How is anhydrous magnesium chloride prepared from magnesium chloride hexahydrate?

Explanation

Magnesium chloride hexahydrate on heating gets hydrolysed by its own water of crystallisation and hence anhydrous `MgCl_(2)` cannot be prepared by heating `MgCl_(2).6H_(2)O`.
`Ngcl_(2).6H_(2)Ooverset(Delta)(rarr)MgO+2HCl+5H_(2)O`
However,if `MgCl_(2).6H_(2)O` is heated in atmoshphere of `HCl` gas, it prevents the above hydrolysis reaction and `MgCl_(2).6H_(2)O` loses its water molecule of crystallisation.
`MgCl_(2).6H_(2)Ooverset(HCl gas)underset(Delta)(rarr)MgCl_(2)+6H_(2)O`