JP4966938B2 - Silicon regeneration method - Google Patents

Description

  The present invention relates to a method for purifying silicon. More specifically, the present invention relates to silicon-containing solids containing silicon dioxide and any one of silicon carbide, silicon nitride, or diamond as particulate impurities, obtained from waste slurry produced in the production process of silicon wafers, etc. The present invention relates to a silicon purification method for obtaining recycled silicon from recovered silicon.

Generally, about 60% of the raw material silicon is obtained by cutting, chamfering or polishing in the manufacturing process of a thin plate (hereinafter referred to as “silicon wafer”) made of silicon single crystal or polycrystal widely used for IC chips and solar cells. It will be contained in the waste slurry.
That is, in the manufacture of silicon wafers, only about 40% of the raw silicon is commercialized, and the remaining 60% is contained in the waste slurry. Therefore, the cost burden on the product and the environmental burden associated with disposal are large. It is a problem. In practice, the above-mentioned waste slurry is generally disposed of in landfill after concentration treatment and recovery of some materials.

  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 slurry generated during the production of a silicon wafer such as cutting or polishing has been proposed.

  For example, in Japanese Patent Application Laid-Open No. 2001-278612 (Patent Document 1), solid content is recovered from waste slurry discharged in a cutting or polishing process of silicon, and the recovered solid content is washed with an organic solvent. Organic solvent cleaning step for removing the dispersant contained in the substrate, and separation step for removing the metal, silicon oxide and abrasive grains from the solid content after the removal of the dispersant to obtain a silicon-based powder A method for recovering silicon including the following is disclosed.

  In addition, the separation process in Patent Document 1 includes an acid cleaning process for supplying an acid solution to the solid content to remove at least metal or silicon oxide, a classification treatment for the solid content, and silicon powder. It also describes including a classification step for separating the abrasive grains.

  Further, Patent Document 1 describes that the classification step is performed by an air classifier that throws powder into an air stream and separates it.

  In other words, mixed metals and silicon dioxide can be removed by acid cleaning, etc., but the abrasive grains are composed of silicon carbide, silicon nitride, diamond, etc., so they are chemically inert and use chemical reactions. Chemical removal due to the difference in the components is difficult, and classification based on the difference in particle size, particle size and density using an airflow classifier is required.

  However, the airflow classifier is a large-sized device compared to a general acid cleaning device or a heating device, and the use of this device increases the manufacturing and use costs.

  In addition, when applying the air classifier to the silicon-containing solid content, the abrasive particles and silicon particles are aggregated due to the coolant remaining in the silicon-containing solid content, or an example is shown in the table below. In addition, since the difference in particle size between the abrasive grains and the silicon grains is small, there is a limit to the classification accuracy, and the effect of removing the abrasive grains by this method is not necessarily large.

JP 2001-278612 A

  An object of the present invention is to provide a method for purifying silicon that efficiently removes the particulate impurities from recovered silicon containing silicon dioxide and any of silicon carbide, silicon nitride, and diamond as particulate impurities. It is said.

  The present inventors diligently studied about efficient removal of the particulate impurities from the recovered silicon. As a result, it was found that when the recovered silicon was heated and melted, the recovered silicon, that is, most of the abrasive grains contained in the silicon-containing solid content were contained as slag in the molten silicon generated by melting.

  Furthermore, the present inventors confirmed by analysis that the slag is composed of abrasive grains and silicon dioxide, pulverized the silicon lump and slag obtained by cooling the molten silicon, and then washed with a hydrofluoric acid aqueous solution. It has been found that slag can be dissolved and removed together with the abrasive grains, and the present invention has been completed.

However, according to the present invention, the melting step of heating silicon dioxide and recovered silicon containing silicon carbide, silicon nitride or diamond as particulate impurities to obtain a silicon melt;
A crushing step of crushing a silicon lump obtained by cooling the silicon melt to obtain silicon particles;
An acid cleaning step of cleaning the silicon grains with an acid solution to remove silicon dioxide together with the particulate impurities;
A method for purifying silicon is provided.

