APPARATUS AND METHOD FOR PRODUCING SILICON OF 3N PURITY OR HIGHER BY PURIFYING SILICON OF 2N PURITY

Abstract

The invention relates to an apparatus for producing 3N purity silicon by purifying 2N purity silicon, the purification including the steps of air injection (E1), filtering (E2) and solidification (E3) to obtain 3N purity silicon in the solid state. Figure to be published with the abstract: Fig. 5

Description

Title of the invention: APPARATUS AND METHOD FOR PRODUCING SILICON OF 3N PURITY OR HIGHER BY PURIFYING SILICON OF 2N PURITY FIELD OF THE INVENTION

[0001] The present invention relates to the general technical field of purification of silicon generated from quartz in order to obtain silicon of purity 3N or higher (4N, 5N, 6N, etc.).

[0002] In particular, the invention relates to methods for treating silicon produced by carboreduction of quartz in an electric arc furnace.

[0003] BACKGROUND OF THE INVENTION

[0004] 1. General

[0005] The production of metallic silicon is generally based on the carbothermic reduction of quartz at high temperature and atmospheric pressure. This carbothermic reduction of quartz can be carried out by chemically reducing quartz in the presence of carbon-based reductants in an electric arc furnace where a high-temperature arc heats the reactants (quartz and reductants).

[0006] This makes it possible to produce a so-called “metallurgical grade” silicon (or “MG-Si” acronym for the Anglo-Saxon expression “Metallurgical Grade Silicon”) having a purity of 99% (or “2N” for the total number of “9”).

[0007] Metallurgical grade silicon can be used: • as an alloying element – ​​for example in the aluminium and steel industry – or • as a precursor – for example in the photovoltaic industry (solar panels), for silicones and for energy storage.

[0008] Technological developments in the fields of photovoltaics and energy storage have led to metallurgical grade silicon becoming a strategic material. Therefore, the supply of high-purity metallurgical grade silicon at a reasonable cost has become a necessity.

[0009] Furthermore, more and more technologies need to have silicon with a purity of 99.9% (3N) and 99.99% (4N), or even 99.999% (5N).

[0010] However, one of the disadvantages of conventional metallurgical silicon production devices and processes is the high content of impurities contained in the metallurgical silicon produced.

[0011] 2. Existing purification solutions for conventional production processes- tional

[0012] To limit the high content of impurities contained in the silicon produced from conventional devices and processes, there are different solutions.

[0013] A first solution consists of using a raw material (quartz) with high purity. However, this solution increases the production cost of silicon.

[0014] A second solution consists of implementing acid treatments after the production of 2N purity silicon, in order to obtain 3N purity silicon or higher. Such acid treatments in fact make it possible to dissolve the impurities present at the grain boundaries of 2N purity silicon obtained from conventional metallurgical silicon production devices and processes. However, disadvantages of such a solution concern its high cost, as well as its unfavorable environmental footprint, due to the use of toxic acid(s). Indeed, the implementation of an acid-based post-treatment requires specific adapted installations. Furthermore, the treatment of gases and effluents produced - when subjecting 2N purity silicon to an acid attack - is expensive due to environmental regulations.

[0015] A third solution consists of carrying out, after the production of 2N purity silicon: • a directional solidification step to obtain a silicon ingot, and • a trimming step consisting of removing the upper part of the ingot and not keep only the lower part of greatest purity.

[0016] The directional solidification step allows, by segregation, to concentrate the impurities in the upper part of the silicon ingot resulting from the directional solidification. The trimming step allows this upper part to be removed to keep only the lower part of 3N purity or higher. However, a disadvantage of such a solution is that the directional solidification step is very slow.

[0017] 3. Method according to US 11,267,714 and object of the present invention

[0018] Document US 11,267,714 proposes another solution consisting of implementing the carbothermic reduction of quartz at high temperature under vacuum.

[0019] With reference to [Fig.l], the apparatus according to US 11,267,714 comprises an electric arc furnace FA and a sealed enclosure E containing the arc furnace FA. This arrangement (sealed enclosure E containing the arc furnace FA) makes it possible to control the pressure at which the carbothermic reduction of quartz is carried out.

[0020] Carrying out the carbothermic reduction of quartz under vacuum makes it possible to promote the evaporation of impurities present in the reactants (quartz and reducing agents) as well as in the newly produced liquid metallurgical grade silicon.

[0021] An object of the present invention is to provide a method for purifying silicon produced according to the technique described in US 11,267,714.

[0022] In particular, an aim of the present invention is to propose one (or more) improvement(s) to the apparatus according to US 11,267,714 to facilitate its use on an industrial scale in the context of the production of silicon of purity 3N or higher.

