EP2419380A1

EP2419380A1 - Method and apparatus for purifying a silicon feedstock

Info

Publication number: EP2419380A1
Application number: EP10713973A
Authority: EP
Inventors: Maxime Labrot
Original assignee: SILIMELT
Current assignee: SILIMELT
Priority date: 2009-04-17
Filing date: 2010-04-16
Publication date: 2012-02-22
Also published as: CA2758563A1; CN102459077A; FR2944520B1; FR2944520A1; WO2010119129A1; CN102459077B; US20120090984A1

Abstract

The invention relates to a method for purifying a silicon feedstock in order to obtain silicon with a higher purity. According to the invention, the following steps are carried out: a) a plasma cone generated by a first non-transferred arc torch (6) is directed onto a solid wall (7) of a space having an outlet opening such that the impact of the cone against said solid wall (7) generates a homogeneous plasma flow; b) a silicon feedstock to be treated, consisting of particles and/or grains or crushed, is continuously injected into said homogeneous plasma flow; c) the whole consisting of the homogeneous plasma into which the crushed feedstock was injected is continuously directed from the outlet opening towards a crucible (1) comprising a means for heating and agitating said crushed feedstock in the molten state; d) once the entire crushed feedstock has been injected and a molten pool (13) has been formed in the crucible (1), the reactive plasma jet of at least one second non-transferred arc torch is directed towards the surface of said pool; e) the slag on the surface (17) of said pool is removed, and steps d) and e) are optionally repeated in order to evaporate at least some of the impurities of the pool brought to the surface (17) of said pool by agitation.

Description

Method and apparatus for purifying a silicon-based charge

The present invention relates to a process for purifying a silicon-based filler and an installation for its implementation.

It is known that the production of photovoltaic silicon is currently insufficient to meet the current needs of the market, a market that is growing strongly due to the interest in renewable energies. Its supply has been based for years on electronic silicon waste, the current and future scarcity of this material then causing an increase in prices that is incompatible with the planned extension of this market. Even if research is conducted with materials other than silicon to transform light energy into thermal or electrical energy, silicon remains the reference material.

Also, other photovoltaic silicon production lines are actively explored, either by simplified chemical process compared to that implemented for the production of electronic silicon, or electrochemically at high temperature.

The photovoltaic industry requires high purity silicon to produce photovoltaic cells, or solar cells. This photovoltaic silicon is a polycrystalline silicon having a silicon content of 99.9999%. The complement, 100 ppm, consists of impurities whose respective levels must be contained within the following limits: boron <0.3 ppm, - phosphorus <1 ppm,

- total metal impurities <100 ppm,

- carbon <10 ppm, - oxygen <10 ppm. As metallurgical silicon is a low purity silicon, it is therefore necessary to purify it to produce a photovoltaic grade silicon.

Many metallurgical silicon purification processes are known in order to eliminate, in particular, impurities such as boron and phosphorus. US Pat. No. 4,354,987 discloses a method of compacting, after melting, already purified silicon powder using inductive heating by means of a graphite susceptor.

The patents FR 2 487 608 and FR 2 585 690 also describe the purification of silicon under inductive plasma with a plasmagene mixture of argon, hydrogen and oxygen. US Pat. No. 4,379,777 also describes a plasma metallurgical silicon treatment process using an inductively coupled plasma torch operating with an argon / hydrogen mixture.

It is clear that many developments have been conducted, for several decades, to purify metallurgical silicon, with the objective of a competitive economic cost to photovoltaic quality.

In particular, many studies have been devoted to the use of thermal plasma, sometimes in combination with an electromagnetic or resistive heating mode, for melting and purifying metallurgical silicon.

However, all these methods have one or the other of the following disadvantages:

the degree of purification required is obtained by complex processes requiring several steps. For example, the document

"Thermodynamics of solar grade silicon refining" (Revue Intermetallics 11,

2003, 1111-1117), proposes a complete analysis leading to propose a metallurgical silicon purification process comprising not less than five steps to achieve photovoltaic quality: calcium addition, acid leaching, oxidation refining, treatment under vacuum and controlled solidification, each step being specialized in the preparation and / or extraction of specific impurities. - we can also obtain the desired result by adding fluxes directly in a molten bath of silicon.

In all cases, the quantities of silicon photovoltaic produced remain marginal and unfavorable energy balances since encumbered either by the addition of materials facilitating the removal of impurities, either by the multiplication of the process steps, or both.

It should be noted that to date, no industrial facility is operational and even less on a scale that could be compatible with the needs of the market and the necessary cost reductions.

If one sticks to the use of the thermal plasma, it is certain that the inductive plasma torch, attractive from the point of view of its non-contaminating characteristics, is on the other hand strongly handicapped on the one hand as for its operation altered by the injection of the products to be treated, on the other hand, by a limited power range.

The object of the invention is therefore to provide a method and an installation for purifying a charge based on silicon such as metallurgical silicon, overcoming these disadvantages of the prior art.

