JP2009196849A - Silicon recovery method and silicon recovery apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon recovery method by which a large amount of high purity silicon is simply recovered from waste slurry. <P>SOLUTION: The silicon recovery method is provided with a first solid-liquid separation step for solid-liquid-separating waste slurry or its concentrated material which has silicon scrap mixed in slurry due to cutting or grinding a silicon wafer using the slurry containing abrasive and coolant to obtain a solid portion for silicon recovery containing the silicon scrap; a cleaning step for cleaning the solid portion for silicon recovery using a cleaning solution; and a second solid-liquid-separating step for solid-liquid separating the cleaning solution and the solid portion for silicon recovery, wherein the coolant contains liquid organic material and the cleaning solution is compatible with the liquid organic material and contains a low boiling point solvent having a boiling point lower than that of the liquid organic material and water. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a silicon recovery method and apparatus for obtaining reclaimed silicon having a high silicon content 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 residual coolant, water cleaning to wash away organic solvent, metals (iron, copper, etc.) contained in waste slurry dissolved in acid aqueous solution (hydrofluoric acid aqueous solution, etc.) In this case, acid cleaning for removing them and water cleaning for washing away the acid aqueous solution are performed.
JP 2001-278612 A

However, when a conventional method as disclosed in Patent Document 1, that is, when an acid aqueous solution or pure water is used for cleaning, silicon oxide is generated by oxidation of silicon, the silicon recovery rate is reduced. It has been found that problems such as a drop in purity occur.
In addition, the present inventors have found a problem that in the conventional method, the solid content is agglomerated by a chemical reaction in an acid aqueous solution or water, and the process after washing may be difficult.

  In view of such circumstances, the present invention provides a silicon recovery method that can easily recover a large amount of high-purity silicon from waste slurry.

  According to the silicon recovery method of the present invention, a silicon lump using a slurry containing abrasive grains and a coolant or a silicon lump or silicon wafer is cut or polished to solid-liquid separate a waste slurry in which silicon scrap is mixed or a concentrated component thereof. A first solid-liquid separation step of obtaining solids for collecting silicon containing scraps, a washing step of washing the solids for collecting silicon using a washing solution, and the washing solution and the solids for collecting silicon after the washing. A second solid-liquid separation step for solid-liquid separation, wherein the coolant includes a liquid organic substance, and the cleaning liquid is compatible with the liquid organic substance and has a lower boiling point than the liquid organic substance And water.

The cleaning liquid may further include an inorganic acid.
The ratio of water in the cleaning liquid may be less than 30 wt%.
The low boiling point organic solvent in the cleaning liquid may be composed of one selected from the group consisting of alcohols having 1 to 6 carbon atoms and ketones having 3 to 6 carbon atoms, or may be composed of a mixture of two or more.
The low-boiling organic solvent in the cleaning liquid may be composed of one selected from the group consisting of methanol, ethanol, isopropyl alcohol, and acetone, or a mixture of two or more.
These low-boiling organic solvents are compatible with propylene glycol (boiling point 187 ° C.) widely used as a liquid organic substance in the coolant and have a low melting point of about 50 to 150 ° C. is there.
The coolant may comprise a water soluble coolant.

  Further, the silicon recovery apparatus of the present invention solid-liquid separates the waste slurry in which silicon scrap is mixed into the slurry or the concentrated portion thereof by cutting or polishing the silicon lump or silicon wafer using the slurry containing abrasive grains and coolant. A first solid-liquid separation unit for obtaining silicon recovery solids containing silicon scrap, a cleaning unit for cleaning the silicon recovery solids using a cleaning liquid, and the cleaning liquid and the silicon recovery solid after the cleaning A second solid-liquid separation part that separates the liquid into solid and liquid, the coolant includes a liquid organic material, and the cleaning liquid has a low boiling point that is compatible with the liquid organic material and has a lower boiling point than the liquid organic material A silicon recovery apparatus comprising an organic solvent and water.

As a result of intensive studies, the inventors have found that when a small amount of water is added to a cleaning liquid composed of a low-boiling organic solvent, a dramatic increase in cleaning effect can be seen that can be predicted from the ratio of the added water. As a result, the present invention has been completed.
In addition, since the cleaning liquid in the present invention contains a low-boiling organic solvent, it is possible to suppress silicon oxidation as compared with the case where a cleaning liquid such as an acid aqueous solution is used, and the silicon recovery rate is reduced due to silicon oxidation. And / or purity reduction of recovered silicon can be suppressed.
In addition, since the cleaning liquid in the present invention contains a low-boiling organic solvent, the liquid organic material derived from the coolant remaining in the solids for silicon recovery after the first solid-liquid separation part can be easily removed by dissolving in the cleaning liquid. .