[0023] BRIEF DESCRIPTION OF THE INVENTION

[0024] To this end, the invention provides an apparatus for producing silicon of 3N purity or higher, the apparatus comprising a device for producing silicon of 2N purity including: • a sealed enclosure connected to at least one suction pump to generate a vacuum inside the enclosure when the suction pump is activated, • an electric arc furnace housed inside the sealed enclosure, the arc furnace including a tank for containing a mixture of raw materials, and a set of electrodes configured to heat said mixture in order to produce silicon of purity 2N, remarkable in that the apparatus further comprises a purification device for obtaining silicon of 3N purity, said purification device comprising a casting ladle and an electromagnetic stirrer downstream of the casting ladle: • the casting ladle including: • a tank suitable for containing a bath of silicon in the liquid state, said bath being composed of 2N purity silicon from the production device, • an injection nozzle for injecting a gas into the silicon bath in the liquid state, the injection of the gas making it possible to obtain a refined liquid silicon having a purity higher than the 2N purity silicon from the production device, • a filter arranged upstream of the electromagnetic stirrer to filter the refined liquid silicon, the filtering making it possible to obtain a filtered liquid silicon having a purity higher than the refined liquid silicon, and • the electromagnetic stirrer being configured to receive the filtered liquid silicon and to solidify it at a solidification rate greater than or equal to 5 cm / h, preferably greater than or equal to 10 cm / h, the solidification making it possible to obtain a bell of purified solid silicon forming silicon of purity 3N or higher.

[0025] Preferred but non-limiting aspects of the apparatus according to the invention are as follows: • the electromagnetic stirrer may include a mechanical stirrer and / or an inductive stirrer to induce a movement of the liquid filtered silicon at a speed greater than or equal to 1 ms; the filter may comprise pores whose dimensions are between 75 micrometers and 3 millimeters, preferably between 100 micrometers and 2.5 millimeters, and even more preferably between 500 micrometers and 2 millimeters; the casting ladle may further include a heating system configured to maintain the silicon bath in the liquid state contained in the tank at a temperature greater than or equal to 1600°C, preferably greater than or equal to 1700°C; the nozzle can be supplied with gas by a power source configured to inject said gas at a flow rate of between 2 and 10 m3 / h; the electromagnetic stirrer may be devoid of thermal insulation to induce cooling of the purified solid silicon block at a rate of between 150°C / h and 500°C / h, preferably between 200°C / h and 300°C / h; the 2N purity silicon production device may also comprise a control unit configured to: • during a pre-processing step: • reduce the pressure inside the enclosed space to a value lower than atmospheric pressure, • implement resistive heating of the mixture of raw materials, • during a carbon reduction step: • increase the pressure inside the enclosed space to a value substantially equal to atmospheric pressure, • implement electric arc heating of the mixture of raw materials, • then when the temperature of the mixture is higher than 2000°C: • maintain the pressure inside the enclosed space at a value substantially equal to atmospheric pressure, • maintain electric arc heating of the raw material mixture, • during a post-processing step: • reduce the pressure inside the enclosed space to a value below atmospheric pressure.

[0026] The invention also relates to a process for the production of silicon of 3N purity or higher, the process comprising: • a phase of production of 2N purity silicon from a production device including a sealed enclosure connected to a suction pump, and an electric arc furnace housed inside the sealed enclosure, the production phase comprising the following steps: • load a tank of the electric arc furnace with a mixture of raw materials, • generate a vacuum inside the enclosure by activating the suction pump, • heating the mixture using a set of electrodes of the arc furnace so as to produce silicon of purity 2N, remarkable in that the method further comprises and a purification phase for obtaining silicon of purity 3N from a purification device including a casting ladle and an electromagnetic stirrer downstream of the casting ladle, said purification phase comprising the following steps: • injection, using an injection nozzle of the casting ladle, of a gas into a bath of silicon in the liquid state composed of silicon of purity 2N from the production device, said bath being contained in a reservoir of the casting ladle, the injection of the gas making it possible to obtain a refined liquid silicon having a purity higher than the silicon of purity 2N from the production device, • filtering, using a filter placed upstream of the electromagnetic stirrer, refined liquid silicon to obtain filtered liquid silicon having a purity higher than the refined liquid silicon, and • solidification, using the electromagnetic stirrer, of liquid filtered silicon at a solidification speed greater than or equal to 5 cm / h, preferably greater than or equal to 10 cm / h.

[0027] Preferred but non-limiting aspects of the method according to the invention are the following: • the solidification step may comprise a sub-step consisting of stirring the liquid filtered silicon at a speed greater than or equal to 1 ms; • the filtering step may consist of circulating the refined liquid silicon through a filter having pores whose dimensions are between 75 micrometers and 3 millimeters, preferably between 100 micrometers and 2.5 millimeters, and even more preferably between 500 micrometers and 2 millimeters; • during the injection step, the gas can be injected into the silicon bath liquid at a flow rate between 2 and 10m3 / h; • the 2N purity silicon production phase can include the following steps: • a pre-processing step including the sub-steps consisting of: • reduce the pressure inside the enclosure to a value lower than atmospheric pressure, • implement resistive heating of the mixture of raw materials, • a carbon reduction step including the sub-steps consisting of: • increase the pressure inside the enclosed space to a value substantially equal to atmospheric pressure, • implement electric arc heating of the mixture of raw materials, • then when the temperature of the mixture is higher than 1500°C: • maintain the pressure inside the enclosed space at a value substantially equal to atmospheric pressure, • maintain electric arc heating of the raw material mixture, • a post-treatment step including a sub-step consisting of reducing the pressure inside the enclosed space. Brief description of the drawings

