Effect of pressure on liquid-liquid immiscibility of high temperature aqueous solution mixtures of uranyl sulfate and sulfuric acid, 280–450°C, 75–1800 bars

Abstract

With rising temperature, aqueous uranyl sulfate and uranyl sulfate-sulfuric acid solutions are very unusual in forming two liquid phases at temperatures above 285°C. In this study the temperatures of immiscibility are shown to be raised by as much as 135°C through the application of hydrostatic pressures up to 1800 bars. This range of pressure extended by a factor of six that of an earlier investigation. An optical cell for use to 500°C and 2000 bars is described, and allowed visual determination of the appearances of immiscibility. The rise in temperature of two liquid phase appearance with increase in hydrostatic pressure was found to be 0·075°C/bar near 300°C, where the initial condition is the boundary of immiscibility in equilibrium with vapor. Upon addition of sulfuric acid the temperature of appearance of immiscibility at vapor saturation is raised, and at these higher initial temperatures the increase in temperature of immiscibility with increase in hydrostatic pressure rises to a value near 0·12°C/bar at 360°C. The present results further confirm a straight line relationship of temperature of immiscibility vs pressure, and support the earlier suggestion of an approximate correlation of the slope with the change in temperature with hydrostatic pressure for pure water either at constant density or constant product of dielectric constant times absolute temperature.