Metal powder cooling and conveying process

Abstract

The present invention relates to a metal powder manufacturing process comprising the steps of (i) discharging metal particles from a chamber of a gas atomizer in a conveyor and (ii) simultaneously cooling and transporting the metal particles in the form of a fluidized bed formed in the conveyor. The present invention also relates to an installation thereof.

Description

[Technical Field]

[0001] The present invention relates to a method for producing metal powder, in particular steel powder for additive manufacturing, by gas atomization. The method particularly relates to the cooling and conveying of the metal powder formed in the atomizer, more particularly to the continuous cooling and conveying of the metal powder. The present invention also relates to a metal powder production facility, in particular to a facility for cooling and conveying metal powder. [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 quickly. The atomizer is then opened to collect the powder, which is then transported to the next step in the process. This cooling process is lengthy and incompatible with the need to produce large quantities of metal powder. The transport process must also be improved. Summary of the Invention [Problem to be solved by the invention]

[0004] SUMMARY OF THE INVENTION It is therefore an object of the present invention to ameliorate the shortcomings of prior art equipment and processes by providing a process for efficiently cooling and conveying metal powder. [Means for solving the problem]

[0005] To this end, a first subject of the invention consists of a process for producing metal powder, said process comprising: (i) discharging metal particles from a chamber of a gas atomizer within a conveyor; (ii) simultaneously cooling and conveying the metal particles in the form of a fluidized bed formed within the conveyor; Equipped with.

[0006] The process according to the invention may also have the optional features listed below, considered individually or in combination. Metal particles are continuously discharged from the gas atomizer chamber, The metal particles are discharged directly from the gas atomizer chamber inside the conveyor, Metal particles are discharged from multiple chambers of a gas atomizer, The metal particles exiting the gas atomizer chamber are at a temperature below 300°C. The fluidized bed within the conveyor is formed by injecting air into the conveyor, The fluidized bed within the conveyor is formed by injecting air into at least one section of the conveyor and by injecting an inert gas into at least one other section of the conveyor; The fluidized bed in the conveyor is a foaming fluidized bed, The metal particles in the conveyor are cooled to below 150°C, Prior to the discharge step, the metal particles undergo a first cooling step in the lower section of the chamber by injecting a gas from the bottom of the chamber to form a foaming fluidized bed of metal particles.

[0007] A second subject of the invention consists of an installation, comprising: a gas atomizer comprising a chamber; a conveyor connected to the atomizer chamber, the conveyor comprising: a lower duct for circulating gas; an upper duct for circulating powder material; a porous wall separating the lower duct and the upper duct along substantially their entire lengths; a fluidizing gas inlet disposed on the lower duct; and a flow regulator coupled to the fluidizing gas inlet for fluidizing, cooling, and conveying metal particles discharged from the chamber; Equipped with.

[0008] The installation according to the invention may also have the optional features listed below, considered individually or in combination: the lower duct comprises two separate sections, each section having its own fluidizing gas inlet and flow regulator; the upper duct having two sections separated by a gas dam disposed substantially perpendicular to the section of the lower duct and laterally disposed in an upper portion of the upper duct; The conveyor is connected to the atomizer chamber by a lower portion of the overflow section, and one section of the upper duct accommodates the lower portion of the overflow section; The equipment includes a plurality of gas atomizers and a single conveyor connected to the chambers of the plurality of gas atomizers; the conveyor has a plurality of branches, each branch connected to at least one chamber of the gas atomizer; The gas atomizer further comprises a gas injector disposed at the bottom of the chamber and a flow regulator coupled to the gas injector for fluidizing the metal particles accumulated in the lower section of the chamber to form a foaming fluidized bed of the metal particles.

[0009] As is apparent, the present invention is based on the technology of a fluidized bed for simultaneously cooling and transporting the powder discharged from the atomizer chamber. If a fluidized bed is also added to the lower section of the atomizer, the fluidized powder can be continuously discharged from the atomizer without interrupting the atomization process, and then simultaneously cooled and transported without interrupting its fluidized state.

[0010] Other features and advantages of the present invention will be explained in more detail in the following description.

