Apparatus and method for powder spheroidization by microwave-induced plasma

Abstract

A device for spheroidization of powders by microwave induced plasma comprises a microwave generator (4), a microwave cavity (20), a waveguide (6) connecting the microwave generator to the microwave cavity, a plasma tube (3) partly located in the microwave cavity, a powder supply (2) connected to the plasma tube to deliver a flow of powder precursor thereto, a gas supply (1) connected to the plasma tube to deliver a flow of process gas thereto to form a plasma torch in the plasma tube by coupling the flow of process gas with microwave radiation, and a supply of compressed air (9). The microwave cavity comprises at least one opening (25) for compressed air (91) to enable external cooling of the plasma tube, and the gas supply is connected to the powder supply to bring the process gas (11) with the powder precursor (21) into the plasma tube and into the plasma torch to form a sphere (31) from the powder precursor by melting in flight.

Description

[0001] This application claims priority from patent application number EP20382965.0, filed on November 6, 2020. Technical Field

[0002] This disclosure relates to an apparatus for spheroidizing powder using microwave-induced plasma. The apparatus includes a microwave generator, a microwave cavity, a waveguide connecting the microwave generator to the microwave cavity, a plasma tube partially located within the microwave cavity, a powder supplier connected to the plasma tube for supplying a powder precursor stream therein, and a gas supplier connected to the plasma tube for supplying a processing gas stream therein, so as to form a plasma torch in the plasma tube by coupling the processing gas stream with microwave radiation.

[0003] This disclosure further relates to a method for manufacturing spheres using powder precursors via microwave-induced plasma, comprising the step of forming a plasma torch in a plasma tube by coupling a process gas stream with microwave radiation. Background Technology

[0004] The demand for powder spheroidization stems from various industrial sectors, from powder metallurgy to electronic packaging. The need for industrial processes to produce spherical powder particles arises from the search for the following benefits of spheroidization:

[0005] • Improves powder flowability.

[0006] • Increase the aggregate density of the powder.

[0007] • Eliminate voids and fractures inside the powder.

[0008] • Improves the surface morphology of particles.

[0009] Spheroidization is an in-flight melting process. Angular powder precursors (typically from waste materials) are introduced into an inductively or microwave-induced plasma and immediately melted at the plasma's high temperature. Under the influence of surface tension in the liquid state, the molten powder particles spherize. These droplets are cooled as they exit the plasma torch. The resulting spheres are collected as the spheroidized product.

[0010] Microwave-induced plasma is a type of plasma that emits high-frequency electromagnetic radiation in the GHz range. It can excite electrodeless gas discharge.

[0011] US 2014 / 287,162A1 discloses a microwave plasma apparatus for processing materials. The apparatus includes a plasma cavity, a microwave radiation source, and a waveguide for guiding microwave radiation from the microwave radiation source to the plasma cavity. A processing gas flows through the plasma cavity, and microwave radiation is coupled to the processing gas to generate a plasma torch. The process material is introduced into the plasma cavity, trapped within the plasma torch, and thereby transformed into a droplet stream of product material. A pressurized source provides a shielding gas to the plasma cavity to establish a basic laminar flow of the shielding gas in a helical manner between the processing gas and the inner surface of the plasma cavity. After the plasma torch is ignited, the swirling flow of the shielding gas prevents the plasma jet from adhering to the inner surface, thereby preventing the plasma cavity from melting through conductive heat transfer between the plasma torch and the inner surface. The shielding gas also dissipates some of the heat transferred through radiation from the plasma torch to the inner surface. The shielding gas also helps maintain the laminar flow of the processing gas.

[0012] However, using shielding gas to protect the plasma cavity is complex and expensive. Summary of the Invention

[0013] One object of this disclosure is to provide an apparatus and method to overcome or mitigate the disadvantages of the prior art.

[0014] In a first aspect, an apparatus is provided for obtaining powder spheroidization by means of microwave-induced plasma. The apparatus includes: a microwave generator; a microwave cavity; a waveguide connecting the microwave generator to the microwave cavity; a plasma tube partially located within the microwave cavity; a powder supplier connected to the plasma tube to deliver a powder precursor stream therein; a gas supplier connected to the plasma tube to deliver a processing gas stream therein to form a plasma torch within the plasma tube by coupling the processing gas stream with microwave radiation; and a compressed air supplier, wherein the microwave cavity includes at least one opening for compressed air, allowing the compressed air to cool the plasma tube from the outside, and the gas supplier is connected to the powder supplier to allow the processing gas to carry the powder precursor into the plasma tube, thereby into the plasma torch, to form spheroids by melting the powder precursor during travel.

