Thermodynamic study of uranium peroxide and neodymium oxalate precipitations
Résumé
Many metals are usually separated from complex media by precipitation processes. The driving force of precipitation chemical reactions is given by the supersaturation which corresponds to the gap with the equilibrium state. Supersaturation is the key variable in any precipitation as its value governs the rates of the different mechanisms. It is expressed using activities of the precipitating ions. For concentrated multicomponent systems with a high ionic strength, the deviation from the ideality has to be taken into account. This work focuses on the description of thermodynamic studies of precipitations of neodymium oxalate and peroxide uranium. Oxalate precipitates of rare earths are characterized by very low solubility so that oxalate precipitation is widely used in the separation of rare earths, in analytical chemistry and treatment of high-level liquid wastes, especially to separate rare earths and actinides from nitric acid medium. The uranium precipitation is involved in the processing of ores to recover uranium from purified leach solution as yellow cake. For both neodymium and uranium, a thermodynamic model is developed to estimate the supersaturation during the precipitation reaction using activity coefficients. In the first case, measurements of water activity are experimentally carried out on ternary mixtures under different conditions in a variable-impedance hygrometer to determine the individual contribution of neodymium. In the second case, the specific interaction theory is applied to describe the precipitating system containing several electrolytes in sulfuric acid. The interaction specific contributions are adjusted using experimental data of peroxide uranium solubilities by ICP-MS.