Method for recovering chlorosilanes

Abstract

To provide a method in recovering chlorosilanes by subjecting chlorosilane liquids containing aluminum chloride to a thin-film evaporation device for highly suppressing the deposition of aluminum chloride scale in the evaporation device.SOLUTION: In a method for recovering chlorosilanes by subjecting chlorosilane liquids containing aluminum chloride to a thin-film evaporation device such as a stirring-type device to evaporate the chlorosilanes, a part of an aluminum chloride concentrated slurry discharged from the thin-film evaporation device is continuously or intermittently returned into the thin-film evaporation device to be discharged into the device during operation of the thin-film evaporation device.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for recovering chlorosilanes, and more particularly to a method for recovering chlorosilanes from a chlorosilane liquid containing aluminum chloride, which is produced by, for example, reacting a gas containing chlorine, hydrogen chloride, or silicon tetrachloride and hydrogen with metallurgical-grade silicon. [Background technology]

[0002] Chlorosilanes obtained by chlorinating or hydrochlorinating metallurgical-grade silicon contain aluminum chloride as an impurity due to the chlorination or hydrochlorination of aluminum, which is normally contained in metallurgical-grade silicon in an amount of about 0.01 to 10 mass %. Therefore, aluminum chloride becomes an undesirable impurity in the production process of semiconductor-grade high-purity silicon or photovoltaic-grade silicon, and therefore needs to be separated and removed from the chlorosilanes.

[0003] Because aluminum chloride has a higher boiling point than useful chlorosilanes such as trichlorosilane (hereinafter referred to as TCS) and silicon tetrachloride (hereinafter referred to as STC), it has been common practice to recover chlorosilanes by distillation from an aluminum-containing chlorosilane liquid using a distillation column, thereby separating the chlorosilanes as a distillation residue. This distillation residue is appropriately extracted from the bottom of the distillation column and disposed of.

[0004] However, the distillation residue to be disposed of still contains useful chlorosilanes, such as TCS and STC, at a concentration of typically around 95% by mass. Therefore, it is desirable to recover the useful chlorosilanes by re-evaporation. However, this evaporation and concentration poses a major problem: scale deposition on the inner walls of the evaporator. Specifically, aluminum chloride dissolved in the chlorosilanes in the distillation residue is prone to precipitation due to concentration fluctuations. In particular, when recovering chlorosilanes by distillation, the aluminum chloride concentration in the column bottoms is high, and the aluminum chloride deposition is particularly severe in the distillation residue extracted from the column bottoms. Therefore, when recovering chlorosilanes from the distillation residue by evaporation, the deposition of aluminum chloride significantly leads to scale deposition on the inner walls of the evaporator. Unless this problem is improved, it becomes difficult to continue operating the evaporator for a long period of time.

[0005] Under these circumstances, it has been proposed to use a thin-film evaporator as an evaporator that reduces the deposition of scale on the inner walls of the evaporator due to aluminum chloride precipitates during the evaporation of chlorosilanes from the distillation residue (see Patent Document 1). Here, in a method using such a thin-film evaporator, it is proposed that the aluminum chloride concentrate discharged from the apparatus after thin-film evaporation can be disposed of after being subjected to a known detoxification method (section

[0046] ). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-261324 Summary of the Invention [Problem to be solved by the invention]

[0007] The method of applying the thin film evaporation described above is significant in that it effectively suppresses the deposition of aluminum chloride precipitates inside the thin film evaporator. However, from the viewpoint of recovering chlorosilanes from distillation residues on an industrial level, the suppression effect is still not sufficient.

[0008] That is, according to the above method, deposition of aluminum chloride scale on the inner wall heating surface where evaporation of chlorosilanes occurs in the chlorosilane liquid, and on the rotating blade surfaces to which centrifugal force is applied when the thin-film evaporator is a stirring type, can be kept to a fairly low level. However, in the region below the heating section in such thin-film evaporation, a concentrate with an increased aluminum chloride concentration flows down due to the evaporation of chlorosilanes, and the liquid temperature also drops, so that the deposition of the scale increases and cannot be sufficiently suppressed. Therefore, the severity of scale deposition in the region below the heating section is particularly noticeable when the thin-film evaporator is a stirring type.

[0009] In light of the above, an object of the present invention is to develop a method for more effectively suppressing the deposition of aluminum chloride scale in a thin-film evaporator when the aluminum chloride-containing chlorosilane liquid is fed to the thin-film evaporator to recover chlorosilanes. [Means for solving the problem]

[0010] In view of the above problems, the present inventors have conducted extensive research and have found that the above problems can be solved by continuously or intermittently returning a portion of the concentrated aluminum chloride slurry discharged from a thin-film evaporator to the inside of the thin-film evaporator during operation of the thin-film evaporator, thereby completing the present invention.

