Solidification of Ln Oxides Containing Volatile Chlorides from Pyrochemical Process

Abstract

The waste generated from a pyrochemical process to recover uranium and transuranic elements has been one of the problematic wastes because of high volatility and low compatibility with silicate glass. For the minimization of final waste, an oxidative precipitation by sparging oxygen has been under development, and the waste containing rare earth oxides (REOs) and volatile salt is expected to be generated. This study intended to find a way to immobilize these kinds of wastes under the limitations of a processing temperature (~1200°C) and a waste loading (~20 wt%). From a series of consolidation experiments, it was induced that Ca-rich silicate glass is effective in consolidating the REOs at relatively low temperature. Based on this result, CaO-SiO2-P2O5 (CaPS) was designed to provide a way to control the volatility of waste and to avoid glass effects in the consolidation at a given temperature. By using a CaPS, REOs were consolidated, regardless of glass composition. At a high content of metal chlorides, CaPS can control the volatility up to 1200°C, but it has a low ability to immobilize alkali metal elements. For this, SiO2-Al2O3-P2O5 (SAP) was suggested to treat LiCl-KCl salt in precipitate. This composite can also control the volatility up to 1200°C, and it converted the REOs into monazite at 650°C, where the entire metal elements in chloride form are changed into relatively stable compounds. The leach test by the product consistency test-method A confirmed the immobilization ability of SAP for waste with a high content of metal chlorides. In conclusion, this study suggests the approach concept to treat a waste containing volatile compounds. For a lower content of metal chloride, CaPS are more favorable, and for a high content of metal chlorides, SAP is more effective to fabricate a wasteform for final disposal.