  As described above, according to the present invention, high-purity silicon can be recovered at a high recovery rate from waste slurry containing silicon waste generated by cutting or polishing silicon, which is currently discarded.

  Hereinafter, various embodiments of the present invention will be exemplified.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

  According to the silicon recovery method of the present invention, a silicon lump using a slurry containing abrasive grains and a coolant or a silicon lump or silicon wafer is cut or polished to solid-liquid separate a waste slurry in which silicon scrap is mixed or a concentrated component thereof. A first solid-liquid separation step of obtaining solids for collecting silicon containing scraps, a washing step of washing the solids for collecting silicon using a washing solution, and the washing solution and the solids for collecting silicon after the washing. A second solid-liquid separation step for solid-liquid separation, wherein the coolant includes a liquid organic substance, and the cleaning liquid is compatible with the liquid organic substance and has a lower boiling point than the liquid organic substance And water.

  This method can be implemented using the silicon recovery apparatus of one embodiment of the present invention shown in FIG. Hereinafter, the description will be focused on the silicon recovery apparatus of the present embodiment. The following description is basically applicable to the silicon recovery method of this embodiment.

  The silicon recovery apparatus of the present embodiment solid-liquid separates the waste slurry in which silicon waste is mixed into the slurry or the concentrated portion thereof by cutting or polishing the silicon lump or silicon wafer using the slurry containing abrasive grains and coolant. A first solid-liquid separation unit 1 that acquires silicon recovery solids containing silicon scrap, a cleaning unit 3 that cleans the silicon recovery solids using a cleaning liquid, and the cleaning liquid and the silicon recovery after the cleaning And a second solid-liquid separation unit 4 for solid-liquid separation of the solid content. The coolant includes a liquid organic material, and the cleaning liquid includes a low-boiling organic solvent that is compatible with the liquid organic material and has a boiling point lower than that of the liquid organic material, and water.

  Moreover, the silicon collection | recovery apparatus of this embodiment is provided with one or more of the drying part 7, the classification part 5, the metal waste removal part 9, the shaping | molding part 11, the heating part 13, and the refinement | purification part 15 as needed. A solidifying unit 17 may be provided instead of the purifying unit 15.

  This will be described in detail below.

1. Waste Slurry, Concentration of Waste Slurry Before describing the embodiment of the silicon recovery apparatus, first, the waste slurry and its concentration will be described.

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 recovery apparatus according to the present embodiment is for recovering silicon scraps mixed in waste slurry to obtain recycled silicon. 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.

  The silicon lump or the silicon wafer is cut or polished using a cutting device or a polishing device, and the used slurry discharged from the cutting device or the polishing device is a waste slurry.

  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, travels while supplying slurry containing abrasive grains and coolant to the wire, and presses and cuts an object to be cut against the wire. . When a silicon ingot is cut using such MWS, silicon scraps, crushed abrasive grains and non-crushed abrasive grains, and metal scraps that are wear pieces of wires are mixed in the slurry. Become.

  In MWS, a slurry is usually used repeatedly, but the proportion of silicon or the like contained in the slurry increases with use. When these ratios increase, for example, when the silicon ratio in the slurry is 5 wt% or more, various problems such as defects such as thickness unevenness and warping occur in the silicon wafer, and wire breakage may occur. It has been known. For this reason, part or all of the slurry is appropriately discharged out of the MWS as a waste slurry, and a new slurry is supplied to the MWS. The waste slurry discharged out of the MWS is processed by the silicon recovery apparatus of this embodiment.

