GB2281233A

GB2281233A - Apparatus for and methods of producing a particulate spray

Info

Publication number: GB2281233A
Application number: GB9416096A
Authority: GB
Inventors: William Hopkins
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-08-09
Filing date: 1994-08-09
Publication date: 1995-03-01
Anticipated expiration: 2014-08-09
Also published as: GB9416096D0; GB9316522D0; GB2281233B

Abstract

Apparatus for producing particulate material comprises an atomisation die 9 connected to receive a supply of gas and positioned about the circumference of a stream 5 of liquid material. The die is operable to direct a jet of the gas onto a freely falling stream of the liquid material to produce a plume 20 of liquid particles. Flame generation means is provided and is operable to raise the temperature of the gas. The flame may be generated by an arc between electrode 19 and lip 12 of the gas outlet. In the alternative electrode 19 may be placed in the heated melt in crucible 2. Alternatively the gas supplied may be oxygen plus a fuel gas which burns at a high temperature. In either case a supersonic stream of atomising gas is provided to produce particles of less than 20 microns in size. The melt may be metal, glass or ceramic. <IMAGE>

Description

Apparatus for and Methods of Producing a Particulate Spray This invention relates to apparatus for and methods of generating particulate material. More especially the invention relates to apparatus for and methods of producing by an atomising technique a spray or plume of liquid particles from which a powder, solid product or coating onto a substrate may be produced. The particles of the spray or plume may be, for example, metallic, glass, ceramic or mixtures of these materials.

Atomizers for producing a spray of metallic particles are known in which one or more jets of gas are directed onto the surface of a freely falling liquid stream or the tip of a guide tube of a container from which liquid metal is emerging. Hitherto, the mean particle size produced using such conventional atomizers has been significantly larger than that required for many applications, particularly those in which a mean particle size less than 20 microns is necessary.

One object of this invention is to provide atomising apparatus capable of consistently producing sprays or plumes of liquid particles having mean particle sizes less than those presently produced using conventional techniques and typically less than 20 microns. A further object of the present invention is to provide a method of consistently producing sprays or plumes of liquid particles having mean particle sizes less than 20 microns.

In one aspect, the present invention provides apparatus for producing particulate material which comprises an annular atomisation die connected to a source of gas and positioned about the circumference of a stream of liquid material, the atomisation die including at least one nozzle through which a jet of the gas can pass to strike the material stream and disintegrate the same, and flame generating means positioned within the path of the gas.

In a further aspect, there is provided apparatus for producing particulate material which comprises atomising means including a die connected to receive a supply of gas and positioned about the circumference of a stream of liquid material the die being operable to direct a jet of the gas onto a freely falling stream of the liquid material to produce a plume of liquid particles and flame generation means operable to raise the temperature of the gas.

The nozzle may be positioned such that the jet issuing therefrom will strike the liquid stream as the stream emerges from a guide tube of a vessel containing a supply of the liquid material. The nozzle may be positioned such that the jet issuing therefrom will partially impinge upon the lower extent of the guide tube. The flame generated within the path of the gas jet may be a plasma flame; alternatively, the flame may comprise an ignited high velocity oxy-fuel jet. Other flame generators may be employed. In the case of a plasma flame, an electrode connected to a source of electrical power may be positioned within a manifold of an atomisation die connected to receive gas under pressure from a source of such gas, the electrical circuit including the electrode being completed via a connection to an exposed surface of the atomisation die or a second electrode of the apparatus or a conductive substrate positioned below the atomisation die.

In an alternative arrangement, an electrode connected to a source of electrical power is immersed within liquid material present in the vessel which communicates with the guide tube, the electrical circuit including the electrode being completed by a connection to an exposed surface of the atomisation die or a second electrode of the apparatus or a conductive substrate positioned below the atomisation die.

In a further aspect, the invention provides a method of producing a plume of particulate material which comprises the steps of directing onto a surface of a liquid mass of the material a high-pressure gas jet within the path of which is generated a high-temperature flame.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which: Figure 1 is a side view partly in section of apparatus in accordance with the invention; Figure 2 is a section taken through an atomizer which forms part of the apparatus illustrated in Figure 1; Figure 3 is a section taken through an alternative atomizer to that illustrated in Figure 2; Figure 4 is a section taken through a further alternative atomizer to that illustrated in Figure 2; Figure 5 is a section taken through a still further alternative atomizer to that illustrated in Figure 2; Figure 6 is a section taken through a still further alternative atomizer in accordance with this invention; and Figure 7 is a section taken through a still further alternative atomiser in accordanc with the invention.

