Wet-milled ferrite magnetic powder having a crystal orientation enhancement effect and a manufacturing process

Abstract

The application discloses wet-milling ferrite magnetic powder with crystal orientation enhancement effect and a preparation process, and belongs to the technical field of magnetic materials. The powder is composed of spinel or magnetoplumbite ferrite grains, has sub-micron particle size and narrow distribution characteristics, and the grains are highly preferentially oriented along [111] or [001] crystal direction. The preparation process comprises the following steps: preparing slurry by mixing precursor powder with initial texture, specific orientation inducer and dispersant; in an alkaline environment, using the asymmetric rotation speed configuration of a planetary ball mill to generate a directional shear flow field, using the synergistic effect of selective adsorption of the orientation inducer and fluid torque generated by the flow field to simultaneously realize particle superfine crushing and active enhancement of crystal orientation in the wet-milling process; and finally obtaining high orientation degree powder through separation, washing and drying. The application solves the technical contradiction that particle refinement and crystal orientation maintenance are difficult to be compatible in the wet-milling process, and the obtained powder is an ideal precursor for preparing high-frequency low-loss ferrite magnetic cores.

Description

Technical Field

[0001] This invention relates to the field of magnetic materials technology, specifically to wet-milled ferrite magnetic powder with crystal orientation enhancement effect and its preparation process. Background Technology

[0002] With the rapid development of modern electronic information technology, electronic components are continuously accelerating towards higher frequencies, miniaturization, and higher energy efficiency, placing unprecedented performance demands on core magnetic materials, especially soft magnetic ferrites. In inductors, high-frequency transformers, and various electromagnetic compatibility components, ferrite magnetic powder serves as a fundamental functional raw material, and its microstructural characteristics directly determine the upper limit of macroscopic magnetic properties. Among these, the preferred orientation of grains (i.e., the degree of crystal orientation order), as a key intrinsic factor affecting permeability, coercivity, and high-frequency eddy current and hysteresis losses, has become one of the core bottlenecks restricting the development of high-performance magnetic cores. Wet milling technology, with its advantages in preventing material oxidation, effective heat dissipation, promoting particle dispersion, and obtaining a narrower particle size distribution, has been widely applied in the preparation process of high-performance ferrite precursor powders. However, the traditional wet milling process is essentially a physical pulverization behavior dominated by mechanical energy input. While achieving particle refinement, it often inevitably introduces strong lattice disturbances and local stress fields, causing the original grain orientation information to be randomized or even completely destroyed in the liquid phase environment. As a result, the final sintered body is difficult to form an effective magnetocrystalline anisotropy synergistic effect.

[0003] The invention patent CN106163701B proposes a method for screening low-iron-loss metallic iron powder based on KAM (Kernel Average Misorientation) values. It uses electron backscatter diffraction (EBSD) technology to quantitatively control the crystal orientation disorder of the core cross-section after pressing, effectively suppressing iron loss in specific application scenarios. The innovation of this method lies in incorporating microstructure parameters into the material quality evaluation system, which has significant engineering guiding significance. However, it is essentially a back-end screening strategy, relying on indirect characterization after high-pressure forming, and cannot actively intervene in the evolution of crystal orientation during powder synthesis or processing. More importantly, this method targets body-centered cubic metallic iron powder, whose crystallographic behavior is fundamentally different from ceramic ferrites with spinel (such as Mn-Zn, Ni-Zn ferrites) or magnetoplumble (such as Ba / Sr ferrite) structures, and it completely ignores the crucial wet grinding process, making it difficult to transfer to the orientation control practice of ferrite systems.

[0004] On the other hand, the HDDR (hydrogenation-disproportionation-dehydrogenation-recombination) process disclosed in the invention patent with publication number CN1293435A, although successfully utilizing the Fe2B phase as an orientation template to induce R2Fe... 14Epitaxial recrystallization of the B-phase master phase achieves high anisotropy in rare-earth permanent magnet powders. However, its technical logic is rooted in the solid-state phase transition mechanism of intermetallic compounds in a high-temperature hydrogen atmosphere. This not only results in demanding process conditions and high costs but also completely excludes the liquid environment, making it unsuitable for the complex conditions of dynamic shearing, collision, and solvent interaction encountered by ferrite precursors during wet milling. Furthermore, this method does not consider the destructive effects of grain boundary slip, dislocation multiplication, and non-equilibrium interface reactions commonly present in wet ball milling on crystal orientation stability, nor does it provide any technical means to maintain or enhance the preferred grain arrangement in a liquid medium.

[0005] The two representative technical solutions mentioned above represent typical approaches to post-evaluation and solid-phase induction, respectively. However, they both overlook a deep-seated technical contradiction: in wet grinding, a necessary process aimed at achieving ultrafine particle size and uniform dispersion, how to simultaneously suppress the tendency of lattice disorder caused by mechanical energy input and actively guide the grains to align in an orderly manner along a specific crystal axis. This contradiction stems from a fundamental conflict between the energy input mode of traditional wet grinding processes (such as random collisions in high-energy ball milling) and the directional stress field or interfacial energy gradient required for the ordered crystal orientation.

[0006] In the absence of an effective orientation-inducing mechanism, even if the initial precursor has a certain degree of texture, it is easily eliminated during long-term grinding. Once the grain orientation is randomized, even if anisotropy is partially restored through magnetic field orientation or heat treatment, its efficiency and extent are severely limited by the original powder microstructure.

[0007] Therefore, achieving in-situ preservation and even enhancement of crystal orientation during the wet grinding stage has become the key to breaking through the current bottleneck in the high-frequency performance of ferrite cores. How to simultaneously achieve particle ultrafineness and crystal orientation enhancement during wet grinding is currently a challenge in this field. Summary of the Invention

[0008] The purpose of this invention is to provide wet-milled ferrite magnetic powder with crystal orientation enhancement effect and its preparation process, which solves the technical problem that it is impossible to simultaneously achieve particle ultrafineness and in-situ maintenance and enhancement of preferred crystal orientation in the process of wet milling to prepare ferrite magnetic powder.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0010] The powder is composed of spinel-type or magnetoplumb-type ferrite grains with an average particle size of 0.3~1.2μm and a particle size distribution standard deviation of less than 0.15.

