CC BY
11
UDC 537.6
O. N. Sorokina, A. L. Kovarski, S. N. Podoynitsyn, G. E. Zaikov
SEPARATION OF MICRO PARTICLES ON PERFORATED HIGH-GRADIENT FERROMAGNETIC MEMBRANE UNIT
Keywords: ferromagnetic membrane, ferromagnetic resonance, high- gradient magnetic separation, magnetite nanoparticles.
The concept of magnetic separator designed and suitable for analytics and scientific research was presented. The ferromagnetic foil perforated by laser beam was used as a membrane separating unit. The water suspension of magnetite nanoparticles adsorbed on the grains of hydroxyapatite was used to test the magnetic separator designed. To evaluate the separation efficiency the suspension magnetization and the particles sizes were measured by ferromagnetic resonance and dynamic light scattering respectively. It was shown that during the separation all particles larger than 500 nm were captured by the membrane, whereas the total magnetization of the separated fraction was halved.
Ключевые слова: ферромагнитная мембрана, ферромагнитный резонанс, высокоградиентная магнитная сепарация, наноча-
стицы магнетита.
Представлена концепция магнитного сепаратора, разработанного и применимого для аналитических и научных исследований. В качестве мембранного разделительного устройства использована ферромагнитная фольга перфорированная лазерным лучом. Для тестирования разработанного магнитного сепаратора использовали водную суспензию наночастиц магнетита, адсорбированных на зернах гидроксиапатита. Для оценки эффективности разделения суспензии, намагниченность и размеры частиц измеряли при помощи соответственно ферромагнитного резонанса и динамического рассеяния света. Показано, что при разделении все частицы с размером более 500 нм удерживались мембраной, в то время как общая намагниченность выделенной фракции была удвоена.
Introduction
Magnetic separation is widely used in metallurgy, coal industry, for wastewater treatment to extract and concentrate of the most strongly magnetized particles from powder or suspension. High-gradient magnetic separation (HGMS) merged due to development of conventional magnetic separation technology. HGMS regarded as a highly productive approach to extract micro particles with the similar magnetic susceptibility values. The most commonly used basic separating unit for HGMS is a ferromagnetic wire [1] variously packed: as a grid, chaotic tangle or ordered spatial structure with the regular wire arrangement collinear to each other [2]. The magnetic separation and flow hydrodynamics near the wire separating unit are complex and ambiguous processes since mechanical or diamagnetic capture of particles is very probable [2], which complicates the selection detained on the separator fraction it's cleaning for reuse. Despite the problems the HGMS has been effectively applied to separate various bio objects like paramagnetic and diamagnetic erythrocytes, red blood cells infected with the malaria parasite, magneto bacteria and magnetic sorbents conjugated with cells, proteins and other biological components [3]. Recently the magnetic separators of new designs were developed.
Microelectronics technologies are widely used to create new separators able to capture superfine and weakly magnetized particles. The on-chip made separators based on magnetic field flow fractionation or magnetic chromatography can separate Brownian particles according to their magnetic properties and can be applied for analytical purposes [4, 5]. Apart from the on-chip construction a magnetic sifter prepared with the microelectronics techniques can also be used as a separating membrane [6].
Materials and Methods
The potential of the designed membrane separa-
tor (Fig. 1) to capture certain particles (ferromagnetic particles here) was tested in the work. In this study we tested the possibility of particles separation using a magnetic membrane separator (Figure 1). The separator design includes two chambers for separated suspension partitioned by a separating element - membrane (unit 2 in Figure 1). The magnetic properties of the membrane were activated by means of a magnetic field generated by the permanent SmCo5 magnets with a magnetic field of 0.3 T at the surface (unit 1 in Figure 1). The sizes of the magnet were 40x40 mm. Magnets were placed in a housing made of Teflon (unit 2 in Figure 1).
The membrane was set perpendicular to the magnetic field lines between the permanent magnets and it was equidistant from the magnets' poles at a distance of 10 mm as it was shown in Figure 1. The initial suspension was separated as it flowed through the membrane.
3 —-
\
fc--: / F=
л
1 -V
Fig. 1 - Scheme of the membrane magnetic separator. 1 - permanent magnets, 2 - ferromagnetic membrane, 3 - Teflon separator body. The arrows indicate input and output of the separated suspension
The ferromagnetic separating unit (Figure 2) with the diameter of 19 mm was made of the foil of magnetic alloy - permendure (Fe-Co). Laser perforation was used to form regular holes of 20 ^m in diameter in the foil (50 ^m in thikness). The distance between the holes was 80 ^m.
