Continuous molten silicon for downstream processing

Abstract

The invention relates to a method for the continuous production of a silicon melt and subsequent immediate further processing of the silicon melt, comprising the following steps: (A) providing solid silicon in a melting vessel; (B) heating the melting container to form a silicon melt; (C) supplying solid silicon, wherein the supplied solid silicon is melted; and (D) immediately supplying the obtained silicon melt to a processing unit for melting silicon. The invention also relates to a processed product of silicon and to the use thereof.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for the continuous production of molten silicon and immediate subsequent further processing, comprising the steps of: (A) providing solid silicon in a melting vessel; (B) heating the melting vessel to form molten silicon; (C) feeding solid silicon and melting the fed solid silicon; and (D) directly feeding the obtained molten silicon to a processing unit for the molten silicon. The present invention further relates to products of silicon processing and uses thereof. BACKGROUND

[0002] The molten silicon can be further processed in a variety of ways, for example by dispersion / atomization, by casting, by stirring to form an alloy or by granulation.

[0003] Processing products based on molten silicon, such as silicon particles obtainable by atomization of molten silicon, have a broad potential of applications. However, as summarized below, the melting and processing operations described in the prior art are generally batch processes and require very complex process management.

[0004] Batch processes have several disadvantages, such as low space-time yield and high material loss due to conventional heating and cooling operations. Thus, large-scale industrial utilization of processing products based on molten silicon requires a continuous production method to be able to provide the required amounts inexpensively.

[0005] EP 3841225 A1 does indeed describe a method for the continuous production and processing (here: atomization) of a metal melt, but the liquid melt has to be transferred through several vessels before it can be further processed by atomization. This complex transfer from smelting before further processing results in high operating costs and complex plant construction and at the same time is very susceptible to defects.

[0006] EP 0372918 B1 likewise relates to a method for the continuous production of silicon powder, wherein silicon is melted in a melting furnace, cast at the top end of the melting furnace and refined and alloyed in a further reaction unit, is stored in a holding furnace in between and then transferred to a processing unit (in this case an atomization device). The method described here requires a very complex processing system, wherein the molten silicon (the processing of which is very complex in any case due to its high temperature) has to be transported through several reactor chambers before the melt processing is carried out. Since the molten silicon is poured at the top end of the melting furnace together with any formed slag floating on top of the melt, a further purification step of the silicon is necessary. Overall, the method described in EP 0372918 B1 is thus very complex and laborious and therefore also susceptible to errors.

[0007] Thus, with regard to the further processing of molten metals by dispersion or atomization of the molten metal, there is a broad prior art, which also includes the following: As early as 1987, DE 3533964 C1 describes a method for producing spherical metal ultrafine powders having a diameter of 5 to 30 pm. It proposes producing a molten metal in a crucible and atomizing a melt jet exiting from the crucible in a Laval nozzle system with a flow of propellant gas. The molten metal can be produced in batches by placing a metal ingot in the crucible, melting it therein and keeping it in the melt. There is an alternative description of a continuous process, in which the metal ingot is pushed from above (further) into the crucible by a complex supplementary device, so that the ingot is always in contact with the melt and melts on the surface. This document thus discloses a very complex and error-prone continuous process concept, in which the metal to be melted must be directed directly to the melt or directly to the Laval nozzle. Furthermore, it is not stated that the method proposed by it is also suitable for silicon.

[0008] DE 3311343 A1 from the same inventors and applicants as the above document emphasizes that the metal particles obtainable by melting and atomizing a molten metal have a smooth surface. However, such particles are not sufficient for the demanding applications of the present invention, for example as a thermally conductive material, as will be explained in detail below.

[0009] DE 3737130 A1 is a further development of DE 3533964 C1 on this system, in which the melt is produced in a melting device which is spatially separate from the Laval nozzle system and is transported to the Laval nozzle system via a U-shaped tube in a complex manner. The goal of DE 3737130 A1 is thus that the melt jet to be atomized has a particularly homogeneous material flow. In this way, metal particles can be produced in a particularly reproducible manner.

[0010] The method described in this way, however, also requires a very complex reaction concept and is not described in conjunction with silicon.

[0011] The method just described is improved again in EP 1042093 B1. An optimized Laval nozzle with a gas nozzle of special arrangement reduces the gas consumption and increases the production output. The only atomizable metal melts mentioned are those of tin solder, Cr-Mo steel, silver and aluminum. There is no indication regarding silicon melts. The resulting particles are spherical, with a particle size of 8.5-10.1 pm, which is very small.