  According to the present invention, particulate impurities can be more efficiently removed from recovered silicon containing silicon dioxide and any of silicon carbide, silicon nitride, and diamond as particulate impurities.

  The term “recovered silicon” used in the present invention is a waste slurry mixed with silicon scrap generated when cutting or polishing a silicon lump (hereinafter also referred to as an ingot) or a silicon wafer using a slurry containing abrasive grains and a coolant. Or the silicon containing solid content obtained by carrying out solid-liquid separation of the concentrated part is meant.

Said slurry consists of an abrasive grain and the coolant which disperse | distributes it.
Examples of the abrasive grains include silicon carbide (SiC), diamond, cubic boron nitride (CBN), and alumina. The grain size of the abrasive grains is preferably 0.3 μm or more and 20 μm or less.
Therefore, the “particulate impurities” used in the present invention means abrasive grains in a slurry composed of any one of silicon carbide, silicon nitride, and diamond.

  The above-mentioned coolant includes, for example, an oil-based coolant (oil based on mineral oil), an aqueous coolant (water based glycol solvent (for example, ethylene glycol, propylene glycol or polyethylene glycol), surfactant, organic acid) And the like).

  The coolant is mainly composed of an organic solvent (water-soluble organic solvent) such as ethylene glycol, propylene glycol or polyethylene glycol, and additives such as organic acid and bentonite are not more than 10% by weight (preferably not more than 3% by weight). ) It may be added. Note that the expression “having an organic solvent as a main component” as used herein means that, for example, 20% by weight or less (preferably 15% by weight or less) of water may be contained in the coolant.

The above-mentioned waste slurry is a slurry in which silicon dioxide or silicon waste generated during cutting or polishing of a silicon ingot or silicon wafer using a slurry containing abrasive grains and coolant is mixed in the slurry.
The concentrated portion of the waste slurry is obtained by concentrating the waste slurry by solid-liquid separation or solvent distillation.

According to a more specific form of the present invention, the silicon purification method of the present invention causes the recovered silicon to melt during the process of cutting or polishing the silicon ingot or silicon wafer, Alternatively, the above-mentioned particulate impurities can be taken into the molten silicon dioxide contained as impurities during the natural oxidation of the raw silicon surface and / or metal grade silicon, and can be removed together with the silicon dioxide. Silicon purification becomes possible.
The above-mentioned “metal grade silicon” means silicon obtained by direct reduction from silica sand, usually lower in purity than 99.99%, but less expensive than semiconductor grade or solar cell grade silicon. It means silicon that can be obtained.

  In particular, in the silicon-containing solid content obtained from the waste slurry, since the silicon in the slurry is fine particles, the surface area per weight of the silicon is large, and thus silicon dioxide that can be formed on the surface of the silicon is Can be included relatively much.

  Hereinafter, as an example of recovered silicon containing silicon dioxide and any one of silicon carbide, silicon nitride, and diamond as particulate impurities, the silicon-containing solid content obtained by solid-liquid separation of the waste slurry or its concentrate is taken up. Here, a method for purifying silicon from which abrasive grains are removed will be described.

The shape of the silicon ingot is not particularly limited, and examples thereof include a cylindrical shape and a quadrangular prism shape.
Examples of the cutting device for cutting the silicon ingot include a multi-wire saw device (hereinafter referred to as “MWS”) widely used as a silicon ingot cutting device.

  The MWS is generally a wire wound between a plurality of rollers and wound, and a slurry containing abrasive grains and coolant is fed to the wire while the wire is run, and the object to be cut is pressed against the wire for cutting. It is a cutting device. 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-pulverized abrasive grains, and metal scraps that are wear pieces of wires and polishing wheels are contained in the slurry. It will be mixed.

Next, an embodiment of the present invention will be described. In the present invention, the silicon particles obtained by pulverizing the silicon lump obtained by melting the recovered silicon and then cooling are washed with an acid solution to dissolve and remove silicon dioxide. And a method for purifying silicon characterized by removing abrasive grains.
In the present invention, when recovered silicon is heated and melted, the recovered silicon, that is, most of the abrasive grains contained in the silicon-containing solid content are contained as slag in the molten silicon generated by melting.

