RU2406592C2 - Method and device to produce nanopowders using transformer plasmatron - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to plasma chemical synthesis using transformer plasmatron to produce high-quality nanopowders of diverse substances. In compliance with this method, initial stock is fed in liquid or vaporous state, or gaseous state in induction discharge or in plasma jet. Plasma chemical reaction is performed at 1.01×10⁵ Pa and higher, induction discharge frequency of 100-400 kHz and temperature of up to 10000 °C in atmosphere of reducing or oxidising gas. Produced powder is tempered and extracted from reaction zone. Proposed device comprises initial stock feed system, transformer plasmatron, nanopowders tempering system, water-cooled chamber, bag filter and nanopowders collector, all being interconnected.

EFFECT: high-quality nanopowders, efficiency, simple design, safe use.

5 cl, 1 dwg

Description

The invention relates to the plasmachemical industry, including plasmachemical synthesis using a transformer type plasmatron, which makes it possible to organize the production of metal powders (tungsten, molybdenum, vanadium, silicon, aluminum, silver, etc.) and other substances and their compounds (oxides, nitrides , borides and carbides) with sizes not exceeding 80 nm, suitable for use in various fields of industry and technology.

The problem of obtaining nanopowders of substances and their compounds is solved in world practice by a variety of methods (separately and in combinations). The plasma-chemical method for producing nanopowders has advantages over other methods for high productivity, and, in addition, it can be used to obtain a very wide range of materials, which cannot be obtained differently, in particular refractory metals and compounds (including hard alloys). The essence of the method is that if we put the material we need into the plasma stream, up to the most refractory ones (tungsten, tantalum, etc.), then physical and chemical transformations begin to occur with it. All this happens in an extremely short time - hundredths and even thousandths of a second. In this case, a sharp temperature difference occurs, up to 10 ⁵ -10 ⁷ degrees per second. As a result, the material cools and crystallizes very quickly. Moreover, it is possible to create such conditions that this crystallization will occur in the form of nanoparticles. As a result of the experiments, it turned out that in this way you can get the widest range of materials with particle sizes from 10 to 100 nm.

There are four different methods for producing powders by the plasma-chemical method [Plasma-chemical technology / V.D.Parkhomenko, P.I. Soroka et al. - Novosibirsk: Science. 1991 .-- 392 p. - (Low-temperature plasma. T.4)]: processing of gaseous compounds; processing liquid dispersed compounds; processing of solid particles suspended in a plasma stream; processing of solid particles in a stationary layer.

To heat the starting material in plasma-chemical methods, both arc (electrode) and electrodeless plasmatrons are used, in which energy is supplied to the discharge plasma without using electrodes. At the same time, electrodeless plasmatrons have a number of significant advantages over arc ones: a significantly longer service life; high purity of plasma and the resulting powder (not contaminated by erosion products of electrodes); the ability to work with aggressive chemicals (for example, the synthesis of nanopowders of oxides from chlorides).

Currently, two types of electrodeless induction plasmatrons are used to obtain nanopowders in plasma: high-frequency or microwave plasmatrons.

A known method and installation for producing nanopowders of metal oxides [Patent US 6994837, 2003.02.25, IPC C01G 23/047; B01J 23/00; C01G 25/02; C01G 27/02]. The installation includes a plasma reactor and a filter. The plasma reactor includes an HF plasmatron, a reactor chamber, consisting of a vertical cylindrical section and a tapering section, designed to collect the synthesized nanopowder of metal oxide. The working gas and metal powder are fed into the plasma torch chamber through two different inlets. A temperature is created in the plasma jet at which a reaction occurs with the formation of nanosized metal oxide particles, which are then rapidly cooled in the cooling zone of the reaction products by supplying a cooling gas (air, oxygen, nitrogen) through many flat nozzles located in the same plane below the upper edge of the vertical cylindrical section of the reactor chamber.

The advantages of this invention are: the use of the minimum discharge power of the formation of induction plasma, 30 kW, and the lowest possible frequency, 3 MHz; creation of intensive turbulization, 20-30%, in the zone of hardening of reaction products due to the peculiarities of the organization of injection of a gas stream.

The disadvantages include the fact that the invention is intended for the production of a limited number of nanopowders, namely nanopowders of metal oxides.