[0012] The patent specification DE 103 40 606 B4 discloses melting a metal in a crucible and atomizing it through a Laval nozzle into a metal powder. It is essential here that the melt connection for introducing the molten metal in the crucible into the nozzle is thermally shielded from the cold atomizing gas, since otherwise there is an excessive cooling of the melt which significantly worsens the quality of the produced powder (particle shape, particle size, particle size distribution width) or makes atomization impossible. DE 103 40 606 B4 therefore proposes an appropriate shielding. The challenge is to design the shielding such that it does not adversely affect the flow distribution of the dispersing gas before it enters the nozzle, since this flow distribution also has a considerable influence on the quality of the produced powder. The described method envisages a discontinuous method in which a stopper rod first closes the crucible at the bottom and is pulled upwards after the melt has been produced to clear the crucible from outflow in the atomization direction. A continuous operation is only recommended in combination with low-melting metals (Pb, melting point approximately 327°C). Silicon is not mentioned.

[0013] EP 3083107 B1 describes a crucible-free melting and atomization method for metals in which a metal rod specially prepared for this purpose is melted within an induction coil to produce a melt flow which is then introduced directly into an atomization nozzle. This method requires complex lifting devices to hold, lift and lower the metal rod in order to constantly adjust the position of the rod within the induction coil. The advantage of this method is that no crucible is required which can contaminate the metal to be melted therein. However, the method is relatively complex and requires the production of corresponding metal rods. These rods must also be produced with the required purity, since it is no longer possible to purify the metal to be atomized in this process. The document discloses, for example, aluminum, stainless steel and titanium as suitable metals; silicon is not disclosed.

[0014] As mentioned above, a common factor in the methods known to date for melting a metal and subsequently further processing the melt is that very complex and laborious method concepts are necessary and, if the silicon used is intended to be purified at all before further processing, this is carried out in separate method steps. Furthermore, there is no known efficient method for a continuous processing concept, in particular using silicon as starting material.

[0015] It is therefore desirable to provide a simple and inexpensive method for preparing and further processing molten silicon, while having the possibility of purification of the silicon, which overcomes the above-mentioned disadvantages and can be operated in a particularly inexpensive and robust manner.

[0016] The resulting processing should also be able to provide high-grade silicon processing products which are used in demanding applications and in particular for example high thermal conductivity. SUMMARY

[0017] The underlying object of the present invention is achieved by the first aspect of the present invention, which relates to a process for the continuous production of molten silicon and immediate further processing, the process comprising the following steps in the order stated: (A) providing solid silicon (initial silicon) in a melting vessel; (B) heating the melting vessel to a processing temperature using suitable heating means and melting the solid silicon to form molten silicon; (C) feeding solid silicon into the melting vessel and melting the fed solid silicon; and (D) feeding the silicon melt obtained after step (C) directly via a melting vessel outlet on the bottom side of the melting vessel to a processing unit for the molten silicon for further processing of the molten silicon.

[0018] In the context of the present invention, "immediately" means that the silicon melt obtained in step (C) is fed directly to the processing unit without being transferred to, for example, an intermediate vessel.

[0019] For example, the feeding can be contactless, by free outflow or via a conduit made of a suitable material. Suitable materials that are resistant to molten silicon are known to the skilled person.

[0020] The process of the present invention has several key advantages compared to prior art processes: By the process of the present invention, liquid silicon can be provided in a continuous manner for downstream processing, for example granulation or dispersion (referred to as atomization). This allows the processing operation of the liquid silicon to be performed for a long time without interruption, which increases, for example, the processing efficiency, the product uniformity and the product quality, and significantly reduces the susceptibility to errors.

[0021] The continuous melting of silicon on a small scale also avoids complex and dangerous handling and transfer of large amounts of melt compared to standard melting processes. The processing reliability is thus significantly increased.

[0022] The continuous process also increases the production volume, since the downstream process (for example granulation or atomization) can also be operated continuously. Since no heating or cooling times are required, the costs of the granular end product are reduced.

[0023] The continuous process is also associated with lower technical complexity.

[0024] The process of the present invention also offers the advantage that the impurities in the starting material can be removed by separating any slag formed containing impurities and simultaneously processing and purifying the silicon in a single processing step.

[0025] When using silicon as starting material, it has been found to be surprisingly advantageous to discharge the molten metal at the bottom end of the melting vessel. Until it is completely molten, solid silicon which is subsequently charged into the melt does not sink to the bottom in the melt, but floats on the melt and melts itself. Any dross formed which contains impurities of the silicon also floats on the liquid melt. If the molten silicon is subsequently discharged at the bottom of the melting vessel, this is done by spatial separation of the un-molten silicon and the dross. The discharged melt thus has an increased purity. The method of the present application thus surprisingly enables a purification directly during the processing of the silicon. The high-purity processed product of the silicon is particularly suitable for high-performance applications of the present application, such as heat transfer technology.

