Apparatus and method for powder spheroidisation

Abstract

An apparatus for powder spheroidisation comprises a microwave cavity (20), a plasma chamber (35) that has an inner wall and is located in the microwave cavity, a confinement tube (3) connected to the plasma chamber, a receptacle for a mixture of a process gas and a powder precursor, the receptacle being connected to the confinement tube to feed said mixture as an incoming gas flow to the plasma chamber, in order to produce a plasma torch therein by coupling the gas flow with microwave radiation, and a flow divider (110) that is located within the confinement tube at the entrance of the plasma chamber, the flow divider providing a broad passageway (118) and a narrow passageway (113), and causing the incoming gas flow to separate into a central gas flow through the broad passageway, and a peripheral gas flow through the narrow passageway.

Description

[0001] An apparatus for powder spheroidisation by microwave-induced plasma comprises a generator of microwave radiation for a microwave cavity, a plasma chamber that has an inner wall and is located in the microwave cavity, a confinement tube connected to the plasma chamber upstream and downstream thereof, a receptacle for a mixture of a process gas and a powder precursor, the receptacle being connected to the confinement tube to feed said mixture as an incoming gas flow to the plasma chamber, in order to produce a plasma torch therein by coupling the gas flow with the microwave radiation.

[0002] A method to make spheroids from a powder precursor, by means of a microwave-induced plasma, comprises the steps of forming a plasma torch in a plasma chamber by coupling an incoming flow of a process gas with microwave radiation, and of carrying the powder precursor into the plasma torch by means of the process gas acting as a carrier gas, in order to make spheroids from the powder precursor by in-flight melting.BACKGROUND ART

[0003] Different industrial fields require the use of spheroids made from powders of different materials. The need for an industrial process to yield spherical powder particles comes from seeking some of the following benefits resulting from the spheroidisation process: Improve the powders flowability. Increase the powders packing density. Eliminate powder internal cavities and fractures and thus improving the quality of a piece made with such powder. Improve the surface morphology of the particles.

[0004] Spheroidisation is a process of in-flight melting and controlled solidification. The powder precursor of irregular (angular or non-uniform) shape (often from waste materials) is carried into an induction or microwave-induced plasma and is melted immediately in the high temperatures of plasma. The melted powder particles assume a spherical shape under the action of the surface tension of the liquid state. These droplets are cooled down when they fly out of the plasma torch. The resulting spheroids are thus collected as the spheroidisation products.

[0005] A microwave-induced plasma is a type of plasma that has high frequency electromagnetic radiation in the GHz range. It is capable of exciting electrodeless gas discharges.

[0006] EP3996472A1 discloses an apparatus for powder spheroidisation 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 in the microwave cavity, a powder supply connected to the plasma chamber to feed a powder precursor flow thereinto, a gas supply connected to the plasma chamber to feed a process gas flow thereinto, in order to form a plasma torch in the plasma chamber by coupling the process gas flow with the microwave radiation, and a compressed air supply for cooling the plasma chamber. The microwave cavity comprises at least one opening for the compressed air so that the latter can cool the plasma chamber from outside, and the gas supply is connected to the powder supply to let the process gas carry the powder precursor into the plasma tube, and so into the plasma torch, in order to make spheroids from the powder precursor by in-flight melting.

[0007] With the setup of EP3996472A1, it has been observed that some powder may adhere to the inner wall of the plasma chamber. This may be undesirable for several reasons: the adhered powder can form a coating that may shield the chamber to the microwaves needed to produce the plasma; the adhered powder can be hotter than the melting point of the chamber wall and may damage it; the hot adhered powder might impair the cooling of the plasma chamber. SUMMARY

[0008] It is an object of the present disclosure to provide an apparatus and a method that overcomes or mitigates the drawbacks of the art.

[0009] In a first aspect, an apparatus is provided to procure powder spheroidisation by microwave-induced plasma, comprising a generator of microwave radiation, a microwave cavity, a plasma chamber that has an inner wall and is located in the microwave cavity, a confinement tube connected to the plasma chamber upstream and downstream thereof, a receptacle for a mixture of a process gas and a powder precursor, the receptacle being connected to the confinement tube to feed said mixture as an incoming gas flow to the plasma chamber, in order to produce a plasma torch therein by coupling the gas flow with the microwave radiation, and a flow divider that is located within the confinement tube at the entrance of the plasma chamber, the flow divider providing a broad passageway and a narrow passageway, so that the flow divider would cause the incoming gas flow to separate into a central gas flow through the broad passageway, and a peripheral gas flow through the narrow passageway.

[0010] In this way, the central gas flow can be driven to produce the plasma torch in the plasma chamber, and the peripheral flow can be driven to cool and clean the inner wall thereof.

