A device for separating ferromagnetic particles from a suspension has a reactor (2) through which the suspension can flow, with at least one magnet (3, 4) arranged on the outside of the reactor (2), wherein the reactor (2) has an interior space (7) and an exterior space (8) surrounding the former, wherein the interior space (7) and exterior space (8) are separated from one another by an insert (6), and the insert (6) has at least one opening (9, 10) near the at least one magnet (3, 4).