Gas atomizer

Abstract

The present invention relates to a process for producing metal powder, the process comprising: (i) supplying molten metal to a chamber of a gas atomizer, (ii) atomizing the molten metal by injection of a gas to form metal particles, and (iii) cooling the metal particles in a lower section of the chamber by injecting a gas from the bottom of the chamber to form a foaming fluidized bed of metal particles. The present invention also relates to that gas atomizer.

Description

[Technical Field]

[0001] The present invention relates to a gas atomizer for producing metal powder, in particular for producing steel powder for additive manufacturing processes. The present invention also relates to a method for producing metal powder by gas atomization. [Background technology]

[0002] Demand for metal powders for additive manufacturing is increasing and manufacturing processes have to be adapted accordingly.

[0003] In particular, it is known to melt a metallic material and pour the molten metal into a tundish connected to an atomizer. The molten metal is forced through a nozzle in a chamber under a controlled atmosphere and impinged by a gas jet, which atomizes the molten metal into fine metal droplets. The latter solidify into fine particles that fall to the bottom of the chamber and accumulate there until the molten metal is completely atomized. The powder is then cooled in the atomizer until it reaches a temperature at which it can be contacted with air without oxidizing too rapidly. The atomizer is then opened and the powder is collected. This cooling process is lengthy and not compatible with the need to produce large quantities of metal powder. Summary of the Invention [Problem to be solved by the invention]

[0004] It is therefore an object of the present invention to ameliorate the shortcomings of prior art equipment and processes by providing a gas atomizer in which the resulting powder can be rapidly cooled within the atomizer chamber.

[0005] Additionally, the prior art processes described above are batch processes that are not suited to the need to produce large quantities of metal powder in a continuous mode.

[0006] It is a further object of the present invention to provide a gas atomizer in which the resulting powder can be discharged from the atomizer chamber without interrupting atomization. [Means for solving the problem]

[0007] To this end, a first subject of the invention consists of a process for producing metal powders, said process comprising: (i) providing molten metal into a gas atomizer chamber; (ii) atomizing the molten metal by injection of a gas to form metal particles; (iii) cooling the metal particles in the lower section of the chamber by injecting a gas from the bottom of the chamber to form a foaming fluidized bed of the metal particles; Equipped with.

[0008] The process according to the invention may also have the optional features listed below, considered individually or in combination. The molten metal is steel obtained through the blast furnace route; The molten metal is steel obtained through the electric arc furnace route, Steps (ii) and (iii) are carried out simultaneously; In step (iii), the metal particles are cooled to below 300°C; In step (iii), the injected gas is extracted, cooled and re-injected, the gas being cooled to below 50°C; The process further comprises the step (iv) of continuously discharging the metal particles from the chamber; Continuous discharge is achieved through the overflow section. The process further comprises the step (v) of conveying the discharged metal particles to a classification station; The discharged metal particles are transported in the form of a fluidized bed.

[0009] A second subject of the invention consists of a gas atomizer comprising a chamber, a gas injector arranged at the bottom of the chamber, and a flow regulator coupled to the gas injector for fluidizing metal particles accumulated in the lower section of the chamber and forming a foaming fluidized bed of metal particles.

[0010] The gas atomizer according to the invention may also have the optional features listed below, considered individually or in combination. the gas injector has an opening in the bottom wall of the chamber; The distance between the bottom of the chamber and the gas injector is preferably less than 10 cm; The gas injector is a sparger, the gas atomizer further comprising a heat exchanger disposed in the lower section of the chamber; the gas atomizer further comprising an overflow portion in the lower section of the chamber; the overflow portion is a pipe that extends at least partially into the lower section of the chamber and passes through the bottom wall of the chamber; a portion of the overflow section outside the chamber comprising a gas inlet; the gas atomizer further comprising a coarse particle collector at the bottom of the chamber; the gas atomizer further comprising a gas extractor in an upper section of the chamber; the gas extractor comprises a cyclone separator for culling the gas extracted from the chamber; The gas extractor is connected to the gas injector for gas recirculation within the atomizer; The connection between the gas extractor and the gas injector comprises a heat exchanger.

[0011] A third subject of the invention consists of an installation comprising a gas atomizer according to the invention and a conveyor, the conveyor comprising a lower duct for circulating gas, an upper duct for circulating powder material and a porous wall separating the lower duct and the upper duct over substantially its entire length.

[0012] The equipment according to the present invention may optionally comprise a conveyor, the conveyor comprising a fluidizing gas inlet and a flow regulator coupled to the gas inlet for fluidizing the metal particles discharged from the gas atomizer and forming a fluidized bed of the metal particles in the upper duct.

[0013] As can be seen, the present invention is based on the technology of fluidized bed for efficiently cooling the powder accumulating at the bottom of the atomizer chamber. If an overflow is added to the lower section of the atomizer, the fluidized powder can be continuously discharged from the atomizer without interrupting the atomization process.

