RU2508947C1 - Method of producing ultra disperse powders with narrow particle size distribution - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to powder metallurgy, particularly, to production of powders of mean grain size in submicron range. Prepared bulk layer of initial powder is displaced by upward gas flow to the area of centrifugal forces developed by centrifugal classifier rotor. Portion of material is recycled to feed coarse particles back from said area to bulk layer. Fine particles are discharged by gas flow from said area. Used is powder material with mean particle size of 1-2 mcm, major portion of said particle is sized to 10-15 mcm. This process proceeds in two steps. For this working gas is, first heated to 90-100°C to disperse the material and dry it in compressed gas flow at 4-6 kg/cm². Moisture content in gas flow at working zone inlet and outlet is continuously measured after release of particles therefrom. Centrifugal acceleration created by classifier rotor is set to (8.5-12)·10⁴ m/s². Mean size of particles in the flow is continuously measured as well as bulk concentration in said flow. Pulsation of bulk concentration is stabilised by increase in centrifugal acceleration to (12-16)·10⁴ m/s². With moisture content at gas flow inlet and outlet equalised, heating is terminated to start the second stage. Working pressure is increased to 6-8 kg/cm². Mean particle size exceeds the preset value, amount of material fed to gas flow is decreased. This is performed by decreasing gradually the height of particle ingress zone to the level of 80-85 % of initial height. At further increase of mean particle size, centrifugal acceleration is increased to (16-19)·10⁴ m/s². At further increase in mean particle size or notable decrease in bulk concentration, the process is interrupted.

EFFECT: jet size reduction and air centrifugal classification to produce powders of mean grain size in submicron range.

15 dwg

Description

The invention relates to the field of powder technology and can be used to obtain narrow fractions of particles with an average size in the submicron range.

Currently, to obtain fine and ultrafine (containing particles with sizes in the submicron range, in particular, nanoscale) powders, several classes of methods have been developed and are used, while the characteristics of the resulting product — particle size distribution and particle shape, impurity content, specific surface area — can vary depending on the method of obtaining in a very wide range. So, depending on the production conditions, the particles can have a spherical, hexagonal, lamellar, dendritic, needle-shaped, amorphous or small-crystalline structure. Production methods are divided into mechanical, physical, chemical. Physical, chemical, and other non-mechanical methods (plasma, electroexplosive, gas-phase synthesis, various chemical, etc.) have a number of disadvantages: they are applicable for a narrow class of materials, are characterized by the complexity of plants and technology, and the resulting powders have both ultrafine particles and a large proportion of particles in the range of more than 1 μm (Fig.4, 11, 12). All these methods for producing ultrafine powders are still inefficient and laborious, and therefore the cost of the powders is very high - about $ 100 per kilogram.

Known mechanical methods for dry grinding powder materials although they give high performance, but also do not allow to achieve a satisfactory result, since the resulting fine powders have a very wide range of particle sizes. This is due to the fact that these methods can efficiently process only sufficiently large initial powders: of the order of 100 μm or more: with decreasing particle sizes, their efficiency decreases significantly, and in the range of the order of 10 μm and below, a positive result is practically absent. This is explained by the fact that it is rather difficult to effectively act on particles of this size mechanically in a dry state: the bulk layer is in an extremely loose state, the particles move after any vortex movements of the gaseous medium in the working volume, which does not allow them to be subjected to effective energy loading. In addition, the task of separating narrow fractions of particles in the ultrafine range from a wide range of particle sizes entering the air-centrifugal classification zone is difficult to achieve: in order to provide a centrifugal force acting on a submicron particle proportional to aerodynamic at the separation boundary, the rotor speed of the classifier is required about 100,000 rpm, for which it is not possible to maintain the dynamic balancing of the rotor in the conditions of rapid wear of the blades from their hard interaction with particles.

At present, wet grinding is used to obtain especially fine powders in industrial quantities, for example, in attritors, bead mills, or ring mills (1). The wet state of the processed powder in the form of a paste or suspension provides a high concentration of particles in the zone of influence of the grinding elements.

