WO2019234072A1

WO2019234072A1 - Device for producing molten silicon

Info

Publication number: WO2019234072A1
Application number: PCT/EP2019/064591
Authority: WO
Inventors: Guy Chichignoud; Jochen Altenberend; Daniel Bajolet
Original assignee: Institut Polytechnique De Grenoble; Centre National De La Recherche Scientifique
Priority date: 2018-06-05
Filing date: 2019-06-05
Publication date: 2019-12-12
Also published as: FR3081856A1; CN112512969B; TW202012310A; TWI844545B; FR3081856B1; EP3802422A1; CN112512969A

Abstract

The invention relates to a device (10) for producing molten silicon (11), comprising an enclosure (12) and comprising, in the enclosure: a crucible (15) intended to receive powder (19) of oxidised particles of silicon, the crucible comprising an internal volume (16) intended to contain silicon in the molten state and silica and a channel (28) for discharging silicon in the molten state out of the internal volume, the crucible comprising at least two holes (32), the straight section of each hole having a maximum dimension that is greater than or equal to a value between 1 mm and 10 mm, or at least one slot, the straight section of which has a maximum dimension that is greater than or equal to a value between 1 mm and 10 mm connecting the internal volume to the crucible; and a heating system (20) at least partly surrounding the crucible.

Description

PRODUCTION DEVICE FOR MOLTEN SILICON

Field

The present invention relates to a device for producing molten silicon and a silicon purification plant comprising such a device, in particular for the manufacture of electric energy production cells by photovoltaic effect.

Presentation of the prior art

Currently, the majority of silicon for photovoltaic applications is produced by chemical treatments similar to those used for the production of silicon for electronic applications. These chemical processes are well controlled but require very heavy investments and lead to high production costs. The pressure on photovoltaic energy costs has led to the search for purification processes that are alternative to the chemical process. Such a process comprises the production of molten silicon from a powder of silicon particles, which may correspond to waste from sawing silicon blocks in the microelectronics or photovoltaic industry, a process for grinding silicon or a process for manufacturing polycrystalline silicon by fluidized bed, the molten silicon can then be recrystallized to form a silicon block. However, it is difficult to melt such a powder of silicon particles in a conventional melting device comprising a crucible in which the powder is introduced, melted and recrystallized. Indeed, the silicon particles may contain a high oxygen content, between 1% and 5% by weight, due to the silica layer forming naturally on the surface of the particles. Indeed, at the melting temperatures of silicon, the silica does not melt, tends to become pasty and forms a sponge-shaped structure. In addition, the filling rate of the crucible by the powder of oxidized silicon particles is low so that the ingot produced after the recrystallization of the molten silicon is fragile.

summary

Thus, an object of an embodiment is to provide a device for producing molten silicon from a powder of oxidized silicon particles which overcomes at least some of the disadvantages of the devices described above.

According to another object of an embodiment, the device for producing molten silicon makes it possible to separate the silica from the silicon from the powder of oxidized silicon particles.

According to another object of an embodiment, the device for producing molten silicon makes it possible to produce molten silicon continuously or semi-continuously.

According to another object, the device for producing molten silicon has a productivity compatible with exploitation on an industrial scale.

Thus, an embodiment provides a device for producing molten silicon, comprising an enclosure and comprising in the enclosure:

a crucible for receiving powder of oxidized silicon particles, the crucible having an internal volume for containing molten silicon and silica and a channel for discharging silicon in the molten state out of the volume internal, the crucible comprising at least two holes, the cross section of each hole having a maximum dimension greater than or equal to a value of between 1 mm and 10 mm, one of the holes being located above the other hole or at least one vertical slot whose cross section has a maximum dimension greater than or equal to a value of between 1 mm and 10 mm connecting the internal volume to the crucible; and

a heating system surrounding at least part of the crucible. According to one embodiment, one of the holes is located above the other hole.

According to one embodiment, the minimum dimension of the cross section of each hole or slot varies from 0.5 mm to 5 mm.

According to one embodiment, the minimum dimension and the maximum dimension of the cross section of the evacuation channel varies from 1 mm to 50 mm.

According to one embodiment, the crucible comprises a bottom and a side wall and the holes or slot open on the side wall.

According to one embodiment, the minimum distance between the hole closest to the bottom or between the slot and the bottom is greater than 10% of the height of the crucible.

An embodiment provides an installation comprising:

the device for producing molten silicon as defined above;

a crucible feed system of the device for producing molten silicon powder of oxidized silicon particles; and

a system for solidifying the molten silicon supplied by the device for producing molten silicon in a silicon block.