[0028] Other advantages and characteristics of the device and the method for producing silicon according to the invention will emerge more clearly from the following description of several variant embodiments, given as non-limiting examples, from the appended drawings in which: • [Fig.l] is a schematic representation of a prior art 2N purity silicon production device, • [Fig.2] is a schematic diagram of a silicon production apparatus of purity 3N or higher including a device for producing silicon of purity 2N and a purification device for obtaining silicon of purity 3N, • [Fig.3] is a schematic representation of different phases of a process silicon production from the production device illustrated in [Fig.2], • [Fig.4] is a schematic representation illustrating a separation front between a liquid phase and a solid phase, • [Fig.5] is a schematic representation of different phases of a process 3N purity silicon production. DETAILED DESCRIPTION OF THE INVENTION

[0029] Various embodiments of the invention will now be described with reference to the figures. In these various figures, equivalent elements are designated by the same numerical reference.

[0030] 1. General

[0031] With reference to [Fig.2], the apparatus according to the invention comprises: • a device for producing 2N purity silicon, and • a 2N purity silicon purification device (produced by the device production) to obtain silicon of 3N purity or higher.

[0032] 2. Silicon production device

[0033] The silicon production device comprises: • an electric arc furnace 1, • a waterproof enclosure 2, and possibly • a cover 3 adapted to cover a tank 11 of the electric arc furnace 1, • the arc furnace 1 and the cover 3 being housed in the enclosure 2.

[0034] 2.1. Arc furnace

[0035] The arc furnace 1 allows: • on the one hand to contain the reagents - namely quartz and reducing agents - for the production of 2N purity silicon, and • on the other hand, heating said reagents to high temperature to induce the carbothermic reduction reaction of quartz.

[0036] The arc furnace 1 comprises in particular the tank 11 and electrodes 12.

[0037] The tank 11 is intended to contain the reagents, but also the liquid silicon resulting of the carbothermic reduction reaction of quartz. The tank has a bottom, one (or more) side wall(s), and an upper opening (covered by the lid 3 in [Fig.2]) opposite the bottom.

[0038] A-A' is a longitudinal central axis of the device which is vertical when the device is placed on a horizontal flat surface. The (or the assembly composed of) the side wall(s) of the arc furnace 1 is substantially centered (centered) on the axis A-A'.

[0039] In the following, the description is detailed by considering that the terms “upper” and “high” correspond to a direction generally parallel to the axis A-A' and going from the bottom towards the opening of the tank 1, while the terms “lower” and “lower” correspond to an opposite direction.

[0040] The tank 11 also comprises a casting channel (or several channels) 111 for the evacuation of the metallurgical grade silicon produced (in the liquid state) to the purification device. A closure system - such as a valve or a nozzle or a plug made of carbonaceous material - may be provided for closing the casting channel (or channels) 111 during the carbothermic reduction reaction of the quartz. At the time of casting, the tank 11, which may be mounted for example on a tilting cradle, is inclined towards the casting ladle so as to gradually send the metallurgical grade silicon in the liquid state through the casting channel (or channels). The casting channel (or channels) 111 then being open, the liquid metallurgical grade silicon can then flow out of the tank and be collected in the casting ladle. Of course, other configurations are possible to allow the extraction of the metallurgical grade silicon.For example, the bottom of the container may have a non-zero slope towards the casting channel(s) so as to send the metallurgical grade silicon towards the casting ladle when the sealing system is removed from the channel (a method known as “tapping” in English).

[0041] The electrodes 12 can be powered either from a direct current source or from an alternating current source. These electrodes 12 allow the formation of an electric arc allowing the heating of the reactants (quartz and reducing agent) to induce the production of silicon of 2N purity.

[0042] Preferably, the electrodes 12 are placed in the axis of the arc furnace 1 so as to be stressed in a relatively symmetrical manner. More precisely, each electrode 12 may consist of a cylindrical graphite bar extending vertically.

[0043] 2.2. Enclosure

[0044] The sealed enclosure 2 makes it possible to define an enclosed space in which it is possible to generate a depression, that is to say a space in which it is possible to reduce the pressure to a value lower than atmospheric pressure.

[0045] Such an enclosure 2 may have different shapes and be made from different materials to ensure this function. Thus, in the context of the present invention, the term "watertight enclosure" means a box composed of at least one wall - for example made of steel - on either side of which there is a pressure difference. Thus, the watertight enclosure makes it possible to delimit two zones (i.e. an internal zone and an external zone) between which there is a pressure difference.

