Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
  • C01B33/037—Purification
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
  • C01B33/021—Preparation
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/08—Compounds containing halogen
  • C01B33/107—Halogenated silanes
  • C01B33/1071—Tetrachloride, trichlorosilane or silicochloroform, dichlorosilane, monochlorosilane or mixtures thereof
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2002/00—Crystal-structural characteristics
  • C01P2002/50—Solid solutions
  • C01P2002/52—Solid solutions containing elements as dopants
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2006/00—Physical properties of inorganic compounds
  • C01P2006/80—Compositional purity

Definitions

• the present invention relates to the silicon production line for the photovoltaic industry. It relates in particular to silicon granules, derived from the recycling of silicon ingot cutting waste ("kerfs") and especially adapted for the preparation of trichlorosilane (TCS).
• kerfs silicon ingot cutting waste
• TCS trichlorosilane
• the high-purity silicon manufacturing chain required for the photovoltaic industry is a complex succession of metallurgical and chemical processes.
• metallurgical silicon comes from a carboreduction reaction of quartz mixed with reducing agents such as reactive coal or charcoal, always accompanied by wood .
• the mixture is heated to very high temperatures in an electric arc furnace. It undergoes different refinements until solidified metallurgical silicon is obtained, either in the form of ingots, which will then be crushed, or in the form of granules, of a few hundred microns in average diameter.
• Metallurgical silicon has many impurities:
• Doping type for example phosphorus (P) and boron (B), in proportions greater than 20 ppm; of organic type (C) or oxygen (O), in proportions greater than 100 ppm.
• trichlorosilane HSIC13, called TCS
• the chlorination process is carried out in a fluid bed reactor, wherein the granules or crushed MG-Si are contacted with gaseous hydrogen chloride.
• the temperature and the pressure in the reactor are respectively of the order of 300 ° C. and 4 bars.
• TCS is formed in a majority proportion; are also formed of silicon tetrachloride (STC) and other chlorinated compounds containing silicon or impurities.
• the alternative hydrochlorination process is also carried out in a fluid bed reactor at a higher temperature and pressure (450 ° C, 10 to 50 bar). Granules or crushed metallurgical silicon are contacted with STC (SiC14) and hydrogen. At the outlet, TCS is formed in less proportion to the chlorination reaction; other chlorinated compounds containing silicon or impurities are also produced.
• the "impure" TCS resulting from one or other of the chlorination or hydrochlorination processes is then purified by a succession of distillation steps. These steps are extremely lengthy and represent a high share of investment and operating costs in the silicon manufacturing chain. In particular, impurities of the dopant type (P, B) are very difficult to eliminate and a high reflux ratio (typically greater than 100) in the columns of distillation is necessary to achieve the required purity of the TCS.
• the purified TCS can then be processed into high purity silicon chips or grains as the raw material for drawing photovoltaic grade (PV) ingots.
• This raw material is commonly called PCS (Poly Cristallin Silicon).
• a first way to form the PCS consists in the decomposition of the TCS at high temperature (about 1200 ° C.) in a Siemens bell reactor ("bell jar reactor"); the silicon is gradually deposited on a filament disposed inside the reactor, forming at the end of the process a silicon bar of a purity typical of 6N (> 99.9999%) to one. The bar of high purity is then crushed to give large pieces of silicon ("chunks") which will be melted for the drawing of ingots.
• An alternative route is to convert the TCS to monosilane (SiH4) and then to decompose the monosilane in a fluid bed reactor.
• SiH4 contacted with fine silicon seeds allows the formation of high purity silicon grains. Silicon grains larger than 400 microns can be melted with chunks for drawing photovoltaic quality ingots. Silicon grains smaller than 400 microns, which represent approximately 10% of the production, can not be used in particular because of the flights, incompatible with the electrical and mechanical components of the equipment for the ingot draw and because of their high proportion of oxide which reduces the efficiency of the printing processes.
• the PV quality silicon ingots then undergo several cutting steps: firstly, a blank of the ingot blanks to define a parallelepiped block, and secondly slicing the ingot. These cuts are at the origin of significant losses (kerf), of the order of 40 to 50% of the raw material of high purity silicon.
