Explain the role of iodine in refining of zirconium

Zirconium is produced by adding gaseous zirconium tetrachloride to molten magnesium metal. (In the absence of air). The end product is always contaminated with magnesium and it doesn't take much magnesium to make it brittle, etc.

at one temperature iodine would react with zirconium metal to form zirconium tetraiodide and that at a higher temperature the tetraiodide would decompose to zirconium metal and free iodine.

First step, zirconium in the cladding is reacted with elemental iodine at 300–500^oC to form volatile ZrI₄, which, in the second step, is then decomposed at >1100^oC.  
Formation  Zr + 4I₂  → ZrI₄  300-500^oC
Decomposition  ZrI₄  → Zr + 4I₂  >1100^oC