[0046] The sealed enclosure 2 comprises one (or more) through orifice(s), possibly for the passage: • one (or more) channel (channels) for injecting gas into the enclosure 2, each channel being able to be connected to a gas supply source (such as argon), • one (or more) channel (channels) for suctioning gas contained in enclosure 2, each channel being able to be connected to a suction pump to enable the generation of a vacuum inside enclosure 2, • electrodes 12, • one (or more) conduit(s) supplying arc furnace 1 with reagents R (quartz, reducing agents, etc.). • electrical connection means (such as an electrically conductive cable, for example for the electrical connection of a sensor contained in the enclosure, etc.), • of the casting channel (or channels) 111, etc.

[0047] 2.3. Cover

[0048] The cover 3 is configured to cover the upper opening of the tank 11.

[0049] The cover 3 makes it possible to contain the gases, and in particular the gaseous compound of silicon oxide (SiO) - formed during the carboreduction reaction of quartz inside the tank 11 of the arc furnace 1. This makes it possible to limit the risks of fouling of the enclosure 2, in particular at the level of its partitions and the suction channel (or channels). Furthermore, the cover 3 makes it possible to maintain the arc furnace 1 at a temperature (and more precisely at a temperature gradient) promoting the consumption of the reactants. This increases the reaction rate of the device, in particular with respect to the silicon production device described in US 11,267,714.

[0050] The cover 3 may have a truncated cone shape, a dome shape, or a disc shape. The cover 3 may be made of any type of refractory thermal insulation material known to those skilled in the art, such as graphite.

[0051] Advantageously, the cover 3 is movable. More precisely, the cover 3 can be moved between: • a closed position in which the cover 3 completely covers the upper opening of the tank 11 (the edges of the cover 3 being in contact with the edges of the upper opening in the closed position), and • an open position in which the cover 3 does not cover (or partially covers) the upper opening of the tank 11.

[0052] To do this, one (or more) support shaft(s) (not shown) of the cover 3 can be connected to one (or more) motor(s).

[0053] The fact that the cover 3 is movable allows access to the interior of the tank of the arc furnace 1, for example: • for maintenance operations of arc furnace 1, and / or • to load / recharge the interior of tank 11 with reagents necessary for the carboreduction reaction, and / or • to promote the evacuation of certain gases produced during the carboreduction reaction, etc.

[0054] Advantageously, the cover 3 may comprise one (or more) through-light(s) capable of being blocked by one (or more) movable closure component(s). This (or these) through-light(s) form(s) one (or more) exhaust conduit(s) for the gases confined in a region defined between the tank 11 and the cover 3 (in the closed position).

[0055] 2.4. Operating principle

[0056] With reference to [Fig.3], the operating principle of the 2N purity silicon production device is as follows.

[0057] It is assumed that a mixture of raw materials containing for example quartz, and a reducing agent, typically carbon, has been previously positioned in the tank 11.

[0058] 2.4.1. Pre-processing phase

[0059] A PI pretreatment phase allowing the initiation of the carbo-reduction reaction as well as the evacuation of impurities from the mixture of raw materials is implemented. This pretreatment phase comprises the steps: • of generating a depression in the sealed enclosure 2, • resistive heating (using the energy transferred by the electrodes 12) of the mixture of raw materials contained in the tank 11, and • opening the cover 3.

[0060] During this PI pretreatment phase, certain impurities are volatilized, those having a high vapor pressure. Generating a vacuum in the enclosure, and opening the cover while heating the raw materials allows the impurities to escape from the enclosure (these being sucked up by the suction pump).

[0061] 2.4.2. Carboreduction phase

[0062] Once this pretreatment phase PI is completed (when the temperature in the enclosure reaches 1000°C to 1500°C), a carboreduction phase P2 is implemented. This carboreduction phase P2 comprises the steps: • return to atmospheric pressure in the sealed enclosure 2, • closing the cover 3 and heating by electric arc the mixture contained in the arc furnace 1 to a carboreduction temperature, and • half-opening of the cover 3 and maintaining heating of the mixture by electric arc.

[0063] Closing the cover 3 and generating an electric arc in the arc furnace allows the temperature of the mixture to be rapidly increased to 1500 to 2000°C.

[0064] Once the temperature is reached, opening the cover 3 while maintaining the electric arc heating allows heat dissipation from the upper part of the reaction medium: a temperature gradient is then created in the reaction medium. actional (high temperature at the bottom of the tank 11 and lower at the upper opening of the tank 11). The formation of a temperature gradient in the tank 11 of the arc furnace allows the formation of the different reaction intermediates in the different temperature zones as well as the gas exchanges between them for an efficient carboreduction reaction.

[0065] 2.4.3. Post-processing phase

[0066] At the end of the carboreduction reaction P2, when all the raw materials have reacted and the liquid silicon is formed in the tank 2, a post-treatment phase P3 is initiated. The cover 3 is completely raised. The chamber 2 is again depressurized to allow the elimination of impurities present in the liquid silicon and which have a high vapor pressure such as phosphorus, and resistive heating is again implemented.

