JP2016164113A - Method for refining silicon sludge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for refining silicon sludge capable of satisfactorily refining by a simple method.SOLUTION: Provided is a method for refining silicon sludge generated when machine working is performed to the object to be worked made of silicon while using a working liquid including organic matter, comprising: the first removal step where the silicon sludge is heated at the first temperature lower than the melting point of silicon, and the organic matter-containing first gas is exhausted from the silicon sludge to obtain the first treated product; and the second removal step where the first treated product is heated at the second temperature higher than the first temperature and lower than the melting point of silicon in the first atmosphere where oxygen partial pressure is lower than air after the first removal step, and the second gas at least containing one or more kinds selected from carbon and oxygen is exhausted from the first treated product to obtain the second treated product.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for purifying silicon sludge generated when machining a workpiece made of silicon.

  A silicon substrate is generally used for a semiconductor substrate such as a solar cell. A silicon substrate is manufactured by performing machining such as cutting, grinding, and polishing on a silicon lump such as a silicon ingot. Usually, the weight of the silicon substrate obtained after machining is about half of the weight of the ingot. In addition, silicon sludge that is a mixture of silicon powder (silicon scrap) and used processing liquid or the like is generated during the machining process. Usually, silicon sludge is discarded or used as a deoxidizer for metal refining, a raw material for refractories, and the like. However, in recent years, with the growth in demand for silicon substrates for solar cells, there is a concern that the supply of raw material silicon will be insufficient and the price will rise.

  Therefore, a method of refining silicon sludge and using it as raw material silicon for solar cells has been proposed. For example, a method for purifying silicon sludge by performing organic solvent cleaning, water cleaning and acid cleaning on silicon sludge containing abrasive grains and coolant has been proposed (see Patent Document 1 below). In addition, a method for purifying silicon sludge is proposed in which an easily evaporable component is removed by evaporation in a reduced-pressure atmosphere, the easily oxidizable component is oxidized and removed as an oxide gas, and then melted and unidirectionally solidified (described below). (See Patent Document 2). Furthermore, a method is also disclosed in which silicon sludge is purified by solid-liquid separation and washing, and then a powder containing silicon is baked, melted and solidified (see Patent Document 3 below).

JP 2001-278612 A JP 2003-212533 A JP 2008-1105040 A

  However, in the conventional method, a by-product having a melting point higher than that of silicon such as silicon carbide and silicon oxide may be generated due to the reaction between silicon and impurity components such as carbon and oxygen present in the silicon sludge. It was. And this by-product sometimes prevented the melting of silicon or remained as a precipitate in the purified silicon.

  On the other hand, if the silicon melt is kept at a high temperature in order to melt the by-product, the amount of silicon purified by evaporation of silicon is reduced. In addition, since the heat load of the refining apparatus used in this case increases, there is a problem that the apparatus is liable to break down and the apparatus is complicated and enlarged.

  Accordingly, an object of the present invention is to provide a method for purifying silicon sludge that can be well purified by a simple method.

A method for purifying silicon according to the present invention is a method for purifying silicon sludge that is generated when machining is performed on a workpiece made of silicon while using a processing liquid containing organic matter. A first removal step of heating the silicon sludge at a lower first temperature and discharging the first gas containing the organic matter from the silicon sludge to obtain a first treated product, and a large amount after the first removal step. In the first atmosphere having a pressure lower than the atmospheric pressure, the first treatment object is heated at a second temperature that is higher than the first temperature and lower than the melting point of silicon, and at least one of carbon and oxygen from the first treatment object. And a second removal step of discharging a second gas containing seeds or more to obtain a second processed product.

  According to the method for purifying silicon sludge described above, by-products having a melting point higher than that of silicon such as silicon oxide and silicon carbide can be reduced, and a large amount of raw silicon can be easily recovered from the silicon sludge.

FIG. 1 is a flow diagram of a method for purifying silicon sludge according to an embodiment of the present invention. FIG. 2 is a flowchart of a method for purifying silicon sludge according to another embodiment of the present invention. FIG. 3 is a perspective view schematically showing a configuration example of the multi-wire saw device. FIG. 4 is a graph showing the amount of gas generated at each temperature, FIG. 4 (a) is a graph showing the amount of water generated at each temperature, and FIG. 4 (b) is the amount of carbon dioxide generated at each temperature. FIG. 4C is a graph showing the amount of hydrogen generated at each temperature. FIG. 5 is a graph showing the amount of carbon monoxide generated at each temperature. FIG. 6 is a flow diagram of a method for purifying silicon sludge of a comparative example.

Hereinafter, embodiments of a method for purifying silicon sludge according to the present invention will be described in detail with reference to the drawings. In addition, AE-B (however, A and B on the left are numbers) described on the scale of the vertical axis in FIGS. 4 and 5 means A × 10 ^B (for example, 1.00E-03 is 1 × 10 6). ^-3 ).

<< Machine processing and silicon sludge >>
A silicon substrate is manufactured by performing machining such as cutting, grinding, and polishing on a silicon lump such as an ingot. In these machining processes, machining tools using a grindstone, abrasive grains, and the like, and machining fluids such as coolant are used.

  The ingot is formed into a lump shape such as a prismatic shape or a cylindrical shape by a method such as a casting method or a Czochralski method. The manufactured ingot is machined using a band saw device, a polishing device, a grinding device, or the like. This machining includes cutting the outer periphery of the ingot, dividing the ingot, and treating the end face of the ingot. By these processes, a brick having a desired shape and size is obtained from the ingot. A plurality of silicon substrates are obtained by cutting these silicon ingots or bricks using a multi-wire saw device or the like. Further, the silicon substrate is subjected to surface polishing by a polishing apparatus or the like as necessary, or cut into element sizes by a dicing apparatus or the like.

  The silicon sludge refining method of this embodiment can be applied to silicon sludge generated by machining as described above. In particular, a large amount of silicon sludge is generated in a machining process for obtaining a silicon substrate using a multi-wire saw device. For this reason, the refinement | purification method of the silicon sludge of this embodiment is useful for refinement | purification of the silicon sludge collect | recovered, for example from a multi-wire saw apparatus.

  Hereinafter, a method for purifying silicon sludge generated when a multi-wire saw apparatus is used will be described as an example.

