Apparatus and method for powder spheroidization

Abstract

The apparatus for powder spheroidization comprises a microwave cavity (20), a plasma chamber (35) having an inner wall and located within the microwave cavity, a confinement tube (3) connected to the plasma chamber, a receptacle for a mixture of process gas and powder precursor, connected to the confinement tube, which supplies the mixture as an inflow gas flow to the plasma chamber and generates a plasma torch by coupling the gas flow with microwave radiation, and a flow divider (110) located within the confinement tube at the inlet of the plasma chamber, which forms a wide passage (118) and a narrow passage (113) to separate the inflow gas flow into a central gas flow through a wide passage and a peripheral gas flow through a narrow passage.

Description

Technical Field

[0001] The present application claims the benefit of European Patent Application No. 233828383 filed on August 9, 2023.

[0002] An apparatus for powder spheroidization by microwave-induced plasma comprises: a generator for microwave radiation for a microwave cavity; a plasma chamber having an inner wall and positioned within the microwave cavity; confinement tubes connected to the upstream side and the downstream side of the plasma chamber; and a receptacle for a mixture of a process gas and a powder precursor, the receptacle being connected to the confinement tube, supplying the mixture as an inflowing gas stream into the plasma chamber, and generating a plasma torch in the plasma chamber by coupling the gas stream with the microwave radiation.

[0003] A method for producing spheroids from a powder precursor by microwave-induced plasma comprises the steps of: forming a plasma torch in a plasma chamber by coupling an inflow stream of a process gas with microwave radiation; and conveying the powder precursor into the plasma torch by the process gas acting as a carrier gas, so as to produce spheroids from the powder precursor through gas-phase melting.

Background Art

[0004] Different industrial fields require the use of spheroids made from powders of different materials. The need for industrial processes to obtain spherical powder particles arises from the demand for several of the following advantages resulting from the spheroidization process. · Improving the fluidity of powder. · Increasing the packing density of powder. · Eliminating internal voids and cracks in powder, thereby improving the quality of parts produced from such powder. · Improving the surface morphology of particles.

[0005] Spheroidization is a process of melting in air and controlled solidification. Irregularly shaped (angular or non-uniform) powder precursors (often obtained from waste) are transported to an inductive or microwave-inductive plasma, where they are immediately melted at the high temperature of the plasma. The molten powder particles take on a spherical shape under the action of surface tension in the liquid state. These droplets are cooled as they are ejected from the plasma torch. The resulting spheroids are thus recovered as spheroidization products.

[0006] Microwave-induced plasma is a type of plasma that emits high-frequency electromagnetic radiation in the GHz range. This can excite electrodeless gas discharges.

[0007] European Patent Application Publication No. 3996472(A1) discloses an apparatus for powder spheroidization by microwave-induced plasma, comprising a microwave generator, a microwave cavity, a waveguide connecting the microwave generator to the microwave cavity, a plasma chamber located within the microwave cavity, a powder supply unit connected to the plasma chamber for supplying a powder precursor stream to the plasma chamber, a gas supply unit connected to the plasma chamber for supplying a process gas stream to the plasma chamber and forming a plasma torch within the plasma chamber by coupling the process gas stream with microwave radiation, and a compressed air supply unit for cooling the plasma chamber. The microwave cavity is provided with at least one opening for compressed air so that the compressed air can cool the plasma chamber from the outside, and the gas supply unit is connected to the powder supply unit to transport the powder precursor into the plasma tube and then into the plasma torch using the process gas, thereby producing spheroids from the powder precursor by melting in air. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] In the configuration described in European Patent Application Publication No. 3996472(A1), it has been observed that some powder may adhere to the inner wall of the plasma chamber. This can be undesirable for several reasons. The adhering powder may form a coating that can shield the chamber from the microwaves necessary to generate the plasma. • The adhering powder may reach temperatures higher than the melting point of the chamber wall, potentially damaging it. • High-temperature adhering powder can impair the cooling of the plasma chamber.