According to one embodiment, the solidification system comprises an additional crucible receiving the molten silicon provided by the device for producing molten silicon and heating elements of the additional crucible.

Brief description of the drawings

These and other features and advantages will be set forth in detail in the following description of particular embodiments in a non-limiting manner with reference to the accompanying drawings in which: Figures 1 and 2 are sectional views, partial and schematic, of embodiments of a device for producing molten silicon; and

Figure 3 is a partial sectional and schematic sectional view of an embodiment of a silicon ingot production facility.

detailed description

The same elements have been designated by the same references in the various figures. For the sake of clarity, only the elements of the device for producing molten silicon that are necessary for understanding the described embodiments have been shown in the various figures and are detailed. In the remainder of the description, the qualifiers "lower", "upper", "upward" and "downward" are used in relation to an axis D, considered to be vertical. However, it is clear that the axis D may be slightly inclined relative to the vertical, for example an angle less than or equal to 20 °. In addition, the expressions "substantially", "about", "approximately" and "of the order of" mean "to within 10%", preferably to "within 5%". In addition, when the terms "substantially", "about", "approximately" and "of the order of" are used for angles, they mean "within 10 degrees", preferably "within 5 degrees".

In this invention, the at least one vertical slot and / or the at least two holes located one above the other have at least the function of allowing a continuous passage of molten silicon to the discharge channel. In particular, if the silica partially obstructs the passage towards the discharge channel, for example in the lower part of the crucible near the bottom of the crucible, the upper holes or the upper part of the slot make it possible to preserve an unobstructed passage towards the evacuation channel.

This invention is therefore not limited to a slot or holes aligned exactly along a vertical axis D. A vertical slot means a slot that is mainly oriented vertically. However, it may be inclined relative to the vertical axis D by an angle of less than 30 °, preferably less than 20 °, preferably less than 10 °. Similarly, the holes located one above the other are not necessarily aligned along the vertical axis D. They can be arranged staggered, for example.

The term "particle" as used in the context of the present application must be understood in a broad sense and corresponds not only to compact particles having more or less a spherical shape but also to angular particles, flattened particles, flake-shaped particles, fiber-shaped particles, or fibrous particles, etc. It will be understood that the "size" of the particles in the context of the present application means the smallest transverse dimension of the particles. For example, in the case of fiber-shaped particles, the size of the particles corresponds to the diameter of the fibers.

By the term "average size" of particles is meant according to the present application the size which is greater than the size of 50% by volume of the particles and smaller than the size of 50% by volume of the particles. This corresponds to the dsg. Particle size can be measured by laser granulometry using, for example, a Malvern Mastersizer 2000.

An example of a device for producing molten silicon will now be described, the molten silicon being intended in particular to obtain silicon blocks having a degree of purity sufficient for direct use for the production of photovoltaic products. However, the molten silicon can also be used to obtain silicon blocks having a degree of purity lower than the level required for direct use for the production of photovoltaic products and intended to be further processed to have a degree of purity sufficient to the realization of photovoltaic products.

FIG. 1 represents a first embodiment of a device 10 for producing molten silicon 11.

The device 10 comprises a gas-tight enclosure 12 formed by gas-tight walls 13 which isolate the enclosure 12 from the outside. At least one opening, not shown, is provided through the walls 13 and allows to communicate the internal volume of the chamber 12 with the outside. The device 10 may comprise a delivery system, not shown, of a neutral gas or a mixture of neutral gases, for example argon or helium, in the chamber 12.

The device 10 comprises a silicon melting furnace 14 disposed in the enclosure 12. The furnace 14 comprises a crucible 15 delimiting an internal volume 16. The crucible 15 comprises a bottom 17 and a side wall 18. According to one embodiment, the crucible 15 is of a material which is a good thermal conductor. A good thermal conductor is a material whose thermal conductivity is greater than or equal to 5 W / (m ^* K). For example, the crucible 15 is made of graphite. According to one embodiment, the crucible 15 is in addition of a material which is a good electrical conductor. A good electrical conductor is a material whose electrical conductivity is greater than or equal to 1000 S / m. According to another embodiment, the crucible 15 is of a material which is not a good thermal conductor, or which is a good thermal insulator. A good thermal insulator is a material whose thermal conductivity is less than or equal to 5 W / (m ^* K). For example, the crucible 15 is made of silicon oxide, silicon nitride or silicon carbide.