[0011] 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]

[0012] [Figure 1] FIG. 1 shows an installation with two atomizers and a conveyor according to a first variant of the invention. [Figure 2] FIG. 1 shows an installation with two atomizers and a conveyor according to a second variant of the invention. [Figure 3] FIG. 1 illustrates a possible scheme for fluidization. [Figure 4] FIG. 10 shows a gas atomizer according to a modified example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] 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.

[0014] 1 and 2, the installation according to the invention comprises first of all a gas atomizer 1 and a conveyor 22.

[0015] The 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.

[0016] 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.

[0017] 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.

[0018] The conveyor fluidizes the metal particles as they exit the chamber, simultaneously cooling the particles and feeding them into a fluidized bed 24, preferably a foaming fluidized bed. (Bubbling fluidized bed) This type of conveying and cooling is advantageous because it requires minimal ventilation power, can prevent dust emissions, and can ensure continuous operation.

[0019] As shown in Figure 3, 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 become more actively transported. 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 preferred for the present invention due to improved circulation of solid particles within the bed, rapid cooling, and uniform temperature throughout the bed. The gas velocity applied to achieve a given regime and desired bed temperature 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 porous walls, or the size of the conveyor. 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 facility 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.

[0020] Thanks to the foaming fluidized bed, the metal particles are cooled very quickly and very efficiently to the working temperature of the fluidized bed while maintaining a uniform distribution of particle size within the fluidized bed.

[0021] To form a fluidized bed, the conveyor 22 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.

[0022] 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 may be a multi-layer canvas or porous refractory material.

[0023] 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.

[0024] This even distribution of gas over the entire cross section of the upper duct allows only one fan 28 to be used for the entire conveyor, simplifying installation and maintenance.

[0025] The conveyor 22 preferably includes at least one pressure valve 30 at the top of the upper duct 26 so that the pressure in the fluidizing gas in the upper duct can be efficiently regulated. The pressure valve is preferably connected to the upper duct through a cyclone 31 arranged in a cyclone box 32. In this way, the fluidizing gas exiting 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.

[0026] 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.

[0027] 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 1. 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 2. 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.

[0028] 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.

[0029] The fluidizing gas flows through the porous wall and then exits the atomizer and passes through the particles in the upper duct, forming a fluidized bed. As soon as the velocity of the fluidizing gas in the interstitial spaces between the particles becomes high enough, the particles begin to move and then lift, losing permanent contact with their neighbors. In this way, a fluidized bed 24 is formed in the upper duct.

[0030] Because the fluidized bed of metal particles behaves like a fluid, it remains horizontal within the upper duct. By discharging the fluidized bed from the conveyor to the classification station and / or bagging station at the conveyor overflow section 34, a continuous flow of powder is generated along the conveyor. If the conveyor overflow section 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 section. If the conveyor overflow section 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 section. For example, for steel particles, the overpressure relative to atmospheric pressure can be set between 200 and 600 mbar per meter of upflow pipe.

[0031] If the powder supply from the atomizer is interrupted, the fluidized bed height decreases in the conveyor until it reaches the height of the conveyor overflow. At this point, flow through the conveyor overflow stops. 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 powder supply from the atomizer may only have to be interrupted when the fluidized bed height reaches the top of the upper duct.

[0032] Furthermore, powder transport by this conveyor can be turned on and off very easily by simply turning the fluidizing gas inlet on and off.

[0033] The conveyor 22 is preferably connected to the chamber 2 of the atomizer 1 away from the conveyor overflow.

[0034] The chamber can be an atomization chamber, i.e., a chamber in which a stream of molten metal collides with a high-velocity gas stream, or it can be any other chamber of a gas atomizer suitable for collecting metal particles formed in the atomization chamber.

[0035] The conveyor can be directly connected to the atomizer chamber, in which case the metal particles flow directly from the chamber onto the conveyor, or the conveyor can be indirectly connected to the atomizer chamber, in which case the metal particles pass through other equipment and / or containers as they are transported from the atomizer to the conveyor.

[0036] According to a variant of the invention shown in Figures 1 and 2, the atomizer is designed to fluidize the metal particles accumulated in the lower section of the chamber, form a foaming fluidized bed of metal particles, and continuously discharge the metal particles into a conveyor.