<< Multi-wire saw device >>
As shown in FIG. 3, the multi-wire saw apparatus S includes a plurality of wire rows. The wire 3 is wound around a plurality of rollers 5 having a plurality of grooves for the wire 3 on the surface. In the multi-wire saw apparatus S, the wire 3 is caused to travel while supplying the machining liquid containing abrasive grains to the wire 3 or supplying the machining liquid to the wire 3 to which the abrasive grains are fixed. Then, the workpiece 1 can be pressed against the wire 3 to slice the workpiece 1. At this time, since the workpiece 1 is fixed to the slice base 2, the workpiece 1 is cut and a part of the slice base 2 is cut. A method of cutting while supplying abrasive grains to the wire 3 is called a free abrasive grain system, and a system using a wire to which abrasive grains are fixed is called a fixed abrasive grain system. Further, at the time of cutting, a working fluid such as a coolant is supplied from the supply nozzle 4 to the wire 3 for the purpose of cooling the workpiece 1 and the apparatus member.

  In the multi-wire saw apparatus S, the used processing liquid is usually collected in a container 6 for collecting the processing liquid and repeatedly used. However, the ratio of cutting scraps including silicon contained in the machining liquid increases depending on the frequency of use. If this ratio is increased, it may cause cutting defects of the silicon substrate, disconnection of the wire 3, and the like. For this reason, unnecessary components such as silicon sludge are discharged from the collected machining fluid, and the regenerated remaining machining fluid is supplied to the multi-wire saw device S. At this time, the discharged silicon sludge is processed by the purification method of the present embodiment. In the silicon sludge collected by the multi-wire saw apparatus S, silicon, cutting scraps of the slice base 2, abrasive grains, metal scraps that are wear pieces of the wire 3, etc. are mixed as solid components.

<< Cutting of workpiece >>
A brick is used as the workpiece 1 in order to produce a silicon substrate for a solar cell. The shape of the workpiece 1 is, for example, a rectangular parallelepiped of 156 mm × 156 mm × 300 mm. The workpiece 1 is bonded to a slice base 2 made of a carbon material or an organic resin via an adhesive or the like. As the adhesive, a thermosetting two-component epoxy resin, acrylic resin or acrylate resin, wax, or the like can be used. In order to make it easier to peel the substrate from the slice base 2 after slicing the workpiece 1 to the substrate, it is preferable to use an adhesive whose adhesive strength decreases as the temperature of the adhesive increases. Note that one or a plurality of workpieces 1 bonded to the slice base 2 are arranged in the multi-wire saw device S by the work holder 10. The work holder 10 has a rectangular parallelepiped shape made of stainless steel or the like. The slice base 2 is fixed to the work holder 10 with the adhesive as described above.

  The wire 3 is supplied from the supply reel 7 and taken up on the take-up reel 8. The wire 3 is wound around a plurality of main rollers 5 (for example, a first main roller 5a, a second main roller 5b, and a third main roller 5c) between the supply reel 7 and the take-up reel 8, and the main roller 5 It is stretched so as to form a plurality of rows between them. As the wire 3, for example, a piano wire whose main component is iron or an iron alloy is used. The wire 3 has a wire diameter of 80 to 180 μm, more preferably 120 μm or less. Hereinafter, the case where the fixed abrasive type multi-wire saw apparatus S is used will be described.

  In the fixed abrasive type multi-wire saw apparatus S, the wire 3 uses an abrasive fixed wire in which abrasive grains made of diamond or silicon carbide are fixed to the outer periphery of the wire 3 by plating with nickel, copper, or chromium. In this case, the average particle diameter of the abrasive grains is preferably 5 to 30 μm.

  When the processing liquid is a coolant liquid, the processing liquid includes, for example, a glycol-based water-soluble organic solvent such as ethylene glycol, propylene glycol, or polyethylene glycol, and water. Then, the machining liquid is supplied to the plurality of wires 3 from the openings of the supply nozzle 4 having a plurality of openings. In addition, the machining fluid may be circulated and used. When the circulated machining fluid is supplied to the wire 3, abrasive grains and chips contained in the machining fluid are discharged and removed as silicon sludge. Is done.

  The main roller 5 includes a first main roller 5a and a second main roller 5b disposed below the workpiece 1. The main roller 5 is made of, for example, urethane rubber such as ester, ether or urea, or resin such as ultra high molecular weight polyethylene. The main roller 5 has a diameter of 150 to 500 mm and a length of about 200 to 1000 mm. A large number of grooves for arranging the wires 3 supplied from the supply reel 7 at predetermined intervals are provided on the surface of the main roller 5. The thickness of the substrate cut out from the workpiece 1 is determined by the relationship between the interval between the grooves and the diameter of the wire 3.

  The wire 3 is supplied from the supply reel 7 and is guided to the main roller 5 by the guide roller 9. Then, the wire 3 is wound around the main roller 5 and the adjacent wires 3 are arranged at a predetermined interval. By rotating the main roller 5 at a predetermined rotational speed, the wire 3 can travel in the longitudinal direction of the wire 3. Further, the wire 3 is reciprocated by reversing the rotation direction of the main roller 5. At this time, the length for supplying the wire 3 from the supply reel 7 is longer than the length for supplying the wire 3 from the wire take-up reel 8. Thereby, the unused wire 3 can be supplied to the main roller 5. Note that the workpiece 1 may be cut by traveling in one direction without reciprocating the wire 3.

  The workpiece 1 is cut by lowering the workpiece 1 while supplying the machining liquid from the supply nozzle 4 toward the wire 3 running at a high speed, so that the workpiece 1 is relatively moved to the wire 3. This is done by pressing. The workpiece 1 is divided into a plurality of substrates having a thickness of 200 μm or less, for example. At this time, the tension of the wire 3, the speed at which the wire 3 travels (travel speed), and the speed at which the workpiece 1 is lowered (feed speed) are each appropriately controlled. For example, the maximum traveling speed of the wire 13 is set to 500 to 1000 m / min, and the maximum feed speed is set to 350 to 1100 μm / min.

  At the same time as slicing the workpiece 1, the slice base 2 is also cut, for example, by about 2 to 5 mm, and the wire 3 is pulled out from between the substrates. At this time, the silicon substrate can be taken out while being adhered to the slice base 2. And the silicon substrate isolate | separated from the slice base 2 can be obtained through a washing | cleaning process and a peeling process.

