Demystifying How Zirconium and Hafnium Separation Can Drive a New Era
Zirconium and hafnium, the "twin brothers" in the periodic table, are one of the most difficult element pairs to separate because of their high chemical similarities. However, their nuclear properties are quite different: zirconium has a small neutron absorption cross section and is the material of choice for nuclear reactor cladding; Hafnium, on the other hand, has a large neutron absorption cross section and is used as a control rod for nuclear reactors.
The separation purity of these "twin brothers" is directly related to the safety and efficiency of nuclear reactors. Among them, the content of hafnium in atomic grade zirconium sponge must be less than 0.01% Therefore, the separation technology of zirconium and hafnium has become the key to obtain high-purity nuclear-grade materials, and it is also an indispensable core technology in today's nuclear energy and high-tech fields.
Challenges and needs, why the separation of zirconium and hafnium so difficult?
Zirconium and hafnium always coexist in nature, and their chemical properties are very similar. This is due to the lanthanide shrinkage effect that causes their ionic radii to be almost the same, making the chemical separation method difficult to work.
However, their nuclear performance is very different. The absorption cross section of zirconium for thermal neutrons is only 0.18 target en, while that of hafnium is as high as 105 target en. This huge difference means that even small amounts of hafnium impurities present in nuclear-grade zirconium can seriously affect the operating efficiency and safety of nuclear reactors.
At present, the international requirements for nuclear-grade zirconium materials are extremely stringent, and the hafnium content must be less than 0.01% (100ppm). This strict standard makes zirconium and hafnium separation technology a key core technology in the field of nuclear industry, and it is also an important manifestation of a country's new material research and development capabilities.
1. Traditional Methods, Recrystallization and Ion Exchange
Early separation of zirconium and hafnium mainly relied on recrystallization and ion exchange methods. The recrystallization method utilizes the small difference in the solubility of K ₂ ZrF and K ₂ HfF in aqueous solution for separation.
The solubility of K ₂ ZrF in water at 293 K is 0.0576 mol/L, while the solubility of K ₂ HfF → is 0.083 mol/L, which is 1.45 times that of the former. By controlling the temperature for multiple crystallization, the low-solubility K ₂ ZrF → crystallizes and precipitates first, and the high-solubility K ₂ HfF → stays in the mother liquor.
This method requires 16-18 recrystallization to reduce the hafnium content from 0.5%-2.0% to below 0.01%, which is inefficient and costly. The former Soviet Union used this method to produce atomic-grade zirconia in the 1950s, which has been replaced by more advanced methods.
The ion exchange method is used to separate zirconium and hafnium ions according to the different adsorption ability on the resin. The hydroxide of zirconium and hafnium was dissolved with sulfuric acid, and the original solution was purified and passed through an exchange column equipped with cation exchange resin.
Then it is desorbed with sulfuric acid solutions of different concentrations, first zirconium is desorbed with a solution containing H ₂ SO → 0.5 mol/L, and then hafnium is desorbed with a solution containing H ₂ SO → 1 mol/L. Only the former Soviet Union used this method to further separate zirconium and hafnium from hafnium-rich materials to produce hafnium oxide.
2. Modern technology, the dominant position of solvent extraction technology
Solvent extraction has become the mainstream technology for the separation of zirconium and hafnium, and the multi-stage counter-current centrifugal extraction process represents the level of the field.
· Advantages of multi-stage counter-current centrifugal extraction
The multi-stage counter-current design realizes the reverse flow of the extractant and the material liquid containing zirconium and hafnium through a series of multi-stage centrifugal extraction units, which can maximize the use of the capacity of the extractant, so that zirconium and hafnium can be repeatedly distributed between the two phases, and the separation coefficient is significantly improved.
Centrifugal force field efficiency is another advantage. The centrifugal extractor rotates at a high speed of 3000-5000rpm, and the super-gravitational field generated accelerates the two-phase mixing and delamination, shortening the mass transfer process completed in the extraction tank for several hours to seconds, emulsifying phenomenon, and ensuring purity and stability.
· Extraction system innovation
The continuous innovation of the extraction system has promoted technological progress. Tributyl phosphate (TBP) is widely used because of its high selectivity to zirconium, but it is easy to corrode equipment and emulsify at high acidity.
The innovative solution introduces a synergistic extraction system (such as TBP D2EHPA), which enhances the distribution difference of zirconium and hafnium through the coordination competition mechanism, and the separation coefficient can be increased by 30%-50%.
The DIBK-Cyanex923 system is another green option. The results show that when the acidity of the aqueous phase is 2.97 mol/L, the separation coefficient β can reach 37.1, which is higher than that of the MIBK-HSCN system in the United States (β = 9-10), and thiocyanate, which is easy to decompose into toxic substances, is not needed.
3. New centrifugal extraction equipment for fully anticorrosive reinforced materials
ZKCQ's new high-efficiency lithium battery chemical centrifugal extraction machine is a national standard equipment with high separation factor and strong stability built by Zhongke New Energy based on automation technology and new material applications. For strong acid, strong alkali and strong corrosive occasions, the equipment can be made of fully anticorrosive reinforced materials. The phase equilibrium is short, and it can realize multi-stage series/cross-flow extraction and washing, which is suitable for deep extraction.
II. Equipment Application
New energy and new materials medical chemical production Fine chemicals, resource regeneration, hydrometallurgy, treatment, food processing, bioengineering extraction and various raw material intermediates production/liquid-liquid separation: lithium battery recycling/salt lake lithium extraction, etc.
It is suitable for the occasions without suspended solids, high extraction and separation parameters, and the range of phase flow ratio is relatively wide, and the inner cavity is fully cleaned under the action of centrifugal force. Liquid materials with different density and viscosity can be adjusted by changing weir plates and frequency conversion; Realize multi-stage extraction separation in strong acid, strong alkali and strong corrosive occasions.
4. Prospects for applications, from Nuclear energy to Aviation
High-purity zirconium and hafnium materials have wide application prospects in many high-end fields. In the field of nuclear-grade zirconium preparation, an enterprise has adopted an optimized process, and the hafnium content in zirconium products has been reduced from 0.5% to less than 10ppm, which is fully required for nuclear reactor coating materials.
The aviation field also benefits from this. The tensile strength of high-purity hafnium alloy has increased by 15%, and the high temperature resistance has exceeded 1600 °C, becoming the core material of rocket engine nozzles.
Sanxiang New Materials (603663. SH) and other enterprises are actively carrying out "zirconium-hafnium separation" technology research, and plan to use zirconium oxychloride as raw material to develop high value-added nuclear-grade zirconium materials and high-purity hafnium materials through "zirconium-hafnium separation" technology. Strategic layout.
The hafnium material obtained by "zirconium-hafnium separation" has been widely used in nuclear power, high-temperature alloys, semiconductor High-k materials and other fields.