[0011] It is to be understood that the term 'broad passageway' means that this passageway is significantly wider than that termed as 'narrow passageway', i.e., the cross-section of the broad passageway is unequivocally larger than that of the narrow passageway.

[0012] In an example, the broad passageway may be located at a central region of the flow divider. The broad passageway may be comprised in the flow divider.

[0013] In an example, the narrow passageway may be located at or about a peripheral region of the flow divider. The narrow passage may be comprised in the flow divider or the latter may contribute to forming the narrow passage, for instance in combination with the inner wall of the plasma chamber.

[0014] In a second aspect, a method to make spheroids from a powder precursor, by means of a microwave-induced plasma, comprises the steps of forming a plasma torch in a plasma chamber by coupling an incoming flow of a process gas with a microwave radiation, carrying the powder precursor into the plasma torch by means of the process gas acting as a carrier gas, in order to make spheroids from the powder precursor by in-flight melting, and cooling the plasma tube from inside by separating the incoming gas flow into a central gas flow and a peripheral gas flow, the central gas flow producing the plasma torch in the plasma chamber and the peripheral flow cooling the inner wall thereof.DESCRIPTION OF THE DRAWINGS

[0015] Non-limiting examples of the present disclosure will be described in the following, with reference to the appended drawings, in which: figure 1 is a perspective view of an element to refrigerate the inner wall of a plasma chamber; figure 2 is a sectional elevation view of the element of Figure 1 mounted at the entrance of a plasma chamber; figure 3 is a perspective view of a perforated washer; figure 4 is a partially cut perspective view of the washer of figure 3 mounted at the entrance of a plasma chamber; figure 5 is a sectional elevation view of the element of figure 1 mounted invertedly at the entrance of a plasma chamber; figure 6 is a perspective view of another element to refrigerate the inner wall of a plasma chamber; figure 7 is a sectional elevation view of the element of figure 6 mounted at the entrance of a plasma chamber; figure 8 is a perspective view of an element to centre a plasma torch in a plasma chamber; figure 9 is a sectional elevation view of the element of figure 8 mounted at the exit of a plasma chamber; figure 10 is a perspective view of a ring to be mounted at the exit of a plasma chamber; and figure 11 is a schematic view of an apparatus for powder spheroidisation. DETAILED DESCRIPTION

[0016] An apparatus for powder spheroidisation by microwave-induced plasma is shown schematically in figure 11. This apparatus is similar to that disclosed in EP3996472A1, although it may or may not include the external cooling system described therein, and may include other features such as those introduced in the present specification (making reference to figures 1 to 10). For the sake of completeness, the disclosure of EP3996472A1 is incorporated herein by reference.

[0017] The apparatus of figure 11 comprises a receptacle 1 with a compressed process gas 11, a tank 2 with an irregular powder precursor, a confinement tube 3, a microwave generator 4, a microwave circulator 5, a waveguide 6, a receptacle 7 for the powder spheroids exiting the confinement tube, a filter 8 to intercept the 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.

[0018] The powder precursor and the process gas are mixed in the receptacle 2 and the resulting gas mixture 21 is carried into the confinement tube 3 as a gas flow.

[0019] The circulator 5 protects the microwave generator 4. The waveguide 6 and the waveguide short circuit 10 convey the microwave radiation into the microwave cavity 20, which is crossed by the confinement tube 3.

[0020] 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 made in one piece. However, it may be convenient for the plasma chamber 35 to be a separate piece, in which case the part of the confinement tube located upstream from the plasma chamber may be joined thereto through mechanical junctions 30, 34, 32, e.g. a cover 30, a gasket 34 and a clamp 32.

[0021] A plasma torch is created in the plasma chamber 35 by coupling the process gas flow 21 (carrying the powder precursor) with the microwave radiation that resonates in the microwave cavity. The wall of the plasma chamber 35 can be made of quartz, that is invisible to microwave radiation and resistant to high temperatures, but also of other materials having these properties, like sapphire, mica, boron nitride, alumina, etc.

[0022] The receptacle 7 collects the mixture 31 of gas and powder spheroids exiting the plasma chamber 35. The powder spheroids are left in receptacle 7 to cool down and the gas exits the receptacle as a mixture 71 still having some small powder particles. Filter 8 intercepts this small powder particles and provides clean gas 81 to be reused as process gas.

[0023] It is advantageous that the flow of process gas is laminar or near laminar, in order to keep the powder particles on a given path. The aim is not a perfectly laminar flow but a flow without turbulences, which would impair the plasma.

[0024] Figure 1 shows a first tubular element 110 to be mounted at the entrance of the plasma chamber 35. The first tubular element 110 comprises a central portion 115, two end portions 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 that surrounds the end portion 114. There are some slits 113 between the ring 112 and the end portion 114. The duct 118 may be cylindrical.