[0037] As shown in Figure 4, the gas atomizer includes a gas injector 6 located at the bottom of the chamber, which fluidizes the metal particles accumulated in the lower section of the chamber, creating a foaming fluidized bed of metal particles. This fluidized bed allows the metal particles to efficiently undergo a first cooling step, lowering their temperature below their oxidation window through intense gas-particle heat transfer. The metal particles accumulated in the lower section of the chamber are maintained at the operating temperature of the fluidized bed, while the hot particles falling from the top of the chamber are rapidly mixed in the fluidized bed and cooled to their operating temperature. Furthermore, because this first cooling step occurs directly within the chamber, which is maintained under a protective atmosphere, the metal particles do not oxidize during this first cooling step.

[0038] Thanks to the foaming fluidized bed, the metal particles are cooled very quickly and very efficiently to the working temperature of the fluidized bed while maintaining a uniform distribution of particle size within the fluidized bed.

[0039] 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.

[0040] 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.

[0041] The gas injector may comprise openings in the bottom wall of the chamber through which gas may be injected to fluidize the powder bed.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] In the example shown in Figure 4, 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 of gas recycling are of course possible.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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 arranged 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 inside. The cooling rate can be controlled by adjusting the flow rate of the medium in the heat exchanger. Such a heat exchanger facilitates the first cooling step of the particles in the fluidized bed and their maintenance at the desired temperature. The heat exchanger can also reduce the gas flow required to cool or maintain the particles at the desired temperature.

[0054] 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.

[0055] According to one variant of the invention, once the metal particles have been produced and optionally undergone a first cooling step in 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.

[0056] 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.

[0057] 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 limit the backflow of gas from the conveyor to the atomizer.

[0058] 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 determined 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 present 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.

[0059] 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.

[0060] According to one variant of the invention, the overflow 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. The hat is placed high enough above the top of the overflow so as not to obstruct the flow of powder exiting through the overflow.

[0061] 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.

[0062] The metal particles are discharged from the atomizer in the conveyor by an input opening located in the upper duct 26 of the conveyor. In the variant shown in Figures 1 and 2, the input opening is simply an opening designed to allow the atomizer's overflow section 19 to enter the conveyor. In particular, the lower end of the overflow section enters the upper duct 26. The conveyor can be connected to multiple overflow sections and therefore multiple atomizers. In that case, the overflow sections are distributed along the entire length of the conveyor. If there are multiple pressure valves, they are preferably located between the overflow sections, and a potential gas dam is preferably located adjacent to and upstream of the overflow section.

[0063] The conveyor 22 is preferably a closed device communicating with the outside only through the input opening and conveyor overflow 34 as far as the powder is concerned, and only through the fluidizing gas inlet 29 and pressure valve 30 as far as the fluidizing gas is concerned.

[0064] The conveyor 22 is preferably horizontal. It can also be made up of different sections. The conveyor can also have several branches, each connected to at least one chamber of the gas atomizer. These sections and / or branches can be at different levels. The conveyor can therefore easily be adapted to the topography of the site.

[0065] Referring to Figure 5, another variant of the conveyor 22 is shown. This variant differs from the one shown in Figures 1 and 2 in that the conveyor is designed to operate with two different fluidizing gases. As a result, the metal particles discharged from the atomizer can first be cooled in an inert atmosphere to avoid their oxidation and then transported with air.

[0066] According to this variant, the conveyor comprises at least two distinct sections: an inerting section and an air section. The inerting section 36 includes the input opening of the conveyor, such as the atomizer overflow section 19. The air section 37 is located between the inerting section 36 and the conveyor overflow section 34.

[0067] The lower duct 25 thus comprises two distinct sections: an inert section and an air section. They are separated by a wall that prevents gas injected into one section from passing through the other. Each of the inert and air sections of the lower duct has its own fluidization gas inlet 29 and flow regulator 28. As a result, different gases can be injected in each section, and the flow can be independently adjusted in each section. According to one preferred variation of the invention, inert gas is injected into the inert section of the conveyor, and air is injected into the air section of the conveyor. As a result, the metal particles discharged from the atomizer begin to cool in an inert atmosphere that prevents oxidation. Upon exiting the inert section, they are sufficiently cooled so that they can be further transported in the air section within the air-fluidized bed.

