熔盐电解精炼铪的电极过程和电结晶机理研究

Hafnium has a high thermal neutron capture cross section is the ideal control materials used in nuclear reactors; Hafnium oxide film is the best film materials resistance to laser damage and excellent high dielectric constant (high K) materials. The molten salt electrorefining is an effective method for purification of hafnium, some gas impurities (O, N, C) and metallic impurities (Fe, Mg, Mn) can be effectively removed。The subject was first proposed to add NaF to the NaCl-KCl-K2HfF6 system in order to change [F-/Hf4+] ions proportion, making hafnium ions reduced to hafnium metal by one step process, and using cycle reverse technology in hafnium electrorefining process, there is no report about this research so far at home and abroad. The electrode process were studied by linear sweep voltammetry, cyclic voltammetry and square wave voltammetry and the electrocrystallization mechanism was researched by chronoamperometry, scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis. The effect of current efficiency, yield and sticky salt rate, particle size and purity at different electrorefining conditions were studied at the better [F-/Hf4+] ions proportion which determined by one step reduction process and better electrocrystallization condition, and combined with periodic reverse technique. The project aims to reveal the electrochemical mechanism in the process of molten salt electrorefining of hafnium, and provide a theoretical basis for optimizing the parameters in electrorefining.

金属铪具有高的热中子吸收截面是理想的核反应堆控制材料；采用高纯金属铪制备的氧化铪薄膜是最好的耐激光损伤薄膜材料和性能优异的高介电常数（高K）材料。熔盐电解精炼是提纯金属铪的有效方法，对气体杂质（O、N、C）和金属杂质（Fe、Mg、Mn）等元素具有较好的去除效果。本课题首次提出向NaCl-KCl-K2HfF6体系中添加NaF，改变[F-/Hf4+]离子比，使铪离子一步还原到金属铪，同时将周期反向技术应用在电解精炼铪上，迄今国内外尚无报道。通过线性扫描伏安法、循环伏安法、方波伏安法研究电极过程，同时采用计时电流法、扫描电镜（SEM）和能谱分析（EDS）考查电结晶机理。根据一步还原步骤和较好电结晶条件下的[F-/Hf4+]离子比，结合周期反向技术，研究电解精炼工艺条件对电流效率、产率、粘盐率以及粒度和纯度的影响。本项目旨在揭示熔盐电解精炼铪过程中的电化学机理，为优化电解精炼工艺条件提供理论依据。

高纯金属铪是制备耐激光损伤薄膜材料和性能优异的高介电常数（高K）材料的重要原料。熔盐电解精炼是提纯金属铪的重要方法，对气体杂质(O, N, C)和金属杂质(Fe, Mg, Mn)都具有较好的去除效果。向NaCl-KCl-K2HfF6体系中添加NaF，改变[F-/Hf4+]离子比，使铪络合离子一步沉积，同时将周期反向脉冲技术应用在电解精炼铪上，获得了较好的研究结果。. 通过循环伏安法和方波伏安法分析得到Hf4+络合离子在1023k下NaCl-KCl-K2HfF6-NaF熔体中的还原机理为：当[F-/Hf4+]<6时，Hf4+ + 2e → Hf2+，Hf2+ + 2e → Hf；。当18<[F-/Hf4+]<22时，Hf4+ + 4e → Hf；Hf4+络合离子的阴极沉积电位随着NaF浓度的增加而逐渐负移。计时电流法、扫描电镜(SEM)和能谱(EDS)分析研究结果表明：Hf4+络合离子在Pt电极和铪镀层上的成核机理是连续成核方式，长大方式是扩散控制的三维半球形长大过程。电解精炼铪的最佳工艺条件为：[F-/Hf4+]比例为19、电解精炼温度为750℃、电流密度为0.35A/cm2和K2HfF6浓度为20%，在此最佳工艺条件下的电流效率可达到92.78％。以低品位海绵铪为阳极料在最佳工艺条件下电解精炼得到了符合工业级标准的金属铪，其中对金属元素Fe、Cr、Ni、Mg、Mn和非金属杂质O具有较好的提纯效果，Fe由0.14%降为0.038%、O由0.2%降为0.13%。. 研究获得周期反向脉冲工艺电解精炼铪的最佳工艺条件为：正向平均电流密度为0.35 A/cm2、正向占空比为50%、正向工作时间80ms、反向电流密度为0.035 A/cm2、正向占空比为50%、反向工作时间10ms、正反向脉冲频率均为1000Hz。得到了粒度（D50）最大为285.89μm的产品，粘盐率也降低为21.5%。项目揭示了熔盐电解精炼铪的电极过程和电结晶机理，为优化电解精炼工艺条件提供了理论依据。获得了最佳电解精炼工艺，结合周期反向脉冲技术获得了粒度大、纯度高的产品。为下一步开展直接电镀制备镀膜用高纯金属铪的研究提供了研究基础。