FI75673B - APPARAT FOER MAETNING AV PARTIKELSTORLEKSFOERDELNINGEN HOS PULVER. - Google Patents

Description

! 75673

Device for measuring the particle size distribution of powders

The present invention relates to the analysis of powdered substances and more particularly to an apparatus for measuring the particle size distribution of powders.

The invention can be used to analyze the particle size distribution in micropowders with a size range of 63-0.5 μm in the pharmaceutical, perfume and chemical industries as well as in the production and use of products such as talc, kaolin, chalk, quartz, mica, porcelain, cement and abrasives. .

Devices for studying the physical properties of industrial aerosols and various powdery substances are known from the prior art, the operating principle of which is based on the initial electrification of particles followed by the processing of electrical signals obtained in particle charge measurement (cf. e.g. USSR Inventor. No. 372 483 / 02, 1973, U.S. Patent No. 3,718,029, Classes 73-28, 1973).

The device comprises a gas duct for producing a stream of suspended particles, an electric accumulator consisting of an electrode capable of producing a corona discharge, giving the particles a charge proportional to their surface area, and a meter for measuring the total particle charge.

However, such a device is mainly intended to measure the concentration of particles in industrial aerosols or powders per unit volume, whereas it is very difficult to use the device to analyze the particle size distribution (dispersion composition) of such substances.

In addition, the analysis of the particle size distribution of the powder in such devices takes from several hours to several days, which greatly complicates the production process that requires such analysis.

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Also known in the art is a device for measuring the particle size distribution of powders, comprising a device for converting an analyte powder into a gaseous stream of discrete particles (otherwise known as a powder sample nebulizer), comprising a hopper housing and an electromagnet on the outer surface of the housing. Said housing is arranged in series after the funnel with a net-based measuring cup connected to the main vibrator by a drawbar located inside the housing and connected to the main vibrator through a hole in the housing crank, an inlet funnel and a tube bent into a single helix.

Both the funnel and the tube are rigidly connected to a sensor with an annular brush on the outer surface, by means of which the anchor is forced tightly by means of a spring against the outer surface of the bottom of the housing. Said device also comprises an additional vibrator consisting of said anchor and a coil and an air blower connected to the interior of the housing and adapted to force air through said device.

The device also comprises a charge chamber connected in succession to the powder conversion device in communication with said device, adapted to give the particles a charge proportional to their surface area, a precipitation capacitor in which the charged powder particles are separated into size fractions, the number of charged particles a number of switching devices for reading information on the relative proportion of each size fraction from each sensor, an electrometric amplifier connected to said sensors via the switching device and intended to form a pulse train at its output characteristic of the particle size ratio of the powder to be analyzed. The known device also has an additional air blower for forcing air through the charging device and the precipitation condenser, a static charge neutralizing unit and high voltage sources for the charging chamber and the precipitation condenser (cf. e.g. USSR Inventor. No. 530 229, class G01N 159, class G01 .

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In a known device, a sample of the powder to be analyzed is placed in the measuring cup of the device which converts the powder into a gaseous particle stream through a feed hopper. The first sizing of the powder particles is performed by mechanically screening the powder sample through the mesh bottom of the measuring cup. This is achieved by applying an alternating voltage with a frequency of a few Hz to the winding of the main vibrator, whereby the measuring cup is made to oscillate by means of a drawbar. At the outlet of the measuring cup, the powder is carried in the area of the inlet funnel by the air flow forced by the air blower into the sample sprayer into a single-helix bent tube, where the final size classification of the powder fractions takes place by means of high speed blown air. To prevent powder particles from settling in the area of the hopper and inside the tube, an additional vibrator comprising an anchor and an anchor reel is used. When an alternating voltage with a frequency of 50 Hz is applied to the winding of the auxiliary vibrator coil, the anchor and the inlet funnel and the tube rigidly connected to it are periodically vibrated, the vibration frequency of the anchor being much lower than the measurement cup oscillation frequency.

In this way, the powder sample is converted at the outlet of the device into a gaseous stream formed by separate particles, which is fed into the charging chamber as an aerosol stream. The aerosol particles entering the chamber then receive an electrical charge when they are affected by a corona discharge with a unipolar space charge, so that particles of the same size receive similar charges. The stream of subsequently charged particles is fed to a precipitation capacitor, where the dispersed aerosol phase is separated by electrostatic field forces into size fractions which precipitate at different distances from the stream inlet to the precipitation capacitor. The sensors then detect signals, each corresponding to the charge amount of the particles in a given fraction, which are characteristic of the particle size. These signals are then fed to an electrometric amplifier, the output of which has a pulse train characteristic of the particle size distribution of the powder to be analyzed. Once the signals are registered, the dust generated is removed and the static charges are neutralized.

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One disadvantage of the above process is that due to the difference between the frequencies of the voltages applied to the main and auxiliary vibrator coils, some of the powder particles precipitate on the hopper and inside the tube, causing the accumulated powder layers to move during subsequent analysis. , which has a very high concentration, which in turn considerably reduces the accuracy of the apparatus used for the particle size distribution analysis of the powders.

