Zirconium and Hafnium Separation Technology Principle
Zirconium and hafnium are used in different aspects of the nuclear industry due to their significant differences in neutron absorption cross-sectional areas. Generally, in zirconium-hafnium alloys used in atomic reactors, the two are "harmful components" to each other. In order to maintain the nuclear properties of zirconium and hafnium alloys, certain requirements are put forward for the content of zirconium and hafnium alloys, that is, the content of hafnium in zirconium shall not be higher than 100 ppm, and the content of zirconium in hafnium shall not be higher than 2%. In nature, zirconium and hafnium are always produced together, and there is no zirconium or hafnium existing alone. Therefore, the separation of zirconium and hafnium has become the key to the preparation of nuclear-grade zirconium and hafnium. In industry, many experts and scholars have successively proposed different methods for separating zirconium and hafnium, which can be roughly divided into the following two categories: pyro separation and wet separation.
1. Pyro separation method of zirconium and hafnium
The pyro separation of zirconium and hafnium has also been an important topic of research by scientific researchers in various countries. According to statistics, there are as many as 16 types of pyro separation of zirconium and hafnium, among which the most representative ones are distillation and selective reduction.
Distillation method
Distillation method is based on the fact that some compounds of zirconium and hafnium, such as chlorides and complex chlorides generated by chlorides of zirconium and hafnium and phosphorus oxychloride, have different boiling points, and the separation of the two is achieved through distillation. Distillation method can be divided into two categories: high-pressure fractionation method and molten salt distillation method. At present, only molten salt distillation method has been successfully applied in industrial production, and the most widely used molten salt distillation system is KCl-AlCl3 and NaCl-KCl. This method uses the difference in vapor pressure of zirconium and hafnium tetrachlorides in solvents such as (molten salt KAlK4) to separate them in a distillation tower.
Selective reduction method
This method is based on the fact that under certain conditions, zirconium tetrahalides are selectively reduced to trihalides or disproportionated to dihalides by zirconium alone, while hafnium tetrahalides are not or rarely reduced, thereby widening the vapor pressure difference between zirconium and hafnium halides, and then separating zirconium and hafnium from each other through distillation. The process is mainly divided into three stages. In the first stage, ZrCl4 undergoes a reduction reaction at 390-405℃ under normal pressure; in the second stage, a disproportionation reaction occurs at 420-450℃. The above two stages are mainly for purifying zirconium. The third stage is for purifying hafnium. After purification, the hafnium content in the raw material increases from 50% to 70%.
The pyrometallurgical separation of zirconium and hafnium process directly uses zirconium tetrachloride and hafnium as raw materials, which can be directly connected with the metal reduction process, eliminating the complex process of intermittent operation of pyrometallurgy and water method, and simplifying the process flow. However, this method needs to be carried out at a higher temperature (350-500℃), which has high requirements for equipment materials, and the process has the disadvantages of being difficult to completely purify impurities and large investment, and is only suitable for large smelters.
2. Wet separation process of zirconium and hafnium
Due to the similar outer electron layer structure and lanthanide contraction, zirconium and hafnium are very similar in chemical properties. They have strong complexing ability with oxygen, so they are very easy to hydrolyze and polymerize in aqueous solution to form different types of complexes, which also increases the difficulty of zirconium and hafnium separation. However, there are also some slight differences in zirconium and hafnium in different media. Based on these slight differences, domestic and foreign researchers have successively proposed a series of wet separation methods for zirconium and hafnium. According to its classification, it can be mainly divided into the following categories: solvent extraction, adsorption separation, membrane separation, micro-solvent extraction, two-phase extraction, fractional crystallization and precipitation, among which solvent extraction separation is the most common and studied method.
Solvent extraction, also known as liquid-liquid extraction, is a method of separating and purifying solutes by using the different distribution of solutes in two immiscible or partially miscible solution phases. It has the advantages of large production volume, simple equipment, easy automation, safe and fast operation, and low cost, and is widely used in the separation of substances. Solvent extraction method Since Fisher first used MIBK to separate zirconium and hafnium in thiocyanate solution in 1947, solvent extraction separation method has made long-term progress and development, and different extraction systems and extractants have been developed successively. At present, several relatively mature nuclear-grade zirconium and hafnium solvent extraction separation processes have been developed successively: MIBK-HSCN system, improved TBP system and TOA/N235-H2SO4 system.
MIBK-HSCN system
The MIBK-HSCN method uses the difference in the complexing ability of Zr4+ and Hf4+ with SCN- ions to preferentially extract hafnium, and zirconium remains in the aqueous phase, thereby achieving the separation of zirconium and hafnium. Since the 1970s, the MIBK method has been the most widely used zirconium and hafnium separation production process in the world, and nearly 1/3 of the world's nuclear-grade zirconium and hafnium are produced by this method. However, the MIBK method has some disadvantages: (1) MIBK has a high solubility in water (1.7%), resulting in large solvent losses; (2) The decomposition of ammonium thiocyanate in industrial wastewater produces hydrogen sulfide, mercaptans and cyanide ions, which are harmful to the environment; (3) MIBK has a certain odor, which makes the operating workshop environment poor.
TBP system
The TBP method was originally invented by Frenchman J V Kerrigan. After years of continuous research and improvement by domestic and foreign scholars, its process parameters and conditions have changed greatly compared to before. At present, the TBP-HNO3-HCl mixed acid system is mainly used in industry. This system directly uses zirconium tetrachloride as the raw material and adds nitric acid to directly prepare a nitric acid-hydrochloric acid extraction solution of zirconium (hafnium). After the improvement, the separation coefficient of zirconium to hafnium has been greatly improved, up to 30~40, and atomic-level zirconium dioxide and hafnium dioxide can be obtained at the same time after one extraction. However, due to the high acidity of the TBP system, it corrodes the equipment severely and is easy to emulsify during extraction, which directly affects the normal operation of the extraction operation.
TOA/N235-H2SO4
The TOA method is another zirconium and hafnium separation process after the MIBK method and the TBP method. This method uses sulfuric acid as the medium, preferentially extracts zirconium, and the separation coefficient of zirconium and hafnium is 8~10. The TOA method has the advantages of low pollution, concentrated radioactive materials, easy handling, and low investment costs, but the extraction capacity of zirconium and hafnium is small and the separation coefficient is not high. In view of the limitations of TOA, scientific researchers have conducted a series of studies and improvements on this method.
Although the above processes can achieve the requirements of zirconium and hafnium separation, they have some disadvantages, such as high water solubility of MIBK, low boiling point, large solvent loss, serious environmental pollution, etc.; TBP process has serious corrosion to equipment and is easy to emulsify, etc.; TOA method and N235 method have small extraction capacity and low separation coefficient, which limits their industrial application. Improving traditional processes and developing new zirconium and hafnium separation processes with high separation coefficients are the main research goals and development directions of current solvent extraction separation methods.
