Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F9/02—Making metallic powder or suspensions thereof using physical processes
  • B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
  • B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
  • B22F9/10—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying using centrifugal force
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
  • C01B33/037—Purification
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F2009/001—Making metallic powder or suspensions thereof from scrap particles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F9/02—Making metallic powder or suspensions thereof using physical processes
  • B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
  • B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
  • B22F9/082—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid
  • B22F2009/0892—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid casting nozzle; controlling metal stream in or after the casting nozzle

Definitions

• the present invention relates to the field of granulation of molten metal.
• the invention relates more particularly to equipment and a method for obtaining metal granules from molten metal. It will find for advantageous but not limiting application the production of silicon granules.
• the production of granules from molten metal can advantageously be used for the recycling of silicon powders.
• silicon powders are generally obtained from the cutting of silicon ingots during the production of silicon wafers, for example in a production line for solar cells. Up to 50% of the ingots can be reduced to powder and lost.
• a shaping solution consists in melting these powders then in forming solid metal granules from the molten material. This solution is called granulation.
• This method is however not suitable for industrial granulation of silicon, and more generally for the granulation of metals of which the oxide is not passivating.
• Document US 5094832 discloses a method for producing silicon powders by atomizing a continuous jet of molten silicon by a flow of pressurized gas.
• a disadvantage of this process is its cost of implementation.
• the use of a pressurized gas flow requires a fluid network whose maintenance cost is high.
• the consumption of gas during atomization also increases the operating cost of such a process.
• Another disadvantage of this method is rapid degradation of the mechanical parts of the equipment.
• the mechanical elements of a disc rotating at this speed of rotation are subjected to strong mechanical stresses, and can undergo rapid wear.
• the reliability of this method is therefore reduced. Its implementation has a high operating cost.
• Another disadvantage of this method is the complex management of the cooling system in the rotating disc.
• a cooling system adapted to such a speed of rotation of the rotating disc is in fact particularly complex and costly to produce.
• An object of the present invention is to overcome at least one of the drawbacks mentioned above.
• an object of the present invention is to provide a method for forming solid metal granules whose cost of implementation is reduced and / or whose reliability is improved.
• Another object of the present invention is to provide a process for the formation of solid metal granules compatible with industrial production.
• Another object of the present invention is to provide a method for forming silicon granules from silicon powder resulting from the cutting of silicon ingots.
• Another object of the invention is to provide a system for forming solid metal granules which is reliable and compatible with industrial production of solid metal granules.
• a first aspect of the invention relates to a method for forming granules of metal in the solid state from this metal in the liquid state, called the granulation method.
• this method comprises at least the following steps:
• a pre-granulation step comprising at least the following steps: o Supply the metal in the liquid state in the crucible, o Form a continuous flow of liquid metal at the inlet of at least one capillary connected to said crucible, then
• An atomization step comprising at least the following steps: o Receiving the flow of droplets generated on a receiving surface of a rotating container, said surface being in rotation so as to fractionate the droplets, said surface further having a temperature at at least two times lower, and preferably at least ten times lower, at a melting temperature of the metal, so as to solidify liquid fractions of droplets into solid granules.
• the granulation process according to the invention has a reduced implementation cost.
• the cost of implementing this process is lower than that of a process of atomization by a flow of gas under pressure.
• the centrifugation energy required to fractionate a continuous stream of liquid metal into fractions small enough for them to solidify in the form of granules can advantageously be significantly reduced, in particular compared to the various rapid solidification techniques. presented in the document "SJ Savage et al., Production of rapidly solidified Metals and Alloys, Journal of Metals, April 1984".
• the prior formation of a stream of droplets makes it possible to considerably reduce the centrifugation energy necessary for the fractionation of this stream on the receiving surface of the rotating container.
• the speed of rotation of the receiving surface of the rotating container can be considerably reduced, for example by a factor of ten.
• the granulation process of the present invention therefore has a reduced cost of implementation and improved reliability compared to existing granulation processes.
• the granulation process of the present invention is therefore particularly advantageous for industrial production of solid metal granules.
• the proposed granulation process is part of a so-called "dry" granulation method which does not generate hydrogen.
• a second aspect of the invention relates to a system for forming granules of metal in the solid state, called a granulation system, comprising a device for supplying powder of said metal in the solid state, at the level of an upper part. of the system, a crucible intended to contain said metal in the liquid state, at least one capillary extending from the crucible and configured to allow a flow of the liquid metal, and at least one rotating container having a receiving surface intended to receive the flow of liquid metal and comprising a device for cooling the receiving surface.