  The slag contains the abrasive grains and the silicon dioxide, and after pulverizing the silicon lump and slag obtained by cooling the molten silicon, the silicon slag in the slag is washed with an aqueous hydrofluoric acid solution. It is a silicon purification method characterized in that it can be dissolved and removed together with abrasive grains.

  More specifically, by dissolving silicon dioxide in the slag in which silicon dioxide and abrasive grains are aggregated with the hydrofluoric acid aqueous solution or acid solution, the abrasive grains aggregated with silicon dioxide are dispersed, It remains as a solid with silicon grains. In this case, for example, when pulverization is performed so that the particle size of the silicon particles is in the range of 0.2 mm to 5 mm, the particle sizes of the silicon particles and the abrasive particles are greatly different as shown in Table 2 below.

したがって、上記の固形分として残るシリコン粒の粒径が砥粒の粒径よりも大きいため、酸洗浄工程またはそれに続く水洗工程において、シリコン粒が沈殿し易く、上記の工程の何れかにおいて、例えば、デカントや洗浄溶液または水洗液のオーバーフローなどの浮遊物分別法や、フィルター等を用いるろ過などの方法で容易に分離することができる。

Accordingly, since the particle size of the silicon particles remaining as the solid content is larger than the particle size of the abrasive particles, the silicon particles are likely to precipitate in the acid cleaning step or the subsequent water washing step. In any of the above steps, for example, It can be easily separated by a method for fractionation of floating substances such as decant, washing solution or washing solution overflow, or filtration using a filter or the like.

  In other words, the abrasive grains are agglomerated by the coolant and silicon dioxide through the melting step, the formed slag is pulverized in the pulverization step, and then the coolant and silicon dioxide are removed by the acid cleaning step, thereby eliminating these effects. Since the abrasive grains are easily dispersed and the grain sizes of the abrasive grains and the silicon grains are greatly different, the effect of removing the abrasive grains can be enhanced.

  The reason why most of the abrasive grains are contained in the slag is not necessarily clear, but generally the materials used as the abrasive grains have a higher melting point than the melting point of silicon (1420 ° C.) and the melting point of silicon dioxide (1728 ° C.). ) Is higher than silicon, so by controlling the melting temperature when melting silicon, the abrasive grains and silicon dioxide flow in the silicon melt in a solid state and are taken into the highly viscous slag. The surface tension of the melt is presumed to exist as slag on the silicon melt surface.

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

  As shown in FIG. 1, the silicon refining method of the present invention is a waste slurry in which silicon waste is mixed into a slurry by cutting or polishing a silicon ingot or silicon wafer using a slurry containing abrasive grains and a coolant, or a concentrated fraction thereof. Obtained by solid-liquid separation to obtain a silicon-containing solid content containing silicon scraps, a melting step for heating the silicon-containing solid content to obtain a silicon melt, and cooling the silicon melt It consists of a pulverization step for pulverizing the silicon lump to obtain silicon particles, and an acid cleaning step for cleaning the silicon particles with a cleaning solution comprising an acid solution.

Processes other than these (in this embodiment, a pre-cleaning process, a neutralization / dilution process, a remelting process, and a purification process) are optional processes, and can be appropriately provided as necessary.
However, in order to further increase the purity of the regenerated silicon obtained by the silicon purification method of the present invention, the steps after the melting step in the silicon purification method of the present invention may be repeated one or more times.

1. Solid-Liquid Separation Step First, the waste slurry or its concentrated component is subjected to solid-liquid separation by the solid-liquid separation device 1 to obtain a silicon-containing solid component. The configuration of the solid-liquid separation device 1 is not particularly limited as long as it is a configuration capable of obtaining a silicon-containing solid content by solid-liquid separation of the waste slurry or its concentrated component. The solid-liquid separation device 1 may be configured by, for example, a solid-liquid separation device such as a centrifuge, a filtration device, or a distillation device alone or in combination of two or more thereof in series.