[0067] At the end of the post-treatment phase P3, the tank 11 contains silicon of purity 2N in the liquid state. The casting channel (or channels) 111 is (are) open to evacuate the liquid silicon of purity 2N to the purification device.

[0068] 3. Purification device

[0069] The purification device comprises: • a casting ladle 4 intended to receive 2N purity silicon from the production device, and • an electromagnetic stirrer 5 downstream of the casting ladle 4, the electromagnetic stirrer 5 being intended to receive the silicon from the casting ladle 4.

[0070] 3.1. Casting ladle

[0071] The casting ladle 4 allows: • on the one hand to maintain the silicon it contains in a liquid state, and • on the other hand to filter, using a filter, residues (solid or liquid) contained in the 2N silicon from the production device.

[0072] The casting ladle 4 comprises: • a reservoir 41 delimited by a refractory lining and intended to receive the liquid silicon of purity 2N from the production device, • optionally, a heating system 42 associated with the reservoir 41 to maintain the silicon contained in the reservoir 41 in the liquid state, • a gas injection nozzle 43 opening into a lower part of the tank 41, • a filter 44 placed in the tank 41, and • an outlet channel 45 for transferring the silicon contained in the reservoir 41 to the electromagnetic stirrer 5.

[0073] 3.1.1. Tank

[0074] The reservoir 41 is of a type known per se to those skilled in the art. It comprises a base, one (or more) lateral partition(s) and a filling orifice delimited by the edge(s) of the lateral partition(s) opposite the base.

[0075] In particular, the reservoir 41 is composed of a carcass in which a lining of refractory material is arranged. Inside this lining is arranged a cavity intended to receive the 2N purity silicon from the production device.

[0076] 3.1.2. Heating system

[0077] The heating system 42 may be of the resistive type or of the inductive type or a gas burner.

[0078] In the embodiment illustrated in [Fig.2], the heating system 42 comprises one (or more) graphite resistor(s) arranged in an upper part of the 4L container

[0079] In certain variant embodiments, the heating system 42 may also comprise one (or more) graphite resistor(s) arranged at the level of the lateral partition(s) of the container 4L. This allows better control of the temperature inside the container 41 over its entire height.

[0080] The heating system 42 may also comprise one (or more) graphite resistor(s) arranged in a lower part of the container 41, in particular under the base of the container 4L.

[0081] Advantageously, the heating system is configured to maintain the liquid silicon contained in the reservoir at a temperature greater than 1600°C, and preferably greater than or equal to 1700°C.

[0082] Of course, the casting ladle can also be without a heating system to limit production costs.

[0083] 3.1.3. Injection nozzle

[0084] The injection nozzle 42 may consist of a graphite nozzle extending projecting towards the interior of the reservoir 41 from its base.

[0085] The injection nozzle 42 makes it possible to introduce gas - for example air or an air / Oxygen mixture (to limit production costs) or an inert gas - into the liquid silicon contained in the reservoir 41 in order to oxidize the impurities, typically Aluminum and Calcium. Advantageously, the flow rate and the speed of the gas introduced into the liquid silicon are respectively between 2 and 10 Nm3 / h, and 0.1 and Im / s. Advantageously, the reaction of the Oxygen in the gas with the Silicon is exothermic, so it makes it possible to heat the Silicon and prevent it from cooling too quickly, which would induce solidification.

[0086] The introduction of the gas makes it possible to eliminate certain impurities - calcium (Ca) and aluminium (Al) - contained in the molten silicon (reduction by a factor of two or three of the initial concentrations of Al and Ca contained in the molten silicon) in forming a slag on the surface of the molten silicon. In fact, oxygen reacts mainly with the aluminum and calcium contained in the molten silicon, which eliminates a significant portion of these impurities.

[0087] In particular, treatment by injection of air (or by injection of an inert gas) makes it possible to obtain a mixture composed of two phases: refined silicon on the one hand, a slag containing calcium and aluminum on the other hand, the slag extending on the surface of the refined silicon.

[0088] The slag can then be separated from the refined liquid silicon by any technique known to those skilled in the art, for example by decantation or by means of tools such as a plane.

[0089] The implementation of gas injection stirring makes it possible to obtain refined silicon in the liquid state in which: the aluminum concentration of refined silicon is less than 0.2% ([Al]<0.2%), and the calcium concentration of refined silicon is less than 0.02% ([Ca]<0.02%).

[0090] 3.1.4. Filter

[0091] The filter 44 makes it possible to retain impurities contained in the refined silicon to obtain a filtered liquid silicon intended to be transferred into the electromagnetic stirrer 5. For this purpose, the filter 44 can be housed in the reservoir 41, between the filling orifice and the outlet channel 45 extending at the base of the container 4L. Thus, the filtering of the impurities is carried out during the flow of the liquid silicon towards the outlet channel 44.

[0092] These impurities can be solid (unreacted raw materials, by-products generated during the carboreduction reaction, etc.), or high-viscosity liquids (slag residues contained in the liquid refined silicon, etc.).