Here, the composition and composition of the slurry will be described. The slurry consists of abrasive grains and coolant that disperses them. 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 as long as it includes liquid organic matter (liquid organic matter such as oil or glycol solvent). For example, an oil-based coolant (oil based on mineral oil) or a water-soluble coolant (water And a glycol solvent such as ethylene glycol, propylene glycol or polyethylene glycol, a surfactant, an organic acid, etc.). The coolant is mainly composed of an organic solvent such as ethylene glycol, propylene glycol or polyethylene glycol, wherein an organic acid aqueous solution for adjusting the pH is preferably 10 wt% or less, and an additive such as bentonite is preferably 10 wt% or less. Preferably 3 wt% or less may be added. Here, “having an organic solvent as a main component” means that, for example, the coolant may contain moisture of preferably 20 wt% or less, more preferably 15 wt% or less.
The coolant is preferably a water-soluble coolant. In this case, there is an advantage that the liquid organic substance is easily removed by dissolving in a low boiling point organic solvent.

2. The first solid-liquid separation unit The configuration of the first 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 content to obtain a solid content for silicon recovery, The 1st solid-liquid separation part 1 is comprised, for example by combining solid-liquid separation apparatuses, such as a centrifuge, a filtration apparatus, or a distillation apparatus, individually or two or more 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 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.

  Here, the structural example of the 1st solid-liquid separation part 1 is demonstrated using FIG. FIG. 2 is a block diagram showing the configuration of the first solid-liquid separator 1. The first solid-liquid separation unit 1 of this configuration example includes a primary centrifuge 19, a secondary centrifuge 21, and a distillation device 23.

  The primary centrifuge 19 separates the waste slurry or its concentrated component into a primary liquid component and a primary solid component by primary centrifugation. The primary centrifugation is performed at a relatively low speed, for example, 100 G or more and 1000 G or less. Since the primary solid content is mainly composed of abrasive grains, it can be reused as regenerated abrasive grains as it is or after processing such as washing and drying. The primary liquid is sent to the secondary centrifuge 21. The primary liquid may be sent directly to the distillation apparatus 23 instead of being sent to the secondary centrifuge 21. In this case, the secondary centrifuge 21 can be omitted.

  The secondary centrifuge 21 separates the primary liquid component into a secondary liquid component and a secondary solid content by secondary centrifugation. The secondary centrifugation is performed at a relatively high speed, for example, 2000 G or more and 5000 G or less. Secondary solids mainly contain silicon and also contain abrasive grains that could not be separated by primary centrifugation. The secondary solids may be discarded or part or all of it may be used for silicon regeneration. Since silicon is mainly contained in the secondary liquid, it is possible to obtain a silicon recovery solid content mainly composed of silicon by distilling the secondary liquid. The secondary liquid is sent to the distillation device 23. In addition, you may send a secondary solid content to the distillation apparatus 23 instead of a secondary liquid component. Moreover, you may send to the distillation apparatus 23 what mixed a part of secondary liquid part and secondary solid content, and what mixed a part of secondary solid part and secondary liquid part.

  The distillation apparatus 23 separates the secondary liquid component into a distillate liquid component and a distilled solid content by distillation. The distillation is preferably performed under reduced pressure (for example, 5 Torr to 20 Torr). This is because distillation at a relatively low temperature and / or high speed becomes possible because the boiling point of the liquid is lowered by the reduced pressure. The distillate can be reused as it is (distilled coolant) or separately as a regenerated coolant by MWS or the like. The distilled solid content is sent to the cleaning unit 3 as a solid content for silicon recovery.

3. Next, the cleaning unit 3 will be described. The cleaning unit 3 cleans the silicon recovery solids using a cleaning liquid. The cleaning liquid contains a low-boiling organic solvent that is compatible with the liquid organic substance contained in the coolant and has a lower boiling point than the liquid organic substance, and water. The cleaning unit 3 includes, for example, a container for storing the silicon recovery solid content and the cleaning liquid, and a stirring unit for stirring the silicon recovery solid content and the cleaning liquid in the container.

The solid content for silicon recovery usually contains about 5 wt% to 20 wt% of a liquid organic material derived from a coolant such as a glycol solvent or oil, and as it is, it causes a decrease in the purity of the regenerated silicon. Hereinafter, the liquid organic substance derived from the coolant remaining in the solid content for silicon recovery is referred to as residual coolant.
The residual coolant forms SiC when the silicon recovery solid content is melted by the heating unit 13 and causes unnecessary SiC to be generated in the silicon ingot formed by solidifying the molten silicon. Therefore, in order to reduce the residual coolant concentration, the silicon recovery solids are washed.

  In the cleaning in this step, the residual coolant is dissolved in the cleaning liquid. Specifically, the cleaning can be performed, for example, by mixing and stirring the solid content for silicon recovery and the cleaning liquid. In this embodiment, since the low boiling point organic solvent in the cleaning liquid is compatible with the liquid organic substance, the residual coolant can be dissolved in the cleaning liquid relatively easily.

