GB2484209A

GB2484209A - Plasma Furnace

Info

Publication number: GB2484209A
Application number: GB1118362.1A
Authority: GB
Inventors: Georgy Vasilievich Lysov; Igor Anatolievich Leontiev; Alexandr Gennadievich Paveliev; Evgeny Alexeevich Petrov
Original assignee: OBSCHESTVO S ORGRANICHENNOI OTVETSTVENNOSTYU TVINN
Current assignee: OBSCHESTVO S ORGRANICHENNOI OTVETSTVENNOSTYU TVINN
Priority date: 2009-03-24
Filing date: 2010-03-23
Publication date: 2012-04-04
Anticipated expiration: 2030-03-23
Also published as: GB2484209A8; EA020329B1; EA201171158A1; RU2009110298A; GB201118362D0; GB2484209B; WO2010110694A1

Abstract

The invention relates to devices for the plasma processing of powdered raw material which contains metal. A furnace comprises an arc melting chamber, a means for collecting the finished product, a hollow cylindrical cathode and anode mounted on the central axis of the furnace, a microwave energy feed means, an outer and an inner tube and a magnetic system around the melting chamber. The furnace is equipped with a conical nozzle, which is attached to the lower part of the outer tube and connects the cathode thereto, said nozzle, together with the tubes and the microwave energy feed means, forming a microwave discharge chamber situated along a common axis with the arc melting chamber. A first vortex generator is mounted on the wall of the arc melting chamber below the roof thereof and is connected to a reducing gas inlet pipe. A gas permeable dielectric insert is situated between the aforementioned tubes with a first inlet for a plasma forming gas being arranged above the insert and a second gas vortex generator with a second inlet for a plasma forming gas being arranged below the insert on the outer tube.

Description

PLASMA FURNACE

The present invention relates to plasmachemical metallurgy, in particular, to devices for the plasma processing of powdered raw material which contains a metal, for example for direct reduction of metals and production of ultrafine nanopowders of metals and compounds thereof.

Arc discharge devices for direct reduction of metals are described in technical literature, for example, a known plasma furnace for direct metal reduction comprises a cooled working chamber having a roof and a final product collecting unit, a charge feeder, a charge input device and means for injecting a reducing gas into the working chamber, a hollow cathode movably mounted on the central axis, an anode combined with the collecting device, and a magnetic system (RU 2007463).

In this device, the charge and the reducing gas are supplied through the cathode cavity; in this case consumption of the cathode (usually graphite one) is determined primarily by two factors: chemical -due to partial recovery of ore by the electrode material, and thermal -due to evaporation of the cathode material caused by uneven heating thereof by the arc current on the cathode. At the same time, the process of narrowing the cathode cavity occurs due to deposition on the cathode of carbon formed as a result of pyrolysis when hydrocarbons are used as a reducing gas. At the initial stage of the melting period, when the arc burns on the cold metal, arc extinctions occur frequently (A.V. Smirnov. Control of Electrode Movement in Arc Furnace in Extreme Situations.

"Electrometallurgy", 2001, No.6, p.21) The closest prior art of the present invention is a plasma furnace comprising a melting discharge chamber having a roof, a wall and a bottom, means for collecting the final product, means for injecting reaction gases into the working chamber, a hollow cylindrical cathode and an anode both mounted on the central axis of the apparatus, means for inputting microwave energy into a discharge chamber, an outer tube with a cavity accommodating a charge feeding means formed as a cooled electro conductive inner tube, a plasma-generating gas inlet pipe on the outer tube, a gas flow vortex generator, means for electrically insulating the cathode against the anode, and a magnetic system around said melting chamber (RU 2315813, IFC C21B 13/12, priority of 2006).

The furnace comprises a coaxial line, an outer tube and an anode combined with a collecting means. Charge is fed through the inner tube, and microwave energy is inputted through the coaxial line formed by the cathode and the outer tube. The basic arc discharge burns between the cathode and the anode; microwave energy is delivered to it from the outside, the microwave energy being inputted along the entire length of the plasma channel, but only part of it affects the area at the end of the cathode. As with increasing the basic discharge current the electrical conductivity of the plasma channel created by it increases, the skin layer (the depth of penetration of microwave fields into plasma) reduces, the effect of microwave electromagnetic fields on the processes occurring in the near-cathode area decreases, the life time of the cathode reduces, and stability of the arc in the anode-cathode space deteriorates. Thus, the deficiency of the prior art structure is that the furnace productivity is reduced in view of the restrictions imposed upon the amount of electric energy inputted into the plasma.