[0026] In order to limit the number of pages of the description of the present application, only the preferred embodiments of the individual features are stated hereinafter.

[0027] However, the expert reader should clearly understand this way of disclosure, so that any combination of different preferred levels is thus also clearly disclosed and clearly intended.

[0028] The melting vessel preferably has a top end and a bottom end which define a melting vessel height therebetween, wherein the upper side preferably has an opening through which the solid silicon charged in step (C) is fed.

[0029] In a preferred implementation, the melting in step (B) forms a molten silicon on which a dross floats, which particularly contains impurities of the silicon used.

[0030] As has been elucidated, the continuous melting method of the present application enables a simultaneous purification of the starting material without performing complex additional process steps. When using raw silicon with a low purity, or even in the case of pre-purified silicon with a purity of at least 99%, the dross, which particularly contains impurities of the silicon used, floats on the surface of the melt and is thus separated from the purified silicon which flows out at the bottom. This is the main advantage of the method. There is no need to separate out the impurities in a complex manner, by decanting or a reactive gas atmosphere, as is often used in the art.

[0031] Suitable heating devices are, for example, induction heaters.

[0032] Preferably, the processing temperature is at least 10°C, preferably at least 20°C, more preferably at least 30°C, above the melting temperature of the solid silicon used.

[0033] In a preferred embodiment, the method is set during step (C) to readjust the treatment temperature set in step (B). This is necessary, in particular, when the temperature of the mixture of the silicon melt and the solid silicon charged into the melting vessel decreases below the desired processing temperature.

[0034] Preferably, step (C) is started once most of the initial silicon has been melted, for example once at least 50% by weight of the initial silicon is in molten form.

[0035] The feeding of solid silicon in step (C) can be continuous or in portions, preferably continuous. More preferably, the feeding of solid silicon in step (C) is arranged such that as much solid silicon is fed as molten silicon leaves the melting vessel via the melting vessel outlet.

[0036] The feeding in step (C) can be carried out by means of suitable devices, for example by direct dosing onto the melt or into the melt below the surface with a star feeder by gravity.

[0037] The initial silicon and / or the solid silicon fed in step (C) is preferably metallurgical silicon.

[0038] Preferably, the initial silicon and / or the solid silicon fed in step (C) has an average particle size in the range of 0.5 mm to 20 cm, preferably 1 mm to 20 mm.

[0039] The initial silicon and / or the solid silicon fed in step (C) is commercially available, for example as pre-crushed material or obtainable by grinding methods using a protective screen with suitable mesh sizes, for example not more than 2 cm.

[0040] Surprisingly, the grain size of the present application leads to particular benefits in the processing scheme: small-grained silicon particles float on the molten silicon and cannot melt fast enough. Silicon particles with a larger particle size than according to the present application also do not melt fast enough. Only particles with the inventive particle size show a suitable melting distribution for the inventive process.

[0041] In a preferred process, the solid silicon fed in step (C) has less than 10% by weight with < 150 pm, preferably less than 5% by weight with < 100 pm, more preferably less than 3% by weight with < 80 pm, determined by sieve analysis according to DIN 66165.

[0042] The compliance with small amounts of ultra-small particles makes a particular contribution to occupational health and safety. If the particle size distribution of the present application is observed, no complex dust protection measures, for example dust extraction devices on the melting vessel, are necessary.

[0043] The solid silicon fed in step (C) is particularly fed in such a way that it comes into direct contact with the molten silicon in the melting vessel.

[0044] This is particularly advantageous because it enables better heat transfer between the already molten silicon and the subsequently dosed solid silicon and a high melting rate can be achieved, which promotes a continuous process sequence.

[0045] Preferably, the purity of the initial silicon and / or the solid silicon dosed in step (C) is > 96 %, preferably > 98 %, more preferably ≥ 99 % purity.

[0046] A preferred method configuration is such that the filling level of the melt within the melting vessel is sufficiently high to prevent clogging of the outlet and / or the discharge of slag, in particular at least 10 cm high.

[0047] Alternatively, a preferred method configuration is such that the filling level of the melt within the melting vessel is at least 10 %, more preferably 20 % to 80 %, in particular 30 % to 70 % of the height of the melting vessel.

[0048] It is particularly advantageous to maintain the melt level within the melting vessel at the height according to the application, because thereby undesirable clogging of the outlet by slag and / or the discharge of slag via the melting vessel outlet (if the level of the melting vessel is too low) or the top end of the melting vessel (when the melt level height is exceeded according to the application) is avoided. When the filling level of the melting vessel is too low, it is also possible to prevent clogging of the melting vessel outlet by un-melted silicon.