  A cutting fluid containing abrasive grains may be used as the processing fluid. Such a working fluid can be used, for example, in a free-abrasive grain type multi-wire saw device in which abrasive grains are placed in the working fluid without fixing the abrasive grains to the surface of the wire 3. In this case, the wire 3 to be used is wound around the main roller 5. As the wire 3, for example, a piano wire whose main component is iron or an iron alloy may be used. The wire diameter of the wire 3 is 80 to 180 μm, more preferably 120 μm or less. The processing liquid is configured by mixing wrapping oil composed of abrasive grains such as silicon carbide, aluminum oxide, diamond, mineral oil, surfactant and dispersant. Further, the machining liquid is supplied to a plurality of wires 3 stretched from a supply nozzle 4 having a plurality of openings. As an average particle diameter of an abrasive grain, what is 5-20 micrometers and a particle size distribution is narrow, for example. The supply flow rate of the processing liquid supplied to the supply nozzle 4 is appropriately set depending on the size and number of the workpieces 1. Further, when the working fluid is circulated and used, new abrasive grains may be appropriately added to the used working fluid and supplied to the wire 3.

<< Silicon sludge purification method >>
<Example 1>
An example of a method for purifying silicon sludge will be described with reference to FIG. First, the basic flow of this embodiment will be described. Silicon sludge generated when machining is performed on a workpiece made of silicon while using a machining fluid containing organic matter. For this reason, at least the first removal step S2 and the second removal step S3 may be performed sequentially.

  In the first removal step S2, the silicon sludge is heated at a first temperature lower than the melting point of silicon, and a first gas containing organic substances is discharged from the silicon sludge to obtain a first processed product.

  In the first removal step S3, after the first removal step S2, in a first atmosphere containing oxygen and having a pressure lower than atmospheric pressure (1 atm), the second removal step S3 is performed at a second temperature higher than the first temperature and lower than the melting point of silicon. A 1st processed material is heated and the 2nd gas containing at least 1 or more types of carbon and oxygen is discharged | emitted.

  Here, as shown in FIG. 1, you may perform drying process S1 which dries a water | moisture content from silicon sludge before 1st removal process S2. Further, after the second removal step S3, a melting step S4 in which the second processed material is heated to a melting point of silicon or higher to be melted, and a solidification step S5 in which the melt is cooled and solidified in one direction to segregate impurities. May be performed.

  The silicon sludge recovered from the multi-wire saw apparatus S includes, in addition to the waste processing liquid that is a liquid component, silicon cutting waste, abrasive grains, cutting waste from the slice base 2, metal scrap that is a wear piece of the wire 3, and the like. Contains solid components.

  The abrasive grains are made of, for example, silicon carbide, diamond, or aluminum oxide. The processing fluid is, for example, oil-based coolant (oil based on mineral oil), aqueous coolant (water-based glycol solvent (for example, ethylene glycol, propylene glycol or polyethylene glycol), surfactant added, etc.) ). The slice base 2 is made of, for example, a carbon material or an organic resin. The wire 3 is made of, for example, iron or an iron alloy. Accordingly, the above components may be contained in the silicon sludge.

  As a device for recovering the silicon sludge, a centrifugal separator, a filtration device, a classification device, or a combination thereof is used. For example, the collected used processing liquid can be separated into reusable processing liquid and silicon sludge by centrifuging. Further, for example, the centrifugal separation step, the filtration step, and the classification step are performed by, for example, centrifuging a component having a density higher than that of silicon in the first centrifugation step and then centrifuging the silicon sludge from the remaining processing liquid component. It may be carried out more than once to recover the silicon sludge.

  Below, each process is explained in full detail.

Drying process:
The silicon sludge recovered from the multi-wire saw device S that uses an aqueous coolant contains a large amount of water. Therefore, a drying step S1 is performed in which silicon sludge is put in a container and dried. The drying step S1 may be natural drying, but it is preferable to evaporate moisture by heating at 80 to 120 ° C. for several hours using a hot plate or the like. Moreover, drying process S1 may be under atmospheric pressure, and may be performed under pressure reduction of a pressure lower than atmospheric pressure. In addition, drying time can be shortened by heating at a temperature exceeding the boiling point of water (100 ° C. if atmospheric pressure). However, when heating at atmospheric pressure, the drying temperature is preferably not more than the flash point of the organic solvent contained in the coolant (for example, 111 ° C. for ethylene glycol).

  The container for storing the silicon sludge used in the drying step S1 may be any material that does not easily deform at the use temperature (for example, 80 to 120 ° C.), is chemically stable, and does not easily contaminate the silicon sludge. For example, glass, ceramics, quartz, heat resistant resin, etc. can be used.

First removal step:
Subsequently, the container containing the silicon sludge is heated in the air using a clean oven or the like at a first temperature lower than the melting point of silicon (350 to 600 ° C.) for several hours to evaporate (for example, The boiling point of ethylene glycol is 197 ° C.), and organic components are removed as volatile components using thermal decomposition, oxidation / combustion (for example, the flash point of ethylene glycol is 410 ° C.).

  Here, the first temperature is higher than the flash point of the organic substance having the largest content among organic substances such as adhesives and organic solvents contained in the silicon sludge, and is lower than the melting point of silicon, preferably Is a temperature that is 50 to 150 ° C. higher than the flash point of the organic substance with the highest content. The reason why the flash point is 50 to 150 ° C. higher than the flash point of the organic matter having the largest content is to vaporize, pyrolyze, and oxidize even when the organic matter changes in the cutting of the workpiece by the multi-wire saw device S or the like.・ To burn. The heating time is preferably about 1 to 2 hours after almost the entire amount of silicon sludge contained in the container reaches the first temperature. Further, the atmosphere in the clean oven is preferably exhausted by flowing air at an appropriate flow rate of, for example, about 1 to 10 liters / minute so that decomposition products of organic substances and the like do not adhere to the silicon sludge.

  By the first removal step, the first gas containing the organic matter and the decomposition product of the organic matter is discharged from the silicon sludge. Thereby, a 1st processed material can be obtained.

  The container containing the silicon sludge used in the steps after the first removal step S2 is also preferably made of a material that is not easily deformed at the use temperature, is chemically stable, and hardly contaminates the silicon sludge. For example, silicon carbide, silicon oxide or the like can be used as the container. In particular, a container made of silicon carbide is preferable.

