JP2009149480A - Silicon regenerating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon regenerating method capable of efficiently removing phosphorus. <P>SOLUTION: The silicon regenerating method comprises the steps of solid-liquid separating a waste slurry or its concentrate in which silicon refuse is mixed by cutting or polishing silicon ingots or silicon wafers using a slurry containing abrasive grains and a coolant, and acquiring a silicon-recovering solid content containing the silicon refuse; cleaning the silicon-recovering solid content with a cleaning solution composed of an acid solution; and after the cleaning, baking the silicon-recovering solid content at a temperature of ≥200°C and ≤1,000°C. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a silicon regeneration method for obtaining reclaimed silicon having a high purity as a solar cell material from waste slurry used in a silicon wafer manufacturing process or the like.

  In the manufacturing process of a thin plate made of silicon single crystal or polycrystal (hereinafter referred to as “silicon wafer”) widely used for IC chips and solar cells, about 60% of the raw material silicon is drained by cutting, chamfering or polishing. The cost to the product and the environmental impact associated with disposal (this waste liquid is generally disposed of in landfill after concentrating and collecting some materials) is a major problem. It has become.

  In particular, in recent years, the production of solar cells has been steadily increasing, and the demand for raw material silicon has been increasing rapidly. For this reason, the shortage of silicon for solar cells has become apparent.

  Therefore, conventionally, a method for recovering silicon from waste liquid generated during the production of a silicon wafer such as cutting or polishing has been proposed.

  For example, in Patent Document 1, solid content is recovered from waste slurry discharged from a process of cutting or polishing a silicon single crystal or polycrystalline ingot using a slurry in which abrasive grains are dispersed in a coolant, and the recovered solid content In contrast, organic solvent cleaning to remove coolant, water cleaning to wash away organic solvent, metals (iron, copper, etc.) contained in waste slurry are dissolved in acid aqueous solution (hydrofluoric acid aqueous solution, etc.) Acid cleaning for removing the water and water cleaning for washing away the acid aqueous solution are performed.

However, Patent Document 1 describes that the solid content inevitably remains in the solid content obtained by the disclosed recovery method, and further purification is necessary to obtain silicon for solar cells therefrom. It is stated that there is.
JP 2001-278612 A

  However, the present inventors have found that the solid content for silicon recovery contains a lot of phosphorus that adversely affects the characteristics of the solar cell. Thus, it has been found that it is difficult to obtain regenerated silicon that can be preferably used as solar cell silicon simply by removing metal (iron, copper, aluminum, etc.) from the solid content for silicon recovery.

  This invention is made | formed in view of such a situation, and provides the silicon | silicone regeneration method which can remove efficiently the phosphorus contained in solid content for silicon | silicone collection | recovery.

  In addition, phosphorus contained in the solid content for silicon recovery is derived from, for example, metal scraps such as wire scraps generated from the wire used for cutting the silicon wafer (which cannot be completely removed even by cleaning and remain unavoidably) This is considered (specifically, described later).

  In the silicon recycling method of the present invention, a silicon slurry using a slurry containing abrasive grains and a coolant or a silicon lump or silicon wafer is cut or polished to solidify the waste slurry mixed with silicon waste or the concentrated component thereof into a solid-liquid separation. Obtaining solids for collecting silicon containing scraps, washing the solids for collecting silicon with a cleaning solution comprising an acid solution, and firing the solids for collecting silicon at a temperature of 200 ° C. to 1000 ° C. after the washing. A process is provided.

  According to the present invention, phosphorus contained in the solid content for silicon recovery can be removed relatively easily. This will be described below.

  Conventionally, as a method for removing phosphorus in silicon, a method of evaporating and removing phosphorus by melting silicon and holding it under reduced pressure for a certain time has been widely used.

  However, when this conventional method is applied to silicon recovery solids, metal waste containing phosphorus is melted together with silicon, so that phosphorus is melted into silicon. While metals melted in silicon (such as iron) can be removed relatively easily by unidirectional solidification, etc., in order to remove phosphorus melted in silicon, as described above, the silicon is melted and held for a long time. Since it is necessary, a large amount of energy is required and there is a risk that the regeneration cost will be greatly increased.

