WO2012152434A1

WO2012152434A1 - A method for treating metallurgical silicon

Info

Publication number: WO2012152434A1
Application number: PCT/EP2012/001982
Authority: WO
Inventors: Louis C. PAROUS III; Frank Weber
Original assignee: Centrotherm Photovoltaics Ag
Priority date: 2011-05-08
Filing date: 2012-05-08
Publication date: 2012-11-15
Also published as: DE102011112662B4; DE102011112662A1

Abstract

A method for generating particles of metallurgical silicon comprises the following steps: melting metallurgical silicon or silicon having specific additives, having for example an amount of copper between 0.01 to 2% by weight; introducing the molten silicon in a process chamber, wherein upon introduction into the process chamber the molten silicon is atomized via a gas flow and subsequently falls to the floor of the process chamber or a collection container attached thereto, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon or the alloy is adjusted or controlled, such that the atomized silicon at least partially solidifies during the free fall and in substance spherical silicon particles having a mean particle size of 20 μιη to 425 pm are generated.

Description

A method for treating metallurgical silicon The present invention relates to a method for treating metallurgical silicon.

Silicon is a widely used material in different technologies, such as the semiconductor industry or the photovoltaic industry. In the semi-conductor industry, high purity single crystal silicon is used, while in the photovoltaic industry typically high purity polycrystalline silicon is used.

Silicon, both for semi-conductor applications and photovoltaic applications, is typically brought via the Siemens method to a degree of purity of approximately 99.9999 % (6N) or higher. The silicon, however, has to be already conditioned prior to its use in the Siemens method, in order to be usable therein.

It is therefore an object of the present invention to provide a method for conditioning metallurgical silicon and to provide specific silicon particles, which may be used for further processing, in particular for a trichlorosilane synthesis. The term metallurgical silicon denotes a material having a silicon portion of at least approximately 98 % to approximately 99 %.

In accordance with the invention, this object is solved by a method according to claim 1 or 2 and by particles of metallurgical silicon according to claim 24. Further embodiments of the invention may be found in the respective dependent claims.

A first method for treating metallurgical silicon comprises the following steps: melting the metallurgical silicon or silicon having specific additives, having for example an amount of copper between 0.01 to 2 % by weight, or of a metallurgical silicon-copper-alloy having an amount of copper of between 0.01 to 2% by weight; introducing the molten silicon in a process chamber, wherein the molten silicon upon introduction into the process chamber is atomized via a gas flow and subsequently falls to the floor of the process chamber or to a collection container mounted thereon, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon, is controlled such that the atomized silicon upon its free fall at least partially solidifies and in substance spherical silicon particles having a mean particle size of 20 pm to 425 pm are generated .

A second method for generating particles of metallurgical silicon comprises the following steps: melting of metallurgical silicon or silicon having specific additions, having as for example an amount of copper between 0.05 to 5 % by weight, introducing the molten silicon in a process chamber, wherein the molten silicon upon introduction into the process chamber is atomized via a gas flow and is subsequently transported into a collection container, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon and/or for transporting the molten silicon are controlled such that the atomized silicon after atomization at least partially solidifies on its way to the collection container, and substantially spherical silicon particles having a mean particle size of 20 pm to 425 pm are generated. This method differs from the above in particular with respect to the possible composition and with respect to the solidification which may not only be controlled during the free fall in the process chamber, but also during conveyance to a remote collection container or a collection container which may for example be mounted to a side of the process chamber. A respective gas flow for transporting the silicon to a collection container may at least partially be formed by the gas flow for atomizing the molten silicon, but may also be a completely different gas flow.

The above method enables production of fine silicon particles having a homogeneous surface structure, which may be a beneficial starting material for subsequent processes. In particular, the particles may have a highly activated surface for subsequent processes, in particular for a trichlorosilane (TSC) synthesis during a hydro-chlorination of silicon or in a direct chlorination process. Furthermore, the particles have good flow or filling characteristics and they may thus be supplied to subsequent processes in a homogeneous and controlled manner with low amounts of dust being generated. By using metallurgical silicon including copper or in general silicon having specific additions of copper, the copper may act as a catalyst. Preferably, the amount of copper should be between 0.01 to 2% by weight in metallurgical silicon and may be present up to 5% as a specific addition. Preferably, the amount of copper, however, should be at approximately 0.05 to 2% by weight. Instead of copper, the metallurgical silicon may also include a different component, which may act as a catalyst for, for example, a TSC synthesis. Such a different catalyst would again be included in an amount of between 0.01 to 2% by weight and in particular between 0.05 to 1 % by weight. A catalyst is particularly useful when the TSC synthesis is achieved via a hydro-chlorination . In a direct chlorination a catalyst may also be advantageous. The term silicon having specific additions should in particular also include an alloy of silicon and the additive. The term alloy is used in the following in such a way that it includes silicon having specific additives.

