EP4247756A1 - Method and plant for purifying silicon from a mixture obtained by cutting silicon bricks into wafers - Google Patents

Abstract

The invention relates to a method for reducing the metal, oxygen and carbon content, in a single melting and solidification cycle, of the micron-sized particles of silicon in the solid mixture resulting from the cutting of silicon bricks into wafers (also referred to as kerf), said method involving a device for filtering slag consisting of SiO₂ and SiC. After filtration of these solid residues, the liquid phase undergoes a step of segregating the metallic impurities of the silicon by directional solidification. The filtration system has the advantage that it can be adapted in particular to a standard industrial DSS (Directional Solidification System) furnace as used in the photovoltaic industry.

Description

DESCRIPTION

METHOD AND PLANT FOR PURIFYING SILICON FROM A MIXTURE FROM THE CUTTING OF SILICON BRICKS INTO PLATES

Technical field of the invention

The invention relates to a process making it possible to reduce the contents of metals, oxygen and carbon, in a single melting and solidification cycle, of the micron particles of silicon in the solid mixture resulting from the cutting of silicon bricks into wafers (called also kerf), said method involving a device for filtering slag consisting of SiO2 and SiC.

After filtration of these solid residues, the liquid phase undergoes a stage of segregation of the metallic impurities from the silicon by directed solidification. The filtration system has the advantage of being able to adapt in particular to a standard industrial furnace for directional solidification DSS (Directional Solidification System) as used in the photovoltaic industry.

Technical background

Photovoltaic cells are made from silicon wafers obtained by sawing silicon bricks. The brick sawing step was traditionally carried out using a steel wire and an abrasive mixture, called slurry, made up of SiC particles about ten microns in diameter. Currently, the dominant cutting technology is using diamond wires. The sawing of silicon bricks generates material losses of the order of 30 to 40% due to the saw cut and this in the form of micron particles of silicon, also called kerf. The recycling of this production scrap therefore represents an issue of cost reduction and loss of raw material.

Kerf has a much lower purity than silicon before cutting. Indeed, the kerf is highly contaminated with metals coming from the cutting wire as well as from the support on which the silicon brick is glued, which is generally a polymer containing hardening particles. In addition to metals, kerf also contains high carbon and oxygen contents, several percent by weight, from this same medium. as well as cutting fluid. In the case of slurry cutting, the kerf also contains carbon in the form of SiC particles.

Without taking into account carbon and oxygen, kerf has a purity ranging from 98% to 99.9% by weight of silicon, and cannot be recycled as is in the field of photovoltaics which requires a higher purity. (at least 99.9999% by weight of silicon). For this application, the recycling of kerf therefore requires prior purification at a lower cost in order to reduce the contents of metallic elements, oxygen and carbon.

Numerous processes have been implemented to purify the kerf and reduce the content of impurities, in particular metallic elements. We can cite, for example:

- chemical attack with acids to reduce the metal content in the kerf, with, in some cases, the use of hydrofluoric acid to eliminate the SiO2 film around the Si particles,

- electrostatic separation of solid impurities,

- the magnetic separation of iron, nickel and their oxides,

- evaporation/condensation of silicon,

- treatment by slag.

These methods are well known to those skilled in the art.

Acid attack does not significantly reduce the carbon and oxygen contents in the kerf, but other processes can reduce the concentration of these two elements. As such, mention may be made, for example, of heat treatment in air or inert gas, carbothermy under vacuum, purification by metallurgical means based on the reaction SiC + 0.5 SiO2 1.5 Si + CO making it possible to obtain a metallurgical grade silicon, separation of solid impurities using a mechanical stirrer, or even melting followed by casting in a segregation crucible.

The issue of carbon and oxygen also exists for metallurgical grade silicon (hereinafter Si MG) produced by carbothermy by reduction of silica by carbon. Indeed, the Si MG obtained contains oxygen and carbon but in lower proportions than kerf (0.3% by weight of oxygen and 0.06% by weight of carbon, compared to total weight of silicon according to D. Sarti et al., Silicon feedstock for the multicrystalline photovoltaic industry, Solar Energy Materials & Solar Cells 72 (2002) p. 27-40).

Plasma purification is another method for reducing oxygen and carbon concentrations in Si MG as described by D. Sarti et al. (Silicon feedstock for the multi-crystalline photovoltaic industry, Solar Energy Materials & Solar Cells 72 (2002) p. 27-40).