Advantageously, the present invention is based on an optimized use of the arc thermal plasma, limiting, without energetic rupture, the steps of a process resulting in photovoltaic quality. It also makes it possible to produce, in industrial operation, large quantities of photovoltaic grade silicon from metallurgical silicon.

For this purpose, the subject of the present invention is a process for purifying a silicon-based filler to obtain silicon of higher purity. According to the invention, this method comprises the following successive steps: a) a plasma dart generated by a first non-transferred arc torch is sent against a solid wall of a volume having an outlet orifice so that the impact of said stinger against said solid wall inside said volume generates a homogeneous plasma flow, b) a silicon-based charge consisting of particles and / or grains is introduced continuously or ground into said homogeneous plasma flow, c) the assembly formed by the homogeneous plasma flow into which said crushed feed has been introduced is continuously directed the outlet orifice of said volume to a crucible having side walls and a bottom and an open top, said crucible comprising means for heating and stirring said mulled charge, d) the entire crushed charge to be treated having been introduced and a melt being formed in said crucible, directs the reactive plasma jet of at least a second non-transferred arc torch on the surface of said bath to volatilize at least some of the impurities in the bath of melting present on the surface of said bath, e) the slag present at the surface of said bath is removed, and steps d) and e) are optionally repeated to volatilize at least some of the impurities in the bath brought to the surface of said bath by stirring, f) discharging said melt.

The purification process of the invention therefore aims to treat crushed silicon-based fillers or silicon-based charges consisting of particles and / or grains in said homogeneous plasma flow in batch form. Preferably, the treatment of a crushed batch of charge ensures the filling of the crucible.

These silicon-based fillers are preferably fillers of silica, silicate, quartz, metallurgical silicon or combinations of these elements.

As a purely illustrative example, these silicon-based fillers consisting of particles and / or grains may comprise sand having a granulometry smaller than 5 mm, and preferably between 0.4 mm and 1.3 mm. These silicon-based fillers may further comprise one or more additives such as carbon black resulting from, for example, biomass combustion.

The homogenization of the plasma dart generated by the first non-transferred arc torch makes it possible to create a homogeneous plasma flow, particularly in terms of temperature. This homogeneity of the plasma flow allows a uniform treatment of the introduced crushed feedstock.

Advantageously, at the outlet of the introduction chamber, the assembly obtained by introducing said ground feed into said homogeneous plasma flow has a sufficient size so as not to cause projections from the melt. Preferably, this assembly is sent into the central portion of the upper opening of the crucible and the reactive plasma jet generated by at least one other non-transferred arc torch is sent away from the walls of the crucible so as not to create hot spots on these walls. In order to ensure a treatment of some of the impurities contained in the melt, the latter is electromagnetically stirred so that its impurities accumulate on the surface of the melt to be vaporized by one or more emitted plasma streams. by one or more other non-transferred arc plasma torches. This electromagnetic stirring can be provided by any electromagnetic mixer such as inductive heating means. Typically, the reactive plasma jet (s) will interact with the surface of the melt to allow the volatilization of some of the bath impurities present on the surface of this bath. The electromagnetic stirring of the bath ensures the renewal of this interface to be purified on the surface of the bath. Advantageously, the non-transferred arc plasma torch (s) are fed with redox plasmagene gas such as H ₂ , CO ₂ , O ₂ , HCl, HF and combinations of these elements, in order to deliver at high temperature Oxidative reducing chemicals that promote the removal, by vaporization, of some of the impurities in the melt.

In various particular embodiments of this purification process, each having its particular advantages and susceptible to numerous possible technical combinations:

said at least one reactive plasma jet is a homogeneous and reactive plasma flow obtained by impact of the reactive plasma dart generated by said at least second non-transferred arc torch against a solid wall of another volume having an exit orifice, said second torch being connected to said other volume,

the particle size of said ground feedstock is between 10 and 500 μm, and more preferably between 80 and 150 μm,

in step b), the milled feed being introduced by means of a carrier gas, the ratio of the mass of the milled feedstock to the carrier gas mass is greater than 20, Preferably, this ratio is between 20 and 100 so as not to cool the plasma dart generated by the non-transferred arc plasma torch placed in the introduction chamber.

the carrier gas being a reactive gas in contact with the homogeneous plasma flow, a first purification of the ground feedstock is carried out in this homogeneous plasma flow,

The carrier gas becomes reactive in contact with the homogeneous plasma flow by transferring energy from the latter to the carrier gas. For purely illustrative purposes, this carrier gas is a gas comprising chlorine such as HCl. before step a), the distance separating said solid wall from the outlet orifice of the non-transferred arc torch is advantageously adjusted. Advantageously, this solid wall is placed relative to the outlet orifice of said torch. non-transferred arc, in an area where the plasma dart temperature measured in the axis of said plasma dart and in the absence of the solid wall would be equal to or substantially equal to half the value of the mean temperature peak of the measured plasma dart at the output of the non-transferred arc torch.