[0014] Other features and advantages of the present invention will be explained in more detail in the following description.

[0015] The invention will be better understood from reading the following description, which is provided for illustrative purposes only and is not intended to be limiting in any way. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 10 shows a gas atomizer according to a modified example of the present invention. [Figure 2] FIG. 1 illustrates a possible scheme for fluidization. [Figure 3] FIG. 1 shows an installation with two atomizers and a conveyor according to a first variant of the invention. [Figure 4] FIG. 1 shows an installation with two atomizers and a conveyor according to a second variant of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] It should be noted that the terms "lower", "beneath", "inward", "inwards", "outward", "outwards", "upstream", "downstream", ... used in this application refer to the locations and orientations of different components of the apparatus when the apparatus is installed in a plant.

[0018] Referring to Figure 1, a gas atomizer 1 is a device designed to atomize a stream of liquid metal into fine metal droplets by colliding it with a high-velocity gas stream. The gas atomizer 1 consists primarily of a closed chamber 2 maintained under a protective atmosphere. The chamber has an upper section, a lower section, a top, and a bottom.

[0019] The upper section of the chamber contains an orifice, and a nozzle 3 is typically located centrally at the top of the chamber, through which the molten metal stream is extruded. The nozzle is surrounded by a gas atomizer 4 for injecting gas at high velocity into the liquid metal stream. The gas atomizer is preferably an annular slot through which pressurized gas flows. The gas atomizer is preferably coupled to a gas regulator 5 for controlling the flow and / or pressure of the gas prior to injection. The gas regulator may be a compressor, fan, pump, pipe section reducer, or any suitable device.

[0020] The lower section of the chamber is primarily a receptacle for collecting metal particles that fall from the upper section of the chamber. It is designed to facilitate collection and discharge of the powder through a discharge opening, usually located at the bottom of the chamber. Therefore, it is usually in the form of an inverted cone or an inverted frustum of a cone.

[0021] The gas atomizer comprises a gas injector 6 arranged at the bottom of the chamber, which is capable of fluidizing the metal particles accumulated in the lower section of the chamber and forming a foaming fluidized bed of metal particles. (Bubbling fluidized bed)Thanks to this fluidized bed, the metal particles are efficiently cooled below their oxidation window by vigorous gas-to-particle heat transfer. Metal particles accumulated in the lower section of the chamber remain cooled, and hot particles falling from the top of the chamber are very rapidly mixed and cooled within the fluidized bed. Furthermore, because cooling occurs directly within the chamber, which is maintained under a protective atmosphere, the metal particles do not oxidize during their cooling.

[0022] As shown in Figure 2, there are several fluidization regimes. Fluidization is the process of suspending solid particles in a gas or liquid to create a fluid state. Particle behavior varies depending on the fluid velocity. In a gas-solid system such as the present invention, as the flow rate increases, the particle bed progresses from a fixed bed to minimal fluidization, bubbling fluidization, and slugging, where agitation becomes more intense and solids move more actively. In particular, as the flow rate increases beyond minimal fluidization, instabilities due to gas bubbling and channeling are observed. At this stage, the fluidized bed is in the bubbling regime, which is necessary for the present invention to achieve good circulation, rapid cooling, and uniform temperature of the solid particles within the fluidized bed. The gas velocity applied to achieve a given regime and desired temperature in the fluidized bed depends on several parameters, such as the type of gas used, the size and density of the particles, the gas pressure drop provided by the gas injector, or the size of the chamber. This can be easily controlled by those skilled in the art. Furthermore, in the bubbling regime, the bed does not expand significantly beyond the solid volume, which helps keep the equipment within a reasonable size. The concept of a foaming fluidized bed is defined in "Fluidization Engineering" by Daizo Kunii and Octave Levenspiel, 2nd Edition, 1991, especially pages 1 and 2 of the Introduction.

[0023] Thanks to the effervescent fluidized bed, and in contrast to other forms of fluidized bed, the metal particles are cooled very quickly and very efficiently to the operating temperature of the fluidized bed while maintaining a uniform distribution of particle size within the bed. As a result, there is no need to use powdered coolants to help cool the metal particles.

[0024] In the context of the present invention, "located at the bottom of the chamber" means that the gas injector 6 is located in the lower section of the chamber, close enough to the chamber bottom 7, so that substantially all particles formed in the atomizer are fluidized. Solidified droplets resulting from the initial non-atomized metal stream and / or coarse particles may not be fluidized and may fall below the gas injector, i.e., below the fluidized bed. The distance between the bottom of the chamber and the gas injector is preferably less than 10 cm, more preferably less than 4 cm, and even more preferably 1-3 cm.

[0025] A gas injector 6 injects gas from the bottom of the chamber towards the top of the chamber so that particles at the bottom of the chamber are lifted and form a fluidized bed.

