Study On The Adsorption Separation Of UF₆ And MoF₆ In The Fluoride Volatility Process

In recent years,global energy and environmental problems have been extremely prominent.The demand for clean energy has led to the rise of research on molten salt reactors.In order to realize the thorium and uranium fuel cycle of the reactor,the treatment and recovery of fuel by pyroprocessing technology has received attention,especially for recovery of uranium from the spent fuel is very important.While fluoride volatility process in the pyroprocessing is an important step in the recovery of UF₆.The removal of other low-boiling fission products fluoride impurities volatilized together with UF₆ is the key to purify UF₆,especially MoF₆,which has similar physical and chemical properties to UF₆.The removal of MoF₆ has a greater impact on the purity of UF₆,and it is more difficult to remove.Therefore,it has received more attention from the counterparts.The use of Na F adsorption-desorption to separate U and Mo has also become a research hotspot.The separation of U and Mo by Na F adsorption-desorption method is currently a relatively common method,but previous studies at home and abroad have shown that it is difficult to achieve both high recovery rate and high purity of U using this method.In view of the above problems,this research investigated the adsorption and desorption behavior of UF₆ and MoF₆ gas on the self-made Na F adsorbent on the dynamic and static adsorption device independently developed by the laboratory,and compared and unearthed the discrepancies between the adsorption and desorption behavior of the two;further resorting to several characterization methods(XRD,in-situ high temperature Raman,EXAFS,TEM,etc.)in-depth study of the thermal stability of the adsorption products was conducted,so as to clarify the reasons for the discrepancies in adsorption and desorption behavior and the mechanism of the desorption process.Finally,the optimal temperature range for the separation of UF₆and MoF₆ was inferred and the adsorption and separation effect of UF₆ and MoF₆ gas on Na F was evaluated.The main research results are as follows:(1)The static adsorption behavior of UF₆ in Na F adsorbent was investigated,and experiments were mainly carried out on the influence of the initial pressure and temperature of UF₆ gas in the range of 20 Torr-80 Torr on the equilibrium adsorption capacity and adsorption time.The adsorption kinetic simulation of the experimental results shows that the adsorption of UF₆ on Na F conforms to the pseudo-second-order kinetic model.The results show that at a certain pressure(60 Torr),the initial adsorption rate(h₀)and equilibrium adsorption capacity(q_e)increase significantly with the increase of temperature,the h₀ rises from 403 mg?g^-1?min^-1 to 591mg?g^-1?min^-1 with the temperature increasing from 50?to 150?and the q_e rises from about 540 mg UF₆/g Na F to about 550 mg UF₆/g Na F;however,the continued increase in temperature is not conducive to the progress of the exothermic adsorption reaction.At 200?,h₀ and q_e decrease to 503 mg?g^-1?min^-1 and about 520 mg UF₆/g Na F,respectively.Therefore,both h₀ and q_e first increased and then decreased.In addition,the reaction rate constant(k)also has the phenomenon of first rising and then falling.The difference in static adsorption behavior of UF₆and MoF₆ on Na F adsorbent was compared.It is found that the initial reaction rate of UF₆ on Na F adsorbent under the same conditions is 10³ times that of MoF₆,and the equilibrium adsorption capacity is 3 times that of MoF₆.There is a certain degree of competitive adsorption between the two on the Na F adsorbent.The post-adsorbed UF₆ will replace about 33.6%-38.3%of MoF₆,and on the contrary,there is no obvious effect of post-adsorbed MoF₆ to UF₆.Because the post-adsorbed UF₆ mainly enters the internal pores of the Na F crystal;the static co-adsorption experiment shows that compared with the respective adsorption,no new substances are formed in the co-adsorbed UF₆ and MoF₆.However,due to the long adsorption equilibrium time(6 h)of MoF₆,the co-adsorption equilibrium time was extended from about 10 min for UF₆ alone to 6 h.The above results indicate that there are obvious differences in the adsorption behavior of UF₆ and MoF₆ on the self-made Na F,and it is expected that the separation of the two can be achieved by adjusting the process conditions.