Question

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

Answer 1

Separation of $Zr$ and $Hf$ are quite difficult because of lanthanoid contraction. Due to lanthanoid contraction, they have the almost same size ($Zr$ = 160 pm and Hf =159 pm) and thus, similar chemical properties. That’s why it is very difficult to separate them by chemical methods.