How can anhydrous magnesium chloride be prepared from magnesium chloride hexahydrate?

Explanation

Anhydrous magnesium chloride cannot be prepared by simply heating `MgCl_2. 6H_2O` because it gets hydrolysed by its own water of crystallisation.
`MgCl_2. 6H_2O overset(Delta)to MgO + 2HCl + 5H_2O`
However, when hydrated `MgCl_2` is heated at 650K in presence of HCl, its hydrolysis is prevented and it loses its water of crystallisation to form anhydrous `MgCl_2`.
`MgCl_2 . 6H_2O underset(650 K)overset(HCl gas)to MgCl_2 + 6H_2O`