Uranium reduction rates

The equation provides the means for determining the conditions necessary to obtain a desired uranium concentration in the aqueous effluent stream through the electrolytic reduction process. Any change in variables and parameters included in the equation will change the uranium transfer rate to the aqueous phase. The higher the transfer rate, the higher the reduction efficiency and the content of uranium in the aqueous phase leaving the column (9). This will affect the uranium-plutonium partition with respect to process requirements. 

Uranium deposited by wet or dry precipitation will be deposited on land or in surface waters. If land deposition occurs, the uranium can be reincorporated into soil, resuspended in the atmosphere (typically factors are around 10 ), washed from the land into surface water, incorporated into groundwater, or deposited on or adsorbed onto plant roots Gittle or none enters the plant through leaves or roots). Conditions that increase the rate of formation of soluble complexes and decrease the rate of sorption of labile uranium in soil and sediment enhance the mobility of uranium. Significant reactions of uranium in soil are formation of complexes with anions and hgands (e.g., COj, OH ) or humic acid, and reduction of U" " to U. Other factors that control the mobility of uranium in soil are the oxidation-reduction potential, the pH, and the sorbing characteristics of the sediments and soils (Allard et al. 1979, 1982 Brunskill and Wilkinson 1987 Herczeg et al. 1988 Premuzie et al. 1995). 

The rate-determining step in an ion-exchange process is the diffusion of the adsorbable ions into the resin matrix. Retention times of 2-10 min are used in the uranium industry to attain full equilibrium. The metal ion to be recovered must almost completely occupy the resin fimctional sites to attain a very high degree of selectivity. Resins should be useful for at least two years if clean clarified leach liquors containing no poisonous ions are used. A drastic reduction in the usefulness of the resins is observed in the presence of such ions. Everest et al. (Ell) studied in detail the deleterious effects of thiocyanates, polythionates and sulfur, cobalt... 

Initial reduction is carried out at lower temperatures to prevent liquefaction of uranium (IV) chloride. After sintering can no longer occur, a higher temperature increases the reaction rate. A sintered product may contain unreacted and occluded uranium(IV) chloride. 

The mechanism of this unique H-D exchange is of considerable importance. The usual mechanism proposed for H-D exchange in d-block transition metals involves a series of reductive-elimination, oxidative-addition cycles. The prerequisite in this type of process is the ability of the metal atom to shuttle between two readily available oxidation states. This type of mechanism could be invoked to explain the exchange reaction in the uranium hydride since uranium (VI) is a well-known oxidation state. The observation that the thorium hydride also undergoes exchange at a comparable rate shows that such a process is not viable as thorium (VI) is unknown. Scheme III outlines an... 

Reduction with tetravalent uranium. Newton [N3] found the rate of reduction of hexavalent neptunium to pentavalent to be rapid and given at 25°C by... 

Reduction with hydroxylamine. No comparable rate data for reduction of Np(V) by hydroxylamine are available. Barney [B2] reported that the initial rate of reduction of Pu(IV) by 0.1 M hydroxylamine nitrate (HAN) was about one-fourth the initial rate of reduction of Pu(lV) by U(IV) at 25°C. If the same ratio applies to reduction of Np(V) by HAN or U(IV), a reaction time of around (4X35) = 140 min might be required. Use of hydroxylamine would have the advantage of not requiring reduction of uranium to U(IV) and its subsequent recycle. 

---