Study on molten salt oxidation process of simulated Co doped cation exchange resins

Abstract

Cation exchange resins (CERs) are widely applied to purify waste liquids generated during the operation of nuclear reactors. The radioactive nuclides 60Co and 58Co are important corrosion activation products in reactor cooling water. In this study, the simulated Co doped CERs were oxidized with ternary carbonate. According to the thermogravimetric analysis (TG), the decomposition of Co doped CERs includes three processes: 1. Elimination of the osmotic water; 2. Pyrolysis of sulfonic acid group; 3. Destruction of styrene–divinylbenzene copolymer. The X-Ray Diffraction (XRD) patterns indicate that sulfur mainly exists in the form of sulfate in waste salt. The Co2+ undergoes the path of CoS2 → Co3O4 with the increase of temperature and the transition point is 650 °C. Combined with Fourier Transform Infrared Spectrometer (FT-IR) spectra and X-ray Photoelectron Spectroscopy (XPS) analysis, sulfonic acid groups begin to decompose at 350 °C. During the molten salt oxidation process, most of the sulfur in sulfonic acid groups is entrapped by carbonate as the form of sulfate, and a little of which remains as sulfone group, sulfoxide group and sulfur bridge in residue. When the resins are oxidized at 800 °C, the retention rate of Co2+ is 97.3%, indicating that the molten salt oxidation can effectively remain Co2+ and convert it into a more stable substance.