RU2541058C1 - Method of obtaining ultradisperse silicon nitride powder - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method is based on method of self-propagating high-temperature synthesis (SHS - process), in which as charge applied is mixture of powders of preliminarily activated nitride-forming component with average size of particles less than 5 mcm and silicon nitride as diluent with average size of particles less than 1 mcm and width of distribution not higher than 2 in quantity smaller than 25% of the total weight, as nitride-forming component applied is ferrosilicon, after sinter crushing the following milling (disaggregation) of powder is carried out in a jet of pressure gas, supplied in bulk layer, and powder-gas stream is recirculated both inside working volume, separating large particles from working gas by inertial and air-centrifugal separation, and outside it, ejecting separated from stream by cyclone separator small particles and returning them into working volume, and simultaneously above bulk layer and in zone of separation of particles of cyclone separator area of magnetic field is created, into which powder-gas stream is supplied and contact of particles with surface of magnet is realised, and during the entire process voltage, induced by iron particles in powder-gas stream, output from bulk layer, is measured, and, when it reaches minimal (specified) value, the process is continued for not less than three recirculations of material, remaining in working volume, then ejection is stopped, and product is classified into fractions.

EFFECT: obtaining of silicon nitride powders with ultradisperse composition from industrial grades of ferrosilicon with considerably reduced costs for chemical purification from iron admixtures.

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Description

The present invention relates to powder metallurgy and can be used to obtain highly pure ultrafine powder of silicon nitride and the manufacture of ceramic products from it, satisfying modern technical requirements for hardness, heat resistance and resistance in aggressive chemical environments.

The main industrial methods for the synthesis of silicon nitride are: "furnace" (PS), plasma-chemical (PCS), gas-phase and the method of self-propagating high-temperature synthesis (SHS process).

Currently, to obtain silicon nitride, the SHS process is widely used, which is most optimal in terms of energy consumption, preservation of chemical purity and simplicity of hardware design / 1 /. However, due to the fact that the temperature in the combustion front is high in the SHS process, local combustion of the reagent and coagulation of particles occur when the combustion is uneven due to the inhomogeneous particle size distribution of the charge, and after passing through the combustion front and the formation of Si ₃ N ₄ in a very hot material sintering of Si ₃ N ₄ particles begins, which leads to the formation of high-strength large aggregates. Therefore, for grinding and preserving the purity of the product, significant energy costs and expensive equipment with special lining of surfaces are required.

These disadvantages are eliminated if you use a method of producing a mixture for the synthesis of silicon nitride / 2 /, in which powders of a pre-activated nitride-forming element (silicon) with an average particle size of less than 5 microns and a diluent (silicon nitride) with an average particle size of less than 1 micron and a width are mixed distribution function no more than 2, in the amount of 10-25% of the total mass. As a result, after the SHS process, an easily crushed cake is obtained, which provides the conditions for further production of ultrafine silicon nitride powder when it is disaggregated without contamination with milling products.

However, this method has the following disadvantages.

So, to reduce the cost of the final product, ferrosilicon can be used as the initial nitride-forming element, the cost of which, for example, FS-75 according to the data of RZM-Metallurgy LLC (Chelyabinsk), published on the website www.uralforum.ru (GOST 1415-93) , amounts to 69 thousand rubles per 1 ton, while the cost of silicon crystalline grade Kp1 (GOST 2169-69) is 103 thousand rubles / ton, but the product will be contaminated with an admixture of iron, which will require additional expensive cleaning operations: in industry for purification of bulk materials from jelly inclusions it is customary to use magnetic separation, followed by etching with a solution of 15-30% hydrochloric acid (then the insoluble precipitate is washed in water and dried).

However, as our experiments have shown, the separation of iron particles by magnetic separation from various ultrafine powders directly, passing through magnetic separators existing in industry, is extremely inefficient due to the significant cohesion of such powders (the effect of cohesion), which prevents the exit of iron particles from the layer to the magnet.

In addition, as can be seen from the photograph (figure 2), the product also contains aggregates of crystals of silicon nitride with internal inclusions from molten iron, which also impairs the quality of the final product and requires additional etching with hydrochloric acid. As a result, a serious problem after etching is the formation of a large amount of harmful waste, which requires significant additional costs for their disposal.

