A process for removing iron impurities, inter alia iron or iron compounds, from petroleum oil distillation residues is disclosed in which a high gradient magnetic separator incorporates a pack of ferromagnetic fillers in the form of a generally flat or curved sheet-like strip. The ferromagnetic filler is a Fe--Cr alloy of a selected composition and the strip is of selected geometric characteristics such that the rate of removal of iron impurities can be maintained substantially at a maximum throughout the separation mode of operation prior to and after washing of the ferromagnetic filler.