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

[prasanna]

Separation of Zr and Hf are quite difficult because of lanthanoid contraction. Due to lanthanoid contraction, they have 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.