The main disadvantage of installations with RF induction plasmatrons is that they require the use of expensive high-frequency power supplies, as well as protective shields, since the discharge is generated at a current frequency of the megahertz range.

A known installation for producing nanodispersed powders [RF Patent 2311225, 2006.04.05, IPC B01J 19/00], consisting of interconnected plasmatrons, an input unit for gaseous, liquid or powder raw materials, a reactor, a filter, and an exhaust gas purification unit.

The reactor has certain ratios of geometric dimensions, connecting the output diameter of the plasma torch nozzle, the diameter and length of the reactor. The input of raw materials is taken outside the channel of the plasma flow into the reactor. The reactor surfaces on which the resulting nanopowder is deposited have special cleaners to remove the nanopowder. Removal of nanopowder deposits from different surfaces of the reactor is carried out in different collectors, thereby eliminating the possibility of ingress of powder cakes. The reactor allows you to increase the temperature of the plasma for the implementation of the processes, while preventing sintering of the resulting nanopowders, to obtain a nanopowder without contamination with coarse inclusions of cakes. In the installation, an arc, RF, microwave generator, or a combination thereof can be used.

The facility allows one to obtain a wide range of nanopowders of elements and their compounds from various types of raw materials, without overgrowing the plasma reactor with sintering nanopowders, and also without contaminating the nanopowder with coarse particulate inclusions. The proposed reactor design makes it possible to increase the plasma temperature for the implementation of processes requiring a high temperature for their implementation, while avoiding sintering of the resulting nanopowders.

The main disadvantage of the microwave plasmatron is the relatively low life of the magnetron, several thousand hours.

The general disadvantages of installations with microwave and high frequency plasmatrons include the formation of plasma torch chambers on heated quartz walls, as well as the walls of the reaction chambers of nanopowder cakes that violate the established operating mode of the equipment. This leads to a decrease in the yield of the target nanopowder and its contamination with coarse inclusions. Additional devices for cleaning the walls of the cakes significantly complicate the design, increase the energy consumption of installations. The disadvantages include the need for expensive high-frequency power sources. When using RF and microwave plasmatrons, it is necessary to install protective screens that absorb microwave radiation, because The discharge is generated at a current frequency in the megahertz and gigahertz ranges, respectively, which also complicates the design. HF and microwave plasmatrons operate at a pressure below atmospheric, at a plasma temperature of not more than 6000-8000 ° C, it is impossible to work with gases that require a high electric field to maintain a discharge, which significantly reduces their capabilities, in particular, in installations with plasmatrons of this type it is impossible to obtain nanopowders of various refractory metals, compounds and nanostructured hard alloys, it is impossible to obtain plasma of any molecular gases, including aggressive ones, such as chlorine, fluorine, oxygen.

Electrodeless plasmatrons operating on the principle of a low-frequency induction discharge of a transformer type can be successfully used in the plasma chemical industry for the synthesis of nanopowders, instead of high-frequency and microwave plasmatrons [I.M. Ulanov, M.V. Isupov and A.Yu. Litvinsev. Experimental study of transformer-coupled toroidal discharge in mercury vapor. J. Phys. D .: Appl. Phys. 40 (2007) 4561-4567, I.M. Ulanov, A.Yu. Litvintsev, P.A. Mishchenko, S.V. Krotov. Transformer plasmatron with a power of 50 kW, with a current frequency of 100 kHz. Materials of the All-Russian (with international participation) conference Physics of Low-Temperature Plasma-2007. Petrozavodsk. 2007.V.1. S.240-245].

A small number of transformer plasmatrons of various designs are described in the patents [A.S. 574100, 1976, Н05В 7/18, SU 957744, 1980, Н05В 7/18, Н05Н 1/24, RU 2094961, 1989, Н05В 7/18, RU 2022917, 1989, С01В 21/24, RU 2056702, 1990, Н05 B7 / 18, RU 2093459, 1995, СВВ 13/11, US 6150628, 2000, В23К 10/10] and scientific and technical literature [Ulanov I.M. Study of the possibility of creating transformer type plasmatrons. TVT, t.31, No. 4, 1993, I.M. Ulanov. 200-KW transformer Plasmachemical reactor. Int. Congress "Electromagn. Processing of material" Paris, May, 1997, Ulanov et al.]. A distinctive feature of transformer-type induction discharges is the low-frequency range of discharge generation. Efficient generation of transformer type induction discharges is possible in the kilohertz range of current frequencies. Due to this, the design of the power source is significantly cheaper, the task of matching the power source and the load (discharge) is simplified, the level of electromagnetic interference is reduced. The service life of a transformer plasma torch is tens of thousands of hours, and with the help of a transformer plasma torch plasma of any molecular gases, including aggressive gases (chlorine, fluorine, oxygen) can be obtained.