Another method consists in filtering the liquid Si MG in a graphite crucible with a density of less than 1.85 g/cm ³ provided with orifices in its bottom as described in DE341 1955. The solid impurities to be filtered are silica ( SiO2), carbon and silicon carbide (SiC). The diameter of the openings at the bottom of the crucible is very small, in the range of a few microns, so that the liquid silicon flows through the openings under the effect of capillary forces and that the solid particles of size greater than 10 microns are retained in the crucible. The graphite is not necessarily a crucible, it can be in the form of a film.

EP0160294 describes a method for separating solid reaction products, such as SiO2 and SiC, from molten Si MG produced in an arc furnace. The separation is carried out using a filter consisting of a SiC/Si plate placed at the bottom of a graphite crucible. The method is characterized in that it consists in adjusting the SiC content of the SiC/Si composite material so as to form, during filtration, in the SiC/Si layer, channels with a diameter of less than 3 microns. .

Zhang et al. (Recycling of solar cell silicon scraps through filtration, Part I: Experimental investigation, Solar Energy Materials & Solar Cells, 92 (2008) pp. 1450-1461 ), seek to recycle the upper part of the crystallized photovoltaic silicon ingots which is rejected because containing a large quantity of SiC and SiaIXk inclusions originating, respectively, from contamination by the crystallization furnace and by the crucible coated with an anti-adherent layer of SiaIXk. The inclusions present on the surface of the ingot trimmings have a size of a few millimeters, while inside the trimmings, the SiaIXk inclusions have a diameter of the order of 20 microns, and the SiC inclusions have a size less than 500 microns. The liquid silicon is purified of its inclusions thanks to a particle filter consisting of a SiC foam containing 15% AhOa used as a binder. The foam is placed at the bottom of a graphite crucible pierced with an orifice. The pore size of the foam is 0.5-3 mm, even more than 5 mm from the photographs of cross-sections given in Zhang et al. This process makes it possible to effectively filter SiaIXk particles in the form of needles several millimeters long which remain on the surface of the foam because their size is comparable to that of the pores. The SiC inclusions, mostly smaller than 200 microns, penetrate inside the foam and are retained in the pores. The filtration of these particles of size smaller than that of the pores is explained by the tortuosity of the porosity. The tortuosity creates, in fact, a flow of the liquid having recirculation loops which transport the particles towards the walls of the filter on which they adhere and remain fixed. The structure of the foam is therefore essential for the filtration of small particles. Although the filtration process is effective, the filter and the crucible lead to significant contamination of the liquid silicon, due in particular to the presence of AhOa in the filter, and the authors conclude that it is necessary to develop a process that is not contaminant to consider development on an industrial scale.

WO201 2113461 relates to a process for obtaining high purity silicon, comprising the treatment of a silicon melt. The molten silicon is poured into a filter device comprising a porous molded body whose surface consists of SiO2. The porous molded body with an SiO2 surface is prepared by impregnating a carrier consisting of zirconium oxide with SiCk and subsequent hydrolysis of the SiCk with formation of an SiO2 coating on the carrier.

JP 2014076927 describes a process for obtaining high purity silicon from a silicon sludge using a system of three crucibles and two filters. The impurity removal furnace is divided into two zones separated by a wall: a first zone whose temperature is from 1530° C. to 1650° C., comprising a first crucible and a first filter placed below the first crucible; and a second zone whose temperature is from 1410°C to 1450°C, comprising a second and a third crucible with a second filter placed below the second crucible.

The first filter allows the filtration of particles whose size is between 1 and 5mm and the second filter allows the filtration of particles of size between 0.1 and 0.5 mm.

The first and second "crucibles" described in JP 2014076927 are not crucibles in the sense of a solidification crucible but rather a receptacle provided at its bottom with a first filter and a second filter, respectively.

Despite the various existing processes, there remains a real need for a process making it possible to significantly reduce the contents of impurities such as carbon, oxygen and metallic elements, in particular oxygen and carbon, in the mixture with solid state, in particular in the form of powder, resulting from the cutting of silicon bricks into wafers, which mixture comprises silicon, oxygen, carbon and metals,

- which is simple to make, and/or

- which does not generate additional contamination of the molten silicon, and/or

- which is effective, that is to say which makes it possible to obtain silicon having a purity suitable for the field of photovoltaic but also for any other field where silicon having a purity greater than or equal to 99.9999% by weight of silicon is required (disregarding oxygen and carbon). In particular, there is a real need for a process which makes it possible to significantly reduce the contents of impurities such as carbon, oxygen and metallic elements, in particular oxygen and carbon, in the mixture in the solid state, in particular in the form of powder, resulting from the cutting of silicon bricks into wafers, which mixture comprises silicon, oxygen, carbon and metals, which implements an effective filtration system making it possible to obtain a mixture of purity high containing 99.9999% by weight of silicon (without taking oxygen into account) and not producing contamination by itself, in particular by metals such as Al, Fe, Ti, Cr, Zr, Ni etc. and/or other doping elements such as B, P etc., which process can be used in industrial processes for the purification and/or recycling of silicon.