As a purely illustrative example, the distance separating the exit orifice of the non-transferred arc plasma torch from this solid wall is of the order of three to five times the diameter of the plasma dart measured at the exit of this torch connected to the introductory speaker.

in step c), the electromagnetic stirring of the melt is carried out,

in step d), the crucible having a diameter D and a height H such that D / H> 5, the reactive plasma jets of at least a second and a third non-transferred arc torch are sent onto the surface of the melting bath to volatilize at least some of the melt impurities present on the surface of this bath,

in step e), a single jet of reactive plasma is oriented on the surface of the melt to give the slag a quantity of movement able to direct it towards at least one discharge orifice placed on the side walls of said crucible this reactive plasma jet being generated in turn or not by other non-transferred arc torches,

While one of the torches is operating and emits a jet of reactive plasma, the rest of the other non-transferred arc torches used to treat the impurities on the bath surface are shut down. Advantageously, the alternative shutdown of each non-transferred arc torch emitting a plasma jet allows a gradual evacuation of the slag in preferred directions.

the spatial volume of the homogeneous plasma flows is increased to avoid projections in said crucible in step c) and to treat a surface of said larger melt in step d)

in step f), the melt thus purified is discharged from its impurities by plasma by controlling its extraction rate, its extraction temperature and the quantity extracted, thus, this solidification being carried out without an additional melting step, ingots having an impurity-enriched silicon outer shell and a silicon-containing core of higher purity are obtained. It then remains to remove this envelope to obtain the silicon of higher purity. The method of the invention therefore allows in a single step of cooling the melt to obtain ingots having an impurity-enriched silicon outer shell while the core of these ingots comprises the silicon of higher purity desired. These ingots preferably have a solar-grade silicon bar shape. They can, for example, have a section of 40x40 cm ² .

The invention also relates to a purification plant for carrying out the purification process as described above. According to the invention, this installation comprises: an insertion chamber comprising at a first end a non-transferred arc plasma torch having a main axis, said torch being intended to generate a plasma dart having an axis of propagation substantially centered on the main axis of said torch,

said insertion chamber comprising a bent part having an outlet orifice, this bent part placed downstream of said plasma torch comprising a solid wall such that said plasma dart collides with said solid wall to form a homogeneous plasma flow,

this introduction chamber comprising at least one insertion port placed downstream of said plasma torch for introduction into continuous operation of a milled feedstock for mixing with said homogeneous plasma flow,

the outlet orifice of said introduction chamber is placed above a crucible having side walls and a bottom and an open top, said crucible being intended to continuously receive said assembly formed by the plasma flow; homogeneous in which said milled feedstock was introduced until complete introduction of the milled feed, to form a melt,

said crucible comprising means for heating and stirring said molten melt, one or more extraction orifices placed on its side walls for evacuating the slag and at least one discharge orifice for discharging said melt,

said installation comprising one or more other non-transferred arc plasma torches each for generating a jet of reactive plasma sent on the surface of the melt in order to volatilize at least some of the surface impurities of said melt.

The crucible preferably has a cylindrical or ovoid shape or any other geometry having an axis of symmetry. Advantageously, the internal volume of this crucible is delimited by walls comprising a non-polluting refractory material with respect to the silicon to be purified, for example, ultrapure silica. The crucible can be rotatable about a vertical axis to be inclined to facilitate the evacuation of the slag. This inclination can be of a few degrees. The extraction orifices of the slag are, for example, evenly distributed around the periphery of the crucible opposite the zones of intersection of the reactive plasma jets with the surface of the melt. As a purely illustrative example, these extraction orifices comprise three notches for overflow of the slag film positioned in the wall of the crucible, being distributed at 120 ° and diametrically opposite to the points of intersection with the surface of the melt of the plasma darts generated. by other non-transferred arc torches for the treatment of surface impurities.

Of course, said at least one discharge port comprises means for ensuring its obstruction such as valves or electromagnetic means. In different particular embodiments of this installation, each having its particular advantages and susceptible to many possible technical combinations:

this installation comprises means of visualization of the melt to determine the opportune moment to evacuate the slag,

the gas of said non-transferred arc torch placed on said introduction chamber is a neutral gas or a reactive gas such as H ₂ , CO ₂ , O ₂ , HCl, HF and combinations of these elements,

the non-transferred arc torches each comprise a downstream electrode which is an electrode flared so as to increase the spatial volume of the plasma dart or the reactive plasma jet generated,

Preferably, each downstream electrode is a conical electrode. Its taper angle can be from 1 ° to 2 °.

the bent portion comprises at least one portion of flared shape to enable the flow of the ground feedstock introduced into said homogeneous plasma flow to be absorbed, this portion of flared shape comprising at its end the outlet orifice of the enclosure of Introduction, The wall of the bent portion with which collides the plasma dart may be inclined relative to the axis of propagation of the plasma dart so as to limit the energy transfer to said wall.

