RECYCLING PROCESS FOR SILICON INGO CUTTING WASTE

Abstract

The invention relates to a process for recycling silicon ingot cutting waste comprising the following steps: a) the recovery of a first sludge, from said waste, formed by an aqueous mixture comprising silicon particles and organic species, but devoid of abrasive particles of the carbide type; the first sludge having an initial mass percentage of organic species, b) the chemical treatment of the first sludge so as to reduce the initial mass percentage of organic species and to form a second sludge having a residual mass percentage of organic species of between 0.8% and 1.5%; c) the drying and associated granulation of the second sludge, by heating under vacuum and agitation, to form granules including the silicon particles and the organic species, said granules having dimensions of which an average value is greater than or equal to 150 µm and a moisture content less than or equal to 2%;d) the introduction of granules into a liquid silicon casting ladle, at the outlet of a silica reduction furnace, to participate in cooling the liquid silicon to a refining temperature and / or to a casting temperature. No figure;

Description

Title of the invention: METHOD FOR RECYCLING SILICON INGO CUTTING WASTE FIELD OF THE INVENTION

[0001] The present invention relates to the silicon manufacturing chain. It relates in particular to a process for recycling waste from the cutting of silicon ingots (“kerfs”) by diamond wires, which provides for reintroducing the treated silicon into the metallurgical silicon manufacturing chain by remelting. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Silicon wafers, intended for the semiconductor or photovoltaic industry, are manufactured from silicon ingots, using wire cutting processes with the addition of carbide-type abrasives ("slurry sawing" according to Anglo-Saxon terminology) or diamond wires.

[0003] The cutting wire width is increasingly smaller, but remains proportional to the thickness of the cut wafers; these sawing processes therefore still generate a significant amount of silicon waste: on the order of 25 to 40% of the total mass of an ingot. The cutting is carried out under a continuous flow of cutting fluid, which removes heat and chips. The fluid consists of water and additives such as PEG (polyethylene glycol), antifoaming agents, and surfactants. The cutting waste is found in the form of a very fine powder (micro- or nanoparticles of silicon with an oxidized surface), mixed with the liquid additives, metallic contaminants, organic or inorganic species, and, in the case of abrasive cutting, abrasive particles (for example, SiC).

[0004] Document EP2712844A1 proposes a process for recycling silicon ingot cutting waste comprising abrasive SiC particles. This process includes a purification step (using acidic or basic solutions) of said waste, so as to obtain purified silicon particles and carbon-containing particles, the latter containing less than 0.25 ppm of boron and less than 0.5 ppm of phosphorus. The particles are then agglomerated into pellets (preferably, having an average diameter greater than 2–5 mm), using a binder, for example, a carbohydrate or a resin. The process then includes a step of using said pellets as reactive materials in a reduction (oxygen removal) process producing silicon.

[0005] Let us recall that a reduction process is usually carried out from quartz (SiO2) which reacts with silicon carbide or carbon, at temperatures around 2100°C, to give silicon.

[0006] According to document EP2712844, SiO2, SiC or carbon can be added to the pellets to obtain appropriate proportions of initial reactive materials in the reduction furnace, at very high temperatures.

[0007] The liquid silicon, obtained from the reduction furnace, is then collected in casting ladles and then cooled, to form blocks of metallurgical grade silicon. SUBJECT OF THE INVENTION

[0008] The present invention relates to a method for recycling silicon ingot cutting waste from diamond wire sawing, the waste thus being free of abrasive particles, particularly carbides. The recycling method involves treating and shaping said waste for reintroduction into the metallurgical silicon manufacturing process by remelting in the ladle of a metallurgical silicon production furnace. BRIEF DESCRIPTION OF THE INVENTION

[0009] The present invention relates to a process for recycling silicon ingot cutting waste comprising the following steps:

[0010] a) the recovery of a first sludge, derived from said waste, formed by an aqueous mixture comprising silicon particles and organic species, but devoid of abrasive particles of the carbide type; the first sludge having an initial mass percentage of organic species defined in relation to the mass of solid matter contained in the first sludge,

[0011] b) the chemical treatment of the first sludge so as to decrease the initial mass percentage of organic species and to form a second sludge having a residual mass percentage of organic species between 0.8% and 2% relative to the mass of solid matter contained in the second sludge;

[0012] c) drying and associated granulation of the second slurry, by heating under vacuum and agitation, to form granules including silicon particles and organic species, said granules having dimensions of which an average value is greater than or equal to 150 qm and a moisture content less than or equal to 2%;

[0013] d) the introduction of granules into a liquid silicon casting ladle, at the outlet of a silica reduction furnace, to participate in the cooling of the liquid silicon to a refining temperature and / or to a casting temperature.

