JP6004531B2 - Reduction furnace cleaning method - Google Patents

Description

  The present invention relates to a cleaning method for a bell jar type reduction furnace used in the production of polycrystalline silicon by the Siemens method. More specifically, the present invention relates to a reduction furnace cleaning effective for reducing the amount of impurities in the produced polycrystalline silicon and thereby improving the quality. Regarding the method.

  In the production of polycrystalline silicon by the Siemens method, the surface of a silicon core material that is energized and heated by the pyrolysis reaction and hydrogen reduction reaction of chlorosilane by passing a mixed gas of chlorosilane and hydrogen gas through a reduction furnace called a bell jar furnace. Silicon is deposited in a vapor phase to produce a polycrystalline silicon rod. At this time, various chlorides are by-produced and adhere to the inner surface of the bell jar furnace as a polymer, which mainly causes a decrease in the production efficiency and productivity of polycrystalline silicon.

  That is, the bell jar furnace used for the production of polycrystalline silicon has a structure in which a bell-shaped bell jar is detachably mounted on the bottom of the disk-like furnace, and the chloride produced as a by-product in the production stage of polycrystalline silicon. The product polymer mainly adheres to the inner surface of the bell jar, lowers the glossiness, and lowers the radiation effect in the furnace, thereby reducing the reaction rate.

  For this reason, it is customary to repeatedly clean the inner surface of the bell jar for each polycrystalline silicon production batch. As a cleaning method, there is dry ice blasting disclosed in Patent Document 1. Dry ice blasting is a method in which small particles of dry ice called dry ice pellets are blown against and collided with the surface to be cleaned to mechanically remove foreign substances adhering to the surface to be cleaned. Is suitable for cleaning the inner surface of the bell jar of a bell jar furnace for producing polycrystalline silicon in that it instantly vaporizes after impact and does not remain as a source of contamination.

  However, with respect to the chloride polymer adhering to the inner surface of the bell jar, the dry ice blast has a small impact force due to the collision of the blast particles, and thus the cleaning power is not sufficient. For this reason, recently, high-pressure water cleaning, which has a high detergency for chloride polymers and is simple in terms of technique, has been frequently used for bell jar inner surface cleaning. Hydrochloric acid is generated when high-pressure water washing is performed on chloride polymers, but the bell jar can be separated from the bottom of the furnace and can be washed at a different location away from the bottom of the furnace, and the inner surface has a smooth shape. For this reason, there is no concern about residual hydrochloric acid, and the generation of hydrochloric acid during cleaning does not become a problem.

  By the way, polycrystalline silicon is a raw material for solar cells, and since it is a raw material for producing silicon single crystals, the required purity of products has been increasing year by year.Therefore, various purity improvement measures have been taken from various aspects. Cleaning the inner surface of the bell jar is a measure for improving the production efficiency and at the same time a measure for improving the purity of polycrystalline silicon. However, even if many countermeasures are combined, it is becoming very difficult to satisfy the latest high required purity, and a high-purity countermeasure from a new viewpoint is required.

Japanese Patent No. 3167191

  An object of the present invention is to provide a reduction furnace cleaning method that is effective as a countermeasure from a new viewpoint for reducing the amount of impurities and improving the quality of polycrystalline silicon produced.

  The present inventor has been working on improving the quality of products, especially reducing the amount of impurities from various viewpoints, but for further reduction of impurities, he believes that measures from a new perspective different from the conventional ones are necessary. We paid attention to the cleanliness of the surface of the bottom of the bell jar furnace that was not omitted.

  The bell jar combined with the bottom of the bell jar furnace is separable from the bottom of the furnace, so it can be moved to a cleaning room away from the bottom of the furnace, and the internal shape is simple, so cleaning is easy and hydrochloric acid etc. As a result, it has been clear that the cleanliness of the inner surface has a direct influence on the radiation effect.

  On the other hand, the bottom of the furnace is fixed at a fixed position and cannot be moved to the cleaning chamber, and the surface shape is complicated by a large number of electrodes and gas nozzles. In addition, if considering the positional relationship with the product that the complex surface is located below the polycrystalline silicon, it is difficult to think that the surface properties of the furnace bottom will have a significant effect on the product quality and radiation effect, There was no surface cleaning. Even if surface cleaning is planned, due to the complicated shape of the surface, residual by-product hydrochloric acid is unavoidable in the case of high-pressure water cleaning, which is considered to be one of the reasons for neglect.

  Under such circumstances, the present inventor considers that contamination due to chloride polymer or the like gradually accumulated on the bottom surface of the bell jar furnace should not be irrelevant to the production of polycrystalline silicon, and a method for removing the contamination That is, the cleaning method of the furnace bottom surface was examined. As a result, the following facts were found.