This bent portion may include a flare towards the outlet of the conical shaped enclosure whose half-angle at the top is between 10 and 30 °.

the bent portion of each of the homogenization enclosures comprises at least one portion of flared shape at the end of which is placed the outlet orifice of the corresponding homogenization enclosure,

the installation comprises means for individually adjusting the distances separating the outlet orifices of the introduction chamber and the homogenization chambers, the bottom of the crucible or the surface of the said melt to optimize the energy balances and the extraction of impurities,

each of said other non-transferred arc torches generating a reactive plasma jet is connected to a corresponding homogenization chamber comprising a bent part placed downstream of said corresponding plasma torch, said bent part comprising a solid wall so that said jet of reactive plasma generated by said torch correspondingly collides with said solid wall to form a homogeneous and reactive plasma flow,

said solid wall with which the plasma dart collides is placed relative to the outlet orifice of said torch connected to said introduction chamber, in an area where the temperature of the plasma dart measured in the axis of said plasma dart and in the absence of said solid wall is equal to or substantially equal to half the mean temperature peak value of the plasma dart measured at the output of said non-transferred arc torch, this solid wall is mobile in translation relative to the outlet orifice of said non-transferred arc torch,

Alternatively, the plasma torch placed at one end of the introduction chamber is movable in order to make it possible to adjust the position of the solid wall with respect to the outlet orifice of this non-transferred arc plasma torch.

the ground feedstock being introduced with a carrier gas, said at least one port for introducing the milled feedstock comprises at least one nozzle allowing introduction of said milled feedstock in rotation,

This introduction in rotation makes it possible to lengthen the residence time of the milled feedstock in the homogeneous plasma flow in order to ensure a good thermal transfer of the plasma flow towards the milled feedstock. In the case of a reactive carrier gas, this good heat transfer also makes it possible to start the purification process.

the plasma power of each of the non-transferred arc torches can be continuously adjusted by power adjustment means in order to optimize the energy balances and the elimination of impurities, moreover avoiding the need to create thermal shocks at the walls of the melting / purification crucible,

in-line measurement means make it possible to continuously determine the degree of purity of the material being purified in the melt,

the crucible has a diameter D and a height H such that D / H> 5, For purely illustrative purposes, this ratio D / H may be equal to 15. Advantageously, this shallow depth of the crucible for a large diameter increases the interface of the melt rich in impurities to vaporize and facilitates the return to the surface of the bath by mixing, impurities contained in the bath for the renewal of this interface. Preferably, in this embodiment, the installation comprises at least two other non-transferred arc plasma torches. For example, since the installation comprises three other non-transferred arc plasma torches, the zones of intersection of the reactive plasma jets emitted by these other torches with the surface of the melt are placed at 120 ° from each other on a circle of radius between a quarter and three quarters of the radius of the crucible.

the means for heating and stirring comprise one or more inductive coupling means such as one or more induction coils,

the other torches are orientable so as to move the jet of reactive plasma that it generates to the surface of said melt,

This displacement of the plasma jet on the surface of the melt allows in particular to push the slag towards the extraction orifices. this installation comprises means for adjusting the composition of the plasma gas of each of the non-transferred arc torches during the operation of the latter,

the discharge orifice (s) being placed in the bottom of the crucible, this installation comprises receptacles for collecting the melt, these receptacles being placed on means of scrolling so as to be presented one by one under this or these orifices of discharge until said crucible is empty.

These scrolling means may comprise a linear scroll chain or a rotatable carousel. Advantageously, the gravitational flow is initiated and interrupted according to the sequential presentation, under the crucible, of containers. To each of these containers has been added a controlled atmosphere chamber which is itself connected to the crucible for the transfer of the melt to the container.

Preferably, the installation further comprises sealing connection means for connecting each of these discharge ports with the corresponding container in which the melt is to be discharged.

The invention will be described in more detail with reference to the accompanying drawings in which: - Figure 1 shows schematically a sectional view of the purification plant according to a particular embodiment of the invention;

FIG. 2 is an enlarged view of the crucible of the installation of FIG. 1 showing a slag extraction orifice with its slag recovery device, FIG. 2 a) is a perspective view of this extraction orifice and FIG. 2b) is a sectional view;

FIG. 3 is a view from above of the installation of FIG. 1;

- Figure 4 is an enlarged view of the lower part of the installation of Figure 1 showing the means for scrolling containers under the discharge port;

Figure 1 shows more particularly a sectional view of a plasma purification plant according to a particular embodiment of the invention which will be described here in the context of the metallurgical silicon treatment.

This installation comprises a melting / purifying crucible 1 of cylindrical shape coupled to a melting / purification chamber 2, also cylindrical in shape and sealed with respect to the crucible 1. The crucible 1 and the chamber 2 may however have any other shape, for example ovoid. This melting / purification chamber 2 comprises a conduit 3, or chimney, for the evacuation of the gases present in the enclosure 2.