Second removal step:
Next, the container containing the first treated product of silicon sludge is held in a furnace of an apparatus that can be evacuated by a rotary pump or the like. Then, the first processed product is heated at a second temperature higher than the first temperature and lower than the melting point of silicon in a furnace in a first atmosphere under reduced pressure (for example, 100 Pa or less, preferably about 10 to 20 Pa). Then, a second removal step is performed to discharge the second gas containing at least one or more of carbon and oxygen from the first processed product to obtain the second processed product. At this time, it is heated for about several minutes to several hours at a second temperature of the first temperature to 950 ° C. using a resistance heater or the like. In this temperature region, the physically adsorbed water and the chemically adsorbed water that have not been desorbed in the first removal step S2 are volatilized and desorbed from the silicon sludge (first processed product). In addition, hydrogen generated by the decomposition of moisture is also eliminated. Further, the carbon content including diamond that has not volatilized in the first removal step S2 and the carbonized tar content contained in the silicon sludge react with the moisture and oxygen content contained in the silicon sludge to mainly react with the dioxide. Volatilizes and desorbs as carbon. These gases are particularly desorbed in a specific temperature range. Due to this desorption, the pressure in the furnace of the apparatus once rises. For this reason, until the silicon sludge is kept in a certain temperature range and the gas desorption is reduced and the pressure in the furnace of the apparatus starts to decrease again or becomes lower than the pressure before being held at the second temperature. It is good to keep the temperature. If the temperature is not maintained when the desorption of the gas increases, the desorption gas containing carbon or oxygen reacts with the silicon in the silicon sludge, so that a solid such as silicon carbide or silicon oxide is formed on the silicon surface. Ingredients are easily formed. And generation | occurrence | production of these solid components can be reduced by hold | maintaining silicon sludge at 2nd temperature. Further, in the second removal step S3, it is necessary to continue exhausting in order to quickly exhaust the desorbed gas outside the furnace. By this second removal step S3, a second gas containing at least one of carbon and oxygen is discharged to obtain a second processed product.

  More specifically, in the second removal step S3, in a range where the second temperature from 550 ° C. to 950 ° C. can be obtained, the second removal step S3 is performed stepwise to each predetermined temperature increased by about 100 ° C. (for example, 80 to 120 ° C.). Make it high. Further, at each of these temperatures, each temperature is maintained until the desorption of gas decreases and the pressure in the furnace starts to decrease again or becomes lower than the pressure before being held at each temperature. For this reason, it is desirable to carry out by dividing into the following steps. Here, the temperature is increased by about 100 ° C. If the temperature interval is made higher than this, there is a possibility that moisture and carbon components to be desorbed and discharged from the first processed product may remain in the second removal step. This is because even if the temperature interval is lower than this, man-hours are required and inefficiency occurs.

2-1 Removal step:
This step is performed at a temperature around 600 ° C. (550 to 650 ° C.). For example, the container containing the first processed material at 600 ° C. is reduced until the gas desorption is reduced and the pressure in the furnace of the apparatus starts to decrease again or becomes lower than the pressure before being held at 600 ° C. Hold at temperature.

  As shown in FIG. 4A, in the temperature range near 600 ° C. (550 to 650 ° C.), the moisture (physical adsorption) that could not be desorbed in the first removal step S2 from the first treated product of silicon sludge. The amount of desorption of water and chemically adsorbed water) is increasing. In tests repeatedly conducted by the inventors, the moisture desorbed in the vicinity of 600 ° C. is dissolved in the moisture adsorption state and moisture even when held at a higher temperature (for example, about 700 ° C. or 800 ° C.). It remains without detachment due to the difference in the components. And it has been found that there is a case where it reacts with a component in the first processed product to form a solid. Therefore, instead of abruptly raising the temperature to, for example, about 700 ° C. or 800 ° C., as much water as possible is easily desorbed at a temperature near 600 ° C. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 600 degreeC.

  Similarly, as shown in FIG. 4B, in the temperature range near 600 ° C., the carbon sludge bonded to the surface of carbon and silicon that could not be desorbed in the first removal step S2 from the first treated product of silicon sludge. Desorption of carbon dioxide reacted with hydroxyl groups is observed. Even when the carbon dioxide desorbed in the vicinity of 600 ° C. is held at a temperature higher than this (for example, about 700 ° C. or 800 ° C.), the carbon dioxide is adsorbed and bonded to other components in the first processed product. There may be cases where it remains without desorption due to a difference in state and reacts with components in the first processed product to form a solid. Therefore, instead of rapidly increasing the temperature to, for example, about 700 ° C. or 800 ° C., as much carbon dioxide as is easily desorbed at a temperature in the vicinity of 600 ° C. is desorbed as much as possible. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in a furnace starts decreasing again, or becomes lower than the pressure before hold | maintaining at 600 degreeC. Note that the peak of the carbon dioxide desorption amount seen in the vicinity of 400 ° C. in FIG. Since the carbon dioxide peak has been removed in the first removal step described above, it is not a target of the second removal step. The carbon dioxide generated by the combustion of the carbonized part of the carbon dioxide desorbed on the higher temperature side near 600 ° C. is the target of the second removal step.

  For example, the temperature may be maintained at a substantially constant temperature of 600 ° C., or may be gradually increased (over a certain time) in a temperature range of 550 to 650 ° C.

2-2 Removal step:
This step is performed at a temperature around 700 ° C. (650 to 750 ° C.). For example, the container containing the first processed material at 700 ° C. is reduced until the gas desorption is reduced and the pressure in the furnace of the apparatus starts to decrease again or becomes lower than the pressure before being held at 700 ° C. Hold at temperature.

  As shown in FIGS. 4A and 4B, in the temperature range near 700 ° C., many desorptions of moisture and carbon dioxide from the first treated product of silicon sludge are observed. Moisture and carbon dioxide desorbed in the vicinity of 700 ° C. are difficult to desorb at a lower temperature (for example, near 600 ° C.). In addition, even when held at a higher temperature (for example, 800 ° C. or 900 ° C.), the moisture and carbon dioxide remain without being desorbed, and react with the components in the first processed product to form solids. May form. Therefore, instead of abruptly raising the temperature to, for example, 800 ° C. or 900 ° C., as much water and carbon dioxide as are easily desorbed at a temperature in the vicinity of 700 ° C. are desorbed as much as possible. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 700 degreeC.

Furthermore, in the 2-2 removal step, as shown in FIG. 4C, the desorption of hydrogen that tends to desorb near 700 ° C. is also an object. Further, the temperature may be maintained at a substantially constant temperature of 700 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 650 to 750 ° C.

