Zirconium and hafnium separation from chloride solutions by organophosphorous compounds

The zirconium/hafnium separation, as currently practiced, is mostly carried out by solvent extraction in which the aqueous chloride solution of metals is contacted with an immiscible organic phase containing a reagent that removes hafnium leaving zirconium in the aqueous solution. Hafnium is extracted as its thiocyanate complex using methyl isobutyl ketone (MIBK). There are environmental consequences of this system that were acceptable when the process was developed but should become a serious problem as legislation becomes more stringent. These problems are primarily the high aqueous solubility of MIBK and the potential for toxic gas generation. This work has tested four organophosphorous reagents to separate zirconium from hafnium without the need of thiocyanate, avoiding toxic gas formation. The extractants and diluent have very low aqueous solubility, removing effluents control problems. We see these features as representing a "zero waste" process. Additional advantages over MIBK process are lower vapor pressure and higher flash point (lower fire risk). This work presents the lab-scale experimental results using mainly two reagents designated by Cytec Canada Ltd as Cyanex 923 and Cyanex 925. The key feature of this work is replacement of MIBK and thiocyanate with one of a family of organophosphorous extractants that were unavailable in the 1950's at the start of the MIBK process. This work shows they can be efficiently applied to hafnium/zirconium separation in aqueous chloride solution, with minimum adjustments of the present industrial practice.