Preferably, during introduction of the molten silicon or the alloy into the process chamber, a substantially inert gas atmosphere is maintained and the gas flow for atomizing the molten silicon or the alloy is generated from an inert gas. In so doing , for example, an oxidation of the surface of the particles being formed within the process chamber may be reduced. In particular, particles formed in this way may have a smaller Si0₂ surface layer compared to conventional particles, in particular broken particles, such that a TSC synthesis may be performed quicker. This may increase the throughput of the TSC synthesis and may reduce the operating costs for a respective TSC synthesis apparatus. In particular, atomization may be generated via a gas flow of at least one of argon and nitrogen.

It is also possible to maintain a reactive gas atmosphere within the process chamber, which reacts with contaminants in the molten silicon or the alloy, in order to remove the contaminants from the molten silicon or the alloy, which may consist of a reactive process gas. In so doing, simultaneously with the formation of the particles, cleaning thereof may be achieved. For cleaning of metallurgical silicon, for example, at least one of SiCI₄, chlorine, HCI , silane, halides and/or mixtures thereof are taken into consideration.

Prior to introduction into the process chamber, the molten silicon or the alloy is preferably exposed to at least one of the following conditioning steps: holding the molten silicon or the alloy in a gas atmosphere made up of at least one of oxygen, air and N₂ or conducting such a gas through the molten silicon, in particular for removing carbon from the molten silicon or the alloy; introducing silicate for forming slag or a different material for forming slag into the molten silicon or the alloy; holding the molten silicon or the alloy in a vacuum, in particular for removing phosphorus from the molten silicon or the alloy; conducting a reactive gas through the molten silicon or the alloy, which gas reacts with contaminants in the molten silicon or the alloy in order to remove the contaminants therefrom; conducting a reactive gas through the molten silicon or the alloy in order to achieve good mixing thereof; introducing alloying components, in particular copper into the molten silicon, in order to obtain an alloy.

In so doing , the composition of the particles may be enhanced. I n particular, undesired contaminants may be at least partially removed by these methods and/or a desired alloy may be obtained in situ. In accordance with one embodiment, at least one of a partially upwardly directed , a circular and a spiral shaped gas flow is generated within the process chamber. In so doing, the time for the free fall of the particles being formed in the process chamber may be extended , thereby enabling use of a process chamber having a reduced height. Via the time of the free fall, furthermore the structure of the particle crystalline/amorphous may be influenced . The gas flow for atomizing the molten silicon or the alloy is preferably a supersonic gas flow in order to achieve a sufficiently fine atomization. The atomization may be supported by exposing the molten silicon or the alloy to ultrasound or megasound prior to atomization thereof.

In order to preserve resources, gas may be exhausted from the process chamber, cleaned and at least partially recirculated for atomization of the molten silicon or the alloy into the process chamber. The cleaning may comprise at least one of a distillation and a chemical absorption, in order to remove contaminants contained in the gas. This is particularly true when a reactive gas is used. Exhaustion of gas may also be used for controlling the pressure in the chamber. During the exhaustion, typically smaller silicon particles may be exhausted together with the gas flow, which may be separated from the gas flow in a fine dust separator. These particles typically have a smaller mean diameter than the particles, which collect at the bottom or floor of the process chamber. These particles could be introduced into the molten silicon for recycling. In another embodiment these particles may also be reacted at an elevated temperature of for example 900 to 1350°C with SiCU, to generate SiCI₂. In such a reaction, a layer of amorphous silicon would deposit on the surface of the silicon, which would be beneficial for a subsequent TSC synthesis. These SiCI₂ particles could then be added to the spherical silicon particles collected at the bottom of the process chamber and may be mixed therewith. The process conditions may be set such that at least a major portion of the particles obtain a crystalline structure in the center and have an amorphous surface layer, wherein the amorphous surface layer is preferably limited to a depth of 10 pm. In order to control the atomization, the molten silicon or the alloy may for example be introduced via a valve having an induction coil, which induction coil spatially limits the flow of the molten silicon or of the alloy. In so doing, in particular the width of the flow may be adjusted, which may influence the atomization and thus the particle size.

After solidification the particles may be exposed to at least one of a vacuum and a reactive gas, which reacts with contaminants in the silicon to remove the contaminants from the silicon. In so doing, the particles may be further cleaned. Via the vacuum, in particular, phosphorus may be pulled from the particles, whereby the small particle size is beneficial in this process. The small spherical particles offer a large surface are to the reactive gas. The particles may in particular be exposed to a vacuum of smaller <1 0^"3 mbar, in particular smaller <10^"4 mbar, in order to pull phosphorus therefrom.