[0009] The purpose of this disclosure is to provide apparatus and methods for overcoming or mitigating the shortcomings of the prior art. [Means for solving the problem]

[0010] In a first embodiment, an apparatus for powder spheroidization by microwave-induced plasma is provided, comprising: a microwave radiation generator; a microwave cavity; a plasma chamber having an inner wall and located within the microwave cavity; a confinement tube connected to the upstream and downstream sides of the plasma chamber; a receptacle for a mixture of process gas and powder precursor, connected to the confinement tube, which supplies the mixture to the plasma chamber as an inflow gas flow and generates a plasma torch within the plasma chamber by coupling the gas flow with microwave radiation; and a flow divider located within the confinement tube at the inlet of the plasma chamber, which forms a wide passage and a narrow passage so that the flow divider separates the inflow gas flow into a central gas flow through a wide passage and a peripheral gas flow through a narrow passage.

[0011] In this way, the central gas flow can be driven to generate a plasma torch within the plasma chamber, and the peripheral flow can be driven to cool and clean the inner wall of the plasma chamber.

[0012] It should be understood that the term "wide passage" means that this passage is significantly wider than what is called a "narrow passage," that is, the cross-section of the wide passage is clearly larger than the cross-section of the narrow passage.

[0013] In one example, a wide passage may be located in the central region of a flow divider. A wide passage may also be contained within the flow divider.

[0014] In one example, a narrow passage may be located in or near the periphery of a flow shunt. The narrow passage may be contained within the flow shunt, or the flow shunt may, in combination with, for example, the inner wall of a plasma chamber, contribute to forming a narrow passage.

[0015] In a second embodiment, a method for producing spheroids from a powder precursor by microwave-induced plasma includes the steps of forming a plasma torch in a plasma chamber by coupling an inflow of process gas with microwave radiation; transporting a powder precursor into the plasma torch with a process gas acting as a carrier gas in order to produce spheroids from the powder precursor by melting in air; and cooling the plasma tube from the inside by separating the inflow of gas into a central gas flow and a peripheral gas flow, wherein the central gas flow generates a plasma torch in the plasma chamber and the peripheral gas flow cools the inner wall of the plasma chamber.

[0016] Non-limiting examples of this disclosure are described below with reference to the attached drawings. [Brief explanation of the drawing]

[0017] [Figure 1] This is a perspective view of the elements for cooling the inner wall of a plasma chamber. [Figure 2] This is a cross-sectional view of the element shown in Figure 1, which is attached to the entrance of the plasma chamber. [Figure 3] This is a perspective view of a perforated washer. [Figure 4] This is a partial cross-sectional perspective view of the washer shown in Figure 3, which is attached to the entrance of the plasma chamber. [Figure 5] It is an elevational cross-sectional view of the element of Fig. 1 inverted and attached to the inlet of a plasma chamber. [Figure 6] It is a perspective view of another element for cooling the inner wall of a plasma chamber. [Figure 7] It is an elevational cross-sectional view of the element of Fig. 6 attached to the inlet of a plasma chamber. [Figure 8] It is a perspective view of an element for centering a plasma torch within a plasma chamber. [Figure 9] It is an elevational cross-sectional view of the element of Fig. 8 attached to the outlet of a plasma chamber. [Figure 10] It is a perspective view of a ring attached to the outlet of a plasma chamber. [Figure 11] It is a schematic diagram of an apparatus for powder spheroidization. DETAILED DESCRIPTION OF THE INVENTION

[0018] Fig. 11 schematically shows an apparatus for powder spheroidization using microwave-induced plasma. This apparatus is similar to that disclosed in European Patent Application Publication No. 3996472 A1, and may or may not include the external cooling system described therein, and may include other features such as those introduced in the present specification (see Figs. 1 to 10). For the sake of completeness, the disclosure of European Patent Application Publication No. 3996472 A1 is incorporated herein by reference.

[0019] The apparatus of Fig. 11 comprises a receptacle 1 holding a compressed process gas 11, a tank 2 holding an irregular powder precursor, a confinement tube 3, a microwave generator 4, a microwave circulator 5, a waveguide 6, a receptacle 7 for powder spheroids exiting the confinement tube, a filter 8 for blocking powder exiting the receptacle 7, a waveguide short circuit 10, and a microwave cavity 20 between the waveguide 6 and the waveguide short circuit 10.