[0049] Any slag formed in step (C) can be collected on the surface of the melt and / or partially or continuously separated from the surface, for example by decanting into a collection vessel for slag.

[0050] In a particular implementation, the removal of the slag floating on the surface of the melt is designed such that the filling level of the melt is increased, for example via the dosing of solid silicon, to such an extent that the filling level reaches the top of the melting vessel. In this case, the top of the melting vessel has an open design such that the slag, possibly together with a small portion of the melt, overflows and is thereby removed from the melting vessel.

[0051] The overflowing slag can subsequently be collected in an overflow channel, for example surrounding the melting vessel and close to the top end of the melting vessel.

[0052] A preferred implementation of the method is carried out at a melting rate of at least 45 kg of silicon per hour, preferably 60 kg of silicon per hour, more preferably 75 kg per hour.

[0053] The processing unit can be selected from a dispersion unit, a casting unit, a stirring unit, a granulation unit or a purification unit, in particular a dispersion unit.

[0054] In the case of a casting unit, the molten silicon from the melting vessel outlet is poured directly into a mold for shaping, in which the silicon solidifies.

[0055] In the case of a stirring unit, the molten silicon from the outlet of the melting vessel is poured directly into a stirring tank, where the molten silicon is then added to, for example, a metal melt in the stirring tank to form an alloy.

[0056] In the case of a granulating unit, the molten silicon from the outlet of the melting vessel is directly transferred to a quenching vessel filled with water, where the molten silicon solidifies in the form of granules in the water.

[0057] In the case of a purification unit, for example, the molten silicon is fed to a Czochralski process, in which a high-purity single crystal is obtained from the silicon melt with the aid of a silicon crystallization seed.

[0058] In a particularly preferred embodiment, the immediate subsequent processing of the molten silicon is the production of silicon granules.

[0059] The processing unit in this case is more preferably a dispersing unit for the molten silicon, and the method is configured such that the molten silicon is dispersed in the form of a spray jet immediately after melting and solidifies to form silicon granules, preferably mainly smooth, spattered and / or nodular silicon granules with an irregular surface morphology.

[0060] The molten silicon is directly fed to the dispersing unit, i.e. the melt is directly fed to the dispersing unit without being transferred to an intermediate tank.

[0061] The dispersing in step (D) can be carried out, for example, with an inert gas as dispersing gas, which is selected from the group consisting of nitrogen and argon, in particular nitrogen.

[0062] The dispersing unit preferably comprises at least one atomizing unit. Gas atomization technology is widely used in industry for the atomization of metal melts. Various different nozzle designs are used, all of which have the common factor that a pressurized atomizing gas escapes from one or more gas nozzles and approaches the melt, resulting in the dispersion of the melt.

[0063] The atomizing unit can thus be a close-coupled unit or a Laval nozzle, in particular a Laval nozzle.

[0064] In the close-coupled unit, the molten silicon that exits the melting vessel via the outlet of the melting vessel is immediately contacted with a dispersing gas released from a plurality of gas nozzles close to the outlet of the melting vessel upon discharge in order to achieve shearing of the melt and thus atomization.

[0065] When a Laval nozzle is used, the melt flows through the nozzle, which is surrounded by a coaxial stream of dispersing gas in the same flow direction. This causes the melt to be subjected to shear stress, which the melt yields to in the form of expansion, which achieves the dispersion.

[0066] Laval nozzles for atomizing molten metal are well known in the art and are described, for example, in US6481638B1.

[0067] Atomization is preferably carried out at a throughput rate of at least 45 kg / hour, preferably 60 kg / hour, more preferably 75 kg / hour.

[0068] When the dispersion unit is surrounded by a housing, the dispersion in step (D) is preferably arranged such that the molten silicon is essentially completely solidified before impinging on any housing part of the dispersion unit.

[0069] The spray jet from the expansion nozzle comprising predominantly round, splashed and / or nodular silicon particles in solidification is preferably continuously maintained for at least 60 minutes, more preferably for at least 90 minutes, more preferably for more than 120 minutes.

[0070] Predominantly round, splashed and / or nodular silicon particles having the particular surface morphology of the particles which are of particular advantageous properties can be obtained especially via the inventive process of continuously producing molten silicon which is subsequently immediately processed in the form of a dispersion operation.

[0071] The object of the present invention is also achieved by another aspect of the present invention, which relates to silicon particles having a predominantly round, splashed and / or nodular particle shape, wherein the median particle size d 50 is in the range of 20 to 200 µm, in particular 30 to 150 µm, wherein the particles have an irregular surface morphology characterized by a raspberry-like surface structure.