An increased temperature would be beneficial during this process, in order to enhance diffusion out of the silicon. Preferably, the particles may be exposed to the vacuum and the reactive gas at a higher pressure than the vacuum in an alternating manner. In one embodiment, the particles are exposed to at least one of the vacuum and the reactive gas atmosphere outside of the process chamber or in a holding area of the process chamber, which was previously isolated from the rest of the process chamber.

The invention also relates to particles of metallurgical silicon, which are produced by gas atomization and which comprise a substantially spherical shape having a mean particle size of 20 μιτι to 425 pm. Such particles have the advantages already set forth above. Preferably, the particles have an amount of copper therein between 0.01 to 2% by weight, wherein the copper may in particular act as a catalyst for a TSC synthesis.

In one embodiment, the particles have a crystalline structure in their center and an amorphous surface layer, wherein the amorphous surface layer is preferably limited to a depth of 10 pm. Particularly the amorphous silicon structure has a high number of none saturated bonding sites or dangling bonds, which may be connected to copper atoms for forming starting points or reaction sites for catalytic reactions, in particular during the TSC synthesis. Since the amorphous surface layer is limited with respect to the thickness, the above effect may particularly occur at the surface. The none-saturated bonding sites or dangling bonds may also be free and may thereby provide a highly activated surface for a further process, in particular a TSC synthesis. The particles may also have a substantially pure crystalline or amorphous structure.

For a subsequent TSC synthesis the particles preferably have the following composition; at least 98 % by weight silicon, 0.01 to 2 % by weight copper, not more than 1.0 % by weight, preferably not more than 0.3 % by weight of aluminum, not more than 0.02 % by weight, preferably less than 0.005 % by weight calcium, other metallurgical contaminants of less than 0.5 % by weight, carbon at a concentration of less than 400 ppm, preferably less than 50 ppm, boron at a concentration at most 15 ppm and phosphorus at a concentration of at most 15 ppm, preferably less than 5 ppm.

The small amounts of phosphorus and boron reduce the waste gas flow during polysilicon processes. The small amount of carbon prohibits formation of harmful CCI₄ during subsequent processes, in particular a TCS synthesis.

The invention will be described in more detail herein below with reference to Fig. 1 , which shows a schematic sectional view of an apparatus for removing contaminants from metallurgical silicon. The relative terms, such as left, right, above and below used in the following description refer to the drawings and should not limit the application, even though they may refer to preferred arrangements.

Fig. 1 shows a schematic sectional view of an apparatus 1 for generating spherical particles of metallurgical silicon. The apparatus 1 comprises a melting unit 3, a process chamber unit 5 and a gas circulation unit 7. The melting unit 3 includes in substance a housing 8, a melting crucible unit 9 and a heating unit 10. The housing 8 is made of a suitable material, which is durable at the required process temperatures for melting silicon material and which also does not provide contaminants for molten silicon. The housing 8 may comprise an insulating material and may also be formed as a vacuum housing, which is in substance gas tight. The housing 8 may be connected to a vacuum pump (not shown), in order to evacuate an interior of the housing 8, in which the melting crucible unit 9 and the heating unit 10 are received. The housing 8 has a loading/unloading opening (not shown) for loading of metallurgical silicon into the melting crucible unit 9.

The melting crucible unit 9 consists in substance of a melting crucible 12 having a conduit element 13 attached thereto. The melting crucible 12 consists of a suitable material, which is durable at the temperatures required for melting metallurgical silicon and furthermore does not provide contaminants for the silicon. For example, a melting crucible made of graphite or silicon nitride (Si₃N₄) may be provided . Obviously, other materials may be used for the melting crucible 12. The melting crucible 12 has a circumferential sidewall as well as a bottom wall, which conically tapers downwards. In the bottom wall, an opening is provided at the lowest point thereof, which is fluidly connected with the conduit element 1 3. The skilled person will recognize that molten silicon, which is melted within the melting crucible 12, may flow from the melting crucible 12 via the conduit element 1 3.

The conduit 13 is again made of a material, which is durable at the required temperatures for melting metallurgical silicon and which does not introduce detrimental contaminants into the molten silicon.