• the system comprises a device for generating a discontinuous flow of liquid metal from a continuous flow of liquid metal at the inlet of the at least one capillary, so as to generate a flow of droplets of liquid metal falling in outlet of said at least one capillary, the rotating container is configured so that the receiving surface is rotated and the cooling device is configured so that the receiving surface has a temperature at least two times lower, and preferably at least ten times lower, at a metal melting temperature, so as to solidify liquid fractions of droplets into solid granules.
• This system advantageously makes it possible to implement the granulation process according to the first aspect of the invention.
• the technical effects and advantages of this system correspond mutatis mutandis to the technical effects and advantages of the method according to the first aspect of the invention.
• FIG. 1 illustrates an enlargement of part of the system illustrated in Figure 1.
• the process includes, after the atomization step, ejection of the granules by centrifugation.
• the method comprises, after ejection of the granules, a step of collecting the ejected solid granules.
• the atomization step is configured so that a speed of rotation of the reception surface is between 100 revolutions / min and 3000 revolutions / min, preferably substantially equal to 500 revolutions / min.
• Such an atomization step requires reduced processing energy, in particular compared to techniques requiring speeds of rotation more than ten times higher.
• the step of generating the discontinuous flow comprises a step of destabilizing the continuous flow by applying a modulated magnetic field on said continuous flow of liquid metal.
• Such a destabilization step advantageously makes it possible to form a stream of droplets comprising droplets of homogeneous size.
• This flow of droplets of homogeneous size gives rise, after the atomization step, to solid metal granules of homogeneous size.
• the modulated magnetic field is applied to the continuous flow of liquid metal at least partially contained within the at least one capillary the modulated magnetic field is applied to the continuous flow of liquid metal falling at the outlet of the at least one capillary.
• the magnetic field is frequency modulated, for example according to a frequency between 100 Hz and 10 kHz.
• Adjusting the frequency in this range allows you to control the size of the droplets. Such an adjustment also makes it possible to produce a stream of droplets of homogeneous size from different metals in the liquid state.
• the generation of the discontinuous flow is done by capillarity within the at least one capillary.
• the generation of the discontinuous flow is done by capillarity within the at least one capillary and by application of a modulated magnetic field to the continuous flow.
• the method further comprises a step of feeding the crucible with a powder of the metal in the solid state.
• the process advantageously makes it possible to recycle metal powders in the solid state.
• the metal is one of silicon, aluminum, an aluminum-silicon alloy and gallium.
• the process advantageously makes it possible to produce granules of one of silicon, aluminum, an aluminum-silicon alloy and gallium.
• the metal is one of platinum (Pt), tungsten (W), rhodium (Rh), iridium (Ir), tantalum (Ta).
• the process advantageously makes it possible to produce granules of one of platinum, tungsten, rhodium, iridium, tantalum.
• the recycling of powders from these so-called noble metals is of significant economic interest. These metals also exhibit a high melting point and / or a thermal behavior similar to that of silicon.
• the process parameters for example rotation speed, cooling temperature, droplet flow) determined for the recycling of silicon powders can therefore be easily and advantageously transposed and adapted to the recycling of powders of these noble metals with high melting point.
• the invention according to its second aspect notably comprises the following optional characteristics which can be used in combination or alternatively: the rotating container is configured so that the receiving surface has a rotation speed of between 100 revolutions / min and 3000 revolutions / min, preferably substantially equal to 500 revolutions / min.
• Such a speed limits the wear of rotating mechanical parts. Reliability is thus increased and the cost of maintaining the system is reduced. Such a speed also allows simplified management of the device for cooling the receiving surface. The cost of the cooling device is also reduced.
• the device for generating the discontinuous flow comprises at least one of at least one capillary and a device for producing a modulated magnetic field.
• the device for producing a modulated magnetic field is configured to destabilize the continuous flow of liquid metal by applying to said continuous flow a magnetic field modulated at a frequency between 100 Hz and 10 kHz.
• This device allows precise control of the droplet size for different metals in the liquid state.
• the device for producing a modulated magnetic field is configured to cooperate at least partially with the at least one capillary so that said magnetic field generates instability in the flow of liquid metal within and / or outside of the at least one capillary, in order to form droplets of uniform size at the outlet of said at least one capillary.
• This device for producing a magnetic field can advantageously be placed around the at least one capillary, for example in order to improve the homogeneity of the granules formed according to the invention.
• the receiving surface is concave.
