熔盐萃取锆铪分离和核级锆电解精炼的机理研究

Zirconium has a very low absorption coefficient for neutrons, which makes this metal suitable as construction material in nuclear reactors. It is also an ideal material for nuclear waste disposal. In nature zirconium is nearly always found in combination with hafnium. Contrary to zirconium, hafnium has a high absorption coefficient for neutrons, which makes this metal suitable as neutron absorber such as control rod in nuclear reactors. Chemically zirconium and hafnium strongly resemble to each other. In the application as construction material of reactors, the zirconium metal should have as low a content of hafnium as possible. So far, there is no very efficient and dominant process for Zr-Hf separation in production of reactor grade zirconium. The commonly used solvent extraction and distillation technologies often involve long processing routes with multi-steps separation and high environmental burden. .Recently a compact process for more efficient production of pure zirconium with a sufficiently low content of hafnium have been developed and patented by the applicant. The process consists essentially 3 steps: (1) Production of Hf-bearing crude Zr into liquid Cu-Sn alloy through molten salt electrolysis,(2) Zr-Hf separation through molten salt-metal equilibrium,(3)Production of (Hf free) zirconium through molten salt electro-refining. In this new production route, Zr-Hf separation is the key innovative step, which is based on the reactions between molten CuCl2-based chloride salt and the Zr-Hf bearing low melting Cu-Sn alloy. In the salt-metal equilibrium, Hf in the metal phase will prior to Zr transfer to the chloride salt, resulting in an almost Hf-free Zr in the Cu-Sn alloy. The results of laboratory experiments confirmed a very high Hf removal efficiency with a separation factor up to 600 (only 10-30 with solvent extraction). The electro-refining step was also experimentally investigated in the lab scale, and it requires sufficiently high Zr content in the Cu-Sn alloy for improving the process efficiency. The preliminary results have proved the technical feasibility of the process, but indicate a great necessity to better understand the reaction mechanisms, so as to provide fundamental knowledge for optimization of the reaction systems. .The main objectives of this project are to establish the scientific fundamentals of this new production method. It includes the following research activities: .(1).To investigate the reaction kinetics and mechanisms of the Zr-Hf separation process, to provide fundamental understanding for optimization of the process parameters for higher separation efficiency..(2).To investigate the electro-chemical behavior of Zr in the molten salt electro-refining system and control of the impurity of the refined Zr product, to achieve a good scientific understanding of the system (e.g. anodic and cathodic processes, effects of electrolyte, cell voltage, and current efficiency).

锆和铪在化学性质上很相似，在自然界中共生，核级锆要求铪含量小于0.01%。目前，工业上生产核级锆采用的溶剂萃取法和蒸馏法，都具有分离过程繁杂和环境负担过重等缺点。申请人开发了紧凑型的火法锆铪分离生产核级锆的工艺,具有分离效率高，可实现大规模连续生产等优点。前期研究表明，在熔盐中加入CuCl2可以有效地将粗锆中的铪从低熔点Cu-Sn-Zr（Hf）合金中提取分离出来,之后经熔盐电解精炼得到金属锆。本项目拟就该生产方法中所涉及的主要物理化学过程做系统的基础研究：（1）研究Zr-Hf分离的反应动力学，深入探讨冶金反应机理,提高Zr-Hf分离效率;（2)研究从液态合金中电解精炼锆的电化学现象，考察分析影响电极过程，电流效率及阴极锆质量的因素，如温度、电解质成分、电流密度等。这一项目的成功实施，将为核级锆新技术的开发提供理论基础和技术支撑，从而实现工业上核级锆的规模化生产。

铪与锆在自然界共生，且与锆有着截然相反的核性能。因此，Zr-Hf分离一直是核工业领域的重要研究课题。本研究采用熔盐萃取的方法对Zr-Hf进行分离，得到的无铪锆合金通过熔盐电解精炼可一步得到核级纯锆。对不同熔盐体系对Zr-Hf分离效率的影响以及锆在不同熔盐体系中的电化学氧化还原过程及机理进行了深入的理论探讨和实验研究。（1）熔盐萃取Zr-Hf分离：热力学研究表明，较低的温度将有利于锆铪分离；在温度低于1000℃时，CuF2对Hf具有更好的选择性。对于低熔点Sn-Cu-Zr-Hf合金体系，以30%NaCl-65%CaCl2-5%CuF2熔盐体系作为萃取剂，在控制Sn/Cu =0.25（质量比）操作温度~950℃及CuF2/Hf化学计量比为2-4时得到很好的分离效果，Hf的脱除率以及Zr-Hf分离系数高达99%和85， 且Zr损失率较低，可一步制备出符合成分要求的核级锆。为提高Zr在合金中的溶解度，对Cu-Zr-Hf合金体系也进行了大量实验研究，采用NaF-CaF2-CuF2熔盐体系，在操作温度1000℃及CuF2/Hf化学计量比为2时，铪的脱除率仅达到47%。研究表明，在合金体系中，Sn的存在可以有效地降低合金熔点，有利于Zr-Hf的分离过程，但同时也会降低锆在合金中的溶解度。溶解度研究表明，900℃及Sn/Cu=0.25时Zr在Cu-Sn合金中的最高溶解度为6 wt%。（2）核级锆电解精炼的机理：在LiF-KF-ZrF4纯氟盐体系中， Zr(IV)的还原是一个“三步过程”，分别为Zr(IV)/Zr(II)，Zr(II)/Zr(I)和Zr(I)/Zr；计算得到的扩散系数为8.31×10-6 cm2s-1。在LiF-NaF-K2ZrF6体系，Zr(IV)的还原是一个“两步过程”- Zr(IV)/Zr(II)和 Zr(II)/Zr； Zr(IV) 和Zr(II)在熔体中的扩散系数分别为1.13×10-5 和2.42×10-5 cm2s-1。Zr的还原步骤与含Zr化合物的形态或复杂性有一定关系。金属Zr在氯化物体系中的阳极溶解过程中不会像在氟化物熔盐体系中在电极表面生成钝化层，阳极产生的锆离子可以顺利地扩散到熔盐中，更适合做锆合金电解精炼的电解质。电解实验结果表明，从合金体系直接精炼锆是可行的，然而阳极合金组元及锆的浓度对产物纯度也有非常重要的影响， 有待于进一步的深入探讨。