In principle, it is also possible to crystallize:
Iron sulfate tetrahydrate FeSO₄ 4H₂O
Anhydrous iron sulfate FeSO₄
To design crystallization plants for FeSO₄ 4H₂O, however, a small temperature range must be maintained, which is why such plants can only have one stage.
To crystallize anhydrous FeSO₄, on the other hand, evaporation must occur at very high temperatures or high sulfuric acid concentrations. This is not common in practice due to the issue of corrosion.