Study On The Process And Mechanism Of N263 Selective Extraction And Separation Of Zirconium And Hafnium

Zirconium and hafnium have similar physicochemical properties,symbiotic and hafnium mass content of 1 to 3%of the total zirconium and hafnium,is one of the elements difficult to separate,but their nuclear properties are different,to achieve the separation of zirconium and hafnium is the technical key to the preparation of nuclear-grade zirconium and nuclear-grade hafnium materials for nuclear reactors,and is important to achieve the goal of carbon peak and carbon neutral.The only industrial solvent extraction method for the separation of zirconium and hafnium is the MIBK process,but there is a large loss of water solubility of the extractant MIBK,SCN^-under acidic conditions is prone to decomposition of toxic substances such as H₂S and cyanide and serious pollution of the environment,etc.,to improve the stability of SCN^-has been its urgent need to solve the technical bottleneck.In this paper,the cationic extractant N263was selected to stabilize SCN^-in the extraction system,and its extraction process,extraction mechanism,kinetics and thermodynamics were studied for the extraction and separation of hafnium zirconium to provide theoretical basis and technical support for the industrial application of the system.Firstly,the acidic medium type,diluent type and extractant concentration,salting agent type and concentration,extraction time,and concentration of thiocyanate in the feed solution on the selective extraction and separation of zirconium hafnium for the N263 system were studied,and optimize the process of extraction and separation of zirconium hafnium for the N263 system.The optimized process parameters for the N263 extraction system are:acidic medium is thiocyanic acid,amyl acetate as the diluent,30 vol%N263 in organic phase,0.6mol/L（NH₄）₂SO₄ as the salting-out agent,2.5 mol/L thiocyanic acid in feed solution,15 min as the extraction time,and O/A=2/1 as the phase ratio.Under the optimized process conditions,one extraction of 110 g/L of zirconium-hafnium stock solution result in 94.49%extraction of hafnium and 40.65%extraction of zirconium,and the separation coefficient of hafnium relative to zirconium is 19.4.The nuclear grade zirconium solution was obtained by two-stage extraction.Secondly,the effects of the type and concentration of the N263 system of reverse extractant,reverse extraction time and compared to O/A on the reverse extraction of zirconium and hafnium in the loaded organic phase were studied.The results show that the sulfuric acid solution has selective reverse extraction ability for zirconium in the loaded organic phase;the ammonium carbonate solution has weaker selectivity but stronger reverse extraction ability for zirconium and hafnium in the loaded organic phase.The optimized process parameters for the reverse extraction are as follows:under the controlled conditions of O/A=2/1 and 10 min of reverse extraction time,the zirconium in the loaded organic phase is washed four times with3.0 mol/L H₂SO₄ solution（containing 0.2775 g/L of hafnium）,and then the hafnium in the loaded organic phase is eluted once with 3.0 mol/L（NH₄）₂CO₃ solution,and the total washing and elution rates of zirconium and hafnium are up to 97.72%and 91.47%,respectively.The reverse-extracted organic phase can be regenerated cyclically after acidification with 2.0 mol/L hydrochloric acid and pre-saturation with 2.2 mol/L thiocyanate solution,and the high extraction separation performance is still maintained after three cycle extraction tests.Finally,the extraction mechanism of hafnium by the N263 system was investigated by the slope method and infrared spectroscopy analysis.The results show that the composition of the extractive complex of hafnium extracted by the N263 system is（R’R₃N⁺）₁₂·(Hf O²⁺)₃（H⁺）_（12x-2）（SCN^-）_（16+12x）,and during the extraction process,the N in the co-extractant HSCN coordinates with hafnium to form the extractive complex;the equation of the extraction reaction of hafnium can be expressed as follows:3Hf O_aq²⁺+4SCN_aq^-+12[（R’R₃N⁺）（H⁺）_x（SCN^-）_（1+x）]_org=[（R’R₃N⁺）₁₂·(Hf O²⁺)₃（H⁺）_（12x-2）（SCN^-）_（16+12x）]_org+2[H⁺]_aq.The kinetics and thermodynamics of the extraction of zirconium hafnium by the N263 system were investigated by the Le cell method,and the influence laws of stirring speed,interfacial area and extraction temperature on the extraction and separation of zirconium hafnium were explored.The results show that the extraction process of zirconium hafnium by the N263 system is controlled by diffusion when the stirring speed is lower than 130 rpm,and the extraction process of zirconium hafnium is controlled by chemical reaction when the stirring speed is 130-170 rpm.The extraction rate constants of zirconium and hafnium keep increasing with the increase of the boundary area,indicating that the extraction reaction occurs mainly at the phase interface.The kinetic equation of the extraction of hafnium by the N263 system is:R=K×[Hf O²⁺]_aq³×[SCN^-]_aq⁴×[（R’R₃N⁺）（H⁺）_x（SCN^-）_（1+x）]_org¹².The N263 system The apparent activation energies for zirconium and hafnium extraction are 42.06 k J/mol and 31.77 k J/mol,the enthalpy changes are 39.59 k J/mol and 29.30 k J/mol,the entropy changes are-147.53J/（mol·K）and-174.38 J/（mol·K）,respectively,and the Gibbs free energy changes at room temperature 298 K are 83.57 kJ/mol and 81.29 kJ/mol,respectively.