Specific examples of combinations include
(1) Centrifuge and distillation device,
(2) A centrifuge and a filtration device or (3) a filtration device and a distillation device.

  In the above (1) to (3), two or more centrifuges, filtration devices, or distillation devices may be included, respectively. Each solid-liquid separation device may send either the separated liquid or solid content to the next solid-liquid separation device, a part of liquid and a mixture of solids or a part of solid and liquid. The mixture may be sent to the next solid-liquid separator.

2. Pre-cleaning step Next, in the pre-cleaning apparatus 2, the silicon-containing solid content is cleaned with a cleaning liquid made of an acid solution. In one example, the pre-cleaning device 2 includes a cleaning bowl and a stirrer provided in the cleaning bowl.

  This pre-cleaning process consists of (1) dissolving and removing residual organic substances derived from coolant such as glycol solvents and additives contained in the silicon-containing solid content, and (2) dissolving metal scraps that are wear pieces of wires. However, this may be omitted depending on the configuration of the apparatus.

As the particle size of the silicon-containing solid content, 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, it is difficult to recover the solids after cleaning the particle size decreases, practically, the particle size of the silicon-containing solid is preferably in the range of less than 0.01μm than 10 mu m. 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.
Further, the “powder having a particle size of less than X μm” means a powder having a particle size of 98% of the particles having a particle size of less than X μm.
Further, the “powder having a particle size of Y μm or more and less than Z μm” means a powder remaining after removing “a powder having a particle size of less than Y μm” from “a powder having a particle size of less than Z μm”.

As the cleaning liquid used in the pre-cleaning step, (1) an acid aqueous solution or (2) an organic solvent is used alone or in combination of two or more stages. The acid aqueous solution is preferably an inorganic acid aqueous solution that does not oxidize with respect to silicon. Examples of such an inorganic acid aqueous solution include hydrochloric acid, hydrofluoric acid, sulfuric acid, and the like.
The acid aqueous solution may be a mixture of a plurality of types of acid aqueous solutions.
The organic solvent is preferably compatible with the coolant and has a boiling point lower than that of the coolant. For example, an alcohol having 1 to 6 carbon atoms or a ketone having 3 to 6 carbon atoms is preferable.

Specific examples of the alcohol include methanol, ethanol, isopropyl alcohol, butyl alcohol and the like.
Moreover, acetone and methyl ethyl ketone are mentioned as a specific example of said ketone.
The organic solvent may be a mixture of a plurality of types of organic solvents.

3. Melting Step Next, in the melting apparatus 3, the silicon-containing solid content obtained by performing the pre-cleaning step is heated to a melting point (1420 ° C.) or higher of pure silicon to be melted, and then solidified to form a silicon lump. And
The melting temperature is preferably 1410 ° C. or higher and 2000 ° C. or lower, and more preferably lower than the melting point of silicon dioxide, that is, 1728 ° C. or lower in order to easily form slag containing abrasive grains and silicon dioxide.

The melting step is preferably performed in a vacuum or an inert gas atmosphere in order to suppress silicon oxidation in the step.
Therefore, the melting device 3 is preferably a sealed system, and further preferably has a pressure reducing port by a vacuum pump or an introduction part of an inert gas.

  The heating means in the melting apparatus is not particularly limited as long as a predetermined temperature can be obtained. For example, a resistance heater, induction heating, electron beam, arc melting, plasma melting, or the like can be used.

4). Pulverization process Next, in the pulverizing apparatus 4, the silicon lump obtained by melting is pulverized to a specific particle size. The pulverization step refers to all known methods for pulverizing a silicon lump to a specific size, and apparatuses such as a ball mill, a jet mill, and a vibration vacuum dryer can be used.

The pulverization is preferably performed so that the silicon particles have a particle size of 0.2 mm or more and 5 mm or less, and more preferably 0.5 mm or more and 2 mm or less.
When the particle size is larger than 5 mm, the surface area per mass is small, so that the area in contact with the acid solution is small in the acid treatment step described later, and the effect of removing silicon dioxide is small, which is not preferable. On the other hand, if the thickness is less than 0.2 mm, the surface area is increased, and oxidation or dissolution of silicon is promoted in the acid treatment step, which is not preferable.