The conduit element 13 extends from the bottom of the melting crucible 12 to an upper portion of the process chamber unit 5 and opens into the same, as will be explained in more detail herein below. The heating unit 1 0 has one or more heating elements 15 which are arranged laterally with respect to the melting crucible 12. In Fig. 1 two separate heating elements are shown which may be of any suitable type, which is capable of heating metallurgical silicon contained within the melting crucible 12 to a temperature above its melting point. In particular, the heating element 1 5 may also be formed as an inductive coil, which heats the metallurgical silicon via induction. Alternatively, the heating element 15 could also be a resistance heating element, in order to heat the metallurgical silicon contained in the melting crucible 12 for example via heat radiation. Although the heating element 1 5 is shown spaced from the melting crucible 12, it may also directly contact the same.

In the area of the melting crucible an optional unit (not shown) for introducing alloying components, in particular copper may be provided . Furthermore, a gas supply conduit may be provided, which allows conveying gas into the melting crucible to thereby provide good mixing of the contents of the melting crucible, in particular of alloying components. The gas conveyed into the melting crucible may also be a reactive gas to remove contaminants from the molten silicon by the gas.

The process chamber unit 5 comprises a housing 17, which forms a process chamber 1 8 therein, as well as a collection container 19. The housing 17 has a flat upper wall 21 as well as a circumferential sidewall 22. The upper wall 21 has a passage for the conduit element 13, which extends through the upper wall 21 in a sealed manner. The upper wall 21 is connected in a sealed manner to the circumferential sidewall 22. The sidewall 22 preferably has a circular cross section, but may also have a different configuration. The sidewall 22 has an upper, tapering section 22a, a central, vertically extending section 22b as well as a lower, tapering section 22c. The upper section 22a of the sidewall 22 is tapering towards the upper wall 21 starting from the central section 22b. The lower section 22c tapers downwards, again starting from the central section 22b, which taper merges into a vertically extending section 22d. With this configuration of the sidewall 22, the process chamber 18 thus forms from top to bottom an expanding section, followed by a section having a constant diameter, followed by a tapering section having a funnel shape, which then merges into an outlet section having a constant diameter.

In the sidewall 22 an outlet opening 23 is formed in the central section 22b, which, as will be explained in more detail herein below, is fluidly connected to the gas circulation unit 7. The outlet opening 23 is preferably arranged above a center line of the process chamber 18 (in vertical direction). In the sidewall 22 a plurality of gas inlet openings 24 is provided. These are also fluidly connected to the gas circulation unit 7, as will be explained in more detail herein below. The gas inlet openings 24 are preferably arranged below a center line of the process chamber 1 8 (in vertical direction). The process chamber preferably has a height of between 8 to 20 meters.

At a lower end of the circumferential sidewall 22, a mounting flange 25 is provided. This mounting flange 25 may work together with a mounting flange 26 of the collection container 19 for mounting the collection container 19 in a gas tight manner to the housing 17. The collection container 1 9 comprises the previously mentioned mounting flange 26 and a downwardly extending bowl shaped housing 28 for forming a collection chamber 29, which opens upwards. Via the mounting flanges 25, 26 the collection container 1 9 may be detachably mounted to the housing 17 in a gas tight manner, such that the process chamber 18 and the collection chamber 29 form a closed, gas tight space.

The collecting container 19 may optionally be isolated from the process chamber 18 via suitable means (not shown), and at least one of a vacuum, a reactive gas and an inert gas may be applied thereto. Since the collecting container 19 has a substantially smaller volume than the process chamber 1 8, it is easier to achieve a high vacuum therein compared to achieving such high vacuum in the process chamber 18. In particular, means may be provided, which may generate a pressure of < 10^"3 mbar, in particular <10^"4 mbar in the collection container. The collection container 1 9 may also be optionally detached from the process chamber 1 8 - this may preferably be done in such a manner that the collection space is isolated with respect to the surrounding - in order to remove silicon particles collected therein from the process chamber

1 8 collected therein. The collection container 19 may also act as a transport and process container. Thus, a plurality of collection containers 19 may be allocated to a housing 17, which, as long as they are not mounted to the housing 17, may act as at least one of a transport container, a storage container and a process container. For this purpose, the collection containers

19 may be filled with an inert gas, to suppress oxidation of the silicon particles while they are contained therein. At least one heating/cooling unit (not shown) may be provided to heat/cool the process chamber to a predetermined temperature.

The gas circulation unit 1 7 comprises in substance a fine dust separator 31 , a first pump 33, a gas conditioning unit 35, a second pump 37, a first inlet unit 38, a third pump 41 and a second inlet unit 42.