The main disadvantage of these methods is that they are suitable for grinding mainly soft materials, since for hard and especially hard materials, wear and tear of equipment elements occurs, as a result of which the product loses its original chemical purity. Another drawback is the need for additional operations to dry and disaggregate particles, which for ultrafine powders is rather laborious and possible on specialized (rather complicated) equipment.

The positive side of mechanical grinding methods is the comparative simplicity of plants and technology, the ability to grind various materials and get with great productivity. The disadvantages of the method include the possibility of contamination of the crushed powder with abrasive materials, as well as the difficulties in obtaining powders with a narrow particle size distribution.

Known inkjet methods for grinding powders in which these disadvantages are partially eliminated. Despite the significantly higher specific energy consumption, they are widely used to obtain fine powders in the dry state without contamination with milling products. Inkjet methods are cost-effective for grinding powders with an initial size of less than 100-200 microns (depending on the hardness of the processed materials).

A known method of grinding particles in oncoming jets in a fluidized bed with simultaneous air-centrifugal classification, implemented in devices for various purposes by the corporation "Hosokawa Micron Ltd" (2).

The disadvantage of this method is that its effectiveness is significantly reduced when processing ultrafine powders. It is known that a bulk layer having an ultrafine composition and a complex morphology of particles is characterized by their significant cohesion, which is the reason for the high porosity of the layer. (For explanation, Fig. 2 shows the dependence of the bulk density of the powder on the dispersed composition for alumina, which shows that in the region of submicron sizes there is a sharp drop in bulk density.) In addition, vortex flows from the rotating rotor of the classifier and several counter jets create a highly turbulent zone in which ultrafine particles cannot form into a sufficiently concentrated mass at the intersection of the jets (which would ensure an effective hour collision egg and destruction).

A known method of producing activated aluminum, including the formation of a bulk layer of powder material, activating it by applying an upward gas stream from the bulk layer to the centrifugal force action zone created by the rotor of the centrifugal classifier, recirculating part of the powder material by returning a large fraction from the centrifugal force action zone to the bulk layer and removing a small fraction of the material by a gas stream from the center of the centrifugal force, while the starting material is use finely dispersed aluminum powder (3).

The specified method is selected for the prototype.

However, this method does not allow to obtain narrow fractions of ultrafine particles for cohesive powder materials characterized by significant polymorphism of particles (especially in the submicron range), for example, ceramic. This is due to the fact that with an ultrafine particle size, the presence of moisture in the starting material being processed or in the working gas is absolutely unacceptable, since this greatly enhances the effect of autohesion, since ultrafine particles (obtained by any method) form aggregates and agglomerates due to the developed surface, and the presence of moisture on the surface of the particles makes their adhesion much stronger. This significantly degrades the efficiency of both grinding and classification.

However, as is known, with adiabatic expansion of the gas in the effluent, a sharp decrease in temperature occurs (Fig. 3), which leads to the condensation of the moisture present in it and its contact with the surface of the particles. For large powders with a particle size of the order of 100 μm or more or fine powders whose particles have a spherical shape, this does not significantly affect the circulation of the layer and particle grinding, but for particles with a developed surface (ultrafine) there is a sharp decrease in the efficiency of the grinding process (energy the jet is spent on re-dispersing the agglomerates) and a violation of the classification process (due to the high windage of the agglomerates from large particles clad in ultrafine particles, large x particles in the product).

Based on experimental studies for cohesive powders, it was determined that even with a small amount of moisture (1-2%), the grinding process slows down and the quality of classification deteriorates. However, the moisture content in the initial finely divided raw materials often reaches 3-5%, and the relative humidity of the working gas, even after the compressor moisture separator, can reach 15% or more, depending on the season and weather conditions, which significantly worsens the processing process. For example, in summer conditions, situations often arise with a sharp increase in humidity during heavy rains, when compressor dehumidifiers do not provide the necessary level.

