Magnetofluidized Bed Research Articles

The influence of the particle size on rheological properties of magneto fluidized bed of strontium hexaferrite

Powder metallurgy processes are in general less polluting and more sustainable than the traditional mechanical manufacturing. In order to organize technological processes with the ferromagnetic powders, one usually applies special methods to change the coefficient of inner friction of particulate systems. We studied the regimes of the magnetic fields that form magneto fluidized bed from the strontium hexaferrite (SrFe12O19) powder and provide the decrease of the inner friction coefficient. The magneto fluidized bed was formed by the effect of the crossed constant and alternating gradient magnetic fields. The results of electromagnetic parameters influence on rheological properties of the magneto fluidized bed formed from strontium hexaferrite powders with various average particle sizes are presented in the paper. The dependencies of the strontium hexaferrite stimulated fluidity speed through a 2 mm diameter hole from the electromagnetic field parameters were studied. The obtained results show the possibility of using dry ferromagnetic powders in the permanent magnets production in powder metallurgy.

Diagnostic method for the selection of regimes of electromagnetic effect on the particulate ferromagnetic material

The recycling of obsolete permanent magnets, the processing and reusing of the obtained powder for a new magnets production has both economic and environmental benefits. The processing of recycled magnetic slur to the particle size distribution required for the magnets production has a problem specific for fine magnetic powders – particle aggregation limiting the milling efficiency. One way of solving this problem is milling the magnetic material with the formation of the magneto fluidized bed. In the present work we study the behavior of the recycled from old electromotor stators strontium hexaferrite powders with average particle sizes 56.8 µm and 7.5 µm in the magneto fluidized bed formed by mutually perpendicular constant and alternating gradient magnetic fields. The desired state of the fluidized bed for milling is the one with the maximal particle motion intensity that prevents particle aggregation. The intensification of particle motion could be done by the increase of the alternating magnetic field gradient, but this also increases energy consumption of the process. In the present paper we propose using the inductive method for estimating rheological state of the strontium hexaferrite powder in the magneto fluidized bed. Such estimation allows finding the optimal parameters of electromagnetic fields forming the fluidized bed for the efficient powder milling process.

Recycled strontium hexaferrite powder behavior in the magneto fluidized state in dependence of the chamber fill factor

The magnetic powders affected by external magnetic fields can be transferred in the pseudo fluidized state to intensify the process of permanent magnets production. The behavior of the fluidized fine strontium hexaferrite powder with an average particle size 0.93 μm obtained by recycling old electric engine stators was studied. Mutually perpendicular constant and alternating gradient magnetic fields were used to form the magneto fluidized bed from the powder. The rheology state of the magnetic powder can be controlled by the parameters of the electromagnetic fields. The powder was affected by a constant field with induction values from 3 mT to 43 mT and alternating gradient magnetic field with induction gradient values from 87 mT/m to 317 mT/m. The chamber bulk fill varied from 4% to 100%. To study the rheological state of the powder in the chamber, we used an inductive method – an inductive coil was put on the chamber and the moving powder particles induced the current in the coil. The values of the current allowed to estimate the mobility of the powder particles in the magneto fluidized bed. The paper presents the results of experimental studies how electromagnetic fields regimes influences on the rheology of the fluidized powder. Methods of choosing the fields regimes providing the rheological state optimal for the required technological process depending on the chamber fill factor are described.

Milling Duration Influence on Strontium Hexaferrite Technological Characteristics

Fine powders of strontium hexaferrite are widely used in powder metallurgy for the production of permanent magnets resistant to atmospheric oxygen and high working temperatures. Obtaining powders with predefined technological characteristics in minimal time and with minimal energy consumption is an actual problem of powder metallurgy. The paper provides the results of experimental studies of technological characteristics of strontium hexaferrite powder (SrFe12O19) during milling in a beater mill. Mechanical milling of coarse strontium hexaferrite was carried out in the mill with the system of rotating beaters for 120 minutes without and with the creation of a pseudo fluidized bed. The fluidization was formed by a perpendicular constant and alternating magnetic field with induction gradients of 150 and 210 mT/m. Average particle size and powder bulk density dependencies from milling time were studied. Experimental data show that milling with the formation of a magneto fluidized bed allows intensifying the process. Beginning from 70 minutes, the dependencies of average particle size and bulk density come to almost asymptotic behavior making further milling rather ineffective. Carried out research allows choosing optimal milling duration for obtaining the required average particle size.