5. Next, the silicon lump crushed in the crushing step is washed in the acid washing apparatus 5. In one example, the acid cleaning device 5 includes a cleaning bowl and a stirrer provided in the cleaning bowl.

As the cleaning liquid used in the pre-cleaning process, in order to dissolve the silicon dioxide contained in the slag generated by the melting process, the hydrofluoric acid aqueous solution or the hydrofluoric acid aqueous solution contains any of hydrochloric acid, sulfuric acid or nitric acid. An acid solution is used. In particular, a hydrofluoric acid aqueous solution having a concentration of 2% by weight to 25% by weight is preferable, and any one of hydrochloric acid, sulfuric acid, and nitric acid in a hydrofluoric acid aqueous solution having a concentration of 2% by weight to 25% by weight is 1% by weight or less. The mixed acid aqueous solution contained in is preferable. A hydrofluoric acid aqueous solution having a concentration of 5% by weight to 15% by weight is more preferable.
If the concentration of the hydrofluoric acid aqueous solution is 2% by weight or more and 25% by weight or less, the oxidation and dissolution of silicon hardly proceed and silicon dioxide can be dissolved effectively. Furthermore, the inclusion of 1% by weight or less of any one of hydrochloric acid, sulfuric acid, and nitric acid has the effect of further promoting dissolution of silicon dioxide and dissolving and removing metal scraps that could not be removed.

  The solvent of the acid solution preferably consists essentially of water, but may contain, for example, hydrogen peroxide. In this case, an effect of removing metal impurities that cannot be completely removed before melting can be expected. The ratio of hydrogen peroxide is, for example, 0.1 to 5% by weight.

  The silicon grains subjected to the acid cleaning step are preferably further subjected to a water washing step for removing the remaining acid aqueous solution and abrasive grains.

6). Neutralization / dilution step Next, the cleaning solution in the acid treatment step and the silicon-containing solid content in the cleaning solution are transferred to the neutralizer 6 to neutralize the acid solution. This neutralization treatment step is performed for the purpose of preventing corrosion of the device in the subsequent steps, 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. Further, the neutralization / dilution treatment may be performed in the acid cleaning device 5.

  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.

  After washing, the washing liquid is removed by a known method to obtain a solid content. Although the method is not particularly limited, for example, a filter, distillation, centrifugation, heat drying treatment or the like can be used.

7). Re-melting step Next, in the re-melting device 7, the solid content obtained after washing is heated and melted again. This remelting step is performed as a pretreatment for the purification step described later, but may be omitted depending on the configuration of the apparatus.

8). Purification Step Next, in the purification device 8, impurities contained in the silicon-containing melt obtained by melting the silicon-containing powder are further removed. The purification unit removes impurities using, for example, various known purification techniques (for example, removal of phosphorus under reduced pressure melting and removal of segregated impurities by unidirectional solidification) during conventional polycrystalline silicon casting. As a result, a silicon lump from which impurities are removed is obtained, but may be omitted depending on the configuration of the apparatus.
Further, this purification step may be performed in the re-dissolution apparatus 7.

The regenerated silicon grains from which impurities obtained by the refining device 8 are removed can be reused as silicon.
However, in order to further increase the purity of the regenerated silicon, the silicon may be purified by being subjected again to the steps after the “3. dissolution step”.

Although not particularly illustrated, a molding process, a classification / separation process, or a drying process can be incorporated before and after each of the above-described processes as necessary.
The molding process can be performed as a pretreatment for melting in order to increase the bulk specific gravity and increase the transportation efficiency, or to increase the thermal conductivity and facilitate melting, and pressurizes the 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 classification / separation step is a method of classifying particles 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. and so on.
In addition, the drying process includes a method of evaporating moisture and the like by heating, decompression, or a mixture thereof.