[0026] The gas injector may comprise openings in the bottom wall of the chamber through which gas may be injected to fluidize the powder bed.

[0027] The gas injector may comprise a pipe 8 passing through the side wall of the chamber. The part of the gas injector located inside the chamber may closely follow the shape of the bottom wall, as shown in the example in Figure 1.

[0028] The gas injector can include a porous metal plate, a sintered metal plate, or a canvas. The gas injector preferably includes a sparger, which is a component such as a pipe perforated with many small holes to provide dispersion of the injected gas. Spargers are preferred for gas velocities greater than 10 cm / s because they provide sufficient pressure drop. The sparger is more preferably a porous sparger. This type of sparger ensures gas distribution within the bed of metal particles through thousands of small holes.

[0029] Each sparger may include a grommet seal (a compression fitting) that allows the sparger to be inserted into and removed from the atomizer while the atomizer is operating.

[0030] The gas injector is coupled to a flow regulator 9. The latter controls the flow of gas injected through the gas injector and therefore the velocity of the gas within the chamber, since the chamber cross-section is known. The gas flow can therefore be adjusted so that the metal particles are fluidized and the resulting fluidized bed is maintained in a foaming manner. The gas regulator may be in the form of a fan. The fan speed is adjusted to control the flow of gas injected through the gas injector. The flow regulator is connected to a gas source. The gas source can be a gas inlet 10 designed to admit fresh gas and / or a gas extractor providing recirculated gas, as described below.

[0031] The gas atomizer 1 preferably includes a gas extractor 11 for compensating the gas injection through the gas injector 6 and the gas atomizer 4. The gas extractor is preferably located in the upper section of the chamber so as not to interfere with the fluidized bed and / or so that particles above the fluidized bed, due to the scattering of gas bubbles, reach the high gas velocity region where they fall by gravity back to the bed and are entrained in the gas extractor. The gas extractor may be in the form of a single pipe or multiple pipes connected to the chamber on one side and to a dust removal means 12 on the other. The dust removal means remove the finest particles from the extracted gas. They may include electric filters, bag filters, or cyclone separators. Cyclone separators are preferred as they have a relatively low pressure drop and no moving parts.

[0032] Preferably, the gas extractor 11 is designed so that the gas injected into the chamber and extracted via the gas extractor can be recirculated, thereby minimizing gas consumption. The gas extractor is therefore preferably connected to the gas injector 6, the gas atomizer 4, or both. In particular, the dust removal means 12, connected on one side to the chamber, is connected on the other side to the gas regulator 5 coupled to the gas atomizer 4, or to the flow regulator 9 coupled to the gas injector 6, or to both.

[0033] In the example shown in Figure 1, one dedusting means 12 in the form of a cyclone separator is connected to the gas regulator 5 for injecting gas into the metal stream so that the gas injected into the chamber to atomize the metal is recycled. Another dedusting means 12 in the form of a cyclone separator is connected to the gas regulator 5 for injecting gas into the bottom of the chamber so that the gas used to fluidize the powder bed is recycled. In both cases, filters can be added to clean the recycled gas. Other designs for gas recycling are of course possible.

[0034] The connection between the gas extractor 11 and the gas injector 6 preferably comprises a heat exchanger 13 so that the gas can be cooled to the temperature at which it must be injected into the chamber if the heat loss in the connection is not sufficient to bring the gas back to the desired temperature and / or if heat recovery is desired.

[0035] The connection between the gas extractor 11 and the gas injector 6 may also be provided with a gas inlet 10 in case some fresh gas has to be introduced into the system, especially to compensate for gas losses.

[0036] The connection between the gas extractor 11 and the gas atomizer 4 is preferably equipped with a heat exchanger 13 so that the gas can be cooled to the temperature at which it must be injected onto the molten metal stream if the heat losses in the connection are not sufficient to bring the gas back to the desired temperature and / or if heat recovery is desired.

[0037] The connection between the gas extractor 11 and the gas atomizer 4 may also be provided with a gas inlet 10 in case some fresh gas has to be introduced into the system, especially to compensate for gas losses.

[0038] According to one variant of the invention, the gas atomizer further comprises a heat exchanger 14 located in the lower section of the chamber. This is positioned so that the foaming fluidized bed 15 formed with the chamber is in contact with the heat exchanger. The heat exchanger can be located at least partially within the chamber or can be a cooling jacket around the lower section of the chamber. The solid particles, kept in motion by the injection of gas through the gas injector 6, come into contact with the heat exchanger and release the captured heat to the transfer medium circulating therein. The flow rate of the medium in the heat exchanger can be adjusted to control the cooling rate. Such a heat exchanger facilitates cooling of the particles in the fluidized bed and maintaining them at the desired temperature. The heat exchanger can also reduce the gas flow required to cool or maintain the particles at the desired temperature.

