JP2001278612A - Method of recovering silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of recovering high purity silicon powder through the treatment of drained slurry generated by cutting silicon crystal using a wire-saw. SOLUTION: This method of recovering silicon comprises the steps of filtering drained slurry generated by cutting silicon crystal using a wire-saw, recovering sludge, washing the sludge with an organic solvent to dissolve the contained dispersing agent, separating the liquid fraction to remove the dispersing agent and to obtain a cake comprising mainly silicon chips, metal powder and abrasive grains, washing the cake with acid solution comprising hydrogen fluoride and an inorganic acid, filtering to separate silicon oxide and metal components, washing with water, drying to obtain a solid mass, disintegrating the mass into powder which comprises mainly silicon and abrasive grains (Silicon carbide), and classifying the powder utilizing the difference of individual density to recollect the high purity silicon powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recovering a powder having a high silicon content from waste slurry generated when cutting silicon with a wire saw, for example.

[0002]

2. Description of the Related Art An apparatus called a wire saw is used as one of the techniques for obtaining thin plates such as wafers for ICs and wafers for solar cells from ingots of silicon single crystal or silicon polycrystal. This device constitutes a wire group formed by winding a single wire between a plurality of rollers at a predetermined pitch, for example, and presses the ingot against the wire while rotating the wire at a high speed by rotating the rollers, and also forms a slurry containing abrasive grains on the ingot. To the contact part with the wire
A wafer is obtained by slicing a silicon crystal.

[0003] The above-mentioned slurry is used in a circulating manner. However, if the process is repeated, the amount of silicon chips and crushed abrasive particles increases, and cutting (including a small amount of metal powder which is a worn piece of wire) is performed. Since the performance is reduced and the slicing accuracy is degraded, the slicing accuracy is maintained by discharging a part of the slurry as needed and replenishing a new slurry to reduce the solid content concentration of chips and the like in the slurry. The waste slurry discharged from the wire saw is either disposed of by dissolving the dispersant or discarded, or is treated by self-combustion.However, the abrasives must be collected in a collection device, and the remaining solids must be removed as sludge and discarded. There is also.

[0004]

The waste slurry and sludge described above contain wasteful silicon, which is becoming scarce, but is wastefully discarded.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for recovering silicon from waste generated by processing silicon crystals such as cutting and polishing.

[0006]

According to a method of recovering silicon according to the present invention, a slurry formed by dispersing abrasive grains in a dispersing agent is supplied while discharging a single crystal or polycrystal of silicon by cutting or polishing. The solid content after solid-liquid separation of the waste slurry to be used is used, the solid content is washed with an organic solvent, and an organic solvent washing step of removing a dispersant contained in the solid content and a dispersant removal are performed. And removing a silicon oxide and abrasive grains from the solid content after the separation to obtain a powder containing silicon as a main component.

According to such a method, high-purity silicon powder is obtained from waste slurry generated when cutting (slicing) a single crystal or polycrystal of silicon or polishing a wafer after slicing. Can be recovered.

Specifically, waste slurry generated when a large number of wafers are obtained by cutting an ingot made of single crystal or polycrystal of silicon with a metal cutting means, for example, a group of wires in which a number of wires are stretched in parallel. Alternatively, high-purity silicon powder can be collected from the sludge after the abrasive grains have been collected from the waste slurry and the dispersant has been removed.

The separating step includes an acid washing step for supplying an acid solution to the solid content to remove at least silicon oxide, and a classification process for the solid content to separate silicon powder and abrasive grains. And a classifying step. In the acid washing step, it is preferable to use hydrogen fluoride as the acid solution, and by doing so, it is possible to dissolve and remove the solid oxide separated from the waste slurry or the silicon oxide contained in the sludge. In the classification step, for example, an airflow classification device can be used.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention described below is a method for recovering silicon as a powder from waste slurry or sludge generated when a single crystal or polycrystal of silicon is sliced by a wire saw. First, before describing a recovery method, a process of obtaining a waste slurry to be treated using a wire saw will be briefly described with reference to FIG.

The wire saw is formed by winding and stretching a single wire as a cutting means at a predetermined pitch between a plurality of, for example, three rollers 12. A slurry obtained by dispersing abrasive grains made of silicon (SiC) in a water-soluble or oil-based dispersant is supplied from a slurry tank 13 to a supply pipe 14 provided with a pump P and a valve V, and a slurry supply means 15. The ingot 16 is circulated and supplied from the upper side of the wire group 11, and the ingot 16 of the single crystal silicon or the polycrystalline silicon, which is the object to be cut, is lowered so as to pass through the wire group 11 to cut (slice) the ingot 16 In addition, it is configured to simultaneously obtain a large number of wafers. Here, the water-soluble dispersant includes, for example, those based on water to which glycols, surfactants, higher fatty acids and the like are added, and the oil-based dispersants include, for example, lubricants for oils such as mineral oils. And the like can be mentioned.