The apparatus illustrated in Figures 1 and 2 comprises a melting chamber 1 including a furnace comprising a crucible 2 and induction coil 3. A stopper rod 4 is provided selectively to open and close an outlet nozzle 5 of the crucible 2. It is to be understood that the melting chamber and furnace illustrated are merely exemplary of melting apparatus which can be employed. Positioned immediately below the melting chamber 1 is an atomising chamber 6 including a primary hopper 7.

The outlet nozzle 5 opens into a guide tube 8 in communication with the upper end of the atomising chamber 6. Positioned within the chamber 6 and about the outlet of the guide tube 8 is an atomisation die 9 comprising an annular manifold 11 connected to a source of high pressure gas through a duct 10 and including an annular array of inwardly inclined jet nozzles 12 whose axes converge on a point positioned immediately below the outlet of the guide tube 8. Typically, four or more evenly spaced nozzles 12 are provided, each nozzle having an internal passageway which is of convergent/divergent configuration to assist acceleration of gas passing therethrough to supersonic speeds. The nozzle passageways exit adjacent the lower lip 18 of the guide tube 8. The jet nozzles 12 may be inclined at angles other than that illustrated. The primary hopper 7 communicates via ducting 14 with a cyclone 15 and 9 secondary hopper 16. Waste gases leave the apparatus via an off-take 17.

The entire apparatus is capable of being evacuated by vacuum pumps (not shown) to remove air and water vapour.

The flow passageway of each nozzle 9 may alternatively be of parallel sided configuration.

Electrodes 19 of a refractory material such as tungsten protrude into the manifold 11. Only one electrode is shown in Figure 2. Each electrode is positioned adjacent the inlet of one of the nozzles 12, the electrode being inclined so that its longitudinal axis is generally coincident with the longitudinal axis of the respective nozzle 9. Each electrode 19 is insulated from the manifold 11 and is connected to a negative terminal of a DC power supply 21 via a lead 22, the positive terminal of the power supply 21 being connected via a lead 23 secured to an exposed surface of the manifold 11. Each electrode is connected to the negative terminal of its own DC power supply. Thus, each electrode 19 is connected to a power source separate from the power sources of the other electrodes. The individual electrodes and nozzles 12 are preferably evenly spaced about the manifold 11.

Maximum voltages may typically be of the order of several hundred volts and maximum currents may typically be of the order of several hundred amperes.

Typically, the high pressure gas supplied to the manifold 11 is an inert gas such as argon, nitrogen, helium or mixtures of such gases including possibly an additive such as hydrogen. The pressures of the gas supplied may range from near ambient to several tens of bar.

In operation, a superheated melt of metal, glass or ceramic present within the crucible 2 is caused to flow through the guide tube 8 to enter the atomising chamber 6.

At the same time (or shortly before or after) gas under pressure is supplied to the manifold 11 and leaves via the nozzle 12 at sonic or supersonic velocity. Simultaneously an arc is struck between each electrode 19 and the manifold 11. The arcs may be initiated by any convenient means, e.g. by moving the electrodes 19 to touch the manifold 11 and then withdrawing them to a suitable distance or by superimposing high voltage, high frequency sparks on to the power supplies 21. The effect of the arcs is to generate a series of plasma flames which emerge from the nozzles 12 to impinge upon the molten material as it emerges from the guide tube 8. As shown, the plasma flame within each gas jet makes partial contact with the external surface of the exit geometry of the guide tube.

The rapidly moving, extremely hot, emerging plasma flames and gas jets disrupt the flow of molten material into a spray or plume 20 of fine liquid droplets which solidify as they travel through the chamber 6 to collect in the primary hopper 7. The powder product is subsequently removed from the primary hopper 7 and the waste atomising gas and fines pass via the cyclone 15 to the secondary hopper 16 for subsequent collection.

In this embodiment the plasma flame operates in what is known as a "non-transferred" mode in that the plasma arcs are not transferred electrically to any part of the apparatus remote from the atomizer assembly.

One advantage of the high temperature plasma flame emerging from the nozzles 12 being in contact with the lip 18 of the guide tube 8 is that the guide tube lip is heated to reduce the tendency of liquid material to solidify on or around the lower end of the guide tube 8. Such solidification problems normally impose technical limitations when using close-coupled gas atomization techniques when a high ratio of atomising gas to liquid material is used (i.e. in excess of 10:1 mass flow ratio) to attempt to produce the finest powders possible.