[0011] Statistical analysis using electron backscatter diffraction under a tolerance angle of 10° shows that, for spinel structures, the grain area fraction with preferred orientation along the

[111] crystal direction is not less than 65%, or for magnetoplumbago structures, the grain area fraction with preferred orientation along the

[001] crystal direction is not less than 65%.

[0012] The specific surface area of ​​the powder is 8~15m². 2 / g, tap density is 2.1~2.8g / cm³ 3 ;

[0013] The powder is suitable for pressing and molding under no external magnetic field conditions and sintering at 1150~1300℃ to obtain the initial magnetic permeability of the spinel system at a frequency of 1MHz. Remanent magnetic flux density of the magnetoplumb system at ≥2500 or 100 kHz ≥420mT, and total power loss at 1MHz ≤350kW / m 3 The magnetic core.

[0014] Furthermore, the surface of the grains is adsorbed with a monolayer film of orientation-inducing agent molecules, wherein the orientation-inducing agent is an amphiphilic organic molecule having the following general structural formula: C n H 2n+1 -C6H4-(COOH)2, where Furthermore, the two carboxylic acid groups are symmetrically distributed on the aromatic ring in either the para or meta position.

[0015] Furthermore, the orientation inducer is selected from at least one of octadecyl phthalic acid, hexadecyl isophthalic acid, or eicosyl phthalic acid.

[0016] Furthermore, the orientation inducing agent molecule interacts with the Fe atoms on the {111} or {001} crystal planes of the ferrite grains via its carboxylic acid group. 3+ The site forms a bidentate coordination structure with Fe—O bond length of 2.1~2.3 Å, O—C—O bond angle of 120~130°, rigid aromatic ring core parallel to the crystal plane, and long-chain alkyl tails extending outward.

[0017] This invention also discloses a process for preparing wet-milled ferrite magnetic powder with crystal orientation enhancement effect, used to prepare wet-milled ferrite magnetic powder with crystal orientation enhancement effect as described above, comprising the following steps:

[0018] Provide initial ferrite precursor powder with the chemical composition MFe₂O₄ or NO·6Fe₂O₃, where M is Mn 2+ Zn 2 + Ni 2+ At least one of them, where N is Ba2+ or Sr 2+ Average particle size ( The texture factor is 2~5 μm, as measured by EBSD. The texture factor is not less than 0.35. Defined as: ;in To define the grain area oriented within a 10° tolerance angle, The total grain area, The maximum pole density of the EBSD pole map. The value is 1;

[0019] The precursor powder, zirconia grinding media, orientation inducer, dispersant, and deionized water are mixed to prepare a slurry with a solid content of 35-50 wt%, where the solid content refers to the percentage of the precursor powder mass to the total slurry mass; wherein the amount of orientation inducer added is 0.8-2.0 wt% of the precursor powder mass, and the amount of dispersant added is 0.5-1.5 wt%.

[0020] The slurry was placed in a planetary ball mill and directional wet milling was carried out under the conditions of pH value of 8.5~9.5 and temperature not exceeding 45℃. During the milling process, the main disc speed was 300-450 rpm, the ball mill jar rotation speed was 180~270 rpm, the speed ratio of the two was kept constant at 5:3, and the milling time was 60~120 min.

[0021] The grinding media are separated, and the slurry is subjected to solid-liquid separation, washing, and drying to obtain the wet-milled ferrite magnetic powder.

[0022] Furthermore, the zirconium oxide grinding media are Y2O3-stabilized ZrO2 balls with a diameter distribution of 0.3~0.8 mm and a filling rate of 60~75% of the effective volume of the grinding jar.

[0023] Furthermore, the dispersant is sodium polyacrylate or triammonium citrate; the pH value of the slurry is maintained at 8.5~9.5 through an online monitoring and adjustment system, so that the orientation inducer exists in the form of deprotonated anions and the zeta potential of the ferrite particles is reduced to -25mV to -30mV.

[0024] Furthermore, the inner wall of the ball mill jar is provided with a spiral guide groove with a depth of 0.5 mm and a pitch of 20 mm. The spiral direction is consistent with the rotation direction of the main disk, which is used to enhance the regularity of the directional shear flow field.

[0025] Furthermore, during the directional wet milling process, a spiral shear flow field is formed inside the slurry, and the curl of its velocity gradient tensor is not zero, thereby applying a continuous torque to the grains modified by the orientation inducer, driving their {111} or {001} crystal planes to align parallel to the main shear plane of the flow field.

[0026] Furthermore, the solid-liquid separation is performed by centrifugal sedimentation at a speed of 4000 rpm for 10 min; or by membrane filtration with a pore size of 0.2 μm; the washing is performed three times using an ammonia solution with a pH of 9.0.

[0027] The orientation enhancement factor OEF achieved by the process satisfies ,in The texture factor of the powder after wet milling. The texture factor of the precursor powder. The average particle size of the powder after wet milling ( ), The average particle size of the precursor powder ( ), It represents a logarithm with base 10.

[0028] Furthermore, the process is integrated into a continuous wet milling production line. The slurry flows through multiple series-connected grinding chambers at a flow rate of 0.5~2.0 L / min, with each stage independently controlling the rotation speed and temperature. The outlet slurry is monitored in real time by an online laser particle size analyzer and a Zeta potential meter, and the amount of orientation inducer added is adjusted accordingly to ensure the texture factor variation coefficient of the final product powder. ≤8%.

[0029] Furthermore, the initial ferrite precursor powder is prepared by co-precipitation or solid-state reaction, and the planetary ball mill is equipped with a dual drive system that independently controls the rotation speed of the main disc and the tank, as well as a PTFE-lined stainless steel tank with a lining thickness of ≥5mm.

[0030] Furthermore, the orientation inducer molecule also contains a second active functional group, which is at least one of a phosphate group, a silanol group, or a catechol group; in the orientation inducer molecule, the ratio of the number of carboxyl functional groups on the aromatic ring to the number of the second active functional group is 1:1 to 2:1.

[0031] Furthermore, the orientation inducer is selected from 2-carboxy-4-octadecyloxyphenyl phosphate or 3-carboxy-5-octadecyl-2-hydroxybenzoic acid.