  Examples of the silicon regeneration apparatus and the purification method of the present invention will be described using specific numerical values. In this embodiment, silicon is regenerated using the silicon regenerating apparatus shown in FIGS. 1 and 2, and the description will proceed with reference to FIGS.

  In this example, about 15% by weight of water, a dispersing agent for facilitating dispersion of abrasive grains, and an organic acid as a pH adjuster were added to propylene glycol in an amount of about 1% by weight. Polycrystalline silicon was cut using a slurry in which abrasive grains were mixed at a weight ratio of 1: 1 to the coolant, and the waste slurry discharged from the MWS was used. Here, SiC having a particle size of 10 μm or more and 30 μm or less was used as the abrasive grains.

  The waste slurry contains about 10 wt% to 12 wt% of cutting waste made of silicon.

  First, solid-liquid separation of the waste slurry was performed in the solid-liquid separation unit 1 to obtain a silicon-containing solid content. As the solid-liquid separator 1, a unit including a primary centrifuge 9, a secondary centrifuge 10, and a distillation apparatus 11 was used. Solid-liquid separation was performed by combining primary centrifugation, secondary centrifugation and distillation. Details will be described below.

(1) Primary Centrifugation Step First, the waste slurry is put into a basket type made by Tanabe Wiltech Co., Ltd., and the centrifugal force is 500 G (relatively low centrifugal force, generally called “primary separation”). By operating the primary centrifuge so that the abrasive grains become the primary solid content (heavy specific gravity liquid) of the main component and the coolant and chips (mainly containing silicon) become the primary liquid content of the main component (low specific gravity liquid) separated.

(2) Secondary Centrifugation Step Next, the primary liquid (low specific gravity liquid) is put into a large-capacity cooling centrifuge (9900) manufactured by Kubota Corporation, and the centrifugal force is 3500G (relatively high centrifugal force). In general, this is called “secondary separation”) by operating the secondary centrifuge so that the secondary liquid component is mainly composed of coolant and the secondary solid content is mainly composed of chips and abrasive grains. Separated.

  Here, for the secondary liquid component and the secondary solid component, an ICP mass spectrometer (ELAN9000) manufactured by PerkinElmer Japan Co., Ltd. and a fluorescent X-ray analyzer (SEA1200VX) manufactured by SSI Nanotechnology Co., Ltd. are used. Table 3 shows the results of measurement using these. The unit of numerical values in Table 3 is% by weight unless otherwise specified.

(3) Distillation step The secondary liquid was put into an industrial distillation apparatus, and the silicon-containing solid content and the regenerated coolant were obtained by performing distillation on the secondary liquid at a final vacuum of 10 Torr and 160 ° C. The obtained silicon-containing solid content components are shown in Table 4. The unit of numerical values in Table 4 is% by weight unless otherwise specified.

  The solid content thus obtained was added with a 20% by weight hydrochloric acid aqueous solution in a polypropylene washing layer, washed with stirring for 4 hours, and filtered through a filter cloth (PP9F) manufactured by Nakao Filter Industry Co., Ltd. The solid content was recovered. About the solid content after washing | cleaning, it dried by heating to 150 degreeC in the air of 1 atm using the industrial resin dryer, and hold | maintaining for 2 hours.

  Next, it melt | dissolved by hold | maintaining at 1800 degreeC for 2 hours by argon 1atm atmosphere using the external heating type vacuum blast furnace, and after extracting to a casting_mold | template with tilting hot water, it cooled and the silicon lump was obtained. Table 5 shows the components of the silicon mass. The unit of numerical values in Table 5 is% by weight unless otherwise specified.

Next, the silicon lump was pulverized to 0.5 mm or more and 2 mm or less using a hammer mill crusher (D810) manufactured by Takeuchi Iron Works.
Next, 10% by weight hydrofluoric acid aqueous solution was added to the pulverized material in a polypropylene cleaning layer, and the mixture was washed with stirring in the cleaning layer for 4 hours.
This was separated into solid and liquid by filtration, and further washed with water until pH 4 was reached. This time, no neutralization / dilution process was performed. The components of the silicon mass after washing thus obtained are shown in Table 6. The unit of numerical values in Table 6 is% by weight unless otherwise specified.
Compared with Table 5, it can be seen that the SiC concentration is particularly decreased.