On the other hand, the application of a voltage of the same frequency (a few kHz) to both the main and auxiliary vibrator coils causes the following negative phenomena: - first the anchor gets stuck due to spring slowness and possibly stops oscillating, and - the amount of powder used increases sharply.

In addition, the known device is characterized in that the screening of the powder through the network of the measuring cup takes a long time due to the precipitation of the powder on the network, which as a whole adversely affects the accuracy and usability of the particle size distribution of the powder to be analyzed.

The first and main object of the present invention is therefore to provide some improvements in the powder particle size distribution device, which are achieved by making some changes in the structure of the powder gaseous flow device so as to prevent powder deposition over the entire device, screening time and powder consumption. maintained at the previous level.

The object is achieved in that a device for measuring the particle size distribution of powders, comprising a device for converting the powder into a gaseous stream of discrete particles, comprising a housing with a hopper and a network-based measuring cup connected in series with an electromagnetic vibrator. a tube helically bent into a sex helix, wherein both the funnel and the tube are rigidly connected to an anchor having an annular brush and forced by a spring against the bottom of the housing, an air blower adapted to force air through the device, successively after the powder changing device the powder particles receive a charge proportional to their surface area, a precipitation capacitor in which the charged particles are separated into size fractions, a number of charged particle sensors equal to the particle size fractions and electrically connected to an electrometric amplifier, and additional air In a powder changing device according to the invention, a vibrator coil is arranged in a housing made of magnetic material, the bottom of which has a hollow cylinder arranged coaxially with a measuring cup connected to the vibrator made of magnetic material. a rod freely attached to the wall of the housing, the free end of which supports the measuring cup and adapted to close the opening in the bottom of the hollow cylinder by a current pulse applied to the coil of the vibrator coil, the flexible membrane having an opening for removing excess air from the housing along the direction of movement.

It is suitable that the device for converting the powder into a gaseous stream formed by separate particles is provided with a flat synchronous start unit for an air blower and an electromagnetic vibrator.

The device according to the invention for measuring the particle size of powders has the following advantages.

1. The device can be used to prevent the precipitation of powder in the area of the powder atomizer and thus to the removal of a powder with a very high concentration during repeated measurements.

6 75673 2. The device can be used to prevent the escape of very high concentration powder at the beginning of the measurement.

3. The device enables a significant reduction in the screening time of the powder in the device for converting the powder into a gaseous stream formed by individual particles.

Due to the above-mentioned advantageous properties, the analytical accuracy is much higher and the time spent on the analysis of the particle size distribution of the powders is considerably reduced.

The invention will become more apparent from the following detailed description of an embodiment with reference to the accompanying drawings, in which, according to the invention, Fig. 1 is a functional block diagram of a powder particle size distribution device, and Fig. 2 is a longitudinal sectional view of a powder.

The device for measuring the particle size distribution of powders comprises a device 1 for converting the powder to be analyzed into a gaseous stream formed by separate particles (Figure 1) and a chamber 2 connected to the device 1 for giving each powder particle a charge proportional to its surface area. The charging chamber 2 is of a conventional type and may comprise, for example, two corona discharge compartments and a charging compartment. The electrostatic precipitator 3 for air purification and air flow lamination is connected to the charging chamber 2 and communicates with a precipitation capacitor 4 comprising a number of charged particle sensors which can be electrically connected to the electrometric amplifier via switching devices 5. The number of said sensors is equal to the number of size fractions into which the stream of charged particles is separated. The device also comprises an air blower 7 for forcing air through the charging chamber 2, the electrostatic precipitator 3 and the precipitating capacitor 4, a device 8 neutralizing static charges in the precipitating condenser 4, high voltage sources 7 756793 9 and 10 charging chambers 2 for the precipitating condenser 11 12 a device for converting the powder into a gaseous stream formed by separate particles for a smooth synchronous start of the air blower 11 and the main vibrator.

The unit 12 is conventional in construction and may comprise e.g. a transistor and a capacitor connected to the base of the transistor. When the unit 12 is energized, its output voltage varies steplessly from zero to the saturation value. The time of the stepless increase of the output voltage depends on the form of the voltage used and is a function of the value of the capacitor capacitance. The device for converting the powder into a gaseous stream of discrete particles according to Figure 2 comprises a housing 13 made of magnetic material with a hollow cylinder 14 at the bottom, a coil of a main vibrator coil 15 housed in the housing and supplied with energy from a 300-500 Hz audio generator, and a rod 16 made of magnetic material, freely attached to the inner wall of the housing 13 and having a free end supporting a mesh-based measuring cup 17. The cup 17 is coaxially arranged with the hollow cylinder 14, while the rod 16 is intended to close an opening in the bottom of the hollow cylinder 14 15 by the applied voltage pulse. The housing 13 is tightly closed by a lid 18, in the middle of which there is a feed hopper 19 coaxial with the measuring cup 17 provided with a lid 20. An air blower, not shown in Fig. 2, communicates with the housing 13 via an inlet sleeve 21 in the wall of the housing 13.