• a concave receiving surface makes it possible to increase the contact time between the cooled surface and the fractions of droplets of liquid metal, in particular before ejection of the granules by centrifugation.
• the cooling of the droplet fractions is faster.
• the rapid solidification of the droplet fractions into granules is improved.
• the receiving surface has a center of rotation offset by a distance d relative to an axis of flow or drop of the droplets at the outlet of the at least one capillary, the distance d preferably being greater than half a radius of the receiving surface.
• An offset center of rotation avoids an accumulation of droplets and / or granules in the center of the receiving surface, where the speed is zero.
• the receiving surface is coated with a barrier material configured to limit contamination of the droplets of liquid metal by the material constituting the rotating container.
• the height is taken in a direction parallel to the free flow of a flow of liquid metal falling by gravity.
• metal means a material exhibiting metallic behavior in the liquid state. This material can be in the form of a simple body or in the form of an alloy. Silicon is thus considered to be a metal in the present application.
• the metals treated by the process and / or the granulation system of the present invention preferably have a high melting point, for example greater than 1400 ° C.
• the following metals can be advantageously treated by the process and / or the granulation system of the present invention: silicon, platinum (Pt), tungsten (W), rhodium (Rh), iridium (Ir), tantalum (Ta).
• Silicon can be presented in elementary, compound or alloyed form.
• the silicon designated here corresponds to a material whose elementary silicon content is at least 90% by mass.
• the main impurities, of metallic type (Fe, Cu, Al for example) or light (C, O, N for example) can represent, taken in isolation, a few percent of the silicon composition and, taken together, up to 10% en masse of its composition.
• the present invention aims in particular to transform solid metal powders into solid metal granules.
• Powders and granules are sets of particles that differ in their respective particle size ranges.
• the powders comprise particles whose size, that is to say the largest dimension, is preferably greater than a few hundred nanometers, for example 500 nm, and less than a few hundred micrometers, for example 500 pm.
• the granules comprise particles the size of which is preferably greater than 500 ⁇ m and less than a few millimeters, for example 15 mm.
• the granules also preferably have a spheroidal shape. Their size therefore corresponds to their average diameter or their maximum diameter.
• the particle sizes of the granules are greater and preferably much greater, for example by at least a factor of 10, than the particle sizes of the powders.
• capillary is understood to mean a tube of very small internal diameter, for example between 0.1 mm and 5 mm, and preferably between 0.5 mm and 5 mm.
• the capillary in particular makes it possible to reduce the pressure of a fluid flowing through it.
• carrier material is understood to mean a material which is chemically inert with respect to liquid metals. Such a material inserted between a liquid metal and a surface carrying this metal advantageously forms a barrier to the interdiffusion of species between the species of liquid metal and the material or materials constituting said surface.
• Passivating means the quality of a metal oxide forming a protective film on a solid metal.
• a granule of a metal whose oxide is passivating can for example be cooled in water without the granule oxidizing further.
• FIG. 1 illustrates an embodiment of a granulation system according to the invention making it possible to implement a granulation method according to the invention. The following description is therefore based on this FIG. 1, to describe both the parts of the granulation system and the stages of the granulation process.
• the granulation process according to the invention comprises at least one pre-granulation step intended to form a flow of liquid metal droplets, followed by an atomization step intended to form solid metal granules from the flow of droplets of liquid metal.
• the granulation system according to the invention comprises at least one crucible 1 having a diameter preferably between 5 cm and 50 cm, capable of receiving a liquid metal M
• This crucible 1 can be based on graphite for example.
• the walls of the crucible 1 are preferably chemically inert with respect to the metal, in order to avoid contamination or pollution of the liquid metal M
• This crucible 1 can in particular receive liquid silicon, or liquid aluminum for example, or any other metal whose metal oxide is not passivating.
• Such crucibles 1 are widely known to those skilled in the art.
• the granulation system is preferably confined in an enclosure 100 at atmospheric pressure.
• This atmosphere can be controlled, for example by vacuuming or by filling with a neutral gas such as argon.
• Such a controlled atmosphere advantageously makes it possible to purge the gases formed during the melting of the solid metal into liquid metal for example.
• Such a controlled atmosphere also makes it possible to avoid the oxidation of the metal contained in the enclosure 100.
• the crucible 1 is preferably first pre-filled with the solid metal, in the form of powder M pow for example, before melting this metal to obtain a bath of liquid metal M
• This pre-filling step can also be carried out in part with a block of solid metal for more efficiency. It is thus possible to mix in crucible 1 the powder M pow of solid metal and the block of solid metal.
