WO2012152435A1

WO2012152435A1 - Method and apparatus for removing contaminants from metallurgical silicon

Info

Publication number: WO2012152435A1
Application number: PCT/EP2012/001983
Authority: WO
Inventors: Louis C. PAROUS; 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: DE102011100884B4; DE102011100884A1

Abstract

A method and an apparatus for removing contaminants from metallurgical silicon are disclosed. In the method, metallurgical silicon is molten and subsequently introduced into a process chamber, wherein the molten silicon upon introduction into the process chamber is atomized via a gas flow. The thus atomized molten silicon subsequently falls to the floor of the process chamber. A reactive atmosphere is generated in the process chamber, wherein the reactive atmosphere contains a process gas, which reacts with contaminants in the molten silicon, in order to remove containments from the molten silicon. The apparatus comprises a melting unit connected via a conduit to a process chamber. Furthermore, a gas circulation unit is provided for circulating gas evacuated from the process chamber through a gas conditioning unit for cleaning the gas and then back into the process chamber via a gas inlet, which is used for atomizing molten silicon exiting the conduit between the melting unit and the process chamber.

Description

Method and apparatus for removing contaminants

from metallurgical silicon

The present invention relates to a method and an apparatus for removing contaminants from metallurgical silicon.

Silicon is a widely used material in different technologies, such as the semi- conductor 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. In the photovoltaic industry, typically the requirements with respect to purity of the silicon is substantially lower compared to the semi-conductor industry, but still too high so that typical metallurgical silicon cannot be used. The term metallurgical silicon denotes silicon having a purity of approx. 99 %.

Silicon for photovoltaic applications is typically produced via the Siemens method to a degree of purity of approx. 99.99999 % (N5) or higher. The Siemens method, however, requires a high energy input and also generates substantial amounts of waste material.

It is therefore an object of the present invention to provide a method for removing contaminants from metallurgical silicon, which method has lower energy requirements and furthermore produces less waste material, in order to allow its use in the photovoltaic industry, possibly after further processing of the material.

In accordance with the invention, this object is solved by a method in accordance with claim 1 and an apparatus in accordance with claim 12. Further embodiments of the invention may be found in the respective dependent claims. The method for removing contaminants from metallurgical silicon comprises melting the metallurgical silicon and 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. Within the process chamber, a reactive gas atmosphere is generated which comprises a process gas, which reacts with contaminants in the molten silicon and thereby removes at least a part of the contaminants from the molten silicon. The atomization of the molten silicon leads to the silicon having a large surface area, which comes into contact with the reactive gas, thereby generating a large reaction surface. This enables good cleaning of the metallurgical silicon, in particular removal of metallic contaminants. In a preferred embodiment of the invention, the molten silicon is atomized via a gas flow, in particular a gas flow of the reactive process gas, said gas flow having a supersonic velocity. This enables a good atomization effect and at the same time a good contact between the reactive process gas and the droplets of molten silicon generated by the atomization. Process gas may be evacuated from the process chamber, cleaned and recirculated for the atomization of the molten silicon on a continuous basis. Thus, a substantially closed circulation of the process gas may be provided, thereby reducing the amount of waste products, in particular of harmful waste products. The cleaning may for example comprise a distillation and/or chemical absorption, in order to remove contaminants residing in the gas.

In one embodiment, prior to and/or during introduction of the molten silicon, negative pressure or a vacuum is generated in the process chamber. The negative pressure may pull contaminants to the surface of the droplets of liquid silicon generated during the atomization. In so doing, a good cleaning effect in particular with respect to boron, phosphorus and metallic contaminants may be achieved. Preferably, a negative pressure of <50 mbar, in particular <15 mbar is generated in the process chamber, in order to pull the contaminants within the atomized droplets of molten silicon to the surface. In a further embodiment of the invention, prior to introducing the molten silicon, a reactive gas, which reacts with contaminants in the molten silicon, may be passed through the molten silicon material, in order to enable at least a partial removal of contaminants prior to the atomization. Also, in this case the reactive gas may be cleaned and recirculated after passage through the molten silicon. The reactive gas used within the molten material may be a different gas or the same gas which is used for atomizing the molten silicon.

In order to facilitate atomization of the molten silicon, and possibly reducing the droplet size, the molten silicon may be exposed to ultra sound or mega sound prior to atomization thereof. Mega sound denotes sound frequencies between 400 kHz and 2 MHz, while ultra sound comprises frequencies of approx. 10 kHz to 400 kHz.

In order to increase the falling time of the atomized silicon droplets of molten silicon within the process chamber (i.e. the time it takes for the silicon droplets to reach the floor of the process chamber) and thus the time the droplets are in contact with the reactive process gas, an at least partially upwardly directed gas flow may be generated within the process chamber. In order to increase the falling time, additionally or alternatively, a circular or spiral shaped gas flow may be generated within the process chamber.

