In this situation it helps to consider an ice cube. As the ice cube forms, the water crystallizes from the outside in, as the water assumes it's crystallline structure it pushes impurities(air, salts etc...)toward the middle. The easiest way to separate the pure iron is by using a seed crystal of pure iron and slowly cooling the molten solution so that the iron expands around the seed crystal pushing all of the aluminum oxide out of the iron crystal. The key is slowly cooling the molten solution around a pure seed crystal.