• the melting of a block of solid metal is advantageously easier to achieve than the melting of a powder of this metal, in particular if the powder is partially oxidized. Therefore, the fusion of metal is first initiated at the level of the metal block.
• the liquid metal from the molten metal block can thus wet the surrounding metal powder and facilitate the melting of the metal powder.
• iq resulting from the melting of a block of solid metal can be greater than the volume of liquid metal resulting from melting a powder of this metal, in particular because the density of the metal block is greater than the density of the metal powder.
• the system preferably comprises a heating device configured to melt the solid metal, preferably directly within the crucible 1.
• This heating device can be configured to heat the solid metal by radiation and / or conduction of the walls and the bottom of the crucible 1. It can alternatively be configured to directly heat the metal by induction or resistively.
• Coils 12 can for example be placed around the crucible 1, and separated from the crucible 1 by an insulating element 13, so as to generate an electromagnetic induction phenomenon within the metal and, consequently, to melt this metal.
• the heating of the solid metal initially contained by the crucible 1, in the form of a block and / or powder makes it possible to obtain an initial bath of liquid metal.
• the crucible 1 preferably has an outlet orifice 10 at the bottom of the crucible 1, so that the liquid metal M
• This orifice 10 is preferably connected to a capillary 2, so as to control the flow of the liquid metal.
• the process can therefore advantageously be initiated.
• the crucible 1 can then be refilled with the solid metal, the solid metal can then be melted, so that the liquid metal flows again through the capillary 2.
• This process is preferably continuous.
• the crucible 1 is preferably supplied with a powder of the metal M pow at an upper part.
• the system may include a powder container 1 1 or another device 11 for feeding powder M pow at the top.
• the positioning of the feed device 1 1 in the upper part allows a powder feed by gravimetry.
• the fine particle size of the powder in fact requires a suitable supply device 11, preventing or limiting the aggregation of the particles by electrostatic bonding.
• a pressurized feed device promotes electrostatic bonding and is not suitable for a powder feed.
• the powder supply stage of the crucible can be configured to deliver the metal powder M pow continuously or intermittently.
• the device 1 1 for feeding powder M pow is preferably configured to deliver a very high volume flow rate of powder, for example greater than 1 kg. h 1 .
• the feed device 1 1 in powder M pow is preferably configured to prevent the powder M pow from clogging said feed device 1 1.
• the feed device 1 1 may comprise at least one passage having a passage section of the powder M pow large enough, typically strictly greater than 3 cm, preferably greater than or equal to 5 cm, to avoid a pressure variation in said passage which would promote aggregation or bonding of the powder M pow on the walls of the passage.
• the walls of the passage also preferably have roughness at large and small scales respectively R g and R p optimized to avoid any onset of accumulation of powder M pow on said walls.
• the roughness at large and small scales can be such that R p ⁇ 0.4 pm and R g ⁇ 0.3. R p .
• the heating of the metal within the crucible 1 is preferably maintained so as to maintain a bath of liquid metal in the crucible 1.
• the pre-filling step which is only optional and optional, makes it possible to melt the metal powder M pow coming from the reservoir 1 1 more quickly.
• the metal powder M pow melts more easily by contact with the initial bath of liquid metal, than by the sole effects of contact with the crucible 1.
• the initial bath of liquid metal therefore makes it possible to more quickly form and maintain the bath of liquid metal M
• iq In order for the liquid metal to flow through the capillary 2, the liquid metal bath M
• Figure 2 illustrates the flow conditions of the liquid metal bath.
• the liquid metal flows through a cylindrical capillary 2 of radius R, if the gravitational force associated with the weight of the column 20 of liquid metal is greater than the surface tension forces at the circumference of this column 20.
• Column 20 has a total height H and is located partly in crucible 1 and partly in capillary 2.
• the powder feed rate is preferably configured so that the height of the liquid bath in the crucible 1 is always greater than H min .
• the granulation process can be continuous.
• the surface tension g d , 1450 C is equal to 730 mN / m, according to “F. Millot et al, The surface tension of liquid Silicon at high temperature, Materials Science and
• the liquid silicon at 1450 ° C. will therefore flow as soon as the height H of column 20 is greater than the value H min indicated in the table below:
• the surface tension g A i 660 C is equal to 1040 mN / m, according to “V. Sarou-Kanian, Surface Tension and Density of Oxygen-Free Liquid Aluminum at High Temperature, International Journal of Thermophysics, (2003) Vol . 24, No. 1 ", and the density p Ai 660 C is equal to 2.38 g. cm 3 , according to https: //www.aqua- calc.com/page/density-table/substance/liquid-blank-aluminum for example.
