| A one-dimensional, analytical model of mass flow and reaction is derived for the leaching of uranium from sandstone-type ores by acidic, oxidizing solutions. Identified are the chemical rate processes which control oxidant consumption and uranium production for conditions relevant to in situ uranium leaching and also to understanding uranium extraction in heap- and dump-leaching operations.;The model incorporates experimentally determined rate expressions to describe chemical processes which take place as a fluid flows through a porous matrix while simultaneously reacting in, and/or with, that matrix. Specific rate expressions were determined for pyrite and uraninite oxidation by ferric iron and hydrogen peroxide and for the catalytic decomposition of hydrogen peroxide by ferric ion. These rates of oxidant consumption and mineral reaction are expressed by sets of mole balance equations which give oxidant concentration and masses of reactive minerals, as a function of time, distance the fluid has traveled through the ore, and fluid velocity. Dispersion of the fluid species along the flow path is neglected, as are changes in porosity due to reaction. For each reaction, the mole balance for oxidant consumption is related to the dissolution rate for each reacting mineral by a stoichiometric coefficient. From this relationship, sets of partial differential equations are coupled for mass flow and the rates of the oxidation reactions. These differential equations are solved numerically by a finite-difference scheme, the generalized Newton-Raphson method.;The model is evaluated by comparing its predictions to results obtained from a series of column-leaching experiments which were conducted on synthetic ores of quartz sand, pyrite, and uraninite. Model predictions closely match most experimental results for the range of ferric iron and hydrogen peroxide concentrations, and flow rates used in the leaching experiments. Significant discrepancies between predicted and experimental rates were found for the leaching of uraninite plus pyrite ores by hydrogen peroxide. A more detailed rate law for pyrite oxidation by hydrogen peroxide, one which takes into account aqueous speciation, is necessary to sufficiently describe this system. Column-leaching experiments and model predictions show that both ferric iron and hydrogen peroxide are effective oxidants for leaching uraninite at low pH. However, the use of H(,2)O(,2) may be limited in solutions containing dissolved iron due to the rapid decomposition of H(,2)O(,2) by reactions catalyzed by ferric ion. |