In a first phase, called pre-purification, a milled silicon feedstock is introduced continuously with a carrier gas into an introduction chamber 4, by means of an injector 5 whose orifice opens into the wall of the introduction chamber 4. The latter comprises at one end a non-transferred arc plasma torch 6, which delivers a plasma dart. This stinger collides with a solid wall 7 of the introduction chamber 4 to generate a homogeneous plasma flow. This flow mixes with the milled silicon feedstock and the carrier gas, to produce a two-phase dart 8 at the outlet of a flared section 9 of the introduction enclosure 4. The two-phase dart 8 is oriented along the axis 10 crucible 1, substantially vertically towards the melting / purification crucible 1. The injector 5, which is positioned so that the crushed silicon charge follows a main trajectory along the axis of the two-phase dart, advantageously makes it possible to give this main trajectory secondary components, for example a rotating component 11, to increase the residence time of the ground silicon charge in the homogeneous plasma flow / carrier gas mixture. This configuration has the advantages of being able to deliver a controlled milled silicon feed stream in its flow rate, independently of the flow rate of the plasma dart generated by the non-transferred arc torch 6 connected to the introduction enclosure 4, while being treated in its totality in the homogeneous plasma flow. It also makes it possible to start the melting / purification process as soon as the crushed silicon feed is injected with adjustable residence times.

The non-transferred arc plasma torch 6 supplies the energy which is partially transferred on the one hand to the milled silicon feedstock and on the other hand to the carrier gas, this carrier gas, heated to high temperature, constituting the starting chemical reagent. the silicon purification process heated within the two-phase dart 8. The ground silicon charge having a particle size of between 10 and 500 microns, and more preferably between 80 and 150 microns, the silicon particles have a maximum exchange surface. The ground silicon charge, confined and conveyed by the diphasic dart 8, fills the melting / purification crucible 1, bringing it to the pre-melted state due to the continuous supply of energy by the arc plasma torch not transferred 6, while the purification process is still active. A high frequency electromagnetic field, produced by an induction coil 12, brings the silicon contained in the crucible 1 in the molten state, creating a stirred melt 13.

In addition to the non-transferred arc plasma torch 6 connected to the input enclosure 4, whose power is reduced when the induction coil 12 is put into operation, three other non-transferred arc torches 14, 15 and 16 (Fig. 3) are put into service to supply the chemical reagents of the plasma which they produce and which are necessary for the continuation of the purification process on the surface 17 of the melt 13.

This surface 17 is fed continuously by the residual impurities due to the electromagnetic stirring generated by the induction coil 12. The three other non-transferred arc plasma torches 14, 15 and 16 are respectively connected to bent portions 18, 19 and 20 (Fig. 3) which respectively deflect the reactive plasma jets generated by these torches by causing a collision between each jet of reactive plasma and a solid wall of the corresponding bent portion to create homogeneous and reactive plasma flows 21, 22 (Figure 1). These bent portions respectively comprise flared sections 23, 24 orienting the homogeneous and reactive plasma flows substantially vertically towards the surface of the bath 17.

The non-transferred arc plasma torches 3, 14, 15 and 16 are each connected to the enclosure 2 by sealed devices (not shown), which furthermore allow orientation of the homogeneous plasma flows 8, 21, 22 with respect to the vertical of a maximum angle of inclination of 10 °. The plasma torches 14, 15 and 16 and their associated bent portions 18, 19 and 20 are concentric with the outlet orifice of the introduction chamber 4, the intersections of the axes 25, 26 of the homogeneous plasma flows 21, 22 with the surface 17 being distributed at 120 ° on a circle whose radius is between one quarter and three quarters of the radius of the crucible 1. The distance between the torches 3, 14, 15, 16 and the surface 17 of the bath, or still the bottom of the crucible, is adjustable by moving the crucible 1 relative to the chamber 2, while maintaining the seal between the chamber 2 and the crucible 1. This mobility increases the thermal efficiency and thermochemical torches relative to on the surface of the bath 17.

The slag film which can form on the surface of the bath, to the detriment of the extraction efficiency of the impurities, is evacuated at regular intervals. The slag is received in three notches 27-29, arranged in the crucible 1 just below the surface of the bath 17 when the crucible 1 is filled (Figure 2). These notches 27-29 face, during the movement of the crucible relative to the chamber 2, interfaces 30 fixed to the chamber 2 and made of a material identical or similar to that of the crucible 1. In the mode of fusion / purification operation, the interfaces 30 are housed respectively in the notches 27-29 to maintain the level 17 of the bath 13. In the slag evacuation mode, the vertical downward movement of the crucible 1, a few millimeters, gives off openings that allow the passage of slag. The notches 27-29 are respectively diametrically opposed to the impact zones of the homogeneous plasma flows 21, 22 with the surface of the bath 17. In order to allow the slag to escape, only one of the three non-transferred arc torches 14, 15 and 16 is turned on at the same time and in turn to cause, respectively by mechanical effect of their homogeneous plasma flow 21, 22, the passage of the slag in the openings of the corresponding notches 27-29. This operation being repeated as much as necessary. The slag is collected in receptacles (not shown). It should be noted that the height of each notch is adjusted to take into account the decrease of the level 17 of the molten bath 13 during the evacuation of the slag. Thus, the notch 29 is deeper than the notch 28, which itself is deeper than the notch 27.

