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.


Introduction
Strontium hexaferrite powders are widely used in the powder metallurgy as materials for electric motor stators due to their magnetic stats and thermal resistance [1][2][3][4][5]. The recycled old magnets and the waste of the magnet manufacturing can be reused [6][7][8]. This decreases the pollution and increase the level of sustainability of the production. Pseudo fluidization is widely used in powder metallurgy to intensify manufacturing process by changing the rheological state of powder systems, destroying particle aggregates, reducing the inner friction [9][10][11]. In case of magnetic powder, the application of electromagnetic fields is the most effective. One use various approach to fluidize the powder, there are known methods applying the rotating magnetic fields [12][13][14], the alternating field [15] and the combination of constant and alternating fields [16]. The electromagnetic effect is applied in mixing powder materials, dry milling, activation of particle surfaces, changing physical and technological properties of the powders, providing the intensification of the technological processes [17,18]. The electromagnetic effect transfers the powder into magneto fluidized state that in its turn is affected by a fill factor of the work chamber. The rational organization of technological process in the devices applying magneto fluidized bed requires this state to be controllable [19]. The fluidized state with intense particle motion is used in the technological processes that require the powder to act as a fluid, like milling, mixing, press-molds filling. Applying strong magnetic field during the fluidization allows to easier align particle magnetic moments and can be used before pressing.
In the current study, the magneto fluidized bed was formed by a constant and alternating gradient magnetic field with mutually perpendicular induction lines and provided stable pseudo fluidized state of magnetic powders [20]. The rheology is affected both by magnetic fields topology and their parameters. The working chamber filling factor has also a great influence on the magneto fluidized bed efficiency and should be considered when choosing the optimal regimes of the electromagnetic effect.
The main goal of the paper is to find out the regularities of the working chamber filling factor and the electromagnetic effect regimes influence on the rheological state of the strontium hexaferrite magneto fluidized bed.

Experiment
We used SrFe12O19 powder obtained by crushing and subsequent fine milling of the obsolete electric motor stators for the study. The recycled powder possessed 0.93 µm average particle size, 0.76 µm median particle size and 1.1 g/cm 3 bulk density. The powder was put in the cylindrical plastic working chamber situated between the poles of constant and alternating field electromagnets. The induction lines of the alternating magnetic field were vertical and parallel to the chamber axis with the stronger gradient area in the top part of the chamber. The induction lines of constant magnetic field were horizontal.
To make an integral appreciation of the rheological characteristics of the magneto fluidized bed, we used the inductive method. The working chamber was placed inside an inductive probe. The probe coils axis was parallel to the working chamber axis and the induction lines of the alternating magnetic field. Both the alternating magnetic field and the moving magnetic particles induce current in the inductive coils of the probe, that is registered by a voltmeter. For distinguishing the background signal induced by the magnetic field in the probe from the signal induced by the fluidized bed magnetic particles, we have measured separately the signal with an empty chamber and the signal with powder filled chamber. The difference between these values () was caused only by the moving particles. Higher values correspond to the more intense reciprocal particle motion. So, the comparative analysis of the electromagnetic effect parameters influence on the strontium hexaferrite powder in dependence from the working chamber filling factor allows to find the fields regimes providing the most intense particle motion.
The magneto fluidized bed was formed in the working chamber by effecting on the strontium hexaferrite powder with the constant magnetic field having induction values from 3 mT to 43 mT and the alternating magnetic field having 50 Hz frequency and induction gradient values of 87 mT/m to 317 mT/m. The range of chamber bulk fill with the powder was from 4% to 100%.
Fine magnetic powders, like the strontium hexaferrite with 0.93 µm average particle size we used, possess the tendency to the particle aggregation, significantly changing powder technological characteristics. The aggregation is caused both by a cohesion and magnetic interactions between particles. The particles form aggregates with circular magnetic moments alignment in order to minimize the summary moment. Applying magnetic fields to the particulate system one the one hand helps to destroy these aggregates on the other hand can lead to a formation of secondary aggregates with magnetic moments alignment along the resulting external magnetic field. The experimental results on figures below allow finding the regimes of the electromagnetic effect and the chamber filling factor providing the most intensive particle motion in the magneto fluidized bed. Figure 1 shows that at an alternating field induction gradient dB/dy = 87.0 mT/m, the maximal inductive signal  = 11.5 mV was at the constant magnetic field induction Bc = 9.86 mT and the 44 % chamber fill factor. Figure 2 shows that at dB/dy = 169.7 mT/m the maximal inductive signal reaches  = 43.5 mV when fill factor is 52 % and Bc = 11.90 mT. The maximal signal  = 69.2 mV (Figure 3) is observed at the gradient dB/dy = 253.3 mT, constant field induction values in the range from 9.86 mT to 11.90 mT and 60 % chamber fill factor. And at the Figure 4 we can see that at the dB/dy = 317.5 mT/m induction gradient, the maximal signal value  = 84.2 mV happens at the same 60 % fill factor and the same constant field induction values range from 9.86 mT to 11.90 mT.  These experimental results show that the critical chamber filling factor for the strontium hexaferrite powder with an 0.93 µm average particle size is 60% and it is reached at the 253.3 mT/m alternating field induction gradient.
The changes in the rheological state of the magneto fluidized bed at critical chamber fill factor 60 % can be followed by the experimental results presented at the Figure 3. At the induction gradient value 253.3 mT/m and the increase of the constant electromagnetic field induction from 3.72 mT to 9.86 mT there is an increase of inductive signal . This can be explained by the growing quantity of powder particles involved in the magneto fluidized bed, by the natural particle aggregate destruction and the formation of secondary aggregates with aligned magnetic moment and by the increase of particulate system motion intensity. Further increase of the constant magnetic field induction from 11.90 mT to 42.60 mT leads to decreasing inductive signal , that corresponds to the decreasing of particulate system motion because of formation of chain-like structural elements aligned by the strong constant magnetic field.

Conclusion
The presented results show that the rheological properties of the magneto fluidized bed depend on the degree of ferromagnetic particles motion constraint that can be defined by the chamber fill factor and the electromagnetic effect parameters. The proposed induction method allows finding out the optimal parameters of electromagnetic fields and chamber fill factor that should provide required rheological state of the ferromagnetic particulate system .