[0039] According to one variant of the invention, the gas atomizer 1 further comprises a coarse particle collector 16 below the bottom of the chamber. As mentioned above, solidified droplets resulting from the initial non-atomized metal flow and / or coarse particles are not fluidized and may fall below the gas injector at the bottom of the chamber, i.e., below the fluidized bed. The coarse particle collector allows these unwanted particles to be discharged from the atomizer without interrupting atomization. The coarse particle collector preferably comprises a valve 17 and a collection chamber 18. The collection chamber can be connected to the movable chamber via a second valve. In this way, the movable chamber can be replaced without compromising the pressure in the chamber.

[0040] According to one variant of the invention, once the metal particles have been produced and cooled by the fluidized bed, they are discharged through a discharge opening located at the bottom of the chamber, which, depending on the technology of the discharge opening, can be done once the batch of molten metal has been atomized or without interrupting the atomization.

[0041] According to another variant of the invention, the gas atomizer comprises an overflow 19 in the lower section of the chamber, the purpose of which is to discharge the powder from the chamber 2. In particular, the fluidized powder in the lower section of the chamber can be discharged from the gas atomizer in continuous mode as soon as the level of the fluidized bed reaches the top of the overflow 19. The atomizer can therefore be operated continuously.

[0042] The overflow section 19 preferably extends at least partially into the lower section of the chamber and passes through the bottom wall 7 of the chamber. It may be in the form of a downcomer, or more preferably a pipe. Its section is preferably adapted to the powder flow exiting the chamber. In particular, it is adapted to the molten metal flow exiting the nozzle so that there is no accumulation of powder in the lower section of the chamber over time. If coarser particles formed in the atomizer collect at the bottom of the chamber, the section of the overflow section is preferably adapted to the molten metal flow exiting the nozzle, so that the coarser particles are removed. The cross section of the pipe is preferably constant, i.e., does not decrease along the pipe or at its upper end, to promote uniform discharge of the metal powder and avoid clogging. In one variant of the invention, the overflow section, or, if applicable, the pipe, is equipped with a valve for adjusting the powder flow exiting the chamber. In one variant of the invention, the lower end of the overflow section has a reduced cross section to further restrict the flow of gas from the outside to the inside of the atomizer.

[0043] The height of the overflow section is defined as the vertical distance between the top of the overflow section and the bottom of the chamber, i.e., the vertical length of the portion of the overflow section extending into the chamber. The height of the overflow section is preferably set so that the volume of the fluidized bed is large enough to cool the metal powder to the desired temperature. The volume of the fluidized bed is actually substantially defined by the cross section of the lower section of the chamber and the height of the overflow section. If the height of the overflow section is short, the volume of the fluidized bed is small and the residence time of the particles in the fluidized bed is short. As a result, the discharged particles are still hot. If the height of the overflow section is very long, the volume of the fluidized bed is large and the residence time of the particles in the fluidized bed is long. As a result, the discharged particles are cold. Based on these principles, a person skilled in the art can select the height of the overflow section depending on the dimensions of the chamber and the desired temperature of the discharged particles. In one variant of the invention, the overflow section, or if applicable, the pipe, is equipped with a height adjustment means so that the height of the overflow section can be adjusted on the fly, particularly to adjust the cooling of the powder and therefore the temperature of the powder discharged from the chamber.

[0044] Thanks to the overflow, the residence time of particles in the fluidized bed is uniform regardless of particle size, in contrast to other solutions such as valves or pipes at the bottom of the chamber, where coarser particles are discharged first and allowed to cool to the working temperature of the fluidized bed. Furthermore, because the amount of gas leaving the chamber through the overflow is low, most of the injected gas is used to fluidize the bed, contributing to a very stable fluidized bed. Furthermore, the overflow is not a mechanical component, which limits its wear due to particles.

[0045] According to one variant of the invention, the overflow section 19 is covered by a hat 20. This prevents the hot metal powder falling from the upper section of the chamber from directly entering the overflow section. The hat is positioned high enough above the top of the overflow section so as not to obstruct the flow of powder exiting through the overflow section. The hat and the top of the overflow section can be positioned substantially perpendicular to the nozzle 3, and the hat can be equipped with an impact pad. In this configuration, the flow of molten metal that is not atomized at the start of the atomization process hits the impact pad and is dispersed into small particles that do not adversely affect the process.

[0046] According to one variant of the invention, the overflow section 19, preferably the part of the overflow section outside the chamber, further comprises a gas inlet 21. As a result, a gas, preferably a gas used to fluidize the powder in the chamber, can be injected into the overflow section. This helps to keep the expelled powder in a fluidized form and prevents the atmosphere downstream of the overflow section from entering the chamber.

[0047] The powder discharged from the chamber through the overflow can be collected by a chamber, a container or a conveyor 22. The conveyor is part of the installation comprising the gas atomizer 1. Preferably, it transports the powder to a classification station 23 and / or a bagging station. The conveyor can in particular be a vacuum-pneumatic conveyor, a pressure conveyor or a suction-pressure conveyor.