Plasma purified silicon is transferred to a controlled solidification device (not shown in Fig. 1 for the sake of clarity) by semi-continuous casting, positioned in the axis of the bottom 30 of the crucible 1, for example by reheating by an electromagnetic field produced by the coil 31. Said controlled solidification device is positioned under the crucible 1 and is sealed relative to the latter, by the interface 32, when casting is allowed.

The volume of the solidification device being more limited than that of the crucible 1, several controlled solidification devices 33-37 (Fig. 4) will be successively presented. This can be achieved, for example, by horizontal displacement of the latter and set up under the crucible 1 by a vertical movement. As an illustration, these solidification devices 33-37 are mounted on a carriage 38 for presentation in line or radiating. Means of measurement and control can capture the temperature and pressure in the melting / purification chamber 2, the level of the melt 13 and the degree of purification of the material.

In this plant, the purification process comprises the following steps: a) a milled metallurgical silicon stream is transported, for example by means of a carrier gas and is injected substantially vertically downwards in a homogeneous plasma flow obtained by the collision of a plasma dart emitted by a non-transferred arc torch, supplied with a neutral gas, for example argon, operating substantially at its nominal power, with a solid wall. The preheated silicon stream is simultaneously subjected to purification "in flight" (first purification) of vaporizable impurities, by the choice of carrier gas raised at high temperature by the homogeneous plasma flow, for example chlorine or chloride of hydrogen, the plasma / silicon exchange surface being maximized by the finely divided state of the incoming millstock, b / the two-phase mixture is directed substantially vertically downward in a melting / purifying crucible (second purification ), substantially along the vertical axis thereof, to form a melt pre-purified material, the crucible having a volume defined by walls made of an ultra pure material with high temperature resistance and non-contaminating with respect to the silicon charge, c) when the crucible is filled with a molten bath under the action of the plasma in neutral gas, the injection of the ground silicon metallurgic charge that ceases, the plasma power of the non-transferred arc torch fed with neutral gas is reduced, the chemical function of the carrier gas is still active to continue the purification in the crucible, d / electromagnetic heating ensures the complete melting of the bath and its maintaining temperature, further inducing a stirring for the homogenization of the latter and the diffusion of impurities towards the surface of the bath, e / then delivers, vertically above the bath and towards the surface thereof, a second dedicated homogenized plasma, by the choice of an evolutionary plasmagene gas in its composition, for example a mixture of oxygen, hydrogen, carbon dioxide, possibly hydrogen chloride, to the elimination, on the surface of the bath, of the vaporizable impurities still present on the surface of the silicon bath (third purification), by specific thermochemical reactions. This plasma is also generated by one or more other non-transferred arc plasma torches. It is also used for its mechanical effects so as to evacuate, laterally and at regular intervals, the slag film which can form on the surface of the bath, because of additional reactions between the plasma and the silicon, in particular oxides of silicon. This film must be evacuated because it reduces the efficiency of the plasma dedicated to the surface removal of impurities. There is provided a variant for evacuating the slag film, preferably its vaporization by the use of specific plasma gases having the ability to vaporize this film, a "cleaning" chemical at regular intervals. f / when the silicon bath is purified of all vaporizable impurities by the actions of the thermal plasma, the casting is initiated to transfer the bath to crystallization means, the electromagnetic induction heating being maintained in a suitable power range, and that, if necessary, the plasma in neutral gas at reduced power. The process described above therefore allows a first elimination of impurities (first purification) on the particles of the milled feed "in flight" within the mixture of plasma gas neutral / carrier gas of the central torch fed with neutral gas, followed by a second purification on the surface of the bath continuously supplied with residual impurities by the electromagnetic stirring.

For a charge of metallurgical silicon, the impurities extracted are phosphorus and metal impurities, for example Fe, Ti, in the form of gaseous chlorides. Several lateral torches fed by the reactive mixture of plasmagene gas, of the oxygen, hydrogen, carbon dioxide and even hydrogen chloride type allow the surface vaporization of the bath of other impurities by oxidizing them. Boron is converted into a gaseous compound of chemical formula BOH, while carbon is oxidized to carbon monoxide. It results from these plasma treatments, a silicon still containing impurities, but only consisting of metallic chemical elements whose contents are compatible with the last extraction by segregation in the controlled solidification process, in particular Cu, V, Al, Cr. It should be noted that the erosion of the electrodes of the arc torches produces Cu and Cr type metallic elements, in minimum quantities, which are eliminated by segregation during the controlled solidification phase.

In the context of the treatment of a charge of metallurgical silicon, this process advantageously allows an optimized use of the plasma for the following reasons: pre-fusion thermal function of the metallurgical silicon, initially little thermal conductor, "in flight" and in the crucible , function of first and second purification coupled to the thermal function, in the initial phase of injecting the material into the plasma and into the crucible, by a combined action with the carrier gas,

function of third purification at the level of the surface of the bath constituted by a specific plasma, evolutionary as to its composition,

- continuity between all the thermal and chemical functions.