The technical result of the claimed invention is to obtain powders of silicon nitride (Si ₃ N ₄ ) with an ultrafine composition from industrial grades of ferrosilicon with significantly lower costs for chemical cleaning of iron impurities.

The specified technical result is achieved as follows.

To solve this problem, the mixture is obtained by mixing powders of a pre-activated nitride-forming component with an average particle size of less than 5 microns and a diluent - silicon nitride with an average particle size of less than 1 micron and a distribution width of not more than 2 in an amount of less than 25% of the total mass, as nitride-forming component use ferrosilicon, carry out the nitriding of the mixture in the SHS process, after crushing the cake into powder to a particle size (aggregate) of less than 0.1 mm, carry out further measurements lchenie (disaggregation) in the compressed gas stream fed to the bulk powder bed at an excess pressure of 4 ÷ 6 kg / cm ^2, and after reduction an average particle size in the bulk layer up to 15 ÷ 20 microns operating pressure is increased to 7 ÷ 8 kg / cm ² while the dust and gas stream is recycled both inside the working volume, returning large particles to the bulk layer due to inertial and air-centrifugal separation, and outside it, ejecting small particles extracted from the stream by a cyclone separator and returning them to the working volume, while simultaneously above bulk the layer and in the separation zone of the particles of the cyclone separator create a magnetic field, into which the dust and gas stream is supplied and the particles are contacted with the surface of the magnet, and during the whole process the voltage induced by iron particles in the dust and gas stream leaving the bulk layer is measured and, when when he reaches the minimum (predetermined) value, the process is continued for at least three more recirculations of the material remaining in the working volume, then the ejection is stopped and the product is classified fractionated.

In order for the product of the SHS process (sinter) to be easily crushed and ground into an ultrafine powder (without contamination with milling products), it is necessary to nitrate using a highly homogeneous mixture of an activated nitride-forming component with a main particle size of less than 5 microns and a diluent powder (silicon nitride) with an average size particles less than 1 μm distribution width of not more than 2 and in an amount of less than 25% of the total mass, which excludes local melts in the reaction zone during the SHS process.

To ensure low production costs during the SHS process, as the starting material in the proposed method, it is necessary to use industrially produced ferrosilicon with a silicon content of 50 ÷ 95%. Alloys containing less than 50% are difficult to grind due to the high iron content. An alloy containing more than 95% is impractical to use because of its high cost.

The flow of the jet into the bulk layer creates a zone with intense vortex movement of the working gas, which captures particles, accelerates to high speeds (depending on the particle size from several meters per second to hundreds) and provides fine dispersion of particles to micron and submicron levels. In this jet self-grinding particles eliminates grinding and the appearance of additional impurities.

The initial coarse powder after preliminary crushing consists mainly of rather fragile large aggregates, the particles of which have a large number of weak internal bonds, and therefore it is economically feasible at the beginning to disaggregate at a lower working pressure - about 4 ÷ 6 kg / cm ² , and then - after reducing the size of the aggregates to the level of 15 ÷ 20 microns - their strength increases significantly and it is necessary to strengthen the mechanism of self-abrasion of particles, that is, increase the pressure to 6 ÷ 8 kg / cm ² , while the flow of working gas can be l is set by the same or slightly increased (by no more than 10–45%) so as not to significantly change the particle separation regimes. Thus, at the beginning, large aggregates are destroyed mainly when they hit a bump surface, and then smaller ones, which are more difficult to fracture, are small when impacted in microvortices, because in opposite directions the relative particle velocity reaches hundreds of meters per second (about 500 m / s ), which allows one to obtain particles of an ultrafine level (<< 1 μm).