[0020] The powder precursor and process gas are mixed in the receptacle 2, and the resulting gas mixture 21 is transported as a gas stream into the confinement tube 3.

[0021] The circulator 5 protects the microwave generator 4. The waveguide 6 and waveguide short circuit 10 transport microwave radiation into the microwave cavity 20, through which the confinement tube 3 crosses.

[0022] Figure 2 shows a plasma chamber 35 formed in the portion of the confinement tube 3 that crosses the microwave cavity 20. The confinement tube 3, including the plasma chamber 35, can be fabricated as a single unit. However, it may be advantageous for the plasma chamber 35 to be a separate component, in which case the portion of the confinement tube located upstream of the plasma chamber can be joined to the plasma chamber via mechanical joints 30, 34, 32 (e.g., cover 30, gasket 34, and clamp 32).

[0023] A plasma torch is formed within a plasma chamber 35 by coupling a process gas stream 21 (which carries the powder precursor) with microwave radiation that resonates within a microwave cavity. The walls of the plasma chamber 35 may be made of quartz, which is permeable to microwave radiation and resistant to high temperatures, but may also be made of other materials with these properties, such as sapphire, mica, boron nitride, or alumina.

[0024] Receptacle 7 collects the mixture 31 of the gas and powder spheroid exiting the plasma chamber 35. The powder spheroid is left inside receptacle 7 for cooling, and the gas exits the container as mixture 71, still containing some small powder particles. Filter 8 blocks these small powder particles, producing a clean gas 81 that is reused as a process gas.

[0025] To maintain powder particles along a predetermined path, it is advantageous for the process gas flow to be laminar or nearly laminar. The goal is not perfect laminar flow, but rather a flow free from turbulence that would impair the plasma.

[0026] Figure 1 shows a first tubular element 110 that is attached to the inlet of the plasma chamber 35. The first tubular element 110 comprises a central portion 115, two ends 114 and 116 that are wider than the central portion 115, and an axial duct 118. The first tubular element 110 further comprises a ring 112 surrounding the ends 114. Several slits 113 are present between the ring 112 and the ends 114. The duct 118 may be cylindrical.

[0027] Referring to Figure 2, which further shows the first tubular element 110 attached to the inlet of the plasma chamber 35 within the confinement tube 3. A clamp 32 clamps the confinement tube 3 to the plasma chamber 35, and a gasket 34 is positioned between the cover 30 and the plasma chamber 35 for airtightness. Most of the gas mixture 21 enters this portion of the confinement tube 3 through the duct 118, while some passes through the slit 113. This configuration separates the gas mixture 21 into two gas flows: a central gas flow (through the duct 118) and a peripheral gas flow (through the slit 113).

[0028] The central gas flow becomes the plasma torch within the plasma chamber 35. The peripheral gas flow is faster than the central gas flow because it passes through a narrow passage, i.e., first through the slit 113 and then through the section 119 remaining between the inner wall of the confinement tube 3 and the relatively wide end 116 of the first tubular element 110. The faster peripheral flow achieves two important results within the plasma chamber 35: namely, it cools the walls of the plasma chamber and pulls out any powder deposited or adhering to the walls.

[0029] Figure 3 shows a perforated washer 120 having a large central hole 124 and several smaller axial perforations 122 distributed around the central hole 124. Figure 4 shows the perforated washer 120 installed between the joining cover 30 and the gasket 34 at the inlet of the plasma chamber 35. Most of the gas mixture 21 enters the plasma chamber 35 through the hole 124, but some passes through the perforations 122. In this case as well, this configuration separates the gas mixture 21 into a central gas flow and a peripheral gas flow to at least a sufficient degree due to the effects described above. Furthermore, this configuration is simpler than the configuration in Figure 2.