During the cutting process, the slurry supply pipe 15
The slurry supplied to the wire group 11 is stored in the slurry tank 13 via the receiving tank 17, but when the ingot 16 is sliced, the chips of the ingot 16, the crushed abrasive particles and the Slicing accuracy deteriorates when metal powders, which are wear pieces, are mixed in and the amount of these solids increases. For this reason, a part of the slurry is appropriately discharged as waste slurry to a waste slurry tank 19 via a discharge pipe 18. The waste slurry generated here contains, as a main component, about 50% by weight of a dispersing agent, for example, abrasive grains having a size of 10 to 30 μm, crushed abrasive grains having a size of 5 to 15 μm crushed by cutting, Includes cutting powder of ingot having a size of μm, metal powder coming out of a wire, and the like. In addition, this waste slurry, for example, to collect the abrasive grains by a decanter or the like, and to separate the cuttings, crushed abrasive grains and metal powder into a sludge and a dispersant by a filter such as a nonwoven fabric for the residue. May be.

Next, a process of recovering silicon from waste slurry and sludge obtained as described above will be described with reference to FIG.
This will be described with reference to FIG. First, since the waste slurry contains a large amount of the dispersant as described above, it is filtered in the solid-liquid separation step, for example, the first filtration device 2 composed of a centrifugal separator to obtain a clay-like solid content (cake). Is separated. Here, the filtering device used in the present embodiment will be described. The same filtering device is used as the first filtering device and the filtering device used in the subsequent process. , A vacuum filtration device and a decanter can be used.

The composition of the solid obtained in this way will be described. For example, a solid content of 70 to 50% by weight contains a dispersant in an amount of 30 to 50% by weight. The composition of the main substances in silicon is silicon (Si) and silicon oxide (SiO2)
Is 10 to 30% by weight, a metal content is 5 to 10%, and silicon carbide (SiC) as abrasive grains is 85 to 60% by weight. SiO2 contained in the silicon powder is a substance covering the surface of the silicon powder, which is generated because silicon is exposed to a high temperature during the cutting process. The metal powder is a metal used for the wire group 31, for example.
Although iron (Fe), copper (Cu), aluminum (Al), and the like are considered to be included, for the sake of convenience, the present embodiment will be described as being formed of iron powder and copper powder.

Then, the solid content is put into a washing tank 32 provided with a stirrer 31, and an organic solvent such as acetone, which is equal to or more than the solid content, is supplied thereto. The solid content is washed in the organic solvent washing step by stirring in the washing tank 32. The dispersant and the thickener contained in the solid content are dissolved in acetone, and the solid content becomes sandy. . The organic solvent is not limited to acetone, but may be kerosene, ether, toluene, or the like.

As described above, after the abrasive grains are collected from the waste slurry on the wire saw equipment side, the dispersant may be removed to obtain sludge. In some cases, the sludge is used as a starting material in an organic solvent washing step. The washing treatment may be performed, or the solid matter separated by the first filtration device 2 may be mixed. Although the abrasive grains are recovered from the waste slurry, the uncrushed abrasive grains are collected. In the sludge, the abrasive grains mainly crushed as described above are, for example, 40%.
% By weight.

After the solid content is washed, the mixed liquid of the sandy solid content and acetone in the washing tank 32 is sent to the second filtration device 33 to perform solid-liquid separation (solid-liquid separation step). Here, since the dispersant and the like in the solid content are taken out as a liquid component together with acetone, the solid content is mainly silicon chips (Si, S).
iO2), metal powder (Fe, Cu), and abrasive grains (SiC). This solid content is washed with water in the washing tank 34, and the dispersant and the organic solvent remaining in the solid content are washed away. The solids washed with water are supplied to a third filtration device 3
After 5 the cake is taken out and the next process (* in Fig. 3)
Sent to.

On the other hand, the liquid fraction separated by the second filtration device 33 is sent to a distillation device 36, where acetone is recovered by distillation and the acetone is supplied to a washing tank 32 to be reused for washing solids. I do. In this distillation, the dispersant dissolved in the liquid component is taken out as a component having a high boiling point and discarded.