2-3 removal process:
This step is performed at a temperature around 800 ° C. (750 to 850 ° C.). For example, the container containing the first processed material at 800 ° C. is kept at this temperature until the desorption of gas decreases and the pressure in the apparatus starts to decrease again or becomes lower than the pressure before holding at 800 ° C. Hold.

  As shown in FIGS. 4A, 4B, and 4C, in the temperature range near 800 ° C., the 2-1 removal process and the 2-2 removal process from the first treated silicon sludge are performed. Moisture, carbon dioxide, and hydrogen that could not be desorbed are observed. Moisture, carbon dioxide, and hydrogen desorbed near 800 ° C. are difficult to desorb at a lower temperature (eg, 550 to 750 ° C.). In addition, even when held at a higher temperature (for example, 900 ° C. or 1000 ° C.), the moisture, carbon dioxide, and hydrogen remain without being desorbed and react with the components in the first processed product. Solids may be formed. Therefore, instead of abruptly raising the temperature to, for example, 900 ° C. or 1000 ° C., as much water, carbon dioxide, and hydrogen as are easily desorbed at temperatures near 800 ° C. are desorbed as much as possible. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 800 degreeC.

  The temperature may be maintained at a substantially constant temperature of 800 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 750 to 850 ° C.

2-4 removal process:
This step is performed at a temperature in the vicinity of 900 ° C. (850 to 950 ° C.). For example, the temperature of the container containing the first processed material at about 900 ° C. is decreased until the gas desorption is reduced and the pressure in the apparatus starts to decrease again or becomes lower than the pressure before being held at 900 ° C. Hold on.

As shown in FIGS. 4A to 4C, in the temperature range near 900 ° C., the 2-1 removal process, the 2-2 removal process, and the 2-3 removal from the first treated product of silicon sludge. Desorption of water, carbon dioxide, and hydrogen that could not be eliminated in the process is observed. Moisture, carbon dioxide, and hydrogen desorbed near 900 ° C. are unlikely to be desorbed at lower temperatures (for example, 550 to 850 ° C.). Moreover, even when it is held at a temperature higher than this (for example, 1000 ° C. or 1100 ° C.), it may remain without desorption and react with components in the first processed product to form a solid. Therefore, instead of abruptly raising the temperature to, for example, about 1000 ° C. or 1100 ° C., water, carbon dioxide, and hydrogen that are easily desorbed at temperatures near 900 ° C. are desorbed as much as possible. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes smaller than the pressure before hold | maintaining at 900 degreeC.

  The temperature may be maintained at a substantially constant temperature of 900 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 850 to 950 ° C.

  In the second removal step S3 in general, in each temperature range (for example, 550-650, 650-750 ° C, 750-850 ° C, 850-950 ° C) from 550 ° C to 950 ° C every 80-120 ° C Increase the temperature (slowly increase the temperature). In addition, gas desorption may decrease in each temperature range, and the pressure in the furnace may decrease again, or may be held in each temperature range until lower than the pressure before being held in each temperature range. .

  From the silicon sludge collected when the brick is obtained using a bandeau saw device, the second processed material obtained by the above-described purification method is used as a raw material, and the brick is produced by the casting method. A silicon substrate may be produced. When a brick is produced using the second processed material as a raw material, the raw material to be put into the casting furnace may be about 20% by mass of the raw material as the second processed material, and the remainder being polysilicon. With the silicon substrate thus obtained and 100% of the raw material put into the casting furnace as polysilicon, the silicon substrate manufactured in the same manner as described above has characteristics such as photoelectric conversion efficiency as a solar cell element substrate. The inventors have confirmed that there is no difference.

Melting process:
Next, the second processed product obtained in the second removal step S3 is heated to a melting temperature equal to or higher than the melting point of silicon (about 1420 ° C.) in a decompression device to form a silicon melt. The pressure in the apparatus in the melting step S4 is preferably 100 Pa or less. The melting temperature is equal to or higher than the melting point of silicon and is preferably lower. For example, if the melting temperature is 1500 ° C. or less, the reduction in the amount of silicon regenerated by silicon vapor can be further suppressed, and the generation of powder and foreign matters due to reaction with oxygen in the atmosphere can be further suppressed. The melting step S4 may be performed in an inert atmosphere such as argon or nitrogen or in a reducing gas atmosphere such as hydrogen. Thereby, the production | generation of the said solid component is further reduced.

Solidification process:
The silicon melt melted in the melting step S4 is cooled and solidified. At this time, it is preferable to solidify the silicon melt in one direction by cooling a part (for example, the bottom surface) of the container containing the silicon melt more strongly than the other parts. Thereby, by utilizing the segregation of the impurity component when silicon is solidified, impurities having a segregation coefficient smaller than 1 can be concentrated in the region to be solidified last.

  Thus, by cutting and removing the region where the impurities are segregated at a high concentration, a regenerated silicon lump that can be used as the raw material silicon for solar cells is obtained even in the case of silicon sludge collected from the multi-wire saw device S in particular. It is done. Even when a silicon substrate is produced using the silicon lump as described above, it can be used as a substrate for a solar cell element. A recycled silicon lump obtained by cutting and removing a region where impurities are segregated at a high concentration may be used alone as raw material silicon, or may be used as a mixture with other raw material silicon. For example, the regenerated silicon of this embodiment may be used in a mixture of 1 to 30% by mass with respect to 100% by mass of raw material silicon for solar cells.

<Example 2>
Next, another method for purifying silicon sludge will be described. As shown in FIG. 2, Example 2 is the same as Example 1 except that a third removal step S32 is added after the second removal step S3.

Third removal step:
After performing 2nd removal process S3 of Example 1, 3rd removal process S32 which obtains a 3rd processed material is performed. In this step, the second treatment product obtained from the silicon sludge is subjected to a third temperature that is higher than the second temperature and lower than the melting point of silicon in a second atmosphere under reduced pressure (for example, 100 Pa or less, preferably about 10 to 20 Pa). To heat the second processed product. For example, by heating to a third temperature of 950 to 1350 ° C., the oxygen component, carbon component, moisture and hydrogen in the second treated product of silicon sludge that could not be removed in the second removal step S3 can be more reliably removed. . For example, the oxygen component remaining in the second processed product reacts with silicon and volatilizes and desorbs as silicon monoxide. Further, the carbon component remaining in the silicon sludge reacts with silicon and oxygen to volatilize and desorb as silicon carbide, carbon monoxide and carbon dioxide. The hydroxyl group on the silicon surface in the second processed product is also decomposed and desorbed into oxygen and hydrogen. Each of these gases is particularly desorbed in a specific temperature range. Then, since the pressure in the apparatus once increases due to desorption, the silicon sludge is held in a predetermined temperature range, and the desorption of gas decreases, and the pressure in the apparatus starts to decrease again or is maintained at the third temperature. It is good to hold | maintain in a predetermined temperature range until it becomes lower than the previous pressure. If the temperature is not maintained when gas desorption increases, the desorption gas reacts with silicon in the silicon sludge, and solid components are likely to be formed on the silicon surface. By maintaining the silicon sludge at the third temperature, the generation of these solid components can be reduced.