In the context of the present invention, when the purity of a mixture is expressed in % by weight of silicon "without taking into account oxygen and carbon", this means that the mixture comprises the % by weight of silicon and possibly impurities other than oxygen and carbon, such as for example metallic elements or doping elements.

Summary of the invention

The purpose of the present invention is precisely to meet these needs by providing a process for purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, which mixture comprises silicon , oxygen, carbon and metals, with a silicon content of at least 96% by weight, an oxygen content greater than or equal to 1% by weight and a carbon content greater than or equal to 0.1 % by weight, relative to the total weight of the mixture, characterized in that it comprises the steps in which a) the mixture is brought to a temperature between 1450 and 1650°C; b) the mixture is filtered through a filtration device comprising a bottom provided with through holes with a diameter of between 0.5 and 2 mm and preferably between 1 and 1.5 mm, and a surface density of between 0, 2 and 2 cm ^-2 , preferably between 0.5 and 1 cm ^-2 ; and c) the filtered liquid phase of the mixture is subjected to controlled solidification. By "through holes" is meant holes which do not exhibit tortuosity from one end to the other, and through which one can see.

The mixture in the solid state is introduced into the filtration device and to be brought there to a temperature between 1450 and 1650°C.

The invention also relates to an installation for the purification of silicon from a solid mixture, in particular in the form of powder, resulting from the cutting of silicon bricks into wafers and comprising silicon, oxygen, carbon and optionally metals, the silicon content being at least 96% by weight, the oxygen content being greater than or equal to 1% by weight and the carbon content being greater than or equal to 0, 1% by weight, relative to the total weight of the mixture, said installation comprising

- means capable of heating the mixture beyond the melting point of the silicon; and

- Filtration means capable of filtering the mixture, the filtration means comprising through-holes with a diameter of between 0.1 and 5 mm, preferably between 0.5 and 2 mm.

The installation of the invention further comprises means capable of carrying out a directed solidification of the filtered liquid phase of the mixture.

In the method of the invention, the filtration is advantageously carried out through a single filtration device comprising a bottom provided with through holes preferably all having the same diameter, between 0.5 and 2 mm and preferably between 1 and 1 .5mm.

In addition, advantageously, the installation according to the invention comprises only one heating zone.

Another object of the invention relates to the use of a process for purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, according to the invention. , or an installation according to the invention, for the manufacture of photovoltaic cells.

The invention also relates to a method for manufacturing photovoltaic cells implementing a step of purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, according to the method of the invention, or an installation according to the invention.

Brief description of figures

Other characteristics and advantages of the invention will appear during the reading of the detailed description which will follow for the understanding of which reference will be made to the appended drawings in which: [Fig 1] represents an embodiment of the method of the invention in which the filtration device in the form of a crucible is placed above another crucible within which the directed solidification of the mixture is carried out.

[Fig 2] represents an embodiment of the method of the invention in which the filtration device in the form of a crucible is placed at the bottom of another crucible within which the directed solidification of the mixture is carried out.

[Fig 3] represents an embodiment of the method of the invention in which the filtration device in the form of a crucible is coupled to a continuous kerf supply system.

[Fig 4] represents an embodiment of the method of the invention in which the filtration device comprises a plurality of unitary filtration devices in the form of crucibles.

[Fig 5] represents an embodiment of the method of the invention in which the filtration device in the form of a crucible comprises an inert gas blowing system consisting of a graphite tube placed in its center. This system being above the mixture/kerf, it will favor the evacuation of the oxygen dissolved in the mixture in the form of the volatile species SiO.

