A continuous, hybrid field-gradient device for magnetic colloid-based separations

A fully continuous, hybrid field-gradient device is developed for colloidal magnetic affinity separation. The device consists of a 1.0 cm internal diameter 1.2 m long, axially rotating horizontal glass chamber, with four repeating magnetic units distributed axially along the tube. Each magnetic unit consists of a stationary alternating-current solenoid that surrounds the chamber, followed by a direct current flowing, computer-controlled electromagnet, placed at a distance of 4.0 cm from the end of the solenoid. The alternating-current-carrying solenoids impart translational and rotary oscillations to the magnetic particles, enhancing mixing. The computer-controlled electromagnets draw the magnetic particles to the chamber walls, and increase their residence time in the chamber. By manipulating the on-off cycle for these electromagnets, the exiting solution can be switched between one that contains a negligible concentration of target-bound magnetic particles and one that has a high concentration of the target, without interrupting the feed. The operation of this device is demonstrated for the removal of cadmium ions from a cadmium sulfate solution. 1-10 mum diameter anion-exchange-resin-coated magnetic particles at a concentration of 0.5 mg particles/ml were used as the mobile solid support. The feed consisted of a 10.0 mg/l cadmium sulfate solution, at a flow rate of 25 ml/min. The device was operated as a three-stage cross-current cascade. For this system, 45%, 58% and 63% of the entering cadmium ions are removed at each stage, respectively, with stage efficiencies that vary. between 57% and 65%. Thus, more than 90% of the entering cadmium ions are removed. While this device has been demonstrated for metal ion removal from an aqueous feed, it can potentially be applied to several other affinity-based separations. (C) 2002 Elsevier Science B.V. All rights reserved.