Electrochemical Detection Of Precipitation Of Purification Of Moten Salt And Vitrification Of Waste

With further research of the spent fuel dry processing,the importance of purifying molten salts has become increasingly prominent.In view of the difficulties in removal of rare earth ions from waste salts and in detection rare earth's concentration in molten salts,the molten salts was purified by precipitation method and the concentration of rare earth ions was monitoring by electrochemical method.The details was follows:(1)The electrochemical behavior of Dy(?)ions was studied in LiCl-KCl molten salts by a series of electrochemical techniques,such as cyclic voltammetry,square wave voltammetry,chronopotentiometry and open circuit chronopotentiometry.The results showed that at scan rate lower than 100 mV s-1,the reduction of Dy(?)on tungsten electrode was reversible and diffusion controlled process.The diffusion coefficient of Dy(?)was also calculated using the Berzins-Delahay equation.The reaction of Dy(?)on tungsten electrode was found to be three electron transfer process step:Dy(?)+ 3 e(?)Dy.Then,the concentration of Dy(?)in molten salts was analyzed quantitatively.The relationship of Dy(?)concentration with peak current height in square wave voltammograms was investigated in temperature of 400 ?-550 ?.After the measurement,we obtained a calibration curve about the relationship of Dy(?)concentrations and peak current densities,which used to detect the concentration of the rare earth Dy(?)ions in the molten salts.The results indicated that the detection range of Dy(?)concentration in the molten salts increased with increasing temperature.(2)In order to remove the rare earth ions from the molten salts and make the molten salt to be recycled,anhydrous potassium silicate and anhydrous sodium phosphate were used to purify the molten salts in the temperature range from 400 ? to 550 ?.Square wave voltammetry was carried out to detect the concentration change of Dy(?)in molten salts.The experimental results show that the peak height in square wave voltammogram decreases gradually with the addition of precipitant.It was found that when the addition of anhydrous sodium phosphate in molten salts,the decrease of peak height in square wave voltammogram was more obvious than that of anhydrous potassium phosphate.Moreover,the peak current density was linearly related to the amount of sodium phosphate added in the molten salts,which could monitor the precipitation reaction of rare earth ions and sodium phosphate in.However,the linear relationship of peak height between square wave voltammogram and the amount of potassium silicate was not found.In the process of purification of molten salt,anhydrous sodium phosphate as precipitant was added into the molten salts,the yellow precipitate was formed.And then the precipitate was filtered,dried and characterized by the X-ray diffraction.The results indicated that the precipitation was DyPO4.The reaction in molten salts was found to be Dy3+PO43-=DyPO4?.The purifying rate could reach more than 99%.(3)The glassification conditions of fission products containing rare earth elements were explored under the condition of laboratory,and the waste were precipitated during the purification process of molten salts.The glass product was obtained by the following process route:taking salt-filtration-adding alumina and borax-grinding-high temperature melting-cooling process.Then,leaching tests were carried out on glass products.It is showed that the leaching rate of metal components in glass was under 1‰.Thus,the geological disposal of glass products will not dissolve in water and pollution of the natural environment.