Leaching Behavior and Surface Alteration of Cesium Aluminum Silicate under Static and Dynamic Conditions.

Abstract

Leach tests have been carried out on solid forms of CsAlSi5O12 in deionized water under static and dynamic conditions. The normalized mass loss (NL) of Si under static conditions at 90°C increased linearly with leaching time at a ratio of specimen surface area (SA, cm2) to solution volume (V, cm3) of 0.1 cm−1, while the leach rate decreased rapidly at SA/V ratios of larger than 0.33 cm−1 owing to the saturation of Si in the leachate. In all cases, the NL values of Al were less than 10−4 g/cm2, and fine particles of aluminum hydroxide precipitated onto the surface at larger than 0.33 cm−1. The leach rate of Cs was larger than that of Si in the early stage within 14d, however, it greatly lowered after 14d. Under dynamic conditions, the NL values of Si and Cs increased linearly with leaching time, yielding normalized leach rates of 6.3×10−6 g/cm2 day for Si and 3.1×10−6 g/cm2 day for Cs, respectively.Under hydrothermal conditions, the leached amounts of Si and Al increased with temperature up to 300°C, while that of Cs greatly decreased over 200°C. The precipitation of crystalline phases of pollucite (CsAlSi2O6) onto the surface resulted in the lowering of Cs concentration in the leachate.