[0048] According to a variant of the invention shown in Figures 3 and 4, the powder discharged from the chamber 2 is conveyed in the form of a fluidized bed 24, preferably a foaming fluidized bed. This type of conveyance is advantageous because it requires minimal ventilation power, prevents dust emissions and ensures continuous operation.

[0049] The conveyor 22 preferably comprises a lower duct 25 for circulating a fluidizing gas, an upper duct 26 for circulating the powder, and a porous wall 27 separating the lower and upper ducts over substantially their entire length.

[0050] The porous wall allows the fluidizing gas to pass through it. Such a porous wall is designed to provide a sufficient pressure drop as the gas passes through it to ensure uniform distribution of the gas across the cross section of the upper duct. The porous wall can be multi-layer canvas or porous refractory.

[0051] The lower duct is supplied with fluidizing gas by a fluidizing gas inlet 29 coupled to a flow regulator 28. The fluidizing gas inlet can be in the form of a fluidizing gas inlet conduit, and the flow regulator can be in the form of a fan. The flow regulator controls the flow of gas injected into the lower duct, and therefore the velocity of the gas in the upper duct, due to the known surface of the porous wall. Thus, the gas flow can be adjusted to fluidize the metal particles in the upper duct. If the flow regulator is a fan, its velocity is adjusted to control the flow of fluidizing gas injected into the lower duct. The flow regulator is connected to a gas source. The gas source can be a gas inlet designed to admit fresh gas and / or a conduit supplying recirculated gas.

[0052] This even distribution of gas across the cross section of the upper duct allows only one flow regulator 28 to be used for the entire conveyor, simplifying installation and maintenance.

[0053] The conveyor 22 is provided with at least one pressure valve 30 at the top of the upper duct 26 so that the pressure of the fluidizing gas in the upper duct can be adjusted. The pressure valve is preferably connected to the upper duct via a cyclone 31 arranged in a cyclone box 32. In this way, the fluidizing gas leaving the upper duct through the pressure valve is filtered, i.e., bed particles entrained by the flow of fluidizing gas are separated from the gas and fall into the fluidized bed. The cyclone box is preferably located above the height of the top of the upper duct to minimize entrainment of particles in the cyclone.

[0054] Preferably, the conveyor 22 comprises a plurality of pressure valves 30 distributed along the length of the upper duct, which restricts horizontal circulation of the fluidizing gas above the fluidized bed and further stabilizes the fluidized bed. More preferably, the plurality of pressure valves are combined with gas dams 33, each dam being located in the upper portion of the upper duct and laterally between two consecutive pressure valves 30. These gas dams further restrict horizontal circulation of the fluidizing gas above the fluidized bed.

[0055] The conveyor 22 has a conveyor overflow section 34 at one end for discharging powder from the classification station 23 and / or the bagging station. The conveyor overflow section can be located at the end section of the upper duct, as shown in Figure 3. In this case, the powder flows to the classification station and / or the bagging station as soon as the level of the fluidized bed reaches the level of the conveyor overflow section. The conveyor overflow section can also be located above the end of the conveyor, as shown in Figure 4. In this case, it is connected to the upper duct via an upward pipe 35. The method of discharging powder from the conveyor in this case will be described later. This configuration is very convenient for supplying classification stations and / or bagging stations that may not be located completely below the conveyor.

[0056] The conveyor 22 is preferably connected at its other end to the atomizer overflow 19. In particular, the lower end of the overflow is connected to the upper duct 26. The conveyor can be connected to multiple overflows and thus multiple atomizers. In that case, the overflows are distributed along the entire length of the conveyor. If there are multiple pressure valves, they are preferably located between the overflows, and a potential gas dam is preferably located adjacent to and upstream of the overflow.

[0057] The conveyor 22 is preferably a closed device which communicates with the outside only as far as the powder is concerned, by the overflow of the atomizer and the overflow of the conveyor, and as far as the fluidizing gas is concerned, by an inlet conduit, preferably a single and pressure valve.

[0058] The conveyor 22 is preferably horizontal. It can also be made up of different sections, which can be at different levels. The conveyance can therefore be easily adapted to the topography of the site.

[0059] To operate the conveyor 22, a fluidizing gas is introduced at a given flow rate below the porous wall 27 which separates the lower duct 25 and the upper duct 26 of the conveyor.

[0060] The fluidizing gas flows through the porous walls and then passes between the particles that lie in the upper duct, forming a fluidized layer. As soon as the velocity of the fluidizing gas in the interstitial spaces between the particles becomes high enough, the particles move and then lift, losing permanent contact with their neighbors. In this way, a fluidized bed 24 is formed in the upper duct.