Thus, the instability of the circulation of the layer in the initial period is due to the connectedness of the initial mass of ultrafine particles and is manifested by short-term local hangs of the layer, fluctuations in the flow of material into the grinding zone and into the zone of air-centrifugal classification. In addition, a change in the fractional composition of the crushed material during processing towards particle enlargement (due to the removal of marketable particles), a change in the morphology of the particles during grinding, a decrease in the effect of aggregation due to the drying of the material - all this together significantly changes the conditions of both grinding and classification and requires continuous change of the operating geometric parameters.

The technical result of the claimed invention is to obtain using jet grinding and air-centrifugal classification of narrow fractions of ultrafine particles with an average size of less than 1 micron.

The specified technical result is achieved as follows. In the method, comprising the formation of a bulk layer of the initial powdery material, its movement by an upward gas flow into the zone of action of the centrifugal forces created by the rotor of the centrifugal classifier, recirculation of a part of the material by returning coarse particles from the zone of action of the centrifugal forces into the bulk layer and removing fine particles by the gas stream from the center zone of action of centrifugal force, using raw material powder having an average particle size (δ ₅₀₎ of less than 1.2 microns, the bulk weight the particles (δ ₉₀₎ of less than 10-15 microns, wherein the process is carried out in two stages, in which the working gas to a temperature of 90 ÷ 100 ° C is first heated, the material was dispersed and then dried in a stream of pressurized gas at a pressure of 4 ÷ 6 kg / cm ^2, continuously measure the moisture content in the gas stream at the entrance to the working area and at the exit after particles are separated from it, while the value of the centrifugal acceleration created by the classifier rotor is set in the range (8.5 ÷ 12) · 10 ⁴ m / s ² , the average is continuously determined particle size δ ₅₀ in a stream exiting the center of the centrobus forces using, for example, a laser diffractometer, and the volume concentration of particles σ _h in a given stream by measuring, for example, its dielectric constant, stabilize the pulsations of the volume concentration σ _h by increasing centrifugal acceleration to (12 ÷ 16) · 10 ⁴ m / ^2, after aligning the moisture content at the inlet and outlet of the gas flow of the working gas heating is switched off and start the second stage of: increasing the operating pressure of up to 6 ÷ 8 kg / cm ² and at exceeding a predetermined value δ ₅₀ simultaneously start to decrease the amount of mother la flowing into the gas stream by gradually reducing the height of the particle inlet region therein to a level of 80 ÷ 85% of the original height, with a further increase δ ₅₀ increases the value of centrifugal acceleration to (16 ÷ 19) × 10 ⁴ m / s ^2, while a further increase in δ ₅₀ or a significant decrease in σ _h , for example, by a factor of 1.5-2, the process is stopped, the coarse residue is removed and repeated on a new portion of the starting material; where δ ₅₀ is the parameter of the dispersed composition having a value less than which the mass fraction of particles is 50% (average particle size), δ ₉₀ is the parameter of the dispersed composition having the value less than which the mass fraction of particles is 90%, σ _h is the volume concentration of particles in gas flow at the exit of the classification zone.

As we experimentally established, to solve the problem - the allocation of narrow fractions of particles in the submicron range, the following conditions are required.

1. The starting material should be ultrafine: the particle size distribution is optimal with an average particle size of up to 1-2 microns (δ ₅₀ <1 ÷ 2 microns) and a bulk of particles (90%) of size up to 10-15 microns (δ ₉₀ < 10 ÷ 15 μm). (Such material can be relatively easily obtained by other methods.)

The specified dispersed composition provides effective self-grinding of particles in the jet, since this size is characterized by a small lag of the particles from the gas flow, as a result of which high-speed impact interaction of particles occurs in microvortices - the interaction speed reaches up to 500-600 m / s in opposite courses.