In the apparatus illustrated in Figure 3, (where similar integers to those illustrated in Figures 1 and 2 have been given the same reference numerals), the nozzle 12 is formed as a single annulus, the internal passageway of which is of convergent/divergent configuration. This passageway could also be parallel-sided. In this embodiment one or more sets of electrodes 19a and 19b are provided, each with its own power supply 21. If more than one electrode set is provided, these are spaced evenly and arranged circumferentially within the manifold 11.

In use, arcs are struck between each pair of electrodes 19a and 19b and the effect of the gas passing through the manifold 11 and the nozzle 12 is to generate a series of plasma flames which are at a higher speed and velocity than would be the case for relatively cooler gas jets of conventional atomisers.

In the apparatus illustrated in Figure 4, an electrode 25 is immersed in liquid material present in the crucible 2. Of course, in this arrangement, the material must be electrically conducting. Also in this embodiment, the positive terminal of the DC power supply 21 is connected to a collector electrode 26 which comprises a plate of refractory electrically conducting, temperature-resistant material e.g. tungsten or water-cooled copper formed with a central orifice through which the spray 20 of particulate material passes.

It will be appreciated that in the embodiment illustrated in Figure 4, the column of liquid material in the guide tube 8 defines one electrode, the other electrode comprising the collector electrode 26.

In this arrangement, the gas jet flowing through the annular nozzle 12 forms a plasma flame at the tip of the guide tube which entrains and disrupts the liquid metal stream to form droplets. Although the plasma arc connects electrically with the plate 26, the flux of gas and liquid droplets pass substantially through the central orifice of the plate.

Figure 5 shows the use of a plasma flame in the transferred mode in which the spray of droplets are caused to coat a substrate 27 with a continuous layer 28 formed by the molten or semi-solid droplets falling out the substrate 27, physically joining together and solidifying. In this arrangement, the arc is struck between the liquid metal in the guide tube 8 and the substrate 27 and, like the arrangement illustrated in Figure 4, the gas flowing through the nozzle 12 forms a plasma flame between the tip of the guide tube 18 and the substrate 27 which is electrically conducting. By varying the shape of the substrate, plate, tube or solid cylindrical bodies may be produced.

The substrate may be stationary or may be moved either continuously or intermittently below the spray or plume of particles.

Operating conditions for the apparatus illustrated in Figures 4 and 5 are generally different from those for the non-transferred plasma apparatus illustrated in Figures 1 to 3. In the embodiment of Figures 4 and 5, because the plasma arc is struck between the continuously moving column of molten material present in the guide tube 8 and the collector electrode 26 or the substrate 27, significantly higher current and lower voltages may be employed.

As jets of atomising gas emerge from the nozzle 12, a plasma flame is formed which extends between the lip 18 of the guide tube 8 and the collector electrode 26 or substrate 27. This plasma flame acts to entrain and disrupt the liquid material leaving the guide tube 8 to form the require spray of liquid droplets.

In the embodiment illustrated in Figure 6, an electrode 33 is employed which is immersed in molten metal present in crucible 2. The electrode 33 is connected to the negative terminal of the DC power supply 30, the positive terminal of which is connected to the flared outlet 29 which is preferably produced from water-cooled copper. The nozzle 12 is of annular configuration. In this embodiment, the length of the contoured outlet 29 extends downwardly to a greater distance to more closely follow the shape of the plume of liquid particles and to increase the heating effect of the generated flame.

Electrical insulation 32 is again provided between this connection and the manifold. In this arrangement, the liquid metal flowing through the guide tube 18 effectively becomes one of the electrodes by virtue of being connected electrically the electrode 33. The contoured outlet 29 becomes the other electrode. the arc is struck between and through the liquid metal emerging from the guide tube 18 and contoured outlet 29. The effect of the gas flowing through the nozzle 12 is to form a plasma flame within the outlet 29 thus entraining and disrupting the liquid metal stream to form droplets, the liquid metal effectively becoming a consumable constantly replaced electrode.

In all of the embodiments described and illustrated, the flame generated within the gas jet is a plasma flame.

Other means of generating flames may, however, be employed, these including high velocity oxy-fuel spraying. High velocity oxy-fuel spraying uses an internal combustion process rapidly to heat and accelerate a gas to a hypersonic velocity of typically 1800 ms-l and a combustion temperature of above 2800"C. Typical combustion fuel gases include propylene, acetylene, propane and hydrogen.

