The Research On Oxidation Of Monomethylhydrazine By Nitrous Acid And Reaction Kinetics Of Heptavalent Technetium With Salt-Free Reagents In Nitric Acid Medium

In the separation step of plutonium from uranium in Purex process, the existence of Tc(â…¦) decreases the separation efficiency of Pu from U by consumption of a great deal of reductants and brings difficulty into the following process because Tc(â…¦) flows into the uranium stream when U(â…£) is used as reductant for tetravalent plutonium and N₂H₄ as supporting reductant to scavenge HNO₂. However, when N, N -dimethylhydroxylamine (DMHAN)-monomethylhydrazine (MMH) are used as reductants, almost all of Tc comes into aqueous phase with plutonium. Therefore, it is necessary to investigate the reaction of heptavalent technetium with DMHAN and MMH including the stability of DMHAN in nitric acid medium, the reaction kinetics between MMH and HNO₂ and so on. These facts play an important role in the application of DMHAN-MMH in the reprocessing process.According to the discussion above, the research subjects are chosen as follows:●the stability of DMHAN in nitric acid solution;●the reaction between MMH and HNO₂;●the kinetics of the reaction between Tc(â…¦) and MMH in nitric acid medium;●the kinetics of the reaction between Tc(â…¦) and DMHAN in nitric acid medium.The research results in the paper are summarized as follows :1, The purity of DMHAN has been analyzed and the results indicate that the self-synthesized DMHAN contains few impurity and meets the needs of the experiment. A titration method for determination of the concentration of DMHAN has been established based on the potassium dichromate titration after the ferric oxidation. The ratio of DMHAN to ferric in the oxidation of DMHAN is found to be 1∶4. Methanol, formaldehyde, and formic acid, which are possible oxidized products of DMHAN, do not influence the result of analysis. Using this titration method, the stability of DMHAN in nitric acid has been studied. The results indicate that the decomposition of DMHAN in nitric acid depends on temperature and the concentration of nitric acid. The decomposition of DMHAN increases with the increasing of acidity and temperature although the concentration of DMHAN has little change under certain acidity and temperature. DMHAN is decomposed quickly when the temperature is higher than 35℃and the acidity is higher than 3.0M.2, The reaction kinetics of MMH and nitrous acid is studied. The results are indicated as follows.The reaction rate equation of MMH and nitrous acid in perchloric acid medium is -d[HNO₂]/dt=k[H⁺]^0.94[MMH]^1.11[HNO₂], where k=(46.0±2.7)(mol/L)^-2.05·S^-1 when the temperature is 4.5℃and the concentration of perchlorate is 0.5mol/L. The activation energy of the reaction is Ea=(43.4±0.1)kJ/mol.The reaction rate equation of MMH and nitrous acid in nitric acid medium is -d[HNO₂]/dt=k[H⁺][NO₃^-]^1.14[MMH]^1.08[HNO₂], where k=(115±2)(mol/L)^-3.22·S^-1 when the temperature is 2.6℃and the concentration of nitrate is 0.5mol/L. The activation energy of the reaction is Ea=(37.8±0.1)kJ/mol。3, The research result of the reaction between DMHAN and heptavalent technetium in nitric acid indicates that, in the studied range, DMHAN does not react with heptavalent technetium, which means that, heptavalent technetium does not directly consume DMHAN.4, The research result of the reaction between MMH and Tc(â…¦) in nitric acid indicates that, this reaction is composed of three stages, induction period, fast reaction period and the termination of the reaction.The length of the induction period is related to the acidity, the concentration of MMH, and that of Tc(â…¦) in nitric acid solution The rate equation of the induction period is -d[Tc(â…¦)]/dt=k₁[H⁺]^0.64[MMH]^0.45[Tc(â…¦)], where k₁=(0.029±0.002)(mol/L)^-1.09·h^-1 when the temperature is 40℃and the concentration of nitrate is 3.0mol/L. The activation energy of this reaction is Ea=(56.5±0.2)kJ/molThe rate equation of the fast reaction period is -d[Tc(â…¦)]/dt=k₂[H⁺]^0.47[MMH]^0.22[Tc(â…¦)], where k₂=(0.186±0.002)(mol/L)^-0.69·h^-1 when the temperature is 40℃. The activation energy of this reaction is Ea=(51.6±0.4)kJ/mol. The ionic strength has no influence on the reaction rate.After termination of the reaction, low valence technetium will be oxidized to heptavalent technetium. The rate of the oxidization reaction rate depends on the acidity and the concentration of the reductant.Under the similar conditions, the rate of the reaction between MMH and Tc(â…¦) is much slower than that of between N₂H₄ and Tc(â…¦). Therefore, Tc(â…¦) will not directly consume a great deal of MMH.