Molten Salt/liquid Metal (Pb/Bi-Li) Reduction Extraction Of Rare Earth Gd

Molten salts reductive extraction has been widely applied in many fields, especially in spent fuel dry reprocessing, in which reductive extraction methods were employed to separate actinides from fission products in LiCl-KCl eutectic salts in order to realize the recycling of nuclear fuel. This work was undertaken to investigate the separation of the gadolinium from the molten salt by reductive extraction in LiCl-KCl/liquid metal (Pb-Li or Bi-Li) system which could be a promising technique for the treatment of spent nuclear objects. The main research works are as follows:1. The electrochemical behavior of Gd(?) ions in LiCl-KCl-GdCl3 melt on an inert W electrode was investigated at 873 K by electrochemical techniques, such as cyclic voltammetry, square wave voltammetry, chronopotentiometry and open ciruit chronoptentimetry. The results from cyclic voltammogram and square wave voltammogram showed that the electrochemical reaction of Gd(?) to Gd(0) was an one-step 3-electrons exchanged. A series of CVs obtained at different scan rate indicated that the reduction of Gd(?) was controlled by mass transfer and revisable process at scan rate lower than 0.09 V s-1. And the diffusion coefficient of Gd(?) was calculated at 773K. The diffusion coefficients of Gd in liquid metal Pb were 1.107×10-5 cm2 s-1 at 823 K by anode chronopotentiometry.2. For the preparation of extraction agent, the electrochemical behaviors of Li(?) on W and Pb-coated W electrodes was investigated, respectively, in the LiCl-KCl eutectic by cyclic voltammetry (CV) , square wave voltammetry (SWV) and open circuit chronopotentiometry(OCP) techniques. On an inert W electrode, the reduction of Li(?) takes place at about -2.45 V in a single soluble-insoluble electrochemical step Li(?)/Li(0). In contrast, the electrochemical reduction of Li(?) on a Pb-coated W electrode was observed at less cathodic potentials than at the inert W electrode. The potential shift was mainly caused by the formation of Li-Pb intermetallic compounds, in which the activity of Li was largely decreased compared to that in pure Li metal. From CV results, five peaks corresponding to the formation of Li-Pb intermetallic compounds were observed. By the OCP technique, five plateaus corresponding to the co-existence of two phases of Li-Pb intermetallic compound such as LiPb-Pb,Li5Pb2-LiPb,Li3Pb-Li5Pb2,Li10Pb3-Li3Pb,Li4Pb-Li10Pb3 were observed. The activities and relative partial molar Gibbs energies of Li were obtained from the measured emf for various Pb-Li intermetallic compounds. The standard Gibbs energies of formation for the Pb-Li intermetallic compounds were calculated. The formation of Pb-Li / Bi-Li alloys were carried out by galvanostatic electrolysis on liquid Pb/Bi electrode at 673 K. All the Pb-Li / Bi-Li alloys were characterized by XRD and ICP-AES. The XRD results showed that the electrolysis product were PbLi and BiLi.3. This study addresses the separation of the gadolinium from the molten salt by reductive extraction in LiCl-KCl/liquid Pb or Bi metal system at some temperatures from 673 to 873K. Sampling for the molten salt and liquid metal,all the samples were characterized by ICP-AES. The yield coefficient(Y) and reduction rate of gadolinium(R),the extraction rate(E) and the distribution coefficients(D) of gadolinium between molten LiCI-KCl eutectic salt and liquid lead/bismuth were calculated from the ICP-AES results.The values of Y, R, E, and D increases with increasing temperatures.The Pb-Gd / Bi-Gd alloys prepared by reductive extraction were characterized by SEM, EDS, XRD, ICP-AES.The XRD results showed that the extraction product were Pb3Gd and BiGd. The influence of temperature and Li content on reductive extraction was also investigated by on-line monitoring method.