In addition, one of the main conditions for the severity of separation of ultrafine particles is their sufficiently large volume concentration in the classification zone, which ensures the effect of bundle formation during rotation of the rotor. The optimal volume concentration (density of the “tow”) depends on the particle size distribution of the processed material, the amount of ejected material by a gas jet, the spin speed created by the classifier rotor, and is a condition for the effective retention of large particles in the working volume. This effect is most pronounced when small particles dominate in the classification zone, since then dust and gas tows have a sufficient density (concentration) of particles that minimizes the probability of breakthrough of large particles. Thanks to this factor, there is no need for super-large revolutions of the classifier rotor. (For example, in Hosokawa Micron Ltd devices, the rotor speed is about 18000-22000 rpm). This is especially true for industrial applications, since at high speeds the rotor blades must be extremely resistant to abrasion, and constant monitoring of the rotor vibration during operation and regular rather laborious balancing of the rotor on complex equipment is also necessary. Since the edge of the thin blade is easily worn out and, as a result, the rotor balancing is impaired, instead of the blades in the rotor design, the Hosokawa Micron Ltd analog devices use rods made of durable ceramic. However, rod rotors (even at high speeds) do not give the required acuity for the classification of ultrafine powders.

It should be noted that the initial powdery material with the indicated dispersed composition minimizes product contamination due to abrasive wear of the blades, since at this size the mass component of the force of interaction of the particle with the rotor blade is much less than the aerodynamic component, which defines a safe (for the blade) direction of particle movement in it boundary layer - along the surface of the scapula.

2. The grinding process of ultrafine materials is very sensitive to the presence of moisture in the processed material, since particles stick together, and the energy of the jet is spent on re-dispersing the material. Therefore, in order to exclude condensation of vapors from the working gas onto the particle surface, as well as to remove it from the starting powder material, the working gas is heated to a temperature of 90-120 ° C before entering the working zone. This temperature is sufficient to prevent moisture condensation on the particles and to dry the original powdery material. In this case, the moisture content of the gas is measured at the entrance to the working area - m _in and exit from the process - m _out , that is, after the separation of particles from the gas.

At the first stage of the process with a working gas stream at a pressure of 4 ÷ 6 kg / cm ² , soft grinding of the powdered material is carried out — disaggregation (deagglomeration) and, as mentioned above, its dewatering. If m _in > m _out , then the drainage process has not yet ended and gas heating continues. When m _in = m _out heating is turned off, while maintaining continuous monitoring of the moisture content of the gas. If the moisture content of the working gas at the outlet has decreased - m _o <m _in , that is, the absorption of moisture by particles has begun, then the heating is turned on again.

3. For the initial ultrafine powder materials, the volume concentration of particles σ _h in the jet zone and in the rotor zone changes significantly during the processing process. This is due to a gradual decrease in the fraction of small particles in the layer. As a result, the nature of the movement of the bulk layer changes - from unstable with local stops to a more uniform one, which significantly increases the mass of particles entering the stream. This leads to a slowdown of grinding, on the one hand, and to an increase in the ejected mass of particles entering the classification zone. As a result, the conditions of qualitative classification worsen: the dispersed composition is enlarged and the mass of material that needs to be “twisted” to the rotor is increased. A decrease in the fraction of small particles in the classification zone leads to a weakening of the separation properties of the “tow”: it holds large particles worse.

This factor — the layer circulation velocity — can be monitored by the change in the volume concentration of particles σ _h in the layer or in the dust and gas stream at the outlet of the classifier by measuring, for example, its dielectric constant. Based on the studies, a more objective (total) control of σ _h is possible when measuring at the exit from the classifier, and not at local points of the layer. The stability of the circulation of the layer is manifested as the constancy of the average value of the pulsations σ _h in a fairly narrow range, the value of which is determined experimentally and set as the base for a particular material.

In addition, the value of σ _h characterizes the grinding efficiency: a significant decrease in σ _h , for example, by a factor of 1.5–2, indicates that the process of self-abrasion of particles becomes inefficient, since the largest particles remain at the end of the operation, the grinding of which requires more time and , accordingly, significantly increases energy consumption. Therefore, it is more economical to grind the coarse residue, for example, on a ball or bead mill, and then continue grinding by the specified method.