Turning now to FIG. 3, the steps after * will be described. The cake obtained in the previous step is put into a washing tank 42 provided with a stirrer 41, and an acid solution is supplied thereto to wash the cake (acid washing step). This acid washing step is performed for the purpose of, for example, dissolving the silicon powder (SiO2) and the metal powder (Fe, Cu) in the cake and separating them into liquid components. Is a mixed solution of 1 to 5% by weight of hydrofluoric acid (hydrofluoric acid: HF) for dissolving SiO2 and an inorganic acid for dissolving metal components, for example, 1 to 5% by weight of sulfuric acid (H2 SO4) Can be used. The reaction between the cake and the acid solution proceeds, for example, as shown in the following equations (1) to (3). The inorganic acid used here is not limited to sulfuric acid, and nitric acid or hydrochloric acid can also be used. SiO2 + 6HF → H2SiF6 + 2H2O (1) Fe + H2SO4 → FeSO4 + H2 (2) Cu + H2SO4 → CuSO4 + H2 (3) Thereafter, solid-liquid separation was performed in the third filtration device 43, and SiO2 and Fe dissolved in the acid solution as described above. And Cu are removed. The residue (solid content) thus obtained is mainly silicon chips (Si) and abrasive grains (SiC).

Finally, the step of extracting a powder containing silicon (Si) as a main component from the solid content will be described. After the solid content is washed with water in a washing tank 51, a fifth filtration device is used. It is sent to 52 and the water is blown off, and then dried by the dryer 53, and the solid particles are combined to form a lump. As the dryer 53, for example, a flash dryer that puts an object to be dried (solid content) in a stream of heated air and dries the object while transporting the pipe is used.
A dryer may be used in which the object to be dried is brought into direct contact with a heating tube or a heating wall. Examples of such a dryer include a drum type, a screw type, and a paddle-disk type in which the method of moving the object to be dried is different. Drum type dryers tilt the drum body, and screw type and paddle / disk type dryers rotate the screw or paddle to move the drying object that comes into contact with the heating tube or heating wall in the drying basic body. Things.

The lump solids thus obtained are put into the pulverizer 54 and the crushing blades 55 are rotated. The lump solids collide with the inner wall surface of the pulverizer 54 and become powdery (crushing step). Separation of silicon powder and abrasive grains is performed on the powder. The density of silicon (Si) is 2.3, and the abrasive grains (Si
Since the density of C) is 3.2, the powdery material is classified by using, for example, an airflow classification device 56 using the density difference (classification step), and separated into a silicon component and an abrasive component. An airflow classification device is a device that classifies powders by using a phenomenon that, when powder is put into an airflow flowing at a certain speed or more, heavy particles fall quickly, and lighter particles fly farther.

Here, "separation step" referred to in the claims
In this embodiment, the term "steps" means steps after the third filtration device 35. Silicon powder (SiO2) is removed from a solid (cake) from which a dispersant has been removed in an organic solvent washing step or the like. ), Metal powder (Fe, Cu), abrasive (SiC)
Corresponds to this.

In this way, high-purity silicon powder is recovered from waste slurry or sludge. The silicon powder contains, for example, silicon (Si) in an amount of 95 to 98% by weight or more.
The remainder is silicon carbide (SiC) and a trace amount of metal, for example, 10 ppm or less, which is an unavoidable residue.

As described above, according to the present embodiment, the waste slurry or sludge generated when a single crystal or polycrystal of silicon is sliced using a wire saw is used to remove a dispersant with an organic solvent and to remove a metal with an acid. After removing the silicon oxide and silicon oxide, the classification process is performed using the density difference between the silicon powder and the abrasive grains. Obtainable. This high-purity silicon powder is a raw material silicon having a high added value. For example, a raw material for silicon crystal or polycrystal can be obtained by handing over to a refiner for further purification. Accordingly, since silicon having a problem with a shortage of raw materials can be recovered from waste slurry or sludge that has been disposed up to now, resources can be effectively used.

In the above embodiment, sludge (cake) from which the organic solvent washing step and the like have been completed and from which the dispersant has been removed
On the other hand, the order of performing the acid washing step and the classifying step may be reversed, that is, for example, the cake obtained by the third filtration device 35 is dried by the dryer 53, and the solid content after drying is decomposed. It is also possible to perform the crushing step and the classifying step, and send the powder thus obtained to the washing tank 42 to perform the acid washing step. By doing so, S
Since iC is removed to some extent, the load in the acid washing step is reduced, and for example, there is an advantage that the amount of the supplied acid solution can be reduced.