• the present invention relates to an alternative solution for recycling silicon waste.
• the invention relates in particular to silicon granules derived from the recycling of kerfs and in particular adapted to the preparation of TCS.
• the invention also relates to a method of manufacturing silicon granules.
• the invention relates to a silicon granule, especially suitable for the preparation of trichlorosilane (TCS), which has a size of between 10 and 500 microns, and which comprises:
• Dopants among which phosphorus or boron, in a mass fraction of less than 5 ppm;
• At least one co-catalyst chosen from iron, aluminum and calcium, in a mass fraction of between 1 and 2500 ppm;
• Metal impurities with the exclusion of at least one co-catalyst, in a mass fraction of less than 50 ppm.
• the mass fraction of oxygen is less than 100 ppm.
• the invention also relates to a pulverulent preparation comprising silicon granules such as above.
• the average size of the granules in said powdery preparation is between 50 and 400 microns.
• the size of the silicon granules in the pulverulent preparation is greater than 50 microns.
• the invention also relates to a method of manufacturing silicon granules as above, comprising:
• Step e) comprises a step of granulation by rapid cooling of drops of liquid silicon
• Step e) comprises pouring the liquid silicon into a mold configured to allow rapid cooling to form a solidified silicon block
• the solidified silicon block is crushed to form the silicon granules
• the manufacturing method comprises a step f) of separation by sieving or by flight, for sorting the silicon granules by size;
• step d) the at least one cocatalyst chosen from iron, aluminum and calcium is introduced in the form of metal or metal alloy in the liquid silicon bath;
• Step a) comprises the supply of silicon grains smaller than 400 microns, originating from a fluid bed reactor based on the monosilane decomposition, the grains being mixed with the silicon waste;
• step c) grains of silicon smaller than 400 microns, derived from a fluid bed reactor based on the monosilane decomposition, are melted with the powder of the silicon particles;
• step a) includes the supply of decommissioned substrates from the microelectronics or photovoltaic industry, crushed and mixed with the silicon waste resulting from diamond-wire cutting of photovoltaic quality ingots; step c) crushed pieces of decommissioned substrates from the microelectronics or photovoltaic industry are melted with the powder of the silicon particles.
• the invention finally relates to a process for obtaining trichlorosilane (TCS) by chlorination or hydrochlorination, using a pulverulent preparation as above.
• Table 1 shows the typical composition of metallurgical silicon according to the state of the art, composition measured by mass spectrometry glow discharge;
• Tables 2a, 2b, 2c show examples of silicon granule compositions according to the invention, composition measured by glow discharge mass spectrometry;
• Table 3 shows the typical composition of silicon particles in a powder resulting from the chemical treatment step of the manufacturing method according to the invention, composition measured by glow discharge mass spectrometry;
• FIG. 1 schematically shows the chemical treatment step of the manufacturing method according to the invention
• FIG. 2 shows an example of size distribution of silicon particles in a powder resulting from the chemical treatment step of the manufacturing method according to the invention.
• the invention relates to a silicon granule specially adapted for optimizing the production of trichlorosilane (TCS) in chlorination or hydrochlorination processes.
• TCS trichlorosilane
• granule in the context of the present invention should be understood in the broad sense, that is to say corresponding to a grain or particle of small size, likely to have different shapes including spherical, rounded, elongated or angular.
• the silicon granule according to the invention has a size of between about 10 microns and about 500 microns. What is called granule size here is its "equivalent diameter of Sauter".
• the "equivalent diameter of Sauter” is the diameter of the sphere which would behave identically during a grain size measurement by a defined technique. By way of example, mention may be made of a Malvern laser diffraction measurement technique.
• the silicon granule comprises a small amount of dopants, in particular phosphorus and boron dopants; each dopant represents a mass fraction less than 5 ppm. Note that the unit "ppm" (part per million) will be used in the remainder of the description as always relating to a mass fraction.