Detailed description of the invention

The present invention relates to a method for purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, which mixture comprises silicon, oxygen, carbon and metals, with a silicon content of at least 96% by weight, an oxygen content greater than or equal to 1% by weight and a carbon content greater than or equal to 0.1% by weight, based on the total weight of the mixture, characterized in that it comprises the steps in which a) the mixture is brought to a temperature between 1450 and 1650°C; b) the mixture is filtered through a filtration device comprising a bottom provided with through holes with a diameter of between 0.5 and 2 mm and preferably between 1 and 1.5 mm, and with a surface density of between 0.2 and 2 cm ^-2 , preferably between 0.5 and 1 cm ^-2 ; and c) the filtered liquid phase of the mixture is subjected to controlled solidification. The solid mixture, in particular in powder form, resulting from the cutting of silicon bricks into wafers, comprises silicon, oxygen, carbon and metals, with a silicon content of at least 96% by weight, an oxygen content greater than or equal to 1% by weight and a carbon content greater than or equal to 0.1% by weight, relative to the total weight of the mixture. In the following description, the term "kerf" can also be used to designate this mixture.

The mixture used as defined above is solid, in particular in powder form. The particle size of this powder is generally such that the average diameter of the particles can range from 0.1 to 10 microns. The method comprises a step c), in which the liquid phase filtered at the end of step b) is subjected to directed solidification.

During step c), the liquid phase filtered at the end of step b) undergoes an operation of segregation of metallic impurities by directed solidification. The segregation of metallic impurities by directional solidification is a process well known to those skilled in the art. Directed solidification can be implemented, for example, according to the HEM method (Heat Extraction Method in English) described by Khattak and Schmid (Growth of silicon ingots by HEM for photovoltaic applications, Silicon Processing for Photovoltaics II, edited by C.P. Khattak and K.V. Ravi, Elsevier Science Publishers B.V., 1987). In the case of silicon, it is known that this segregation is effective if the solidified silicon is single-phase, that is to say not containing precipitates or inclusions. The segregation of the metallic impurities will therefore be inoperative if the mixture, in particular the silicon which is present therein, contains oxygen and carbon contents in the percent range. Indeed, given the fact that these contents are well above the solubility limit of oxygen and carbon in the silicon in the molten state, the liquid will contain inclusions of SiO2 and SiC.

The process of the invention relates to the purification of silicon from the mixture or from kerf containing silicon contents of at least 96% by weight, oxygen contents greater than or equal to 1% by weight and carbon contents greater than or equal to 0.1% by weight, relative to the total weight of the mixture. The mixture/kerf can come from the cutting of silicon bricks, with slurry or with diamond wire. The method consists in carrying out a filtration of the solid particles of SiO2 and SiC, then a segregation, by directed solidification, of the silicon metals present in the filtered liquid, the filtration and segregation operations advantageously being able to be carried out during the same cycle. heating/cooling in a standard DSS (Directional Solidification System) industrial furnace used in the photovoltaic industry.

The geometry of the filtration device used is adapted to the volume of kerf or mixture.

The filtration device used in the process of the invention has the advantage of being able to adapt in particular to a standard industrial furnace for directed solidification DSS (Directional Solidification System) as used in the photovoltaic industry.

Preferably, the filtration device is in the form of a crucible. In a preferred embodiment of the invention, for the filtration step, a filtration device is used in the form of a crucible comprising a bottom provided with through holes with a diameter of between 0.5 and 2 mm, and preferably between 1 and 1.5 mm.

In the case where a small quantity of kerf is used, which, upon melting, would form a drop, the filtration device can be in the form of a plate. In this case, a continuous supply of kerf can be considered. By "through holes" is meant holes through which one can see and which exhibit no tortuosity from one end to the other.

In the mixture (or kerf), in solid form, in particular in powder form, the silicon particles have a micron size, which is also the case for compounds based on carbon and oxygen.

By "micron size" particles is meant particles having a diameter of 0.1 to 10 microns.

Indeed, the oxygen is mainly found in the form of a SiC film around the Si particles. The carbon coming from the cutting fluid is in the form of organic species grafted onto the Si particles. The carbon is also found in the form of micron particles of polymer which constitutes the support for the silicon bricks and ingots during cutting and which is partially cut. For slurry cutting, the carbon is also in the form of SiC particles, the size of which is around 10 microns. Against all expectations, millimetric holes allow effective filtration of the precipitates present in the molten kerf. Indeed, during heating and melting, oxygen and carbon react with silicon to form SiO2 and SiC which are stable compounds at high temperature in molten silicon. The SiO2 particles agglomerate between them during fusion, and the SiC particles adhere to SiO2, the whole forming large agglomerates, from a few mm to a few cm which are retained inside the filter even if the holes are millimeter in size. The filtration device, in particular in the form of a crucible, is made of a material chosen from the group consisting of graphite, silicon carbide (SiC), silicon nitride (SiNa), silica (SiC), or a mixture of these materials, or a mixture of graphite with silicon carbide (SiC), silicon nitride (SiNa), and silica (SiC). In the case where the filtration device is made of a mixture of graphite with silicon carbide (SiC), silicon nitride (SiNa), and silica (SiOa), it may be in the form of a multilayer. As such, one can, for example, cite a graphite filter with a thickness of a few millimeters to 1 cm, covered, either on all its faces, or on its internal faces in contact with the kerf, with a layer of SiC or SiaIXk or SiO2, 10 to 500 microns thick. This type of filter is described in patent application FR1661788.