[0061] The powder discharged from chamber 2 through overflow 19 in upper duct 26 is maintained in a fluidized state within the conveyor. Because it behaves like a fluid, it remains horizontal within the upper duct. A continuous flow of powder is generated along the conveyor by discharging the fluidized bed from the conveyor to the classification and / or bagging stations at conveyor overflow 34. If the conveyor overflow is located at the end section of the upper duct, a continuous flow is achieved as soon as the height of the fluidized bed reaches the height of the conveyor overflow. If the conveyor overflow is connected to the upper duct by an upflow pipe 35, the fluidizing gas pressure in the upper duct is set slightly above atmospheric pressure, causing the fluidized bed to rise through the upflow pipe to the conveyor overflow. For example, for steel particles, the overpressure relative to atmospheric pressure can be set between 200 and 600 mbar per meter of upflow pipe.

[0062] If the atomizer overflow is interrupted, the fluidized bed level will decrease in the conveyor until it reaches the level of the conveyor overflow. At this point, flow through the conveyor overflow will cease. Conversely, if the conveyor overflow must be temporarily closed for some reason, the fluidized bed level will rise in the conveyor. In that case, the atomizer overflow may only have to be interrupted when the fluidized bed height reaches the top of the upper duct.

[0063] Furthermore, powder transport by this conveyor can be turned on and off very easily by simply turning the fluidizing gas inlet on and off.

[0064] The fluidizing gas can be air if the powder is sufficiently cool and will not oxidize on contact with air. If the powder needs to be protected from the atmosphere, the fluidizing gas can be an inert gas such as argon or nitrogen. In that case, it is preferable to recirculate the inert gas.

[0065] From a process point of view, the cooling of the powder in the atomizer chamber 2 is made possible by the process for producing metal powder, which process: (i) supplying molten metal to the chamber 2 of the gas atomizer 1; (ii) atomizing the molten metal by injection of a gas to form metal particles; (iii) cooling the metal particles in the lower section of the chamber by injecting gas from the bottom of the chamber to form a foaming fluidized bed 15 of metal particles; Equipped with.

[0066] Preferably, the process is for the continuous production of metal powders, as described in more detail below.

[0067] The metals to be atomized may be, in particular, steel, aluminum, copper, nickel, zinc, iron, alloys, including in particular carbon steel, alloy steel and stainless steel.

[0068] The metal can be fed to the atomizer in solid form and melted in a tundish connected to the atomizer via a nozzle, or it can be melted in a previous step and poured into the tundish.

[0069] According to one variant of the invention, the molten metal to be atomized is steel obtained through a blast furnace process. In this case, pig iron is poured from a blast furnace and, optionally after being sent to a hot metal desulfurization station, is transferred to a converter (or a basic oxygen furnace (BOF)). The molten iron is refined in the converter to form molten steel. The molten steel from the converter is then poured from the converter into a collection ladle and preferably transferred to a ladle metallurgical furnace (LMF). Thus, the molten steel can be refined in the LMF, in particular by deoxidation, and primary alloying of the molten steel can be carried out by adding strong alloys, silicide alloys, nitride alloys, pure metals, or mixtures thereof. In certain cases where demanding powder compositions must be produced, the molten steel can also be treated in a vacuum tank degasser (VTD), a vacuum oxygen decarburization (VOD) vessel, or a vacuum arc degasser (VAD). These devices allow, in particular, to further limit the hydrogen, nitrogen, sulfur, and / or carbon content.

[0070] The refined molten steel is then poured into multiple induction furnaces. Each induction furnace can be operated independently of the others, particularly when shut down for maintenance or repair while the others are still operating. It can also be fed with ferrous alloys, scrap, direct reduced iron (DRI), silicide alloys, nitride alloys, or other pure elements in different amounts than the induction furnaces.

[0071] The number of induction furnaces is adapted to the flow of liquid steel coming from the converter or refined liquid steel coming from the ladle metallurgy furnace and / or the desired flow of steel powder at the bottom of the atomizer.

[0072] In each induction furnace, the molten steel is alloyed by adding iron alloys or silicide alloys or nitride alloys or pure metals or mixtures thereof to adjust the steel composition to that of the desired steel powder.

[0073] For each induction furnace, molten steel of the desired composition is then poured into a dedicated reservoir connected to at least one gas atomizer. By "dedicated," we mean that the reservoir is paired with a given induction furnace. However, multiple reservoirs can be dedicated to a given induction furnace. For clarity, each induction furnace has its own production stream with at least one reservoir connected to at least one gas atomizer. With such parallel and independent production streams, the process for producing steel powder is versatile and can be easily made continuous.

[0074] The reservoir is primarily a storage tank, which can be atmospherically controlled, can heat the molten steel, and can be pressurized.

[0075] The atmosphere in each dedicated reservoir is preferably argon, nitrogen or a mixture thereof to avoid oxidation of the molten steel.