  On the other hand, according to the present invention, since phosphorus (or phosphorus compound) is directly removed from metal scrap at a temperature below the melting point of silicon without melting metal scrap containing phosphorus in silicon, solid content for silicon recovery The phosphorus contained in can be removed relatively easily.

  As a result of diligent research, the present inventors have found that the metal impurities remaining in the solids for silicon recovery contain a lot of phosphorus that adversely affects the characteristics of the solar cell. Has been found to greatly deteriorate. Furthermore, after the silicon recovery solid content is washed with a cleaning solution comprising an acid solution, the silicon recovery solid content is baked at a temperature of 200 ° C. or higher and 1000 ° C. or lower, thereby efficiently reducing phosphorus in the silicon recovery solid content. The inventors have found that it can be removed, and have completed the present invention. The reason why phosphorus can be efficiently removed by the method of the present invention is not necessarily clear, but the high boiling point phosphorus (or phosphorus compound) contained in the solids for silicon recovery is a substance that is easily vaporized at low temperatures by an acid solution. It is guessed that it became.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG.

  As shown in FIG. 1, the silicon recycling method of the present invention is a waste slurry in which silicon waste is mixed into a slurry by cutting or polishing a silicon lump or a silicon wafer using a slurry containing abrasive grains and a coolant, or a concentrated portion thereof. Solid-liquid separation step for obtaining solid content for silicon recovery containing silicon waste by solid-liquid separation, an acid cleaning step for washing solid content for silicon recovery with a cleaning solution comprising an acid solution, and a temperature of 200 ° C. or higher and 1000 ° C. or lower It comprises a firing step of firing the solids for silicon recovery after acid cleaning at a temperature.

  Processes other than these (in this embodiment, a neutralization treatment process, a separation / water washing process, a washing process, and a drying process are illustrated) are optional processes, and can be appropriately provided as necessary.

  Before explaining the embodiment of the silicon regeneration method, first, the waste slurry and its concentrated part will be explained.

  The waste slurry is obtained by mixing silicon scraps into the slurry by cutting or polishing a silicon lump or silicon wafer using a slurry containing abrasive grains and coolant. The concentrated portion of the waste slurry is a concentrate of the waste slurry.

The silicon lump is a lump of silicon, for example, a silicon ingot. The shape of the silicon lump is not particularly limited, but in an example, it is a columnar shape or a quadrangular prism shape. An example of the cutting device is a multi-wire saw device (hereinafter referred to as “MWS”) that is widely used as a silicon ingot cutting device. In general, MWS is a cutting device that wraps and winds a wire between a plurality of rollers, runs slurry while supplying slurry containing abrasive grains and coolant, and presses an object to be cut against the wire for cutting. . An example of a polishing apparatus is a wheel-type polishing apparatus, which is an apparatus that performs polishing by rotating a wheel having abrasive grains fixed with an adhesive and moving a silicon ingot. When a silicon ingot is cut or polished using these devices, silicon scraps, crushed abrasive grains and non-crushed abrasive grains in the slurry, and metal scraps that are worn pieces of wires and polishing wheels, etc. It will be mixed. Here, Table 1 shows the results of analyzing the composition of typical silicon ingots, wires and metal wheels. Although silicon contains almost no metal impurities and phosphorus, it shows that the wire and the polishing wheel contain a lot of phosphorus in addition to the metal impurities.

  Here, the composition and composition of the slurry will be described. The slurry is composed of abrasive grains and a coolant that disperses the abrasive grains. The type of abrasive grains is not limited and is made of, for example, SiC, diamond, CBN, alumina, or the like. The type of the coolant is not limited. For example, an oil-based coolant (oil based on mineral oil), an aqueous coolant (a glycol solvent based on water (for example, ethylene glycol, propylene glycol or polyethylene glycol), surfactant) Or an organic acid added thereto). The coolant is mainly composed of an organic solvent (water-soluble organic solvent) such as ethylene glycol, propylene glycol or polyethylene glycol, and an additive such as organic acid or bentonite is added thereto at 10 wt% or less (preferably 3 wt% or less). It may be. Note that the phrase “having the organic solvent as a main component” here means that, for example, the coolant may contain 20 wt% or less (preferably 15 wt% or less) of water.