[0032] Furthermore, the directional wet grinding process is carried out using a periodically alternating pulse speed modulation mode. Within a complete modulation cycle T, the first half-cycle uses a first speed ratio, and the second half-cycle uses a second speed ratio that is the reciprocal of the first speed ratio. The modulation cycle T is 60 seconds to 180 seconds. The first speed ratio is 5:3.

[0033] Furthermore, the directional wet grinding process is carried out synchronously or intermittently under the condition of applying a low-frequency ultrasonic field; the frequency of the low-frequency ultrasonic field is 20 kHz to 40 kHz, and the acoustic power density is 50 W / L to 150 W / L; when the intermittent application method is adopted, the ultrasonic field application time is 1 minute to 2 minutes, and the intermittent time is 4 minutes to 9 minutes.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention combines molecular interface engineering with fluid dynamics control to construct a synergistic mechanism for orientation induction and particle refinement in a wet grinding system, thereby effectively solving the fundamental technical problem that has long existed in the field of maintaining or even enhancing the preferred orientation of ferrite crystals during ultrafine grinding.

[0036] Regarding powder material properties, this invention enables the preparation of ferrite precursor powders that combine ultrafine particle size, narrow distribution characteristics, and high crystal orientation. This powder not only possesses excellent physical properties, such as superior dispersibility and high tap density, but more importantly, its internal grains exhibit a highly consistent and preferred arrangement along a specific easy magnetization axis. This inherent high-texture characteristic directly provides high-quality raw materials for subsequent processes, making it possible to perform pressing and molding without the need for an external magnetic field for orientation, significantly simplifying the production process and reducing manufacturing costs.

[0037] In terms of process principles and technological advancements, this invention achieves interfacial energy modulation at the target crystal plane by introducing an orientation inducer with specific molecular configuration and crystal facet selectivity, and effectively adsorbing it under a precisely controlled alkaline environment. Simultaneously, by designing asymmetric grinding kinetic parameters, a flow field with directional shear characteristics is actively constructed within the slurry. This flow field can apply a continuous, directional fluid torque to the surface-modified particles, thereby driving their ordered alignment.

[0038] The resulting sintered ferrite core exhibits significantly optimized high-frequency magnetic properties. Due to the high uniformity of grain orientation, the domain flipping resistance is greatly reduced, which directly translates into a significant increase in permeability and a simultaneous reduction in eddy current and hysteresis losses in high-frequency applications. Therefore, this invention provides a novel and reliable advanced material solution for producing key magnetic components suitable for high-frequency, high-efficiency, and miniaturized electronic devices, and has significant industrial application prospects. Attached Figure Description

[0039] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0040] Figure 1 This is a simplified flow chart of the wet grinding process of the present invention.

[0041] Figure 2 This is a flowchart of the pulse-ultrasound coordinated modulation process of the present invention. Detailed Implementation

[0042] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the embodiments of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0043] The following is in conjunction with the appendix Figure 1 and Figure 2 The embodiments of the present invention will be described in detail below.

[0044] Example 1: This example discloses a wet-milled ferrite magnetic powder with crystal orientation enhancement effect. The specific preparation method is as follows:

[0045] Preparation of initial ferrite precursor powder:

[0046] Taking spinel-type Mn-Zn ferrite as an example, a co-precipitation method was used to prepare Mn-Zn ferrite with the following chemical composition. 0.7 Zn 0.3 Fe2O4 precursor powder.

[0047] The specific operation was as follows: 0.7 mol of analytical grade MnSO4·H2O, 0.3 mol of ZnSO4·7H2O, and 2.0 mol of FeSO4·7H2O were dissolved in deionized water to prepare a mixed solution with a total metal ion concentration of 1.5 mol / L. Under nitrogen protection, air was introduced at a flow rate of 2.0 L / min for oxidation, while 25 wt% ammonia solution was slowly added dropwise to control the pH of the reaction system to be stable at 9.8 ± 0.2. The reaction temperature was maintained at 65℃, the stirring speed was 300 rpm, and the reaction lasted for 4 h. The resulting precipitate was filtered, washed with deionized water until the conductivity was lower than 10 μS / cm, and then dried in an oven at 120℃ for 12 h. Subsequently, the dried powder was pre-calcined in air at 850℃ for 3 h to obtain a precursor powder with a single spinel phase.

[0048] Laser particle size analysis showed that its average particle size The particle size distribution span is 3.2 μm. The value is 1.1.

[0049] For magnetoplumb-type Sr ferrite, SrFe is prepared by solid-state reaction method. 12 O 19 Precursor. SrCO3 (1.0 mol) and Fe2O3 (6.0 mol) were weighed according to stoichiometric ratio, and anhydrous ethanol was added as the dispersion medium. The mixture was wet-milled in a planetary ball mill at 300 rpm for 4 hours. The slurry was vacuum-dried at 60℃, calcined in air at 1250℃ for 2 hours, and then naturally cooled to room temperature. The resulting blocky product was coarsely crushed in a jaw crusher, then dry-milled in a vibrating mill for 30 minutes, and passed through a 200-mesh sieve to obtain the average particle size. The powder has a particle size of 4.1 μm. Analysis confirmed it to be a pure magnetic leadstone phase (JCPDS No. 33-1340). For the spinel-type Mn-Zn ferrite precursor used in subsequent wet milling, EBSD measured its texture factor in the

[111] direction. It is 0.38.

[0050] Preparation of wet grinding slurry:

[0051] Taking Mn-Zn ferrite precursor as an example, 1000g of the precursor powder prepared above was weighed and deionized water was added until the solid content was 42wt%, i.e., the total slurry mass was 2381g; sodium polyacrylate (PAA-Na, molecular weight 8000, Sigma-Aldrich) was added as a dispersant at an amount of 1.0wt% of the precursor powder mass, i.e., 10g; orientation inducing agent: p-octadecyl phthalic acid (C 18 H 37 -C6H4-(COOH)2), the synthesis method of which is as described above, is to add the orientation inducer to the slurry at a ratio of 1.2wt% (i.e., 12g); finally, add zirconium oxide grinding media (Y2O3 stabilized ZrO2, purity ≥99.9%, Vickers hardness 1250HV, density 5.85g / cm³). 3 The media diameter distribution is 0.3~0.8mm (of which 0.5mm accounts for 60%, and 0.3mm and 0.8mm each account for 20%), and the filling rate is 68% of the effective volume of the ball mill jar. The above materials are placed in a PTFE-lined stainless steel ball mill jar (lined with a thickness of 5mm), sealed, and prepared for grinding.