[0076] The steel composition injected into each reservoir is heated above its liquidus temperature and maintained at this temperature. This heating prevents clogging of the atomizer nozzle 3. The reduced viscosity of the molten composition also helps to obtain powders with a good particle size distribution, no satellites, and high sphericity.

[0077] Finally, when the dedicated reservoir is pressurized, the molten steel can flow from the reservoir to at least one of the gas atomizers connected to the reservoir.

[0078] According to another variant of the invention, the metal to be atomized is steel obtained via an electric arc furnace (EAF) route. In this case, raw materials such as scrap, metallic ores, and / or metal powders are fed into an electric arc furnace (EAF) and melted into a heated liquid metal at a controlled temperature. Impurities and inclusions are removed as a separate liquid slag layer. The heated liquid metal is removed from the EAF into a ladle, preferably a passively heatable ladle, and transferred to a refining station, where it is preferably placed in an induction-heated refining holding vessel. There, a refining step, such as vacuum oxygen decarburization, is performed to remove carbon, hydrogen, oxygen, nitrogen, and other undesirable impurities from the liquid metal. The ladle containing the refined liquid metal can then be transferred, under controlled vacuum and inert atmosphere, above a closed chamber containing a heated tundish of the atomizer. The ladle is connected to a feed conduit, through which the refined liquid metal is then fed to the heated tundish.

[0079] Alternatively, the ladle containing the refined liquid metal is transferred from the refining station to a separate, induction-heated atomizing vessel, which is positioned at the door of the atomizing station, which contains an injection zone with a heated tundish for the gas atomizer under a controlled vacuum and inert atmosphere. The induction-heated atomizing vessel is then introduced into a receiving zone, where the vacuum and atmosphere are adjusted to match those of the injection zone. The vessel is then introduced into the injection zone, where the liquid metal is injected at a controlled rate into the heated tundish and atomized by the atomizer.

[0080] In both variations, molten metal is maintained at atomization temperature in a tundish until it is forced through a nozzle 3 in a chamber 2 under a controlled atmosphere (step (i)) and impinged by a jet of gas which atomizes it into fine metal droplets (step (ii)).

[0081] In step (ii), the gas injected through the gas atomizer 4 to atomize the metal stream is preferably argon or nitrogen. Both of these increase the melt viscosity more slowly than other gases, such as helium, which promotes the formation of smaller particle sizes. They also control the purity of the chemistry, avoid undesirable impurities, and play a role in the morphology of the powder. Finer particles can be obtained with argon than with nitrogen, since the molar weight of nitrogen is 14.01 g / mol compared to 39.95 g / mol for argon. Meanwhile, the specific heat capacity of nitrogen is 1.04 J / (gK) compared to 0.52 for argon. Therefore, nitrogen increases the cooling rate of the particles.

[0082] The gas flow influences the particle size distribution and microstructure of the metal powder. In particular, the higher the flow rate, the higher the cooling rate. Therefore, the gas flow rate (m 3 The gas to metal ratio, defined as the ratio of the gas flow rate (kg / h) to the metal flow rate (kg / h), is preferably kept between 1 and 5, more preferably between 1.5 and 3.

[0083] Once metal particles are obtained from atomization of the molten metal in the chamber, the resulting powder is cooled in the lower section of the chamber by injecting gas from the bottom of the chamber so as to form a foaming fluidized bed 15 of metal particles (step (iii)). This step is preferably performed simultaneously with the atomization step, more preferably sequentially and simultaneously with the atomization step. In this way, the atomizer can operate continuously.

[0084] During this step, the metal particles are preferably cooled below their oxidation window. In the case of steel powder, the metal particles are preferably cooled below 300°C, more preferably below 260°C, and even more preferably between 150 and 260°C. Such cooling allows the powder to then be operated in air in the next step of the process. Cooling can be adjusted depending on the sensitivity of the steel composition to oxidation and / or the purity of the gas. To limit the gas flow required to cool the powder, it is preferable not to overcool the powder, e.g., below 150°C. In a continuous mode, the gas flow is adjusted to maintain the fluidized bed at a constant temperature while a portion of the particles is continuously discharged from the chamber and new hot particles are continuously added to the bed. In this case, the fluidized bed is maintained below 300°C, more preferably below 260°C, and even more preferably between 150 and 260°C.

[0085] The gas injected through the gas injector 6 to fluidize the powder bed is preferably argon or nitrogen, and more preferably the same gas used to atomize the molten metal stream. It is preferably injected at a velocity between 1 and 80 cm / s, which requires low ventilation power and therefore reduces energy consumption. The gas flow is preferably regulated by a flow regulator 9, such as a fan.

[0086] The gas is preferably injected at a temperature comprised between 10 and 50° C. This further improves the cooling of the metal particles.

[0087] The injected gas is preferably extracted from the chamber to maintain a constant pressure in the chamber. The gas flow in the gas extractor 11 is adjusted accordingly. The pressure in the chamber 2 is preferably set between 5 and 100 mbar.