Figure 7 illustrates how an internal combustion process can be used to generate within the gas supplied to the manifold 11 a high velocity, high temperature flame as an alternative to plasma but without departing from the basic principal of passing liquid metal into the centre of an annular flame.

In this embodiment, oxygen is passed under pressure into an annular mixing chamber 32 through a pipe 33. A combustion gas such as propylene, acetylene, propane or hydrogen enters the mixing chamber 32 through a separate pipe 34. The gases are mixed in the chamber 33 and pass through annular nozzle 12 which may be either parallelsided or of convergent/divergent configuration. The high temperature jets emerging from the nozzle 12 disrupt the stream of molten material into liquid droplets. The velocity and temperature of the gas at this point of emergence are typically of the order of 1800ms'l and 28000C.

These typical values are significantly higher than values achieved with conventional close-coupled gas atomisation.

The die 9 is cooled by virtue of internal passages supplied with coolant.

It will be appreciated that when multiple jets are used, they should be of sufficient number and size to cover completely the tip of the guide tube with high velocity gas, that is to say although the jets are discrete at their point of emergence, by virtue of the reducing diameter of the conical section of the guide tube tip and because they spread out when they impact the tip, they overlap and join together.

One important advantage of the process described is that conventional close-coupled gas atomisation can only practically produce gas velocities in the region of 400-500 ms-l and the gas temperature may be sub zero. With plasma and oxy-fuel flames the velocities and temperatures are significantly higher thus allowing much better break up of the liquid into smaller droplets.

It will be appreciated that the foregoing is merely exemplary of apparatus in accordance with the invention and that modification can readily be made thereto without departing from the true scope of the invention.

Claims

1. Apparatus for producing particulate material which comprises an annular atomisation die connected to a source of gas and positioned about the circumference of a stream of liquid material, the atomisation die including at least one nozzle through which a jet of the gas can pass to strike the material stream and disintegrate the same, and flame generating means positioned within the path of the gas.

2. Apparatus for producing particulate material which comprises atomising means including a die connected to receive a supply of gas and positioned about the circumference of a stream of liquid material the die being operable to direct a jet of the gas onto a freely falling stream of the liquid material to produce a plume of liquid particles and flame generation means operable to raise the temperature of the gas.

3. Apparatus as claimed in Claim 1 or Claim 2 wherein the nozzle is positioned such that the jet issuing therefrom will strike the liquid stream as the stream emerges from a guide tube of a vessel containing a supply of the liquid material.

4. Apparatus as claimed in any one of Claims 1 to 3 wherein the nozzle is positioned such that the jet issuing therefrom will partially impinge upon the lower extent of the guide tube.

5. Apparatus as claimed in any one of the preceding claims wherein the flame generation means is operable to produce a plasma flame.

6. Apparatus as claimed in any one of Claims 1 to 4 wherein the generated flame comprises an ignited high velocity oxy-fuel jet. Other flame generators may be employed.

7. Apparatus as claimed in any one of Claims 1 to 5 further comprising an electrode connected to a source of electrical power positioned within a manifold connected to receive gas under pressure from the source of gas.

8. Apparatus as claimed in Claims 7 wherein the electrical circuit including the electrode is completed via a connection to an exposed surface of the atomisation die.

9. Apparatus as claimed in Claim 7 wherein the electrical circuit is completed through a second electrode.

10. Apparatus as claimed in Claim 7 wherein the electrical circuit is completed through a conductive substrate positioned below the atomisation die.

11. Apparatus as claimed in any one of Claims 1 to 5 further comprising an electrode connected to a source of electrical power and immersed within liquid material present in the vessel, the electrical circuit including the electrode being completed by a connection to an exposed surface of the atomisation die.

12. Apparatus as claimed in Claim 11 wherein the electrical circuit is completed through a second electrode.

13. Apparatus as claimed in Claim 11 wherein the electrical circuit is completed through a conductive substrate positioned below the atomisation die.

14. A method of producing a plume of particulate material which comprises the steps of directing onto a surface of a liquid mass of the material a high-pressure gas jet within the path of which is generated a high temperature flame.

15. Apparatus for providing particulate material substantially as herein described and as described with reference to Figures 1 and 2; or Figure 3; or Figure 4; or Figure 5; or Figure 6; or Figure 7 of the accompanying diagrammatic drawings.