Magnetofluidization of fine magnetite powder

Abstract

The behavior of a fluidized bed of fine magnetite particles as affected by a cross-flow magnetic field is investigated. A distinct feature of this naturally cohesive powder, as compared to noncohesive magnetic grains usually employed in magnetofluidized beds, is that the fluidized bed displays a range of stable fluidization even in the absence of an external magnetic field. Upon application of the magnetic field, the interval of stable fluidization is extended to higher gas velocities and bed expansion is enhanced. We have measured the tensile strength as affected by application of the external magnetic field according to two different operation modes.In the H off-on operation mode, the bed is driven to bubbling in the absence of external magnetic field. Once the gas velocity is decreased below the bubbling onset and the bed has returned to stable fluidization due to natural cohesive forces, the field is applied. In the H on-on mode, the field is maintained during the whole process of bubbling and return to stable fluidization. It is found that the tensile strength of the naturally stabilized bed is not essentially changed by application of the field ͑H off-on͒ since the magnetic field cannot alter the bed structure once the particles are jammed in the stable fluidization state. Magnetic forces within the bulk of the jammed bed are partially canceled as a result of the anisotropic nature of the dipole-dipole interaction between the particles, which gives rise to just a small increment of the tensile strength. On the other hand, when the field is held on during bubbling and transition to stable fluidization ͑H on-on mode͒, the tensile strength is appreciably increased. This suggests the formation of particle chains when the particles are not constrained due to the dipole-dipole attractive interaction which affects the mechanical strength of the stably fluidized bed. Experimental data are analyzed in the light of theoretical models on magnetic surface stresses.

Effect of particle size polydispersity on the yield stress of magnetofluidized beds as depending on the magnetic field orientation

“…For instance, important differences can be observed for the porosity of MFBs depending on whether the magnetic field is imposed to a previously fluidized bed or to a settled bed which is subsequently fluidized in the presence of the magnetic field [108,57,71,109]. The later is known as Magnetization FIRST operation mode and the structure of the bed remains unchanged until it is broken up by a sufficiently large gas flow rate.…”

Influence of cohesive forces on the macroscopic properties of granular assemblies

“…The friction forces are influenced by the surface properties and by the chemical nature of the grains. The cohesive forces are induced by the presence of liquid bridges [2,3], electric charges [4,5], van der Waals interactions [6] and magnetic dipole-dipole interactions [7,8,9,10,11]. The predominance of one of these cohesive forces depends on both the environmental condition and the physicol-chemical properties of the grains.…”

Mesoscopic structuring and yield stress of magnetofluidized fine particles

“…4 (inset) as a function of the magnetic-field strength externally applied H. It is observed that the field effect becomes noticeable for H 2 kA/m. At this field strength the interparticle attractive force due to the magnetic interaction prevails against the natural van der Waals force [8]. As H is further increased, v c can be fitted by a linear function ∆v c αH.…”

“…On the other hand, the bed settled in the absence of magnetic field also possesses a yield stress due to the natural cohesiveness of the fine powders used. In order to investigate the yield stress purely induced by the magnetic field, the pressure drop cycle can be reversed from a fluidization state close to marginal stability (v g v c ), in which the stabilized bed is unconsolidated and the natural cohesion is negligible [8]. Alternatively, the natural yield stress of the settled bed, measured in the absence of magnetic field, can be subtracted from the total yield stress, measured in the presence of magnetic field.…”

“…• tilted chain of magnetic spheres with susceptibility χ p = 5.33 (as obtained for magnetite beads from their measured bulk susceptibility and using Bruggeman mixing rule [8]). Since χ p is large, the field lines are guided throughout the chain at interparticles contacts, where the field becomes almost normal to the contact surface and its strength is intensified.…”