[0015] The fact that the plasma tube is cooled with air simplifies the apparatus and makes it and its operation cheaper. Furthermore, the fact that cooling occurs outside the plasma tube creates an atmospheric pressure within the microwave cavity (provided by compressed air), thus preventing powder leakage due to any cracks in the plasma tube.

[0016] Furthermore, since the plasma tube is cooled externally with compressed air, the cooling pipes surrounding the plasma tube can be eliminated. Additionally, because the cooling air cools the plasma tube from the outside, this air cannot mix with the processing gas, thus preventing contamination of the plasma torch.

[0017] Therefore, this device is cheaper, safer, and easier to maintain.

[0018] In a second aspect, a method for fabricating spheres using powder precursors via microwave-induced plasma includes the step of forming a plasma torch in a plasma tube by coupling process gas flow with microwave radiation, and further includes the following steps:

[0019] • The powder precursor is carried into the plasma torch by means of a processing gas that acts as a carrier gas, so as to form spheres by melting the powder precursor in transit;

[0020] • The plasma tube is cooled from the outside by injecting compressed air into the microwave cavity surrounding it.

[0021] The statement "microwave cavity surrounding plasma tube" does not mean that the microwave cavity completely includes the plasma tube, because it should be understood that some parts of the plasma tube extend outside the microwave cavity, including upstream and downstream of the microwave cavity.

[0022] There is no need to use processing gas to cool plasma tube 3, as the cooling function is left to compressed air.

[0023] Further advantages, performance, aspects, and features of this disclosure may be derived from the examples described below. If desired, the foregoing features and / or features disclosed in the claims and / or the descriptions of the examples below may be combined with each other, even if not explicitly described in detail. Attached Figure Description

[0024] Non-limiting examples of this disclosure will now be described with reference to the accompanying drawings, in which:

[0025] Figure 1 This is a schematic diagram of a device for spheroidizing powder;

[0026] Figure 2 This is a perspective view of the microwave cavity through which the plasma tube passes;

[0027] Figure 3 yes Figure 2 A front view of the microwave cavity; and

[0028] Figure 4 yes Figure 2 Another perspective view of the microwave cavity. Detailed Implementation

[0029] An exemplary apparatus for spheroidizing powder using microwave-induced plasma includes a tank 1 containing compressed processing gas 11, a tank 2 containing an irregular powder precursor, a plasma tube 3 (or a closed tube, or a discharge tube, or a plasma cavity), a microwave generator 4, a microwave circulator 5, a waveguide 6, a container 7 for powder spheres 31, a filter 8 for intercepting powder exiting from the container 7, a tank 9 containing compressed air 91, a waveguide short circuit 10, and a microwave cavity between the waveguide 6 and the waveguide short circuit 10.

[0030] A wall that is transparent to microwave radiation may exist between waveguide 6 and microwave cavity 20 or between waveguide short circuit 10 and microwave cavity 20. Any of these walls may be made of mica.

[0031] The powder precursor and the processing gas are mixed in tank 2, and the processing gas carries the mixture 21 into plasma tube 3.

[0032] Circulator 5 protects microwave generator 4. Waveguide 6 and waveguide short circuit 10 deliver microwave radiation into microwave cavity 20, through which plasma tube 3 passes. A plasma torch is formed in plasma tube 3 by coupling process gas flow 21 (carrying powder precursor) with the microwave radiation resonating in the microwave cavity. The plasma torch is ignited by a consumable electrode (not shown) or any suitable means.

[0033] Compressed air 91 from tank 9 is introduced through at least one groove 25 in microwave cavity 20 to cool plasma tube 3 from the outside. Other grooves may be provided on waveguide 6 or waveguide short circuit 10 to introduce more compressed air into microwave cavity.

[0034] Container 7 collects a mixture 31 of gas and powder spheres exiting from plasma tube 3 (and the plasma torch therein). The powder spheres are left in container 7 to cool, while the gas leaves the container as mixture 71, still containing some small powder particles. Filter 8 intercepts these small powder particles and provides clean gas 81 for reuse as processing gas.