(2)On the basis of initially obtaining the difference of static adsorption behavior,the difference of dynamic adsorption-desorption behavior of the UF₆ and MoF₆ was further studied,which is helpful to obtain the best conditions for the adsorption and separation of the two.The adsorption behavior of a mixture of UF₆ and MoF₆ on Na F adsorbents was carried out at 100?,150?and 200?.The results showed that the adsorption capacity of UF₆ increased slightly(from 329 mg/g Na F rise to 364 mg/g Na F)with the increase of temperature.Because the adsorption capacity(about 1 mg/g Na F)of MoF₆ itself was too low,the adsorption capacity of MoF₆ did not change significantly,so the molar ratio of the two increased from 164 to 235with the increase of temperature from 100?to 200?.Therefore,it is speculated that by increasing the temperature within a certain temperature range,U and Mo can be separated more effectively in the adsorption stage.Furthermore,the temperature-increasing desorption process of co-adsorbed UF₆ and MoF₆ on Na F was investigated.On-line monitoring by infrared spectroscopy found that MoF₆ began to desorb at around150?,while UF₆ desorption was found at 170?,so we designed the experiment again.The above results laid the foundation for the design and optimization of conditions in the subsequent adsorption and separation process.(3)In view of the difference between the static and dynamic adsorption-desorption behaviors of UF₆ and MoF₆ above,with the help of high-temperature in-situ Raman analysis technology,XRD,EXAFS and other characterization methods,the thermal stability of the adsorption products of UF₆ and MoF₆ on the Na F adsorbent under argon atmosphere from room temperature to 450?was investigated,and the decomposition(desorption)mechanism of UF₆ was inferred.The structure of Na₂UF₈ is relatively stable within 200?.The decomposition process of Na₂UF₈ after heating is as follows:in the first stage(250?-300?),Na₂UF₈becomes unstable,and a small part decomposes slowly according to the following equation:Na₂UF₈?2Na F+UF₆?,and a small amount of UF₆ desorbs and volatilizes.In the second stage(320?-450?),Na₂UF₈ becomes very unstable and the decomposition speed increases,most of Na₂UF₈ is decomposed according to the above equation,while a small part of Na₂UF₈ is decomposed according to the following equation:3Na₂UF₈?2Na₃UF₇+UF₆?+2F₂?,the generated Na₃UF₇ exists stably below 450?and becomes the desorption residue of U on the Na F adsorbent.In the fluorine gas state,it is not conducive to the second reaction and is conducive to reducing the residual U.While the Na₂MoF₈ complex,the product of MoF₆ adsorbed on Na F,can be completely desorbed at 400?in Ar atmosphere,and the presence of MoF₆ does not affect desorption of UF₆ from Na F adsorbent.From a microscopic point of view,the difference in desorption behavior between the two and the reasons for the generation of desorption residues are clarified.It is also proved that the residue of U can be reduced under fluorine gas atmosphere,which is more conducive to desorption of UF₆and the recycling of Na F adsorbent,and reduces generation of secondary solid wastes.(4)Based on the results of the above research on the difference of the adsorption-desorption behavior and adsorption mechanism of UF₆ and MoF₆ on Na F adsorbents,the experimental conditions were designed to evaluate the separation effects at different adsorption temperatures.The experimental results show that the separation effect of UF₆ and MoF₆ on Na F is better at 150?than at 100?.When U:Mo(molar ratio)is about 10,the molar recovery rate of UF₆ increases from 98.6%at100?to 99.1%at 150?,the molar removal rate of MoF₆ increases from 97.2%to98.3%,and the decontamination coefficient increased from 31.5 to 53.1.Therefore,the separation effect of UF₆ and MoF₆ adsorption at 150?is better than that adsorption at 100?,which is more consistent with the expected result.And on the basis of the higher intake content of MoF₆,it has achieved a better separation and recovery effect than the domestic and foreign counterparts.In addition,in the follow-up work,reducing the carrier gas flow rate so that the gas and Na F have sufficient contact time will improve the recovery rate of UF₆ and the adsorption separation effect.