For nuclear applications, zirconium metal (Zr) must adhere to stringent specifications, where a low hafnium (Hf) content, i.e., <100 ppm, is one of file most important requirements due to the opposing nuclear characteristics of the two elements. Hf-purified Zr is however not easily obtained, as the two entities occur together in nature and they are exceptionally difficult to separate clue to their close chemical and physical similarities. In this study, cooling crystallization of K2Zr(Hf)F-6 from aqueous solutions, i.e., fractional crystallization, was investigated as a separation method. The influence of the solvent composition oil the selectivity of fractional crystallization as separation method was studied, wherein KF and HF were used as additives. Using scanning electron microscopy and X-ray diffraction analysis of the product crystals, it is shown that crystallization from KF solutions resulted in a decrease in the crystal size leading to the formation of K3Zr(Hf)F-7 With increasing KF concentration. These effects resulted in a decreased selectivity, which was Suppressed by the addition of 2.5 mol/L HF to the crystallization solution. In addition, it is shown that selectivity improved with all increasing relative excess amount of solute with respect to the Solubility at the lower crystallization temperature, which could be favorably regulated by adjusting the KF concentration of a 2.5 mol/L HF solvent solution. Consequently, the most efficient separation was obtained using a solution consisting of 0.45 mol/L KF and 2.5 mol/L HF.