[0068] Similarly, the upper duct 26 preferably comprises two sections, arranged generally perpendicular to the lower duct section and separated by a gas dam 33 arranged laterally in the upper part of the upper duct. The gas dam at least partially limits horizontal circulation of the fluidizing gas above the fluidized bed and thus mixing of the atmospheres of the two sections. The inerting section of the upper duct includes an input opening, such as the atomizer overflow 19, through which the metal particles are discharged from the atomizer. The air section preferably includes a pressure valve 30 and a cyclone 31 arranged in a cyclone box 32, as previously described. The pressure valve located only in the air section prevents air from being drawn into the inerting section from the air section.

[0069] As shown in Figure 5, the conveyor can include multiple inerting sections and multiple air sections. The inerting sections are adjacent to the atomizer input opening or overflow 19, and the air sections are located between the inerting sections and between one inerting section and the conveyor overflow 34. In the example shown, each section has its own fluidizing gas inlet 29 and flow regulator 28. Nevertheless, it is also possible, for example, to connect all the inerting sections to a single fluidizing gas inlet 29 and / or a single flow regulator 28 on the one hand, and all the air sections to a single fluidizing gas inlet 29 and / or a single flow regulator 28 on the other hand.

[0070] From a process perspective, efficient cooling and transport of metal powders is enabled by the metal powder production process, which: (i) discharging metal particles from a chamber of a gas atomizer within a conveyor; (ii) simultaneously cooling and transporting metal particles in the form of a fluidized bed formed within the conveyor; Equipped with.

[0071] Preferably, the process is for the continuous production of metal powders, as described in more detail below.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] The reservoir is primarily a storage tank, which can be atmospherically controlled, can heat the molten steel, and can be pressurized.

[0080] The atmosphere in each dedicated reservoir is preferably argon, nitrogen or a mixture thereof to avoid oxidation of the molten steel.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] Alternatively, the ladle containing the refined liquid metal is transferred from the refining station to a separate induction-heated atomizing vessel, which is located at the door of the atomizing station, which contains an injection area with a heated tundish for the gas atomizer under controlled vacuum and inert atmosphere. The induction-heated atomizing vessel is then introduced into a receiving area where the vacuum and atmosphere are adjusted to match those of the injection area. The vessel is then introduced into the injection area, where the liquid metal is injected at a controlled rate into the heated tundish and atomized by the atomizer.

[0085] 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 and struck by a jet of gas which atomizes it into fine metal droplets.

[0086] 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.

[0087] 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.

[0088] Once the metal particles are obtained from the atomization of the molten metal in the chamber, they can be subjected to a first cooling step in the atomizer, particularly 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. This step is preferably carried out simultaneously with the atomization step, and more preferably, it is carried out continuously and simultaneously with the atomization step. In this way, the atomizer can operate continuously.

[0089] During this first cooling step, the metal particles are preferably cooled below their oxidation window. In the case of steel powder, the metal particles are preferably cooled to below 300°C, more preferably below 260°C, and even more preferably between 150 and 260°C. After such a first cooling step, the powder can then be operated in air for the next step of the process. Depending on the sensitivity of the steel composition to oxidation and / or the purity of the gas, the cooling can be adjusted. To limit the gas flow required to cool the powder in the atomizer, the powder is preferably not overcooled, 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.

[0090] The gas injected through the gas injector 6 to fluidize and cool the powder bed is preferably argon or nitrogen, 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.

[0091] The injected gas is preferably extracted from the chamber in order 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. The injected gas is preferably recirculated. In that case, it is more preferable to cool it after removing it from the chamber.

[0092] Once the metal particles have been formed and optionally undergone a first cooling step, they are discharged from the atomizer (step i). This step is preferably carried out simultaneously with the atomization step and the optional first cooling step. Continuous discharge can be achieved via the overflow 19, as previously described.

[0093] As mentioned above, the conveyor can be directly or indirectly connected to the atomizer chamber, thereby allowing the metal particles to be directly or indirectly discharged from the gas atomizer chamber into the conveyor.