Diagnostic of Structural Changes in Ferromagnetic Powders during Milling in Beater Mill

Problem of obtaining fine powders of strontium hexa-ferrite is actual because of its wide applications. The paper provides the results of studies of particle size distribution and structural characteristic changes of strontium hexa-ferrite powder (SrFe12O19) during milling in impact mill and after its consequent annealing. Mechanical processing of coarse particulate system was carried out in the mill for 120 minutes without electromagnetic effect and with creation of magneto fluidized bed, formed by perpendicular constant and alternating magnetic field with induction gradient of 210 mT/m, providing reciprocating motion of particles and aggregates with sizes of 3 – 4 mm. It was shown that milling of coarse strontium hexa-ferrite with average particle size 1558.5 μm and the most possible size 1500 μm in magneto fluidized bed allowed to intensify milling process and to provide a significant increase of powder particle sizes uniformity. It was found out, that milling in magneto fluidized bed leads to a great decrease of coherent scattering regions sizes and an increase of lattice micro-deformations and relative dislocation density. Consequent annealing of the powder for 2 hours at 850°C refined structural characteristics significantly. The carried out research allows to choose the optimal milling duration for solution of practical problems of powder metallurgy.

Stimulated fluidity of Nd-Fe-B powder in magneto fluidized bed

Experimental studies results of electromagnetic effect influence on stimulated fluidity of Nd-Fe-B powder are presented in the paper. Fine Nd-Fe-B powder doesn’t have natural fluidity. To make it flow through hopper nozzle, the powder was affected by constant and alternating gradient magnetic fields. Constant magnetic field induction lines were horizontal, and alternating field lines were vertical with gradient towards efflux direction. We studied influence of magnetic fields on powder fluidity speed to find optimal magnetic fields parameters that form steady efflux of Nd-Fe-B powder with an average particle size 4.6 μm through nozzles with diameters 1 and 2 mm.

X-ray Diffraction Analysis by Williamson-Hall Method of Strontium Hexaferrite Lattice Features after Mechanical Milling

The paper presents the result of experimental studies of strontium hexaferrite milling in beater mill. SEM studies showed that milling of strontium hexaferrite in beater mill with formation of magneto fluidized bed leads to significant intensification of the process. Williamson-Hall method was used to determine structural characteristics changes in the strontium hexaferrite powder from milling time in magneto fluidized bed. Carried out studies showed that milling intensification is significant only to a certain dispersity of processed powder. However, during milling with formation of magneto fluidized bed, decrease of particle sizes and structural characteristics changes speed happens at finer disperse compound.

Study of Electromagnetic Effect Parameters Influence on Fluidity of Fine Nd-Fe-B Powder

The paper experimentally proves the efficiency of electromagnetic effect on stimulated fluidity of magneto-hard powders. Fine Nd-Fe-B powder doesn’t have natural fluidity. To make it flow through hopper outlet we effected on the powder by constant and alternating gradient magnetic fields. Induction lines of constant magnetic field were horizontal, and alternating field lines were vertical. In order to form stimulated powder fluidity, higher induction gradient was situated below the powder receiver chamber. We experimentally found optimal magnetic fields parameters that form steady magneto fluidized bed from Nd-Fe-B powder, with an average particle size 1 μm. The study results of influence of magneto fluidized bed electromagnetic parameters on Ne-Fe-B powder efflux speed through outlets with diameters 1 and 2 mm are presented. Gradient field topology and parameters of electromagnetic effect providing minimal efflux time of 1000 mg of powder are defined.

Particle Size Distribution and Structural State Analysis of Mechanically Milled Strontium Hexaferrite

Article presents an experimental study result of milling coarse strontium hexaferrite in beater mill with formation of magneto fluidized bed and without it. Magneto fluidized bed is formed by mutually perpendicular constant and alternating gradient magnetic fields. We studied the dynamics of particle size distribution from milling time and parameters of magnetic fields. Microstructure dynamics of strontium hexaferrite powder particles milled in various regimes was studied by X-ray diffraction methods. Milling efficiency and energy efficiency of milling process were studied in conditions with and without powder fluidization by magnetic fields. Analysis of experimental data showed advantages of milling in magneto fluidized bed in increased efficiency, particle size distribution homogeneity and powder chemical activity because of lattice micro-stresses.

The Influence of Milling Regimes in Beater Mill on Particle Size Distribution and Structural Characteristics of Strontium Hexaferrite Powder

The paper experimentally substantiates effectiveness of method of milling particulate ferromagnetic materials in magneto fluidized bed. Comparative results of particle size distributions and structural parameters of strontium hexaferrite SrFe12O19 powder obtained by milling coarse material in beater mill without electromagnetic effect and in same mill with formation of magneto fluidized bed from mill material are presented. The magneto fluidized bed is formed by constant and alternating gradient magnetic fields with induction lines that are mutually perpendicular and parallel to the plane of rotating beaters. It is shown that application of electromagnetic effect to milling coarse material in beater mill allowed to greatly intensify that process, significantly increase powder quality: increase particle size distribution uniformity and decrease average particle size from 1558.50 μm to 0.56 μm after 120 minutes of processing in the mill. X-ray diffraction analysis showed that milling in beater mill in magneto fluidized bed leads to reduction of coherent-scattering region size, increase of lattice microstrain and dislocation density, making powder more active during sintering process.