  In the third removal step S32, it is necessary to continue exhausting in order to quickly exhaust the desorbed gas to the outside of the furnace. At a third temperature of 1000 to 1350 ° C., silicon in the second processed product reacts with hydroxyl groups on the silicon surface, chemically adsorbed water, and the like to produce silicon oxide SiO 2. At the temperature near 1400 ° C. at the time of melting, the following reaction occurs, and SiO vapor is generated.

SiO ₂ + Si → 2SiO ↑
The SiO vapor thus generated cools immediately and adheres to the silicon melt as SiO ₂ particles. And it remains as an impurity. In order to reduce this remaining, the desorbed gas should be quickly discharged out of the furnace, and the exhaust should be continued. In order to make the reaction less likely to occur, oxygen components such as hydroxyl groups and chemically adsorbed water on the silicon surface should be desorbed and discharged as much as possible at a temperature of 1200 ° C. or lower and at most 1350 ° C. A third removed product is obtained by the third removing step.

  More specifically, in the third removal step S32, in a range where the third temperature from 950 ° C. to 1350 ° C. can be taken, the temperature is gradually increased to about 100 ° C. (for example, 80 to 120 ° C.), The process is divided into the following steps to maintain each temperature until gas desorption decreases at each temperature and the pressure in the furnace starts to decrease again or becomes lower than the pressure before holding at each temperature. It is desirable to do this.

3-1 Removal step:
This step is performed at a temperature around 1000 ° C. (950 to 1050 ° C.). For example, the container containing the second processed material at 1000 ° C. is kept at this temperature until the desorption of gas decreases and the pressure in the apparatus starts to decrease again or becomes lower than the pressure before being held at 1000 ° C. Hold.

As shown in FIGS. 4 (a), (b) and FIG. 5, in the temperature range near 1000 ° C., the moisture, carbon dioxide, and the amount that could not be desorbed from the second treated product of silicon sludge in the second removal step. A small amount of carbon oxide desorption is observed. Moreover, as shown in FIG.4 (c), the big detachment | desorption of hydrogen is seen. The hydrogen, moisture, carbon dioxide and carbon monoxide desorbed in the vicinity of 1000 ° C. are not easily desorbed in the second removal step at a lower temperature. Further, even when held at a temperature higher than this (for example, about 1100 ° C. or 1200 ° C.), the gas remains without being desorbed and reacts with the components in the second processed product to form a solid. There is a case. For this reason, hydrogen, moisture, carbon dioxide, and carbon monoxide, which are easily desorbed at a temperature in the vicinity of 1000 ° C., are desorbed as much as possible, rather than rapidly increasing to a temperature of about 1100 ° C. or 1200 ° C., for example. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 1000 degreeC.

  The temperature may be maintained at a substantially constant temperature of 1000 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 950 to 1050 ° C.

3-2 Removal process:
For example, if the container containing the second processed material is about 1100 ° C., the gas desorption is reduced and the pressure in the apparatus starts to decrease again. Hold at this temperature until lower than the pressure before holding at 1100 ° C.

As shown in FIGS. 4 (a), 4 (b) and FIG. 5, in the temperature range near 1100 ° C., the moisture, carbon dioxide, and the amount that could not be desorbed in the second removal step from the second treated product of silicon sludge. A small amount of carbon oxide desorption is observed. Moreover, as shown in FIG.4 (c), the big detachment | desorption of hydrogen is seen. The hydrogen, moisture, carbon dioxide, and carbon monoxide desorbed in the vicinity of 1100 ° C. are not easily desorbed in the second removal step and the 3-1 removal step at lower temperatures. Even when kept at a temperature higher than this (for example, about 1200 ° C. or 1300 ° C.), it remains difficult to desorb. And what remains may react with the component in a 2nd processed material, and may form a solid substance. Therefore, it is preferable to desorb as much hydrogen, moisture, carbon dioxide, and carbon monoxide as possible as possible to be desorbed at a temperature near 1100 ° C., rather than suddenly raising the temperature to about 1200 ° C. or 1300 ° C., for example. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 1100 degreeC.

  The temperature may be maintained at a substantially constant temperature of 1100 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 1050 to 1150 ° C.

3-3 Removal step:
This step is performed at a temperature around 1200 ° C. (1150 to 1250 ° C.). For example, the container containing the second processed material at 1200 ° C. is kept at this temperature until the desorption of gas decreases and the pressure in the apparatus starts to decrease again or becomes lower than the pressure before being held at 1200 ° C. Hold.

  As shown in FIGS. 4 (a), (b), (c) and FIG. 5, in the temperature range near 1200 ° C., the moisture that could not be desorbed in the second removal step from the second treated product of silicon sludge, Desorption of carbon dioxide, hydrogen and carbon monoxide is observed. Moisture, carbon dioxide, hydrogen and carbon monoxide desorbed in the vicinity of 1200 ° C. are not easily desorbed in the second removal step, the 3-1 removal step and the 3-2 removal step at lower temperatures. Further, even when the temperature is maintained at a higher temperature (for example, about 1300 ° C. or 1400 ° C.), the gas remains difficult to desorb. And what remains may react with the component in a 2nd processed material, and may form a solid substance. For this reason, hydrogen, moisture, carbon dioxide, and carbon monoxide, which are easily desorbed at a temperature in the vicinity of 1200 ° C., are desorbed as much as possible, rather than rapidly increasing to a temperature of about 1300 ° C. or 1400 ° C., for example. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 1200 degreeC.

  The temperature may be maintained at a substantially constant temperature of 1200 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 1150 to 1250 ° C.