A transformer plasmatron is known [RF Patent RU 2094961, 1989, IPC Н05В 7/18], comprising a magnetic circuit with a primary winding and a closed water-cooled discharge chamber enclosing a magnetic circuit made of electrically insulated metal sections from one another, with gas inlet and plasma outlet assemblies located on opposite parts of the camera. The gas input unit is equipped with a swirl for vortex stabilization of the arc, and the magnetic circuit has a primary winding consisting of several turns of a conventional conductor.

In this plasmatron, due to the design features, a stable discharge is achieved in an inert and molecular gas medium at pressures up to atmospheric pressure, a decrease in the specific energy consumption and an increase in productivity.

In known transformer plasmatrons, the maximum value of the electric field is limited, since electrical breakdowns can occur between the individual metal sections of the plasma torch chamber.

As a prototype, the invention was selected “Installation and method for producing nanodispersed powders in microwave discharge plasma” [RF Patent No. 2252817, 2003.12.23, IPC B01J 19/08, B01J 19/12, Н05В 6/80, Н05Н 1/24, B22F 9 /fourteen]. The installation contains a technologically interconnected microwave generator, a microwave plasmatron, a gas flow former, a discharge chamber, a microwave absorber, a reaction chamber, a heat exchanger, a filter collector of the target product, and a device for introducing the starting reagents in a powder or vapor state. The installation further comprises a device for introducing into the reaction chamber the initial reagents in a liquid droplet state, containing a dispenser in the form of a cylinder, a piston with a gear-screw mechanism of an electric drive, regulating the speed of the piston, an evaporation chamber with a thermostatic housing to control the temperature inside the chamber, which connected to the reagent input unit in the vapor state, and to the reagent input unit in the liquid-droplet state, the input unit is made with 6-12 holes, from which are enclosed in the volume of the reaction chamber at an angle of 45-60 degrees to the axis of the chamber, which consists of at least two sections, the first of which is connected with the reagent inlet nodes to the discharge chamber, the plasma torch, with a microwave fan installed between it and the generator, and the lower flange, through subsequent sections connected to a heat exchanger, while the reaction chamber contains an electric water-cooled insert rotated by an electric motor and a metal scraper located along it to cut off the powder deposits of the target the product formed on the walls of the reaction chamber, and the heat exchanger is made of two water-cooled coaxial cylinders, the axes of which are perpendicular to the axis of the reaction chamber and installed with a gap for the passage of the cooled stream, and a knife located in the gap, rotating around the cylinder axis and cleaning the working surfaces of the cylinders from powder growth , a powder filter collector comprising inside a filter sleeve of chemically and thermally resistant material on which the powder of the target product is deposited and gas stream, at the top of the flange is connected to the heat exchanger and the bottom of the filter is provided with a device for periodic cleaning of the material by its deformation and the device with a valve for sealing the internal volume of the filter.

Advantages are the versatility of this installation, increasing productivity, increasing the duration of continuous operation, as well as increasing the yield of nanodispersed powder and expanding the technological capabilities of the method.

The disadvantage is the impossibility of increasing the plasma temperature, which is due to the design features of the reactor, namely the fact that the diameter of the plasma torch channel is practically the same as the diameter of the reactor. With increasing plasma temperature, the temperature of the deposited nanopowder layer will increase. This will lead to sintering of the nanopowder and loss of the required properties. The use of a technically sophisticated solution for cleaning the walls of the reactor from precipitated nanopowder due to the rotation of the internal water-cooled insert relative to the stationary scraper ensures the continuity of the installation, however, it complicates the design of the installation and increases its energy consumption. This installation also has all the disadvantages associated with the use of microwave plasmatrons.