[0014] According to advantageous features of the invention, taken alone or in any feasible combination: • the initial mass percentage of organic species is between 3% and 6%, typically around 4%; • the residual mass percentage of organic species is 1.3% + / - 0.2%; • step c) is carried out at a temperature between 40°C and 100°C; • the average size of the granules is approximately 0.8mm; • the moisture content of the granules is 1.5% + / - 0.2%; • the mass proportion of granules introduced into the casting ladle is between 1% and 5%; • the chemical treatment of step b) is carried out with a dilute solution of hydrogen peroxide; • the chemical treatment of step b) is carried out with a dilute solution of a mineral peroxide; • step b) includes at least one, two, or even three successive washes with pure water, after chemical treatment, to obtain the second sludge. DETAILED DESCRIPTION OF THE INVENTION

[0015] The invention relates to a process for recycling waste from the cutting of silicon ingots using diamond wires. The silicon contained in this waste is found in the form of a very fine powder (silicon nanoparticles at least partially oxidized on the surface), mixed with liquid additives, metallic contaminants, and organic or inorganic species. This waste essentially contains liquid matter; the mass percentage of silicon (which constitutes the majority of the solid matter) is between 2% and 10%.

[0016] The first step a) of the process consists of recovering a first sludge, from said waste, formed by an aqueous mixture including in particular silicon nanoparticles (also called silicon particles hereafter) and organic species, and of course free of abrasive particles of the carbide type (in particular SiC).

[0017] The term "sludge" is used to describe a substance typically comprising more than 40% (mass percentage) of solid matter (consisting mainly of silicon nanoparticles), mixed in an aqueous solution. The first sludge can be obtained from the cutting waste by a known solid / liquid separation method chosen from filtration (e.g., vacuum filtration) or tangential flow filtration, sedimentation, centrifugation, or cyclonic separation. Advantageously, in step a), the first sludge comprises approximately 50% solid matter and 50% liquid matter. The term "approximately" here means that the mass percentage value is within + / -10% (absolute: that is, a mass percentage of approximately 50% could vary between 40% and 60%), or even within + / - 5% (absolute).

[0018] Generally, the size distribution of the silicon particles contained in the The first mud is between approximately 1 pm and 5 pm, typically centered on 0.6 pm.

[0019] The first sludge has an initial percentage of organic species, typically between 3% and 6%, for example around 4%. Note that in the context of this description, we are referring to mass percentages of organic species, defined in relation to the mass of solid matter contained in the sludge.

[0020] These organic species come essentially from additives such as PEG (Polyethylene glycol), antifoam, surfactants, introduced into the cutting liquid and whose volume loaded at each cutting operation is precisely known, and therefore in each volume of waste recovered.

[0021] The recycling process then includes a step b) comprising a chemical treatment of the first sludge, which aims to reduce the initial mass percentage of organic species and to form a second sludge having a residual mass percentage of organic species of between 0.8% and 2%, this percentage being defined in relation to the mass of solid matter contained in the second sludge. Preferably, the residual mass percentage of organic species is 1.3% + / - 0.2%.

[0022] Advantageously, the chemical treatment is carried out with a dilute solution of hydrogen peroxide (H2O2) or of a mineral peroxide, for example based on potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), etc. By dilute solution is meant a solution consisting of peroxide and pure water (deionized water or ultrapure water, having respectively a resistivity of a few hundred kohms.cm and a resistivity greater than 18.2 Mohms.cm).

[0023] The diluted solution may have a mass concentration of between 1% and 35% peroxide, the additional percentage being water.

