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

Explanation

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