[0030] Figure 5 shows a modified version of the example in Figure 2. The elements in Figure 5 are substantially the same as those in Figure 2, except that in Figure 5, the first tubular element 110 is mounted in an inverted position relative to its orientation in Figure 2, i.e., upside down. This configuration is intended for cases where the central flow gas is different from the peripheral flow gas. In such a case, the central flow gas flows downstream through the containment pipe 3, while the peripheral flow gas may flow laterally from, for example, a different conduit 38.

[0031] Figure 6 shows a second tubular element 130, which is a modified form of the first tubular element 110. In Figure 6, the second tubular element 130 is shown in an inverted position relative to the position of the first tubular element 110 in Figure 1. The second tubular element 130 comprises two ends 134 and 136, a central section 135 that is narrower than the ends 134 and 136, and a duct 138. The second tubular element 130 further comprises a holder 132 for support at its central section 135. The duct 118 may be cylindrical.

[0032] Figure 7 shows a second tubular element 130 attached to the inlet of the plasma chamber 35 within the confinement tube 3. A gasket 139 between the cover 30 and the end 136 ensures airtightness at the joint between the confinement tube 3 and the plasma chamber 35. The configuration shown in Figure 7 is similar to the configuration in Figure 5 and similarly intends for a case where the central flow gas is different from the peripheral flow gas. In such a case, the central flow gas flows downstream through the confinement tube 3, while the peripheral flow gas may enter the confinement tube 3 laterally from a different conduit 38.

[0033] In the example shown in Figure 7, the peripheral flow enters the plasma chamber 35 through the section 137 remaining between the inner wall of the confinement tube 3 at the inlet of the plasma chamber 35 and the relatively wide end 134 of the second tubular element 130.

[0034] In the configuration shown in Figure 7, the perforated washer 120 is installed between the end 134 and the gasket 34 and can be clamped by the clamp 32 at the inlet of the plasma chamber 35. This configuration can facilitate the splitting of the gas mixture into a central gas flow and a peripheral gas flow.

[0035] Figure 9 shows a tapered outlet 140 attached to the inlet of the plasma chamber 35. Figure 8 shows the tapered outlet 140 alone. The tapered outlet 140 comprises a flange 142 and a cone 144. A clamp 33 attaches the flange 142 to the plasma chamber 35, and its airtightness is ensured by a gasket 36 located between the plasma chamber and the outlet 140. The flange 142 has a plurality of holes 146 for attachment to the clamp 33 (e.g., via bolts).

[0036] The portion of the confinement tube 3 located downstream of the plasma chamber 35 can be joined to the plasma chamber 35 via mechanical joints 33, 36, and 142 (i.e., clamp 33, gasket 36, and flange 142).

[0037] The cone 144 causes mixing of the central gas flow downstream from the plasma torch and the surrounding gas flow, increasing the resulting flow velocity, which helps to axially center the plasma torch within the plasma chamber 35 and prevent powder deposited on its inner wall from adhering to it.

[0038] Figure 10 shows a ring 150 having a first central hole 154 and a second central hole 153, at least two side holes 152, and a plurality of axial holes 156. The second central hole 153 is narrower than the first central hole 154 and is located downstream of the first central hole 154. The axial holes 156 may be distributed corresponding to the holes 146 of the tapered outlet 140. The ring 150 may be mounted around the cone 144 by several bolts between the holes 146, 156, in the configuration shown in Figure 9.

[0039] Another gas can be introduced through the hole 152 of the ring 150 and flowed around the cone 144 to cool it. If this gas flows at high speed, it draws the mixture 31 of the gas and powder spheroid (see Figure 11) out of the plasma chamber 35 and increases the velocity of this gas flow 31 (its velocity is also increased by the tapering of the cone 144). The annular portion between the cone 144 and the second central hole 153 can be narrowed, thus increasing the velocity of the gas introduced through the hole 152.

[0040] Gaskets 34, 36, and 139 can be made of graphite.

[0041] The external cooling disclosed in European Patent Application Publication No. 3996472(A1) may be combined with the internal cooling described herein.