The dust and gas stream is recirculated, locking large (underfinished) aggregates of particles in the working volume using inertial and air-centrifugal separation (using, for example, a conical reflector and a classifier with a high-speed blade rotor), and small ones (micron and submicron) are isolated from the stream on a cyclone separator and using ejection is returned to the working volume. Moreover, at the same time, the entire dust and gas stream from dispersed (disaggregated) particles is passed through the magnetic field, where they provide tangential contact of the particles with the surface of the magnet. As a result of this, iron particles are released on the surface of the magnet. In this case, a constant “bombardment” of the layer adhering to the magnet by a dust and gas stream occurs in the internal volume of the grinding chamber, as a result of which aggregates of particles of silicon nitride crystals with iron particles, which are more weakly susceptible to magnetic field attraction, are knocked off the magnet surface and returned to the grinding zone. Such small aggregates, which could pass through the air-centrifugal classification, are captured in the outer loop by a cyclone separator and are returned to the grinding volume again by ejection.

The magnetic separation process is monitored by measuring the voltage induced by iron particles contained in the dust and gas stream leaving the bulk layer. The induced voltage is proportional to the number (concentration) of iron particles, therefore, due to the strongly expressed magnetic properties of iron, the process control is highly efficient. After the measured values reach the specified (minimum) level, the process is continued for at least three material recycling in the working volume in order to grind (disaggregate) the product purified from iron as much as possible. Then the ejection is stopped, and the product is classified into fractions. The amount of recirculation is determined by the requirements for the final particle size distribution according to particle size, distribution width.

The invention is illustrated by drawings.

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

Figure 2 is a photograph of the product of the SHS process "Fe-Si-N".

Figure 3 - analysis of the results of x-ray examination of the material "Fe-Si-N": the initial and after magnetic separation and air-centrifugal classification.

Figure 4 is a photograph of the fraction of particles containing iron, isolated on magnet in the process of disaggregation and magnetic separation of the product of the SHS process in the working volume of the installation.

Figure 5 is a photograph of a large fraction of particles remaining in the working volume of the installation after disaggregation and magnetic separation of the product of the SHS process.

6 is a photograph of the average fraction of particles isolated in a cyclone separator after disaggregation and magnetic separation of the product of the SHS process.

Fig. 7 is a photograph of the fine particles separated on the fine filter of exhaust gas (air).

Figs. 8-12 are histograms of the particle size distribution of Fe-Si-N samples: initial powder and coarse, medium and fine fractions.

A device that implements the claimed method is presented in figure 1.

The device consists of a vertically mounted cylindrical-hopper 1, a circulation pipe 2 located along the central axis of the hopper 1, a nozzle 3 located in the lower conical part of the hopper 1 coaxially with the pipe 2, a reflector 4, a blade rotor for air-centrifugal classification 5, mounted on an electric drive coaxial to the hopper 1, cyclone separator 6, the forbunker of which 8 is equipped with an ejector 7 connected to the working volume of the hopper 1, the accumulators of the finished material 9, the bag filter 10 for fine cleaning spent th gas, the control panel 11, the magnetic inductive sensor 12. Moreover, in the cylindrical parts of the hopper 1 and forbunker 8 are installed annular zones of magnetic elements 13.

The method is as follows. After nitriding by the SHS process, a mixture obtained on the basis of ferrosilicon, for example, FS-75, by the above method and crushing the Fe-Si-N cake, a powder with a particle size of less than 1 mm is loaded into hopper 1. Then through nozzle 3 to the bottom part of the bulk layer of this powder material is supplied with a working gas at a pressure of 4 ÷ 6 kg / cm ² , as a result of which particles captured by the jet after exiting the circulation pipe 2 collide with the reflector 4 and partially collapse. Large particles return to the bulk layer, while smaller particles enter the zone of air-centrifugal classification of the blade rotor 5. Separated on the reflector 4 and on the blade rotor 5, large particles are returned to grinding, and those passing through the rotor of the classifier 5 are fed to the cyclone separator 6. Separated by particles enter the hopper 8, and then the concentrated stream of larger particles with the help of the ejector 7 is returned to the hopper 1, and the dust and gas stream containing ultrafine particles (submicron and nanoscale s), is fed to the bag filter 10. On the control panel 11, the required speed of rotation of the rotor of the classifier 5 is set, the pressure of the gas supplied to the grinding nozzle 3, the pressure of the gas supplied to the bearing assembly of the rotor of the classifier 5, the pressure of the gas supplied to the zone between the rotating the rotor of the classifier 5 and the surface of the hopper 1, and the pressure of the gas supplied to the ejector 7, and also controls the change in voltage at the sensor 12, induced by iron particles during magnetic separation.

