In-situ Mineralization Of Pyrolysis Furnace Ash For Waste Organic Solvent Tributyl Phosphate In The PUREX Process

Aiming at the disadvantages of current cement immobilization technology in the treatment of waste organic solvent tributyl phosphate pyrolysis furnace ash,such as increase of waste volume and high leaching rate of nuclide,this thesis proposes an in-situ mineralization technology for the treatment of waste organic solvent tributyl phosphate pyrolysis furnace ash.The core idea of the above technology is to realize in-situ mineralization of waste organic solvent tributyl phosphate pyrolysis furnace ash and the lattice immobilization of simulated nuclide Nd in in-situ mineral phase fluorapatite by adding Ca F₂ and Na₂CO₃ into simulated radioactive pyrolysis furnace ash[Ca₂P₂O₇,Ca（OH）₂,Nd₂O₃].In this thesis,in-situ mineral fluorapatite[Ca₁₀（PO₄）₆F₂]and Nd doped fluorapatite solid solutions[Ca_10-2xNd_xNa_x（PO₄）₆F₂]were synthesized by the pressureless calcination process.Nd doped fluoroapatite ceramic waste form[Ca₈Nd₁Na₁（PO₄）₆F₂]was prepared by the hot pressing sintering process.The crystal phase composition,PO₄^3-group,element distribution,element composition and Vickers hardness of the samples were characterized by XRD,FTIR,Raman,SEM and EDX.According to the MCC-1 standard,the leaching experiment of Nd doped fluoroapatite ceramic waste form was conducted.The ion concentrations of Nd³⁺,Ca²⁺and Na⁺in leachates were measured by ICP-MS,ICP-OES and AAS.The chemical stability of the Nd doped fluorapatite ceramic waste form and leaching behavior of Nd³⁺,Ca²⁺and Na⁺in the Nd doped fluorapatite ceramic waste form were investigated.This thesis mainly includes the following three parts:（1）Low temperature controllable synthesis of in-situ mineral phase fluorapatite[Ca₁₀（PO₄）₆F₂]in air atmosphere.The study found that the introduction of secondary calcination based on the traditional pressureless calcination process and the additional 5wt.%addition of calcium fluoride based on the original stoichiometric molar ratio,which could help to synthesize in-situ mineral phase fluorapatite under the optimal experimental conditions,such as calcination temperature of 900℃and holding time of 4 h.The synthesized in-situ mineral phase fluorapatite only contains adsorbed water and PO₄^3-group.The results show that the synthesized fluorapatite crystallizes well,the boundaries of grain are clear,and the size of grains are 1-3μm.The chemical elements such as Ca,P,O and F are uniformly distributed in the in-situ mineral phase fluorapatite.（2）Pressureless calcination and characterization of Nd doped fluorapatite solid solution[Ca_10-2xNd_xNa_x（PO₄）₆F₂].The study found that the optimal process parameters for synthesis Nd doped fluorapatite solid solution are calcination temperature of 900℃and holding time of 4 h.The results show that the synthesized Nd doped fluorapatite solid solution Ca_10-2xNd_xNa_x（PO₄）₆F₂ is a finite solid solution,and its limit solid solubility is x=1.1formula unit（weight percentage:14.367 wt.%）.When the Nd solid solubility x>1.1,the second phase neodymium fluoride（Nd OF）will appear in the sample.Different Nd solid solubilities（x=0.8-1.1）have no obvious effects on the microstructure of Nd doped fluorapatite solid solution Ca_10-2xNd_xNa_x（PO₄）₆F₂.（3）The hot pressing sintering and chemical durability of Nd doped fluoroapatite ceramic waste form[Ca₈Nd₁Na₁（PO₄）₆F₂].The study found that the optimal preparation process parameters for Nd doped fluorapatite ceramic waste form are hot pressing sintering temperature of 1100℃and holding time of 2 h.The Vickers hardness of the Nd doped fluoroapatite ceramic waste form[Ca₈Nd₁Na₁（PO₄）₆F₂]reaches 5.999 GPa.The Nd doped fluorapatite ceramic waste form[Ca₈Nd₁Na₁（PO₄）₆F₂]can be severely etched in acidic and alkaline leachates.Especially,the etching degree of the waste form in the pH=3 leachate is the most serious and a new calcium containing monazite crystal(Ca_0.19Nd_0.99PO_3.71)is formed on the surface of the waste form.The results show that the crystal phase composition and microstructure of Nd doped fluorapatite ceramic waste form have no obvious change after being leached in neutral leachate for 42 days,which indicates that Nd doped fluorapatite ceramic waste form[Ca₈Nd₁Na₁（PO₄）₆F₂]possesses superior chemical durability in neutral leachate.The normalized leaching rate of simulated nuclide Nd is as low as～10^-5 g·m^-2·d^-1,which is two orders of magnitude lower than the normalized leaching rate(10^-3 g·m^-2·d^-1)in the borosilicate glass waste form which has been applied in the industry.The in-situ mineralization treatment technology proposed in this thesis can not only realize the volume reduction and resource utilization of waste organic solvent tributyl phosphate pyrolysis furnace ash,but also prepare Nd doped fluorapatite ceramic waste form with high mineral purity and excellent chemical durability.The related research results can provide scientific basis and technical support for the preparation of radionuclide ceramic waste form with excellent integrated performance,and lay a foundation for the safe immobilization treatment of waste organic solvent tributyl phosphate pyrolysis furnace ash in the PUREX process.