[0052] Implement directional wet grinding treatment.

[0053] The ball mill jar containing the slurry was installed on a planetary ball mill (model: QM-3SP4, Nanjing Nanda Instrument Factory, this equipment was modified to have independent control over the rotation speed of the main disc and the jar). The main disc speed was set to 400 rpm, and the jar rotation speed was set to 240 rpm, with the speed ratio strictly maintained at 5:3. Simultaneously with starting the equipment, the jacket cooling system was activated, circulating 5°C deionized water at a flow rate of 2 L / min to ensure the slurry temperature never exceeded 45°C. The pH value of the slurry was monitored in real time using a built-in online pH electrode (such as a Mettler Toledo InPro 3250i). If it deviated from the set range (8.5~9.5), 0.1M ammonia or dilute hydrochloric acid was automatically added via a micro-injection pump for fine-tuning. In this embodiment, the initial pH was adjusted to 9.0 and remained stable. The grinding duration was 90 minutes. Under this asymmetric speed configuration, a stable helical shear flow field was formed inside the ball mill jar.

[0054] In a preferred embodiment, the inner wall of the ball mill jar is machined with a right-handed spiral guide groove, 0.5 mm deep, with a pitch of 20 mm, and the spiral direction is consistent with the rotation direction of the main disk. This structure can guide the fluid to flow along a predetermined path, suppress the generation of turbulent vortices, and make the streamlines more regular. Experiments show that the sample with the guide groove structure has a maximum polar density value of 4.8 for its {111} pole figure, while the control group without the guide groove is only 3.1, proving that this structure significantly improves the orientation alignment efficiency.

[0055] Separation and washing.

[0056] After grinding, the slurry was poured through a 200-mesh stainless steel sieve to separate the zirconia grinding media. The resulting wet grinding slurry was then subjected to solid-liquid separation using centrifugal sedimentation: the centrifuge speed was set to 4000 rpm, and the centrifugation time was 10 min to obtain wet powder. Subsequently, the wet powder was washed three times with an ammonia solution (0.01M) with a pH=9.0, with each washing solution being 5 times the mass of the powder. After thorough stirring, the powder was centrifuged again. After the final washing, the wet powder was transferred to a vacuum drying oven and dried at 60℃ and an absolute pressure ≤10 kPa for 12 h to obtain dry wet-milled ferrite magnetic powder. The powder was grayish-black, had good flowability, and showed no obvious agglomeration.

[0057] The physical properties of the obtained powder were characterized as follows:

[0058] The average particle size was measured using a laser particle size analyzer (Malvern Mastersizer 3000). The particle size distribution standard deviation is 0.78 μm. The specific surface area was 0.12; the BET specific surface area meter (Micromeritics ASAP 2460) measured the specific surface area to be 11.3 m². 2 / g; the tap density test result (ASTM B527) is 2.45 g / cm³. 3 EBSD texture analysis (conditions: accelerating voltage 20 kV, step size 0.05 μm, tolerance angle 10°, confidence index > 0.1) showed that the grain area fraction with preferred orientation along the

[111] crystal direction was 76.3%, and the texture factor was [missing information]. Increased to 0.69; according to the formula Calculated The value is much greater than 1, indicating that a significant enhancement of crystal orientation was achieved while refining the grains.

[0059] Furthermore, the mechanism of action of the orientation inducer was verified.

[0060] Zeta potential testing (Malvern Zetasizer NanoZS) showed that, at pH 9.0, the Zeta potential of the Mn-Zn ferrite particles without the inducer was +15.2 mV, while the Zeta potential dropped to -28.5 mV after adding 1.2 wt% p-octadecyl phthalic acid, confirming the effective adsorption of the anionic inducer. Density functional theory (DFT) calculations (using the VASP software package, PAW pseudopotential, and PBE functional) showed the adsorption free energy of this molecule on the {111} crystal plane of the Mn-Zn ferrite. The efficiency is -42.7 kJ / mol, while it is only -21.3 kJ / mol on the {100} facet, demonstrating its high crystal plane selectivity.

[0061] Molecular dynamics simulations (LAMMPS software) further revealed that the carboxylate group forms a bidentate coordination structure with the surface Fe atoms, with a Fe-O bond length of 2.18 Å, an O-C-O bond angle of 124°, a rigid aromatic ring parallel to the crystal plane, and long-chain alkyl tails extending outward to form an ordered molecular monolayer.

[0062] As another preferred embodiment, the orientation inducer can be replaced with hexadecyl isophthalic acid (C... 16 H 33 -C6H4-(COOH)2).

[0063] Synthetic method: Esterification of isophthalic acid with hexadecyl alcohol at 150 °C for 6 h yielded 78%. The molecule satisfies the structural constraints: two carboxyl groups on the aromatic ring, arranged in a meta-symmetrical pattern; alkyl chain length... The critical micelle concentration (CMC) was determined to be 0.12 mmol / L ≤ 0.15 mmol / L. It was used at a ratio of 1.5 wt% in SrFe. 12 O 19 precursor ( =4.1μm, The wet milling process was performed with a particle size distribution of 0.41 (particle size = 0.41). The process parameters were: main disc 420 rpm, tank 252 rpm, pH = 9.2, and time 100 min. The average particle size of the resulting powder was... The texture factor is 0.92 μm. =0.76, calculated according to the formula. The powder was pressed into a toroidal magnetic core (outer diameter 20 mm, inner diameter 12 mm, height 8 mm) under a pressure of 100 MPa and sintered in air at 1220 °C for 2 h. Magnetic property testing (using a SY-8232B-H analyzer) showed the following results: remanent magnetic induction intensity... =435mT, coercivity =210kA / m, maximum magnetic energy product =38.5kJ / m 3 .

[0064] To systematically evaluate the technical effects of the present invention, a comparative experiment was conducted using a comparative example.