[0072] d 50 may be determined by methods known to the person skilled in the art, for example by laser diffraction. In laser granulometry, the particles are dispersed in a liquid or gaseous medium. The particle size distribution is then determined on the basis of the diffraction of a laser beam directed at the sample medium. 50 means that 50% of the particles are smaller than the reported value.

[0073] The silicon particles produced according to the preferred melting and dispersion process of the present invention have a predominantly round, splashed and / or nodular particle shape, i.e. a raspberry-like surface structure. This means that the silicon particles are formed from predominantly round, splashed and / or nodular primary particles, preferably nodular or predominantly round primary particles, more preferably predominantly round primary particles, on which a large number of secondary particles are present.

[0074] The average diameter of the primary particles is greater than the average diameter of the secondary particles, wherein the average diameter of the primary particles is preferably at least 2 times, preferably at least 3 times greater than the average diameter of the secondary particles. The secondary particles can likewise be spherical or in the form of prisms, prismatic structures or cones protruding from the surface of the primary particles. Preferably, at least 5% of the surface area of the primary particles is covered with secondary particles, preferably at least 10%, more preferably at least 20%, as can be confirmed via scanning electron microscope images. Completely unexpectedly, in the silicon particles according to the application having a preferred raspberry-like surface, the secondary particles are firmly fused to the primary particles and are partially immersed therein, preferably to an average extent of at least 10% of the diameter of the secondary particles, as discernible via electron micrographs. In this way, the silicon particles according to the application having a preferred raspberry-like surface differ from agglomerates of primary particles and secondary particles, wherein the secondary particles adhere to the surface of the primary particles only by physical interaction and can thus easily be separated therefrom.

[0075] The term "substantially circular" (cf. Figure 1A and 1B ) means a shape comprising a small deviation from a spherical shape, for example a slightly ellipsoidal shape.

[0076] "Spattered" is a term known in the art of granules and means that the primary particles have, for example, cracked and possibly jagged surfaces.

[0077] Preferably, the spattered primary particles are formed from agglomerates of ellipsoidal and / or spherical individual particles.

[0078] "Nodular" (cf. Figure 2A and 2B ) is likewise a term known in the art of granules and means that the primary particles are formed, for example, from nodular agglomerates of irregular individual particles.

[0079] It has been completely unexpectedly found that the silicon particles produced by the preferred method according to the application exhibit the advantageous properties of the present application, in particular a relatively high thermal conductivity. The high thermal conductivity of the particles of the present application is particularly useful, for example, in the production of thermally conductive plastics in which the particles of the present application are embedded in a plastic matrix, for example a silicone matrix.

[0080] In this connection, Figure 3A a cross-section of the interface between a particle of the present application having an irregular surface morphology (bottom) and a plastic matrix (top) is shown. In Figure 3B it is clear that the heat transfer from the particle into the plastic surprisingly passes through the secondary particles, i.e. the raspberry-like surface structure, into the plastic matrix. The particles of the present application are surprisingly outstanding due to the excellent heat transport properties.

[0081] Due to their low heat transfer tolerance, the particles of the present invention are particularly well-suited as thermally conductive fillers for various matrices, such as plastics.

[0082] In the prior art, Si particles used as fillers are typically obtained by grinding and are prismatic in shape (see [link]). Figure 4 It also has an angular surface.

[0083] The disadvantage is that such particles have a large surface area and, as a complex with the polymer, bind a very large amount of polymer. This significantly increases the viscosity of the plastic composition. Only mixtures with relatively low filler content and low thermal conductivity can be produced. At higher filler contents, the composition becomes very hard and can no longer be processed by conventional methods, such as dispensers. It has also been found that plastic compositions containing abrasive silica particles with angular surfaces exhibit relatively severe abrasion during processing.

[0084] In experiments, it was found completely unexpectedly that plastics containing the silicon particles of the present invention exhibit significantly improved thermal conductivity.

[0085] The irregular surface morphology of the particles significantly improves thermal conductivity.

[0086] Preferably, the particles of the present invention have an aspect ratio (b / l) of at least 0.6, preferably at least 0.7.

[0087] Aspect ratio is used as an index to describe particle shape. Previous techniques often described it as the ratio of length to width (l / w). This gives a value greater than or equal to 1. In newer literature, such as according to ISO 9276-6, the aspect ratio is calculated as the inverse ratio of width to length (w / l). This gives a value no greater than 1.