[0093] The filter may be made of ceramic (preferably silicon carbide SiC) and consist of a body - such as a plate - including pores. Advantageously: the thickness of the body can be between 1 and 15 centimeters, preferably between 2 and 10 centimeters and even more preferably between 4 and 8 centimeters, the median diameter of the pores can be between 75 micrometers and 3 millimeters, preferably between 100 micrometers and 2.5 millimeters, and even more preferably between 500 micrometers and 2 millimeters.

[0094] These dimensions of the filter 44 are defined to guarantee effective separation of impurities while ensuring sufficiently rapid filtering of the refined silicon to avoid any risk of solidification of the silicon during this filtering.

[0095] 3.1.5. Output channel

[0096] The outlet channel 45 allows the filtered liquid silicon to be evacuated outside the casting ladle 4 for transfer to the electromagnetic stirrer 5.

[0097] In certain embodiments, the filtered liquid silicon pours from the outlet channel 45 into an intermediate container - such as an intermediate casting ladle - prior to its introduction into the electromagnetic stirrer 5. In other embodiments, the filtered liquid silicon is solidified rapidly in an ingot mold to purify it; it will therefore be necessary to remelt it in an induction furnace before transferring it to the electromagnetic stirrer.

[0098] 3.2. Electromagnetic stirrer

[0099] The electromagnetic stirrer 5 allows: • on the one hand to concentrate impurities in the liquid phase of silicon by segregation, and • on the other hand to produce a silicon bell of 3N purity or higher.

[0100] The electromagnetic stirrer 5 comprises: • a crucible 51 including a sole 511 and one (or more) lateral panel(s) 512 whose upper edge(s) opposite the sole 511 define(s) an inlet mouth: • the crucible can be made of graphite to allow its reuse, • the angle between the sole and the side panel(s) may be greater than or equal to 105° to facilitate the demolding of the solidified bell of silicon produced, • one (or more) inductor winding(s) (not shown) intended to stir, by induction, the silicon charge contained in the crucible 51.

[0101] As indicated previously, the electromagnetic stirrer 5 makes it possible to purify the filtered silicon from the casting ladle 4.

[0102] To purify the silicon, a solidification step under strong stirring is implemented in the electromagnetic stirrer 5. With reference to [Fig.4], this solidification takes place from the bottom 511 of the crucible 51 and from the side walls of the crucible towards the mouth according to a rising solidification front (solid / liquid interface) 513. This solidification front 513 is highly concave, the solidification of the silicon taking place both by the bottom 511 and by the lateral side(s) 512 of the crucible 51.

[0103] The solidification makes it possible to obtain a bell of purified solid silicon 514, and a liquid residue of impure silicon 515 in which the majority of the impurities are accumulated.

[0104] This accumulation of impurities in the liquid residue of impure silicon is obtained by segregation. Segregation is a physical phenomenon occurring during the solidification of a material. During the solidification of the material, the majority of impurities (particularly metals) are released into the liquid phase.

[0105] In fact the distribution of impurities follows Scheil's law:

[0106] Cs =

[0107] With k = Cs / Cl the impurity-specific segregation coefficient in Silicon. The specific segregation coefficient is known to those skilled in the art and will not be described in further detail below.

[0108] Thus, during the solidification of silicon, the impurities preferentially remain in the liquid phase 515. This makes it possible to obtain a solid silicon bell 514 of purity 3N or higher. The more the solid fraction 514 increases, the greater the concentration of impurities in the liquid phase 515.

[0109] Advantageously, the segregation phenomenon can be improved by effective stirring of the silicon bath in the liquid state. Stirring the liquid silicon bath makes it possible to renew the liquid boundary layer at the solid / liquid interface, in order to promote the rejection of impurities in the liquid silicon phase.

[0110] Preferably, this stirring is obtained using an inductive stirrer allowing the application of an electromagnetic field - alternating or rotating or sliding - to the liquid silicon bath. Advantageously, the inductive stirrer can be powered by an alternating current having a frequency between 1 and 100Hz, preferably of the order of 10Hz for a crucible of 1 meter in diameter. Such a power supply frequency in fact allows: • to maximize stirring (speed of the liquid silicon bath > Im / second) thanks to significant penetration of the electromagnetic field inside the crucible 51, and • to minimize the heat transferred to the liquid silicon bath in order to promote rapid solidification of the silicon bell (solidification rate greater than or equal to 5 cm / h, preferably greater than or equal to 10 cm / h).

[0111] Alternatively, this mixing can be obtained using a mechanical stirrer such as a graphite propeller immersed in the liquid silicon bath and driven in rotation by a motor shaft.

[0112] The electromagnetic stirrer makes it possible to obtain a bell 514 of solid silicon of purity 3N or higher and a liquid residue of impure silicon 515 in which the majority of the impurities are accumulated.

[0113] The electromagnetic stirrer may comprise a cooling system (not shown) composed for example of one (or more) heat exchanger(s) in through which a heat transfer fluid circulates. The integration of a cooling system makes it possible to control the cooling speed of the solid silicon bell of 3N purity or higher down to room temperature. The electromagnetic stirrer can also be without a specific cooling system.

