The Study Of Separation Lanthanide And Actinide By Selective Crystallization

With the development of nuclear energy and technology,the storage of spent fuel is increasing sharply.The safe and efficient disposal of spent fuel has become a major concern of nuclear power countries.At present,there are two main strategies of spent fuel disposal in the world including deep geological disposal and spent fuel reprocessing.Spent fuel reprocessing is the spent fuel treatment method selected by most countries with highly developed nuclear power technology.After spent fuel reprocessing,96%of the un-utilized nuclear fuel could be separated and recycled,and the highly toxic and long-lived actinides such as Am3+and Cm3+in the high-level liquid waste are separated and transformed into low-toxic,short-lived elements.After post-treatment,the radiotoxicity of spent fuel could be reduced from millions of years to hundreds of years,and the threat posed by nuclear energy to humans and the environment will be largely released.Therefore,spent fuel reprocessing is one of the key technical methods to ensure the safety development of nuclear power stations.However,in the process of separation and transmutation,the lanthanide fission products featured with large neutron absorption cross section,which are considered as neutron poison in the process of separation and enthalpy,which seriously affects the efficiency of separation and transmutation.Therefore,the efficient separation of trivalent lanthanide(Ln3+)from actinide(An3+)is a key step in spent fuel reprocessing.However,since the lanthanide-contracted effect,trivalent actinides have similar ionic radii and chemical properties to lanthanide ions,the separation of trivalent lanthanides and actinides is considered to be one of the most rigorous challenges in the nuclear science and technology.At present,there are various methods for separating trivalent lanthanide series,including ion exchange method,extraction chromatography,oxidation method,molten salt/metal extraction method,solvent extraction method,etc.,wherein solvent extraction method is widely used at present.However,solvent extraction produces a large amount of organic waste pollution,which is not conducive to the reduction of spent fuel,and the extractants also exhibit less radiation stability and chemical stability.Therefore,it is necessary to develop a new method of separation of lanthanides and actinides.In this paper,the separation between actinides and lanthanides based on lattice difference is studied.The research contents including ligands selection,controlling the reaction conditions to find a reaction system that can amplify the difference of the lattice of the lanthanide series,and determining the crystal structure by X-ray single crystal diffractometer,then generating new periodic trend of lanthanides for the crystallization products;separation experiments based on the selective crystallization of lanthanide in the reaction system be studied,and the influence of kinetics and thermodynamics on the separation results also be studied.Based on the experimental results,this paper proposes a selective separation strategy based on lattice difference.The specific research results are as follows:1.Chapter 2.we report the lattice difference of lanthanides in borate and a method for lanthanides separation by selective borate crystallization.The lanthanides form six different lanthanide borate crystal structures in the boric acid melting reaction,which separates the lanthanides into six different groups,which in turn generates a new lanthanide borate periodic trend,breaking the traditional recognition of lanthanide should be identity.Binary lanthanide separation experiments show that there are two separation modes if controlling the kinetics of separation experiments:The combination of Nd3+/Sm3+ is selected as an important example.When mixed Nd/Sm reaction onlyafford crystalline products in structure type of NdBOCl-2.the presence of Nd in the reaction prevents Sm from formation of its own preferential product;Sm is therefore stays in the water-soluble part,and the separation tractor is 5.32,that giving rise to a potential selective crystallization mechanism.The Nd/Dy reaction represents a completely different story and gives rise to a significant optimization of separation efficiency.The results show that beside crystals of NdBOCl-2,they can form additional crystalline products of DyBOCl-5 and DyBOCl-6 phases.Therefore,the selective flotation method can be utilized because the theoretic density for the relatively porous structure of NdBOCl-2 is significantly lower than that of the dense structures of both DyBOCl-5 and DyBOCl-6,givies a promiding separation factor of 986.On the one hand,this work shows that borate can amplify the slight difference between lanthanides and separate the lanthanides according to their lattice differences.On the other hand,it also shows that selective crystallization has a separation in trivalent ruthenium with good application prospects.2.Chapter 3.we introduces the lattice differences between lanthanides and actinides in organic-inorganic hybrid borate systems and the selective crystal separation of lanthanide elements.A series of organic-inorganic hybrid borate crystal structures were synthesized by boric acid melting reaction.The X-ray single crystal diffraction results showed that the coordination environments around the central metals,the polymerization forms of borate,linker model for H2BDC are various in different compounds,all this resulting in different properties of the compound.Moreover,the reaction system can amplify the difference between the lanthanide metals,and 14 lanthanides(except Pm)can form three different crystal structures under the reaction conditions,forming a unique lanthanide organic-inorganic hybrid borate periodicity.The separaction factor SNd/Dy is 3.3 for Nd/Dy binary lanthanide.This work not only provides a basis for understanding the solid-state chemistry of lanthanides in different valence states.At the same time,they form a unique periodic trend,which has changed the basic understanding of trivalent lanthanides solid chemistry and provided a basis for finding a new selective crystallization system.3.Chapter 4.we study the selective separation of lanthanide(Eu3+)and actinides(UO22+)in highly acidic solutions.Lanthanide crystallizes in an organic ligand N-methyl-2-pyrrolidone and 1.5 M nitric acid mixed solution,and form the lanthanide compound,NH2(CH3)2[Eu(C2O4)2(H2O)].The reaction mechanism research show that,N-methyl-2-pyrrolidone decomposes when heated in a strong acid solution and this process consumes a large amount of H+.UO22+cations are not bonded to the decomposition product under the same conditions.The separation of lanthanide and actinide in strong acid solution was further discussed.The reaction exhibits high selectivity towards Eu3+over UO22+as only lanthanide oxalate compound was formed when lanthanides and actinides are coexisted,and the separation factor up to 201699.8,through a solid-liquid separation model,and the recovery rate of Eu was more than 87%This experiment provides a method for the selective crystallization separation lanthanide and actinide in a highly acid solution features low energy output and high recovery rate.4.In Chapter 5.based on the understanding that lanthanide nitrate crystallizes in the nitric acid solution with the presents of N-methyl-2-pyrrolidone,and form lanthanide oxalate compounds,the lanthanides oxalate periodic trend was studied.The experimental results show that the lanthanide elements form four different crystal structures in the reaction system,and then the lanthanides are divided into eight different groups,which breaks the traditional understanding of the periodic trend of the lanthanides,especially,this reaction system has a lattice discrimination effect on heavy rare earth elements.In addition,we also studied the influence of thermodynamics on the periodic trend.The results show that the lanthanides in the early and middle stages tend to form the same structures if elevated the reaction temperature.However,for heavy lanthanides,although the crystal structure changed,the lattice difference of the late elements still be enlarged in this system.At last,we carried out the binary lanthanide separation experiments based on the obtained periodic trend results.The results show that the lanthanide elements have two separation modes:solid-solid separation and solid-liquid separation in this reaction system.the separation factor for Nd/Dy is 3.354 in solid-liquid separation model weanwhile Nd/Lu givies a promiding separation factor of 50.88 in solid-solid separation model.