Although $Zr$ belongs to the $4d$ and $Hf$ belongs to the $5d$ transition series, it is quite difficult to separate them. Why?

(N/A) This is due to the lanthanoid contraction.
Due to the lanthanoid contraction, the atomic radii of $Zr$ $(160 \text{ pm})$ and $Hf$ $(159 \text{ pm})$ are almost identical.
Because of these similar atomic radii, they exhibit similar chemical properties, making them very difficult to separate.