[0035] Powder materials that can undergo this spheroidization process include black tungsten or tungsten carbide, which have a melting point of approximately 2800°C, as well as bronze, bronze-nickel, iron, aluminum, steel, and iron-tin. This spheroidization process can also be applied to ceramic materials.

[0036] Both the powder precursor particles and the powder spheres are on the micrometer scale.

[0037] The processing gas can be helium, argon, nitrogen, etc.

[0038] The wall of plasma tube 3 can be made of quartz (or ruby ​​or boron nitride), or at least the portion of the wall within microwave cavity 20. Compressed air will affect that portion of the plasma tube, so it is preferable that the temperature of the compressed air is not lower than 10-15°C to avoid damaging the tube material.

[0039] This device can be directly applied to surface treatment machines to combine some processing steps together.

[0040] Advantageously, the flow of the processing gas is laminar or near-laminar to keep the powder particles on a specific path. This apparatus achieves sufficient laminar flow and a non-turbulent flow of the processing gas at any rate because, among other things, there are no obstructions at the gas inlet into the plasma tube 3, and the relevant surfaces have almost no roughness. The carrier gas (i.e., the processing gas) and the precursor powder are mixed at a distance (not less than 1 meter) from the plasma tube. Typically, the flow rate is between 3 L / min and 30 L / min, which usually gives a relatively low Reynolds number. Furthermore, the inner surface of the tube leading to the plasma cavity is smooth, and its cross-section is constant. The aim is not perfect laminar flow, but rather a flow without turbulence, as turbulence would impair the plasma. The typical length of the tube leading to the plasma cavity is between 1 m and 3 m, preferably between 2 m and 2.5 m, to avoid turbulence.

[0041] This apparatus and process allow for the processing of small quantities of powder, such as approximately 2 kg, which is ideal for laboratory use. Regarding the amount of powder to be processed, the aim is to achieve a balance between microwave power, gas flow rate, and the mass of the precursor powder to achieve the desired conversion rate. All other things being equal, greater microwave power means greater powder conversion.

[0042] This process can achieve 70%-80% powder spheroidization. Generally, this is achieved by using different carrier gases, increasing microwave power, and using an automated system to match the device in its "hot state" (in...). By adjusting parameters such as "hot" and instantaneous impedance, better performance can be achieved. This automated system, which adjusts impedance at every moment, seeks a better operating point whenever the properties or quantity of the processing gas or precursor powder change. This is typically done manually and is therefore slow and inefficient, usually starting processing at an equilibrium point and maintaining that equilibrium throughout the process. Experiments have shown that nitrogen mixtures as processing gases produce better results, and by adding approximately 5%–15% hydrogen, the number of free electrons increases, along with the ability to melt the precursor powder. In plasma, energy is provided by releasing electrons from the processing gas (there would be no combustion without oxygen). Other methods to improve plasma reactivity are not excluded, such as optimizing waveguides, increasing microwave power, and using other gas mixtures.

[0043] Microwave generator 4 can operate at relatively low power, for example, less than 1 kW, because plasma ignition starts from 0.6 kW. Generally, microwave generators can start operating from 10% of their full power. However, the higher the power, the larger the plasma torch, because more electrons are released from the processing gas. But care must be taken to prevent the plasma torch from reaching the tube wall, because, for example, the melting point of quartz is 1100°C, while the melting point of powder precursors can be as high as 2200°C or higher, and the temperature inside the plasma torch should be at least that high.

[0044] The length of plasma tube 3 can be varied as needed, but as mentioned above, maintaining a constant tube cross-section is important to avoid interfering with the processing gas flow. Shorter plasma tubes are more robust than longer ones. Furthermore, plasma tube 3 only needs to be transparent to microwave radiation inside microwave cavity 20; outside the microwave cavity, the tube can be made of a less opaque material. If, for any reason (e.g., to cool the sphere), tube 3 must be lengthened, several short tubes can be connected together, taking care to ensure a smooth joint and maintain a constant cross-section; this also applies to connecting tube sections made of different materials. Quartz tubes are suitable for laboratory experiments, but are less suitable for industrial applications due to the fragility of quartz. To overcome this drawback, plasma tube 3 can be made of quartz inside microwave cavity 20 (because quartz is transparent to microwave radiation) and can be completed using connecting parts made of another material or other materials with greater mechanical strength (such as steel, stainless steel, aluminum, copper) or any other heat-resistant metal or material (designed to extend from upstream and downstream of the microwave cavity). In fact, the portion of plasma tube 3 extending outside microwave cavity 20 can be made of any material capable of resisting heat conducted to it by the quartz tube located inside the microwave cavity; the joint or connection between the quartz tube portion and the adjacent tube portion should be mechanical, as adhesive bonding would not be able to withstand the associated heat.