The crucible 15 comprises, for example, a circular base of axis D whose external diameter can vary from 100 mm to 800 mm. The crucible 15 has, for example, a height ranging from 100 mm to 800 mm. The crucible 15 rests on a support not shown. The support may be made of a refractory material, for example a refractory concrete associated with a stack of materials ensuring good thermal insulation of the bottom of the crucible 15. In operation, powder 19 of oxidized particles of silicon is poured into the crucible 15 .

The device 10 further comprises a silicon heating system 20 in the crucible 15. According to one embodiment, the heating system 20 is an induction heating system. The heating system 20 comprises, for example, a coil 22 surrounding the crucible 15. The coil 22 may be hollow and include an internal opening 24 used for cooling the coil 22 by the circulation of a coolant. The crucible 15 can then be surrounded by insulating walls 26 thermally and electrically, for example flexible or rigid graph felt. In particular, an insulating lid, not shown, can cover the crucible 15, the lid comprising an opening for the introduction into the crucible 15 of the powder 19 of oxidized particles of silicon. The support, the walls 26 and the insulating cover promote the maintenance of a homogeneous temperature in the crucible 15 and reduce heat losses. The maintenance of the liquid silicon at the desired temperature in the crucible 15 is obtained by the generation of currents induced by the coil 22 in the crucible 15, when it is made of an electrically conductive material, and / or in silicon. Alternatively, the heating of the crucible 15 and the silicon contained in the crucible 15 can be carried out by an electric heating system comprising resistors, these being arranged around the crucible 15 and thermally insulated from the enclosure 12 by means of thermal insulation elements.

The crucible 15 comprises a channel 28 for discharging the molten silicon 11 present in the internal volume 16 of the crucible 15. Preferably, the evacuation channel 28 extends through the bottom 17 and / or into the side wall 18 of the crucible 15 and opens through an orifice 30 of a nose 31 provided on the underside of the crucible 15. The discharge channel 28 may be substantially vertical. The discharge channel 28 may have a circular, square or rectangular cross section. The minimum dimension of the cross section of the discharge channel 28 varies from 1 mm to 30 mm. The maximum dimension of the cross section of the discharge channel 28 varies from 10 mm to 50 mm. For example, the discharge channel 28 may have a circular cross section with a diameter greater than 5 mm, preferably ranging from 10 mm to 15 mm. The crucible 15 comprises at least two holes 32 opening on the side wall 18 of the crucible 15, preferably at least three holes, more preferably at least four holes, each hole connecting the internal volume 16 of the crucible 15 to the discharge channel 28. The holes 32 may be arranged in a substantially vertical column. Each hole 32 may have a circular, square or rectangular cross section. The maximum dimension of the cross-section of each hole 32 varies from 5 mm to 15 mm and the minimum dimension of the cross section of each hole 32 varies from 0.5 mm to 5 mm. According to one embodiment, one of the holes 32 opens on the side wall 18 while being substantially tangent to the bottom 17. In the case where the discharge channel 28 and the holes 32 have a circular cross section, the diameter of each hole 32 is preferably less than or equal to the diameter of the discharge channel 28. According to another embodiment, the holes 32 are replaced by a vertical or inclined slot. The cross-section of the slot has a maximum dimension which varies from 5 mm to 15 mm and a minimum dimension which varies from 0.5 mm to 5 mm. According to another embodiment, the holes 32 or at least a portion of the holes 32 are located on the bottom 17 of the crucible 15.

The faces of the crucible 15 delimiting the nose 31 form between them only angles smaller than 120 °. The device 10 may comprise heating elements, not shown, of the nose 31.

The operation of the device 10 is as follows. An inert atmosphere is maintained in the chamber 12. According to one embodiment, the pressure in the chamber 12 is between 0.1 atm (10132.5 Pa) and 1 atm (101325 Pa). Preferably, the pressure in the chamber is substantially equal to atmospheric pressure. Powder 19 of oxidized silicon particles is introduced into the crucible 15 by means not shown in FIGS. 1 and 2. According to one embodiment, the average size of the oxidized silicon particles is less than 300 miti, preferably between 100 nm and 100 μm. The feed of the crucible 15 with the powder 19 of oxidized silicon particles may be carried out in a continuous flow or in successive batches separated by periods of absence of powder supply. For a feed rate of the powder 19 of oxidized silicon particles of less than 5 kg / h, a discontinuous feed with a higher flow rate makes it possible to better distribute the powder 19 of oxidized silicon particles on the bottom 17 of the crucible 15.