  Since the bottom of the bell jar furnace is fixed at a fixed position and cannot move, and a large number of electrodes and the like are erected, removal of by-product hydrochloric acid is difficult when high-pressure water washing is applied. For this reason, it is difficult to apply high-pressure water cleaning applied to bell jar inner surface cleaning. In view of this, attention was paid to dry ice blasting, which has poor cleaning power but has no fear of causing secondary adverse effects such as residual hydrochloric acid, and an attempt was made to actually clean the furnace bottom surface with dry ice blasting. As a result, it was confirmed that even though the shape was complicated, it could be cleaned uniformly with almost no blind spots, the amount of impurities in the product was reduced, and the quality was improved. In addition, problems due to secondary adverse effects such as residual hydrochloric acid did not occur. However, the impact force due to the collision of the blast grains is small, and there is a problem in the cleaning performance and the quality improvement effect.

  Therefore, the present inventor planned to increase the cleaning ability by dry ice blasting, and intensively studied the measures. As a result, if the temperature at the bottom of the furnace is raised during dry ice blasting, the chloride polymer adhering to the furnace bottom surface softens, and even with dry ice blasting, chloride from the surface of the furnace bottom having a complicated shape It has been found that the contamination of the polymer and the like is effectively and efficiently removed, and as a result of the removal of the contamination, the quality of the produced polycrystalline silicon is higher than expected and is greatly affected by heating of the furnace bottom.

  The reduction furnace cleaning method of the present invention was developed on the basis of such knowledge, and the bell jar type reduction furnace used for the production of polycrystalline silicon by the Siemens method is opened after the operation, and after the product is taken out, the furnace The surface of the furnace bottom is cleaned by dry ice blasting while the bottom is heated.

  In the reduction furnace cleaning method of the present invention, when the furnace bottom part surface of the bell jar furnace is cleaned with dry ice blasting, the chloride polymer adhering to the furnace bottom part surface is softened by heating the furnace bottom part. Even if the dry ice blasting is small, the chloride polymer can be removed efficiently and effectively. Dry ice blasting is also effective for the furnace bottom surface having a complicated shape. Also, dry ice pellets, which are blasted particles, are instantly vaporized and disappear at the same time as the collision. Even if it exists, there is no possibility of causing secondary problems due to residual by-products.

  In addition, the bottom of the bell jar furnace has a cooling water flow path for cooling during operation, and heating is easily performed by passing a heating fluid such as steam, heated water, and heated dry air. be able to. Although the cooling water passage does not lead to the electrode standing at the bottom of the furnace, heating the furnace bottom body heats the electrode by heat conduction, and the chloride polymer adhering to it can be easily removed. There is no particular need to heat the electrodes independently.

  About the timing which heat-operates a furnace bottom part, you may heat before a blast start, may heat during a blast, and may heat before a blast before a blast. From the viewpoint of cleaning ability, it is preferable to perform heating before starting blasting. In short, dry ice blasting is performed with the bottom of the furnace higher than room temperature, and the timing of the heating operation is not particularly limited.

  The heating temperature at the bottom of the furnace is preferably 30 to 200 ° C, more preferably 40 to 150 ° C, and still more preferably 50 to 100 ° C in terms of surface temperature. Heating at a high temperature exceeding 200 ° C. may deteriorate the packing, and it cannot be said that it is preferable in terms of thermo-economics. A heating temperature that is too low has little effect on improving the cleaning ability. Here, the surface temperature of the furnace bottom is an average value of four points moved from the center of the furnace bottom by four distances in the east, west, north and south directions.

  The standard of cleaning with dry ice blasting is preferably 30 to 1000, more preferably 50 to 700, expressed by the glossiness after cleaning of the furnace bottom surface. When dry ice blasting is performed on the furnace bottom surface, dirt such as a chloride polymer adhering to the surface is removed, and the glossiness is increased. The glossiness is quantified as a glossiness of 100 when the reflectance of light incident at 60 ° on a glass surface having a refractive index of 1.567 based on JISZ8741 is 10%. Here, as with the surface temperature of the bottom of the furnace, it is represented by an average value of four points moved from the center of the furnace bottom by ½ to the east, west, south, and north.

  If the glossiness after washing is too low, the purity of the produced polycrystalline silicon is insufficiently improved. In addition, the radiation efficiency is increased by washing, so that the production speed of polycrystalline silicon is increased and the power consumption is reduced. However, if the glossiness after washing is too low, this effect becomes insufficient. On the other hand, if the glossiness after cleaning is too high, the labor and cost required for cleaning (blasting time, amount of blast particles used) increase more than necessary, and the economic efficiency decreases.