In addition, with a significant enlargement of the processed material, the classification conditions worsen, and therefore a further increase in δ ₅₀ occurs. In connection with the above changes in the parameters σ _h , δ _{50 the} process is stopped and repeated on a new portion of the starting material.

4. The value of centrifugal acceleration (rotor speed of the classifier) at the first stage is set in the range (8.5 ÷ 12) · 10 ⁴ m / s ² depending on the properties of a particular material: density, particle shape and particle size distribution. For example, for heavy ultrafine materials with a bulk density, acceleration is sufficient - (8.5 ÷ 10) · 10 ⁴ m / s ² , but for most “light” ultrafine powders it is necessary (10 ÷ 12) · 10 ⁴ m / s ² . In addition, so that the layer of “light” materials does not hang - the grinding and classification conditions are not violated, it is necessary to “crush” it. It was experimentally established that the dust and gas vortex from the classifier rotor effectively has a stabilizing effect on the movement of the circulating material with a centrifugal acceleration value (12 ÷ 16) · 10 ⁴ m / s ² .

Thus, at the first stage, preliminary preparation of ultrafine raw materials is carried out: at the same time, drainage, deagglomeration and removal of part of the finished particles from the starting material are carried out. As a result, the circulation of the bulk layer becomes stable - without internal freezes, which ensures the mass constancy of the particles entering the grinding zone and the constant volume concentration in the classification zone, and this is a condition for regulating (optimizing) these processes. At the first stage, the working pressure is necessary only for deagglomeration, and therefore its value is set 4 ÷ 6 kg / cm ² . This is due to the fact that, due to the surface activity of ultrafine particles, agglomerates have a relatively large size and strength, and therefore their effective destruction begins at a pressure above 4 kg / cm ² depends on the properties of a particular material and its dispersion. The use of pressure above 6 kg / cm ² does not increase the efficiency of deagglomeration (for most tested ultrafine materials), and the specific energy consumption increases.

5. At the second stage of the processing process for the efficient use of energy gas, an increase in operating pressure is required up to the level of 6 ÷ 8 kg / cm ² (depending on the hardness of the crushed material). This is due to the fact that the main part of the circulating material is dispersed and consists of monoparticles, which for most materials are always much stronger than agglomerates. In addition, ultrafine monoparticles have a much smaller number of defects and microcracks per unit volume, which also requires an increase in pressure for greater force interaction of the particles.

6. Improving the circulation (fluidity) of the layer by reducing the fraction of small particles in it leads to an increase in the amount of material entering the stream, and, accordingly, to an increase in the dust and gas load on the rotor, which reduces both the grinding efficiency and the quality of classification — δ increases ₅₀ particles. In this regard, it is necessary to continuously adjust the amount of incoming material. This is done by changing the parameters of the ejection zone: the ejection coefficient, which depends on the ratio of the squares of the diameters of the pipe and nozzle, or the height of the zone of contact of the jet with the bulk layer. But without changing the set operating parameters of the working gas (pressure and flow), it can be adjusted by reducing the height of the grinding zone, that is, by reducing the distance between the nozzle and the circulation pipe.

Subsequently, despite the optimization of the mass flow rate of the material passing through the grinding zone, the air-centrifugal classification at the initial value of centrifugal acceleration gradually begins to give a larger average particle size (δ ₅₀ ) at the outlet. This is due to the fact that after a decrease in the fraction of small particles in the layer during the first stage in the second stage, the dispersed composition entering the classification gradually coarsens, and therefore, to maintain the separation boundary (δ ₅₀ ), it is necessary to continuously increase the swirl speed of the dust and gas stream (adjust the centrifugal force acting on particles). In this regard, increase centrifugal acceleration to (16 ÷ 19) · 10 ⁴ m / s ² . For example, with a rotor diameter of 150 mm, this is achieved by a rotor speed of 14,000 ÷ 15,000 rpm.

