Study on Treatment of Spent Mixed Ion Exchange Resins in Bench-scale Molten Salt Oxidation Plant

In order to provide theoretical guidance for the engineering application of molten salt oxidation (MSO) for spent mixed ion exchange resins (MRs) treatment, a bench-scale MSO plant with a treatment capacity of 1.25 kg/batch was used to treat the same type of spent MRs used in nuclear power plants (NPPs). The superiority of MSO in the treatment of spent resins was mainly reflected in the adsorption of acid tail gas (carbonate was consumed, N and S elements were trapped in the form of nitrate and sulfate) and the efficient volume reduction ratio of spent resins. The MRs commonly used in NPPs (consisting of strong basic anion exchange resin and strong acid cation exchange resin) were treated by bench-scale MSO plant. The results show that the acid gas (NO, NO₂, SO₂, H₂S) can decrease to 10 mg/m³ and the main gas product is CO₂ after 0.5 h in the process of MSO. This means that the first 0.5 hours mainly involve the decomposition of functional groups, followed by the destruction of the resin skeleton. Scanning electron microscope (SEM) images show that the resin structure changes from spherical to broken particle after MSO. The content of carbonate in waste salt is 62.1% after MSO. Moreover, the retention rates of sulfur and nitrogen in resins in waste salt are 22.77% and 59.6%, respectively. X-ray diffraction (XRD) results show that the molten carbonate salts could convert sulfur dioxide gas into sulfate in the waste salt. The Gibbs free energy (ΔG) of Li₂CO₃, Na₂CO₃ and K₂CO₃ reacting with SO₂ at different temperatures was calculated thermodynamically. The calculation results show that all the three carbonates could spontaneously react with SO₂ in the temperature range of 200 ℃ to 800 ℃, and the ΔG decreases with the increase of temperature. At 800 ℃, the ΔG of Li₂CO₃, Na₂CO₃ and K₂CO₃ react with SO₂ is -2.800, -41.626 and -69.590 KJ/mol, respectively. Moreover, the ΔG of K₂CO₃ is the lowest among the three carbonates, indicating that K₂CO₃ is the main adsorbent of SO₂. The results also provide a theoretical basis for the adsorption reaction process of SO₂ by ternary carbonate system. After MSO, the degradation rate and the volume reduction ratio (resin volume ratio before and after MSO) of MRs are 98.36% and 6.9, respectively. The above results show that MSO is an ideal treatment method for spent MRs, which has great development potential for industrial scale application.