[0088] These two indices can be converted to each other by forming their reciprocals. In the context of this invention, aspect ratio is defined as the ratio of the particle's width to its length (w / l). Particle width is defined as x. c min , is the smallest of all the maximum chords measured in the particle projection, and the particle length is defined as x. Fe max , is the longest Feret diameter among all measured Feret diameters of the particle. More detailed information can be found, for example, in "Operating Instructions / Manual ParticleSize analysis System CAMSIZER®", Retsch Technology GmbH, 42781 Haan; Doc.No. CAMSIZER V0115. This gives the following formula for the aspect ratio: w / l = x c min / x Fe max In a preferred embodiment, the particles of the present application have a sphericity index SPHT of at least 0.68, preferably at least 0.74.

[0089] The sphericity SPHT is calculated from the projected area A of the particles analyzed relative to the area of a circle having the same perimeter P of the projected particles according to the following formula (for more details, reference is made to e.g. "Operating Instructions / Manual Particle Size Analysis System CAMSIZER®", Retsch Technology GmbH, 42781 Haan; Doc. No. CAMSIZER V0115): SPHT = 4πA / P 2 The index SPHT corresponds to the square of the circularity C according to ISO 9276-6.

[0090] The particles of the present application preferably have a particle size distribution with a distribution width SPAN ((d 90 -d 10 ) / d 50 ) of at least 0.4, preferably at least 0.6, more preferably at least 0.8.

[0091] In a preferred embodiment, the SPAN is from 0.6 and 3.0, in particular from 0.8 to 2.5.

[0092] The skilled person knows that the standard deviation is not normalized and is a useful characteristic for assessing the particle size distribution of different samples only when the average particle size of the compared samples is approximately equal. Therefore, in the context of the present application, the relative width of the particle size distribution is described using the width of the particle size distribution (weighted by the median particle size d 50 , the dimensionless distribution width SPAN is defined as: SPAN = (d 90 -d 10 ) / d 50 .

[0093] Within the scope of the present application, the particle size (parameter: median diameter d 50All parameters referring to the particle size distribution (parameters: standard deviation sigma and distribution width SPAN) or the particle shape (parameters: aspect ratio w / l and sphericity SPHT) refer to the volume-based distribution. The mentioned indices can be determined, for example, by dynamic image analysis according to ISO 13322-2 and ISO 9276-6, for example using the Camsizer X2 instrument from Retsch Technology.

[0094] The particles of the present application preferably contain, based on the total amount of silicon particles, not more than 1.5% by weight, preferably not more than 1% by weight, more preferably not more than 0.5% by weight of silicon particles smaller than 2 pm. Especially preferred silicon particles are essentially free of the fraction of particles smaller than 2 pm. By "essentially free" is meant tolerable in the particles of the present application to the extent that such particles are "impurities" and do not destroy their inventive effect.

[0095] The silicon particles of the present application preferably contain, based on the total amount of silicon particles, less than 15% by weight, more preferably less than 10% by weight, especially preferably less than 5% by weight of the fraction of particles having a diameter of less than or equal to 10 pm.

[0096] The silicon particles of the present application preferably contain, based on the total amount of silicon particles, less than 20% by weight, more preferably less than 15% by weight, especially preferably less than 10% by weight of the fraction of particles having a diameter of less than or equal to 20 pm.

[0097] The person skilled in the art is also aware that metallic silicon particles are combustible under certain conditions and that their dust can cause an explosion hazard. The person skilled in the art is also aware that with decreasing particle size the risk of dust formation, combustibility and explosion risk of metallic powders increases significantly. For this reason, a high proportion of very small silicon particles below 20 pm is unsuitable for many applications. Such particles are hazardous to process due to their lower minimum ignition energy and require complex and expensive safety precautions in industrial processing.

[0098] Silicon particles having an average particle size of greater than 200 pm are unsuitable for many applications of thermally conductive plastic compositions, since such large particle sizes of silicon particles are often unsuitable for small gaps which should be filled with, for example, a gap filler.

[0099] A further aspect of the present application relates to the use of the primarily round, splashed and / or nodular silicon particles having a raspberry-like surface morphology of the present application as thermally conductive filler.

[0100] Primarily round, splashed and / or nodular silicon particles with a raspberry-like surface morphology have a significant application potential, especially in the field of electric vehicles as thermally conductive filler for gap fillers or potting compounds. Such silicon particles provide a significantly higher thermal conductivity, reduce sealing and reduce production costs compared to conventional prior art thermally conductive fillers. BRIEF DESCRIPTION OF DRAWINGS

[0101] Figure 1A Figures A and B show electron micrographs of primarily round particles with a raspberry-like surface structure.

[0102] Figure 2A Figures A and B show electron micrographs of nodular particles with a raspberry-like surface structure.