[0025] Continuing with figure 2, it further shows the first tubular element 110 mounted at the entrance 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 located between a cover 30 and the plasma chamber 35 for the sake of gas tightness. Most of the gaseous mixture 21 enters this portion of the confinement tube 3 through the duct 118, but some goes through the slits 113. This arrangement separates the gaseous mixture 21 into two gas flows: a central gas flow (through duct 118) and a peripheral gas flow (through slits 113).

[0026] The central gas flow becomes the plasma torch inside the plasma chamber 35. The peripheral gas flow is faster than the central gas flow because it goes through narrow passageways: first through the slits 113 and then through a section 119 left between the inner wall of the confinement tube 3 and the relatively wide end portion 116 of the first tubular element 110. The fast peripheral flow achieves two important results in the plasma chamber 35: it cools the wall of the plasma chamber and it drags the powder deposited on said wall or adhered thereto.

[0027] Figure 3 shows a perforated washer 120 provided with a big central hole 124 and a plurality of small axial perforations 122 distributed around the central hole 124. Figure 4 shows the perforated washer 120 mounted between the junction cover 30 and the gasket 34 at the entrance of the plasma chamber 35. Most of the gaseous mixture 21 enters the plasma chamber 35 through the hole 124, but some goes through the perforations 122. Again, this arrangement separates the gaseous mixture 21 into the central gas flow and the peripheral gas flow, with the effect explained above, at least to a sufficient extent. Besides, this arrangement is simpler than that of figure 2.

[0028] Figure 5 shows a variant of the example of figure 2. The elements of figure 5 are substantially the same than those of figure 2 but, in figure 5, the first tubular element 110 is mounted in an inverted position with respect to its position in figure 2, i.e. upside down. This arrangement is intended for the cases in which the gas of the central flow is different from the gas of the peripheral flow. In such cases, the gas of the central flow runs downstream through the confinement tube 3, and the gas of the peripheral flow may enter sideways, for instance, from a different conduit 38.

[0029] Figure 6 shows a second tubular element 130 which is a variant of the first tubular element 110. In figure 6, the second tubular element 130 is represented in an inverted position with respect to the position of the first tubular element 110 in figure 1. The second tubular element 130 comprises two end portions 134 and 136, a central portion 135 that is narrower than the end portions 134 and 136, and a duct 138. The second tubular element 130 further comprises a holder 132 to support it by its central portion 135. The duct 118 may be cylindrical.

[0030] Figure 7 shows the second tubular element 130 mounted at the entrance of the plasma chamber 35, within the confinement tube 3. A gasket 139 between the cover 30 and the end portion 136 insures the gas tightness of the junction between the confinement tube 3 and the plasma chamber 35. The arrangement shown in figure 7 is similar to that of figure 5 and, analogously, is intended for the cases in which the gas of the central flow is different from the gas of the peripheral flow. In such cases, the gas of the central flow runs downstream through the confinement tube 3 and the gas of the peripheral flow may enter the confinement tube 3 sideways from a different conduit 38.

[0031] In the example of figure 7, the peripheral flow enters the plasma chamber 35 through a section 137 left between the inner wall of the confinement tube 3, at the entrance or the plasma chamber, and the relatively wide end portion 134 of the second tubular element 130.

[0032] The perforated washer 120 can be mounted in the setup of figure 7 between the end portion 134 and the gasket 34, clamped by the clamp 32 at the entrance of the plasma chamber 35. This arrangement may reinforce the division of the gas mixture into a central gas flow and a peripheral gas flow.

[0033] Figure 9 shows a tapering outlet 140 mounted at the exit of the plasma chamber 35. Figure 8 shows the tapering outlet 140 by itself. The tapering outlet 140 comprises a flange 142 and a cone 144. A clamp 33 attaches the flange 142 to the plasma chamber 35 and the gas tightness thereof is insured by a gasket 36 located between the plasma chamber and the outlet 140. The flange 142 comprises a plurality of holes 146 for its attachment to the clamp 33 (e.g. through bolts).

[0034] The part of the confinement tube 3 that is located downstream from the plasma chamber 35 may be joined thereto through the mechanical junctions 33, 36, 142, i.e. the clamp 33, the gasket 36 and the flange 142.

[0035] The cone 144 causes the mixing of the central gas flow and the peripheral gas flow downstream from the plasma torch and increases the speed of the resulting flow, thereby helping to keep the plasma torch axially centered within the plasma chamber 35 and the inner wall thereof free of deposited powder.

[0036] Figure 10 shows a ring 150 provided with 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 that the first central hole 154 and is located downstream therefrom. The axial holes 156 may be distributed correspondingly with the holes 146 of the tapering outlet 140. The ring 150 can be mounted in the setup of figure 9, around the cone 144, by means of some bolts between the holes 146 and 156.

