grandstaff, D. E. 1976. A kinetic study of the dissolution of uraninite. Economic Geology, 71, 1493-1506. 

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

Janeczek, J. Ewing, R. C. 1992. Dissolution and alteration of uraninite under reducing conditions. Journal of Nuclear Materials, 490, 133-156. 

Fig. 9. Experimental solubilities as total uranium concentration in solution for experiments on dissolution of uraninite samples from Oklo and Cigar Lake. Solid lines correspond to the calculated solubilities. Calculations performed with PHREEQC geochemical code (Parkhust Appelo 1999) and uranium database taken from Grenthe et al. (1992) and Bruno Puigdomenech (1989).

Aerobic mineral oxidation resulting in mineral degradation and product mobilization Aerobic bacterial oxidation of elemental sulphur (S°), of various mineral sulphides such as pyrite (FeS2), chalcopyrite (CuFeS2), arsenopyrite (FeAsS), sphalerite (ZnS), cobalt sulphide (CoS) and nickel sulphide (NiS) to corresponding metal sulphates, and of uraninite (UO2) to U02 are examples in which oxidizable minerals undergo dissolution of one or more of their constituents, which are thus mobilized (see Ehrlich, 2002a). 

Experiments by Grandstaff (1976, 1980) and more recently by Ono (2002) on the oxidation and dissolution or uraninite can be used to set a rough upper limit of 10 -10 atm on the O2 content of the atmosphere during the formation of the Au-U deposits of the Witwatersrand Basin (Holland, 1984, chapter 7). This maximum O2 pressure is much greater than that permitted by the presence of MIF of sulfur isotopes during the last 0.5 Ga of the Archean the observations do, however, complement each other. 

Ono S. (2002) Detrital uraninite and the early Earth s atmosphere SIMS analyses of uraninite in the Elliot Lake district and the dissolution kinetics of natural uraninite. Doctoral Dissertation, Pennsylvania State University, State College, PA. 

Grandstaff (1976), in examining the kinetics of oxidative dissolution of uraninite concluded that the factors influencing the rate of dissolution are ... 

Many researchers have attempted to measure the solubility of amorphous to crystalline UO2 (uraninite) as a function of pH. Some of this work is summarized in Table 13.4 and Fig. 13.7. Solubility measurements have been complicated by the fact that the UO2 solids were often of different and poorly known crystallinity and particle size. Further, oxygen (and possible CO2) contamination invalidated the results of most early measurements. With oxygen contamination, the measured solubility becomes that of a mixed oxidation-state oxide or a U(VI) solid such as schoepite. Oxygen contamination apparently invalidates the results of Gayer and Leider (1957) and Bruno et al. (1987) who obtained solubilities roughly equal to that of UO3 H2O (as reported by Gayer and Leider 1955) or to the solubility of schoepite as shown in Fig. 13.5. Measurements by Rai et al. (1990), Torrero et al. (1991), and Yajima et al. (1995) (see also Parks and Pohl 1988) indicate that the solubility of amorphous to more crystalline UO2 is independent of pH above about pH 4 to 4.5. This indicates that the dissolution reaction is... 

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. 

---