1. Solid-Liquid Separation Step First, the solid content for silicon recovery is obtained by solid-liquid separation in the solid-liquid separation unit 1 with respect to the waste slurry or its concentrated component. The configuration of the solid-liquid separation unit 1 is not particularly limited as long as it is a configuration capable of solid-liquid separation of the waste slurry or its concentrated component to obtain a solid content for silicon recovery. The solid-liquid separation device such as a centrifuge, a filtration device or a distillation device is used alone or in combination of two or more thereof in series. Specific examples of the combination are (1) a centrifuge and a distillation device, (2) a centrifuge and a filtration device, or (3) a filtration device and a distillation device. In (1) to (3), two or more centrifuges, filtration devices, or distillation devices may be included. Each solid-liquid separation unit may send either the separated liquid or solid content to the next solid-liquid separation device, and a part of the liquid and a mixture of solids or a part of the solids and liquid. The mixture may be sent to the next solid-liquid separator.

2. Acid Cleaning Step Next, in the acid cleaning unit 2, the silicon recovery solid content is cleaned with a cleaning liquid made of an acid solution. In one example, the acid cleaning unit 2 includes a cleaning basket 2a and a stirrer 2b provided in the cleaning basket 2a.

  This acid cleaning step is (1) removing residual organic substances derived from coolant such as glycol solvents and additives contained in the solid content for silicon recovery by dissolving them in an acid solution, and (2) wear pieces of wires. It is performed for the purpose of dissolving and removing metal scraps in an acid solution.

  As the particle size of solids for silicon recovery, the larger the specific surface area, the higher the cleaning effect. Therefore, it is preferable that the particle size is small. On the other hand, when the particle size is small, it becomes difficult to recover the solids after washing. Therefore, in practice, the particle size of the solids for silicon recovery is preferably in the range of 0.01 μm or more and less than 10 mm. More preferably, it is in the range of 0.1 μm or more and less than 5 μm. In the present specification, “particle size” means a value measured by a method based on JIS R1629. The “powder having a particle size of less than X μm” means a powder having a particle size of 98% of particles in the powder of less than X μm. The “powder of powder Y μm or more and less than Z μm” means a powder remaining after removing “powder of particle size less than Y μm” from “powder of particle size less than Z μm”.

  The acid solution is a solution in which an acidic substance is dissolved in a solvent containing water. Although the pH of an acid solution should just be less than 7, 0-4 are preferable. The pH of the acid solution is, for example, 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, or 4. The pH of the acid solution may be within a range between any two of the numerical values exemplified herein. The solvent of the acid solution preferably consists essentially of water, but may contain components other than water. The ratio of water in the solvent of the acid solution is preferably 50 wt% or more, for example, 50, 60, 70, 80, 90, 95, 99, 99.9, 100 wt%. The ratio of water may be within a range between any two of the numerical values exemplified here. Components other than water in the solvent are preferably compatible with the coolant and have a boiling point lower than that of the coolant, for example, 1 to 6 carbon atoms (preferably 1, 2, 3, 4, 5 and 6 is an alcohol or a ketone having 3 to 6 carbon atoms (preferably, a range between any two of 3, 4, 5 and 6). In this case, it becomes easy to remove the residual coolant by dissolving it in the acid solution.

  Examples of acidic substances (substances that dissolve in a solvent such as water and release protons) include inorganic or organic acidic substances. Examples of inorganic acidic substances include hydrogen chloride, sulfuric acid, nitric acid, hydrogen fluoride, and hydrogen bromide. Examples of the organic acidic substance include citric acid, acetic acid, formic acid, oxalic acid, and lactic acid. The acid solution only needs to contain at least one kind of acidic substance, and may contain both an inorganic acidic substance and an organic acidic substance. Hereinafter, an acid solution containing only an inorganic acidic substance is called an inorganic acid solution, and an acid solution containing only an organic acidic substance is called an organic acid solution.

  The acid solution is preferably composed of an inorganic acid solution. This is because the phosphorus compound produced by the reaction with the inorganic acid generally has a lower boiling point than the phosphorus compound produced by the reaction with the organic acid.

  The acid solution is preferably non-oxidizing with respect to silicon. In the present invention, “non-oxidizing” means that the oxidizing power of silicon is weaker than that of sulfuric acid. When the acid solution is non-oxidizing with respect to silicon, it is possible to suppress a decrease in silicon recovery rate due to silicon oxidation. Examples of the acid solution that is non-oxidizing with respect to silicon include hydrochloric acid, hydrofluoric acid, citric acid, and an aqueous ammonium fluoride solution.