[0024] The diluted solution is added to the first sludge, thus forming a first mixture. Note that no other acidic product is added to form the first mixture. The first mixture therefore consists of the first sludge, a peroxide (hydrogen or mineral), and water.

[0025] The respective proportions of dilute solution and first sludge in the first mixture will depend on the peroxide concentration and the type of peroxide. For example, for a hydrogen peroxide concentration of 35%, the first mixture preferably comprises one volume of dilute solution for one volume of first sludge; for a hydrogen peroxide concentration of 10%, the first mixture preferably comprises three volumes of dilute solution for one volume of first sludge. As another example, for a potassium peroxide concentration of 6.7% K2S2O8 relative to the mass of solid matter in the first sludge, the first mixture preferably comprises one volume of dilute solution for a volume of first mud.

[0026] The chemical treatment can be carried out by agitating the first mixture, so as to homogenize the distribution of the dilute solution in the middle of the silicon nanoparticles and other organic species or metallic contaminants of the first sludge.

[0027] The silicon particles in the first slurry predominantly have a silicon oxide layer on their surface; moreover, they are totally or partially "covered" by layers formed of long chains of organic species. The organic species are, in fact, mostly attached to the surface of the nanoparticles in the first slurry. Hydrogen peroxide or another mineral peroxide, through an oxidation reaction of the organic species, will induce the segmentation of the long organic chains, which promotes their detachment from the surface of the silicon particles.

[0028] Advantageously, the first mixture comprises approximately 5% to 10% of solid matter and an additional percentage of liquid matter (by mass percentage): this liquid consistency promotes the suspension of silicon nanoparticles, organic species and metallic contaminants in the first mixture.

[0029] Agitation of the first mixture then makes it possible to homogenize the distribution of silicon nanoparticles and other species suspended in the first mixture; agitation also makes it possible to increase the efficiency of the oxidation reaction segmenting the organic chains.

[0030] The chemical treatment can be carried out at a temperature between 20°C and 95°C, for a duration of 10 min to 5 h.

[0031] At the end of the chemical treatment, the first aqueous mixture comprises homogeneously suspended particles, including silicon nanoparticles, organic species partially in the form of segmented chains (other organic species may still be attached to the surface of nanoparticles) and metallic contaminants.

[0032] A solid / liquid separation sequence of the first mixture is carried out to obtain, on the one hand, a second sludge and, on the other hand, a liquid containing organic species and metallic contaminants. The second sludge is composed of at least 40% solid matter. The liquid is discharged and treated as liquid effluent.

[0033] At this stage, additional treatment with soda can optionally be carried out to obtain a pH greater than or equal to ten, which makes it possible to eliminate most of a potential contamination by aluminium (often used to collect the cutting liquid).

[0034] Step b) may optionally include at least one, or even two or three, successive washes with pure water of the first sludge or an intermediate sludge obtained before the final solid / liquid separation sequence leading to the formation of the second mud.

[0035] The residual quantity of organic species depends on the chemical treatment (peroxide dilution, time, and other application conditions of the treatment to the first sludge) and the water dilution rate of successive washes. Indeed, these washes will likely result in a greater or lesser quantity of organic species present in suspension in the liquid portion of the first mixture. The residual quantity of organic species can be assessed and monitored by balances based on the measurement of total organic carbon (TOC) or by LECO-type techniques using IGA (Inductively Gas Analysis).

[0036] The washing (or washings) are followed by a solid / liquid separation to obtain a sludge comprising at least 40% solid matter.

[0037] The solid / liquid separation(s) of step b) can be carried out by known techniques such as sedimentation, centrifugation, cyclonic separation, filtration, or other.

[0038] Although the removal of some of the organic species from the first sludge by the use of a dilute peroxide solution has been particularly described, this removal can be carried out by other techniques known from the prior art, such as extraction by a solvent, said solvent being able in particular to be chosen from toluene, dichloromethane, alcohol and acetone.

[0039] Note that the process according to the invention does not require that step b) include a chemical treatment using an acidic solution to purify the silicon particles from the first sludge. In the process according to the invention, it is not necessary for all the metallic contaminants present in the first sludge to be specifically removed. Some of them are eliminated during step b), along with the excess organic matter, but it is acceptable for a residual portion of metallic contaminants to remain in the second sludge.