• the silicon granule also contains in a small amount (less than 5 ppm) other dopants, for example arsenic or antimony.
• the silicon granule according to the invention further comprises at least one co-catalyst chosen from iron, aluminum and calcium, the mass fraction of which is adjustable between 1 and 2500 ppm, advantageously between 100 and 2000 ppm.
• a co-catalyst is an impurity whose presence is necessary in the silicon matrix to favor in particular a chlorination or hydrochlorination reaction. These co-catalysts are generally present in intermetallics at the silicon grain boundaries.
• the metallic impurities, apart from the at least one co-catalyst, such as, for example, titanium, nickel, zinc, chromium, magnesium, manganese, vanadium, etc., are present in the low silicon granule. amount, corresponding to mass fractions for each impurity less than 50 ppm, or even less than 30 ppm, or even less than 10 ppm.
• a pulverulent preparation comprising silicon granules according to the invention is particularly favorable for the manufacture of TCS by the chlorination or hydrochlorination processes, for the reasons which will now be set forth.
• the small mass fractions of dopants and metallic impurities (excluding co-catalysts) contained in the silicon granules make it possible to drastically limit the number of distillation cycles necessary for the purification of the TCS.
• the distillation of phosphorus and boron compounds is particularly long and complex: the granules or crushed metallurgical silicon usually used in incoming materials for chlorination or hydrochlorination processes typically contain 70 to 100 ppm of phosphorus and 50 to 70 ppm of boron. (Table 1).
• the silicon granules according to the invention comprise less than 5 ppm of each of these dopants.
• Table 2a shows a table illustrating an example of a composition of a silicon granule according to the invention, measured by glow discharge mass spectrometry (GDMS): boron is present at 0.4 ppm and phosphorus at 2 ppm is more than 30 times lower than in metallurgical silicon.
• GDMS glow discharge mass spectrometry
• the reactivity of the silicon granules in the fluid bed, during the chlorination and hydrochlorination reactions is a function of several parameters other than pressure and temperature.
• pressure and temperature are parameters other than pressure and temperature.
• the reaction is as follows: Si + 23 ⁇ 4 + 3 STC 4 TCS + impurities and co-products.
• the most commonly used catalyst is copper.
• the aluminum and iron contents of the granules of the pulverulent preparation according to the invention are then optimized for maximum reaction rate and highest TCS selectivity.
• a pulverulent preparation adapted to a chlorination process will advantageously have a limited iron content, since the iron slows down the selectivity in TCS.
• the granules of the pulverulent preparation may, for example, have the composition of Table 2b.
• the most active co-catalysts will be iron and aluminum, the amounts of iron to be significantly higher than for chlorination. Copper is also commonly used to catalyze the reaction.
• a pulverulent preparation adapted to a hydrochlorination process may have the composition of Table 2c.
• silicon with a low aluminum content for example 1000 ppm
• the silicon granules comprise the (at least one) co-catalyst in a mass fraction of between 1 and 2500 ppm so as to catalyze efficiently, but without unnecessarily re-polluting the silicon: the process for manufacturing the TCS at This type of granule is thus more efficient, thanks to improved reactivity and faster purification, allowing significant gains in energy and production capacity.
• the average size of the silicon granules in the pulverulent preparation is between 50 and 400 microns.
• medium size also called d50, we means the size that is greater than the size of 50% by volume of the granules and smaller than the size of 50% by volume of the granules.
• the reactivity of the silicon granules in the fluid bed of chlorination or hydrochlorination is also a function of the specific surface area of the particles, linked to their d50.
• a d50 between 50 and 400 microns is quite suitable for a chlorination process.
• the pulverulent preparation according to the invention contains only silicon granules greater than 50 microns in size, the finer granules being incompatible with the hydrochlorination process for practical reasons of clogging. heat exchangers.
• the silicon granules according to the invention are particularly suitable for the manufacture of TCS by chlorination or hydrochlorination, in that they have a size and a chemical composition favoring the efficiency and the reactivity of the chlorination and hydrochlorination reactions in bed. fluid; the low content of dopants and metallic impurities "not useful for the reaction" (that is to say outside co-catalysts) also limits the subsequent steps of purification of the TCS.