Influence of electromagnetic effect parameters on ferromagnetic powders fluidity

The paper presents result of experimental studies of ferromagnetic powders fluidity management by effecting on particulate medium with constant and alternating gradient magnetic fields. Induction lines of constant magnetic field are perpendicular to symmetry axis of powder hopper. Alternating field is created by electromagnet with one tapering pole and has higher induction gradient values near hopper outlet. These fields form magneto fluidized bed from powder in hopper allowing powders of barium and strontium hexaferrite to flow through the hopper outlet. Dependencies of efflux speed from parameters of electromagnetic fields are presented for barium hexaferrite powder with average particle size 1 μm and strontium hexaferrite powders with average particle sizes 1 and 9 μm.

Technological processes intensification in devices with magneto-fluidized bed

The paper shows the efficiency of application of magneto-fluidized bed formed of ferromagnetic materials in press-forms dosed filling and milling devices. For powders without natural fluidity, dosing device is situated between electromagnets creating constant magnetic field with horizontal induction lines and alternating gradient magnetic field with vertical induction lines and higher induction gradient in the lowest part of chamber with powder. This configuration allows development of uniform efflux speed of ferromagnetic powders from 2 mm diameter opening. For example, mass efflux speed of strontium ferrite powder with 1 μm average particle size in magneto-fluidized bed reached 181.1 mg/s and the dosing time of 2000 mg of this powder was 11.1 s. For practical usage of magneto-fluidized bed in the milling device, beater mill is situated between electromagnet poles in the way that induction lines of constant and alternating gradient magnetic fields were mutually perpendicular and parallel to the plane of rotating beaters. Milling of particulate strontium ferrite with 1558.5 μm average particle size during 120 minutes in magneto-fluidized bed allows increasing milling degree in 16.2 times by comparison with processing without electromagnetic effect.

Stabilization of gas-fluidized beds of magnetic powders by a cross-flow magnetic field

In this paper we present an experimental study of the stabilization of gas-fluidized beds of magnetic powders by application of a cross-flow magnetic field. The powders tested consist of magnetite and steel powders in a range of particle size dp between 35 and 110 μm, allowing us to investigate the effect of particle size and material properties on magnetic stabilization. In the operation mode employed by us the magnetic field is applied to the unstable bubbling bed and the gas velocity is slowly decreased. According to our observations, the bed is stabilized at a critical gas velocity by the jamming of particle chains formed during bubbling because of the attractive forces induced between the magnetized particles, which are thus responsible for stabilization. Although the magnetic field is applied in the horizontal direction, these chains are mechanically stable at orientations close to the gas flow direction, in agreement with the prediction of an unconfined chain model based on the balance between gas flow shear and interparticle magnetic force fm. Since fm is increased as dp is increased, the critical gas velocity at marginal stability vc for a fixed field strength B is seen to increase with dp. As the gas velocity v0 is decreased below vc, there is a rearrangement of the structure depending on particle size. Restructuring of the bed depends on particle size as derived from measurements of its permeability to the gas flow, which causes the yield stress to be a function of particle size. It is also inferred from our results that natural agglomeration of fine particles (in the absence of a magnetic field) due to van der Waals forces enhances the yield stress of the magnetically stabilized bed. From our experimental results it is concluded that structural effects, as affected by operating conditions and material properties, play a main role in the rheology of the stabilized magnetofluidized bed (MFB).

CORRELATION BETWEEN MICROSTRUCTURE AND YIELD STRESS IN MAGNETICALLY STABILIZED FLUIDIZED BEDS

A magnetofluidized bed consists of a bed of magnetizable particles subjected to a gas flow in the presence of an externally applied magnetic field. In the absence of magnetic field, there is a given gas velocity at which naturally cohesive fine particles can form a network of permanent interparticle contacts capable of sustaining small stresses. This gas velocity marks the jamming transition of the fluidized bed. For gas velocities above the jamming transition, the bed resembles a liquid. Below the jamming transition, the bed behaves as a weak solid and it has a nonvanishing tensile strength. In the absence of magnetic field, the tensile strength of the solidlike stabilized bed has its only origin in nonmagnetic attractive forces acting between particles. In the presence of a magnetic field, the gas velocity at the jamming transition and the tensile strength of the bed depend on the field strength as a consequence of the magnetostatic attraction induced between the magnetized particles. In this work we present experimental measurements on the jamming transition and tensile strength of magnetofluidized beds of linearly magnetizable fine powders. It is shown that powders with similar magnetic susceptibility but different strength of the nonmagnetic forces show a different response to the magnetic field. This finding can be explained by the influence of the nonmagnetic natural forces on the network of contacts. Thus, our experimental results reported in this paper reinforce the role of short-ranged interparticle contact forces on the behavior of the system, which contrasts with the usual modeling approach in which the magnetofluidized bed is viewed as a continuum medium and a fundamental assumption is that the fields can be averaged over large distances as compared with particle size.