[0037] Another gas can be introduced through the holes 152 of the ring 150 and made to flow around the cone 144 in order to cool it. If this gas flows at high speed, it will suck the mixture 31 of gas and powder spheroids (see figure 11) out of the plasma chamber 35, adding speed to this gas flow 31 (the speed of which is also increased by the tapering of cone 144). The annular section between the cone 144 and the second central hole 153 can be narrow, thus adding speed to the gas introduced through the holes 152.

[0038] The gaskets 34, 36 and 139 may be made of graphite.

[0039] The external cooling disclosed in EP3996472A1 can be combined with the internal cooling described herein.

[0040] Although only a number of examples have been disclosed herein, other alternatives, modifications, uses and / or equivalents thereof are possible. Furthermore, all possible combinations of the described examples are also covered. Thus, the scope of the present disclosure should not be limited by particular examples, but should be determined only by a fair reading of the claims that follow. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.

Claims

1. Apparatus for powder spheroidisation by microwave-induced plasma, comprising a generator (4) of microwave radiation for a microwave cavity (20), a plasma chamber (35) that has an inner wall and is located in the microwave cavity, a confinement tube (3) connected to the plasma chamber upstream and downstream thereof, a receptacle (2) for a mixture of a process gas and a powder precursor, the receptacle being connected to the confinement tube to feed said mixture as an incoming gas flow (21) to the plasma chamber, in order to produce a plasma torch therein by coupling the gas flow with the microwave radiation, characterized by further comprising a flow divider (110; 120; 130) that is located within the confinement tube at the entrance of the plasma chamber, the flow divider providing a broad passageway (118; 124; 138) and a narrow passageway (113, 119; 122; 137), so that the flow divider would cause the incoming gas flow (21) to separate into a central gas flow, through the broad passageway, and a peripheral gas flow, through the narrow passageway.

2. Apparatus according to claim 1, wherein the flow divider comprises a tubular element (110; 130) and the broad passageway (118; 138) is a duct in the tubular element.

3. Apparatus according to claim 2, wherein the tubular element (110; 130) comprises a central portion (115; 135) and two end portions (114, 116; 134, 136) that are wider than the central portion.

4. Apparatus according to claim 3, wherein the tubular element (110) comprises a ring (112) that surrounds one of the end portions (114) and leaves a plurality of slits (113) between the ring and said end portion, said slits being a part of the narrow passageway.

5. Apparatus according to claim 4, wherein the tubular element (110) is placed axially inside the confinement tube (3) with the ring (112) at the end portion (114) of the tubular element that is furthest from the plasma chamber (35).

6. Apparatus according to claim 4, wherein the tubular element (110) is placed axially inside the confinement tube (3) with the ring (112) at the end portion (114) of the tubular element that is closest to the plasma chamber (35).

7. Apparatus according to claim 3, wherein one of the end portions (136) of the tubular element (130) is wider than the other end portion (134) thereof.

8. Apparatus according to claim 7, wherein the tubular element (130) is placed axially inside the confinement tube (3) with the widest end portion (136) at the end of the tubular element furthest from the plasma chamber (35).

9. Apparatus according to any of claims 2 to 8, wherein the tubular element (110; 130) is placed axially inside the confinement tube (3) and the narrow passageway is formed, at least partially, in an annular section (119; 137) left between an inner wall of the confinement tube (3) and the end portion (116; 134) of the tubular element (110; 130) that is closest to the plasma chamber (35).

10. Apparatus according to claim 1, wherein the flow divider comprises a perforated washer (120) having a central hole (124) as the broad passageway and a plurality of small axial perforations (122) as the narrow passageway.

11. Apparatus according to claims 8 and 10, wherein the perforated washer (120) is placed downstream from the tubular element (130).

12. Apparatus according to any of the preceding claims, comprising an outlet (140) located at the exit of the plasma chamber (35), the outlet comprising a tapering cone (144).

13. Apparatus according to any of the preceding claims, comprising a ring (150) located downstream from the plasma chamber (35), the ring comprising a plurality of side holes (152).

14. Apparatus according to claims 12 and 13, wherein the ring (150) is placed around the tapering cone (144).

15. Method to make spheroids from a powder precursor by means of a microwave-induced plasma, comprising the step of forming a plasma torch in a plasma chamber by coupling an incoming flow of a process gas with a microwave radiation, characterized by further comprising the steps of: - carrying the powder precursor into the plasma torch by means of the process gas acting as a carrier gas, in order to make spheroids from the powder precursor by in-flight melting; - cooling the plasma tube from inside by separating the incoming gas flow into a central gas flow and a peripheral gas flow, the central gas flow producing the plasma torch in the plasma chamber and the peripheral flow cooling the inner wall thereof.