[0040] The recycling process includes a subsequent step c), corresponding to the drying of the second sludge. The parameters of this drying, combined with the presence of organic species (respecting the chosen residual mass percentage) in the second sludge, induce granulation of the solid particles in said sludge. The drying / granulation is carried out by heating under vacuum and agitation to form granules including the silicon nanoparticles and the residual organic species. Performing this step under vacuum (on the order of 20–80 mbar) avoids the phenomenon of evaporation, which disperses the particles and hinders agglomeration. The heating is carried out in the temperature range of 40°C–100°C. The heating and agitation bring the silicon particles into contact with the organic species, the latter forming a binder and causing the particles to agglomerate in the form of granules.The drying / granulation stage can be carried out in a vacuum dryer. such as, for example, the Pan Dryer model offered by the company De Dietrich for other applications.

[0041] By way of example, the drying / granulation step can be carried out on a Pan Dryer type piece of equipment under the following conditions: - Heating surface area (bottom and walls of the equipment tank): approximately 1.25m2, of which 0.17m2 is heated bottom, - After the second sludge is introduced into the tank, the pressure is reduced (vacuum cycle) to a pressure of 70 mbar in the equipment. - Heating the tank to 80°C, - Initial temperature of the second sludge: approximately 48°C; initial moisture content of the second sludge: approximately 50%, - Operation of the agitator and lump breaker to obtain the desired particle size of the granules; - Final temperature of the granules: approximately 65°C; final moisture content of the granule powder: approximately 1.5%.

[0042] Adjusting the pressure, heating temperature, temperature rise, time, agitation parameters, combined with the residual organic species content in the second sludge, allows the formation of granules having mostly dimensions greater than 70 pm and a moisture content less than or equal to 2%.

[0043] Advantageously, the moisture content of the granule powder is 1.5% + / - 0.2%. The moisture content can be measured on a sample taken from the granules by differential weighing before and after heating with an infrared lamp.

[0044] The granules will generally have a polyhedral or spherical shape; the dimension or size of a granule can be defined as its "equivalent Sauter diameter." The "equivalent Sauter diameter" is the diameter of the sphere that would behave identically during a particle size measurement using a defined technique. A Malvem-type laser diffraction measurement technique may be cited as an example.

[0045] In the context of the present invention, it is desirable to obtain a powder of granules whose size distribution is substantially in the form of a Gaussian distribution; the maximum of the Gaussian distribution is typically centered on an average granule size greater than or equal to 150 µm, ensuring an absence or a very low proportion of granules smaller than or equal to 70 µm. This requirement arises from the fact that the presence of granules smaller than 70 µm in a powder predominantly composed of silicon poses a risk in terms of the powder's explosiveness. Advantageously, the maximum of the Gaussian distribution of granule size is centered on an average size of approximately 0.8 mm. In general, the The average size of the granules (average of the dimensions of all the granules) is between 150 pm and 1 mm.

[0046] To maintain its moisture content of less than or equal to 2%, the granule powder is conditioned and stored under a nitrogen atmosphere, after step c).

[0047] The process then includes a step d) corresponding to the introduction of the granules into a liquid silicon casting ladle, at the outlet of a silica reduction furnace, to participate in the cooling of the liquid silicon to a refining temperature (aiming to reduce the levels of aluminum and calcium contained in the raw liquid silicon) and / or to a casting temperature.

[0048] Liquid silicon, resulting from the reduction reaction of silica, arrives in the pouring ladle at a temperature of approximately 1700°C. Generally, it then passes through a refining temperature, around 1550°C, then through a pouring temperature, around 1480°C, before being poured into ingots.

[0049] The granule powder from step c), added to the pouring ladle, cools the liquid silicon to one or both of these temperatures. The mass proportion of granules introduced into the pouring ladle is preferably between 1% and 5%. Mass proportion refers to the ratio between the mass of the granules incorporated and the mass of liquid silicon in the pouring ladle.