  The blasting conditions may be appropriately set so that the above glossiness is satisfied. For example, the average particle size of the dry ice pellets is preferably 0.1 to 20 mm, and the carrier gas pressure is 0.1 to 1.0 MPa. Is preferred. If these are small, the cleaning ability becomes insufficient, and if they are too large, the economic efficiency is lowered, or conversely, the cleaning ability is lowered. The blast amount is preferably 0.3 to 3 kg / min. If the amount is small, the cleaning efficiency is lowered. If the amount is large, the blasting apparatus is unnecessarily increased in scale, which causes a reduction in economic efficiency. The shape of the dry ice pellet is not limited to a sphere, but includes a cylindrical body and the like, and the particle diameter of the other than the sphere is expressed in terms of the diameter of a sphere having the same volume.

  The reduction furnace cleaning method of the present invention focuses on the cleanliness of the surface of the bottom of the furnace having a complicated structure of a bell jar furnace, which has not been omitted so far, in improving the quality of the produced polycrystalline silicon. Can be easily and efficiently and effectively improved by dry ice blasting combined with heating, so that further improvement in the quality of polycrystalline silicon can be easily achieved, and productivity can be improved. .

It is explanatory drawing of the reducing furnace cleaning method which shows one Embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

  As shown in FIG. 1, a reduction furnace targeted by the reduction furnace cleaning method of the present embodiment, that is, a bell jar type reduction furnace (bell jar furnace) for producing polycrystalline silicon, includes a disk-shaped furnace bottom 10 and a furnace bottom 10. A bell-shaped bell jar 20 to be covered is a main constituent member. While the furnace bottom 10 is installed in the operation room, the bell jar 20 can be separated from the furnace bottom 10 and can be reciprocated from the operation room to the cleaning room or the like by a transfer mechanism (not shown).

  A large number of electrodes 11 are dispersed and installed on the surface of the furnace bottom 10 for the purpose of supporting and energizing the silicon core material, as well as a gas introduction nozzle and an unreacted gas for introducing the raw material gas into the furnace. The gas discharge nozzles for discharging the byproduct gas are dispersed and opened so as not to interfere with the electrode 11. Due to these electrodes and nozzles, the surface of the furnace bottom portion 10 is configured such that complex structures are densely packed.

  On the other hand, a cooling water passage (cooling water passage) is provided inside the furnace bottom portion 10 in order to protect the furnace bottom portion 10 from heating during operation. A cooling water supply source and a heating steam supply source are selectively connected to the water passage by a switching mechanism (not shown).

  In the operation room where the furnace bottom 10 is installed, a blasting device 30 is disposed together with the furnace bottom 10. The blast device 30 is configured to discharge small particles of dry ice (dry ice pellets), which are blast particles, from the tip of a downward nozzle 31 by using a dry gas (for example, nitrogen, argon, dry air) as a carrier gas. ing. The blasting device 30 can be moved in the operating room, and by moving the nozzle 31 and changing the angle of the nozzle 31, the dry ice pellets can be sprayed on the entire surface including the surface of the structure such as the electrode on the furnace bottom 10. It is configured.

  Next, the reducing furnace cleaning method of this embodiment will be described.

  In the polycrystalline silicon manufacturing operation, a raw material gas composed of a mixed gas of chlorosilane and hydrogen is introduced into the furnace from the gas introduction nozzle at the furnace bottom 10 while the silicon core attached to the electrode in the bell jar furnace is energized and heated. . Thereby, the silicon core material in the furnace grows by precipitation of polycrystalline silicon on the surface, and becomes a polycrystalline silicon rod as a product. At this time, the exhaust gas containing unreacted source gas and by-products is discharged out of the furnace from the gas discharge nozzle around the furnace bottom 10. Further, in order to prevent overheating of the furnace bottom portion 10, cooling water is circulated through the cooling water flow path in the furnace bottom portion 10.

  When the manufacturing operation of the polycrystalline silicon rod is finished, the raw material gas is switched to only hydrogen gas while cooling water is circulated through the cooling water flow path of the furnace bottom 10 to cool the inside of the furnace. When the furnace temperature is lowered to such an extent that the bell jar furnace can be opened, the bell jar 20 is separated from the furnace bottom 10 and the manufactured polycrystalline silicon rod is exposed to the outside air to promote cooling of each rod. The bell jar 20 separated from the furnace bottom 10 is transported from the operation room to the washing room, where the inner surface is washed with high pressure water to remove deposits such as chloride polymer and restore the glossiness.

  When each rod is cooled to about room temperature where the polycrystalline silicon rod can be handled, the rod is removed from the electrode. At the stage where the rod is removed, that is, the product is collected, the furnace bottom 10 is cooled to room temperature.