The kind of the low boiling point organic solvent is not limited as long as it is compatible with the liquid organic substance contained in the coolant and has a lower boiling point than the liquid organic substance. In the case where a plurality of types of liquid organic substances are contained in the coolant, it is sufficient that the coolant has compatibility with at least one of the plurality of types of liquid organic substances and has a lower boiling point than the liquid organic substances. It is preferable that they are compatible with all kinds of liquid organic substances and have a boiling point lower than any of those liquid organic substances. Even in the former case, at least one liquid organic substance in the coolant can be dissolved and removed in the low boiling point organic solvent, and in the latter case, all kinds of liquid organic substance in the coolant can be dissolved in the low boiling point organic solvent. This is because it can be removed.
The low boiling point organic solvent is, for example, an alcohol having 1 to 6 carbon atoms (preferably, a range between any two of 1, 2, 3, 4, 5 and 6) or 3 to 6 carbon atoms (preferably , 3, 4, 5 and 6). Specific examples of such alcohol include methanol, ethanol, isopropyl alcohol, butyl alcohol and the like. Specific examples of such ketones include acetone and methyl ethyl ketone. The low boiling point organic solvent may be a mixture of a plurality of types of organic solvents. Moreover, it is preferable that a low boiling-point organic solvent consists of methanol, ethanol, isopropyl alcohol, acetone, or a mixture of two or more of these. These have the advantage of improving the characteristics of the solar cell by being easily removed when heating the solid content for silicon recovery obtained by washing.
From another point of view, the low-boiling organic solvent preferably has a boiling point lower by 50 ° C. or more (preferably 60 ° C., 70 ° C., 80 ° C., 90 ° C. or 100 ° C. or more) than the liquid organic material. This is because the low-boiling organic solvent is usually removed by evaporation in a later step, but if it has a low boiling point, it is easily evaporated.
The ratio of the low boiling point organic solvent in the cleaning liquid is, for example, 50 to 99.9 wt%, specifically, 50, 60, 70, 71, 72, 73, 74, 75, 80, 90, 95, 96. 97, 98, 99, 99.5, 99.9 wt%. The ratio of the low boiling point organic solvent in the cleaning liquid may be within a range between any two of the numerical values exemplified here.

In addition, the solid content recovered from the waste slurry contains iron and copper, which are wire chips, as well as alkali metals such as sodium and calcium, and alkaline earth metals. These metal impurities exist not only as a simple substance but also as a salt with a compound, particularly an organic acid contained in the coolant. Since these compounds are generally poorly soluble in organic solvents and soluble in water, water is added to the cleaning solution. This makes it easy to dissolve the compound in the cleaning solution during the cleaning in this step. The ratio of water in the cleaning liquid is, for example, 0.1 to 50 wt%, specifically, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 26, 27, 28, 29, 30, 40, 50 wt%. The ratio of water in the cleaning liquid may be within a range between any two of the numerical values exemplified here.
In addition, when the proportion of water in the cleaning liquid is increased, the water activity increases and the silicon is oxidized, resulting in a decrease in the silicon recovery rate and agglomeration of the solids for recovery due to a chemical reaction with the alkali metal. It becomes easy. It has been confirmed that agglomeration is likely to occur when the ratio of water in the cleaning liquid is actually 30 wt% or more. Accordingly, the ratio of water in the cleaning liquid is preferably less than 30 wt%.
If agglomeration occurs, the effect of classification and removal of metal debris, which will be described later, cannot be obtained, so that it is necessary to pulverize separately. In such a case, since there is a high possibility that oxidation is promoted, the recovery rate of silicon obtained by melting may be lowered.
In order to increase the silicon recovery rate, the ratio of water in the cleaning liquid is more preferably 1 to 10 wt%, and further preferably 1 to 2 wt%.

Further, it is preferable to further add an acid aqueous solution to the cleaning liquid. Since the metal impurity compound is also soluble in an acid aqueous solution, the metal impurity compound can be further easily dissolved in the cleaning liquid by including the acid aqueous solution in the cleaning liquid. The type of the acid aqueous solution is not particularly limited, and examples thereof include hydrochloric acid and hydrofluoric acid.
The total ratio of water and acid aqueous solution in the cleaning liquid is, for example, 0.1 to 50 wt%, specifically 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 26, 27, 28, 29, 30, 40, 50 wt%.

