1. Field of the Invention
The present invention relates to a method of effectively separating zirconium and hafnium in aqueous solution using a solvent extraction process that uses an acidic extractant D2EHPA (Di-(2-ethylhexyl)phosphoric acid) or PC88A (2-Ethylhexyl phosphonic acid mono-2-ethylhexyl ester), and more particularly, to a separation method of zirconium and hafnium for greatly enhancing the separation effect while reducing the usage of the extractant compared to the related art.
2. Background of the Invention
Zirconium is an essential element used for nuclear fuel rods in nuclear power, and has characteristics such as excellent high temperature strength and corrosion resistance and a very small neutron absorption cross section of about 0.18 barn. On the contrary, the neutron absorption cross section of hafnium is 600 times greater than that of zirconium 600, and thus metallic zirconium used for nuclear fuel rods should have very low content of hafnium.
In general, zirconium ore distributed in the earth's crust contains approximately 0.5 to 2 percent of hafnium, but the physicochemical properties of zirconium and hafnium are so similar, thereby causing much difficulty in mutual separation. The separation methods of zirconium and hafnium known until now may include a fractional crystallization method of fluoride complexes, a distillation method using a difference between the volatilities of chlorides, a solvent extraction method, and the like. Each of those various separation methods has advantages and disadvantages, and thus an appropriate separation method should be selected by taking economic efficiency, operational environment, and the like into consideration.
In case of the solvent extraction process, as a method that has been used since a long time ago to separate zirconium and hafnium, an extractant is used to separate the two metals existing in aqueous solution. At this time, a plurality of extractants such as diethyl ether, methyl isobutyl ketone (MIBK), tributyl phosphate (TBP), trioctyl amine (TOA), trioctyl methylammonium chloride, and the like have been used for the extractant in a single or mixed manner. However, there is only a few cases where the foregoing extractants are actually commercialized and applied to the separation of zirconium and hafnium. It is because there exist rare extractants having a great effect on the separation of zirconium and hafnium as well as economic issues such as the cost of the extractants.
The solvent extraction processes for zirconium and hafnium commercialized up to now may include a process of using methyl isobutyl ketone (MIBK) as an extractant and a process of using tri-butyl phosphate (TBP) as an extractant.
The process of using methyl isobutyl ketone (MIBK) as an extractant has an advantage in that the use amount of the extractant is low since a far less amount of hafnium is selectively extracted from aqueous solution compared to zirconium, but causes an environmental pollution problem due to cyanide compounds since thiocyanate ions (SCN−1) should be added to aqueous solution prior to the extraction process as well as has a problem in that the cost of MIBK is high. Furthermore, there is a disadvantage in that the loss of extractant is high since the solubility of MIBK in water is very high, about 1.8%.
On the other hand, the process of using tri-butyl phosphate (TBP) has an advantage in that cyanide compounds is not required to be added contrary to the MIBK process, but has a problem in that the cost of extractants is high, and the use amount of the extractant is relatively excessive since zirconium rather than hafnium is selectively extracted from aqueous solution.
Accordingly, the technological development of an extractant and its extracting condition capable of satisfying both the economical efficiency and zirconium and hafnium separation effect as well as overcoming the foregoing problems is required, but effective solvent extraction processes of zirconium and hafnium that can substitute the conventional methods have not been developed until now.