The objective to be attained by the present apparatus is to increase the productivity of the furnace.

The technical effect to be achieved consists in elimination of restrictions imposed upon the amount of the inputted electrical energy and in an increase in the efficiency of raw material processing.

Such technical effect is attained owing to the fact that a microwave discharge excitation means is arranged outside of the melting discharge chamber and creates, with the aid of vortex generators, a rotating plasma torch of microwave discharge inside the cathode, while a simultaneous injection of a rotating flow of reducing gas in the opposite direction ensures the existence of a diffuse mode of the discharge at the entire cathode surface.

According to the invention, in a plasma furnace comprising a melting discharge chamber having a roof, a wall and a bottom, means for collecting the final product, means for injecting reaction gases into the working chamber, a hollow cylindrical cathode and an anode both mounted on the central axis of the apparatus, means for inputting microwave energy into a discharge chamber, an outer tube with a cavity accommodating a charge feeding means formed by a cooled conductive inner tube, a plasma-generating gas inlet pipe on the outer tube, a gas flow vortex generator, means for electrically insulating the cathode against the anode, and a magnetic system around said melting chamber, the cathode is mounted in an opening in the chamber roof, said collecting means is a cylinder connected to the bottom of said melting chamber and enclosing a hollow cylindrical anode which is movably fixed therein with an axial gap relative to the cathode, the cylinder having openings around the anode; the reducing gas inlet pipe with a first vortex generator is mounted on the wall of the chamber under its roof; the microwave discharge chamber is mounted on the said melting chamber along a common axis, the microwave discharge chamber being formed by said tubes and a conical nozzle attached to the lower part of the outer tube and connecting the cathode thereto; a gas permeable dielectric insert is provided between said tubes with a first plasma-generating gas inlet pipe arranged above the insert, and a second gas vortex generator with a second plasma-generating gas inlet pipe arranged under the insert on the outer tube.

In an embodiment, the electrically insulating means formed by dielectric rings are provided between the nozzle and the outer tube or between the nozzle and the cathode together with a microwave throttle, as well as between the roof and the wall of the melting chamber and/or between the bottom and the wall of the melting chamber; in another embodiment, the electrically insulating means formed by dielectric rings are provided between the bottom of the melting chamber and said collecting means, as well as between the wall and the rocf of the melting chamber, and said microwave energy inputting means is a coaxial-waveguide junction containing a rectangular waveguide of which the wide wall and axis are perpendicular to the apparatus axis, said waveguide being connected to a microwave energy source, and a coaxial line formed by the outer and the inner tube.

Furthermore, the gap along the axis between the inner tube and the conical nozzle is approximately A/2 (A is the length of the working electromagnetic microwave in the microwave chamber), the cathode protrudes into said melting chamber from its roof at an amount of (0.5-1) the inner diameter of the cathode, the anode protrudes into the space of the melting chamber at a height approximately equal to the outer diameter of the anode; the gap between the anode and the cathode is approximately equal to the inner diameter of the anode, and the total area of openings in the bottom of the melting chamber around the anode is less than the lateral surface area of the axial gap between the anode and the cathode; a microwave discharge initiation device is mounted in the side wall of the microwave discharge chamber; the furnace is equipped with an anode moving mechanism; the inner tube, the cathode, the bottom of the melting chamber and the walls of both said chambers are water-cooled, the charge feeder is connected to the inner tube through an insulator; in an embodiment the inner and the outer tubes are insulated against each other, a microwave throttle is mounted on the outer tube, and the magnetic system is a solenoid.

Terms used: Plasma furnace -a device comprising two or more electrodes between which an electric discharge is excited in plasma-generating gas environment, the gas discharge being controlled by gas-or magnetodynamic methods; the discharge plasma is used for heating gas, for melting and recovery of crude ore material.

Feeder -a device usually comprising a hopper with starting crude ore material and means for feeding the starting material at a predetermined rate.

Charge -a mixture of crude ore material (ore, concentrate), alloying and refining additives.

Crude ore, iron ore raw material -mineral or technogenic raw materials containing one or more oxides, e.g. iron oxides with different iron valences.

Coaxial-waveguide junction -a device that converts a microwave in a coaxial line into a microwave propagating in a waveguide, and vice versa. It usually consists of a coaxial line formed by an outer and inner conductors, and a rectangular waveguide disposed perpendicular to the axis of the coaxial line.

FIG. 1 shows schematically a longitudinal section of a preferred embodiment of the present apparatus.

In a preferred embodiment, a plasma furnace comprises a melting discharge chamber 1 having a roof 2, a wall 3, and a bottom 4, and a microwave discharge chamber 5.