  The cleaning liquid may consist of only the low-boiling organic solvent and water or only the low-boiling organic solvent, water and the acid aqueous solution, and may contain other components. When other components are included, the total ratio of the low-boiling organic solvent and water or the total ratio of the low-boiling organic solvent, water, and the acid aqueous solution is, for example, 50 wt% to 99.9 wt%. 50, 60, 70, 80, 90, 95, 99, 99.5, 99.9 wt%. This total ratio may be within a range between any two of the numerical values exemplified here.

4). The second solid-liquid separation unit The second solid-liquid separation unit 4 has a configuration capable of solid-liquid separating the cleaning liquid and the silicon recovery solid content after the cleaning to obtain the silicon recovery solid content. It is not limited, A structure equivalent to the 1st solid-liquid separation part 1 may be used, and a different structure may be used. However, when an aqueous acid solution is added to the cleaning liquid or when a coolant containing an organic acid is used, the cleaning liquid after cleaning may be acidic due to the aqueous acid solution or organic acid. Therefore, it is preferable to use a material that is resistant to acid as the material of the second solid-liquid separation unit 4.

5. Drying unit The drying unit 7 removes the cleaning liquid remaining in the solid content for silicon recovery after the second solid-liquid separation, and dries the solid content for silicon recovery. The silicon recovery solid content can be dried by, for example, heating the silicon recovery solid content or reducing the ambient atmosphere of the silicon recovery solid content.
The drying unit 7 may be a drying and pulverizing unit having a function of pulverizing the solid content for silicon recovery. Drying and pulverization may be performed at the same time, pulverization may be performed after drying, or vice versa. The pulverization of the solid content for silicon recovery can be performed using a known device such as a pulverizer using a pulverizing blade, a ball mill, a jet mill, or a vibration vacuum dryer.

6). Classification unit The classification unit 5 classifies the solid content for silicon recovery after washing. One of the purposes of classification is to reduce the content of abrasive grains and increase the content of silicon than before classification. Classification is a method of classifying particles based on particle parameters such as particle size and density. The classifying unit 5 can be configured by a sieve, an inertia classifier, a centrifugal classifier, or the like.

7). Metal Waste Removal Unit The metal waste removal unit 9 has a function of removing ferromagnetic metal waste such as wire waste mixed into waste slurry when cutting or polishing a silicon lump or silicon wafer using a magnetic field. Metal debris may be removed with silicon or SiC attached. The metal scrap removing unit 9 is configured by, for example, a magnet and a vibration stirrer.

  The removal of metal debris can be achieved by collecting the solid content for silicon recovery dispersed in the cleaning liquid, or the solid content for silicon recovery in a powder state, the powder being conveyed by airflow during classification, and the solid content for silicon recovery after classification. This can be done for any one or more. Therefore, the metal waste removal unit 9 can be provided in any one or more of the first cleaning unit 3, the drying unit 7, and the classification unit 5, for example. The metal concentration in the recycled silicon can be reduced by removing the metal scrap from the solid content for silicon recovery. Moreover, the wire used for MWS often contains phosphorus, and in this case, the metal waste mixed in the waste slurry also contains phosphorus. Phosphorus is a component that is unnecessary for the production of a P-type solar cell, and therefore it is preferable to remove it before melting. However, according to this embodiment, phosphorus is removed together with removal of metal debris.

8). Forming part The shape of the forming part 11 is not particularly limited as long as it is a device having a function of pressurizing the solid content for silicon recovery and granulating it into a plate shape, block shape, pellet shape or the like. For example, it can be performed using a known apparatus such as a compression granulator or an extrusion granulator.

9. Heating unit The heating unit 13 has a function of heating and melting the solid content for silicon recovery. The heating unit 13 may be any unit that can heat the solids for silicon recovery above the melting point of silicon to melt the solids for silicon recovery, but is preferably 1800 ° C. or higher, more preferably 2000. It is more preferable to have an introduction part of an inert gas that can be heated at a temperature of 0 ° C. or higher.

