C. R. Paige

Solution Equilibria for Uranium Ore Processing: The BaSO4-H2SO4-H2O System and the RaSO4-H2SO4-H2O System

Abstract A radio-tracer determination of the solubility of BaSO4 in H2SO4-H2O mixtures was undertaken at temperatures of 25°C and 60°C. These were subsequently modelled using the Pitzer model for the activity coefficients, and estimates were made for Ba2+-SO42− and Ba2+-HSO4− interaction parameters. The final model yields an excellent description of the solubility of BaSO4 in H2SO4-H2O mixtures up to 6 mol/kg and at both 25°C and 60°C. The logarithm of the thermodynamic solubility product constant (logKsp) for barite obtained by regression analysis was −10.02 ± 0.02 at 25°C and −9.68 ± 0.01 at 60°C. The values of the ion association constant derived from these data are 2.49 ± 0.03 at 25°C and 2.55 ± 0.1 at 60°C. The enthalpy of dissolution for BaSO4 was estimated to be 17.6 ± 0.2 kJ mol−1. Literature solubility data for the RaSO4-H2SO4-H2O system have also been interpreted, using the Pitzer model for the necessary activity coefficients. The logarithm of the thermodynamic solubility product constant (logKsp) for radium sulfate obtained was −10.21 ± 0.06 at 25°C, while the value of the ion association constant derived is 2.76 ± 0.05 at 25°C. The high negative value of the Pitzer parameter β2 obtained by regression analysis of these data suggests that the explicit recognition of a RaSO4(aq) ion association species is to be preferred in this case.

The aqueous dissolution kinetics of the barium/lead sulfate solid solution series at 25 and 60°C

Abstract The dissolution behavior of the BaxPb(1−x) SO4 solid solution series and pure endmembers was investigated. The solids were precipitated from sulfuric acid solutions, then dissolved in slightly acidified water at 25 and 60°C for 24 h. Endmembers equilibrated within several h at 25°C and several min at 60°C. At 25°C the solid solutions dissolved congruently up to 30 min, then the Ba2+ concentration decreased and Pb2+ concentration increased for the duration of the experiments. At 60°C the results were similar except the region of congruent dissolution was over before the first samples were taken at 3 min. Solid solution-aqueous solution (SSAS) reaction paths were followed on Lippmann diagrams. During congruent dissolution the reaction paths dissolved toward stoichiometric saturation. During incongruent dissolution the systems moved toward two-component thermodynamic equilibrium at lower aqueous mole fractions of Ba2+, while maintaining steady states of undersaturation with respect to the stoichiometric saturation. At 60°C the solutions maintained steady states closer to stoichiometric saturation than at 25°C. The 25 °C results were fit to the two parameter kinetic model of Denis and Michard (1983) with an additional parameter to account for the precipitation rate dependence of the distribution coefficient. The 60°C results could not be modelled due to a complete lack of data in the congruent dissolution region. The model was able to generate close fits to our Ba2+ data, but not our Pb2+ data. The model was unable to generate steady states significantly below the stoichiometric saturation given the observed rates of congruent dissolution. This indicates a process occurs which acts to decrease the congruent dissolution rate after the formation of a secondary phase.

Modelling solution equilibria for uranium ore processing: The PbSO4-H2SO4-H2O and PbSO4-Na2SO4-H2O systems

Abstract Solution equilibria in the PbSO4-H2SO4-H2O and PbSO4-Na2SO4-H2O systems have been modelled using the Pitzer formalism for the determination of activity coefficients over the temperature range from 0–60°C, and up to 6 m in H2SO4. The available literature, common ion solubility data, and our own experimental data at 60°C have been analysed to determine the various ion interaction parameters needed to provide a description of the solution equilibria. The resulting array of Pitzer parameters yield an accurate description of the solubility equilibria over the temperature range 0–60°C, and concentration conditions up to 6 m in H2SO4.

The formation of hetero-epitaxial deposits of lead, barium and (barium/lead) sulfates on quartz surfaces

As a continuation of an investigation concerning the fate of226Ra during uranium ore milling and long-term tailings storage we have grown hetero-epitaxial deposits of barium sulfate, lead sulfate and mixed barium/lead sulfate on sawn quartz surfaces of differing crystallographic orientations. The deposits formed readily on the substrate from supersaturated aqueous solutions of the parent ions and appear to be strongly adherent. Calcium sulfate formed deposits on quartz at very much greater levels of supersaturation than lead or barium sulfate.

STUDY OF THE DYNAMIC EQUILIBRIUM IN THE BASO4 AND PBSO4/AQUEOUS SOLUTION SYSTEMS USING 133BA2+ AND 210PB2+ AS RADIOTRACERS

The transport of cations implicated in the National Uranium Tailings Project source model for the geochemical control of 226Ra have been investigated under equilibrium conditions with the aid of 133Ba2+ and 210Pb2+ as radiotracers. The rates of the isotopic exchange between BaSO4 and PbSO4 and their respective saturated solutions appear to exhibit a fast, followed by a slow component. These data permit an estimate of the transport rate constants to be made. It was found that the rate of these exchange processes is retarded by the presence of other ions. In contrast, the crystallographic nature of the faces did not have a large effect on the exchange rate. It was noted that equilibrium (exchange) and nonequilibrium (crystal growth) experiments yield different information about the interfacial kinetics. The rate constants which play the key role in crystal growth are believed to be determined largely by the dehydration of cations, whereas the rate constants for exchange are more likely to be a measure of the activation energy needed in order to break bonds in the solid phase.

KINETICS OF DESORPTION OF IONS FROM QUARTZ AND MICA SURFACES

As part of an investigation concerning the fate of226Ra during uranium ore milling and long-term taillings storage we have investigated the kinetics of the desorption of alkaline earth, lead and sulfate ions from quartz and mica surfaces into water and dilute nitric acid using a radio-tracer technique. The retention times are sufficiently long to permit the growth of the surface deposits hypothesized in the HILEMAN-SNODGRASS model. The kinetics of desorption of most of these ions is found to agree with the model for rate-limiting desorption proposed by CEREFOLINI. In the case of the desorption of Ba2+ from mica surfaces the data suggest some degree of integration of the Ba2+ into the mica.

Uranium ore processing: Kinetics of the growth and dissolution of a potential host solid from a model surface

As part of an investigation concerning the fate of226Ra during uranium ore milling and long-term tailings storage, we have investigated the growth of potential host solids-barium sulfate and lead sulfate-on mica, a model surface. The kinetics of dissolution of barium sulfate-and of a barium sulfate solid containing228Ra as a trace component into 10−3M sulfuric acid were subsequently investigated using a flow-cell and radiotracer techniques. It was found that ther dissolution of both the barium sulfate host and the228Ra trace component was congruent. The dissolution reaction appears to be surface controlled.

The formation of hetero-epitaxial deposits of strontium sulfate on mica surfaces

As a continuation of an investigation concerning the fate of226Ra during uranium ore milling and long-term tailings storage we have grown hetero-epitaxial deposits of strontium sulfate on mica surfaces. The deposits formed readily on the substrate from supersaturated aqueous solutions of the parent ions and appear to be strongly adherent.