FR2611340A1 - Multicathode plasma generator including cathode screening - Google Patents

Abstract

The invention relates to an arc plasma generator including at least one inlet for plasma-forming gas and a plurality of cathodes whose axes converge towards a common anode, and which is intended for the heat treatment of a material in the divided state and which is introduced into the upper part of the plasma, characterised in that each cathode 1 is fitted with at least one concentric screening determining at least one annular passage whose upper part is connected to the source of plasma-forming gas and whose lower part 8 opens out in the vicinity of the end part 5 of each cathode, on which the arc forms. Preferably, the generator includes three cathodes 1 arranged symmetrically around a vertical axis and spaced 120 DEG apart over a horizontal circle whose centre is located on the axis of symmetry of the system. Preferably, the axis of symmetry is vertical and the inclination of each cathode 1 with respect to this axis is fixed between 15 and 60 DEG .

Description

NULTICATHODE PLASMA GENERATOR
INCLUDING CATHODE SHEATHING
TECHNICAL FIELD OF THE INVENTION
The invention relates to a multicathode plasma generator, each cathode of which has a cladding allowing the introduction into the plasma of the plasma gas, the material to be treated being introduced separately from the plasma gas.

STATE OF THE ART
Thermal plasmas are a powerful means of raising the temperature of materials to be treated for chemical or physical transformation.

than.

However, it is known that the introduction of material into an electric arc, whether in powder or gaseous form, poses significant problems.

In fact, plasma is a medium with high viscosity (close to that of water), and the material to be introduced into it only penetrates with difficulty. If it is entrained by a gas flow, its penetration depends on the speed of the flow and the diameter of the particles. Thus, the trajectories in the plasma are not easy to control. On the other hand, the cold entrainment gas locally cools the plasma and thus reduces heat transfer.

A more advanced method uses magnetohydrodynamic pumping, also called the "Maecker effect". This has been used in several plasma productions, notably by SHEER C., KORMAN S., DOUGHERTY TJ,
CHEH HY: "INVITED REVIEW: DEVELOPMENT AND APPLICATION OF THE HIGH
INTENSITY CONVECTIVE ELECTRIC ARC "; CHEM. ENGINEERING COMNUNICATIONS, 19, 1982, p. 1- 47
In this case, the forced flow of gases towards the axis of the arc, near a constriction, forces the material to be treated towards the hot regions of the plasma. However, the material introduction system is located near the base of the arch and therefore subject to very high temperatures. This places severe constraints on this system, and can also cause blocking of the outlet openings, by partial fusion. of the material to be introduced into the plasma.

Another introduction system is the passage of material between three-phase arcs from electrodes located around the trajectory of this material. It has been shown, however, that these three-phase arcs repel each other and therefore only slightly heat the central area of the material passage.

Finally, a system for introducing material into the (vertical) axis of three cathodes located symmetrically around this axis was developed and presented in patent application PUT WU 86/02024. However, these cathodes are directly exposed to the ambient atmosphere, with no flow of particular shielding gas. The three arcs from these cathodes join, in an uncontrolled manner, in the axis and are transferred to a common toric anode.

PROBLEM TO SOLVE
It appears, on analysis of the prior art, that the problem of the introduction of material to be treated into a plasma is not satisfactorily resolved, because, with the devices currently known, it is not known how to carry out reproducibly, and under the conditions of industrial production, a precise introduction of material into the hottest zone of the plasma, without destabilizing or cooling the plasma.

OBJECT OF THE INVENTION
The object of the invention is a multi-cathode plasma generator with three cathodes with a common anode, characterized in that each cathode has an outer sheath delimiting an annular space into which a plasma gas is injected. The material to be treated is introduced separately from the plasma gas.

Preferably, but not exclusively, the generator will include three cathodes arranged at 1200.

Figures 1 to 4 illustrate the invention.

Figure 1 shows, in schematic view, the arrangement, in space, of the three cathodes and the common toroidal anode.

FIG. 2 represents, in very simplified section, a sheathed cathode according to the invention.

FIG. 3 represents, in section, the detail of a cathode according to the invention, comprising a double cooling circuit.

FIG. 4 shows, in top view, the arrangement of the electrodes on the cover of the plasma generator.

Three cathodes (1) are located symmetrically around a vertical axis (2), spaced 1200 on a horizontal circle (3) whose center is on the axis of symmetry. The inclination of each cathode relative to the vertical can be adjusted between 15 and 60 by shims (4) integrated into the electrode holder. This inclination and the radius of the circle are two important parameters for the treatment of matter, as will be explained later.

Each cathode (1) is schematically constituted by a cone (5) with very slightly rounded tip (radius of curvature 0.001 to 1 mm, and preferably from 0.01 to 0.1 min) in thoriated tungsten (normally at 2% of Th), which is the attachment zone of the electric arc (6), and surrounded by a tubular pipe (7) with frustoconical end piece (8) which allows the passage of the sheathing gas (in general Ar or He, possibly mixed with H2).

The tip (5) of the cathode emerges from the gas pipe over a length between O and 50 mm, and preferably between O and 30 mm (Figure 2).