[0094] Also, the conveyor can be connected to multiple atomizers, so that metal particles can be discharged from multiple chambers of the gas atomizer.

[0095] In a second step (step ii), the metal particles discharged from the atomizer are simultaneously cooled and transported by a conveyor in the form of a fluidized bed. This second step is preferably carried out simultaneously with the discharge step (step i) and / or simultaneously with the atomization step and the optional first cooling step.

[0096] The fluidizing gas can be air if the powder has undergone a first cooling step in the atomizer and is not oxidized by 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 this case, it is preferable to recirculate the inert gas. In a variation shown in Figure 5, an inert gas is used to cool and convey the metal particles in the first stage of the cooling and conveying step, and air is used to convey the metal particles in the second stage of the cooling and conveying step, optionally for further cooling.

[0097] The temperature and flow of the gas or gases used to fluidize the bed within the conveyor are preferably adjusted so that the metal powder arriving at the conveyor exit or classification station entrance is cooled to less than 150° C. As a result, conventional classification equipment can be used as opposed to high temperature resistant equipment.

Claims

1. 1. A metal powder manufacturing process comprising: (i) discharging metal particles from the chamber (2) of the gas atomizer (1) in a conveyor (22); (ii) simultaneously cooling and conveying the metal particles in the form of a fluidized bed (24) formed within the conveyor; A process comprising:

2. 10. The process of claim 1, wherein the metal particles are continuously discharged from the chamber of the gas atomizer.

3. 3. The process of claim 1 or 2, wherein the metal particles are discharged directly from the chamber of the gas atomizer in a conveyor.

4. 3. The process of claim 1 or 2, wherein the metal particles are discharged from multiple chambers of a gas atomizer.

5. 3. The process of claim 1 or 2, wherein the metal particles exiting the gas atomizer chamber are at a temperature of less than 300°C.

6. 3. The process of claim 1 or 2, wherein the fluidized bed in the conveyor is formed by injecting air into the conveyor.

7. 3. The process of claim 1 or 2, wherein the fluidized bed in the conveyor is formed by injecting air into at least one section (37) of the conveyor and by injecting an inert gas into at least one other section (36) of the conveyor.

8. 3. The process according to claim 1 or 2, wherein the fluidized bed in the conveyor is a bubbling fluidized bed.

9. 3. The process of claim 1 or 2, wherein the metal particles in the conveyor are cooled to below 150°C.

10. 3. The process according to claim 1 or 2, wherein, before the discharge step, the metal particles undergo a first cooling step 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.

11. It is equipment a gas atomizer (1) comprising a chamber (2); a conveyor (22) connected to the chamber of the atomizer, the conveyor (22) comprising a lower duct (25) for circulating gas, an upper duct (26) for circulating powder material, a porous wall (27) separating the lower duct and the upper duct over substantially the entire length, a fluidizing gas inlet (29) disposed on the lower duct, and a flow regulator (28) coupled to the fluidizing gas inlet for fluidizing, cooling, and conveying metal particles discharged from the chamber; Facilities equipped with:

12. 12. The installation of claim 11, wherein the lower duct comprises two separate sections, each section having its own fluidizing gas inlet (29) and flow regulator (28).

13. 13. The installation according to claim 12, wherein the upper duct (26) has two sections, the two sections being separated by a gas dam (33) arranged substantially perpendicular to the section of the lower duct and laterally disposed in the upper part of the upper duct.

14. 14. The installation according to claim 13, wherein the conveyor is connected to the atomizer chamber (2) by a lower part of the overflow part (19), and one section of the upper duct (26) comprises the lower part of the overflow part.

15. An installation according to any one of claims 11 to 14, comprising a plurality of gas atomizers (1) and one single conveyor (22) connected to the chambers (2) of the gas atomizers.

16. 16. The installation of claim 15, wherein the conveyor comprises a plurality of branches, each branch connected to at least one chamber of the gas atomizer.

17. 15. The installation according to any one of claims 11 to 14, wherein the gas atomizer (1) further comprises a gas injector (6) arranged at the bottom (7) of the chamber and a flow regulator (9) coupled to the gas injector for fluidizing the metal particles accumulated in the lower section of the chamber and forming a bubbling fluidized bed (15) of metal particles.

Images