A method for producing nanodispersed powders in a microwave discharge plasma using this setup involves introducing the starting reagents into the plasma gas stream of the reaction chamber, plasma-chemical synthesis of the reactants, cooling the target product and its separation from the reaction zone through the filter collector, while the starting reagents are introduced into the plasma gas stream having a mass-average temperature of 1200-3200 K, in any state of aggregation: vaporous, powdery, liquid droplet or in any combination thereof, reagents in the shock-like state is introduced in the form of an aerosol with a carrier gas into the reaction chamber through an input unit with an opening opening into the volume of the reaction chamber at an angle of 45-60 degrees to the axis of the chamber, the reagents in a liquid or vapor state are introduced into the reaction chamber through the input nodes, in the form ring collectors, the last of which is made with 6-12 holes opening into the volume of the reaction chamber at an angle of 45-60 degrees to the axis of the chamber, each of which is blown by a satellite gas stream through coaxial channels around the holes, with the consumption of initial reagents, plasma-forming gas, specific power of microwave radiation, the length of the reaction zone, allowing to obtain composite systems and individual substances with desired properties, chemical, phase composition and dispersion.

The method allows to obtain nanopowder of a number of substances with desired properties, chemical, phase composition, dispersion.

The disadvantages when using the known method adopted for the prototype are that the known device lacks technological methods and operating conditions, for example, the maximum possible plasma temperature is not more than 6000-8000 ° C, the inability to work at high electric field intensities, to obtain any plasma molecular gases, including aggressive gases (chlorine, fluorine, oxygen), providing a more efficient way to obtain nanowires of a wide range of substances, including refractory metals, compounds and nano structural hard alloys, with minimal cost during operation.

The problem to which the present invention is directed, is to develop an effective method for producing a wide range of metal nanopowders (tungsten, molybdenum, vanadium, silicon, aluminum, silver, etc.), other substances and their compounds (oxides, nitrides, borides and carbides) with with dimensions not exceeding 80 nm and installations for its implementation using a plasma torch of transformer type.

The problem is solved by using a transformer type plasmatron to obtain nanopowders. Moreover, the new use of a transformer type plasmatron known in the art is not analogous to its use from the technical field, since transformer plasmatrons are used in plasma chemistry and metallurgy for various plasma chemical processes, for example, the synthesis of nitrogen oxides, production of fluorocarbon compounds, etc. , have never been used to obtain nanopowders, and at the same time gives a significant new effect, which consists in the possibility of plasma-chemical synthesis significantly olee wide variety of substances and their compounds to obtain a high quality product, a large resource continuous operation (tens of thousands of hours) and its high productivity. Plant productivity is twice or more than all known plants for the production of nanopowders, depending on the products obtained.

Thermodynamic calculations show that the capacity of a 50 kW plant is: for silicon nanopowder - 5 kg / h;

for tungsten nanopowder - 30 kg / h, for titanium oxide nanopowder - 10 kg / h. Calculations for other types of raw materials also demonstrate high productivity and low cost of the resulting nanopowders.

Thus, the proposed method allows to obtain high-quality nanopowders, economical in terms of energy consumption, continuous, single-stage, highly efficient. The proposed installation is characterized by versatility, simplicity of design, safety of use, high resource of work.

Comparative analysis with the prototype shows that the proposed installation is characterized in that the plasma-chemical reactions in it are carried out in a transformer plasmatron, which allows the use of more efficient technological methods and wider operating conditions. The facility also has structural differences in the organization of the feed system, plasma output, and product quenching.

The proposed method differs from the prototype by the process operating parameters, namely, the discharge is initiated using low frequencies of the order of 100-400 kHz, the reaction is carried out at atmospheric pressure and higher, the temperature in the plasma can reach 10,000 ° C. The most significant difference from the prototype is the introduction of the feedstock directly to the discharge, which can significantly increase the reaction rate and device performance. For quenching of metal nanopowders, argon and nitrogen are used, for quenching of nanopowders of metal oxides - air, for quenching of nanopowders of carbides and nitrides - inert gases.