Identification of the State of a Magneto-Fluidized Bed by Pressure Fluctuations

Local instantaneous pressure signals obtained through a magneto-fluidized bed have been analyzed using both classical and advanced signal analysis methods, which can deliver the necessary information about the presence of the bubbling and turbulent flow pattern. The conventional signal processing tool such as autocorrelation and cross correlation were used as preliminary tools to analyze the data. Evaluation of the dominant bubble frequency was completed using the autocorrelation function and power spectral density function. Mutual information function was used to identify the periodicity and the predictability of the local instantaneous pressure signals. Since it does not demand any particular functional relationships between the data points, it is a better method (compared to autocorrelation function) for measuring the predictability of nonlinear systems.

Investigation of the properties of a magnetofluidized bed and development of a new gas purification technology based on them for simultaneous removal of solid and gaseous substances from hot waste gases

The results of investigation of the influence of the mechanical, thermal, structural, catalytic, and regenerative characteristics of granular oxide catalysts on the efficiency of decontamination of volatile organic substances in the catalyst unit and of the filtration rate, amount, and concentration of dust, the size of filter particles, the direction and intensity of a magnetic field, and other parameters on the behavior of the particles, the structure, the porosity, the resistance, the optimum fluidization rates, and the efficiency of collection of dust in magnetofluidization of magnetic particles have been given. The diagram and operating principles of a new technology for simultaneous decontamination of volatile organic matter and collection of dust have been presented.

Heat Transfer Between a Magnetofluidized Bed and a Surface

Heat Transfer Between a Magnetofluidized Bed and a Surface

HYDRODYNAMICS OF MAGNETOFLUIDIZED ADMIXTURE BEDS

Hydrodynamic behavior of a magnetofluidized bed is investigated over a wide range of magnetic-field intensity values as a function of superficial air velocity. The bed comprises of different proportions of copper and iron particles and is contained in a Plexiglas column of 0.102 m internal diameter. The uniform constant magnetic field is created by a Helmholtz electromagnet. In particular, the bed pressure drop is measured as a function of superficial air velocity and characteristic bed voidage and fluidizing velocities are determined at minimum fluidization and bubbling bed conditions. These characteristic properties of the magnetically stabilized fiuidized beds are reported as a function of magnetic-field intensity, and are correlated by suitable expressions. These will be useful for prediction purposes related to design and operation.

Gas holdup in a three-phase magneto-fluidized bed

The influence of magnetic field on local gas holdup, ε g and average gas holdup ε ̄ g has been studied in a three-phase (gas–liquid–solid) magneto-fluidized bed using electroresistivity probe technique and quick closing valve technique, respectively. The local gas holdup shows a linear variation with dimensionless radius at low gas velocities and parabolic variation at high gas velocities with maximum at the centre and minimum at the wall. Higher values of ε g are obtained in presence of field than in absence of magnetic field. Average gas holdup ε ̄ g decreases with field intensity upto H=52.8 Oe and shows an increase beyond this field intensity. The ε ̄ g values obtained by first fluidizing the bed and then magnetizing it have been found to be higher than those obtained by reversing this sequence.

Heat-transfer rate variations from the surface of a heater probe in a magnetofluidized bed

A cylindrical strip heater heat-transfer probe is designed and employed to measure heat-transfer coefficients in the horizontal configuration at three angular positions (0, 90 and 180° relative to gas flow while submerged in a magnetically stabilized fluidized bed of 1086 μm iron shots. The superficial air velocity is varied up to about 3.8 ms −1, and measurements have been conducted for five values of magnetic-field intensity up to 5662 A m −1. In each case, pressure drops across the bed and a section of it are determined to obtain values of superficial minimum fluidization and minimum bubbling velocities, and gross bed voidage. The heat-transfer coefficient is found to be dependent on the angular position at the horizontal cylindrical probe immersed in the bed. The magnetic field destabilizes the gas pocket at the upstream side of the probe and this characteristically influences the h w values. The magnetic interparticle forces augment the gas flow in the equatorial lateral zones by the formation of a channel at the probe surface leading to gas bypassing. At the downstream side, the variation is somewhat similar to the variation observed for the total heat-transfer coefficient. Experimental minimum bubbling velocities are well corrected by a semitheoretical expression proposed by the authors.