  Further, preferably, before heating at a temperature equal to or higher than the melting point of silicon, the solid content for silicon recovery is reduced at a temperature lower than the melting point of silicon and higher than the boiling point of the cleaning liquid used in the cleaning section 3 in a reduced pressure or in the presence of an inert gas. Can be performed as a pretreatment by baking and removing residual liquid of the cleaning liquid. For example, the solid content for silicon recovery can be heated to a temperature of 300 ° C. or higher and 600 ° C. or lower in an inert gas atmosphere and held for 2 hours or longer to remove impurities.

10. Purification unit The purification unit 15 has a function of removing impurities contained in the silicon-containing melt obtained by melting the solid content for silicon recovery. For example, the purification unit 15 removes impurities using 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. Thereby, a silicon lump from which impurities are removed is obtained.

  The silicon lump from which impurities obtained by the purification unit 15 are removed can be recovered as recycled silicon as it is.

10. Solidified part The solidified part 17 has a function of solidifying by naturally cooling or forcibly cooling the silicon-containing melt, whereby a silicon lump is obtained. This silicon mass can be recovered as recycled silicon.

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

In this example, polycrystalline silicon was cut using a slurry in which a coolant and abrasive grains were mixed at a weight ratio of 1: 1, and waste slurry discharged from MWS was used. The coolant contains water (about 15 wt%), a dispersant for facilitating dispersion of abrasive grains and the like, and an organic acid as a pH adjuster (about 1 wt%), and the balance is made of propylene glycol Was used.
The waste slurry contains about 10 wt% to 12 wt% of cutting waste made of silicon.

1-1. First Solid-Liquid Separation Step First, the solid content for silicon recovery was obtained by performing solid-liquid separation of the waste slurry in the first solid-liquid separation unit 1. As the first solid-liquid separator 1, one including a primary centrifuge 19, a secondary centrifuge 21 and a distillation device 23 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 the primary centrifuge 19 so that the centrifugal force becomes 500 G (relatively low centrifugal force, generally called “primary separation”). By operating the centrifugal separator 19, the abrasive grains were separated into the primary solid content (heavy specific gravity liquid) of the main component and the coolant and chips (mainly containing silicon) were separated into the primary liquid content (low specific gravity liquid) of the main component.

(2) Secondary Centrifugation Step Next, the primary liquid (low specific gravity liquid) is put into the secondary centrifuge 21, and the centrifugal force is 3500G (relatively high centrifugal force. By operating the secondary centrifuge 21 so as to be “separation”, the coolant was separated into the secondary liquid component, and the chips and abrasive grains were separated into the secondary solid content.

  Here, the components of the secondary liquid and the secondary solid are shown in Table 1 below. In the present example, 80 kg of secondary liquid and 100 kg of secondary solid were obtained from 500 kg of waste slurry. The unit of numerical values in Table 1 is wt% unless otherwise specified.

(3) Distillation step Next, the secondary liquid is put into the distillation apparatus 23, and the solid content for recovering silicon and the regenerated coolant are obtained by performing distillation at a final vacuum of 10 Torr and 160 ° C on the secondary liquid. Obtained. The obtained solid components for silicon recovery are shown in Table 2 below. The unit of numerical values in Table 2 is wt% unless otherwise specified.

1-2. Next, in the cleaning unit 3, the solid content for silicon recovery and the cleaning liquid are mixed at a mass ratio of 1:10 using a cleaning liquid in which IPA and pure water are mixed at a weight ratio of 98: 2. And mixed with stirring for 1 hour. During the stirring, the metal waste contained in the stirring solution was removed using the metal waste removal unit 9 made of a magnet having a magnetic force of 1.4T. Thereafter, a small amount of hydrochloric acid was added so that the pH of the cleaning solution was 1.5 to 5, and metal scraps contained in this stirring solution were further removed.

1-3. Second Solid-Liquid Separation Step Next, in the second solid-liquid separation unit 4, the above-mentioned stirred solution was subjected to solid-liquid separation, and the solid content for silicon recovery was taken out. Solid-liquid separation was performed by filtration.

1-4. Next, in the drying unit 7, the solid content for silicon recovery was dried. In this drying, the solid content was dried for 1 hour by heating to 60 ° C. in air at 0.1 atm.

1-5. Classification process Next, the solid content for silicon recovery was classified in the classification unit 5 composed of a centrifugal classifier. Since the slurry used this time contains SiC abrasive grains having a particle size of about 10 μm and silicon cutting waste having a particle size of about 0.1 to 3 μm, the rotational speed is set to separate particles having a particle size of 5 μm or more. The SiC was separated by setting.