Inside the crucible 15, the powder 19 of oxidized silicon particles and the molten silicon 11 are heated directly by the heating system 20 and / or by radiation of the walls of the crucible 15 and / or by conduction of the walls and bottom of the crucible above the silicon melting temperature of 1400 ° C, preferably above 1600 ° C. The silicon present in the powder melts and separates from the silica which forms an agglomerate 33 in the crucible 15. The molten silicon 11 flows through the holes 32 and the channel 28 to flow, for example in the form of drops. 34 of molten silicon, through the orifice 30. Advantageously, the dimensions of the cross section of the discharge channel 28 are sufficient to reduce the pressure required to push a gas bubble in the discharge channel 28. The fact that the faces of the crucible 15 defining the nose 31 form between them angles less than 120 ° allows the silicon does not flow laterally to the insulating walls 26 but falls in the form of drops 34. According to one embodiment , the distance between a droplet 34 escaping through the orifice 30 and the adjacent insulating walls 26 is greater than 5 mm to prevent silicon from infiltrating to the insulating walls 26.

The silica remains in the crucible and accumulates. The holes 32 are distributed between the bottom 17 and the top of the crucible 15 so that, when a hole 32 is plugged with silica, the molten silicon 11 can flow through the hole 32 following. The fact that the diameter of the holes 32 is less than or equal to the diameter of the discharge channel 28 makes it possible to prevent silica from flowing through the holes 32 in the discharge channel 28 and causes clogging in the outlet channel 28. the evacuation channel 28.

The device 10 may comprise systems for stirring the molten silicon 11 in the crucible 15. The stirring may be performed at least partly by induction when the heating system 20 is induction. The frequency of the current supplying the induction coil 22 can then be adapted to promote a stirring of the molten silicon in the crucible 15.

According to one embodiment, in the case where the crucible 15 is made of graphite, a silicon coating of electronic quality can be placed in the crucible 15 so that a layer of silicon carbide is formed on the inner walls of the crucible 15 after the melting of this silicon coating and before the powder 19 of oxidized silicon particles is introduced into the crucible 15. This prevents the graphite of the crucible 15 from reacting with the powder 19 of oxidized silicon particles melting in the crucible 15. The formation of the silicon carbide layer can be accelerated by temporarily reducing the pressure in the chamber 12 below 10 mbar, preferably below 1 mbar . In another embodiment, a silicon carbide coating is formed on the inner walls of the graphite crucible by another method, for example by Chemical Vapor Deposition (CVD).

When the production of molten silicon is complete, after cooling, the silica can be removed by mechanical means. A coating composed of at least one material such as graphite, silicon carbide, silicon oxide and silicon nitride may be placed in the crucible 15 in order to prevent the silica from sticking to the crucible 15.

FIG. 2 is a sectional view of another embodiment of a device 35 for producing molten silicon 11. The device 35 comprises all the elements of the device 10 represented in FIG. 1, with the difference that the hole 32 opening on the side wall 18 closest to the bottom 17 is remote from the bottom 17 by a distance greater than 10% of the height of the crucible. For example, the holes 32 are located in the upper half of the crucible 15.

In operation, a part of the molten silicon 11 remains in the crucible 15 so that the agglomerate 33 of silica floats on the molten silicon 11 and is not in contact with the crucible 15. The silica can then be removed by means mechanical while the crucible 15 is still above the melting temperature of the silicon. The process for producing the molten silicon can be carried out continuously without cooling the crucible 15. In the case where there is no stirring of the molten silicon in the crucible 15, the height of the molten silicon bath 11 present in the crucible 15 may be of the order of 10 mm. In the case where a stirring of the molten silicon in the crucible 15 is performed, the height of the molten silicon bath present in the crucible 15 may be of the order of the diameter of the crucible 15, for example of the order of 200 mm. FIG. 3 represents an embodiment of an installation 40 for producing an ingot or silicon block. The installation 40 comprises the device 10 or 35 for producing molten silicon shown in FIGS. 1 and 2 and further comprises an enclosure 42 formed by gastight walls 44 which isolate the enclosure 42 from the outside. An opening, not shown, may be provided through the walls 44 of the enclosure 42 and allow the enclosure 42 to communicate with the outside. An opening 46 is provided through the walls 13 of the enclosure 12 and the walls 44 of the enclosure 42 and makes it possible to communicate the internal volume of the enclosure 12 with the internal volume of the enclosure 42. The installation 40 comprises a sealed door 48 at the opening 46 to hermetically isolate the internal volume 16 of the chamber 12 of the internal volume of the enclosure 42. The door 48 is, for example of the swinging or sliding type, is actuated by a mechanism not shown.

