The present invention relates to a filtering unit at least partially filled with particles agglomerated from fine-particle iron oxide and/or iron oxyhydroxide by producing an aqueous suspension of fine-particle iron oxides and/or iron oxyhydroxides having a BET surface area of 50 to 500 m.sup.2/g, and removing the water and dissolved constituents by a set of washing drying and filtering steps and processes of using the particles.