For the photovoltaic application, the filter is preferably made of graphite, in particular of isostatic graphite for its mechanical properties, in particular its mechanical resistance during the infiltration of liquid silicon into its porosity. The isostatic graphite may for example be the 2020 grade from Mersen having a porosity of 9% and a bending strength of 45 MPa, or the R7500 grade from SGL having a porosity of 14% and a bending strength of 50 MPa. For applications aiming for a silicon purity of less than 99.9999% by weight of silicon (without taking into account oxygen and carbon), the filter can be made of another refractory ceramic material, such as for example Al2O3 or ZrO2 .

The size of the filter device holes is such that the holes are easily made with standard machining tools. As already indicated, the diameter of the holes is between 0.5 and 2 mm, and preferably between 1 and 1.5 mm.

The surface density of holes must be within a range of optimal values. Indeed, the holes must not be too close together otherwise the filter risks cracking during the machining of the holes or during its use under the effect of the stresses generated by the infiltration of the liquid silicon into the pores of the graphite. and its transformation into silicon carbide. On the other hand, the holes must not be too far apart, otherwise the flow rate of the filtered liquid silicon will be low and the productivity of the purification process mediocre.

The volume flow rate of the liquid Q through the filtration device is given by the formula Q = V.St.nt.Sf, where V is the flow velocity of the liquid, St the cross section of a hole (St = K .dt ² /4, with dt the diameter of the hole), nt the surface density of holes, and St the internal section of the filter.

Assuming an inertial regime, i.e. neglecting the viscous forces and the surface tension forces of the liquid, the maximum value of the flow velocity is given by the formula V = (2 .gH) ^1/2 , where p is the bulk density of liquid silicon, g the acceleration of gravity, and H the height of liquid silicon in the filter. In the field of photovoltaics, the standard masses of silicon ingots manufactured by directional solidification are for example 13 kg (furnace of size called Gen 1), 60 kg (furnace of size called Gen 2) or even 650 kg on an industrial scale (furnace of size called Gen 6). For heights of liquid silicon H in the filter ranging from 18 cm for Gen 1 to 25 cm for Gen 6, the inventors have found that for holes with a diameter of between 0.5 mm and 5 mm, a surface density of holes of between between 0.2 cm ^-2 and 2 cm ^-2 makes it possible to obtain a maximum flow rate of liquid Q at the start of the flow from a few tenths to a few tens of kg per second. For masses of silicon ranging from Gen 1 to Gen 6, this range of flow rates corresponds to a minimum filter emptying time ranging from a few seconds to a few hundred seconds, thus ensuring good productivity of the purification process.

For the filtration step, a filtration device is used in the form of a crucible comprising a bottom provided with a surface density of holes of between 0.2 cm ^-2 and 2 cm ^-2 , preferably between 0.5 cm ^-2 and 1 cm ^-2 . The thickness of the filtration device could condition its mechanical strength. Typical thicknesses can range from 5mm to 20mm. The solid state mixture may be introduced into the filtration device prior to the heating cycle. Alternatively, the method may include a step of introducing the mixture/kerf into the filtration device when the maximum temperature is reached in the furnace.

The mixture is brought to a temperature greater than or equal to the melting point of silicon, in particular to a temperature between 1450 and 1650°C, preferably between 1500°C and 1600°C.

The duration of the plateau at the maximum temperature depends on the maximum temperature and the quantity of kerf/mixture. For example, for a maximum temperature of 1535°C and a mass of kerf/mixture of 10 kg, the stage duration is approximately 1 hour. Those skilled in the art will be able to determine the duration of this plateau based on the maximum temperature and the amount of kerf/mixture.

After filtration step b), the filtered liquid phase, freed in particular of slag consisting of SiO2 and SiC, is subjected to controlled solidification, during which the segregation of the impurities, in particular metal impurities, of the mixture is obtained.

Directed solidification is a method well known to those skilled in the art. It can be carried out according to the HEM method (Heat Extraction Method in English) described by Khattak and Schmid (Growth of silicon ingots by HEM for photovoltaic applications, Silicon Processing for Photovoltaics II, edited by CP Khattak and KV Ravi, Elsevier Science Publishers BV, 1987). The method of the invention may comprise a step d) of cooling, in particular to ambient temperature, of the silicon in the solid state obtained at the end of step c).