The consumption of compressed gas is determined by the operational characteristics of the blade rotor necessary to ensure its optimal operation, and depends on its size, rotation speed and working concentration of particles in the classification zone. At a given pressure, the flow rate of the working gas is determined by the diameter of the nozzle 3. The degree of destruction of the particles (process speed) depends on the speed of their collision with the surface of the reflector 4, which is determined by the gas velocity at the outlet of the circulation pipe 2, that is, when the selected flow rate of the working gas is set by the diameter of the pipe 2. For grinding aggregates of particles of readily destructible cake (obtained in CB synthesis from a mixture prepared according to the claimed method) with a size of up to 100 μm, the flow rate of the gas stream to 4 trazhatel about 20 ÷ 50 m / s.

Since with a decrease in the particle size of aggregates to a level of 15–20 μm, grinding on reflector 4 practically ceases (although the speed of impact interaction of particles in the jet increases, but it is not sufficient for effective destruction of the aggregates), further disaggregation is carried out by self-abrasion of particles in a highly turbulent - by increasing the pressure to 7 ÷ 8 kg / cm ² - a stream of working gas. As mentioned earlier, the speed of impact interaction of particles at this size is hundreds of meters per second.

In the upper part of the hopper 1 above the bulk layer, the dust and gas flow is directed to the periphery by spinning with a blade rotor and passes tangentially along the wall of the hopper - in the field of action of the magnetic field, as a result of which iron particles are attracted to the surface of the magnetic elements 13, that is, removed from the product. A similar picture is observed in the forbunker 8: iron particles are trapped on the surface of the magnetic elements 13, and the remaining part of the material, including the small aggregates that have not been destroyed, is fed through the ejector 7 to the grinding mill in the hopper 1.

After reaching the voltage measured by the sensor 12 of a predetermined (minimum) level, the process is continued for at least three recirculations of the material in the hopper 1, and then the ejector 7 is turned off and the product is finally classified into fractions according to particle size. The amount of recirculation is determined by the product requirements: average particle size, distribution width.

The time of one recirculation is determined as follows:

t = M _zagr / G _mat = M _zagr / (µ × G _gas ) [hour], where

M _zag - the mass of the bulk layer [kg];

With _mat - material consumption through the circulation pipe [kg / h];

G _gas - flow rate of compressed gas through the nozzle [kg / h];

µ = (d ² _mp / d ² _c ) -1 is the ejection coefficient;

d _Tr , d _c , - the diameters of the circulation pipe 2 and nozzle 3.

To further improve the composition of the product by the content of iron particles, before the final purification of the exhaust gas and the collection of the ultrafine fraction, you can use either an additional cyclone separator 6 or a special unit 14, in which: a) due to the construction elements of the honicomb type ∗, the flow flows more laminarly , b) due to the expanded bore of the node 14, the flow rate decreases and - c) due to the implementation of the node 14 in the form of a flat-shaped pipe, the poles of the magnet are brought together as close as possible. All this increases the likelihood of precipitation of even ultrafine particles of iron from the stream, the high “windage” of which made it possible to pass through previous zones of magnetic separation.

As can be seen from the photograph of the powder (figure 2) of the initial product of the SHS process "Fe-Si-N", almost all particles are aggregates of silicon nitride crystals with inclusions from iron melts.

As can be seen from the photograph (figure 4), the dominant fraction of particles separated on the magnet in the hopper 1 in the process of disaggregation is iron particles, except for a small part, which contains both iron and silicon nitride crystals.

From the photograph of the particles remaining in hopper 1 (Fig. 5) after disaggregation and magnetic separation, it is seen that basically all the particles are pure Si ₃ N ₄ crystals, although there is an insignificant amount of large Si ₃ N ₄ crystallites with inclusions from iron particles , and particles of unreacted silicon.

In the photographs of particles (FIGS. 6, 7) extracted after disaggregation and magnetic separation in a cyclone separator (middle fraction), and on a fine filter for exhaust gas (fine fraction), the number of iron particles is even less.