3-4 Removal process:
This step is performed at a temperature around 1300 ° C. (1250 to 1350 ° C.). For example, the container containing the second processed material at 1300 ° C. is kept at this temperature until the desorption of gas decreases and the pressure in the furnace starts to decrease again or becomes lower than the pressure before holding at 1300 ° C. Hold.

  As shown in FIGS. 4 (a), (b), (c) and FIG. 5, in the temperature range near 1300 ° C., moisture that could not be desorbed in the second removal step from the second treated product of silicon sludge, Desorption of carbon dioxide, hydrogen and carbon monoxide is observed. The moisture, carbon dioxide, hydrogen and carbon monoxide desorbed in the vicinity of 1300 ° C. are removed at a lower temperature than the second removal step, the 3-1 removal step, the 3-2 removal step, and the 3-3 removal. It is difficult to desorb in the process. Further, even when kept at a temperature higher than this (for example, about 1400 ° C.), it remains difficult to desorb. And what remains may react with the component in a 2nd processed material, and may form a solid substance. For this reason, hydrogen, moisture, carbon dioxide, and carbon monoxide, which are easily desorbed at a temperature in the vicinity of 1300 ° C., are desorbed as much as possible, instead of being suddenly raised to a temperature of about 1400 ° C., for example. For this reason, it is good to hold | maintain at this temperature until desorption of gas reduces and the pressure in the furnace of an apparatus starts decreasing again, or becomes lower than the pressure before hold | maintaining at 1300 degreeC.

  The temperature may be maintained at a substantially constant temperature of 1300 ° C., for example, or may be gradually increased (over a certain time) in a temperature range of 1250 to 1350 ° C.

  Moreover, in 3rd removal process S32, it raises to 950-1350 degreeC so that it may become monotonously high in each temperature range for every about 100 degreeC. In addition, each temperature range is maintained until the desorption of gas decreases in each temperature range and the pressure in the furnace starts to decrease again or becomes lower than the pressure before being held in each temperature range. May be.

<Others>
In addition, this invention is not limited to the said embodiment, Many corrections and changes can be added within the scope of the present invention.

  For example, the silicon sludge used in the present invention is not limited to the silicon sludge generated in the multi-wire saw device S. For example, silicon sludge recovered from a physical processing apparatus that uses a processing liquid such as a band saw apparatus, a grinding apparatus, a polishing apparatus, or a dicing apparatus may be used.

  For example, the drying step S1 and the first removal step S2 may each be performed in an air atmosphere, under reduced pressure, in an inert atmosphere such as argon or nitrogen, or in a reducing gas atmosphere such as hydrogen. You may go. The second removal step (1) S31 and the second removal step (2) S32 are each performed under reduced pressure, in an inert atmosphere such as argon or nitrogen, or in a reducing gas atmosphere such as hydrogen. Also good.

  Further, after the second removal step (1) S31 or the second removal step (2) S32, solid components such as silicon oxide and silicon carbide formed on the surface of the silicon sludge are made of a clean heat-resistant member. You may remove | eliminate by methods, such as blowing of inert gas, such as a brush and nitrogen, and you may throw in into a melting process. Thereby, further high purity raw material silicon can be obtained.

<Example 1>
The embodiment will be described mainly with reference to FIGS.

  Using a fixed abrasive type multi-wire saw apparatus S, silicon sludge generated when the workpiece 1 was cut was purified to produce silicon for solar cell raw materials as follows.

  First, the workpiece 1 made of rectangular parallelepiped polycrystalline silicon was bonded to the slice base 2 made of carbon plate with an adhesive made of epoxy resin. The workpiece 1 bonded to the slice base 2 was placed in the multi-wire saw device S. The wire 3 was made of stainless steel in which diamond abrasive grains were fixed by nickel plating. Then, the workpiece 3 was run while supplying the machining liquid to the wire 3 to cut the workpiece 1. A glycol aqueous solution was used as the processing liquid. And after completion | finish of a cutting process, the used processing liquid collect | recovered from the multi-wire saw apparatus S was isolate | separated into the reusable processing liquid and silicon sludge using the centrifuge.

  The recovered silicon sludge contained waste processing liquid and solid components as liquid components. As the solid component, a resin component derived from an adhesive, carbon derived from a slice base, diamond derived from abrasive grains, a metal component derived from a wire, and the like were included. As for the composition ratio, the solid component contained about 95% by mass of silicon, about 5% by mass of inorganic carbon component, and a small amount of resin component and metal component excluding oxygen.

  This silicon sludge was dried in the atmosphere at about 100 ° C. for 1 hour to remove moisture (drying step S1).

  Subsequently, the container containing the silicon sludge was heated at about 500 ° C. for 1 hour as a first temperature, and organic components such as glycol and adhesive were volatilized and removed to obtain a first treated product (first product). Removal step S2).

Desorption temperatures of various gases from silicon sludge by thermal desorption spectroscopy (TDS method) using a part of the first treated product of silicon sludge obtained in the first removal step S2. Was measured. The measurement was performed using a high-frequency heating temperature programmed desorption analyzer (IH-TDS1700, manufactured by Denki Kagaku Co., Ltd.). As a result of this measurement, it was confirmed that the main desorbed gases were hydrogen, moisture, carbon monoxide, and carbon dioxide.

  Furthermore, regarding the relationship between each temperature up to 1700 ° C. and each desorption amount of moisture, carbon dioxide, hydrogen, and carbon monoxide, the graphs of FIGS. 4A, 4B, 4C, and 5 were obtained.

  Moreover, about 200 kg of the first treated product of silicon sludge obtained in the first removal step S2 was placed in a silicon carbide container. And this container was hold | maintained in the furnace which can be evacuated and heated, and it heated, hold | maintaining at each temperature of 600 degreeC, 700 degreeC, 800 degreeC, 900 degreeC, 1000 degreeC, 1100 degreeC, 1200 degreeC, and 1300 degreeC. (Second removal step S3, third removal step S32). The pressure in the furnace of the heating apparatus before the start of heating was a steady state at about 20 Pa. Here, the holding time at each temperature was set until the pressure in the apparatus increased by the desorbed gas as the holding temperature was changed and became lower than the pressure before holding the temperature. In this temperature region, gases such as hydrogen, water, carbon monoxide, carbon dioxide, etc. that were not desorbed in the first removal step S2 were volatilized / desorbed from the silicon sludge as described above.

