Purification of Kroll’s Reduction Unreacted Hafnium Oxide Residue

Hafnium is widely used in space and nuclear industry applications because of its strategic material properties. The process of reduction of hafnium chloride generates around 5–10% unreacted hafnium chloride (HfCl₄) residue, which needs to be recycled. The typical elemental composition of major impurities constitutes 3% aluminum, 0.5% chromium, 0.8% iron, and 0.17% silicon. In this work, the removal of aluminum oxide and silicon oxide was studied through alkali (sodium hydroxide) leaching. The optimization of the leaching process parameters was studied using Box-Behnken design (BBD) experiments through the response surface methodology. The impurities such as SiO₂ and Al₂O₃ react with NaOH during the hydrometallurgical leaching process and form sodium aluminate and sodium silicate, respectively, which helps in the process. The leached samples were quantitatively analyzed by an ICP-OES analyzer. It is observed that ~ 85 wt.% SiO₂ and ~ 66 wt.% Al₂O₃ were separated at optimal experimental conditions. The optimal conditions for the removal of impurities using 1 molarity of NaOH concentration, a solid-to-liquid ratio of 1:30 min, and 60 min leaching time are suggested by BBD.