—' ..Ул*-.. :: :: :: :: .•:*..
......
:
........
.......................
Fig. 2 - Ferromomagnetic membrane separating unitt. 1 - ferromagnetic foil; 2 - zone with holes in the foil; 3 - zoom image of holes structure
The particle sizes in the separated suspension were measured by dynamic light scattering (DLS) using Malvern Zetasizes Nano S (UK). The concentration of magnetic phase in suspension before and after separation was determined by ferromagnetic resonance (FMR). FMR spectra were measured using X-band spectrometer Bruker EMX 8/2.7 (Germany). All spectra were measured at room temperature. Radio wave frequency was 1 mW and modulation amplitude was 1 G. The suspension magnetization proportional to FMR signal intensity was determined by double integration of experimental spectra. The powder of magnesium oxide containing Mn2+ was used as the outer standard for comparing the intensity of the FMR signal in samples.
Results and discussion
The suspension of magnetite nanoparticles (MNP) adsorbed on micro particles of hydroxyl apatite (HA) was a model mixture (system) for separation tests. The MNP hydrosol were obtained by Massart method [7] by co-precipitation of Fe(II) and Fe(III) in alkaline conditions (NH4OH). The mean diameter of the particles obtained was ~26 nm according to DLS (Figured 3, line 1). The MNP concentration was 61(1) mg/ml and volume fraction was 0.012. The initial magnetic hydrosol was 10 fold diluted and mixed with the HA suspension in 1:1 ratio thus the resulting MNP concentration was 3.10(5) mg/ml. The pure HA suspension exposed for 20 minutes to select supernatant to mix with the MNP hydrosol. The suspension prepared this way kept stability during the experiment. The maximum size of the HA grains was less than 2 ^m according to DLS. The mixture of the MNP and HA was diluted in 5 times and kept for a day to complete the MNP adsorption. The DLS spectrum of the finished suspension (fig. 3 line 2) did not detect the peak of the separated particles at 26 nm (fig. 3 line 1) thus the majority of the MNP were adsorb on the HA or aggregated with each other. The DLS results were represented in terms of mean volume percent instead of mean number percent terms as usual. The number fraction of the coarse grains of the HA is significantly less than the number fraction of the fine MNP whereas their volume fractions were comparable. Therefore the volume percentage was much more appropriate term for the particles size description in this particular case.
The HA suspension with the adsorbed MNP was shaken and its volume was made up to 100 ml. The MNP concentration in the resulting suspension was 6.11(1) 10-2 mg/ml. The resulting suspension introduced into the vessel attached to the separator inlet with the tubes (Figure
1). Separation was carried out in near diffusion regime. 1 ml of the suspension was pumped through the separator per 5-7 min and no faster. A number of fractions of cleaned suspension were thieved one after another (fraction 1 and fraction 2) for the following analysis. The residue of the suspension concentrated above the membrane was also analysed (residue).
100
Diameter (nm)
Fig. 3 - Average volumetric percentage of particles depending on their sizes: 1) hydrosol of MN, 2) in suspension of MNP adsorbed on HA before separation and 3) in suspension MNP adsorbed on HA after separation
Ferromagnetic resonance and dynamic light scattering were used for the separation monitoring. The DLS curves of the suspension before (line 2) and after separation (line 3) are presented in Figure 3. It is seen that after the separation the particles larger than 500 nm were captured by the membrane and their signal disappeared. Vice versa the volume percentage of the fine particles (size less than 100 nm) increased. These results confirm the efficiency of the membrane to capture coarse magnetic grains.
The potential of the suggested technique was also confirmed by FMR (Figure 4). Area under an absorption curve (FMR spectrum intensity) is proportional to magnetic susceptibility x which relates to magnetisation M as M = x H, where H is an external magnetic field (spectrometer field). Total magnetization of magnetic suspension (Mt) is in a proportion to magnetization of individual particle (Mnp) and to volume concentration of these particles (9).