As experimental studies have shown, providing dynamic balancing of the rotor at speeds above 15,000 rpm is a complex (time-consuming) technical task. In addition, it is not economically feasible, since at such speeds even slight wear of the rotor blades by particles transiently leads to a serious imbalance and damage to the rotor shaft.

The invention is illustrated by drawings.

Figure 1 - diagram of a device that implements the proposed method.

Figure 2 - extrapolated curve of the bulk density of the powder of Al ₂ O ₃ depending on the average particle size in the fraction.

Figure 3 - dependence of the gas parameters (T ₁ , ρ ₁ ) at the exit of the nozzle depending on the working pressure (P _slave ) during adiabatic expansion.

Figure 4 is a photograph with an optical microscope of Al ₂ O ₃ powder obtained by electric explosion. This powder consists of agglomerates of various sizes.

Figure 5 is a differential curve of the volumetric (mass) distribution of particles of the initial Al ₂ O ₃ powder obtained by electric explosion. (The initial particle size distribution of the powder was measured on a Mastersizer 2000 laser diffractometer, on which the sample is subjected to preliminary ultrasonic disaggregation, which explains the smaller particle size in the histogram compared to the photograph).

6 - differential and integral curves of volumetric (mass) distribution of particles in the fine fraction isolated by the claimed method (without heating the working gas) from Al ₂ O ₃ powder obtained by electric explosion after processing by the proposed method.

7 - differential and integral curves of the volume (mass) distribution of particles in the fine fraction isolated by the claimed method (with heating the working gas) from Al ₂ O ₃ powder obtained by electric explosion.

Fig - differential and integral curves of volumetric (mass) distribution of particles in the coarse residue selected by the claimed method from powder Al ₂ O ₃ obtained by electric explosion.

Fig.9 is a photograph from an electron microscope in transmitted light with a magnification of 58,000 times selected fine (nanoscale) fraction of powder Al ₂ O ₃ obtained by the electric explosion method isolated by the claimed method. Corresponds to the size distribution in Fig.7.

Figure 10 - the ratio of the parameters of particle size distribution (δ ₁₀ , δ ₅₀ , δ ₉₀ ) for the starting material, for the coarse residue, which is mainly particles of a fragmented form, and for the fine fraction obtained without heating the working gas and heating the working gas. The parameters of the fractions were measured using a Mastersizer 2000 laser diffractometer.

11 is a photograph from an electron microscope in transmitted light at a magnification of 4800 times the initial Al ₂ O ₃ powder obtained by evaporation of a solid target by a repetitively pulsed CO ₂ laser. (A particle size of approximately 4 × 7 μm is observed).

Fig - photograph with an electron microscope in transmitted light with a magnification of 10,000 times the source powder of Al ₂ O ₃ obtained by evaporation of a solid target pulse-periodic CO ₂ laser. (A particle size of about 1.5-2 microns is observed).

Fig. 13 is a photograph from an electron microscope in transmitted light at a magnification of 58,000 times the fine (nanoscale) fraction isolated by the proposed method from Al ₂ O ₃ powder obtained by evaporation of a solid target by a pulsed-periodic CO ₂ laser.

Fig - ratio of the parameters of particle size distribution (δ ₁₀ , δ ₅₀ , δ ₉₀ ) for the fine fraction, for the middle fraction, for the coarse residue after grinding and classification by the proposed method of powder Al ₂ O ₃ obtained by evaporation of the target by pulse-periodic CO ₂ - with a laser. The parameters of the fractions were measured using a Mastersizer 2000 laser diffractometer.

Fig - differential and integral curves of volumetric (mass) distribution of particles of a fine fraction (nanoscale) fraction of Al ₂ O ₃ powder obtained by evaporation of the target by a pulse-periodic CO ₂ laser.

The method is carried out in the following sequence.