The distance between the cathode (1) and the sheathing pipe (7) is of the order of a millimeter, that is to say between 0.05 and 5 min, and preferably between 0.1 and 2 min; the respective correct centering is very important. The tips (5) of the three cathodes (1) are spaced 10 to 100 mm apart, depending on the use and the power level chosen. The essential cooling circuits are not shown in this simplified section.

Figure 3 shows the construction detail of a cathode according to the invention, comprising two separate cooling circuits, plus cooling of the electrode holder cover. We find, from the central axis to the periphery: the axial tube (10) for the cathode cooling water inlet, the water enters at the upper part at (11) and goes up into the annular space ( 12) and exit via the side nozzle! 13).

The tube (14) forms the cathode proper which is isolated from the head by the ring (15). The cathode power supply, connected to the negative pole of the current generator, is carried out on the fitting (16).

The head of the tube (14) is sealed by the cover (17).

The cathode is sheathed by three concentric tubes;
The first cladding (18), separated from the cathode by the insulating shims (19), is at a floating potential.

The second cladding (20), which plays a simple role of separation, is also at this floating potential.

The third (21) is the external cladding, which is also at this floating potential. These tubes are held in position by the cladding cover (22) in the upper part, and by the edges of the outlet nozzle (23), in the lower part.

The sheathing is cooled by a circulation of water, which enters at (24), descends between the sheath (19) and the sheath (20), rises between the sheath (20) and the outer sheath (21), and leaves in (24A).

Each cathode (1) is placed in the cover. Its angle of inclination relative to the vertical axis (2) is determined by the profile of the electrode holder (25), which rests on the outer sheath (21) by means of insulating rings (26).

The active part of the cathode (cathode tip (5)) consists of a cylinder (27), extended by a cone (28) which has been defined geometrically a little higher.

The cathode, made of thorium tungsten with 2 Z of thorium, is inserted into a cathode sleeve (29), made of copper, itself connected to the end of the cooling tube (10).

The plasma gas is introduced through the nozzle (30), flows into the annular spaces between the cathode ray tube (14) and the first sheath (18), and exits through the orifice (31) of the nozzle (23) .

The material to be treated in the plasma is introduced through the tube (32).

Three electric arcs (6) develop between the cathodes (1) and a common toric anode (33), centered on the axis of symmetry (2) of the assembly at a distance between 50 and 500 imn from the tips (5) cathodes.

This distance determines, together with the nature of the plasma gas and its speed of flow, the voltage drop of the arc. The toroidal anode (33) is made of a metal which is a good electrical and thermal conductor (for example a Cu alloy) and cooled by an internal circulation of water.

The electrode holder cover (25) is cooled by water entering (34) and leaving (35).

Each cathode (1) has its own direct current electrical supply connected by its positive pole to the common anode (33).

This diet is of the conventional type.

Each of the three arcs (6), coming from its respective cathode, joins the other two in the material entry zone where the jets formed by the plasma gas accelerated by the arc, entrain the present material. The material is transported to this entry zone either by simple effect of gravity, or by a low flow rate of carrier gas which cools the plasma only in a negligible manner. The material leaving the tube (32) is then transported, by the plasma gas, in the axial zone (34) where high temperatures prevail, undisturbed by an excessively abundant carrier gas; this is where the desired heat treatment takes place.

Contrary to the teaching of patent application WO 86/02024, it is not the gas injected in the central axis which stabilizes the system of the arcs, but rather the gas injected in the axis of each cathode. The gas injected by the tube (32) in the direction of the central axis, in the present invention, only serves to transport with more precision the material to be introduced into the set of arcs, this material not being accompanied that of a low (or zero) flow of carrier gas, which does not disturb the plasma.

Arcing
Several known methods can be used to strike arcs: - bringing one or more cathodes from the anode to the con
physical tact, to establish a short circuit, and arc stretching
thus obtained until the inter-electrode distance is restored
desired; - short-circuiting by a conductive object (wire) which explodes under
the influence of the strong current which crosses it and thus creates the arc; - bringing one or more cathodes closer together
less than the normal distance, and an arc burst using a
high frequency auxiliary electrical discharge.

Another solution is recommended here, without excluding the methods mentioned above. It consists of a reduction in the pressure of the gas prevailing around the electrodes, by pumping, up to a value between 5 and 100 Pa. At this pressure, a luminescent discharge is easily established between electrodes, even distant from more than 100 min. A pressure rise with a controlled increase in the current of the electrical discharge makes it possible to establish an arc at atmospheric pressure from this discharge, without resorting to mechanical interventions on the electrodes. This solution implies, however, that the plasma generator is placed in a sealed enclosure connected to a device under reduced pressure, which is not always the case.

Influence of different parameters
The system is adjusted using the following quantities - nature of the plasma gas (Ar, He, H2) - flow rate of the plasma gas (1 to 100 Normoliter / min per cathode) - space between cathode and sheathing line (0.05 - 5 mm) - length over which the cathode tip emerges from the pipe
cladding (O at 50 min) - radius of the cathode position circle - inclination of the cathodes relative to the vertical (15 to 60) - azimuth of the cathodes relative to the radius of the circle - distance between cathode tips and anode (50 at 500 mm) - radius of the large circle of the anode torus - intensity of the current of each arc.