Ambient temperature means a temperature of 20°C + 5°C. The method may also comprise a step e) of recovering the silicon in the solid state purified after the cooling of step d). The expression "after cooling" means the return to ambient temperature, i.e. to a temperature of 20°C + 5°C. After filtration and at the end of cooling, a thin layer of silicon remains at the bottom of the capillary filtration device, and the slag adheres little to the surface of said device so that they can be removed manually and the device filtration can be reused. According to one embodiment of the invention, the filtration device is placed above another crucible, on which it rests, in a photovoltaic silicon segregation or crystallization furnace as shown in [Fig 1]. Kerf charge is placed in the filtration device and, on melting, the liquid phase comprising the silicon and the metallic impurities flows through the holes present in the bottom of said device, while the slag is retained in said device. A cooling ramp is then applied to allow the controlled solidification of the silicon within the crucible and the segregation of the impurities present in the dissolved state in the initial mixture comprising the silicon.

In another embodiment of the invention, the filtration device initially rests at the bottom of another crucible as shown in [Fig 2]. The volume of the filtration device is adjusted to contain only the slag to be filtered. The kerf/mixture is loaded into the filtration device and the crucible. At the end of the melting stage, the filtration device is raised above the filtered mixture by mechanical means using rods, in particular made of graphite. The advantage of this embodiment is that for the same volume of crucible, it makes it possible to filter much larger quantities of kerf than in the case of the embodiment illustrated in [Fig 1] where only the filter is loaded with kerf . In another embodiment of the invention, the filtration device is coupled to a continuous kerf supply system. The capacity of the filtration device is in this case reduced, the volume of the device being adjusted to contain only the slag to be filtered. Such a continuous supply system is well known to those skilled in the art, for example for the growth of monocrystalline silicon ingots by Czochralski pulling in order to increase the productivity of the process. [Fig 3] shows an example of this embodiment in which the filtration device is arranged above another crucible, on which it rests, this other crucible being intended for the segregation of the metallic elements of the liquid phase filtered from the mixture.

According to another embodiment of the invention, the filtration device comprises a plurality of unitary filtration devices resting on bars, in particular of graphite, themselves resting on the side plates, in particular of graphite, holding the crucible as shown in [ Fig 4], This filtration device is intended in particular for large size crucibles for the production of industrial size silicon ingots.

In another embodiment of the invention, the interior walls of the filtration device, or the interior and exterior walls of the filtration device, can be covered with an anti-adherent deposit based on SiaIXk powder, such as conventionally used for photovoltaic silicon crystallization crucibles.

In another embodiment of the invention, the filtration device comprises an additional blowing system making it possible to inject neutral gas above the filtered liquid phase contained in the crucible. In this embodiment, the neutral gas can, for example, be injected through a graphite tube placed in the center of the filter as shown in [Fig 5].

So that the kerf powder does not escape through the holes of the filtration device when it is loaded into said device and during the heating step, silicon wafers or pieces of broken silicon wafers can advantageously be placed at the bottom of the device filtration to cover the holes. These wafers are then made of photovoltaic or microelectronic grade silicon. Their thickness is advantageously at least 100 microns.

To increase the amount of kerf loaded into the filtration device and thus improve the productivity of the process, the kerf can be densified beforehand by compaction techniques, for example isostatic compaction, or by agglomeration techniques, for example agglomeration by high shear mixing or agglomeration by spray drying. These techniques are well known to those skilled in the art.

As already indicated, the process of the invention makes it possible to reduce the contents of metals, oxygen and carbon in the kerf, in a single melting and solidification cycle, which is particularly advantageous industrially compared to the processes of the state of the art.

More particularly, in the method of the invention, the filtration device fits into an industrial DSS furnace for the segregation or crystallization of photovoltaic silicon. This is not the case for the melting and pouring process in a segregation crucible, as described in WO201 2113461.