[0114] 4. Operating principle

[0115] We will now describe the operating principle of the purification device with reference to [Fig.4] which illustrates the bubbling E1, filtering E2 and solidification E3 steps of a purification process.

[0116] 4.1. Bubbling

[0117] After pouring the liquid silicon into the casting ladle 4 of the purification device, refining (bubbling step E1) by injecting air (or any other inert gas) into the silicon bath contained in the tank is carried out.

[0118] Refining removes common impurities - such as aluminum and calcium - contained in the 2N purity liquid silicon from the production device. This air injection is carried out using the injection nozzle 42.

[0119] Under the effect of bubbling, the common impurities form a slag containing alumina, lime and silica. This slag floats and is therefore easy to remove using any suitable technique known to those skilled in the art, such as a suction, skimming or decantation technique.

[0120] Advantageously, the flow rate and speed of the air (or inert gas) introduced into the liquid silicon bath are respectively between 2 and 10 m3 / h and 0.1 to Im / s to maximize the elimination of common impurities.

[0121] Furthermore, the temperature of the silicon bath is maintained greater than or equal to 1600°C, and preferably greater than or equal to 1700°C. This makes it possible to guarantee the effectiveness of the filtering carried out subsequently by limiting the risks of untimely solidification of silicon in the filter 44 contained in the tank 4L. Such solidification would in fact risk blocking the filter 44.

[0122] The bubbling step E1 makes it possible to obtain a refined liquid silicon in which the concentration of current impurity is reduced.

[0123] 4.2. Filtering

[0124] Under the effect of gravity, the refined liquid silicon moves towards the outlet channel 45 of the casting ladle 4.

[0125] The refined liquid silicon passes through the filter 44, the pore size of which prevents the passage of solid particles larger than 75 μm. These solid particles (unreacted raw materials, by-products generated during the carboreduction reaction, etc.) constitute impurities.

[0126] The retention by the filter 44 of these solid impurities therefore makes it possible to improve the purity of the filtered liquid silicon having passed through the filter.

[0127] At the end of the filtration step, the filtered liquid silicon flows into the electromagnetic stirrer through the outlet channel 45.

[0128] The reader will appreciate that the bubbling and filtration steps can be carried out simultaneously.

[0129] 4.3. Solidification

[0130] The solidification step can then be initiated, preferably under strong stirring (liquid silicon bath speed > Im / second). The presence of strong stirring makes it possible to eliminate the risk of segregation defects linked to the accumulation of impurities at the solidification front.

[0131] The solidification step is carried out at a high speed, in particular greater than or equal to 5 cm / h, preferably greater than or equal to 10 cm / h. Rapid solidification in fact allows: • on the one hand to minimize the costs linked to the implementation of this solidification step (the faster the solidification, the lower its cost), and • on the other hand to limit the generation of waste and the electrical energy consumed.

[0132] When the desired quantity of silicon has solidified, the solidification step is interrupted: a bell of purified solid silicon 514 and a liquid residue of impure silicon 515 are obtained.

[0133] This liquid residue of impure silicon 515 (i.e. liquid remaining at the end of the solidification step) is poured by tilting the electromagnetic stirrer (rotation along the z axis) into an ingot mold.

[0134] The crucible is then turned over to unmold and remove the silicon bell.

[0135] 5. Conclusions

[0136] The apparatus and method described above make it possible to obtain a purified solid silicon bell of 3N purity or higher.

[0137] In particular, the presence of a purification device makes it possible to eliminate numerous impurities contained in the metallurgical grade silicon obtained using the production device illustrated in [Fig.2].

[0138] The reader will have understood that numerous modifications can be made to the invention described above without materially departing from the new teachings and advantages described here.

Claims

Claims

1. An apparatus for producing silicon of 3N purity or higher, the apparatus comprising a 2N purity silicon production device including: • a sealed enclosure (2) connected to at least one suction pump to generate a vacuum inside the enclosure (2) when the suction pump is activated, • an electric arc furnace (1) housed inside the sealed enclosure (2), the arc furnace (1) including a tank (11) for containing a mixture of raw materials, and a set of electrodes (12) configured to heat said mixture in order to produce 2N purity silicon, characterized in that the apparatus further comprises a purification device for obtaining silicon of 3N purity, said purification device comprising a casting ladle (4) and an electromagnetic stirrer (5) downstream of the casting ladle: • the casting ladle (4) including: • a reservoir (41) adapted to contain a bath of silicon in the liquid state, said bath being composed of silicon of purity 2N from the production device, • an injection nozzle (43) for injecting a gas into the silicon bath in the liquid state, the injection of the gas making it possible to obtain a refined liquid silicon having a purity higher than the 2N purity silicon from the production device, • a filter (44) arranged upstream of the electromagnetic stirrer (5) to filter the refined liquid silicon, the filtering making it possible to obtain a filtered liquid silicon having a purity higher than the refined liquid silicon, and • the electromagnetic stirrer (5) being configured to receive the liquid filtered silicon and to solidify it at a solidification rate greater than or equal to 5 cm / h, preferably greater than or equal to 10 cm / h, the solidification making it possible to obtain a bell of purified solid silicon forming the silicon of 3N purity or higher.