1-6. Molding step Next, in the molding part 11 composed of an extrusion granulator, the solid content for silicon recovery is granulated at 25 ° C. and 30,000 N / m ² to obtain a pellet shape of about 1 mm × 1 mm × 0.5 mm. .

1-7. Next, the pelletized silicon after granulation was baked, melted, and purified in an apparatus that also serves as the heating unit 13 and the purification unit 15.

  Specifically, the pelletized silicon after granulation is placed in a graphite crucible and fired at 600 ° C. for 1 hour by resistance heating under a vacuum of 10 Torr, so that a slight amount of organic matter remaining in the pelleted silicon can be removed. Next, silicon was melted at 1800 ° C. by high-frequency induction heating in an Ar atmosphere, and then the temperature was lowered from below the crucible, so that silicon was unidirectionally solidified to obtain a silicon lump. Furthermore, the upper part (concentration part of a metal impurity) of the obtained silicon lump was cut and removed. This unidirectional solidification and removal of the impurity concentration portion were repeated twice to obtain 4.7 kg of a regenerated silicon ingot.

  Next, the recycled silicon ingot was cut to a thickness of 250 μm with MWS to obtain a recycled silicon wafer (polycrystalline substrate). Using this recycled silicon wafer, a solar cell was fabricated and the photoelectric conversion characteristics were measured.

  Table 3 shows the characteristics of the solar cell using the recycled silicon wafer in this example and the solar cell using a commercially available normal silicon wafer for solar cells. Table 3 shows a relative comparison of the solar cell characteristics in this example, with the characteristics of a solar cell using a normal silicon wafer for solar cells being 100%.

2. Comparative Example 1
In Comparative Example 1, only IPA was used as the cleaning liquid used in the cleaning unit 3, and the rest of the silicon was recovered under the same conditions as in Example 1. At this time, 4.8 kg of regenerated silicon ingot was obtained from 12.8 kg of the solid for recovery.
Next, a solar cell was produced using a recycled silicon wafer obtained by cutting the recycled silicon ingot with a thickness of 250 μm with MWS, and the photoelectric conversion characteristics were measured.

  Table 4 shows the characteristics of the solar cell using the recycled silicon wafer in Comparative Example 1. Table 4 shows a relative comparison of the solar cell characteristics in Comparative Example 1 with the characteristics of a solar cell using a normal solar cell silicon wafer being 100%.

  Comparing Table 3 and Table 4, it can be seen that the maximum output Pmax is superior in the first embodiment than in the first comparative example.

  As described above, the photoelectric conversion characteristics were improved when Example 1 was used, and the characteristics could be brought closer to those of a commercially available ordinary silicon wafer for solar cells. As is clear from FIG. 3 relating to the related experiment described later, this is considered to be due to the increase in the cleaning effect and the reduction of impurities by adding pure water.

  On the other hand, the recovered amount of regenerated silicon obtained from the same amount of recovered solid content was slightly reduced, but more highly regenerated silicon can be obtained.

3. Experiment for demonstrating the effect of including water in the cleaning liquid In this experiment, 500 g of the cleaning liquid was added to 50 g of the solid content for silicon recovery, and the mixture was stirred with a stirrer for 15 minutes, followed by ultrasonic cleaning for 30 minutes. Further, solid-liquid separation by filtration was performed, and the solid content was heated to 60 ° C. in air at 0.1 atm to dry for 1 hour.

  In the above-described method, cleaning was performed with a cleaning solution containing pure water and the balance being IPA. The ratio of pure water in the cleaning liquid was 1, 2, 5, 10, 15, 25, 30, 50 wt%. For comparison, a cleaning solution containing only IPA was also evaluated.

  About each solid content for silicon | silicone collection | recovery, the amount of residual organic impurities was measured by the thermobalance measuring method. Further, the silicon recovery rate was calculated from the silicon weight after melting with respect to the solid content for silicon recovery before washing. FIG. 3 shows the results together with the measurement results.

As shown in FIG. 3, the amount of organic impurities removed is increased by adding pure water as compared with cleaning using only IPA. In particular, it was found that the cleaning effect increases dramatically even when a small amount of about 1 wt% is added. The action is not always clear, but as mentioned above, the solids for silicon recovery contain salts of metals and organic acids, and the ionization of these causes the cleaning solution to become acidic, increasing the cleaning effect. Presumed to be.
Further, the silicon recovery rate was large when the amount of water added was 10 wt% or less, and was larger when the amount of water added was 2 wt% or less. Further, the silicon recovery rate was relatively small when the amount of water added was 15 wt% or more. It is assumed that such a result was obtained because the oxidation of silicon was promoted when the amount of water added was large.