• the suitability therefore results from a combination of the size of the granules and their chemical composition, parameters that can be optimized and adjusted according to the particularities of each chlorination or hydrochlorination process.
• the mass fraction of oxygen contained in the silicon granules is less than 100 ppm.
• Oxygen comes essentially from the oxide layer which usually covers the silicon granules.
• An oxide layer as thin as possible will promote the reactivity of the chemical reaction in the fluid bed of the chlorination or hydrochlorination process, because the catalyst will be more rapidly in contact with a silicon surface and in particular with intermetallics (from co-catalysts).
• the pulverulent preparation according to the invention is advantageously conditioned under a neutral atmosphere, for example under nitrogen, thus limiting any contact with an oxygen-rich atmosphere capable of oxidizing the silicon granules.
• the invention also relates to a method for producing silicon granules.
• the manufacturing process comprises a step a) of supplying silicon waste (kerfs) from diamond wire cutting of photovoltaic quality ingots.
• Said waste comprises silicon particles covered with an oxide layer and mixed with impurities in an aqueous medium; these impurities include metal particles and potentially organic additives.
• step a) may also include the supply of silicon grains from a fluid bed reactor based on the monosilane decomposition. Preferentially, it is the grains of size less than about 400 microns that will be provided in step a), these being incompatible with the processes for drawing the PV ingots, as recalled in the introduction. These grains may be mixed with the silicon waste (kerfs) in step a) of the process. Note that these grains could alternatively be used alone (not mixed with kerfs) and undergo the subsequent steps of the manufacturing process.
• step a) may comprise the supply of decommissioned substrates from the microelectronics or photovoltaic industry.
• Declassified substrate means silicon-based substrates removed from a production line because of a breakage, a defect or other non-compliance, or substrates at the end of their life, and likely to be recycled.
• these decommissioned substrates may be monocrystalline or polycrystalline silicon wafers comprising uniform or patterned insulating or metallic layers forming all or part of components, silicon-on-insulator (SOI) wafers, defective solar panels or end of life, etc.
• SOI silicon-on-insulator
• decommissioned substrates may be crushed and continue the subsequent steps of the process according to the invention, or they may be mixed with the silicon waste (kerfs) in step a) of the process before proceeding with the process steps.
• the manufacturing process then comprises a step b) of chemical treatment of the material supplied in step a).
• This step b) aims on the one hand, to separate the silicon particles from all or part of the impurities, and on the other hand, to dry the silicon particles so as to form a powder.
• the silicon waste is in the form of a suspension of about 5% of silicon particles and metal particles in an aqueous liquid supplemented with soluble organic additives such as PEG for example.
• a first operation usually consists of roughly separating the liquid fraction which may optionally be recycled to the cutting system.
• the residual pasty mixture then typically comprises silicon particles 1 covered with an oxide layer 2 and a layer of organic compounds 3, and particles or metal ions 4 ( Figure la). This pasty mixture then undergoes the chemical treatment step, the sequences of which are as follows:
• the typical chemical composition of this powder is shown in Table 3. Its level of purity is very good: on the one hand, because the silicon particles come from the cutting of ingots of high purity and on the other hand, because the chemical treatment has allowed to isolate the silicon particles of the greater part impurities (metal particles and organic additives) present in the cutting waste.
• decommissioned substrates are introduced as silicon waste in step a)
• said decommissioned substrates may undergo a chemical treatment step b) allowing the removal of all or part of the surface layers that they involve.
• This step may comprise known sequences of dry or wet etchings of said layers, rinses and drying of the substrates decommissioned. They will then be crushed to undergo, alone or mixed with the powder of the silicon particles from other waste, the next step c) of the manufacturing process.
• the manufacturing process then comprises a step c) of metallurgical treatment of the very fine powder obtained in step b).
• Step c) aims at melting the silicon particles of the powder and forming a bath of liquid silicon.
• the metallurgical treatment of the powder is carried out in an induction heating fusion equipment equipped with a graphite crucible.