[0088] The injected gas is preferably recycled, in which case it is more preferably cooled after removal from the chamber, preferably below 50°C, more preferably between 10 and 50°C.

[0089] During step (iii), cooling of the metal particles can be further enhanced by contacting the fluidized bed with a heat exchanger 14 .

[0090] The process according to the invention may further comprise the step (iv) of continuously discharging the cooled metal particles from the chamber, preferably simultaneously with the atomization and cooling steps. The continuous discharge may be via the overflow 19, as previously described.

[0091] The process according to the invention may further comprise a step (v) of conveying the discharged metal particles to a classification station 23 and / or a bagging station, which step is preferably carried out simultaneously with the atomization, cooling and discharge steps.

[0092] The discharged metal particles can be conveyed in the form of a fluidized bed 24, which is preferably a foaming fluidized bed.

Claims

1. 1. A metal powder manufacturing process comprising: (i) supplying molten metal to the chamber (2) of the gas atomizer (1); (ii) atomizing the molten metal by injection of a gas to form metal particles; (iii) cooling the metal particles in the lower section of the chamber by injecting gas from the bottom of the chamber to form a bubbling fluidized bed (15) of metal particles; Equipped with In step (iii), the injected gas is extracted, cooled and reinjected. process.

2. 10. The process of claim 1, wherein the molten metal is steel obtained through a blast furnace pass.

3. 10. The process of claim 1, wherein the molten metal is steel obtained through an electric arc furnace pass.

4. The process of any one of claims 1 to 3, wherein steps (ii) and (iii) are carried out simultaneously.

5. The process of any one of claims 1 to 3, wherein in step (iii) the metal particles are cooled to below 300°C.

6. 10. The process of claim 1, wherein the gas is cooled to less than 50°C.

7. The process of any one of claims 1 to 3, further comprising the step (iv) of continuously discharging the metal particles from the chamber.

8. 8. The process according to claim 7, wherein the continuous discharge is effected by means of an overflow (19).

9. 8. The process of claim 7, further comprising the step (v) of conveying the discharged metal particles to a classification station.

10. 10. The process of claim 9, wherein the discharged metal particles are transported in the form of a fluidized bed (24).

11. A gas atomizer (1) comprising a chamber (2), a gas injector (6) arranged at the bottom (7) of the chamber, and a flow regulator (9) coupled to the gas injector for fluidizing metal particles accumulated in the lower section of the chamber to form a bubbling fluidized bed (15) of metal particles; Further comprising a gas extractor (11) in the upper section of the chamber; Gas atomizer (1).

12. 12. Gas atomizer according to claim 11, wherein the gas injector (6) comprises an opening in the bottom wall of the chamber.

13. 12. The gas atomizer according to claim 11, wherein the distance between the bottom of the chamber (7) and the gas injector (6) is less than 10 cm.

14. 14. The gas atomizer according to claim 11, wherein the gas injector is a sparger that injects the gas in a dispersed manner.

15. A gas atomizer according to any one of claims 11 to 13, further comprising a heat exchanger (14) arranged in the lower section of the chamber.

16. A gas atomizer according to any one of claims 11 to 13, further comprising an overflow (19) in the lower section of the chamber.

17. 17. The gas atomizer of claim 16, wherein the overflow is a pipe that extends at least partially into the lower section of the chamber and passes through a bottom wall of the chamber.

18. 17. A gas atomizer according to claim 16, wherein a part of the overflow outside the chamber comprises a gas inlet (21).

19. A gas atomizer according to any one of claims 11 to 13, further comprising a coarse particle collector (16) at the bottom of the chamber.

20. 12. Gas atomizer according to claim 11, wherein the gas extractor (11) comprises a cyclone separator for dedusting the gas extracted from the chamber.

21. 12. Gas atomizer according to claim 11, wherein the gas extractor (11) is connected to the gas injector (6) for gas recirculation within the atomizer.

22. 22. Gas atomizer according to claim 21, wherein the connection between the gas extractor (11) and the gas injector (6) comprises a heat exchanger (13).

23. A gas atomizer (1) comprising a chamber (2), a gas injector (6) arranged at the bottom (7) of the chamber, and a flow regulator (9) coupled to the gas injector for fluidizing metal particles accumulated in the lower section of the chamber to form a bubbling fluidized bed (15) of metal particles; a conveyor (22) comprising a lower duct (25) for circulating a gas, an upper duct (26) for circulating a powder material, and a porous wall (27) separating the lower and upper ducts over substantially their entire length; Facilities equipped with:

24. 24. The installation according to claim 23, wherein the lower duct (25) of the conveyor (22) comprises a fluidizing gas inlet (29) and a flow regulator (28) coupled to the gas inlet for fluidizing the metal particles discharged from the gas atomizer and forming a fluidized bed (24) of metal particles in the upper duct (26).

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