  The acid solution may contain hydrogen peroxide. In this case, there is an advantage that metal waste can be removed in a short time. The ratio of hydrogen peroxide is, for example, 0.1 to 5 wt%, specifically, for example, 0.1, 0.5, 1, 2, 3, 4, 5 wt%. The ratio of hydrogen peroxide may be in a range between any two of the numerical values exemplified here.

3. Neutralization treatment step Next, the washing solution after washing the solids for silicon recovery is transferred to the neutralization treatment vessel 3 to neutralize the acid solution. This neutralization treatment step is performed for the purpose of preventing the device from corroding in the next solid-liquid separation step, but may be omitted depending on the configuration of the device. Further, instead of neutralization, a solvent such as water may be added to the cleaning solution to dilute the acid solution, thereby reducing the concentration of protons in the acid solution to prevent corrosion of the apparatus.
The neutralization method is not particularly limited. For example, basic substances such as sodium hydroxide, calcium hydroxide, and ammonia (dissolve in a solvent such as water to release hydroxide ions or become proton acceptors). For example, a method of adding a substance) solution (hereinafter referred to as “base solution”) to the cleaning liquid, and a method of adding a basic substance directly to the cleaning liquid.

  Although the pH value of the cleaning liquid after neutralization or dilution is not particularly limited, it is 2 to 4, for example. In this case, it is possible to suppress the precipitation of phosphorus as a hardly soluble solid material that is difficult to remove.

4). Separation / Washing Step Next, in the separation / water washing section 4, the washing liquid after acid washing and the solid content for silicon recovery are separated into solid and liquid, and washed with pure water to obtain the solid content for silicon collection after washing. Note that this step may be omitted and the cleaning liquid may be volatilized in, for example, a drying step or a baking step.
The separation / water washing unit 4 is configured by combining two or more pure water supply devices in series with a solid-liquid separation device such as a centrifuge, a filtration device, or a distillation device, for example.

5. Cleaning Step Next, in the cleaning unit 5, the silicon recovery solids are cleaned with a cleaning liquid. In one example, the cleaning unit 5 includes a cleaning bowl 5a and a stirrer 5b provided in the cleaning bowl 5a. The impurity removal effect can be further enhanced by the cleaning in the cleaning unit 5.

  In this step, for example, a cleaning liquid made of pure water, an inorganic acid solution, or an organic solvent can be used. The inorganic acid solution is preferably non-oxidizing with respect to silicon. The organic solvent preferably has a boiling point lower than the firing temperature in the firing step. In this case, since the organic solvent is removed at a temperature lower than the firing temperature, generation of SiC is prevented. The organic solvent is preferably compatible with the coolant and has a boiling point lower than that of the coolant. For example, the organic solvent has 1 to 6 carbon atoms (preferably any one of 1, 2, 3, 4, 5 and 6). Or a ketone having 3 to 6 carbon atoms (preferably, a range between any two of 3, 4, 5 and 6). In this case, it becomes easy to remove the residual coolant by dissolving it in an organic solvent.

6). Separation and washing process 2
Next, in the separation / water washing section 6, the washed cleaning liquid and the silicon recovery solid are separated into solid and liquid, and washed with pure water to obtain the cleaned silicon recovery solid.
The separation / water washing unit 6 is configured by combining two or more solid-liquid separation devices such as a centrifuge, a filtration device, or a distillation device, and two or more pure water supply devices in series.

7). Drying step Next, in the dryer 7, the solid content for silicon recovery after washing obtained in the steps so far is dried. Drying can be performed, for example, by heating the solid content for silicon recovery or reducing the ambient atmosphere. The solid content for silicon recovery may be naturally dried, or the solid content for silicon recovery may be dried at the same time as another step such as a baking step described later. Therefore, this step can be omitted as an independent step.

8). Baking process Next, in the baking apparatus 8, baking of the solid content for silicon | silicone recovery is performed at the temperature of 200 to 1000 degreeC.