The effects of the main parameters are as follows: a) speed of the gas flow in the sheathing near the cathode;
it depends on the gas flow rate and the passage section near the
cathode tip. It influences two effects
- arc stability: a compromise between excessively high turbulence
aunt of the jet (at too high speed) and lack of training
arc (too low speed) must be found
- the arc voltage which varies in the same direction as the speed; b) geometry of the material entry zone
the opening angle of this area is determined by the tilt angle
birth of cathodes; it influences the dwell time of the material
in the material entry area. The energy density present
in the treatment area decreases with decreasing opening angle
health, and with the increasing distance of the cathodes from the axis.
heat treatment is therefore strongly influenced by this
geometry; c) arc tension
it is determined by
- the nature of the plasma gas: the presence of di- (or poly-) gas
atomic increases it considerably
- the distance between cathode and anode: in the column of the arc, the
electric field is, as a first approximation, constant, and the
tension is therefore proportional to the length of the arc;
- gas flow: the voltage increases with the flow;
- the material introduced: the loss of energy from the arc due to the trans
thermal fertility towards the material is compensated by an increase
tion of the arc voltage. However, the introduction of matiè
re easily ionizable can reduce this voltage.

EXAMPLES OF IMPLEMENTATION
With argon (of technical purity) as plasma gas, a distance between the tips of the cathodes of 30 mm, a distance between the tips of the cathodes and the anode of about 200 min and a space between cathode and sheathing line approximately 0.5 mm, the electrical voltage between each cathode and the anode is between 50 and 200 V, for cladding gas flow rates between 5 and 30 Nl / min per cathode and currents of approximately 200 A.

With nitrogen (of technical purity) as the plasma gas, a distance between the tips of the cathodes of 30 mm, a distance between the tips of the cathodes and the anode of approximately 150 min and a space between the cathode and the sheathing line d '' approximately 0.5 mm, the electrical voltage between each cathode and the anode establishes between 60 and 250 V, for the sheathing gas flows between 5 and 25 Nl / min per cathode and currents of approximately 150 A The zone (34) for entering the material to be treated into the plasma is located at a distance of between 50 and 100 mm below the plane of the horizontal circle (3).

During these tests, the material introduced was successively alumina and then zirconia, obtained by grinding of electrofused products. The plasma treatment made it possible to obtain perfect spheroldization of the grains which originally had very irregular shapes.

The hourly material flow could be adjusted from 5 to 40 kg, the latter value having been obtained for a total electrical power close to 120 kVA.

Claims (9)

1. Arc plasma generator, comprising at least one plasma gas inlet and a plurality of cathodes whose axes converge towards a common anode, intended for the heat treatment of a material in the divided state, that the the upper part of the plasma is introduced, characterized in that each cathode (1) is provided with at least one concentric cladding, determining at least one annular passage, the upper part of which is connected to the source of plasma gas, and the lower part (8) opens in the vicinity of the terminal part (5) of each cathode, on which the arc is formed. 2. Plasma generator according to claim 1, characterized in that it comprises three cathodes (1), arranged symmetrically around a vertical axis (2), and spaced by 1200 on a horizontal circle (3) whose center is located on the axis of symmetry (2) of the system. 3. Plasma generator, according to claim 1 or 2, characterized in that the axis of symmetry (2) is vertical, and that the inclination of each cathode (1) relative to this axis is fixed between 15 and 600. 4. Plasma generator according to one of claims 1 to 3, characterized in that the lower end (5) of each cathode, on which the arc is formed, has a conical shape with a slightly rounded tip, of which the radius of curvature is between 0.001 and 1 mm, and preferably between 0.01 and 0.1 mm. 5. Plasma generator according to any one of claims 1 to 4, characterized in that the tip (5) of the cathode emerges from the cladding (21) of a length between 0 and 30 min, and, preferably Between O and 50 mins. 6. Plasma generator according to any one of claims 1 to 5, characterized in that the space between the external wall of the cathode and the internal wall of the external cladding is between 0.05 and 5 mm and, preferably between 0.1 and 2 mm. 7. Plasma generator according to any one of claims 1 to 6, characterized in that each cathode is constituted by a tube (14) of good conductive metal, cooled by circulation of water, connected at its upper part to the pole negative of the current generator, and at its lower part with a cylindro-conical cathode tip (27,28), and surrounded by a first concentric sheath (18), then by a second concentric sheath (20), and d '' a third external concentric sheath (21), the annular space between the tube (14) and the first sheath (18) being connected to a plasma gas inlet, the annular space between the first sheath (18) and the second sheath being connected to a cooling water inlet, the annular space between the second sheath (20) and the external sheath (21) being provided with a nozzle for discharging this water. 8. Plasma generator according to any one of claims 1 to 7, characterized in that it comprises, in the axis (2), a tube (32) connected at its upper part, to the source of material to process, and at a low-flow source, carrier gas. 9. Plasma generator according to any one of claims 1 to 8, characterized in that the plasma gas flow rate, for each cathode, is between 1 and 100 normo-liters per minute.