The fine dust separator 31 may be of any suitable type, which may be used at the high temperatures present in the process chamber 1 8. The fine dust separator 31 is fluidly connected to the outlet opening in the sidewall 22 of the housing 17 via a conduit 44. Via a conduit 45, the fine dust separator 31 is fluidly connected to the first pump 33, which may be formed as a vacuum pump. As the skilled person will recognize, the process chamber 18 may be evacuated by the first pump 33 via the fine dust separator 31 , in order to bring the same to a negative pressure or vacuum. The pump 33 and the process chamber 18 are matched to each other that a negative pressure of <50 mbar and in particular preferably a negative pressure of <15 mbar may be achieved in the process chamber 18. It is also possible to use another type of pump, if the process performed in the process chamber may be performed at atmospheric pressure. At the bottom of the fine dust separator, a collection space for separated Si-Particles, which are exhausted from the process chamber during operation of the pump 33, is formed. These particles typically have a mean diameter, which is smaller than the mean diameter of the particles collected at the bottom of the process chamber 18. The collection space may optionally be connected via a suitable conduit to a separate chamber (not shown). In such a chamber the particles may be reacted with SiCI₄ at an elevated temperature of for example 900°C to 1350°C to generate SiCI₂. In this reaction, a layer of amorphous silicon would deposit on the surface of the silicon, which layer would be beneficial for a subsequent TCS synthesis. These SiCI₂ particles could be supplied to the silicon particles collected at the bottom of the process chamber 18 and may be intermixed therewith. The supplying step may be achieved directly via a respective conduit from the separate chamber to the silicon particles at the bottom of the process chamber 18. Alternatively, the reaction process including SiCI₄ and the supplying step to the spherical silicon particles may be done offline. Alternatively, the particles from the collection space of the fine dust separator may also be supplied to the melting crucible to recycle the same. The first pump 33 is fluidly connected via conduit 46 to the gas conditioning unit 35, such that gas evacuated from the process chamber 18 is conveyed into the gas conditioning unit 35. Even though not shown, a further conduit or branch may be provided in order to convey gas evacuated from the process chamber via the pump 33 into the surrounding or atmosphere. This may for example be desirable for an initial evacuation of the process chamber 18 after it was opened for removal of silicon. At this time, primarily atmospheric gas is present in the process chamber 18, which does not have to be conveyed into the gas conditioning unit. In the gas conditioning unit 35, a gas, which was evacuated from the process chamber 18, which for example contains volatile metallic contaminants, such as phosphorus, boron and/or similar contaminants, may be conditioned. Such a conditioning may for example provide filtering with respect to particulate contaminants, if filtering by the fine dust separator 31 is not sufficient. Furthermore, volatile contaminants, in particular metallic contaminants, for example in the form of metal halides should be removed in the gas conditioning unit 35. For this purpose the gas conditioning unit 35 may for example comprise a distillation unit and/or a chemical absorption unit.

Even though, it is not shown in detail, the gas conditioning unit 35 may have a storage vessel for cleaned gas or a liquid distilled from the gas. Furthermore, an external supply unit for gas may be provided. Examples of gases which are taken into consideration are inter alia inert gases, in particular argon an nitrogen. If the gases other than providing the atomization action should also provide a cleaning action for metallurgical silicon, as will be described herein below, gases containing for example at least one of SiCI₄, chloride, HCI, silane, halogenide or halides and/or mixtures of the above are take into consideration. At the gas conditioning unit 35 and/or the external supply unit, the base materials may for example be provided in a liquid form and they may be gasified via respective heating prior to introduction into the process chamber, as will be recognized by the skilled person. The gas conditioning unit 35 is fluidly connected via a conduit 47 to the second pump 37, which conveys gas or a precursor for the gas from the gas conditioning unit 35 or also from the external supply unit via conduit 48 to the first inlet unit 38. In or at the conduit 47 a heating unit may be provided, in order to heat the process gas or the precursor and to ensure that, when it is introduced into the process chamber, it has an increased temperature and is in a gaseous stage. It may be beneficial, if the process gas upon insertion into the process chamber has a temperature of above 800°C, preferably above 1000°C. The first inlet unit 38 comprises a housing 50, which has a passage 51 for the conduit element 1 3. In the interior of the housing 50 an annular conduit 52 and a plurality of outlet conduits 53 are formed. The annular conduit 52 is fluidly connected to conduit 48 and a gas may be applied thereto via the pump 37. The annular conduit 52 concentrically surrounds the passage 51 for the conduit element 13.

Starting from the annular conduit 52, a plurality of outlet conduits 53 extends inward towards the passage 51 for the conduit element 13. The outlet conduits 53 are arranged such that they have a radially extending section as well as an obliquely downwardly extending section. Each obliquely downwardly extending section ends at an outlet opening. Each outlet opening is arranged adjacent to the passage 51 for the conduit element 13. The oblique section and the respective outlet opening may thus provide an oblique gas flow, which is directed towards an exit area of the conduit element 13, as will be described in more detail herein below. It is noted that the conduit element 1 3 is received in the passage 51 of housing 50 in such a manner that it ends with a lower surface of the housing 50 or already in the passage 51 itself. In the vicinity of the passage, an inductive coil may be provided in order to control the flow of metallurgical silicon through the passage and in particular to adjust or control the diameter of the flow.