As the baking apparatus 8, a drum type airflow dryer etc. can be used, for example. In this dryer, the solid content for silicon recovery is baked by blowing a gas stream of heated gas to the solid content for silicon recovery while rotating and rotating the solid content for silicon recovery by rotating the drum.
This firing step is performed by (1) removing residual organic substances derived from coolant such as glycol solvent and additives in solids before washing, and (2) recovering silicon. The purpose is to remove phosphorus remaining in the solid content.

  The reason for setting the firing temperature to 200 ° C. or more and 1000 ° C. or less is that when the firing temperature is 200 ° C. or more, phosphorus is easily removed by volatilization of phosphorus (single phosphorus or phosphorus-containing compound). Further, when the firing temperature is 1000 ° C. or lower, the oxidation of silicon is suppressed and the formation of an alloy between phosphorus and silicon is suppressed. As a result, the silicon recovery rate is improved and phosphorus is efficiently removed.

  The firing temperature is 200 ° C. or more and 1000 ° C. or less, for example, 300 ° C. or more and 800 ° C. or less. Specifically, for example, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000 ° C. The firing temperature may be within a range between any two of the numerical values exemplified here.

  The firing time is, for example, 0.5 to 10 hours. Specifically, for example, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8 9, 10 hours. The firing time may be within a range between any two of the numerical values exemplified here. In one example, it is preferable to perform treatment for 1 hour or more per 200 kg of the solid content for recovery.

  In order to further suppress the oxidation of silicon during firing, the firing is preferably performed in an inert gas atmosphere. The inert gas is, for example, a rare gas (eg, argon) or nitrogen gas.

  In order to promote the volatilization of phosphorus, the firing is preferably performed in a reduced pressure atmosphere. The atmospheric pressure at the time of decompression is, for example, 0.1 to 0.9 atm, and specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0 0.7, 0.8 and 0.9 atm. The atmospheric pressure at the time of depressurization may be within a range between any two of the numerical values exemplified here.

  In this way, a solid content for silicon recovery from which phosphorus has been removed can be obtained. The obtained solid content for silicon recovery can be recovered as recycled silicon.

9. Heating / refining step Further, the solids for recovering silicon from which phosphorus has been removed are melted in the heating / purifying device 9 at a temperature equal to or higher than the melting point of silicon (generally 1410 ° C. to 1420 ° C.) and then solidified. Alternatively, a silicon lump may be used. The heating / purification device 9 is preferably a hermetically sealed system and suppresses the oxidation of silicon, and has an inert gas introduction part. The obtained silicon mass can be recovered as recycled silicon.

  The silicon mass can be purified to remove residual metal components. As the purification method, for example, various known purification techniques (for example, removal of segregated impurities by unidirectional solidification) at the time of conventional polycrystalline silicon casting can be used. As a result, a silicon lump from which impurities such as metal components have been removed can be obtained and recovered as recycled silicon.

  The heating / purifying device 9 is configured by combining, for example, two or more vacuum heating and melting furnaces and two or more unidirectional solidification furnaces in series.

  Before and after each of the above-described steps, a pulverization step, a molding step, or a powder separation step can be incorporated as necessary. The pulverization step refers to all known methods for pulverizing the solid content for silicon recovery to a specific size, and devices such as a ball mill, a jet mill, and a vibration vacuum dryer can be used. The molding process can be performed as a pretreatment for melting to increase bulk specific gravity and increase transport efficiency, or to increase thermal conductivity and facilitate melting, and pressurizes silicon-containing powder to form a plate, An apparatus having any configuration can be used as long as the apparatus granulates in a block shape, a pellet shape, or the like. The impurity separation step includes, for example, classification in which particles are classified based on physical parameters such as particle size and density using an inertia classifier or a centrifugal classifier, and a method of removing magnetic impurities such as iron using a magnet. .

1. Example 1
Silicon was regenerated in the above-described process using waste slurry obtained by cutting a solar cell wafer using a wire saw.

  In this example, 500 L of a 15 wt% hydrochloric acid aqueous solution was added to 120 kg solid content obtained by filtering the waste slurry, and the mixture was stirred and washed for 1 hour. The neutralization treatment step was omitted, and the acid solution was filtered as it was to recover the solid content.