次に、一方向凝固が可能な機構を備えた外熱式の真空溶鉱炉を用いてアルゴン１ａｔｍ雰囲気下において前記シリコン塊を１６００℃まで融解し、一方向凝固による精製を２回行い、日平トヤマ製ワイヤーソーを用いてスライス加工して多結晶シリコン基板とし、太陽電池セルを作成したところ、太陽光から電気エネルギーへの変換効率は１１〜１３％となった。

Next, using an externally heated vacuum blast furnace equipped with a mechanism capable of unidirectional solidification, the silicon mass is melted to 1600 ° C. in an argon atmosphere of 1 atm and purified by unidirectional solidification twice. When a solar cell was prepared by slicing using a saw to obtain a polycrystalline silicon substrate, the conversion efficiency from sunlight to electric energy was 11 to 13%.

  According to the present invention, particulate impurities can be more efficiently removed from recovered silicon containing silicon dioxide and any of silicon carbide, silicon nitride, and diamond as particulate impurities. Can be reused as recycled silicon.

It is explanatory drawing for demonstrating the purification method of the silicon | silicone of one Embodiment of this invention. It is a block diagram which shows the 1st structural example of the solid-liquid separation process of FIG.

Explanation of symbols

1: Solid-liquid separator (solid-liquid separator)
2: Pre-cleaning device 3: Melting device 4: Grinding device 5: Acid cleaning device 6: Neutralization device 7: Remelting device 8: Purification device 9: Primary centrifuge 10: Secondary centrifuge 11: Distillation device

Claims (10)

A melting step of heating silicon dioxide and recovered silicon containing silicon carbide, silicon nitride or diamond as particulate impurities to obtain a silicon melt;
A crushing step of crushing a silicon lump obtained by cooling the silicon melt to obtain silicon particles;
An acid cleaning step of cleaning the silicon grains with an acid solution to remove silicon dioxide together with the particulate impurities;
A method for purifying silicon, comprising:   The recovered silicon contains a silicon waste generated when cutting or polishing a silicon ingot or a silicon wafer using a slurry containing abrasive grains and a coolant, and a waste slurry containing silicon dioxide, or a concentrated portion thereof is subjected to solid-liquid separation. The method according to claim 1, wherein the obtained silicon-containing solid content and the particulate impurities are the abrasive grains.   The method according to claim 1 or 2, wherein the melting step is performed in a vacuum or an inert gas atmosphere.   The method according to any one of claims 1 to 3, wherein the crushing step is a step of crushing the silicon lump into silicon particles having a particle diameter of 0.2 mm to 5 mm.   The acid cleaning step comprises cleaning the silicon particles with a hydrofluoric acid aqueous solution or an acid solution containing any one of hydrochloric acid, sulfuric acid or nitric acid in a hydrofluoric acid aqueous solution. The method described in one.   In the acid cleaning step, a hydrofluoric acid aqueous solution having a concentration of 2% to 25% by weight, or a hydrofluoric acid aqueous solution having a concentration of 2% to 25% by weight, and any one of hydrochloric acid, sulfuric acid, and nitric acid at a concentration of 1% by weight or less. The method according to any one of claims 1 to 5, further comprising a water washing step of washing the silicon particles with the acid solution contained to remove the particulate impurities.   The method according to claim 1, wherein the silicon purification method further includes a step of neutralizing or diluting the acid solution after the acid cleaning step.   The silicon purification method obtained after the acid washing step, or the step of re-melting the silicon particles obtained by subjecting the acid solution to the step of neutralizing or diluting is further subjected to the melting step. The method of claim 7 comprising.   The method according to claim 8, wherein the method for purifying silicon includes a pulverizing step and an acid washing step of further pulverizing a silicon lump obtained after the step of re-melting in the melting step.   The process after the melting step is repeated at least once for the silicon particles obtained after the acid washing step or after the step of neutralizing or diluting the acid solution. The method as described in any one of -9.

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