The chamber 1 includes a hollow cylindrical anode 6 movably attached to the bottom 4, a cathode 7 mounted in an opening in the roof 2 of the chamber 1, a first gas flow vortex generator 8 mounted on the wall 3 of the chamber 1 and connected to a reducing gas inlet pipe (not shown), openings 9 for withdrawal of excessive exhaust gas, an opening 10 in the bottom 4 of the chamber 1, and means 11 for collecting the final product in the form of a cylinder with a conical lower part, connected to the bottom 4 of the chamber 1. The microwave discharge chamber 5 includes an inner cooled metal tube 12 and an outer metal tube 13, a conical nozzle 14, microwave energy inputting means in the form of a coaxial-waveguide junction 15 having a coaxial part 16 and a rectangular waveguide 17, a porous dielectric insert 18, a first plasma-generating gas inlet pipe 19, a second vortex

S

generator 20 and a second plasma-generating gas inlet pipe connected thereto (not shown), a microwave throttle 21 and a dielectric ring 22 provided between the nozzle 14 and the outer tube 13. A solenoid 23 is mounted around the chamber 1.

The microwave discharge chamber 5 is formed by the inner tube 12, the outer tube 13 and the conical nozzle 14 secured on the lower part of the outer tube 13. In another embodiment, the roof 2, the cathode 7, the nozzle 14 and the outer tube 13 are electrically integrated. To enable operation of the chamber 5 (in particular, its tube 13) at the Earth potential, as can be dictated by electrical safety requirements, a power source with a high potential arc on the positive electrode should be used, which is not always convenient. In a preferred embodiment, the nozzle 14 is electrically connected to the cathode 7, but it is insulated with the aid of a microwave throttle 21 and ring 22 against the outer tube 13 (or the nozzle 14 is insulated against the cathode 7), thereby enabling the microwave discharge chamber 5 to be always used at the Earth potential. The charge feeding means is combined with the inner tube 12. The size of the gap between the tubes 12 and 13 is fixed by a dielectric insert 18, the gas permeability of which is dictated by the need to prevent the entrance of the discharge plasma into the waveguide through creating above the insert 18, with the aid of a first plasma-generating gas inlet pipe 19, an overpressure with respect to the pressure in the area under the insert 18, which is created by a second plasma-generating gas inlet pipe connected to the second vortex generator 20.

The chamber S can be also attached to the chamber 1 without the conical nozzle, for example, by mounting the outer tube 13 on the roof 2, but in this case additional measures are necessary to direct the charge flow and the microwave discharge plasma into the opening in the cathode 7.

A conventional microwave discharge initiation device 24 in the form of a metal pin briefly introduced into the chamber 5 is placed on the outer tube 13. Electrical insulation of the walls 3 of the chamber 1 against the roof 2 can be provided by a dielectric ring 25 and against the bottom 4 by a dielectric ring 26. If necessary, the bottom 4 can be insulated against the collecting device 11 by a dielectric ring 27.

In the preferred embodiment, the means for inputting microwave energy into the chamber S is a coaxial-waveguide junction 15 consisting of a rectangular waveguide 17 and a coaxial line 16 formed by the tubes 12 and 13. However, microwave energy can be inputted into the chamber 5 by another means as well, for example, by a circular or coaxial waveguide, rather than rectangular one. The recommended gap between the inner tube 12 and the conical nozzle 14, which is about half the length of the working electromagnetic wave in an embodiment of the invention, is indicated as a starting value in designing the furnace and subsequently setting up the microwave units. Due to uncertain boundary conditions, the length of the gap must be adjusted during setting up the chamber 5 based on the criterion of minimum reflected power to provide resonance in the gap region, in the conditions of which the applied microwave energy is completely transferred into the chamber 5. The setting up is performed before starting the operation of the furnace; its results are fixed and will not change.

The relations according to which the cathode 7 protrudes into the melting chamber 1 from its roof 2 at an amount of (0.5 -1) the internal diameter of the cathode 7, the anode 6 protrudes into the space of the melting chamber 1 at a height approximately equal to the outer diameter of the anode 6, and the gap between the anode 6 and the cathode 7 is approximately equal to the inner diameter of the anode 6, have been chosen experimentally. These relations influence on the formation of a vortex gas flow in the chamber 1 and subsequent formation of the plasma channel and are valid for the used size of the chamber 1 (cathode inner diameter 30 mm; anode protrudes at a height of 70 mm, anode inner diameter 40 mm; gap between the anode and the cathode 40 mm), and operation conditions of the furnace: tjarc = 100 V, arc = 400 A, Prnjcrowave = 4 kW; they may be different for other sizes and operation conditions. These relations are also the starting point in designing and may be adjusted in the process of setting up the operation conditions of the chamber 1.