Through the raw material loading means 19 (Fig. 1) and the loading nozzle 2, the hopper 1 is filled with the original powder material 21, compressed gas is supplied through the nozzle 3 of the gas reducer 4 at a pressure of 4 ÷ 6 kg / cm ² and a temperature of 90 ÷ 100 ° С, as a result whereby the powdery material is dispersed and dried. In this case, particles of the aforementioned size (less than 10-15 microns) are effectively crushed when interacting with each other in microvortices formed during the expansion of a gas jet, and are captured by an upward flow of gas. The magnitude of the working pressure is monitored by a sensor 13 placed in the gas reducer 4, and a sensor 12.

A gas stream with particles is fed through a pipe 5 to the centrifugal force zone formed by the rotation of the rotor 8 of the centrifugal classifier 7. The coarse fraction separated by the classifier 7 is returned to the bulk layer 21. The fine fraction passing through the classifier 7 is discharged through the pipe 9, 10 and trapped in dust removal system 20 (cyclone + filter).

The moisture content of the working gas at the inlet to the hopper 1 is continuously measured by the sensor 15 and after it is exited from the fine filter 20 by the sensor 18, the value of the centrifugal acceleration created by the rotor 8 of the classifier 7 is set within (8.5 ÷ 12) · 10 ⁴ m / s ² , continuously measure the parameter of the dispersed composition δ ₅₀ in the stream exiting the center of the centrifugal force using the sensor 17, and the volume concentration of particles σ _h in this stream using the capacitive sensor 16. If the volume pulsations of the volume concentration σ _{h are} greater than a given range, then centrifugal acceleration is increased to (12 ÷ 16) · 10 ⁴ m / s ² .

After reducing pulsations σ _h to a predetermined range and equalizing the moisture content at the inlet and outlet of the gas stream (m _in = m _out ), the working gas heater 14 is turned off. In case of repeated violation of this equality, it is switched on again.

Increase the working pressure at the nozzle 3 to 6 ÷ 8 kg / cm ² , while when exceeding δ _{50 a} predetermined value, the amount of material entering the gas stream is gradually reduced, lowering the vertical pipe 5 with a movable mechanism 6 to the level of 80 ÷ 85% of the original heights; with a further increase in δ _50, an increase in the rotational speed of the rotor 8 provides the value of centrifugal acceleration (16 ÷ 19) · 10 ⁴ m / s ² . And finally, with a significant decrease in σ _h , for example, 1.5-2 times or with a further increase in δ _{50, the} process is stopped, the coarse residue is removed from hopper 1 and repeated on a new portion of the starting material.

The control unit 11 monitors the operation of the device. It can be made in the form of a computer with software that allows you to synchronously monitor the process of loading raw materials into the cylindrical hopper 1, the centrifugal acceleration created by the rotor 8 of the centrifugal classifier 7, the working gas pressure at the nozzle 4, and the process of recirculation of the material on the pulsation scale bulk concentration at the outlet - σ _h, the process of drying the processed material of _Rin m, m _O, the efficiency of the grinding process level σ _h, and the labeling process largest ranulometricheskogo composition fines - δ and ₅₀ control the amount of centrifugal acceleration in the range (8,5 ÷ 19) × 10 ⁴ m / s ^2, and the operating pressure - in the range of 4 ÷ 8 kg / cm ^2.

Currently, many ultrafine powders obtained by both industrial and laboratory methods have a wide particle size distribution and large inclusions from the destruction of the surface of equipment elements. As the most illustrative example of the effectiveness of the proposed method, the results of our work with customers are given. The application of the claimed method was tested on ultrafine alumina powders obtained by two modern methods of creating nanosized powders: the first material was obtained by the method of electric explosion of wires, the second material was obtained by the evaporation of a solid target by a pulsed-periodic CO ₂ laser (Institute of Electrophysics, Ural Division of the Russian Academy of Sciences, Yekaterinburg).

Figure 4, 11, 12 presents photographs of the particle fields of the starting powders. As can be seen from the photographs, in both cases there are agglomerates formed by foreign inclusions from fragments of electrodes (electric explosion method) or fragments of a target (laser evaporation method) and ultrafine particles formed. After processing the proposed method, the product fractions acquired a satisfactory purity from impurities (see Fig.9, 13).

