The Research On The Molten Carbonate Salt Oxidation Process Of Ion Exchange Resins And Simulated Waste Resins With Sr²⁺ Or Cs⁺

Multiple or one resin will be used to form mixed or single beds for radioactive wastewater treatment in pressurized water reactor plants.Spent resin was also produced from nuclear fuel cycle and isotope preparation and usage,which belongs to low and medium radioactive solid waste.The temporary waste resin will be irradiated to produce flammable gases such as methane and hydrogen,which has not been satisfactorily solved so far.Solidification and incineration are common disposals for waste resins treatment,but they have disadvantages such as the increase of capacity and the generation radioactive exhaust gases.Molten salt oxidation（MSO）is considered as the most likely alternative to incineration for waste resin disposal with the advantages of simple operation,large volume reduction,and economic exhaust treatment.The MSO processes of anion exchange resins（AERs）,cation exchange resins（CERs）,mixed resins（MRs）,and Sr or Cs doped simulated waste resins were studied.The influence of time during MSO of AERs in Na₂CO₃-K₂CO₃ system were studied.The degradation and removal efficiency（DRE）value of AERs could be up to 94.6%when the oxidation time was 2 h.The oxidation and MSO behaviors of AERs in Na₂CO₃-K₂CO₃ system were studied by TG.The contents,constructure,and microstructure of MSO products were analyzed by UV,ICP,XRD,FT-IR,Raman,and SEM tests.It indicated that molten carbonate salt could effectively oxidize AERs and convert the NO_x into the nitrate.Linear sweep voltammetry（LSV）test proved that the E_O2 shifted from 0.67 V to 0.22 V with the existence of nitrate,which could increase the oxidation capacity of molten salt and promote the oxidation of residue.The MSO process of CERs in Na₂CO₃-K₂CO₃ were studied.The result of TG and gas mass spectrometer showed that the oxidation and MSO processes of CERs include the drying process（25°C-200°C）,functional group decomposition（200°C-450°C）and residue continue oxidation process（450°C-800°C）with the generation of the gas products of H₂O,SO₂,and CO₂,respectively.FT-IR and XPS analysis indicated that sulfur in the CERs could generate sulfur bridge in the styrene-divinylbenzene（ST-DVB）with the high thermal stability,which is different from AERs.Elemental analysis,XRD,and ICP tests showed that molten carbonate salt could facilitate the removal of sulfur bridge in the residue,which promotes the oxidation of CERs and the production of sulfate.The temperature of sulfate formation in molten salt lagged behind that of sulfonic acid group decomposition,and the optimal oxidation condition was 550°C for 1 h.The results of process simulation experiments proved that 100 mesh was the best particle diameter parameter for the MSO of CERs in the temperature range of 450-750°C.And the influence of the particle diameter and the carbonate system on the DRE of resins could be ignored when the oxidation temperature was higher than 750°C for1.5 h.The MSO behaviors of the MRs（AERs:CERs=50 wt.%:50 wt.%）in Na₂CO₃-K₂CO₃system were studied.The activation energies of the composition process for ammonium and sulfonic groups were calculated by non-isothermal thermogravimetric analysis,which were252.08 k J/mol and 391.77 k J/mol,respectively.And the quaternary ammonium group in MRs decomposed before the sulfonic acid group.According to the thermodynamic calculation results,the reaction of N and S in MRs to nitrate and sulfate in the molten carbonate salt could be spontaneous within the range of the temperature of MSO.The content of nitrate in the waste salt after MSO was only 49.78%of that in AERs.However,the content of sulfate in the waste salt was 5.3%higher than that in CERs.The main factor affecting the DRE and volume reduction ratio of the three kinds of resins after MSO was oxidation temperature.When the temperature was 850°C,the DRE and volume reduction ratio of resins（the ratio of resin to salt was 1:1）in Li₂CO₃-Na₂CO₃-K₂CO₃ were 99.99%and 3.7,respectively,which was the best system for MSO.The MSO behaviors of simulated radioactive waste resins containing Sr²⁺and Cs⁺in Li₂CO₃-Na₂CO₃-K₂CO₃ were studied.LSV tests demonstrated that the introduction of Sr²⁺and Cs⁺could enhance the oxygen ion content in the melt salt and catalyze the resin oxidation when the temperature was above 400°C.Sr²⁺mainly promoted residue oxidation by decreasing the content of-SO₂-and was ultimately trapped in the form of Sr CO₃.Cs⁺could facilitate residue oxidation by promoting the generation of peroxide ions in the melt salt and produce Cs₂SO₄,which could simultaneously trap S and Cs.The addition of AERs and preheating at 550°C for 1 h could increase the sulfate content and improve the Cs⁺trapping rate.ICP results demonstrated that using this method can increase the retention rate of Cs from 85%to 99%at 800°C for 0.5 h.Moreover,over 92%of Cs still could be trapped in the molten salt at high temperature at 800°C for 2.5 h.