To cool the furnace at a low power, air cooling can be used, but at a high power delivered tQ the furnace, its structural elements can be vigorously heated and subsequently fail, so the inner tube 12, the cathode 7, the bottom 4 of the melting chamber 1 and walls of both chambers 1 and 5 must be water-cooled.

Magnetic field can be created by a permanent magnet as well, but in the proposed embodiment a solenoid 23 is used because of the large size of the chamber 1.

The apparatus operates as follows.

Nominal voltages are set at the anode 6 and the cathode 7 (in the preferred embodiment the anode is connected to earth).

Microwave energy is delivered on a TEM wave through a rectangular waveguide 17 from a microwave energy source (not shown) into a coaxial waveguide 16. Plasma-generating gas, such as nitrogen or argon-hydrogen mixture, injected through inlet pipe 19, penetrates through a porous insert 18 and flows along the axis of the gap between the tubes 12 and 13. Gas injected through a second vortex generator 20 is moving tangentially to the surfaces of the tubes 12 and 13. A device 24 initiates microwave discharge under the insert. Initiation of microwave discharge may be accomplished in another way (e.g. by briefly touching the cathode 7 with the inner tube 12 at a low voltage between them; using a start-up torch, high-frequency breakdown, etc.), so in an embodiment the inner tube 12 is movable. Where a microwave discharge is initiated by applying voltage to the tube 12, an insulator between the tube and a mixture feeder (not shown), and a microwave throttle between the tube and the pipe 13 are to be provided.

Microwave discharge plasma torch in the rotating flow of plasma-generating gas is moving towards the chamber 1. By adjusting of the gas flow rate ratio through the pipe inlet 19 and the vortex generator 20 conditions are created for burning and transporting the microwave discharge plasma torch in the direction of the chamber 1. It has been established experimentally that approximately equal flow rate ratio is satisfactory.

Reducing gas is injected into the chamber 1 through the first vortex generator 8, and an arc discharge is initiated in the gap between the anode 6 and the cathode 7 by applying a short-time high-voltage pulse between them, or by high-frequency breakdown, using the ability of the anode 6 to move.

The movement mechanism can be similar to that described in RU 1781306, or it can be formed by a pin on a rod. Ignition of the arc is facilitated by the presence in this gap of microwave discharge plasma, which is inputted by the centrifugal forces under the lower end of the cathode 7. This process is further enhanced by rotation of the arc under the effect of the magnetic field generated by the solenoid 23. The opposite motion of the reducing gas rotary flow creates turbulence of plasma and prevents its exit from under the lower end of the cathode 7 towards the wall 3 of chamber 1. As shown by experimental studies, due to the fact that the microwave discharge plasma flows around the lower end of the cathode 7, the arc discharge spot does not jump on the end face by fixing at some points, but transforms into a diffuse plasma ring at its surface. Thereby the conditions of uniform current extraction from the end face of the cathode 7 are provided.

Then, charge is fed through the inner tube 12 from the feeder (not shown) into the process chamber 1.

The total area of openings 10 in the bottom 4 of the melting chamber 1 around anode 6 should be less than the lateral surface area of the axial gap between the anode 6 and the cathode 7 in order to create an excessive pressure of the reducing gas injected through the reducing gas inlet pipe and the first vortex generator 8 in the area between the wall 3 and the electrodes (cathode 7 and anode 6) with respect to the pressure in the gap between them and the cavity of anode 6.

The smaller total area of openings 10 in the bottom 4 as compared with the area of the lateral surface of the anode 6 -cathode 7 gap ensures that a substantial part of the reducing gas injected through the vortex generator 8 passes through the gap into the anode 6 cavity and draws there the charge coming from the chamber 5.

The presence of openings 10 makes it possible to withdraw the excessive volume of gas injected through the vortex generator 8, and to prevent blow off of the arc, and provides simultaneous cooling of the wall 3 to prevent its destruction.

As the charge is moving towards the collecting means 11, plasmachemical reaction occurs in the anode 6 -cathode 7 gap and the anode 6 cavity between the charge and the reducing gas in the conditions of increasing the gas temperature to the values which ensure melting and recovery of the charge. The final product (reduced metal) is collected in the collecting means 11, and exhaust gases exit through the pipe 9.

