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

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.