Achievable technical result of the claimed invention consists in expanding the technological capabilities of the method of production of nanopowders, in creating a universal installation of continuous operation, which allows to obtain high-quality nanopowders of a wide range of substances in this way, including refractory metals, compounds and nanostructured hard alloys, with minimal cost during operation, which has a high performance at low cost of the resulting nanopowders.

A method of producing a nanopowder in plasma is carried out by performing a series of sequential operations. The method includes pre-supplying a plasma-forming gas to the plasma torch chamber, performing a discharge at a pressure of 1-10 Pa depending on the gas, increasing the pressure in the discharge chamber to atmospheric, supplying the feedstock to the plasma torch chamber simultaneously with a reducing gas, a plasma-chemical reaction to obtain a nanopowder, and quenching of the obtained nanopowder and its separation from the reaction zone through a bag filter into the collector. The introduction of the feedstock in a liquid or vapor or gaseous state is carried out in an induction discharge or a plasma jet. The plasma-chemical reaction is carried out in a transformer plasmatron at a pressure of 1.01 × 10 ⁵ Pa and above, an induction discharge frequency of 100-400 kHz and a temperature of 10000 ° C or less in the presence of a reducing or oxidizing gas. The introduction of the feedstock directly into the induction discharge or plasma jet can significantly increase the reaction rate and productivity. The supply of a number of substances, for example silicon monosilane, for the decomposition of which low temperatures of the order of 800-1000 ° C are required, is carried out directly into the plasma jet. Quenching of the obtained nanopowder is carried out using a quenching system. Hardening is carried out with argon or nitrogen upon receipt of a metal nanopowder, air - upon receipt of a nanopowder of metal oxides or inert gases - upon receipt of a nanopowder of carbides and nitrides. Hydrogen is used as the reducing gas, and oxygen is used as the oxidizing gas.

To implement the aforementioned method for producing nanopowders, a universal continuous installation is proposed, including a feedstock supply system, a transformer plasma torch, a reactor chamber, a target product quenching system, a bag filter, and a target product collection hopper. The use of transformer plasma torch allows you to use effective and wide operating conditions of the proposed method. The installation has design features of the organization of the feed system of the feedstock, the output of plasma, quenching of products. The installation provides for the supply of feedstock in a liquid or vapor state by means of a feed feed system, when the working fluid under pressure of any inert gas from the feeder is fed through the evaporator through the feed inlet directly to the discharge chamber of the plasma torch, or in a gaseous state, when through the same the input node of the feed gas is supplied directly to the discharge chamber of the plasma torch. Oxygen or hydrogen is supplied simultaneously to the feedstock input unit to carry out a reduction or oxidation reaction, depending on the product obtained. The plasma torch chamber is connected to the reactor chamber by means of a nozzle, which is part of the plasma outlet assembly, through which a plasma stream flows into the reactor chamber. To introduce a number of substances directly into the plasma jet, one or more nodes are located on the nozzle. Below these nodes, a nanopowder cooling system is located, which is a set of tubes located radially to the walls of the nozzle, the axes of which lie in the same plane. The number of tubes and their geometric dimensions are selected depending on the product obtained. Such a constructive solution of the nanopowder cooling system makes it possible to bring quenching gas as close as possible to the plasma jet and select the most effective quenching gas supply modes depending on the required rate of quenching of the products, the exit velocity, and the temperature of the plasma jet. The water-cooled chamber of the reactor has the form of a vertically mounted cylinder, the upper end of which is closed with a lid with a plasma outlet unit, and a cone-shaped hopper for collecting the synthesized nanopowder is attached to the lower end.

The proposed installation for producing nanopowders using a transformer plasma torch is illustrated by the drawing shown in figure 1, where all the elements are shown schematically and at an arbitrary scale. The installation includes a power source 1 (100-400 kHz), a feed system in the liquid (vapor) state, which includes a pressure gauge 11, feeder 7 with a level indicator 8, an evaporator 10, a transformer plasma torch, consisting of water-cooled sections of a gas discharge chamber 3, systems ferrite magnetic circuits with primary windings 2, a plasma-forming gas input unit 12, equipped with a flow swirl, a liquid (vapor) or gas input unit 13, input units for inputting a gaseous source 14, and also e of the plasma outlet assembly 15, which is a section of the plasma torch chamber with a transverse nozzle, a water-cooled reactor chamber 4, which is a vertically arranged cylinder, the upper end of which is closed by a lid with the plasma outlet assembly, a cone-shaped hopper for collecting synthesized nanopowder 6, a quenching system is attached to the lower end the target product 16, and a bag filter 5, in which the nanopowder of the target product is deposited from the gas stream into the collection hopper of the target product 6.