[0045] Since the plasma tube 3 inside the microwave cavity 20 is cooled externally, there is no need for an external cooling tube surrounding the plasma tube.

[0046] Since it is only a single tube, plasma tube 3 can be easily replaced.

[0047] Plasma tube 3 is bidirectional, meaning that the gas and powder mixture fed into the plasma tube can enter from either end.

[0048] This device allows the processing of filament materials, not just powder materials, simply by replacing the powder tank 2 with a feeder suitable for filaments.

[0049] While only a few examples are disclosed herein, other alternatives, modifications, uses, and / or equivalents are possible. Furthermore, all possible combinations of the described examples are covered. Therefore, the scope of this disclosure should not be limited by the specific examples but should be determined solely by a fair reading of the appended claims. If reference numerals relating to the drawings are enclosed in parentheses in the claims, they are merely an attempt to increase the comprehensibility of the claims and should not be construed as limiting the scope of the claims.

Claims

1. A device for sphericalizing powder using microwave-induced plasma, comprising: Microwave generator (4); microwave cavity (20); waveguide (6) connecting the microwave generator to the microwave cavity; A plasma tube (3), partially located within the microwave cavity; a powder supply (2) connected to the plasma tube to deliver a powder precursor stream therein; and a gas supply (1) connected to the plasma tube to deliver a processing gas stream therein to form a plasma torch in the plasma tube by coupling the processing gas stream with microwave radiation, characterized by further comprising a compressed air supply (9), wherein the microwave cavity includes at least one opening (25) for compressed air (91) such that the compressed air (91) can cool the plasma tube from the outside, and the gas supply is connected to the powder supply such that the processing gas (11) carries the powder precursor (21) into the plasma tube, thereby into the plasma torch, to form a sphere (31) by melting the powder precursor in transit, wherein the at least one opening is a groove and the groove is parallel to the axis of the plasma tube (3).

2. The apparatus of claim 1, wherein the waveguide (6) includes at least one opening for the compressed air.

3. The apparatus according to claim 1 or 2, comprising a waveguide short circuit (10) connected to the microwave cavity (20), the waveguide short circuit (10) further comprising at least one opening for the compressed air.

4. The apparatus according to claim 1, wherein the plasma tube (3) comprises at least three different tube portions, namely a portion located inside the microwave cavity (20), a portion located upstream of the microwave cavity, and a portion located downstream of the microwave cavity.

5. The device according to claim 4, wherein the tube located inside the microwave cavity (20) is made of quartz.

6. The apparatus according to claim 4 or 5, wherein the tube portions located upstream and downstream of the microwave cavity (20) are engaged with the tube portion located inside the microwave cavity via a mechanical joint (30).

7. The apparatus according to claim 1, wherein the power of the microwave generator (4) is 10-20 kW.

8. The apparatus of claim 1, comprising a container (7) for collecting the sphere (31) exiting the plasma tube (3), and a filter (8) for the gas (71) exiting the container, the filter being configured to intercept powder carried by the gas (71) exiting the container, thereby allowing the gas (81) exiting the filter to be reused as a processing gas.

9. A method for forming spheres from powder precursors in the apparatus of any one of the preceding claims using microwave-induced plasma, the method comprising the step of forming a plasma torch in a plasma tube by coupling a processing gas stream with microwave radiation, characterized in that... Further steps include: The powder precursor is carried into the plasma torch by means of the processing gas, which acts as a carrier gas, so as to form spheres from the powder precursor by melting it in transit; The plasma tube is cooled from the outside by injecting compressed air into a microwave cavity surrounding it, wherein the compressed air is injected through at least one groove parallel to the axis of the plasma tube.

10. The method of claim 9, wherein the compressed air is further injected into a waveguide leading to the microwave cavity.

11. The method according to claim 9 or 10, wherein the compressed air is further injected into a waveguide short circuit connected to the microwave cavity.

12. The method of claim 9, wherein the power for forming and maintaining the plasma torch is 10-20 kW.

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