[0065] Comparative Example 1: Using traditional wet milling process: using the same batch of Mn 0.7 Zn 0.3 Fe2O4 precursor ( =3.2μm, =0.38), a slurry with a solid content of 42wt% was prepared, with only 1.0wt% PAA-Na dispersant added, and no orientation inducing agent added; the pH value was adjusted to 7.0; a symmetrical rotation speed configuration was used, with both the main disc and the tank set to 400rpm; other conditions (grinding time 90min, temperature ≤45℃, media specifications, etc.) were exactly the same as in Example 1. The average particle size of the obtained powder was... It is 0.85 μm, but the texture factor is... When it drops to 0.21, the result calculated according to the formula is... The powder was pressed and sintered under the same conditions as the powder obtained in Example 1 (1250℃, 4h), and the magnetic properties test results are shown in Table 1.

[0066] Table 1: Comparison of magnetic properties of the magnetic cores obtained in Example 1 and Comparative Example 1;

[0067]

[0068] Data shows that, despite similar particle sizes, the sample of this invention exhibits significantly better high-frequency magnetic properties due to its high texture: initial permeability is increased by 48.4%, and power loss is reduced by 38.6%.

[0069] At the level of large-scale production, the process of this invention can be integrated into a continuous wet milling production line.

[0070] In practice, precursor powder, orientation inducer, dispersant, and deionized water are continuously added to a high-speed dispersion reactor in proportion to form a homogeneous slurry. The slurry is then pumped at a constant flow rate of 1.2 L / min to a three-stage tandem grinding chamber (each stage with a volume of 50 L). The first stage has a main disc speed of 450 rpm and a tank speed of 270 rpm, the second stage has 420 rpm and 252 rpm, and the third stage has 400 rpm and 240 rpm, with energy input decreasing progressively at each stage to avoid over-grinding. Each chamber is equipped with an independent temperature control system (adjustable cooling water flow) and an online pH monitoring module. The outlet slurry enters an online detection unit via a pipeline, which includes a laser particle size analyzer (real-time output). Span) and Zeta potentiometer (output) The detection signal is fed back to the central control system, which dynamically adjusts the frequency of the replenishment pump in the orientation inducer storage tank to ensure that the concentration fluctuation of the inducer is ≤±0.1wt%. Finally, the slurry is filtered through a membrane (0.2μm ceramic membrane), washed, and dried to obtain the finished powder.

[0071] Batch data from 72 hours of continuous operation showed that the texture factor of the product powder

[111] The mean is 0.71, the standard deviation is 0.04, and the coefficient of variation is... =5.6%≤8%, which proves that the continuous process has excellent stability and consistency.

[0072] This invention constructs a multi-physics synergistic wet milling system by precisely designing the molecular structure of the orientation inducer (rigid aromatic ring, polycarboxylic acid group, and long alkyl chain), combining it with the directional shear flow field generated by the asymmetric rotation speed configuration, and the interfacial charge regulation under alkaline pH environment. During the mechanical grinding process, this system not only does not destroy the original grain orientation, but also actively induces the grains to align in an orderly manner along the

[111] or

[001] crystal orientation through a dual mechanism of thermodynamics (reducing the surface energy of specific crystal faces) and kinetics (fluid torque driving directional alignment).

[0073] The resulting powder exhibits both submicron particle size (0.3~1.2μm) and narrow particle size distribution. <0.15), high texture ( It also possesses excellent molding properties (tap density 2.1~2.8 g / cm³). 3 This provides an ideal highly oriented precursor for subsequent non-magnetic field pressing and low-temperature sintering, ultimately resulting in a high-frequency, low-loss, high-performance ferrite core.

[0074] Example 2: This example is a further optimization based on Example 1. The aim of this example is to demonstrate how the scheme of Example 1, after introducing an orientation inducer with a second active functional group, can maintain and enhance crystal orientation under extreme high-energy grinding conditions, thus solving the technical problem of insufficient adsorption stability that traditional single carboxylic acid group inducers may face. The bifunctional orientation inducer selected in this example is 2-carboxy-4-octadecyloxyphenylphosphoric acid.

[0075] As electronic components move towards the very high frequency (VHF) band, the requirements for the particle size of ferrite powder are becoming increasingly stringent, often needing to reach the sub-0.5 micrometer level.

[0076] To achieve this goal, wet milling processes must employ higher mechanical energy inputs (such as increasing rotational speed and extending grinding time). However, higher mechanical energy inputs mean stronger shear forces and collision frequencies, which significantly increases the probability of dissociation or displacement of orientation inducer molecules adsorbed on the ferrite grain surface. Once the stability of the inducer molecular layer is disrupted, its function of reducing the surface energy of specific crystal planes and guiding the orderly arrangement of grains will weaken or even fail, leading to the simultaneous loss of crystal orientation during grain refinement.

[0077] Therefore, developing orientation inducers with stronger adsorption and anchoring capabilities under high dynamic shear stress is key to overcoming the bottleneck in the preparation of high-performance ultrafine ferrite powders. This embodiment designs and synthesizes a bifunctional molecule containing both carboxylic acid and phosphate groups, utilizing the phosphate group to interact with metal ions (such as Fe) on the ferrite surface. 3+ It forms stronger and more stable coordination bonds and even some covalent bonds, and constructs a double anchor point fixing mechanism, thereby maintaining the integrity of the orientation inducer monolayer film under extreme grinding conditions and ensuring the continuous performance of the crystal orientation enhancement effect.

[0078] The specific implementation process is as follows:

[0079] Synthesis of the Bifunctional Orientation Inducer: The synthetic route of 2-carboxy-4-octadecyloxyphenylphosphine is as follows: Under nitrogen protection, 2,4-dihydroxybenzoic acid (15.4 g, 0.1 mol), octadecyl alcohol (27.0 g, 0.1 mol), and p-toluenesulfonic acid (0.5 g) were dissolved in 150 mL of toluene and heated to 140 °C for 8 hours. The water generated was removed using a water separator. After the reaction was complete, the mixture was cooled, and toluene was removed by vacuum distillation to obtain the intermediate 2-carboxy-4-octadecyloxybenzoic acid. This intermediate (approximately 38.2 g, 0.1 mol) was mixed with phosphorus oxychloride (18.4 g, 0.12 mol) in anhydrous pyridine (50 mL) at 0 °C with stirring. The mixture was then slowly heated to room temperature and reacted for 12 hours. The reaction mixture was poured into ice water, and the pH was adjusted to 2 with dilute hydrochloric acid, resulting in the precipitation of a white solid. The solid was filtered, washed with cold ethanol, and finally recrystallized from an ethanol-water mixture to obtain the target product, 2-carboxy-4-octadecyloxyphenyl phosphoric acid, in approximately 75% yield. The structure of the product was confirmed by 1H NMR and MS.