It should be noted that the generation of arc plasma is insensitive to the injection of pulverulent metallurgical silicon, which makes it particularly efficient for this type of application. In other words, the operation and the performance of the non-transferred arc plasma torch connected to the introduction chamber are not modified, or very marginally, by the injection of the material, and therefore the efficiency of the The process is solely related, as regards plasma, to optimizing the heat and chemical transfer of the plasma from the torch to the material to be treated. Moreover, the configuration adopted guarantees that all the silicon will be effectively treated. This purification process offers a very favorable energy balance:

on the one hand the plasma and electromagnetic energy sources are respectively used for a maximum transfer of energy to the ground metallurgical silicon charge, namely

by the plasma, a first preheating of the particles "in flight" within the plasma, then a pre-melt in a bath,

by the electromagnetic induction a complete fusion and its maintenance (the electromagnetic thermal transfer is then more effective than the plasma heat transfer),

On the other hand, this method does not exhibit a thermal break (heating / cooling / heating) and in particular no silicon heating other than the initial heating, hence a bath transfer directly to the final solidification process. In a particular embodiment, and for illustrative purposes only, an industrial purification plant has the following main characteristics:

the crucible 1 has an inner diameter of order of 1.5 meters,

the height of the bath is of the order of 0.2 meters, the plasma power of the torch 3 is of the order of one megawatt, while the unit power of each side torch, 14, or 15 or 16 is of the order of 300 kW,

the power of the electromagnetic heating device is of the order of one megawatt,

the flow rate of metallurgical silicon is of the order of 500 kg / hour. Given a material yield of 80% at the outlet of the melting / purification crucible and an overall batch treatment time of 1 hour, the capacity of a purified silicon production unit is of the order of 400 kg / hour.

Claims

1. Process for purifying a silicon-based filler to obtain silicon of higher purity, characterized in that the following steps are carried out: a) a plasma dart generated by a first non-transferred arc torch (6) is sent; ) against a solid wall (7) of a volume having an outlet so that the impact of said stinger against said solid wall (7) within said volume generates a homogeneous plasma flow, b) introduced continues a charge to be treated based on silicon consisting of particles and / or grains, or ground in said homogeneous plasma flow, c) continuously directs the assembly formed by the homogeneous plasma flow in which said charge has been introduced ground, from the outlet of said volume to a crucible (1) having sidewalls and a bottom (30) and an open top, said crucible (1) having means for heating and stirring (12) said charge in the molten state, d) all of the milled feed to be processed having been introduced and a melt (13) being formed in said crucible (1), the reactive plasma jet is directed at least once second torch (14-16) to arc not transferred to the surface (17) of said bath to volatilize at least some of the impurities of the melt (13) present on the surface (17) of said bath, e) the slag is evacuated present on the surface (17) of said bath, and steps d) and e) are optionally repeated to volatilize at least some of the impurities in the bath brought to the surface (17) of said bath by stirring, f) said melt is discharged (13).

2. Method according to claim 1, characterized in that said at least one reactive plasma jet is a homogeneous and reactive plasma flow obtained by impact of the reactive plasma dart generated by said at least second non-transferred arc torch (14-16). against a solid wall (7) of another volume having an outlet, said second torch (14-16) being connected to said other volume.

3. Method according to claim 1 or 2, characterized in that the particle size of said ground feed is between 10 and 500 microns, and more preferably between 80 and 150 microns.

4. Method according to any one of claims 1 to 3, characterized in that in step b), said ground feed being introduced by means of a carrier gas, the ratio of the mass of the milled feedstock on the carrier gas mass is greater than 20.

5. Method according to claim 4, characterized in that said carrier gas being a reactive gas in contact with said homogeneous plasma flow, a first purification of said ground feed is carried out in said homogeneous plasma flow.

6. Method according to any one of claims 1 to 5, characterized in that in step c), electromagnetically stir said melt (13).

7. Method according to any one of claims 1 to 6, characterized in that in step d), said crucible (1) having a diameter D and a height H such that D / H> 5, are sent the reactive plasma jets of at least second and third non-transferred arc torches (14-16) on said surface (17) of said melt (13) to volatilize at least some of the impurities in the melt (13) present on the surface (17) of said bath.

8. Process according to any one of claims 1 to 7, characterized in that in step e), a single jet of reactive plasma is oriented on the surface (17) of said bath to impart to said slag a quantity of movement adapted to direct it to at least one discharge orifice placed on the side walls of said crucible (1), said reactive plasma jet being generated in turn or not by said other non-transferred arc torches.

9. Method according to any one of claims 1 to 8, characterized in that in step f), the molten bath thus purified is discharged from its impurities by plasma by controlling its extraction rate, its temperature of extraction and the quantity extracted.

10. Process according to claim 9, characterized in that said solidification being carried out without additional melting step, ingots having an outer shell rich in impurity-enriched silicon and a core containing the silicon of higher purity are obtained and said envelope is removed. to obtain said silicon of higher purity.