2. Apparatus according to claim 1, wherein the electromagnetic stirrer comprises a mechanical stirrer and / or an inductive stirrer for inducing movement of the liquid filtered silicon at a speed greater than or equal to 1 ms

3. Apparatus according to any one of claims 1 or 2, wherein the filter (44) comprises pores whose dimensions are between 75 micrometers and 3 millimeters, preferably between 100 micrometers and 2.5 millimeters, and even more preferably between 500 micrometers and 2 millimeters.

4. Apparatus according to any one of claims 1 to 3, wherein the casting ladle further includes a heating system configured to maintain the silicon bath in the liquid state contained in the reservoir (41) at a temperature greater than or equal to 1600°C, preferably greater than or equal to 1700°C.

5. Apparatus according to any one of claims 1 to 4, wherein the nozzle (43) is supplied with gas by a supply source configured to inject said gas at a flow rate of between 2 and 10m3 / h.

6. Apparatus according to any one of claims 1 to 5, in which the electromagnetic stirrer (5) is devoid of thermal insulation to induce the cooling of the purified solid silicon block at a rate of between 150°C / h and 500°C / h, preferably between 200°C / h and 300°C / h.

7. Apparatus according to any one of claims 1 to 6, wherein the device for producing 2N purity silicon further comprises a control unit configured to: • during a pretreatment step: • decrease the pressure inside the enclosed space to a value lower than atmospheric pressure, • implement resistive heating of the mixture of raw materials, • during a carboreduction step: • increase the pressure inside the enclosed space to a value substantially equal to atmospheric pressure, • implement electric arc heating of the mixture of raw materials, • then when the temperature of the mixture is higher than 2000°C: • maintain the pressure inside the enclosed space at a value substantially equal to atmospheric pressure, • maintain electric arc heating of the raw material mixture, • during a post-processing step: • reduce the pressure inside the enclosed space to a value below atmospheric pressure.

8. A process for producing silicon of 3N purity or higher, the process comprising: • a phase of production of silicon of purity 2N from a production device including a sealed enclosure (2) connected to a suction pump, and an electric arc furnace (1) housed inside the sealed enclosure (2), the production phase comprising the following steps: • loading a tank (11) of the electric arc furnace (1) with a mixture of raw materials, • generate a vacuum inside the enclosure (2) by activating the suction pump, • heating the mixture using a set of electrodes (12) of the arc furnace (1) so as to produce silicon of purity 2N, characterized in that the method further comprises a purification phase for obtaining silicon of 3N purity from a purification device including a casting ladle (4) and an electromagnetic stirrer (5) downstream of the casting ladle (4), said purification phase comprising the following steps: • injection, using an injection nozzle (43) of the casting ladle (4), of a gas into a silicon bath in the liquid state composed of silicon of purity 2N from the production device, said bath being contained in a reservoir (41) of the casting ladle (4), the injection of the gas making it possible to obtain a refined liquid silicon having a purity higher than the 2N purity silicon from the production device, • filtering, using a filter (44) arranged upstream of the electromagnetic stirrer (5), the refined liquid silicon to obtain a filtered liquid silicon having a purity higher than the refined liquid silicon, and • solidifying, using the electromagnetic stirrer (5), the filtered liquid silicon at a solidification rate greater than or equal to 5 cm / h, preferably greater than or equal to 10 cm / h.

9. The method of claim 8, wherein the solidifying step comprises a substep of stirring the liquid filtered silicon at a rate greater than or equal to 1 ms

10. A method according to any one of claims 8 or 9, wherein the filtering step consists of circulating the refined liquid silicon through a filter (44) having pores whose dimensions are between 75 micrometers and 3 millimeters, preferably between 100 micrometers and 2.5 millimeters, and even more preferably between 500 micrometers and 2 millimeters.

11. Method according to any one of claims 8 to 10, in which during the injection step, the gas is injected into the liquid silicon bath at a flow rate of between 2 and 10m3 / h.

12. Method according to any one of claims 8 to 11, in which the phase of producing silicon of purity 2N comprises the following steps: • a pretreatment step including the sub-steps consisting of • reducing the pressure inside the enclosure to a value lower than atmospheric pressure, • implementing resistive heating of the mixture of raw materials, • a carboreduction step including the sub-steps consisting of: • increasing the pressure inside the enclosed space to a value substantially equal to atmospheric pressure spherical, • implement electric arc heating of the mixture of raw materials, • then when the temperature of the mixture is higher than 1500°C: • maintain the pressure inside the enclosed space at a value substantially equal to atmospheric pressure, • maintain electric arc heating of the raw material mixture, a post-treatment step including a sub-step consisting of reducing the pressure inside the enclosed space.

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