The device also includes an anchor 22 mounted on an inlet funnel 23 and an associated tube 24 bent into a single helix. The funnel 23 and the tube 24 are arranged coaxially with the measuring cup 17.

The outer surface of the anchor 22 has an annular brush 25.

The device further comprises a flexible membrane 26 with an opening 27 through which excess air can escape from the housing 13. The anchor 22 β 75673 passes through a central opening in the flexible membrane 26, and the membrane is forced against the anchor brush 25 by means of a nut 28. The spring 29 forces the annular brush 25 of the anchor 22 tightly against the outer surface of the bottom of the housing 13.

The device also includes an additional vibrator comprising a housing 30 having a central internal metal protrusion 31 into which the holder 32 of the additional vibrator coil 33 is fitted.

The device for measuring the particle size distribution of powders operates as follows.

With the lid 20 (Fig. 2) removed, the powder sample to be analyzed is placed through the hopper 19 in the measuring cup 17 of the device 1 (Fig. 1). The feed hopper 19 (Fig. 2) is closed by the lid 20, after which a voltage is applied to the unit 12 (Fig. 1) which simultaneously causes a smooth start of the air blower 11 and energizes the main vibrator coil 15 (Fig. 2), thus preventing very high concentration powder leakage.

In the initial position, located inside the hollow cylinder 14, the anchor 22 is pressed tightly against the bottom of the housing 13 by the action of the spring 29. As soon as the pressure of the air flow reaches the housing 13 through the sleeve 21, the excess air generated in the housing 13 forces the anchor 22 downwards and some of the air can freely escape from the housing 13 through the opening 27 of the flexible membrane 26.

When a voltage pulse is applied to the winding of the main vibrator coil 15, the hollow cylinder 14 and the anchor 22 are magnetized, as a result of which the rod 16 made of magnetic material is pressed tightly against the bottom of the hollow cylinder 14, closing its inlet and restricting air entry into the hollow cylinder 14. As a result, the air pressure on the anchor 22 decreases, and the anchor is free to return to its initial position and rest with its annular brush 25 against the bottom of the housing 13 so that it fits snugly. The 9 75673 overflow of air generated inside the housing 13 facilitates the passage of powder particles through the network of the measuring cup 17 and prevents the powder from settling on the network, which considerably shortens the screening time.

When the voltage pulse applied to the winding of the main vibrator coil 15 has terminated, the steel bar 16 returns to its initial position. In this case, the regenerated overpressure of the air flow forces the anchor 22 downwards. When the next voltage pulse arrives, the whole cycle is repeated.

Both the measuring cup 17 and the anchor 22 are thus made to oscillate simultaneously. The result is as follows. First, significantly better conditions for powder access through the mesh of the measuring cup 17 are achieved due to the rod 16 striking the bottom of the hollow cylinder 14 and closing the bottom of the hollow cylinder 14 and the resulting air overpressure, significantly shortening the screening time and avoiding possible precipitation of powder particles. Second, much better conditions are achieved for the flow of powder through the hopper 23 and the coil 24 of the tube 24 due to the simultaneous vibration of the measuring cup 17 and the anchor 22.

The nebulizer converts the powder into an aerosol state just as mentioned above. The aerosol stream flowing out of the nebulizer of the device (Fig. 1) enters the charging chamber 2, which consists of two heel-discharge compartments and a charging compartment. In the charging chamber 2, the powder particles receive unipolar high-voltage charges proportional to their surface area (square of their diameter), in which the aerosol flow expands. In addition, an additional air flow is forced through the charging chamber 2 by means of an air fan 7. The particles thus charged are fed from the charging chamber 2 to an electrostatic precipitator 3, which is adapted to register the input coordinate of the charged particles to the precipitation capacitor 4. The capacitor is made into a tube of rectangular cross-section, in which two electrodes, namely a high-voltage and a precipitation electrode isolated from each other by dielectric spacers, generate an electric field. In the middle of the precipitation electrode there are measuring plates, the number of which corresponds to the number of size fractions to be measured.

The powder particles are separated in the precipitation capacitor 4 into size fractions, and particles of different size distribution precipitate in different areas of the precipitation electrode under the influence of electrostatic field forces. As a result, particles with a certain size fraction precipitate on each measuring plate. As long as the particles precipitate and their charges accumulate, the measuring plates are disconnected from the measuring circuit. When the splitter division process is completed, the measuring plates are connected via the switching device 5 to the input of the electrometric amplifier 6, as a result of which the output signal of the amplifier 6 turns out to be proportional to the total charge and thus the number of particles.

When the process of measuring the particle size distribution of the powdery substances is completed, the static charges of the capacitor 4 are neutralized by a charge neutralizing device 8 supplied by a high voltage source 9 using an air blower, while the atomizer area is cleaned using an additional vibrator 33.

Thanks to the better separation of the powder particles in terms of their size and the prevention of particle precipitation in the atomizer area, it is possible to: 1. significantly improve the measurement accuracy and shorten the analysis time, and 2. extend the range of the powder particle size distribution device.

Claims (3)

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