[0050] The moisture content of the granules referred to in step c) is of great importance here: indeed, the decomposition of water at the temperature of the liquid silicon in the pouring ladle can lead to an explosion (hydrogen) if the quantity of water exceeds a few percent.

[0051] As stated previously, it is not critical that a residual amount of metallic contaminants remain in the granules because the waste from diamond wire cutting contains metallic contaminant levels 10 times to 100 times lower than the levels in the casting ladle.

[0052] Furthermore, even if the silicon particles composing the granules contain silicon oxide (covering all or part of the surface of said particles), this oxide will be melted when the powder is introduced into the casting ladle, due to the high temperature (approximately 1700°C) and the presence of a slag of SiO2 / Al2O3 / CaO.

[0053] The process according to the invention thus makes it possible to recycle the silicon contained in the waste from the cutting of ingots by diamond wires, by implementing a reduced number of steps.

[0054] It should be noted that with the decrease in the size of silicon particles in the “kerfs” (from micrometers to nanometers), linked to progress in cutting technologies (reduction in wire diameter), the recycling of these very fine particles can be encountering several difficulties: - the coating of silicon particles by organic species contained in the cutting fluid, which most state-of-the-art processes attempt to eliminate entirely, - the extreme fineness of the particles, which leads to a high specific surface area of ​​the corresponding powder, which induces its explosivity, - the coating of the surface of the particles by a silicon oxide whose melting temperature is higher than the melting temperature of silicon itself (1420°C); the processes of the prior art therefore treat the particles with acid to remove this coating.

[0055] The process according to the invention takes advantage of the composition of the initial slurry obtained from diamond wire cutting waste and adjusts it to promote the shaping of silicon-rich granules. It further defines characteristics of the granule powder particularly suited to remelting said powder in a liquid silicon casting ladle at the outlet of a silica reduction furnace, thus enabling, in addition to silicon recycling, the management of liquid silicon cooling sequences in a completely safe manner and without the consumption of extracted raw materials.

[0056] The invention is not limited to the modes and examples of embodiments described and alternative embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. A process for recycling silicon ingot cutting waste comprising the following steps: a) the recovery of a first sludge, from said waste, formed by an aqueous mixture comprising silicon particles and organic species, but devoid of abrasive particles of the carbide type; the first sludge having an initial mass percentage of organic species defined in relation to the mass of solid matter contained in the first sludge, b) the chemical treatment of the first sludge so as to reduce the initial mass percentage of organic species and to form a second sludge having a residual mass percentage of organic species of between 0.8% and 2% in relation to the mass of solid matter contained in the second sludge;(c) drying and associated granulation of the second slurry, by heating under vacuum and agitation, to form granules including silicon particles and organic species, said granules having dimensions of which an average value is greater than or equal to 150 pm and a moisture content less than or equal to 2%; (d) introduction of the granules into a liquid silicon casting ladle, at the outlet of a silica reduction furnace, to participate in the cooling of the liquid silicon to a refining temperature and / or to a casting temperature.

2. Recycling process according to the preceding claim, wherein the initial mass percentage of organic species is between 3% and 6%, typically in the order of 4%.

3. Recycling process according to any one of the preceding claims, wherein the residual mass percentage of organic species is 1.3% + / - 0.2%.

4. Recycling process according to any one of the preceding claims, wherein step c) is carried out at a temperature between 40°C and 100°C.

5. A recycling process according to any one of the preceding claims, wherein the average size of the granules is about 0.8mm.

6. Recycling process according to any one of the preceding claims, wherein the moisture content of the granules is 1.5% + / - 0.2%.

7. A recycling process according to any one of the preceding claims, in in which the mass proportion of granules introduced into the casting ladle is between 1% and 5%.

8. A recycling process according to any one of the preceding claims, wherein the chemical treatment in step b) is carried out with a dilute hydrogen peroxide solution.

9. A recycling process according to any one of the preceding claims, wherein the chemical treatment in step b) is carried out with a dilute solution of a mineral peroxide.

10. Recycling process according to the preceding claim, wherein step b) comprises at least one, two, or even three successive washes with pure water, after chemical treatment, to obtain the second sludge.