In the invention, it is sought to purify the silicon from a mixture in the solid state, in particular in the form of powder, resulting from the cutting of silicon bricks into wafers, which mixture comprising silicon, oxygen , carbon and metals, with a silicon content of at least 96% by weight, an oxygen content greater than or equal to 1% by weight and a carbon content greater than or equal to 0.1% by weight, by relative to the total weight of the mixture. Such a mixture does not have the same characteristics as MG silicon and scrap crystallization ingots. Si MG contains oxygen and carbon in lower proportions than kerf (0.3% by weight of oxygen and 0.06% by weight of carbon according to D. Sarti et al., Silicon feedstock for the multi- crystalline photovoltaic industry, Solar Energy Materials & Solar Cells 72 (2002) p. 27-40), and scrap from photovoltaic silicon crystallization ingots do not contain precipitates oxygen but precipitates of SiaIXk with a size of 20 microns, and precipitates of SiC mainly with a size of less than 200 microns.

Thus, contrary to the process of the invention where the holes of the filtration device have a diameter in the range of one millimeter, the filters of the state of the art for the filtration of liquid Si MG have holes of small size, of a few microns in DE 3411955 and with a diameter of less than 3 microns in EP 0160294. The silicon seeps into micron-sized holes which can be blocked by inclusions penetrating inside.

In the SiC foam filtration system described in Zhang et al. (Recycling of solar cell silicon scraps through filtration, Part I: Experimental investigation, Solar Energy Materials & Solar Cells, 92 (2008) pp. 1450-1461) for the filtration of scrap ingots from the crystallization of photovoltaic Si, the tortuosity of the Foam porosity is required for filtration of particles that are small in size. The tortuosity in fact creates a flow of the liquid having recirculation loops which transport the particles towards the walls of the filter on which they adhere and remain fixed. However, in the process of the invention, the solid mixture, in powder form, resulting from the cutting of silicon bricks into wafers, initially contains a high oxygen content (an oxygen content greater than or equal to 1% by weight) relative to the total weight of the mixture. Oxygen is transformed into SiÛ2 during heating and melting, and the agglomeration of SiÛ2 leads to large particles, from a few millimeters to a few centimeters. Furthermore, the filtration device used in the process of the present invention is provided with through holes, that is to say holes which do not present any tortuosity from one end to the other, and through which one can see. The filtration device in the method of the invention is therefore simpler to produce than the SiC foam filter described by Zhang et al., and unlike the latter, the filtration device of the invention does not generate contamination. liquid silicon.

The process of the invention makes it possible to obtain a mixture having the purity required for the photovoltaic application (up to 99.9999% by weight of silicon or more, without taking into account oxygen and carbon). The invention also relates to an installation for the purification of silicon from a solid mixture, in particular in the form of powder, resulting from the cutting of silicon bricks into wafers and comprising silicon, oxygen, carbon and optionally metals, the silicon content being at least 96% by weight, the oxygen content being greater than or equal to 1% by weight and the carbon content being greater than or equal to 0.1% by weight, relative to the total weight of the mixture, said installation comprising:

- means capable of heating the mixture beyond the melting point of the silicon; and

- Filtration means capable of filtering the mixture, the filtration means comprising through-holes with a diameter of between 0.5 and 2 mm, and preferably between 1 and 1.5 mm.

The installation of the invention further comprises means capable of carrying out a directed solidification of the filtered liquid phase of the mixture.

Another object of the invention relates to the use of a process for purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, according to the invention. , or an installation according to the invention, for the manufacture of photovoltaic cells.

The invention also relates to a method for manufacturing photovoltaic cells implementing a step of purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, according to the method of the invention, or an installation according to the invention.

The invention, although described for a mixture resulting from the cutting of bricks, could also apply to a mixture resulting from the cutting of ingots. In the case of the cutting of ingots, the mixture used is solid, in particular in the form of powder. The particle size of this powder is generally such that the average diameter of the particles can range from 80 to 100 microns.

Unless otherwise stated, the expression "comprising/comprising a" should be understood as "comprising/comprising at least one". Unless otherwise stated, the expression "between ... and ..." must be understood as included limits.

Unless otherwise stated, the expression “included between ranging from ... to ...” must be understood as inclusive limits.

EXAMPLE

Purification is carried out according to the method of the present invention on kerf containing 4.3% by weight of oxygen, and 1.8% by weight of carbon, relative to the total weight of the kerf. Kerf has a purity of 99.955% by weight of silicon (without taking into account oxygen and carbon), and is previously compacted in the form of granules a few millimeters in diameter.

18 kg of kerf are loaded into a graphite filter (grade R7550 from SGL) resting on a silica crucible coated with a layer of SiaIXk according to the configuration of [Fig 3].

The filter has an internal section of 1246 cm ² and is pierced at its bottom with 529 holes 1.5 mm in diameter. The filter and the crucible are placed in a DSS silicon segregation furnace for photovoltaic application with a capacity of 60 kg.