The oblique section of the outlet conduit 53 provides a flow cross section, which tapers towards the outlet opening in order to provide high flow velocities at the outlet opening. In particular, the pump 37 and the inlet unit 38 are matched to each other such that at the outlet openings of the outlet conduits 53 a high velocity gas flow, in particular a gas flow having supersonic velocity may be generated.

Optionally an ultra sound or mega sound transducer (not shown) may be provided in the housing 50, adjacent to conduit element 13, which is capable of introducing ultra sound or mega sound into the conduit element 1 3 and molten silicon contained therein, respectively. A respective ultra sound or mega sound transducer may also be provided in a different area adjacent to conduit element 13. The gas conditioning unit 35 is fluidly connected to a third pump 41 via a further conduit 57. The pump 41 is fluidly connected to the second inlet unit 42 via a conduit 58. The second inlet unit 42 in substance consists of an annular housing 60 mounted to the outside of sidewall 22 of housing 17. The annular housing 16 comprises an annular conduit 62 as well as a plurality of outlet conduits 63.

The angular conduit 62 in housing 60 is fluidly connected to conduit 58. As the skilled person will recognize, gas from the gas conditioning unit 35 may be conveyed via pump 41 into the annular conduit 62 of the second inlet unit 42. The annular conduit 62 is fluidly connected to the plurality of outlet conduits 63. The outlet conduits 63 each extend obliquely upwards from the annular conduit 62 and open into the inlet openings 24 in the sidewall 22 of housing 17. As the skilled person will recognize, the second inlet unit 42 in combination with pump 41 is thus capable of generating an obliquely upwardly directed gas flow within the process chamber 1 8. Additionally or alternatively, the outlet conduits 63 may also be arranged such that they generate a circular or spiral shaped flow within the process chamber 18. In the following, operation of the apparatus 1 will be described in more detail with reference to Fig. 1 .

Initially, the apparatus 1 is in a starting condition, in which metallurgical silicon is contained in the melting crucible 12 of the melting crucible unit 9. The collection container 1 9 is mounted in a gas tight manner to housing 17. The process chamber 1 8 is evacuated via the vacuum pump 33, wherein the evacuated gas is initially exhausted into the atmosphere. In so doing , the process chamber is for example pumped to a pressure smaller than 50 mbar and preferably smaller than 15 mbar. The metallurgical silicon is melted in the melting crucible via the heating unit 10. Optionally, alloying components may be added thereto and further processing steps may be performed. After the melting step, molten silicon or a silicon alloy, which will be called silicon herein below, is present in the conduit element 13. The molten silicon may optionally be conveyed via a respective valve unit (not shown) towards the process chamber 18. The conduit element 13 may be sized such that the molten silicon would not flow therethrough without the influence of an additional force acting thereon, such as a negative pressure in the process chamber and/or flow effects generated by gas being introduced in the vicinity of the outlet conduits 53.

When molten silicon is present at the conduit element 1 3 and flows towards the process chamber 18, gas, which may contain the previously mentioned components, is conveyed via the pump 37 and the first inlet unit 38 into the process chamber 1 8. In the following, it is assumed that the gas is an inert gas such as argon. The gas, when entering the process chamber 1 8, has a high flow velocity, preferably a supersonic velocity. Molten silicon flowing through the conduit element 1 3 and exciting the same will be atomized by this gas flow into fine droplets. The thus atomized d roplets of molten silicon then fall downwards within the process chamber 1 8 and are subsequently, after they have at least partially solidified during the free fall, received within the collection container 1 9 in the form of silicon particles. The process conditions are adjusted or controlled in such a manner that the particles have a substantially spherical shape and have a mean diameter of 20 pm to 400 pm. This may be achieved by different means, in particular via the atomizing gas flow, which is matched to the flow of molten silicon. In particular, the temperature, the flow velocity and the amount of gas flow of the atomizing gas flow may be adjusted or controlled. The substantially spherical shape comes from the free fall of the droplets and their solidification during the free fall, wherein an upwardly directed gas flow may increase the duration of the fall to for example ensure that sufficient time is available for solidification of the droplets. Further process conditions may be adjusted or controlled, such as the pressure and the temperature within the process chamber. During the above procedure, gas, in particular the introduced gas and if applicable contaminants emanated from the molten silicon (these will in particular be present if a reactive gas is used instead of an inert gas), are continuously evacuated from the process chamber via the pump 33. Obviously, at least some silicon droplets or particles are also evacuated, which may be separated from the gas flow within the fine dust separator 31 and which may be processed as explained above. The gases and the contaminants are conveyed via the pump 33 into the gas conditioning unit 35. I n the gas conditioning unit 35, the gas is cleaned for example by filtering, distillation and/or chemical absorption, in order to remove the contaminants from the gas. Via the pump 37 or 41 the gas can then be recirculated into the process chamber 18.