[0080] Initial precursor preparation: In this embodiment, manganese-zinc ferrite was used ( The precursor powder was prepared by co-precipitation combined with a high-temperature solid-state reaction method. The resulting precursor powder was a pure spinel phase with an average particle size of [missing information]. Texture factor along the

[111] crystal orientation under a tolerance angle of 10° .

[0081] For wet grinding slurry preparation, 1000 grams of the aforementioned precursor powder were weighed and, based on a solid content of 42 wt%, deionized water was added until the total slurry mass was 2381 grams. Sodium polyacrylate (molecular weight 8000) was added to the slurry as a dispersant at a rate of 1.0 wt% (10 grams) of the precursor powder mass. 2-Carboxy-4-octadecyloxyphenyl phosphoric acid synthesized in this embodiment was added as an orientation inducer at a rate of 1.2 wt% (12 grams) of the precursor powder mass. A control group was also set up, using the aforementioned p-octadecyl phthalic acid as an orientation inducer at a rate of 1.2 wt%, and all process conditions (including precursor, grinding parameters, pH, temperature, time, etc.) were the same as in this embodiment, except for the type of orientation inducer. Finally, zirconium oxide grinding media of the same specification were added to both slurry systems. Stablize (0.3-0.8 mm in diameter), with a filling rate of 68% of the effective volume of the ball mill jar.

[0082] This embodiment employs higher mechanical energy input. The prepared slurry is placed in a planetary ball mill equipped with an independent dual-drive system. The main disc speed is set to 480 rpm, and based on a fixed speed ratio of 5:3, the ball mill jar rotation speed is correspondingly set to 288 rpm. The pH value of the grinding environment is maintained at 9.0 ± 0.1 through online monitoring and an automatic replenishment system. The slurry temperature is controlled to remain below 45°C through a jacket cooling system. The total grinding time is 100 minutes. This parameter combination aims to achieve a lower average particle size of the powder. Reducing the size to below 0.4 micrometers poses a severe test to the adsorption stability of orientation inducers.

[0083] After post-processing grinding, the grinding media were separated using a 200-mesh sieve, and the slurry was centrifuged (4000 rpm, 10 minutes) to obtain wet powder. The wet powder was washed three times with an ammonia solution of pH 9.0, with each wash solution being 5 times the mass of the powder. The washed powder was then dried in a vacuum drying oven at 60℃ for 12 hours to obtain the final dry magnetic powder.

[0084] The adsorption free energy of different inducing agents on the {111} crystal plane of manganese zinc ferrite was calculated using density functional theory (DFT). The calculations were performed using the VASP software package, employing a projected plane wave (PAW) pseudopotential and a PBE functional. A periodic surface model of the {111} plane of manganese-zinc ferrite was constructed, and the structure of the adsorbed molecule-surface composite system was optimized and its energy was calculated. The calculation formula is as follows: ,in It is the total energy of the adsorption system. It is the energy used to clean the surface of ferrite. It is the energy of the free orientation inducer molecule. The more negative the value, the stronger and more stable the adsorption.

[0085] The calculation results show that:

[0086] p-Octadecanyl phthalic acid (original inducer): ;

[0087] 2-Carboxy-4-octadecyloxyphenylphosphonic acid (inducer in this example): The calculation results show that the binding energy between the novel inducing agent and the {111} face of the ferrite is significantly enhanced due to the introduction of phosphate groups, which theoretically predicts that it has better resistance to desorption under high-speed shear conditions.

[0088] Powder physical properties and texture characterization: The two powders were systematically characterized:

[0089] Particle size analysis: Measured using a laser particle size analyzer. Average particle size of the control group (original inducer) powder. Example 2 (New Inducer) Powder Average Particle Size Both achieved the target ultrafine particle size (<0.4μm).

[0090] Crystal orientation statistics were performed using electron backscatter diffraction (EBSD). Scanning conditions: accelerating voltage 20 kV, step size 0.05 μm, tolerance angle 10°.

[0091] Control group powder texture factor .

[0092] Example group powder texture factor .

[0093] Orientation strengthening factor calculation and comparison: To quantitatively evaluate the enhancement effect of crystal orientation during grain refinement, the orientation strengthening factor is calculated. .

[0094] Orientation enhancement factor The calculation formula is:

[0095] ;

[0096] in, The texture factor of the powder after wet milling. The texture factor of the precursor powder. The average particle size of the powder after wet milling. The average particle size of the precursor powder is denoted as . This indicates that crystal orientation enhancement is achieved simultaneously with grain refinement, and the larger the value, the more significant the orientation enhancement effect is under the same degree of refinement. Substituting experimental data into the formula (where...) (representing a logarithm to base 10)

[0097] control group :

[0098] ;

[0099] Example group :

[0100] ;

[0101] Two groups The values ​​are all greater than 1, indicating that crystal orientation enhancement was achieved simultaneously with particle refinement. Among them, the example group... The value (1.52) was significantly higher than that of the control group (1.00).

[0102] Furthermore, comparison This ratio: the control group has a ratio of approximately 1.079, indicating that under extreme grinding, the original inducing agent can only barely maintain the original orientation level; while the example group has a ratio of approximately 1.605, proving that the bifunctional inducing agent can still actively and effectively enhance crystal orientation under high-energy shearing conditions.

[0103] Magnetic performance verification involved pressing two groups of powders into toroidal cores under the same conditions (no external magnetic field, pressure of 100 MPa, and sintering at 1250℃ for 4 hours) and testing their high-frequency magnetic properties.

[0104] Control group core: initial permeability at 1MHz Total power loss .

[0105] Example group core: initial permeability at 1MHz Total power loss The magnetic properties data are in high agreement with the texture analysis results. Due to the higher crystal orientation, the sample group showed an increase in magnetic permeability of approximately 34% and a reduction in high-frequency loss of approximately 26%.

[0106] This embodiment enhances the interfacial binding energy between the inducer and the ferrite crystal facets at the molecular design level by introducing an orientation inducer with a second active functional group consisting of a phosphate group. This solves the industry problem of insufficient adsorption stability of single carboxylic acid group inducers under high-energy grinding conditions.