The thermal cycle consists of a rise to 900°C under vacuum (residual pressure of ^10'1 mbar), a rise to 1500°C under argon (partial pressure of 600 mbar) and a plateau at 1500°C for 6 hours. The melting of the silicon in the mixture begins during the ramp-up and ends after a 45-minute plateau. Then, 4.2 kg of solid kerf in the form of granules are recharged in the filter by means of the supply system represented in FIG. 3. This recharge is carried out 10 times in succession, every 25 minutes. At the end of the plateau at 1500°C, the liquid silicon has completely flowed into the crucible through the filter holes. A cooling ramp is then applied allowing a directed solidification of the silicon contained in the crucible at a speed of the order of 1 cm/h.

The solidified silicon ingot has a mass of 43 kg and the material yield, ie the ratio between the mass of filtered silicon and the initial mass of kerf (60 kg), is 72%. The segregated part of the ingot contains 0.003% by weight of oxygen and less than 0.001% by weight of carbon, relative to the total weight of the ingot. The purity of the ingot is 99.9999% by weight of silicon, relative to the total weight of the ingot (without taking into account oxygen and carbon).

Claims

1 . Process for purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, which mixture comprises silicon, oxygen, carbon and metals, with a in silicon of at least 96% by weight, an oxygen content greater than or equal to 1% by weight and a carbon content greater than or equal to 0.1% by weight, relative to the total weight of the mixture, characterized in that that it comprises the steps in which a) the mixture is brought to a temperature of between 1450 and 1650°C; b) the mixture is filtered through a filtration device comprising a bottom provided with through holes with a diameter of between 0.5 and 2 mm and preferably between 1 and 1.5 mm, and a surface density of between 0, 2 and 2 cm ^-2 , preferably between 0.5 and 1 cm ^-2 ; and c) the filtered liquid phase of the mixture is subjected to controlled solidification. 2. Method according to claim 1, characterized in that the filtration device is made of a material chosen from the group consisting of graphite, silicon carbide (SiC), silicon nitride (SiNs), silica (SiC) , or a mixture of these materials, or a mixture of graphite with silicon carbide (SiC), silicon nitride (SiNs), and silica (SiC ). 3. Method according to one of claims 1 or 2, characterized in that the filtration device is in the form of a crucible. 4. Method according to any one of claims 1 to 3, characterized in that the mixture in the solid state is introduced into the filtration device and to be brought there to a temperature of between 1450 and 1650°C. 5. Process according to any one of claims 1 to 4, characterized in that the mixture is brought to a temperature of between 1500 and 1600°C. 6. Method according to any one of claims 1 to 5, characterized in that the filtration device is arranged above a crucible, on which it rests, in a furnace for the segregation or crystallization of photovoltaic silicon. 7. Method according to any one of claims 1 to 5, characterized in that the filtration device initially rests at the bottom of the crucible and is raised above the filtered mixture by mechanical means using rods in particular made of graphite , at the end of the fusion stage. 8. Method according to any one of claims 1 to 7, characterized in that the filtration device is coupled to a continuous kerf/mixture supply system. 9. Method according to any one of claims 1 to 5 and 8, characterized in that the filtration device comprises a plurality of unit filtration devices resting on bars, in particular of graphite, themselves resting on the side plates, in particular of graphite, now the crucible. 10. Method according to any one of claims 1 to 5 and 8, characterized in that the filtration device has in its center an additional blowing system making it possible to inject neutral gas above the filtered liquid phase contained in the crucible during directional solidification. 11 . Installation for the purification of silicon from a solid mixture, in particular in powder form, resulting from the cutting of silicon bricks into wafers and comprising silicon, oxygen, carbon and possibly metals, the silicon content being at least 96% by weight, the oxygen content being greater than or equal to 1% by weight and the carbon content being greater than or equal to 0.1% by weight, relative to the total weight of the mixture, characterized in what it contains - means capable of heating the mixture beyond the melting point of the silicon; - Filtration means capable of filtering the mixture, the filtration means comprising through holes with a diameter of between 0.5 and 2 mm, and preferably between 1 and 1.5 mm; and - means capable of carrying out a directed solidification of the filtered liquid phase of the mixture. 12. Use of a process for purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, according to any one of claims 1 to 10, or from an installation according to claim 11, for the manufacture of photovoltaic cells. 13. A method of manufacturing photovoltaic cells implementing a step of purifying a mixture in the solid state, in particular in powder form, resulting from the cutting of silicon bricks into wafers, according to the method of claims 1 to 10 , or an installation according to claim 11.