When the molten silicon is completely or at least partially introduced into the process chamber in the above manner, the process may be stopped and silicon particles collected in the collection container 1 9 may be removed and may optionally be directly fed to further processes. It is also possible, as already ind icated above, to directly perform further processes in the collection container 1 9, for example by exposing the Si-particles to a vacuum and/or a reactive gas, in particular in an alternating manner. For this purpose, the collection container 19 may preferably be isolated with respect to the process chamber 18, in order to provide a substantially reduced process volume. The collection container may also be used as a transport and/or storage container and may for example be filled with an inert gas.

Optionally, during at least a portion of the above process, gas may also be conveyed through the molten silicon in the melting crucible, in order to provide partial cleaning thereof at this stage. This cleaning may optionally be enhanced by generating a negative pressure or vacuum in the housing 8 of the melting unit 3. The gas introduced into the melting crucible may be a different gas than the gas used in the process chamber to provide different cleaning mechanisms. This gas may optionally also be recirculated after a conditioning thereof. Additionally, in the melting unit 3, additional cleaning of the molten silicon in the melting crucible may be provided by electron beam assisted surface cleaning and/or plasma assisted surface cleaning.

Operation of the apparatus may also be performed in a continuous manner by continuously charging/recharging the melting unit 3 and continuous removal of the collected spherical silicon particles. For this purpose, an intermediate container may for example be provided in the vicinity of the melting crucible, which may be supplied with molten silicon via at least one melting crucible. The molten silicon can than be conveyed to the process chamber 18 from the intermediate container, while further silicon material may be melted in the at least one melting crucible.

The above described process thus provides a means for providing substantially spherical particles of metallurgical silicon having a predetermined diameter, which particles have beneficial characteristics for subsequent processes, in particular for a TCS synthesis.

The invention was described herein above with respect to a preferred embodiment of the invention without being limited to the specific embodiment. In particular, the second gas inlet unit 42 is only optional. Furthermore, it is noted that the molten silicon may also be introduced from the side or the bottom of the process chamber and may for example be directed upwards by a respective gas flow in the process chamber. Furthermore the structure of the melting unit may deviate from the one shown, as was already indicated with reference to the intermediate container. It is also not required that the melting unit 3 is arranged above the process chamber.

Claims

A method for generating particles of metallurgical silicon, said method comprising the following steps:

melting the metallurgical silicon or silicon having specific additives, having for example copper in an amount of between 0.01 to 2% by weight;

introducing the molten silicon into a process chamber, wherein the molten silicon upon introduction into the process chamber is atomized via a gas flow, and subsequently falls to the bottom or floor of the process chamber or a collection container mounted thereto, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon or the alloy are adjusted or controlled such that the atomized silicon at least partially solidifies during the free fall thereof and in substance spherical silicon particles having a mean particle size of 20 pm to 425 pm are generated.

melting the metallurgical silicon or silicon having specific additives, having for example copper in an amount of between 0.01 to 5% by weight;

introducing the molten silicon into a process chamber, wherein upon introduction into the process chamber the molten silicon is atomized via a gas flow, and is subsequently transported to a collection container, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon and/or for transporting the molten silicon is adjusted or controlled such that the atomized silicon after atomization and on its way to the collection container at least partially solidifies and in substance spherical silicon particles having a mean particle size of 20 pm to 425 pm are generated.

3. The method of claim 2, wherein the molten silicon falls into a collection container after atomization or is conveyed thereto via a further gas flow.

4. The method of any one of the preceding claims, wherein the metallurgical silicon or the silicon having specific additives contains copper between 0.01 and 2% by weight, in particular between 0.05 and 1 % by weight.

5. The method of any one of the preceding claims, wherein during introduction of the molten silicon into the process chamber, a substantially inert gas atmosphere is maintained and the gas flow for atomizing the molten silicon or the alloy consists of an inert gas.

6. The method of any one of the preceding claims, wherein the atomization is achieved via a gas flow of at least one of argon and nitrogen .

