Researches On Process And Mechanism Of Extraction And Separation Of Zr And Hf By DIBK-TOPO Extraction System

Nuclear power is a well-developed clean energy source. With the intensification of crisis of energy, fog and haze, it is undoubtedly to become an important direction and trend to develop nuclear power for all around the world in the 21 st century. Nuclear-grade Zr and Hf are ir-replaceable core materials to the development of nuclear power and nuclear submarines, meanwhile, Zr and Hf separation is the key technology for the preparation of nuclear-grade Zr and Hf. But the situation is that the production of nuclear-grade Zr and Hf in China are still lagged behind, although MIBK separation technology was introduced in recent years, there is no property rights ourselves. Therefore, it is urgent to develop Zr and Hf separation technology. In general, the Hf content in Zr chemical products in the market is 1-3%, hence, it is of great theoretical and industrial value to develop an extractant or extraction system of environment-friendly as well as prior to extracting Hf.DIBK and traditional MIBK all belong to neutral ketone extractant with similar structure. But compared to MIBK, it has advantages of higher boiling point, lower water solubility and toxicity, exhibiting a good application prospects. Previous study suggested that extraction percentage of Hf(21.56%) was not high using single DIBK extractant because of strong steric hinderance and hydrophobic in the extracted specials of Hf with DIBK. Therefore, a modifier was needed in replace of the combined water, in order to improve Hf distribution coefficient DHf in organic phase. In this study, a binary extraction system consisting of DIBK and TOPO was proposed, and its extraction properties in the presence of ammonium thiocyanate and stripping behavior as well as extraction mechanism were investigated, respectively.The Zr and Hf extraction and separation by DIBK-TOPO extraction system from ammonium thiocyanate medium was studied, and several variability affecting Zr and Hf extraction, such as, extraction time, pre-saturated acid concentration, mass concentration of TOPO, salting-out agents, ammonium thiocyanate concentration, phase ratio(O/A) and temperature were investigated, respectively. Based on the investigations above, an optimum process conditions for the Hf and Zr extraction and separation was obtained, namely, extraction time,15 min, pre-saturated acid concentration, 2.0-3.0 M, mass concentration of TOPO, 2.5%, ammonium thiocyanate concentration, 3.0 M, ammonium sulfate concentration, 0.8 M, phase ratio, O/A=2:1, and aqueous acidity [H+]aq, 0.9-1.0 M. Under the optimum technological conditions, DHf and β reached to 2.76 and 33.45, respectively. Furthermore, by plotting log D against 1000/T and applying its slopes to the Gibbs-Helmholtz Equation, enthalpy change for Zr and Hf, namely, ΔHZr and ΔHHf were calculated to be-11.53 kJ/mol and-11.22 kJ/mol in DIBK-TOPO extraction system.Additionally, some of acidic mediums and carbonate salts were used to strip Zr and Hf from its loaded organic phase. The results indicated that hydrochloric and sulfuric acid were difficult to strip Zr and Hf from their loaded organic phase effectively, and under the conditions of O/A=1:1, T=303.15 K and 3.5 M hydrochloric acid, a maximum stripping percentage got to 13.47% for Zr and 15.10% for Hf, respectively; While using sulfuric acid as stripping agents, under the conditions of O/A=1:1, T=303.15 K, a maximum stripping percentage of 12.25 % r were got for Zr at 1.0 M sulfuric acid and 33.23% for Hf at 2.0 M sulfuric acid, respectively. Besides, carbonate salts were proved to be effective Zr and Hf stripping agents from the loaded organic of DIBK-TOPO, and under the conditions of phase ratio O/A=1:3 and ammonium carbonate of 2.0 M, Zr and Hf stripping percentage, RfZr and RfHf reached to 60.73% and 97.4% for one-stage stripping, respectively.Moreover, slope method, saturation capacity method and FT-IR infrared spectroscopy methods were taken to study the Zr and Hf extraction mechanism in DIBK-TOPO system. The results showed that the introduction of TOPO promoted Hf extraction as well as its separation from Zr exhibiting a synergist effect, and presented as a solvated mechanism; For the DIBK extraction system, the constitutions of extracted complexes for Zr and Hf were Zr4(OH)8(H2O)12SCN4Cl4.4DIBK and Hf4(OH)8(H2O)12SCN6Cl2 4DIBK, respectively;While for TOPO alone, the compositions of extracted specials for Zr and Hf were Zr4(OH)8(H2O)12SCN4Cl4.4TOPO and Hf4(OH)8(H2O)12SCN4Cl44TOPO, respectively; While for the DIBK-TOPO binary extraction system, the compositions of extracted specials for Zr and Hf were Zr4(OH)8(H2O)8SCN4Cl44S.4B and Hf4(OH)8(H2O)8SCN84S.4B, respectively.(Wherein, B stand for DIBK, S refer toTOPO).The results obtained by the saturation capacity method showed that molar ratio between the amount of TOPO and total amount of extracted specials of Zr and Hf(nTOPO: n(Zr+Hf)) was close to 1:1 for TOPO and DIBK-TOPO extraction systems, which were consistent with the results obtained by the slope method. Moreover, in the FT-IR spectrum of extracted specials of DIBK with Zr and Hf, the presence of absorption peaks in 3450cm-1 and 1620 cm-1 revealed that the extracted specials contained water, and infrared absorption peaks at 2028 cm-1 suggested that S-C≡N- participated in the coordination with Zr and Hf. Furthermore, absorption peak of C=Obond moved to lower wave number of 1712 cm-1, Δυ=18 cm-1, proving that C=O- involved in the coordination with Zr and Hf; While in the FT-IR spectra of extracted specials of TOPO with Zr and Hf, absorption peak of the P=O- bond moved to lower wave number of 1120 cm-1, Δυ=23 cm-1, suggesting that P=O- involved in the coordination with Hf and Zr, and the presence of infrared absorption peaks at 2050 cm-1 and 2025 cm-1 indicated the formation of coordination bond between S-C≡N-and Hf or Zr; Additionally, in the FT-IR spectrum of DIBK-TOPO with Zr and Hf, infrared absorption peak appeared at 2050 cm-1 and 2025 cm-1,indicating that S-C≡Ninvolved in coordination with Hf and Zr, and C=O- absorption peak moved to higher wave number, Δυ =7.2 cm-1 compared to that of C=O- bond without coordinating with Zr and Hf, proving the formation of coordination bonds between Zr or Hf with C=O- bond. In addition, P=Oabsorption peak for the FT-IR spectrum without Zr and Hf divided into two infrared absorption peaks of 1181 cm-1 and 1130 cm-1,indicating that P=O- involved in coordination with Hf and Zr.