  Conventionally, one cycle of operation has been completed here, but in this embodiment, the surface of the furnace bottom 10 is further subjected to surface cleaning by dry ice blasting using the blasting device 30. Specifically, instead of the cooling water, steam is circulated through the cooling water flow path of the furnace bottom 10 to heat the surface of the furnace bottom 10 to a predetermined temperature. In this state, the blasting device 30 is transported near the furnace bottom 10 and the downward nozzle 31 is moved in the horizontal direction (XY direction) on the furnace bottom 10 while changing the angle. This causes the dry ice pellets to collide with the entire surface including the electrode and gas nozzle on the furnace bottom 10.

  The dry ice pellets are soft as blast grains, and not only do not damage the surface of the furnace bottom 10, but also vaporize into carbon dioxide gas at the same time as blasting, so no residue is generated. However, the blasting ability is not high, and the effect of removing surface deposits is low when the furnace bottom 10 is cooled to room temperature. However, since the furnace bottom 10 is heated here, the chloride polymer which is the main substance of the deposit is softened, and the deposit is effectively removed despite the dry ice blasting. The cleaning effect by dry ice blast becomes more remarkable as the heating temperature of the furnace bottom 10 becomes higher.

  And the chloride polymer etc. which adhere to the surface of the furnace bottom part 10 are removed, the amount of impurities in the polycrystalline silicon rod manufactured in the next cycle is reduced, and the purity and quality are improved. In addition, although the surface of the furnace bottom 10 is below the product and has a complicated diffuse reflection structure, the effect of improving the radiation effect in the furnace due to the increase in glossiness of the surface is not small. It can be expected to increase the deposition rate of crystalline silicon, shorten the reaction time, and reduce the power consumption.

  Below, the effectiveness of this embodiment is shown quantitatively.

  The surface of the bottom of a bell jar furnace used for the production of polycrystalline silicon was cleaned by the method described above. The scale of the bell jar furnace is 34 in the production number of polycrystalline silicon. Regarding dry ice blasting conditions, a rice-sized cylindrical dry ice pellet, specifically, a cylindrical dry ice pellet having a diameter of about 3 mm and a length of 10 to 20 mm is used, and the pressure of nitrogen gas as a carrier gas is used. By adjusting the spray amount of dry ice pellets in the range of 0.3 to 3 kg / min in various ways, we found one appropriate condition that achieves both cleanability and economy, Under this condition, the entire surface of the furnace bottom was cleaned over a certain time. In contrast to this normal cleaning, precision cleaning was also performed with a cleaning time of 1.5 times without changing the blasting conditions. The heating of the furnace bottom was performed by supplying steam to the cooling water flow path, and the heating temperature was variously changed by increasing or decreasing the supply amount.

  The effect of surface cleaning by dry ice blasting with heating of the bottom of the furnace, the product quality and productivity of the manufactured polycrystalline silicon rod, when surface cleaning is not performed (Comparative Example 1), the surface cleaning of the furnace bottom is performed The results are shown in Table 1 in comparison with the case where the method was only dry ice blasting (no heating of the furnace bottom) (Comparative Example 2).

  The product quality was investigated by measuring the lifetime with a lifetime measuring instrument after single-crystallizing the manufactured polycrystalline silicon sample by the FZ method, and the result of the comparison is 100 for Comparative Example 1. It was expressed as a relative ratio. The larger this ratio, the higher the purity and quality. Productivity was evaluated in terms of power consumption, and expressed as a relative ratio when Comparative Example 1 was 1000. The smaller this ratio, the higher the efficiency and the economy. The degree of cleaning of the furnace bottom surface was evaluated by the glossiness of the surface after cleaning for some examples with high cleaning levels. The heating temperature at the bottom of the furnace is the surface temperature just before the start of blasting. The glossiness and the surface temperature are measured values by the above-mentioned four-point average method.

  As is clear from Table 1, dry ice blasting involving heating of the furnace bottom is effective for improving the quality and productivity of polycrystalline silicon rods. If the blast condition is constant, the higher the heating temperature at the bottom of the furnace, the higher the cleanliness and the more effective the quality and productivity. The blast cleaning “◎” is a precision cleaning that takes 1.5 times as long as the normal cleaning “◯”, and has a corresponding effect in improving quality and productivity.

10 furnace bottom 11 electrode 20 bell jar 30 blasting device 31 nozzle

Claims (4)

  A reduction furnace cleaning method in which a bell jar type reduction furnace used for the production of polycrystalline silicon by the Siemens method is opened after operation, and the surface of the furnace bottom is cleaned with dry ice blasting while the furnace bottom is heated after taking out the product. .   2. The reduction furnace cleaning method according to claim 1, wherein heating of the furnace bottom is performed by circulating a heating fluid through a cooling water flow path.   The reduction furnace cleaning method according to claim 1 or 2, wherein the heating temperature at the bottom of the furnace is 30 to 200 ° C as a surface temperature.   The reduction furnace cleaning method according to any one of claims 1 to 3, wherein the dry ice blasting is performed until the glossiness of the furnace bottom surface becomes 30 to 1000.

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