• the radius R of the capillary 2 can be between 2 mm and 10 mm.
• the height h 2 of the capillary 2 is not zero, and between 1 mm and 50 mm.
• Such a capillary 2 also called “drop nose” makes it possible to avoid an uncontrolled flow of the liquid metal at the level of the outlet orifice 10 in the bottom of the crucible 1, in particular at the edges of this orifice 10.
• the drop nose also makes it possible to prevent spreading of the droplet of liquid metal from the edges of the orifice 10 on an external face of the bottom of the crucible 1.
• the flow of liquid metal leads, in the presence of a drop nose, to a magnification of the droplet at the outlet of the drop nose or, in the absence of a drop nose, to spreading of the droplet in the form of a liquid film on the external face of the bottom of the crucible 1 (by minimizing the surface energy).
• the drop nose creates a vertical wall favoring a flow in the form of droplets by gravity.
• iq can therefore flow continuously at the outlet orifice 10, at the inlet of the capillary 2, and discontinuously in the form of droplets M dropS at the outlet of the capillary 2.
• Such a height h 1eq is preferably chosen so that the gravitational force associated with the weight of the column 20 of liquid metal of height h 1eq is substantially equal to the surface tension forces at the circumference of this column 20. This height h 1eq therefore corresponds to an equilibrium point for the flow of liquid metal.
• a discontinuous flow can be formed by slightly varying the conditions of the liquid metal bath around such an equilibrium point.
• a column height 20 slightly greater than this height h 1eq will cause a droplet of liquid metal to fall.
• the addition of powder M pow in the crucible 1 will ultimately cause the droplet to fall.
• a column height 20 slightly lower than this height h 1eq will stop the flow of liquid metal.
• the height of column 20 will decrease and the flow of liquid metal will be stopped, in particular until a new addition of powder M pow in the crucible 1 again allows the flow in the form of a droplet.
• This instability can also be induced by a variable magnetic field having a frequency of the order of kHz.
• the magnetic field is applied to the continuous flow of liquid metal, preferably at the level of capillary 2.
• the droplets M drop S formed at the outlet of the capillary 2 advantageously have a homogeneous size.
• the characteristics of the magnetic field in particular its frequency, depend on the properties of the metal considered.
• the production of gallium droplets of homogeneous size occurs for a magnetic field of the order of 320 Hz.
• This magnetic field is such that it generates a distance interval between droplets which corresponds to a wavelength of intrinsic destabilization of the flow of liquid metal. This wavelength depends in particular on the surface tension and the resistivity of the liquid metal.
• the frequency of the magnetic field can be adjusted depending on the metal whose flow is to be destabilized.
• the frequency of the magnetic field can be between 100 Hz and 1500 Hz.
• the granulation system can comprise an electromagnetic coil 21 arranged around the capillary 2, so as to generate this electromagnetic field and, consequently, the instability in the continuous flow.
• the instability is induced in a combined manner by capillarity and by the variable magnetic field.
• the distribution of droplet sizes therefore has a reduced standard deviation.
• the reproducibility of this distribution is further improved.
• the mass flow rate of the droplet flow flowing out of the capillary 2, called the mass flow output can be between 0 and 60 kg. h 1 , preferably between 1 and 20 kg. h 1 , depending on the size of the capillary (ies) 2.
• the powder feed rate can be adjusted according to the desired outlet mass flow rate.
• the formation of the droplet flow at the outlet of the capillary 2 corresponds to the end of the pre-granulation step.
• the following atomization step is intended to form solid granules of metal Mgrains, from the flow of droplets M drops ⁇
• the liquid metal droplets M dropS are preferably collected on the receiving surface 30 in rotation of a rotating disc 3.
• This receiving surface 30 can have a diameter between 10 cm and 50 cm, preferably between 10 cm and 30 cm.
• the droplets preferably fall directly onto the rotating receiving surface 30.
• the drop height of the droplets of liquid metal, taken between the capillary 2 and the surface 30 can be between 1 cm and 1 m.
• This rotating disc 3 makes it possible to atomize the droplets, that is to say to fragment them. This fragmentation makes it possible to obtain droplet fractions which can be solidified quickly by cooling.
• the cooling is preferably carried out directly by contact with the receiving surface 30.
• the contact time depends in particular on the rotation of the surface 30.