[0107] In achieving powder ultrafineness ( At the same time, it significantly improved the crystal texture. This resulted in a manganese-zinc ferrite material with significantly optimized high-frequency magnetic properties. This provides a reliable and effective technical solution for preparing key magnetic materials suitable for next-generation very high-frequency, miniaturized inductors.

[0108] Example 3: This example modulates the energy field (pulse rotation speed and intermittent ultrasound) in the time dimension to solve two major technical problems: uneven processing of high solid content slurry and insufficient fluid driving torque of micro-nano particles, thereby achieving better crystal orientation alignment under harsh process conditions.

[0109] In the large-scale production of ferrite magnetic powders, increasing the solid content of the slurry is key to improving equipment utilization and reducing energy and water consumption. However, high solid content (>45wt%) leads to a sharp increase in slurry viscosity and more complex rheological behavior.

[0110] In traditional wet grinding, this easily leads to uneven flow field distribution within the grinding jar, generating dead zones (low-shear zones) and turbulent vortices, preventing the macroscopically designed directional shear flow field from acting uniformly and effectively on each particle. Furthermore, according to fluid mechanics formulas, the fluid torque acting on spherical particles... With particle radius It is proportional to the cube of ( When the target particle size enters the submicron level (e.g., <0.5μm), the orientation torque on the particles will be significantly attenuated and easily masked by Brownian motion or local turbulent disturbances, resulting in low orientation efficiency at the small particle size end. This embodiment introduces pulsed rotational speed modulation and low-frequency ultrasonic field assistance to synergistically solve the above problems from two levels: macroscopic flow field reconstruction and microscale additional force field, respectively.

[0111] Pulse modulation disrupts the steady-state flow field and eliminates dead zones by periodically reversing the shear direction, forcing grains to continuously adjust to the most stable orientation; while the ultrasonic field utilizes the microjets and acoustic radiation forces generated by the cavitation effect to provide a powerful, directional additional alignment driving force for micro and nano particles.

[0112] The specific implementation is as follows:

[0113] This embodiment uses strontium ferrite ( The precursor powder was prepared via a solid-state reaction method. Analysis showed the obtained precursor powder to be a pure magnetite phase with an average particle size of [missing information]. EBSD analysis revealed its texture factor along the

[001] crystal orientation under a tolerance angle of 10°. .

[0114] Precursor powder was weighed to prepare a slurry with a solid content as high as 48 wt%. The slurry contained 1.5 wt% p-octadecyl phthalic acid as an orientation inducing agent and 1.0 wt% triammonium citrate as a dispersant. Three sets of comparative experiments were set up:

[0115] Group A (Traditional Process Control Group): Constant asymmetric rotation speed was used, with the main disc at 420 rpm and the tank at 252 rpm (5:3 ratio), for 110 minutes of grinding.

[0116] Group B (Pulse Speed ​​Modulation Group): Total grinding time 110 minutes. Pulse modulation mode used: A second constitutes a complete cycle. Within each cycle, the first 60 seconds use a speed ratio of 5:3 (main disc 420 rpm / tank 252 rpm), and the next 60 seconds switch to a speed ratio of 3:5 (main disc 252 rpm / tank 420 rpm), and so on.

[0117] Group C (Pulse-Ultrasound Synergistic Group): Building upon the pulse modulation technique of Group B, intermittent low-frequency ultrasound was simultaneously applied during the last 40 minutes of abrasion. Ultrasound frequency... Sound energy power density The operating mode is: Ultrasound on. Minutes, turn off Minutes, cycle 5 times.

[0118] The pH value of the slurry in all groups was maintained at 9.0±0.2, and the temperature was controlled at ≤45℃. The post-processing steps (separation, washing, drying) were the same as in Example 2.

[0119] Transient simulations of the flow field under Group A (constant rotation speed) and Group B (pulse modulation) processes were performed using ANSYS Fluent software. The simulations focused on the curl of the slurry velocity field within the tank. The uniformity of grain distribution and the magnitude of curl are directly related to the torque that drives the grain rotation.

[0120] Simulation results show that, under constant rotational speed in Group A, there is a large dead zone at the bottom of the tank and near the axis, and the average velocity curl amplitude in this region is [not specified]. It is only about 30% of the average value of the mainstream area of ​​the tank. However, in the Group B pulse modulation mode, due to the periodic reversal of the flow field direction, these dead zones are effectively disturbed and scourned, and the uniformity index of the velocity curl distribution throughout the tank increases. (Defined as the ratio of the amplitude of the minimum curl region to the maximum curl region) increased from 0.3 in Group A to 0.75, greatly improving the uniformity of the flow field.

[0121] The powders obtained from each group were tested:

[0122] Particle size analysis:

[0123] Group A: ;

[0124] Group B: ;

[0125] Group C: ;

[0126] Texture Analysis (EBSD):

[0127] Group A: ;

[0128] Group B: ;

[0129] Group C: ;

[0130] Orientation enhancement factors in each group (in (Representing a logarithm to base 10): Group A: Group B: Group C: ;

[0131] Compare The ratios were approximately 1.268 for group A, increased to approximately 1.732 for group B, and further increased to approximately 1.927 for group C. Meanwhile, according to the definition, The value (must be greater than 0.8) increased from 0.724 in group A to 1.025 in group B, and then to 1.199 in group C. This clearly demonstrates that:

[0132] 1) Pulse modulation significantly improves the overall orientation efficiency by enhancing flow field uniformity, enabling the process to meet and exceed basic requirements. );

[0133] 2) The introduction of the ultrasonic field further enhances the orientation driving force of micro and nano particles, achieving a breakthrough in orientation degree based on pulse modulation.

[0134] Three sets of powders were used to make magnetic cores (sintered at 1220℃ for 2 hours) and their permanent magnetic properties were tested:

[0135] Group A: Residual magnetic flux density coercivity .

[0136] Group B: , .

[0137] Group C: , .

[0138] From Group A to Group C Significant improvement With a moderate reduction, the overall magnetic properties are optimized. In addition, while achieving similar or finer particle sizes, the effective grinding energy required for groups B and C may be relatively optimized due to the improved flow field efficiency, resulting in potential energy-saving benefits.