The installation 40 may comprise a feed system 50 of the melting furnace 14 in powder form of oxidized silicon particles. The supply system 50 may comprise a gas-tight reservoir 52 and a gas-tight system 54 for supplying the oxidized silicon particles supplied by the reservoir 52, comprising, for example, a vibratory feeder or a rotary screw feeder.

The installation 40 further comprises, in the chamber 42, a system 56 for solidifying the molten silicon supplied by the melting furnace 14. The system 56 may comprise a crucible 58 in which the molten silicon solidifies to obtain a block of molten silicon. silicon. The system 56 may further include heating elements 60 provided at the top of the crucible 58. The heating elements 60 may comprise electrical resistors. The system 56 may further comprise cooling and / or heating elements 62 provided in the crucible 58 so as to obtain a solidification of the silicon in the crucible 58 directed from bottom to top. The cooling elements 62 may comprise pipes in which a coolant circulates. Thermally insulating walls 64 may be provided around the crucible 58, the heating elements 60 and the heating elements. In particular, a cover 66 made of a thermally insulating material, for example graphite or silicon nitride, with a hole 68 may be placed above the crucible 58 to prevent splashing of molten silicon from affecting the heating element 60.

The installation 40 may comprise at least one vacuum pump, not shown, connected to each chamber 12 and 42. The pump is adapted to create a controlled atmosphere in the enclosure 12 or 42. According to another embodiment, the installation 40 may comprise, for each chamber 12 and 42, a vacuum pump, not shown, connected to the chamber 12 or 42 and means for injecting an inert gas or inert gas into each chamber 12 or 42, in order to maintain controlled atmospheres, possibly different, in the enclosures 12 and 42.

In operation, the crucible 58 is filled with the molten silicon produced by the device 10 for producing molten silicon, and a controlled solidification of the molten silicon present in the crucible 58 is produced, for example with a solidification front of the silicon which progresses from bottom up. The heating elements 60 provided at the top of the crucible 58 can advantageously be used for heating the nose 31 of the crucible 15.

Various embodiments with various variants have been described above. It will be appreciated that those skilled in the art may combine various elements of these various embodiments and variants without demonstrating inventive step. Particular embodiments of the present invention have been described. Various variations and modifications will be apparent to those skilled in the art. In particular, although FIG. 3 represents an embodiment in which the molten silicon feeds a system 56 for directed solidification of silicon, the molten silicon can feed a crucible in which the silicon solidifies without directed solidification, subsequent treatments, by example purification, being performed on the silicon block.

Claims

1. Device (10; 35) for producing molten silicon (11), comprising a chamber (12) and comprising in the chamber:

a crucible (15) for receiving powder (19) of oxidized silicon particles, the crucible having an internal volume (16) for containing molten silicon and silica and an evacuation channel ( 28) of molten silicon out of the internal volume, the crucible comprising, connecting the internal volume to the evacuation channel, at least two holes (32), the cross section of each hole having a maximum dimension greater than or equal to a value between 1 mm and 10 mm, one of the holes (32) being located above the other hole or at least one vertical slot whose cross-section has a maximum dimension greater than or equal to a value between 1 mm and 10 mm; and

a heating system (20) surrounding at least part of the crucible.

2. Device according to claim 1, wherein the minimum dimension of the cross section of each hole (32) or slot varies from 0.5 mm to 5 mm.

3. Device according to any one of claims 1 to 2, wherein the minimum dimension and the maximum dimension of the cross section of the discharge channel (28) varies from 1 mm to 50 mm.

4. Device according to any one of claims 1 to 3, wherein the crucible (15) comprises a bottom (17) and a side wall (18) and wherein the holes (32) or the slot open on the side wall .

5. Device according to claim 4, wherein the minimum distance between the hole (32) closest to the bottom (17) or between the slot and the bottom (17) is greater than 10% of the height of the crucible (15). .

6. Installation (40) comprising:

the device (10; 35) for producing molten silicon (11) according to any one of claims 1 to 5; a feed system (50) for the crucible (15) of the device for producing molten silicon powder (19) of oxidized silicon particles; and

a system (56) for solidifying the molten silicon supplied by the device for producing molten silicon into a silicon block.

7. Installation according to claim 6, wherein the solidification system (56) comprises an additional crucible (58) receiving the molten silicon supplied by the device (10; 35) for producing molten silicon and heating elements (60). additional crucible.