[0042] While this specification discloses only a few examples, other alternative forms, modifications, uses, and / or equivalents are possible. Furthermore, all possible combinations of the examples described are also encompassed. Therefore, the scope of this disclosure should not be limited by any particular example, but should be determined solely by a fair reading of the following claims. Where reference numerals relating to drawings are placed in parentheses in a claim, the reference numerals are merely for the purpose of enhancing the understanding of the claim and should not be construed as limiting the scope of the claim.

Claims

1. The apparatus for powder spheroidization using microwave-induced plasma comprises a microwave radiation generator (4) for a microwave cavity (20), a plasma chamber (35) having an inner wall and located within the microwave cavity, a confinement tube (3) connected to the upstream and downstream sides of the plasma chamber, and a receptacle (2) for a mixture of process gas and powder precursor, connected to the confinement tube and supplying the mixture to the plasma chamber as an inflow gas stream (21) and coupling the gas stream with the microwave radiation. An apparatus comprising a receptacle (2) that generates a plasma torch in the plasma chamber by doing so, wherein the apparatus further comprises flow dividers (110, 120, 130) located in the confinement tube at the inlet of the plasma chamber, the flow dividers (110, 120, 130) forming wide passages and narrow passages such that the flow dividers separate the inflow gas flow (21) into a central gas flow passing through wide passages (118, 124, 138) and a peripheral gas flow passing through narrow passages (113, 119, 122, 137).

2. The apparatus according to claim 1, wherein the flow divider comprises tubular elements (110, 130), and the wide passages (118, 138) are ducts within the tubular elements.

3. The apparatus according to claim 2, wherein the tubular elements (110, 130) include a central portion (115, 135) and two ends (114, 116, 134, 136) that are wider than the central portion.

4. The apparatus according to claim 3, wherein the tubular element (110) comprises a ring (112) surrounding one end (114), leaving a plurality of slits (113) between the ring and the end, the slits being part of the narrow passage.

5. The apparatus according to claim 4, wherein the tubular element (110) is positioned axially inward of the confinement tube (3) by the ring (112) at the end (114) of the tubular element furthest from the plasma chamber (35).

6. The apparatus according to claim 4, wherein the tubular element (110) is positioned axially inward of the confinement tube (3) by the ring (112) at the end (114) of the tubular element closest to the plasma chamber (35).

7. The apparatus according to claim 3, wherein one end (136) of the tubular element (130) is wider than the other end (134).

8. The apparatus according to claim 7, wherein the tubular element (130) is positioned axially inward of the confinement tube (3) by the widest end (136) of the end of the tubular element furthest from the plasma chamber (35).

9. The apparatus according to any one of claims 2 to 8, wherein the tubular elements (110, 130) are arranged axially inward of the confinement tube (3), and the narrow passage is at least partially formed in the annular section (119, 137) remaining between the inner wall of the confinement tube (3) and the ends (116, 134) of the tubular elements (110, 130) closest to the plasma chamber (35).

10. The apparatus according to claim 1, wherein the flow divider comprises a perforated washer (120) having a central hole (124) as a wide passage and a plurality of small axial perforations (122) as narrow passages.

11. The apparatus according to claims 8 and 10, wherein the perforated washer (120) is positioned downstream of the tubular element (130).

12. The apparatus according to any one of claims 1 to 11, further comprising an outlet (140) located at the outlet of the plasma chamber (35), wherein the outlet comprises a tapered cone (144).

13. The apparatus according to any one of claims 1 to 12, further comprising a ring (150) located downstream of the plasma chamber (35), wherein the ring has a plurality of side holes (152).

14. The apparatus according to claims 12 and 13, wherein the ring (150) is arranged around the tapered cone (144).

15. A method for producing spheroids from a powder precursor using microwave-induced plasma, comprising the step of forming a plasma torch in a plasma chamber by coupling the inflow of process gas with microwave radiation, To produce a spheroid from the powder precursor by melting in air, the process gas acting as a carrier gas is used to transport the powder precursor into the plasma torch. A cooling step comprising: cooling a plasma tube from the inside by separating the incoming gas flow into a central gas flow and a peripheral gas flow, wherein the central gas flow generates the plasma torch within the plasma chamber and the peripheral gas flow cools the inner wall of the plasma chamber; A method for producing a product, further comprising the above.

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