Installation works as follows.

A plasma-forming gas (argon or other inert gas) is supplied to the chamber of the plasma torch 3 through a node 12, the flow rate of which is controlled by the rotameter 9, the discharge and plasma are produced in the discharge chamber of the plasma torch 3 at a pressure of 1-10 Pa depending on the gas, then the pressure in the discharge the chamber is raised to atmospheric and feedstock is introduced in a liquid or vaporous or gaseous state. The feedstock in a liquid or vapor state is supplied directly to the discharge by means of a feedstock feed system, which is designed in such a way that the working fluid under the pressure created by the supply of argon or other inert gas from the feeder 7 enters the evaporator 10, where if necessary evaporate, and then through the input node of the raw material 13 in the discharge. The supply of a number of substances in a gaseous form, the decomposition of which requires low temperatures of the order of 800-1000 ° C, is carried out directly into the plasma jet through one, two or more input nodes of the raw material 14. The plasma-chemical reaction is carried out either directly in the discharge of the chamber of the plasma torch 3, or in the stream plasma. The quenching of the target product is carried out by supplying the corresponding quenching gas into the plasma jet through the quenching system of the target product 16. For quenching of metal nanopowders, argon and nitrogen are used, nanopowders of metal oxides are air, nanopowders of carbides, nitrides are inert gases. A part of the product settles in the nanopowder collection hopper 6 of the reactor chamber 4. The other part of the product is isolated through a bag filter 5 by back-flushing the filter into the product collector 6.

Practical implementation.

An experimental bench has been manufactured that simulates a setup for producing silicon nanopowder from silicon tetrachloride with a power of 20 kW and a frequency of 100 kHz. A series of experiments was carried out and electron microscopy was used to analyze the obtained silicon nanopowder, which showed that 20-40 nm silicon particles were formed in the plasma. The experiments showed that the method implemented in this setup allows one to obtain high-quality nanopowder, it is economical in terms of energy consumption, continuous, single-stage, highly efficient.

Claims (5)

1. A method of producing a nanopowder in a plasma, including supplying the feedstock to the plasma torch chamber, a plasma-chemical reaction to obtain a nanopowder, quenching the obtained nanopowder and its separation from the reaction zone, characterized in that the plasma-chemical reaction is carried out in a transformer plasmatron at a pressure of 1.01 · 10 ⁵ Pa and above, the frequency of the induction discharge of 100-400 kHz and a temperature of up to 10,000 ° C in the presence of a reducing or oxidizing gas, while the feedstock is supplied in a liquid or vapor, or gaseous state radiation into an induction discharge or into a plasma jet. 2. The method according to claim 1, characterized in that the feedstock having a decomposition temperature of 800-1000 ° C is fed into the plasma stream. 3. The method according to claim 1, characterized in that hydrogen is used as the reducing gas, and oxygen is used as the oxidizing gas. 4. The method according to claim 1, characterized in that the hardening is carried out with argon or nitrogen upon receipt of a metal nanopowder, air - upon receipt of a nanopowder of metal oxides or inert gases - upon receipt of a nanopowder of carbides or nitrides. 5. Installation for producing a nanopowder in a plasma, containing a technologically interconnected feed system, a plasma torch, a quenching system of a nanopowder, a water-cooled chamber, a bag filter and a nanopowder collector, characterized in that it contains a transformer type plasmatron with plasma forming input nodes gas and plasma outlet located on opposite sections of the plasma torch chamber connected to the water-cooled chamber by means of a nozzle that is part of the plasma outlet assembly zma, while the feedstock supply system consists of a feedstock feed system in a liquid state containing an evaporator, a feeder with a level indicator and a manometer, and a feed gas supply system containing one or more feed input nodes located on the pipe, below which host the nanopowder quenching system, made in the form of a set of tubes radially located to the nozzle pipe for supplying quenching gas, the axes of which lie in the same plane.