An apparatus for processing magnetic particles comprises a sealed enclosure and a magnetic field source. The sealed enclosure comprises an inlet into the enclosure and an outlet from the enclosure. The configuration of the sealed enclosure and of the inlet and the outlet are such that fluid containing the magnetic particles that is introduced into the enclosure through the inlet exhibits a spiral flow towards the outlet. The magnetic field source is disposed to the enclosure to intermittently apply a magnetic field to the fluid contained therein.