To quantify the iron content in figure 3 presents the analysis of the results of x-ray examination of the material "Fe-Si-N" obtained by the SHS process from ferrosilicon grade "FS-75" - (table 1), and its samples after disaggregation, magnetic separation and air-centrifugal classification: coarse fraction (material remaining in the working case) - table 2, middle fraction (material recovered in the cyclone separator) - table 3, fine fraction (material recovered in the filter) - table 4. As shown in the table 1, iron content in the initial (synthesized) “Fe-Si-N” it was 7.09%, and after disaggregation with magnetic separation and subsequent air-centrifugal classification (see tables 2-4) it was respectively: 3.7% in large fraction (mass the fraction of which was ~ 33%), 1.73% in the cyclone separator (whose mass fraction is ~ 62%) and 0.74% of iron contains material separated in the filter (its mass fraction is ~ 5%).

From the presented histograms (Figs. 8-10) it follows that the initial Fe-Si-N powder (Fig. 8) and the fractions of silicon nitride obtained from it after magnetic separation (Figs. 9-11) differ significantly in particle size ( see, for example, by the parameter D (0.50), which characterizes the average particle size in a fraction), which confirms the effectiveness of this method also for grinding and separation of fractions of the obtained silicon nitride.

Figure 10 shows the particle size distribution of the silicon nitride powder separated after the magnetic separation of iron particles in the cyclone separator upon termination of ejection and return of the material after the first recirculation, and Fig. 11 after the third recirculation. A significant improvement is seen both in the average particle size - D (0.50), and in the content of large particles - D (0.90), D (0.100).

The most satisfactory quality of the product, both in terms of iron content and dispersed composition, has a fraction allocated to the fine filter (see Fig. 12).

Thus, the advantage of this method is the possibility of obtaining a silicon nitride powder with an ultrafine composition at an extremely low iron content, while using not expensive pure silicon as the raw material, but ferrosilicon produced by the industry. The combination of newly introduced features allowed us to solve all the problems encountered in the production of silicon nitride powders by known methods, to create a waste-free, cost-effective, energy-saving and environmentally friendly process for producing ultrafine silicon nitride.

∗ Honicomb - a straightening grating (honeycomb rectifier) for leveling the field of gas flow velocities in a wind tunnel or hydrochannel.

USED SOURCES

1. Merzhanov A.G. In the book: Solid-flame combustion, Chernogolovka city: ISMAN publishing house, 2000, 224 pp.

2. A method of producing a mixture for the synthesis of silicon nitride, patent RU 2465197 C2 dated 10.27.2012, IPC C01B 21/068, (prototype).

Claims (1)

A method of producing an ultrafine powder of silicon nitride, including the preparation of a mixture by mixing powders of a pre-activated nitride-forming component with an average particle size of less than 5 microns and a diluent of silicon nitride with an average particle size of less than 1 micron and a distribution width of not more than 2 in an amount of less than 25% of the total mass characterized in that the resulting mixture, containing as a nitride-forming component of ferrosilicon, is subjected to nitriding in the SHS process, after crushing the cake into powder, yuschy basic weight of the particles (aggregates) in a size less than 0.1 mm, further grinding is carried out (disaggregation) in the compressed gas stream fed to the bulk powder bed at an excess pressure of 4 ÷ 6 kg / cm ^2, and after reduction an average particle size in bulk layer to 15 ÷ 20 μm, the working pressure is increased to 7 ÷ 8 kg / cm ² , while the dust and gas stream is recycled both inside the working volume, returning large particles to the bulk layer due to inertial and air-centrifugal separation, and outside it, ejecting isolated from the cyclone separation stream small particles and returning them to the working volume, while simultaneously above the bulk layer and in the separation zone of the particles of the cyclone separator create a magnetic field, in which a dust and gas stream is supplied and the particles are contacted with the magnet surface, and the voltage is measured throughout the process, induced by iron particles in the dust and gas stream leaving the bulk layer and, when it reaches the minimum (predetermined) value, the process continues for at least three more material recirculations, about having become in the working volume, then the ejection is stopped, and the product is classified into fractions.

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