  In addition, Table 5 shows the presence or absence of agglomeration of the solid content for silicon recovery after drying. As shown in Table 5, when the amount of pure water added was 30 wt% or more and 1 hour or more passed in the solvent, agglomeration gradually occurred. The action is not necessarily clear, but it is assumed that the alkali metal causes a chemical reaction due to an increase in pure water. If the agglomeration progresses remarkably, it is necessary to go through a process such as pulverization, and thus the cleaning conditions are limited. Therefore, the amount of water added is preferably less than 30 wt%. FIG. 3 shows that the amount of water added is more preferably 1 to 10 wt%, and more preferably 1 to 2 wt%.

It is a block diagram which shows the structure of the silicon | silicone collection | recovery apparatus of one Embodiment of this invention. It is a block diagram which shows the example of 1 structure of the solid-liquid separation part of FIG. It is a graph which shows the result of the experiment which investigates the effect of water addition, and shows the relationship between the amount of pure water addition and the amount of residual organic impurities, and the relationship between the amount of pure water addition and a silicon recovery rate.

Explanation of symbols

1: First solid-liquid separation unit 3: Washing unit 4: Second solid-liquid separation unit 5: Classification unit 7: Drying unit 9: Metal scrap removal unit 11: Molding unit 13: Heating unit 15: Purification unit 17: Solidification unit 19: Primary centrifuge 21: Secondary centrifuge 23: Distillation device

Claims (13)

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 A first solid-liquid separation step for obtaining a fraction, a washing step for washing the solid content for silicon recovery using a washing solution, and a second solid-liquid separation for solid-liquid separation of the washing solution and the solid content for silicon collection after the washing A separation step,
The coolant includes a liquid organic material,
The silicon recovery method, wherein the cleaning liquid includes a low-boiling organic solvent having compatibility with the liquid organic material and having a boiling point lower than that of the liquid organic material, and water. The method according to claim 1, wherein the cleaning liquid further contains an inorganic acid. The method according to claim 1 or 2, wherein a ratio of water in the cleaning liquid is less than 30 wt%. The low boiling point organic solvent in the cleaning liquid is composed of one selected from the group consisting of alcohols having 1 to 6 carbon atoms and ketones having 3 to 6 carbon atoms, or a mixture of two or more. 4. The method according to any one of 3. 5. The low-boiling organic solvent in the cleaning liquid consists of one selected from the group consisting of methanol, ethanol, isopropyl alcohol, and acetone, or consists of a mixture of two or more. Method. The method according to claim 1, wherein the coolant comprises a water-soluble coolant. 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 A first solid-liquid separation unit for obtaining a fraction, a washing unit for washing the solid content for silicon recovery using a washing liquid, and a second solid-liquid separation for solid-liquid separation of the washing liquid and the solid content for silicon collection after the washing With a separation part,
The coolant includes a liquid organic material,
The silicon recovery apparatus, wherein the cleaning liquid includes a low-boiling organic solvent having compatibility with the liquid organic material and having a boiling point lower than that of the liquid organic material, and water. The apparatus according to claim 7, wherein the cleaning liquid further contains an inorganic acid. The apparatus according to claim 7 or 8, wherein a ratio of water in the cleaning liquid is less than 30 wt%. The low boiling point organic solvent in the cleaning liquid is composed of one selected from the group consisting of alcohols having 1 to 6 carbon atoms and ketones having 3 to 6 carbon atoms, or a mixture of two or more. The apparatus according to any one of 9. The low boiling point organic solvent in the cleaning liquid is composed of one selected from the group consisting of methanol, ethanol, isopropyl alcohol, and acetone, or a mixture of two or more. apparatus. The solid content obtained in the second solid-liquid separation part is further once using a cleaning liquid consisting of one or a mixture of two or more selected from the group consisting of a low-boiling organic solvent, an acid aqueous solution and water. The apparatus according to claim 7, further comprising a cleaning unit that performs the above cleaning. The apparatus according to any one of claims 7 to 12, wherein the coolant comprises a water-soluble coolant.

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