  500 L of pure water was added to the solid content, and the mixture was stirred and washed again for 1 hour, and filtered to collect the solid content. The solid content thus obtained was dried by heating to 60 ° C. in 1 atm air and holding for 2 hours. Next, the sample was thrown into the drum of a drum-type baking machine that performs baking while rotating the inclined drum slowly. The drum is provided with a heating tube, and the temperature of the firing object can be controlled by feeding heated nitrogen gas into the drum. In Example 1, the firing target was controlled to be in the range of 400 to 500 ° C., and firing was performed for 1.5 hours to obtain a dry powder.

  Next, the dry powder is heated to 1800 ° C. at a rate of 300 ° C./hour in an argon 1 atm atmosphere using an externally heated blast furnace and melted by holding for 2 hours to obtain 60 kg of silicon lump. It was.

  Table 2 shows the results of analyzing the concentration of typical impurities for the solid content obtained from the waste slurry, the solid content after washing twice, the dried powder after firing, and the silicon lump after melting in Example 1. It can be seen that impurities were reduced by washing, but the concentration of phosphorus (P) was greatly reduced by firing. Since the concentration of phosphorus is greatly reduced by firing, the silicon mass after melting can be made into a solar cell material by removing the remaining metal by an appropriate method without removing phosphorus particularly under vacuum melting. .

  Here, purification is performed by unidirectional solidification in an atmosphere of 1 atm of argon, sliced into a polycrystalline silicon substrate, and a solar cell is created. The conversion efficiency from sunlight to electric energy is 13 to 14.2%. A characteristic close to that of a normal commercial product was obtained.

2. Comparative Example 1
In Comparative Example 1, the solid content obtained by filtering the waste slurry was subjected to the same method as in Example 1 up to twice washing, solid-liquid separation, and drying. In this case, melting was performed in the same manner as in Example 1 using an externally heated blast furnace as it was without firing with a firing machine.

  Table 3 shows the results of analyzing the silicon mass after melting in Comparative Example 1. Compared with Example 1, the residue of phosphorus is particularly large. This is presumed to be because phosphorus was taken into silicon and was not efficiently removed because the holding time was below the melting point of silicon.

  In Comparative Example 1, as in Example 1, purification by unidirectional solidification was performed in an atmosphere of 1 atm of argon, and a solar cell was created by slicing to a polycrystalline silicon substrate. 1% or less. This is considered to be improved by going through a dephosphorization step, but in this case, the production cost is remarkably increased.

It is explanatory drawing for demonstrating the silicon | silicone reproduction | regenerating method of one Embodiment of this invention.

Explanation of symbols

1: Solid-liquid separation unit (filtering device) 2: Acid cleaning unit 2a: Washing tank 2b: Stirrer 3: Neutralization bowl 4: Separation / water washing unit (filtration / washing device) 5: Washing unit 5a: Washing tank 5b: Stirrer 6: Separation / water washing part (filtration / washing device) 7: Dryer 8: Firing device 9: Heating / purification device

Claims (10)

Solid silicon for recovery of silicon containing silicon waste by solid-liquid separation of waste slurry mixed with silicon scrap by the cutting or polishing of silicon lump or silicon wafer using slurry containing abrasive grains and coolant or its concentrated component Get the minute
The solid content for silicon recovery is washed with a cleaning solution comprising an acid solution,
A silicon regeneration method comprising a step of firing the solid content for silicon recovery at a temperature of 200 ° C. or higher and 1000 ° C. or lower after the cleaning. The method of claim 1, wherein the acid solution comprises an inorganic acid solution. The solid content obtained by carrying out solid-liquid separation of the washing | cleaning liquid after acid washing | cleaning, and the said solid content for silicon | silicone collection | recovery is further equipped with the process of wash-processing again with a pure water, a non-oxidizing inorganic acid solution, or an organic solvent. 2. The method according to 2. The method according to any one of claims 1 to 3, further comprising a step of neutralizing or diluting the cleaning solution after the acid cleaning. The method according to claim 4, wherein the neutralization or dilution is performed such that the pH of the cleaning liquid after neutralization or dilution is 2 to 4. 5. The method according to any one of claims 1 to 5, wherein the firing temperature is 300 ° C or higher and 800 ° C or lower. The method according to claim 1, wherein the firing is performed in an inert gas atmosphere. The method according to claim 1, wherein the firing is performed under reduced pressure. The method according to any one of claims 1 to 8, further comprising a step of melting the solid content for silicon recovery after the baking. The method according to claim 9, further comprising the step of purifying the silicon recovery solids after the melting.

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