• the temperature of the crucible is raised to about 1500 ° C.
• the fine powder of silicon particles is introduced into the crucible through an upper opening.
• the silicon forming the particles will melt and flow from the "shell” formed by the oxide layer to feed a liquid silicon bath.
• the oxide, remaining in the solid state, will agglomerate and float in the liquid silicon bath.
• At least one orifice arranged at the crucible allows the continuous flow of the liquid silicon in a channel provided for this purpose. We will see later how the liquid silicon at the output of this channel is shaped.
• silicon grains preferably less than 400 microns in size, from a fluid bed reactor based on the monosilane decomposition can be melted with the powder. silicon particles or melted alone. This allows an efficient reintroduction of these small grains (10% of the production as recalled in the introduction) in the production line of PV silicon by fluid bed.
• the manufacturing method further comprises a step d) of introduction into the liquid silicon bath of at least one co-catalyst chosen from aluminum, iron and calcium.
• This step aims to optimize the content of one or more catalyst (s) of silicon granules from the manufacturing process. This optimization is done by controlled additions in the liquid silicon bath, avoiding the addition of dopants (phosphorus and boron) or other "not useful" metallic impurities (in particular for chlorination or hydrochlorination processes) and not desired in the final PCS silicon.
• dopants phosphorus and boron
• other "not useful" metallic impurities in particular for chlorination or hydrochlorination processes
• the nature of the cocatalyst (s) introduced and the quantity depend on the intended use of the silicon granules, in particular for chlorination or hydrochlorination, and particular conditions for carrying out these processes.
• Step d) makes it possible to adjust the nature and the quantity of co-catalyst (s) mixed with the liquid silicon bath, and thus to adjust the mass fraction of cocatalyst (s) in the silicon granules from manufacturing method according to the invention; such an adjustment makes it possible to optimize the reaction rate of the chlorination or hydrochlorination processes and to increase the TCS selectivity and the purity of the TCS produced.
• cocatalyst (s) are advantageously introduced by assaying in the liquid silicon bath, in the form of metal (Al, Fe) or in the form of an alloy (for example FeSi, SiCa, etc.).
• the at least one co-catalyst is introduced at a level of from 1 to 2500 ppm by mass fraction of the liquid silicon, in an amount such that its mass fraction in the silicon granules resulting from the manufacturing process is between 1 and 2500 ppm; advantageously, the mass fraction of the co-catalyst is chosen between 100 and 2000 ppm.
• the mass fraction of cocatalyst in the granules is adjusted according to the specificities of the targeted chlorination or hydrochlorination process.
• the manufacturing process then comprises a step e) of solidifying the liquid silicon to form the silicon granules.
• step e) comprises a rapid cooling granulation step of the drops of liquid silicon leaving the channel of the fusion equipment.
• the drops of silicon fall on a cold surface and undergo a centrifugation force to be dispersed before their grouping in a pulverulent preparation: their solidification (quenching) is rapid, which ensures a uniform concentration of (or) co-catalyst (s) ) as intermetallic compounds in the granules.
• the size of the granules will essentially depend on the size of the liquid silicon drops and the centrifugation speed.
• the granulation step is carried out under a neutral atmosphere (for example argon) so as to avoid or at least limit the oxidation of the silicon granules.
• step e) comprises the discharge of the liquid silicon through the channel of the melting equipment, in a mold configured to allow rapid cooling, so as to form a solidified silicon block.
• the mold has a large thickness (for example 15 cm of cast A 319) while the block of solidified silicon is thin (for example 5 cm), which ensures rapid cooling and therefore a relatively uniform concentration of ( or co-catalysts (as intermetallic compounds) in the solidified silicon block.
• Step e) then comprises a step of crushing the solidified silicon block, under a nitrogen atmosphere, to form the silicon granules.
• the manufacturing method according to the invention may further comprise a step f) of separation by sieving or by flight, for sorting the silicon granules by size.
• this separation step makes it possible to assemble pulverulent preparations devoid of silicon granules smaller than 50 microns in size.

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• 2017
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