Next, the silicon sludge having undergone the second removal step S3 and the third removal step S32 was once taken out of the furnace to remove powders such as silicon carbide adhering to the surface of the silicon sludge. Then, it heated at 1500 degreeC within the pressure-reduced furnace, and formed the silicon melt (melting process S4). Then, it cooled from the bottom face of the container and solidified in one direction in the direction from the bottom face to the opening (solidification step S5). Then, the final solidified region where impurities are segregated to a high concentration was cut and removed. Thereby, the silicon lump of high purity which can be used as raw material silicon for solar cells was obtained. The concentration of carbon in the obtained silicon was about 0.5% by mass. The purity of silicon was 99.9999% by mass or more excluding carbon and oxygen.

  On the other hand, as shown in FIG. 6, as a comparative example, a drying step S1, a first removal step S2, a melting step S4, and a solidification step S5 were sequentially performed on the silicon sludge. However, after the drying step S1 and the first removal step S2, the silicon sludge was heated to a temperature equal to or higher than the silicon melt in the decompression device without maintaining the second temperature (melting step S4). As a result, the gas desorbed from the silicon sludge reacted with the silicon sludge to generate silicon carbide and silicon oxide powder. This covered the surface of the silicon sludge. For this reason, even at about 1500 ° C., part of the silicon sludge remained solid and melted. Therefore, in the comparative example, regenerated silicon that can be used as a solar cell material could not be obtained.

<Example 2>
About 100 kg of the first treated product of silicon sludge obtained in the first removal step S2 is put in another silicon carbide vessel, and this vessel is held in a furnace capable of being evacuated under reduced pressure, and 600 ° C to 1300 ° C. The temperature was continuously raised in the temperature range (second removal step S3, third removal step S32). The pressure in the furnace of the heating apparatus before the start of heating was a steady state at about 20 Pa. Here, the rate of temperature rise is each temperature range from 550 ° C. to 1350 ° C. for each 100 ° C. (550 to 650, 650 to 750 ° C., 750 to 850 ° C., 850 to 950 ° C., 950 to 1050 ° C., 1050 to 1150 ° C. 1150 to 1250 ° C., 1250 to 1350 ° C.) until the pressure in the furnace decreases until the desorption of the gas starts to decrease, and the pressure in the furnace increases. In order to prevent the desorption gas from greatly increasing with the change of the temperature rate, the temperature increase rate was decreased in the temperature range where the desorption gas was large, and the temperature increase rate was increased in the temperature range where the desorption gas was small. The pressure in the apparatus at this time was adjusted so as not to exceed a maximum of about 100 Pa. As a result, gases such as hydrogen, water, carbon monoxide, and carbon dioxide that were not desorbed in the first removal step S2 were volatilized and desorbed.

  Next, the silicon sludge having undergone the second removal step S3 and the third removal step S32 was once taken out of the furnace to remove powders such as silicon carbide adhering to the surface of the silicon sludge. Then, it heated at 1500 degreeC within the pressure-reduced furnace, and formed the silicon melt (melting process S4). Then, it cooled from the bottom face of the container and solidified in one direction in the direction from the bottom face to the opening (solidification step S5). Then, the final solidified region where impurities are segregated to a high concentration was cut and removed. The concentration of carbon in the obtained silicon was about 0.5% by mass. The purity of silicon was 99.9999% by mass or more excluding carbon and oxygen. Thereby, the silicon lump of high purity which can be used as raw material silicon for solar cells was obtained.

1: Workpiece 2: Slice base 3: Wire 4: Supply nozzle 5: Main roller 6: Container 7: Supply reel 8: Take-up reel 9: Guide roller 10: Work holder

Claims (14)

A method for refining silicon sludge that is generated when machining is performed using a working fluid containing organic matter on a workpiece made of silicon,
A first removal step of heating the silicon sludge at a first temperature lower than the melting point of silicon and discharging the first gas containing the organic matter from the silicon sludge to obtain a first treated product;
After the first removal step, the first processed material is heated at a second temperature higher than the first temperature and lower than the melting point of silicon in a furnace containing oxygen and having a pressure lower than atmospheric pressure. A second removal step of obtaining a second processed product by discharging a second gas containing at least one or more of carbon and oxygen from the first processed product;
A method for purifying silicon sludge.   2. The method for purifying silicon sludge according to claim 1, wherein the second removing step includes a step of gradually or gradually increasing the temperature in the furnace while exhausting the inside of the furnace.   The method for purifying silicon sludge according to claim 2, wherein, in the second removing step, the inside of the furnace is maintained at a predetermined temperature. The method for purifying silicon sludge according to claim 2.   4. The silicon according to claim 3, wherein in the second removal step, the pressure in the furnace once increases and then decreases or is maintained at the predetermined temperature until the pressure is lower than the pressure before being maintained at the predetermined temperature. Sludge refining method.   After the second removal step, a melting step for heating the second processed product to a melting point of silicon or higher to form a melt, and a solidification step for cooling the melt to segregate and solidify impurities in the melt. The method for purifying silicon sludge according to any one of claims 1 to 4, further comprising:   After obtaining the second processed product, the second processed product is heated at a third temperature higher than the second temperature and lower than the melting point of silicon in a second atmosphere lower than atmospheric pressure in the furnace. The method for purifying silicon sludge according to any one of claims 1 to 5, further comprising a third removal step for obtaining three processed products.   The method for purifying silicon sludge according to claim 6, wherein the third removing step includes a step of gradually or gradually increasing the temperature in the furnace while exhausting the inside of the furnace.   The method for purifying silicon sludge according to claim 7, wherein, in the third removal step, the inside of the furnace is maintained at a predetermined temperature.   9. The silicon according to claim 8, wherein, in the third removal step, the pressure in the furnace once increases and then decreases, or is maintained at the predetermined temperature until the pressure is lower than the pressure before being maintained at the predetermined temperature. Sludge refining method.   The method for purifying silicon sludge according to any one of claims 1 to 9, further comprising a drying step of heating the silicon sludge at a temperature lower than the first temperature before the first removing step.   The method for purifying silicon sludge according to any one of claims 1 to 10, wherein the processing liquid used in the machining includes a water-soluble organic solvent and water.   The method for purifying silicon sludge according to any one of claims 1 to 11, wherein the second temperature in the second removal step is 550 to 850 ° C.   The method for purifying silicon sludge according to any one of claims 1 to 12, wherein the third temperature in the third removal step is 950 to 1350 ° C.   The method for purifying silicon sludge according to any one of claims 1 to 13, wherein the first temperature in the first removal step is 350 to 650 ° C.

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