In the above, the present embodiment is not limited to a wire saw, and may be used for processing waste slurry and sludge generated when, for example, a single crystal or polycrystalline silicon is sliced one by one using a metal cutting blade. However, in the case of a wire saw, since a large number of wafers are sliced at once, a large amount of waste slurry or sludge is generated, which is particularly effective. Further, the abrasive grains used at the time of slicing may be, for example, BC (boron carbide) or diamond other than SiC. Further, in this embodiment, a waste slurry generated in a so-called lapping process in which a slurry containing, for example, abrasive grains made of SiC and water is supplied to a wafer after slicing, and polishing is performed using, for example, a grindstone made of SiC. And sludge may be treated, and in this case, the same steps as those in the above-described embodiment are performed.

The waste slurry (or sludge) discharged in the lapping process does not contain any metal component, but the surface of the silicon powder to be polished is covered with SiO2, so that the acid cleaning process is also performed. There is a need.

[0028]

EXAMPLES (Example 1) A wafer for IC was manufactured using a wire saw, and an oil-based dispersant was used for the slurry used here. The composition of the waste slurry was 44.0% by weight of the dispersant, abrasive grains 48.7% by weight, silicon 6.4% by weight, and metal content 0.7% by weight. On the other hand, the composition of the sludge separated by the slurry collecting apparatus 19 in FIG. 1 of the present embodiment is as follows: 30% by weight of a dispersant, 40% by weight of abrasive grains,
5% by weight and 5% by weight of metal. When the method for recovering silicon according to the present invention was attempted with these as processing targets,
In both cases of using the waste slurry and the sludge, silicon (Si) 98% by weight, silicon carbide (SiC)
A powder having a composition of 2% by weight was obtained, and the metal content of the unavoidable residue was contained at 100 ppm or less.

[0029]

As described above, according to the present invention, high-purity silicon can be recovered from waste generated by processing silicon crystals such as cutting and polishing by a simple method.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a process up to collecting waste slurry of a wire saw to be processed according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating an embodiment of the method of the present invention.

FIG. 3 is an explanatory view illustrating an embodiment of the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Wire group 13 Slurry tank 16 Ingot 2 1st filtration apparatus 32, 42 Cleaning tank 33 2nd filtration apparatus 43 4th filtration apparatus 53 Dryer 54 Crusher 56 Airflow classification apparatus

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kensho Miyata 1 Shinmeicho, Yokosuka-shi, Kanagawa F-term in Hihei Toyama Technical Center Co., Ltd. 4D031 AC04 4D066 AA06 AB10 BA05 BB10 BB21 BB23 4G072 AA01 BB05 GG03 HH01 JJ15 JJ18 MM22 MM23 MM24 MM28 MM31 UU01

Claims (7)

[Claims] 1. A solid after solid-liquid separation of waste slurry discharged in a process of cutting or polishing a silicon single crystal or polycrystal while supplying a slurry in which abrasive grains are dispersed in a dispersant. Using an organic solvent, the solid content is washed with an organic solvent, and an organic solvent washing step of removing the dispersant contained in the solid content, and removing the silicon oxide and abrasive grains from the solid content after the dispersant is removed. And obtaining a powder containing silicon as a main component. 2. A waste slurry discharged by a process of slicing an ingot made of silicon single crystal or polycrystal by a metal cutting means while supplying a slurry in which abrasive grains are dispersed in a dispersant, A solid-liquid separation step of performing solid-liquid separation on the waste slurry to separate solids, washing the solids obtained in this step with an organic solvent,
An organic solvent washing step for removing the dispersant contained in the solid content, and removing the metal component, silicon oxide and abrasive grains from the solid content after the dispersant removal has been performed, and forming a powder containing silicon as a main component. And a step of obtaining silicon. 3. A slurry from waste slurry discharged in a process of cutting a silicon single crystal or polycrystal ingot by a metal cutting means while supplying a slurry in which the abrasive is dispersed in a dispersant. Using the sludge after collecting and removing the dispersant, washing the sludge with an organic solvent to remove the dispersant contained in the sludge, and the solid after removing the dispersant. And removing a metal component, silicon oxide and abrasive grains from the component to obtain a powder containing silicon as a main component. 4. The method for recovering silicon according to claim 2, wherein the metal cutting means is a group of wires formed by winding one wire between a plurality of rollers at a predetermined pitch. 5. The separation step includes: an acid washing step for supplying an acid solution to the solid content to remove at least silicon oxide; and a classification process for the solid content to separate silicon powder and abrasive grains. The method for recovering silicon according to any one of claims 1 to 4, further comprising a classification step for performing the classification. 6. The method for recovering silicon according to claim 5, wherein the acid solution used in the acid washing step contains hydrofluoric acid. 7. The classifying step of removing abrasive grains from the solid content comprises:
7. The method for recovering silicon according to claim 5, wherein the method is performed by an airflow classifier that separates the powder by introducing the powder into the airflow.