7. The method of any one of claims 1 to 5, wherein a reactive gas atmosphere is maintained within the process chamber, which gas atmosphere reacts with contaminants in the molten silicon, in order to remove these contaminants from the molten silicon, wherein in particular also the gas flow for atomizing the molten silicon contains a reactive process gas.

8. The method of any one of the preceding claims, wherein prior to introducing the molten silicon into the process chamber, the molten silicon is exposed to at least one of the following conditioning steps: holding the molten silicon in a gas atmosphere containing at least one of oxygen, air and/or N₂, or conducting such a gas through the molten silicon, in particular for removing carbon from the molten silicon; introducing a silicate for forming slag or a different material for forming slag into the molten silicon; holding the molten silicon in a vacuum, in particular for removing phosphorus from the molten silicon;

conducting a reactive gas or mixture of reactive gases through the molten silicon, which gas reacts with contaminants in the molten silicon in order to remove the contaminants from the molten silicon; conducting a reactive gas through the molten silicon, in order to achieve a good mixture thereof;

introducing alloying components, in particular copper into the molten silicon to obtain an alloy.

9. The method of claim 8, wherein a gas which is conducted through the molten silicon contains at least one of hydrogen and hydrogen peroxide.

10. The method of anyone of the preceding claims, wherein an at least partially upwardly directed gas flow is generated within the process chamber.

11. The method of any one of the preceding claims, wherein a circular or spiral shaped gas flow is generated within the process chamber.

12. The method of any one of the preceding claims, wherein the gas flow for atomizing the molten silicon or the alloy is a supersonic gas flow.

13. The method of any one of the preceding claims, wherein the molten silicon or the alloy is exposed to ultrasound or megasound prior to atomization thereof.

14. The method of any one of the preceding claims, wherein gas is evacuated from the process chamber, cleaned and at least partially recirculated into the process chamber for atomization of the molten silicon or of the alloy.

15. The method of claim 14, wherein the cleaning includes at least one of a distillation and a chemical absorption to remove contaminants contained in the gas.

16. The method of any one of the preceding claims, wherein the process conditions are controlled or adjusted such that at least a major portion of the particle has a crystalline structure in its center and an amorphous surface layer, wherein the amorphous surface layer is preferably limited to a depth of 10pm.

17. The method of any one of the preceding claims, wherein the molten silicon or the alloy is introduced into the process chamber via a valve having an induction coil, wherein a flow of the molten silicon or of the alloy is spatially limited via the induction coil.

18. The method of any one of the preceding claims, wherein after solidification the particles are exposed to at least one of a vacuum and a reactive gas that reacts with contaminants in the silicon in order to remove the contaminants from the silicon.

1 9. The method of any one of the preceding claims, wherein the particles are exposed to a vacuum of <10^"3 mbar, in particular smaller <1 0^"4 mbar.

The method of claim 18 or 19, wherein the particles are exposed to a vacuum and a reactive gas having a higher pressure than the vacuum in an alternating manner.

The method of one of claims 18 to 20, wherein the particles are exposed to at least one of the vacuum and the reactive gas atmosphere outside the process chamber or in a holding area of the process chamber, which was previously isolated from the remainder of the process chamber.

22. The method of any one of claims 18 to 21 , wherein the particles are heated to a temperature just below the melting point of the silicon prior to and/or during exposure of the particles to at least one of the vacuum and the reactive gas atmosphere.

23. The method of any one of claims 18 to 22, wherein the particles are moved via a gas flow or mechanically while they are exposed to at least one of the vacuum and the reactive gas atmosphere.

24. Particles of metallurgical silicon, which were produced via gas atomization and which comprise a substantially spherical form and have a mean particle size of 20 pm to 425 pm.

Particles according to claim 24, wherein the particles comprise an amount of copper between 0.01 to 5% by weight, in particular between 0.01 to 2% by weight.

Particles of claim 24 or 25, wherein the particles comprise a crystalline structure in their center and have an amorphous surface layer, wherein the amorphous surface layer is preferably limited to a depth of 10 pm.

Particles of claim 24 or 25, wherein the particles have a crystalline or an amorphous structure.

Particles of any one of claims 24 to 27, wherein the particles comprise the following composition:

at least 98% by weight of silicon,

0.01 to 2% by weight of copper,

not more than 1.0% by weight, preferably not more than 0.03% by weight of aluminum,

not more than 0.02% by weight, preferably less than 0.005% by weight of calcium, other metallic contaminants of preferably less than 0.5% by weight, carbon at a concentration of less than 400 ppm, preferably less than 50 ppm,

boron at a concentration of preferably not more than 15 ppm, and phosphorus at a concentration of preferably not more than 15 ppm, preferably less than 5 ppm.