FMR spectra of the initial suspension (curve 1), 1-st and 2-nd fractions of the suspension after separation (curve 2 and 3) and of the residue (curve 4) are presented in fig. 4. The broad line is the signal of the ferromagnetic fraction and narrow multiplet (6 lines) is the signal of the outer standard Mn2+. According to fig. 4 the spectra intensities decreases for the samples after separation i.e. the sample magnetization decreases as well. The relative magnetization (Mfn/M/n/t) of the initial suspension (M/n/t) and after separation (Mfn) was determined using FMR spectra. The initial magnetization which is the magnetization of the suspension before separation was taken as 1. Basing on this assumption the relative magnetization of the fraction 1 was 0.58 and of the fraction 2 was 0.5. The magnetization of the residue accumulated above the membrane was 1.36. These results confirm that the half of magnetic fraction was captured by the membrane.
Fig. 4 - FMR spectra of the mixed suspension of HA with MNP before separation (1), the 1-st fraction after separation (2), the 2-nd fraction after separation (3) and of the accumulated residue of the suspension above the membrane (4)
Theoretical analysis of magnetic separation for a model ferromagnetic membrane would be presented below.
The ferromagnetic membrane is perpendicular to the external magnetic field in the separator (Figure 1). The magnetic field strength above the membrane hole is less than the average uniform field far from membrane surface. For this type of the membrane orientation the diamagnetic particles pushed through the holes with the liquid flow by the magnetic force. Whereas ferromagnetic and paramagnetic particles would be deposited on the membrane surface near the hole wall.
The density of the regular holes can be set arbitrarily. The minimum density corresponds to one hole per entire surface of the foil. For this case one can obtain the analytical expression for the magnetic field along the hole axis (Hx). Assuming that a uniform magnetic field H0 over the intact ferromagnetic foil (without holes) is a sum of the fields given by the foil with a hole and by ferromagnetic rod filling this hole, the following expression would be obtained [8]:
Hx - Hi
1 -
\\
L + x
Jr 2 + (L + x У
Vr 2 + x 2
JJ
And the field Hx gradient along the hole axis is:
dx
R
R
Л2 + x2 У V(r 2 +(L + x У У
Where L is the thickness of the magnetic foil, R is the radius of the hole, x is the axial distance from the foil surface to the magnetic field measuring point within and above the hole, H0 is the magnetic field strength near the foil surface without holes.
The magnetic field above the hole decreases coming close to the magnetic foil surface and becomes minimum in the middle of the foil. The magnetic field within the hole is less than near the ferromagnetic foil surface. Thus, the magnetic field is maximum and comparatively uniform over the entire foil surface without holes, whereas a curvilinear cone of gradient field forms above the hole. The magnetic field value decreases from
the cone vertex above the hole center to its base on the foil surface.
The field gradient along the hole axis is the function of the hole radius (R) and the foil thickness (L). The field gradient above the hole rises with the foil thickness increase and the hole radius decrease. In case of thin ferromagnetic membrane when R >> L the gradient tends to zero and the magnetic separation does not occur in the central area of the hole.
In fact this gradient region of the magnetic field above the hole is the separator working area. The set of such areas acts as a working high-gradient region of magnetic field of the separator in case of many regular holes in a membrane. Thus, all the particles in fluid able to pass through the holes in the membrane overcome the intense gradient magnetic fields and separate according to their magnetic properties and sizes.
The ponderomotive force affecting particle is proportional to the particle volume, to the gradient of the squared magnetic field strength and to the difference between the magnetic susceptibility of the particles and environment. Paramagnetic particles subjected to this force would be drawn from the central area of the hole to its wall and diamagnetic particles would be pushed into the hole center. The particles always move in a specific trajectory and at specific velocities. The resulting particle velocity consists of the particle velocity in the flow along the hole axis - vP and velocity of the force affects particle along the magnetic field gradient - vF. Particle can be captured by the separator when vF > vP. When vF << vP the separation is almost absent. The maximum selectivity of the separation can be achieved at minimum flow velocities vP through the membrane or at zero velocity when the diffusion mode of the separation is realized. The separation capacity (volume of the separated suspension passed through the membrane per time unit) is a direct proportion to the flow velocity within the hole - vP.
The separation selectivity depends on the vF value for specified separation productivity and consequently for the given velocity vP. The velocity of certain particles affected by the force of gradient fields relates to the product of the field strength by gradient. The ferromagnetic separating unit selectivity can be enhanced by the application of stronger magnetic field or by the reduction of separation productivity. The decrease in the hole diameter results in the liner increase in the field gradient. Thus the force acting on particle and particle velocity also increase. The distance of particle translation before its capture by the membrane surface reduces as well. The capture time diminished as an approximately quadratic low.