[0103] Figure 3A Figures A and B show the thermal transfer curve of the silicon particles of the present invention in a plastic matrix.

[0104] Figure 4 Figures A and B show the thermal transfer curve of the silicon particles of the present invention in a plastic matrix. DETAILED DESCRIPTION

[0105] The present invention also relates to the following embodiments described by points 1-28: 1. A method for the continuous production of molten silicon and immediate subsequent further processing, comprising the following steps in the order stated: (A) providing solid silicon (initial silicon) in a melting vessel; (B) heating the melting vessel to a processing temperature using a suitable heating device and melting the solid silicon to form molten silicon; (C) feeding solid silicon into the melting vessel and melting the fed solid silicon; and (D) feeding the molten silicon obtained after step (C) directly via a melting vessel outlet on the bottom side of the melting vessel to a processing unit for the molten silicon for further processing of the molten silicon.

[0106] 2. The method according to point 1, wherein the melting vessel has a top end and a bottom end, the spacing of which defines a melting vessel height, wherein the upper side preferably has an opening through which the fed solid silicon in step (C) is fed.

[0107] 3. The method according to any of the preceding points, wherein the melting in steps (B) and (C) forms molten silicon with a floating dross on top.

[0108] 4. The method according to any of the preceding points, wherein the suitable heating device is an induction heater.

[0109] 5. The process according to any of the preceding points, wherein the process temperature is at least 10 °C, preferably at least 20 °C, more preferably at least 30 °C above the melting temperature of the initial silicon or solid silicon used.

[0110] 6. The process according to any of the preceding points, wherein step (C) is started as soon as a major part of the initial silicon has been melted, for example as soon as at least 50 % by weight of the initial silicon is in molten form.

[0111] 7. The process according to any of the preceding points, wherein the solid silicon is continuously or partially, preferably continuously, fed in step (C).

[0112] 8. The process according to any of the preceding points, wherein the feeding in step (C) is carried out by suitable means, for example by gravity feeding with a star feeder, directly onto the melt or into the melt below the surface.

[0113] 9. The process according to any of the preceding points, wherein the initial silicon and / or the solid silicon fed in step (C) is metallurgical silicon.

[0114] 10. The process according to any of the preceding points, wherein the initial silicon and / or the solid silicon fed in step (C) has an average particle size in the range of 0.5 mm to 20 cm, preferably 1 mm to 20 mm.

[0115] 11. The process according to any of the preceding points, wherein the solid silicon fed in step (C) has less than 10 % by weight having a particle size of < 150 pm, preferably less than 5 % by weight having a particle size of < 100 pm, more preferably less than 3 % by weight having a particle size of < 80 pm, determined by sieve analysis according to DIN 66165.

[0116] 12. The process according to any of the preceding points, wherein the solid silicon fed in step (C) is fed in such a way that it comes into direct contact with the molten silicon in the melting vessel.

[0117] 13. The process according to any of the preceding points, wherein the purity of the initial silicon and / or the solid silicon fed in step (C) is a purity of > 96 %, preferably > 98 %, more preferably > 99 %.

[0118] 14. The process according to any of the preceding points, configured such that the fill level of the melt within the melting vessel is high enough to prevent clogging of the outlet and / or to drain the slag, in particular at least 10 cm high.

[0119] 15. The process according to any one of claims 1 to 13, configured such that the fill level of the melt within the melting vessel is at least 10 %, more preferably 20 % to 80 %, in particular 30 % to 70 % of the height of the melting vessel.

[0120] 16. The process according to any one of the preceding points, wherein any slag formed in step (C) is collected on the surface of the melt and / or removed from the surface batchwise or continuously, for example by decanting into a collection vessel for the slag.

[0121] 17. The process according to any one of the preceding points, which is carried out at a melting rate of at least 45 kg silicon / hour, preferably 60 kg silicon / hour, more preferably 75 kg silicon / hour.

[0122] 18. The process according to any one of the preceding points, wherein the processing unit is selected from a dispersing unit, a casting unit, a stirring unit, a granulating unit or a purifying unit.

[0123] 19. The process according to point 18, wherein the processing unit is a dispersing unit for the molten silicon, and the process is configured such that the molten silicon is dispersed in the form of a spray jet immediately after melting and solidified to form silicon particles, preferably predominantly round, splashed and / or nodular silicon particles having an irregular surface morphology.

[0124] 20. The process according to point 18 or 19, wherein the dispersing in step (D) is carried out with an inert gas as dispersing gas, which is selected from nitrogen and argon, in particular nitrogen.

[0125] 21. The process according to any one of points 18 to 20, wherein the dispersing unit comprises at least one atomizing unit, for example selected from the group consisting of closely coupled units and Laval nozzles, in particular Laval nozzles.