Preferably, the process gas comprises as reactive component at least one of chlorine, another halogen, in particular hydrogen halide or silicon halide and/or a mixture of at least two of these components. The reactive components are particularly suited to form volatile compounds, for example metal chlorides, with metallic contaminants in the silicon, which may be evacuated with the process gas from the process chamber. Argon may be mixed into the process gas. In accordance with an embodiment of the invention, the atomized molten silicon at least partially solidifies on its way to the floor of the process chamber and thus Si-particles collect at the bottom of the process chamber. The method may expose the Si-particles to a vacuum of <10^"3 mbar, in particular <10^"4 mbar, in order to extract certain contaminants, in particular phosphorus from the particles by the vacuum. In so doing, the small size of the particles and their large surface is advantageous. The atomization may advantageously generate for example spherical particles having a mean diameter of 20 pm to 400 pm. Preferably the Si-particles are exposed to a vacuum and a reactive gas atmosphere at a higher pressure than the vacuum in an alternating manner. The Si-particles may be exposed to the vacuum and/or the reactive gas atmosphere outside of the process chamber or within a holding area of the process chamber, which was previously isolated with respect to the remainder of the process chamber.

The apparatus for removing contaminants from metallurgical silicon comprises: a melting unit having a melting crucible for receiving metallurgical silicon and at least one heating unit for melting metallurgical silicon in the melting crucible; a process chamber unit having a process chamber; at least one conduit extending from the melting crucible to the process chamber, which opens towards both; and at least one gas circulation unit. The gas circulation unit comprises: at least one first pump connected to the process chamber and adapted to evacuate a gas from the process chamber; a gas conditioning unit, which is connected to an outlet of the at least one first pump, and adapted to clean or remove contaminants from the gas evacuated from the process chamber; at least one first gas inlet unit connected to the gas conditioning unit, in order to circulate gas from the gas conditioning unit into the process chamber; and at least one second pump for conveying gas from the gas conditioning unit to the at least one gas inlet unit. The at least one first gas inlet unit comprises at least one outlet conduit having an outlet opening adjacent to an end of the conduit opening towards the process chamber, wherein the outlet conduit and the outlet opening are arranged such that it is directed obliquely towards an outlet area of the conduit. Such an apparatus enables performing the method described above in order to achieve the above mentioned advantages. Preferably the first pump is dimensioned such that it is capable of achieving a negative pressure, in particular a negative pressure of <50 mbar in the process chamber.

Preferably, the at least one second pump and the at least one first gas inlet unit are dimensioned such that at the outlet opening of the at least one outlet conduit, a gas flow having supersonic velocity may be generated.

For a good cleaning effect of the gases evacuated from the process chamber, preferably at least one of a filter unit, a distillation unit and/or a chemical absorption unit is provided. In one embodiment of the invention, the gas circulating unit further comprises at least one third pump and a second gas inlet unit. The second gas inlet unit comprises at least one outlet conduit having an outlet opening, opening into the process chamber, wherein the outlet conduit and the outlet opening are directed obliquely upwards into the process chamber.

Preferably, at least one of an ultra sound transducer or a mega sound transducer is provided, which is arranged adjacent to the conduit, such that it may introduce ultra sound or mega sound into the conduit. In order to improve the cleaning of the metallurgical silicon, furthermore, means for introducing a process gas into the melting crucible may be provided. The process gas may be the same, which is used in the process chamber, but it can also be a different one. In one embodiment, the process chamber comprises a holding area at the floor or bottom portion thereof, which may be isolated with respect to the remainder of the process chamber and/or may be detached therefrom, to for example allow transport to a further processing device, or to provide direct processing within the holding area. For this purpose, for example means for generating a vacuum and/or for introducing a process gas into the holding area may be provided in the holding area. 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 removing contaminants from 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 may comprise an insulation and may also be formed as a vacuum housing, which is in substance gas tight. The housing 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 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 nitrite (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 13. 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 13.

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 10 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 15 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 15 is shown spaced from the melting crucible 12, it may also directly contact the same.

The process chamber unit 5 comprises a housing 17, which forms a process chamber 18 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 is guided 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 18 (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 in order to mount the collection container 19 in a gas tight manner to the housing 17. The collection container 19 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 19 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 a vacuum and/or a reactive gas may be applied thereto. Since the collecting container 19 has a substantially smaller volume than the process chamber 18, 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 19 may also be optionally detached from the process chamber 18, preferably in such a manner that the collection space is isolated with respect to the surrounding, in order to remove silicon from the process chamber 18 collected therein, and the collection container 19 may then 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 transport container, storage container and/or process container.