As an example, the material balance for the second powder is presented. The mass of the fine fraction was approximately 33%, with a grain composition: δ ₅₀ ≈0.7 μm, δ ₉₀ ≈1.2 μm; the mass of the middle fraction was approximately 47%, the granular composition: δ ₅₀ ≈ 1.2 μm, δ ₉₀ ≈2.3 μm; the mass of the coarse residue was approximately 20%, granular composition: δ ₅₀ = 2.8 μm; δ ₉₀ ≈ 9.6 μm. The measurements were carried out by Insitek Voyager mobile complex for particle size analysis (Malvern, UK). Refinement of the granulometry of the fine (nanoscale) fraction was carried out on a special device for measuring the zeta potential and the size of nanoparticles - "Zetasizer Nano ZS" (Fig. 15). This curve is consistent with the particle sizes of this fraction shown in the photograph - Fig.13.

As can be seen from the obtained results, this method made it possible to isolate narrow fractions of nanoparticles. Thus, the proposed technical solution allows you to effectively grind (or disperse) ultrafine powders in a dry way and to isolate narrow fractions with an average particle size of less than 1 micron by air-centrifugal classification.

Information sources

1. Prospectus of the company "Buehler MicroMedia", website http://www.buhlergroup.com/global/downloads/P20626_Buehler_MicroMedia_IH_en_low.pdf

2. Hosokawa Micron Ltd Corporation Brochure, http://www.alpinehosokawa.com/02powder/prodpro/machines/machine.asp7sqlTab le = machine_dr_rsig2 & machineID = 2

3. Powdered activated aluminum, a method for its preparation, a device for implementing the method and a device control unit, patent RU 2371284 C2 dated 10.27.2009, IPC B22F 1/00 (prototype).

Claims (1)

A method for producing ultrafine powders with a narrow fractional composition, including the formation of a bulk layer of the initial powdery material, its movement by an upward gas flow into the centrifugal force action zone created by the centrifugal rotor, recirculation of a part of the material by returning coarse particles from the centrifugal force action zone to the bulk layer and removal fine particles with a gas stream from the center of the centrifugal force, characterized in that the original powdered material having an average particle size (δ ₅₀₎ of less than 2.1 microns, the basic weight of the particles (δ ₉₀₎ of less than 10-15 microns, wherein the process is carried out in two stages, for which the working gas is first heated to a temperature of 90 ÷ 100 ° C, the material dispersed and dried in a stream of compressed gas at a pressure of 4 ÷ 6 kg / cm ² , continuously measure the moisture content in the gas stream at the inlet to the working area and at the outlet after particles are extracted from it, while the value of the centrifugal acceleration created by the classifier rotor is set within (8.5 ÷ 12) · 10 ⁴ m / s ² , continuously determined the average particle size δ ₅₀ in the flow leaving the center of the centrifugal force zone is consumed using, for example, a laser diffractometer, and the volume concentration of particles (σ _h ) in this stream by measuring, for example, its dielectric constant, the volume pulsations σ _h by increasing centrifugal acceleration to (12 ÷ 16) · 10 ⁴ m / s ² , after equalizing the moisture content at the inlet and outlet of the gas stream, the heating of the working gas is turned off and the second stage begins: increase the working pressure to 6 ÷ 8 kg / cm ² , moreover when δ ₅₀ exceeds the specified value, they begin to simultaneously reduce the amount of material entering the gas stream by gradually decreasing the height of the particle entry zone into it to the level of 80–85% of the initial height; with a further increase in δ ₅₀ , the value of centrifugal acceleration is increased to (16–19 ) · 10 ⁴ m / s ² , and with a further increase in δ ₅₀ or a significant decrease in σ _h , for example, the process is stopped 1.5-2 times, where δ ₅₀ is the size less than which the mass fraction of particles is 50% (average size particles), δ ₉₀ - the size below which m ssovaya proportion of particles is 90%, σ _h - the volume concentration of the particles in the gas stream downstream of the labeling zone.

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