[0126] 22. The process according to any one of points 18 to 21, wherein the atomizing is carried out at a through-put rate of at least 45 kg / hour, preferably at least 60 kg / hour.

[0127] 23. The process according to any one of points 18 to 22, wherein the dispersing unit is surrounded by a housing, and the dispersing in step (D) is preferably arranged such that the molten silicon is essentially completely solidified before it hits any housing part of the dispersing unit.

[0128] 24. The process according to any one of points 18 to 23, wherein the spray jet from the expansion nozzle containing the spherical silicon particles in solidification is continuously maintained for at least 60 min, preferably at least 90 min, more preferably more than 120 min.

[0129] 25. Silicon particles having a predominantly round, splashed and / or nodular particle shape, having a median particle size d in the range from 20 to 200 pm, in particular from 30 to 150 pm 50 wherein the particles have an irregular surface morphology characterized by a raspberry-like surface structure.

[0130] 26. Silicon particles according to point 25, consisting of predominantly round, splashed and / or nodular primary particles, preferably nodular or predominantly round primary particles, more preferably predominantly round primary particles, with a multitude of secondary particles present on the silicon particles, wherein the primary particles are larger than the secondary particles, and wherein the secondary particles are spherical or protrude in the form of prisms, prism-like structures or cones from the surface of the primary particles.

[0131] 27. Silicon particles according to point 25 or 26, wherein at least 5%, preferably at least 10%, more preferably at least 20% of the surface area of the primary particles is covered by secondary particles.

[0132] 28. Use of the silicon particles according to any one of points 25 to 27 as a thermally conductive filler.

Claims

1. A method for continuously preparing molten silicon and immediately performing subsequent further processing, comprising the following steps in the order stated: (A) Providing solid silicon (initial silicon) in a molten container; (B) The molten container is heated to the processing temperature using a suitable heating device, and the solid silicon is melted to form molten silicon; (C) Feeding solid silicon into the melting vessel and melting the fed solid silicon; and (D) The molten silicon obtained after step (C) is fed directly into a processing unit for molten silicon via the molten container outlet on the bottom side of the molten container for further processing.

2. The method according to claim 1, wherein, The melting in steps (B) and (C) forms molten silicon with slag floating on top.

3. The method according to any one of the preceding claims, wherein, The processing temperature is at least 10°C higher than the melting temperature of the initial silicon or the solid silicon used, preferably at least 20°C, and more preferably at least 30°C.

4. The method according to any one of the preceding claims, wherein, In step (C), solid silicon is fed continuously or in parts, preferably continuously.

5. The method according to any one of the preceding claims, wherein, In step (C), the solid silicon is fed in such a manner that it comes into direct contact with the molten silicon in the molten container.

6. The method according to any one of the preceding claims is configured such that the melt level in the molten vessel is sufficiently high to prevent blockage of the outlet and / or discharge of any slag, particularly at least 10 cm high.

7. The method according to any one of claims 1 to 5, wherein the melt level in the melting vessel is at least 10% of the height of the melting vessel, more preferably 20%-80%, particularly 30%-70%.

8. The method according to any one of the preceding claims, wherein, Any slag formed in step (C) is collected on the surface of the melt and / or removed in batches or continuously from the surface, for example by decanting into a collection container for slag.

9. The method according to any one of the preceding claims, wherein, The processing unit is selected from a dispersion unit, a casting unit, a stirring unit, a granulation unit, or a purification unit.

10. The method of claim 9, wherein the processing unit is a dispersion unit for molten silicon. The method is configured such that the molten silicon is atomized and solidified immediately after melting in the form of a spray jet to form silicon particles.

11. The method according to claim 9 or 10, wherein, The dispersing unit includes at least one atomizing unit, preferably selected from tightly connected units and Laval nozzles, particularly Laval nozzles.

12. Silicon particles having a predominantly spherical, sputtered, and / or nodular particle shape, with a median particle size d in the range of 20 to 200 µm, particularly 30 to 150 µm. 50 The particles described therein have an irregular surface morphology characterized by a raspberry-like surface structure.

13. The silicon particles of claim 12, wherein the silicon particles are formed from primarily spherical, sputtered, and / or nodular primary particles, and a large number of secondary particles are present on the primary particles. The primary particle is larger than the secondary particle, and The secondary particles are spherical or protrude from the surface of the primary particles in the form of prisms, prismatic structures, or cones.

14. The silicon particles according to claim 12 or 13, wherein, At least 5% of the surface area of ​​the primary particles is covered by the secondary particles.

15. Use of silicon particles according to any one of claims 12-14 as a thermally conductive filler.

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