At least one heating unit (not shown) may be provided to heat the process chamber to a predetermined temperature.

The gas circulation unit 17 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 18. 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.

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, in particular a process gas, 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 for cleaned gas or a liquid distilled from the gas. Furthermore, an external supply unit for process gas may be provided. For cleaning of metallurgical silicon, for example at least one of SiCI₄, chloride, HCI, Silanes, halogenides 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 process gas or a precursor for the process 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, in order to slow down solidification of molten silicon.

The first inlet unit 38 comprises a housing 50, which has a passage 51 for the conduit element 13. 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 13 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.

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 13 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 a 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 18. 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 19 is mounted in a gas tight manner to housing 17. The process chamber 18 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, such that molten silicon 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 process gas being introduced.

When molten silicon is present at the conduit element 13 and flows towards the process chamber 18, process gas, which may contain the previously mentioned components, is conveyed via the pump 37 and the first inlet unit 38 into the process chamber 18. In the following, it is assumed that the process gas contains a chlorine containing process gas, in particular SiCI₄. The process gas, when entering the process chamber 18, has a high flow velocity, preferably a supersonic velocity. Molten silicon flowing through the conduit element 13 and exciting the same will be atomized by this gas flow into fine droplets. The thus atomized droplets of molten silicon then fall downwards within the process chamber 18 and are subsequently, after they may have solidified received within the collection container 19 in the form of silicon particles. During this free fall of the droplets, the droplets are in contact with the process gas SiCI₄. The process gas and in particular the chlorine component thereof reacts with metallic contaminants in the droplets and form metal chlorides. Contaminants of boron and phosphorus for example form BCI3 and PCI₃, respectively, which at the temperature in the process chamber 18 are volatile. Similar reactions may also occur with other contaminants, in particular metallic contaminants. The conduit element 13 may for example be additionally heated with a resistance heater or another heater, in order to ensure that the liquid silicon does not solidify during the process or while flowing therethrough.

Due to the negative pressure in the process chamber 18, the metallic contaminants are preferably pulled to the surface of the droplets. By forming the droplets, a very large surface is provided, such that the process gas may effectively react with the metallic contaminants, and to thus remove the same to a high percentage from the molten silicon. In order to increase the falling time within the process chamber 18, an obliquely upwardly directed gas flow may be generated within the process chamber 18 via the second inlet unit 42. This gas flow again consists of the process gas, such that a good mixture of the droplets with the process gas occurs.

During the above procedure, gas, in particular the introduced process gas as well as the volatile components generated during reaction of the process gas with the contaminants, is continuously evacuated from the process chamber via the pump 33. Obviously, at least some particles are also evacuated, which may be separated from the gas flow within the fine dust separator 31 . The gases and the volatile contaminant components are conveyed via the pump 33 into the gas conditioning unit 35. In the gas conditioning unit 35, the gas is cleaned for example by filtering, distillation and/or chemical absorption, in order to remove the metallic 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 processed in the above manner, the process may be stopped and silicon collected in the collection container 19 may be removed and may optionally be fed directly to further processes. It is also possible, as already indicated above, to directly perform further processes in the collection container 19, 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, in order 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 and continuous removal of the cleaned material.

The above described process thus provides an effective cleaning for metallurgical silicon in order to remove in particular metallic contaminants such as boron and phosphorus in an easy and cost effective manner from the silicon. In particular phosphorus may be removed from Si-particles formed after gas atomization and subsequent solidification of Si-particles by a high vacuum. The typically spherical form of the Si-particles having a respective small diameter of for example 20 to 400μηι facilitates removal of phosphorus in vacuum. 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.

Claims

1. A method for removing contaminants from metallurgical silicon, the method comprising:

melting the metallurgical silicon;

introducing the molten silicon in a process chamber, wherein the molten silicon upon insertion into the process chamber is atomized via a gas flow, and wherein subsequently the molten silicon falls towards the floor of the process chamber;

generating a reactive atmosphere in the process chamber, wherein the reactive atmosphere contains a process gas, which reacts with contaminants in the molten silicon in order to remove the same from the molten silicon.

2. The method of claim 1 , wherein the molten silicon is atomized via a gas flow of the process gas, in particular a supersonic gas flow of the process gas.

3. The method of claim 1 or 2, wherein gas is continuously evacuated from the process chamber, wherein the gas is cleaned and at least partially recycled for atomization of the molten silicon and/or for generating the reactive atmosphere in the process chamber.

4. The method of claim 1 , wherein cleaning of the gas comprises a distillation and/or chemical absorption, in order to remove contaminants contained in the gas.