• the speed of rotation of the rotating disc 3 is in particular chosen so that the droplets of liquid metal solidify before leaving the receiving surface 30 of the rotating disc 3.
• This surface 30 is preferably cooled by the circulation of a fluid at room temperature in the rotating disc 3, for example water at 18 ° C.
• this surface 30 is preferably made of metal with high thermal conductivity, for example copper or cast iron.
• the receiving surface 30 of the rotating disc 3 is concave in order to increase the contact time between the cooled surface 30 and the droplets of liquid metal. Cooling is thus optimized.
• the cooling is configured to evacuate a large heat flow, for example greater than 250 W. This makes it possible to cool the metal droplets fast enough so as to obtain a sufficiently low granule temperature, for example less than or equal at half the melting temperature Tf of the metal considered.
• Such “cold” granules advantageously limit the phenomena of solid diffusion (thermally activated) which can occur during contact between the granules and the different walls of the granulation system (rotating disc, receptacle). The contamination of the particles is thus reduced.
• Such cooling also makes it possible to cool droplets having a high calorific capacity c x and / or a high latent heat of fusion, such as silicon droplets (c x ⁇ 1000 J. kg 1. K 1 ).
• Silicon is an example of a material that may require a dimensioned cooling to evacuate a heat flow greater than 400 W. Indeed, to cool a kilogram of liquid silicon from its melting temperature Tf to half of it (Tf / 2 ), it is necessary to evacuate approximately 1, 6.10 e J against 8.5.10 e J for aluminum and 1, 1 10 e J for iron. The use of silicon thus requires evacuating a 50% excess of heat compared to iron and 100% compared to aluminum. In addition, the thermal conductivity of solid silicon is much lower than that of transition metals, with a value between and 20 and 40 Wm 1 .K 1 over the range [Tf / 2 - Tf], against more than 200 Wm 1 .K 1 for aluminum for example. The cooling of the silicon may therefore require evacuating a heat flux greater than approximately 400 W.
• the system according to the invention preferably comprises a cooling device configured to evacuate a heat flow greater than or equal to 400 W.
• the method according to the invention preferably comprises a configured cooling step, mutatis mutandis, to evacuate a flow of heat. heat greater than or equal to 400 W.
• This avoids an extension of the residence time of the hot granules (ie having for example a temperature between Tf / 2 and Tf) on the rotating disc 3, before ejection of the cold granules (ie having for example a temperature below Tf / 2) to a receptacle.
• the mass flow rate of granules produced is thus improved.
• the fragmentation by rotation of a stream of droplets advantageously requires less energy than the fragmentation by rotation of a continuous stream.
• the rotation speed of the rotating disc 3 can therefore be between 100 and 3000 revolutions per minute.
• Such a speed 10 times lower than the rotational speeds of the rapid solidification processes described in the literature advantageously makes it possible to simplify the granulation system, and to make the system and the granulation process more reliable.
• the device for cooling the rotating disc 3 can be relatively simple to implement, unlike a device for cooling a rotating disc at a rotation speed of the order of 35,000 revolutions / min, for which problems cavitation may appear, for example.
• the surface 30 can also be protected by a barrier material in order to limit any contamination between the liquid metal and the surface 30 of the disc 3.
• a barrier material for example in the case of silicon, the surface 30 can be protected by a layer of silicon nitride, silica or graphite.
• a receptacle made of a non-polluting material for example an ultra clean silicon bed resulting from chemical processes of the Siemens or FBR type (fluidized bed reactor), also makes it possible to limit the contamination. .
• the droplet fractions solidify in the form of granules M grai ns.
• the center of rotation of the receiving surface 30 carried by the axis B is offset by a distance d from the axis A of flow of the droplet flow, in order to avoid an accumulation of material at the center of the rotating disc 3 where the rotation speed is zero.
• the distance d is preferably greater than 50% of the radius of the disc.
• the solid metal granules M grai ns can then be collected in a receptacle 4 in the form of a funnel for example, and directed into a removable container 5, for their subsequent use.
• the device and the method according to the invention can be advantageously used for the industrial production of silicon granules from silicon powders. These silicon granules can then advantageously be used in a production chain of the photovoltaic silicon sector.
• the production of granules can have a mass flow of between 0 and 60 kg. h 1 , preferably between 1 and 20 kg. h 1 .
• the metal may be a metal alloy, for example an aluminum-silicon alloy AlSi.
• the axes A of flow of the droplet flow and B of rotation of the rotating disc are not necessarily parallel to each other.

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• Manufacture Of Metal Powder And Suspensions Thereof (AREA)
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