High-gradient magnetically seeded filtration

A two-step magnetically seeded filtration process that includes heterogeneo us flocculation (shear-flow and Brownian) and magnetic filtration is examin ed experimentally. The effects of various parameters - magnetic-field stren gth, size of particles, flow rate, seeding concentration? and solution pH - on the removal efficiency are investigated. A breakthrough model - which c ombines trajectory analysis, a particle buildup model, and a bivariate popu lation-balance model applicable for Brownian flocculation - is developed to predict particle breakthrough in a magnetic filter. Experiments show that the removal efficiency increases as magnetic-field strength and particle si ze are increased and flow rate is decreased. A maximum in the removal effic iency is observed at a certain seeding concentration and at the lower pH va lues, which is explained from competing effects that take place with respec t to magnetic susceptibility and size of aggregates as the seeding concentr ation and solution pH are increased. Modeling results of the trajectory ana lysis show that the effect of hydrodynamic resistance becomes important as Reynolds number and particle size are increased or the magnetic-field stren gth is decreased. Similarly to experimental observations, the modeling resu lts predict that the removal efficiency increases with increasing magnetic- field strength and particle size indicating that the relative importance of magnetic and drag forces and the aggregation rate in the flocculation step play an important role in the magnetically seeded process. The breakthroug h model developed in this study provides a good description of the experime ntal breakthrough data obtained from magnetic filtration of paramagnetic pa rticles and magnetically seeded filtration with Brownian-flocculation. (C) 1999 Elsevier Science Ltd. All rights reserved.