11. Method according to any one of claims 1 to 10 and 2, characterized in that increases the spatial volume of said homogeneous plasma flows to avoid projections into said crucible (1) at step c) and for treating a surface (17) of said larger melt (13) in step d).

12. Purification plant for carrying out the purification process according to any one of claims 1 to 11, characterized in that it comprises:

an insertion chamber (4) comprising at a first end a non-transferred arc plasma torch (6) having a main axis, said torch being intended to generate a plasma dart having an axis of propagation substantially centered on the main axis; said torch, said insertion chamber (4) having a bent portion having an exit orifice, said bent portion disposed downstream of said plasma torch (6) comprising a solid wall (7) so that said plasma dart enters in collision with said solid wall (7) to form a homogeneous plasma flow, - said introduction chamber (4) comprising at least one insertion port (5) placed downstream of said plasma torch (6) for the introduction continuously a crushed feedstock to be treated for mixing with said homogeneous plasma flow,

the outlet orifice of said insertion chamber (4) is placed above a crucible (1) having side walls and a bottom (30) and an open top, said crucible (1) being intended continuously receiving said assembly formed by the homogeneous plasma flow into which said milled feed has been introduced until complete introduction of the milled feed, to form a melt (13), - said crucible (1) comprising means to heat and brew

(12) said melt (13) in the molten state, one or more extraction orifices (27-29) placed on its side walls for discharging the slag and at least one discharge port for discharging said melt (13)

said plant comprising one or more other non-transferred arc plasma torches (14-16) for each generating a reactive plasma jet sent on the surface (17) of the melt (13) to volatilize at least some of the impurities of the surface of said melt (13).

13. Installation according to claim 12, characterized in that the gas of said non-transferred arc torch placed on said introduction chamber (4) is a neutral gas or a reactive gas.

14. Installation according to claim 12 or 13, characterized in that said non-transferred arc torches (6, 14-16) each comprise a downstream electrode which is an electrode flared so as to increase the spatial volume of the plasma jet or jet reactive plasma generated.

15. Installation according to any one of claims 12 to 14, characterized in that said bent portion comprises at least one portion of flared shape (9) to allow to absorb the flow of the ground feed introduced into said homogeneous plasma flow, said flared portion (9) comprising at its end the outlet of said insertion chamber (4).

16. Installation according to any one of claims 12 to 15, characterized in that each of said other non-transferred arc torches (14-16) generating a reactive plasma jet is connected to a corresponding homogenization chamber comprising a bent portion placed downstream of said corresponding plasma torch, said bent portion comprising a solid wall (7) so that said reactive plasma jet generated by said corresponding torch collides with said solid wall (7) to form a homogeneous and reactive plasma flow .

17. Installation according to claim 16, characterized in that said bent portion (18-20) of each of said homogenization enclosures comprises at least one portion of flared shape (23, 24) at the end of which is placed the outlet of said corresponding homogenization chamber.

18. Installation according to claim 16 or 17, characterized in that said installation comprises means for individually adjusting the distances separating the outlet orifices of the introduction chamber (4) and the homogenization chambers from the bottom of the crucible. (1) or the surface (17) of said melt (13) to optimize the energy balances and the extraction of impurities.

19. Installation according to any one of claims 12 to 18, characterized in that said solid wall (7) with which the plasma plunger collides is placed relative to the outlet of said torch connected to said enclosure. introduction (4), in an area where the plasma dart temperature measured in the axis of said plasma dart and in the absence of said solid wall (7) is equal to or substantially equal to half the value of the mean temperature peak of the plasma dart measured at the output of said non-transferred arc torch.

20. Installation according to claim 19, characterized in that said solid wall (7) is movable in translation relative to the outlet of said non-transferred arc torch (6).

21. Installation according to any one of claims 12 to 20, characterized in that said ground load being introduced with a carrier gas, said at least one port (5) for introducing the milled feed comprises at least one nozzle allowing a introducing said milled load into rotation.

22. Installation according to any one of claims 12 to 21, characterized in that said crucible (1) has a diameter D and a height H such that D / H> 5.

23. Installation according to any one of claims 12 to 22, characterized in that said means for heating and stirring (12) comprise one or more inductive coupling means.

24. Installation according to any one of claims 12 to 23, characterized in that said other torches (14-16) are adjustable so as to move the jet of reactive plasma that it generates, to the surface (17) of said bath melting point (13).

25. Installation according to any one of claims 12 to 24, characterized in that said installation comprises means for adjusting the composition of the plasma gas of each non-transferred arc torches during operation of the latter.

26. Installation according to any one of claims 12 to 25, characterized in that said or said discharge ports being placed in the bottom of said crucible (1), said installation comprises containers for collecting the melt (13), said containers (33-37) being placed on scrolling means (38) so as to be presented one by one under said at least one orifice until said crucible (1) is empty.

27. Installation according to claim 26, characterized in that said installation further comprises sealing means (32) sealed to connect each of said discharge ports with said container (33-37) corresponding.