[0139] This embodiment solves the hydrodynamic challenges in high-solids-content slurry processing and micro / nano particle orientation by introducing a pulse-ultrasound synergistic energy field modulation process. Pulse rotation speed modulation macroscopically disrupts the steady-state flow field, eliminating shear dead zones and making directional shearing more uniform and efficient. Intermittent low-frequency ultrasound provides additional, strong directional alignment force microscopically, effectively compensating for the attenuated fluid torque caused by micro / nano particles. The synergy of these two techniques significantly improves crystal orientation enhancement under demanding process conditions such as high solids content and ultrafine particle size targets, without significantly increasing total energy consumption. (Up to 1.927), and eventually transformed into superior magnetic properties.

[0140] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0141] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A wet-milled ferrite magnetic powder with crystal orientation enhancement effect, characterized in that: The powder is composed of spinel-type or magnetoplumb-type ferrite grains with an average particle size of 0.3~1.2μm and a particle size distribution standard deviation of less than 0.

15. Statistical analysis using electron backscatter diffraction under a tolerance angle of 10° shows that, for spinel structures, the grain area fraction with preferred orientation along the [111] crystal direction is not less than 65%, or for magnetoplumbago structures, the grain area fraction with preferred orientation along the [001] crystal direction is not less than 65%. The specific surface area of ​​the powder is 8~15m². 2 / g, tap density is 2.1~2.8g / cm³ 3 ; The powder is suitable for pressing and molding under no external magnetic field conditions and sintering at 1150~1300℃ to obtain the initial magnetic permeability of the spinel system at a frequency of 1MHz. Remanent magnetic flux density of the magnetoplumb system at ≥2500 or 100 kHz ≥420mT, and total power loss at 1MHz ≤350kW / m 3 The magnetic core.

2. The wet-milled ferrite magnetic powder with crystal orientation enhancement effect according to claim 1, characterized in that: The surface of the grains is adsorbed with a monolayer film of orientation-inducing agent molecules, which are amphiphilic organic molecules having the following general structural formula: C n H 2n+1 -C6H4-(COOH)2, where Furthermore, the two carboxylic acid groups are symmetrically distributed on the aromatic ring in either the para or meta position.

3. The wet-milled ferrite magnetic powder with crystal orientation enhancement effect according to claim 2, characterized in that: The orientation inducer is selected from at least one of octadecyl phthalic acid, hexadecyl isophthalic acid, or eicosyl phthalic acid.

4. The wet-milled ferrite magnetic powder with crystal orientation enhancement effect according to claim 2, characterized in that: The orientation inducer molecule interacts with the Fe atoms on the {111} or {001} crystal planes of the ferrite grains via its carboxylic acid group. 3+ The site forms a bidentate coordination structure with Fe—O bond length of 2.1~2.3 Å, O—C—O bond angle of 120~130°, rigid aromatic ring core parallel to the crystal plane, and long-chain alkyl tails extending outward.

5. A process for preparing wet-milled ferrite magnetic powder with crystal orientation enhancement effect, used to prepare the wet-milled ferrite magnetic powder with crystal orientation enhancement effect as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Provide initial ferrite precursor powder with the chemical composition MFe₂O₄ or NO·6Fe₂O₃, where M is Mn 2+ Zn 2+ Ni 2+ At least one of them, where N is Ba 2+ or Sr 2+ Average particle size ( The texture factor is 2~5 μm, as measured by EBSD. The texture factor is not less than 0.

35. Defined as: ； in To define the grain area oriented within a 10° tolerance angle, The total grain area, The maximum pole density of the EBSD pole map. The value is 1; The precursor powder, zirconia grinding media, orientation inducer, dispersant, and deionized water are mixed to prepare a slurry with a solid content of 35-50 wt%, where the solid content refers to the percentage of the precursor powder mass to the total slurry mass; wherein the amount of orientation inducer added is 0.8-2.0 wt% of the precursor powder mass, and the amount of dispersant added is 0.5-1.5 wt%. The slurry was placed in a planetary ball mill and directional wet milling was carried out under the conditions of pH value of 8.5~9.5 and temperature not exceeding 45℃. During the milling process, the main disc speed was 300-450 rpm, the ball mill jar rotation speed was 180~270 rpm, the speed ratio of the two was kept constant at 5:3, and the milling time was 60~120 min. The grinding media are separated, and the slurry is subjected to solid-liquid separation, washing, and drying to obtain the wet-milled ferrite magnetic powder.

6. The wet-milling ferrite magnetic powder preparation process with crystal orientation enhancement effect according to claim 5, characterized in that: The zirconium oxide grinding media are Y2O3 stabilized ZrO2 balls with a diameter distribution of 0.3~0.8 mm and a filling rate of 60~75% of the effective volume of the grinding jar.

7. The wet-milling ferrite magnetic powder preparation process with crystal orientation enhancement effect according to claim 5, characterized in that: The dispersant is sodium polyacrylate or triammonium citrate; the pH of the slurry is maintained at 8.5~9.5 through an online monitoring and adjustment system, so that the orientation inducer exists in the form of deprotonated anions and the zeta potential of the ferrite particles is reduced to -25mV to -30mV.

8. The wet-milling ferrite magnetic powder preparation process with crystal orientation enhancement effect according to claim 5, characterized in that: The inner wall of the ball mill jar is provided with a spiral guide groove with a depth of 0.5 mm and a pitch of 20 mm. The spiral direction is consistent with the rotation direction of the main disk, which is used to enhance the regularity of the directional shear flow field.

9. The wet-milling ferrite magnetic powder preparation process with crystal orientation enhancement effect according to claim 5, characterized in that: During the directional wet milling process, a spiral shear flow field is formed inside the slurry, and the curl of the velocity gradient tensor is not zero. This applies a continuous torque to the grains modified by the orientation inducer, driving the {111} or {001} crystal planes to align parallel to the main shear plane of the flow field.

10. The wet-milling ferrite magnetic powder preparation process with crystal orientation enhancement effect according to claim 5, characterized in that: The solid-liquid separation is performed by centrifugation at 4000 rpm for 10 min; or by membrane filtration with a pore size of 0.2 μm; the washing is performed three times with an ammonia solution at pH 9.0.

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