The present invention uses the physical effect consisting in a diffuse nature of burning the arc, at which current is evenly extracted from the entire surface of the cathode 7 end at the current density much lower than that in the closest prior art; the physical effect is implemented owing to the fact that at the vortex motion of the plasma-generating and reducing gases in the axial magnetic field of the solenoid 26, the microwave discharge and arc discharge plasma is drawn under the lower end of the cathode 7. As shown by experimental studies, in this case uniform extraction of current from the end of the cathode 7 is implemented with no noticeable erosion of the cathode 7.

In addition, the microwave discharge in the near-cathode area of the arc discharge stabilizes the arc, because even with possible jumps of the arc observed in the prior art devices, the microwave discharge plasma filling this area does not allow termination of current with breaking down the basic arc.

Thus, the provision of the diffuse nature of burning the arc by introducing the microwave discharge plasma under the lower end of the cathode 7 allows the removal of restrictions imposed upon on the amount of the inputted electrical energy, and as a result improves the efficiency of processing raw materials. Increase in the delivered electrical power also allows processing more charge per unit tine and increases the productivity of the furnace.

Claims

CLAIMS1. A plasma furnace comprising a melting discharge chamber having a roof, a wall and a bottom, means for collecting the final product, means for injecting reaction gases into the working chamber, a hollow cylindrical cathode and an anode both mounted on the central axis of the apparatus, means for inputting microwave energy into a discharge chamber, an outer tube with a cavity accommodating a charge feeding means formed by a cooled electrically conductive inner tube, a plasma-generating gas inlet pipe on the outer tube, a gas flow vortex generator, means for electrically insulating the cathode against the anode, and a magnetic system around said melting chamber, said plasma furnace characterized in that the cathode is mounted in an opening in the chambet roof, said collecting means is a cylinder connected to the bottom of said melting chamber and enclosing a hollow cylindrical anode which is movably fixed therein with an axial gap relative to the cathode, the cylinder having openings around the anode; the reducing gas inlet pipe with a first vortex generator is mounted on the wall of the chamber under its roof; the microwave discharge chamber is mounted on the said melting chamber along a common axis, the microwave discharge chamber being formed by said tubes and a conical nozzle attached to the lower part of the outer tube and connecting the cathode thereto; a gas permeable dielectric insert is provided between said tubes with a first plasma-generating gas inlet pipe arranged above the insert, and a second gas vortex generator with a second plasma-generating gas inlet pipe arranged under the insert on the outer tube.
2. The apparatus according to claim 1, wherein electrically insulating means formed by dielectric inserts are provided between the nozzle and the outer tube or between the nozzle and the cathode together with a microwave throttle, as well as between the roof and the wall of the melting chamber and/or between the bottom and the wall of the melting chamber.
3. The apparatus according to claim 1, wherein electrically insulating means formed by dielectric inserts are provided between the bottom of the melting chamber and said collecting means, as well as between the wall and the roof of the melting chamber.
4. The apparatus according to any one of claims 1 to 3, wherein said microwave energy inputting means is a coaxial-waveguide junction containing a rectangular waveguide of which wide wall and axis are perpendicular to the axis of the apparatus, said waveguide being connected to a microwave energy source, and a coaxial line formed by the outer and the inner tubes.
5. The apparatus according to claim 1, wherein said gap along the axis between the inner tube and the conical nozzle is approximately A/2 (A is the length of the working electromagnetic microwave in the microwave discharge chamber)
6. The apparatus according to claim 1, wherein the cathode protrudes into said melting chamber from its roof at an amount of (0.5-1) the inner diameter of the cathode.
7. The apparatus according to claim l wherein the anode protrudes into the space of the melting chamber at a height approximately equal to the outer diameter of the anode.
8. The apparatus according to claim 1, wherein the gap between the anode and the cathode is approximately equal to the inner diameter of the anode.
9. The apparatus according to claim 1, wherein the total area of openings in the bottom of the melting chamber around the anode is less than the lateral surface area of the axial gap between the anode and the cathode.
10. The apparatus according to claim 1, wherein a microwave discharge initiation device is mounted in the side wall of the microwave discharge chamber.
11. The apparatus according to claim 1, further comprising an anode moving mechanism.
12. The apparatus according to claim 1, wherein the inner tube, the cathode, the bottom of the melting chamber and walls of both said chambers are water-cooled.
13. The apparatus according to claim 1, wherein the charge feeder is connected to the inner tube through an insulator.
14. The apparatus according to claim 1, wherein the inner and the outer tube are insulated against each other, and a microwave throttle is mounted between them.
15. The apparatus according to claim 1, wherein said inner tube is movable.
16. The apparatus according to claim 1, wherein said magnetic system is a solenoid.