MIBK wet separation of zirconium and hafnium technology has become mainstream

Zirconium and hafnium are two important rare metal materials in the nuclear industry and have good market prospects and technology development space in the field of nuclear power. In addition to its excellent nuclear properties, zirconium also has many other excellent properties, such as extremely stable at room temperature, radiation resistance, corrosion resistance, high strength, and good processing performance. Zirconium has a small thermal neutron capture surface (0.18b (1 b=10 m/atom)), which can prevent uranium radiation from leaking and prevent nuclear radiation; and can ensure that there are enough thermal neutrons in the nuclear reactor to maintain nuclear fission The continuous progress of the reaction has become the preferred material for high-performance fuel element cladding in today's nuclear power plants. Contrary to zirconium, hafnium has a large thermal neutron capture surface (105 b) and a high neutron absorption coefficient. It is often used as thermal neutron absorbing material in nuclear reactors to control the reaction speed of nuclear reactors.

In nature, hafnium is almost entirely dispersed in zirconium minerals, and the two are of the same quality. The content of hafnium in zirconium generally does not exceed 2%. Nuclear-grade zirconium used in nuclear reactors is required to contain as low a hafnium as possible to reduce its neutron absorption coefficient. The hafnium content in nuclear-grade zirconium should be less than 0.01%, and the zirconium content in nuclear-grade hafnium should be less than 2%. But because zirconium and hafnium have very similar chemical properties, separation is difficult. Therefore, the separation of zirconium and hafnium is the key to preparing nuclear-grade zirconium and nuclear-grade hafnium.

Hydrometallurgical separation technologies for zirconium and hafnium mainly include fractional crystallization, graded precipitation, ion exchange and solvent extraction. Among them, the fractional crystallization method and the fractional precipitation method both utilize the different solubility characteristics of a certain compound of zirconium and hafnium in a specific solution to achieve the purpose of zirconium and hafnium separation. Compared with other wet separation methods of zirconium and hafnium, the solvent extraction method has the advantages of large output, simple equipment, easy automation, and low cost. It is the main method for separating zirconium and hafnium in industry.

Solvent extraction method is currently the main method for separating zirconium and hafnium in industry. At present, the solvent extraction methods used in industry mainly include MIBK (methyl isobutyl ketone) method, TBP (tributyl phosphate) method, N235 (tertiary trialkyl amine) method, P204 (bis(2-ethylhexyl) ) phosphoric acid) method, etc. Among them, the MIBK method has the advantages of large extraction capacity, good separation effect of zirconium and hafnium, and large separation coefficient. About two-thirds of the world's nuclear-grade zirconium production uses the MIBK method, such as Western Zirconium Company (owned by Westinghouse Company of the United States) and Huachang Company wait.

MIBK is a saturated fat monoketone with the molecular formula (CH) CHCH COCH. In the molecular structure of MIBK, the chemical properties of the carbonyl group and adjacent hydrogen atoms are very active. MIBK extraction agent is a neutral oxygen-containing extraction agent. In the process of extracting and separating zirconium and hafnium, the extraction mechanism is complex extraction. When using MIBK extraction to separate zirconium and hafnium, they are usually in thiocyanate solution. The main reason is that the stability of the complex formed by zirconium and hafnium with SCN is different. Among them, the complexing ability of hafnium and thiocyanate is more During the extraction process, zirconium preferentially enters the organic phase, while zirconium remains in the aqueous phase in the form of thiocyanate, thereby achieving effective separation of the two. In industry, the MIBK-HSCN system is often used. Generally, NH SCN is added to the aqueous phase, and the organic phase is saturated with HSCN. Among them, the chemical reaction equation of zirconium and hafnium and SCN to form a complex is:

In addition, the distribution ratio and separation coefficient of zirconium and hafnium increase with the increase of SCN- content in the solution. At the same time, sulfate can promote the ability of MIBK to extract and separate zirconium and hafnium.

In industry, zircon is usually directly chlorinated by adding carbon in a fluidized bed reactor to obtain crude zr(Hf)CL4, and then the crude Zr(Hf)CL4 is transferred to the MIBK-HSCN system for extraction and separation. The hafnium separation factor can reach 80. Since MIBK has a high solubility in water, the dissolution loss in water is large. MIBK is volatile and can easily cause fire or explosion, and the complex thiocyanate is unstable and can easily decompose into toxic gases such as hydrogen sulfide and hydrocyanic acid. etc., causing greater environmental pollution

Subsequently, domestic and foreign scholars conducted a series of improvement studies on the MIBK method, such as adjusting the water phase conditions to reduce the dissolution of MIBK, increasing the concentration of the zirconium feed liquid and reducing the acidity, and reducing the concentration of the extraction agent and complexes. Through improvements, the problem of environmental pollution caused by this process has been reduced to a certain extent.