The most efficient particle capture would be carried out at similar sizes of particles and holes. In this case, particle is moved to a minimum distance under magnetic field. The distance of particle translation increases with particle size decrease.
Furthermore, the Brownian motion of submicron particles (radius less than 1 micrometer) makes the description of their movement in terms of trajectories, velocities, etc. impossible.
The diffusion equation, where particles concentrate under external forces, has to be derived:
^ = DV 2 c -V(Vc),
At
x
here V is the velocity obtained from the equality of magnetic force and hydrodynamic resistance force;
kT
FM = Ft = 6n^V; D =-is a diffusion constant; k
6%V[R
is Boltzmann constant, T is absolute temperature, c is particle concentration, n is a coefficient of dynamic viscosity.
Submicron particles can be divided into two types according to their sizes: the particle radius R is greater than the critical radius Rc, then the particles are closely deposited on the ferromagnetic unit; R < Rc then particles form a cloud. For example, refers to [9] radius Rc =1.4 nm for Fe, 2.2 nm for Fe3O4, 10 nm for Mn2P2O7 and 77 nm for Au. Thus, particles are divided into three groups according to their sizes: 1) large particle moving along the trajectory; 2) Brownian particles with radius R meeting the condition 2FR > kT, F is the resulting force; 3) Brownian particles with radius less than the critical. At zero flow velocity the fraction can be separated due to particles diffusion.
The ferromagnetic membrane can be set along external magnetic field. Since a demagnetizing factor of the unit is close to zero the magnetic field near membrane is equal to the external field and exceeds it only near hole. Magnetic field inside the hole is the sum of external field and halved magnetization of the ferromagnetic material. This relation holds for comparatively thick layer of ferromagnetic which is two or three times greater than the hole diameter. Magnetic field of thin ferromagnetic was calculated in [10]. It was shown that the magnetic field decayed sharply near the hole wall and did not cover the entire area of the hole.
When the membrane is set along the field the di-amagnetic particles would be pushed out from the holes and deposited on the membrane surface. In this case the diamagnetic particle is captured by the membrane. Paramagnetic particles would be drawn into the pores, and pass through them with the fluid flow. Ferromagnetic particles highly likely would coagulate and clog holes.
Conclusions
A new design of membrane separator was presented in the work. This separator was successfully applied to separate complex magnetic suspension consisted of magnetite nanoparticles in water including particles adsorbed on hydroxylapatite grains. It was shown that the majority of large particles (greater than 500 nm) were captured by the ferromagnetic membrane and the magnetization of the separated fraction decreases.
The presented concept of the separator can be applied for analytics and scientific research. The main advantage of the technique is the enhanced selectivity of
fractioning against the not high productivity. Indeed, the sample volume of 1 - 3 ml of suspension separated per 10 - 30 min is acceptable for laboratory service. The construction simplicity allows to get the separator ready rather fast.
The magnetic separation can be applied to extract strongly magnetized fraction or vice versa diamag-netic fraction for the following operation, for example to search substances with the unique properties like: ferromagnetics, superparamagnetics, and superconductors, as well as for selecting biological microorganisms accumulating metals, metal-containing proteins and etc.
The obtained results hold out the hope of application of HGMS to extract paramagnetic and diamagnetic particles larger than 0.1 ^m.
Acknowledgment
The authors are grateful to Dr. A.V. Bychkova (Emanuel Institute of Biochemical Physics, Russian Academy of Science, Moscow, Russia) for DLS measurements.
The work was supported by Russian Foundation for Basic Research under Grant № 13-08-01390.
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© O. N. Sorokina - Ph.D., researcher, Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, A. L. Kovarski - Doctor of Chemistry, Full Professor, Head of Magnetic Spectroscopy Department, Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, S. N. Podoynitsyn - Ph.D, Senior Researcher, Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, chembio@sky.chph.ras.ru.
© О. Н. Сорокина - канд. хим. наук, науч. сотр. Института биохимической физики им. Н.М. Эмануэля, Москва, Россия, А. Л. Коварский - д-р хим. наук, проф., руководитель Отдела магнитной спектроскопии того же института; С. Н. Подойни-цын - канд. хим. наук, ст. науч. сотр. того же института; Г. Е. Заиков - д-р хим. наук, проф. каф. технологии пластических масс, КНИТУ, Казань, Россия, chembio@sky.chph.ras.ru.