5. The method of any one of the preceding claims, wherein prior to and/or during the insertion of the molten silicon a negative pressure or vacuum is generated in the process chamber.

6. The method of claim 1 , wherein a negative pressure of <50 mbar, preferably <15 mbar is generated in the process chamber. The method according to any one of the preceding claims, wherein prior to inserting the molten silicon into the process chamber, a reactive gas, which reacts with contaminants in the molten silicon, is conveyed through the molten silicon in order to remove contaminants therefrom.

The method of claim 7, wherein the reactive gas is cleaned and at least partially recirculated.

The method of anyone of the preceding claims, wherein prior to atomizing the molten silicon, ultrasound or megasound is applied thereto.

The method of any one of the preceding claims, wherein an at least partially upwardly directed gas flow, in particular of process gas, is generated within the process chamber.

1 1 The method of any one of the preceding claims, wherein a circular or spiral shaped gas flow, in particular of process gas is generated within the process chamber.

12. The method of any one of the preceding claims, wherein the process gas contains at least one of chlorine, a different halogen , in particular hydrogen halide or silicon halide and/or mixtures of at least two of these components as a reactive component.

13. The method of any one of the preceding claims, wherein argon is mixed into the process gas. 14. The method of any one of the preceding claims, wherein the atomized molten silicon at least partially solidifies on its way to the floor of the process chamber, such that Si-particles are collected at the floor of the process chamber, the method further comprising: exposing the Si-particles to a vacuum of <10 ³ mbar, in particular <10^-4 mbar.

The method of claim 14, wherein the Si-particles are exposed to the vacuum and a reactive gas atmosphere having a higher pressure than the vacuum, in an alternating manner.

The method of claim 14 or 15, wherein the Si-particles are exposed to the vacuum and/or reactive gas atmosphere outside of the process chamber or in a holding area of the process chamber, which was previously isolated with respect to the remainder of the process chamber.

An apparatus for removing contaminants from metallurgical silicon, said apparatus comprising:

a melting unit (3) having a melting crucible (9) for receiving metallurgical silicon and at least one heating unit (10) for melting metallurgical silicon contained within the melting crucible (9);

a process chamber unit (5) having a process chamber (18);

at least one conduit (13) extending from the melting crucible (9) to the process chamber (18) and opening towards both of them; and

at least one gas circulation unit (7), said gas circulation unit (7) further comprising:

at least one first pump (33) which is connected to the process chamber (18) and which is adapted to evacuate gas from the process chamber (18);

a gas conditioning unit (35), which is connected to an outlet of the at least one first pump (33), and which is adapted to remove or clean contaminants from gas evacuated from the process chamber (18);

at least one first gas inlet unit (38), which is fluidly connected to the gas conditioning unit (35), in order to circulate gas from the gas conditioning unit (35) into the process chamber, wherein the at least one first gas inlet unit (38) has at least one outlet conduit (53) having an outlet opening adjacent to the end of conduit (13), which opens towards the process chamber (18), wherein the outlet conduit (53) and the outlet opening are arranged such that they are obliquely directed towards an outlet area of conduit (13); and

at least one second pump (37) for conveying gas from the gas conditioning unit (35) to the at least one gas inlet unit (38).

18. The apparatus of claim 17 wherein the first pump is dimensioned such that it is capable ob achieving a negative pressure in the process chamber, in particular a negative pressure of <50 mbar.

19. The apparatus of claim 17 or 18, wherein the at least one second pump (37) and the gas inlet unit (38) are dimensioned such that at the outlet opening of the at least one outlet conduit a gas flow having supersonic velocity is generated.

20. The apparatus according to one of claims 17 to 19, wherein the gas conditioning unit (35) comprises a filter unit, a distillation unit and/or a chemical absorption unit.

21. The apparatus according to one of claims 17 to 20, wherein the gas circulation unit (7) further comprises at least one third pump (38) and a second gas inlet unit (42), said second gas inlet unit (42) having at least one outlet conduit (63) having an outlet opening into the process chamber (18), wherein the outlet conduit (63) and the outlet opening are directed into the process chamber (18) in an oblique upwards manner.

22. The apparatus of any one claims 17 to 21 , wherein at least one ultrasound-transducer or mega sound transducer is provided, which is arranged adjacent to the conduit (13) such that it may introduce ultrasound or mega sound into the conduit (13).

23. The apparatus according to any one of 17 to 22, said apparatus further comprising means for introducing a process gas into the melting crucible (9).

The apparatus according to any one of claims 17 to 23, wherein the process chamber (18) comprises a holding area (19) at the bottom or floor thereof, which may be isolated with respect to the process chamber. 25. The apparatus according to claim 24, which further comprises means for generating a vacuum in the holding area (19) and/or for introducing a process gas therein.