[0011] That is, the present invention provides a method for recovering chlorosilanes, in which a liquid chlorosilane containing aluminum chloride is supplied to a thin-film evaporator to evaporate and recover chlorosilanes, comprising storing concentrated aluminum chloride slurry discharged from the thin-film evaporator for one hour or more during operation of the thin-film evaporator, and then continuously or intermittently returning a portion of the concentrated aluminum chloride slurry after storage into the thin-film evaporator to recover the chlorosilanes. Thin Film Evaporation This is a method for recovering chlorosilanes, characterized in that the liquid is discharged into the device.

[0012] The present invention also provides a method for producing chlorosilanes by distilling a chlorosilane liquid containing aluminum chloride, which is produced by reacting a gas containing chlorine, hydrogen chloride, or silicon tetrachloride and hydrogen with metallurgical-grade silicon, wherein the chlorosilane liquid containing aluminum chloride obtained as a distillation residue by the distillation is supplied to a thin-film evaporator, and the method for recovering chlorosilanes is carried out during the operation of this thin-film evaporation. [Effects of the Invention]

[0013] The method of the present invention enables the thin-film evaporation of aluminum chloride-containing chlorosilane liquid to continue operation while highly suppressing scale deposition within the equipment, thereby enabling the stable recovery of chlorosilanes over long periods of time. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a process diagram showing a typical embodiment of the method for recovering chlorosilanes of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the thin film evaporator (7) shown in the flow chart of FIG. 1 taken along the line AA. DETAILED DESCRIPTION OF THE INVENTION

[0015] The chlorosilane liquid to be subjected to the recovery method of the present invention is not limited in any way as long as it contains aluminum chloride, but typically it is obtained by condensing a chlorosilane mixed gas produced by reacting metallurgical-grade silicon having a purity of about 90 to 99% with chlorine, or hydrogen chloride, or hydrogen and silicon tetrachloride, or the like.

[0016] The types and proportions of chlorosilanes produced in the above chlorosilane synthesis reactions vary depending on the reaction raw materials and reaction conditions, but the products are chloromonosilanes, typically TCS (SiHCl3), STC (SiCl4), and dichlorosilane (SiH2Cl2).In addition to these, dimeric chlorosilanes such as pentachlorodisilane (Si2HCl5) and hexachlorodisilane (Si2Cl6), as well as polymeric chlorosilanes, may also be present.

[0017] As mentioned above, metallurgical-grade silicon usually contains about 0.01 to 10 mass % of aluminum, and when this aluminum is chlorinated or hydrochlorinated, the chlorosilane liquid contains aluminum chloride. The aluminum chloride content varies depending on the aluminum content in the metallurgical-grade silicon used and the reaction conditions for producing chlorosilanes, etc.

[0018] More specifically, the method for producing the chlorosilane liquid containing aluminum chloride used in the present invention can be described as follows: For example, a method of chlorinating metallurgical-grade silicon powder with a chlorine-containing gas at 100°C or higher, generally 100 to 500°C, a method of chlorinating metallurgical-grade silicon powder with a hydrogen chloride-containing gas at 250°C or higher, generally 250 to 450°C, or a method of hydrochlorinating metallurgical-grade silicon powder with a mixed gas of STC and hydrogen at 400°C or higher, generally 400 to 600°C, is generally known.

[0019] In these reactions, the gases reacted with metallurgical-grade silicon can be mixed together and reacted, or can be mixed with a reactive inert gas such as hydrogen or nitrogen and reacted.

[0020] The chlorosilane liquid obtained by condensing the chlorosilane gas thus obtained typically contains 0.01 to 2 mass% aluminum chloride. Metallurgical-grade silicon, which is the raw material used to produce the chlorosilane liquid, typically contains metal impurities such as iron and titanium in addition to aluminum. These are also chlorinated during the production of chlorosilanes, so the chlorosilane liquid may contain chlorides of these metals (iron chloride, titanium chloride, etc.). The chlorosilane liquid may also contain silicon fine powder.

[0021] The present invention is applicable to the recovery of chlorosilanes from a chlorosilane liquid containing aluminum chloride obtained by a chlorosilane production reaction. While the reaction liquid from which the chlorosilanes have been produced may be used directly as the treated liquid, subjecting the chlorosilanes to thin-film evaporation at a high concentration is inefficient. Therefore, it is usually preferable to first distill the chlorosilanes to recover most of the chlorosilanes by distillation, and then apply the method to the distillation residue. As mentioned above, the distillation residue still contains chlorosilanes such as TCS and STC at a concentration of typically around 95% by mass. Thin-film evaporation has been proposed as an effective means of further recovering these chlorosilanes. When applied to the thin-film evaporation of the distillation residue, the present invention can more effectively prevent scale deposition within the thin-film evaporation apparatus.

[0022] The recovery method of the present invention will be described in detail with reference to the flow chart in Fig. 1, taking as an example a representative embodiment of the present invention in which the liquid to be treated is the distillation residue obtained when a reaction liquid in which chlorosilanes have been produced is distilled. However, the present invention is not limited to these embodiments.

[0023] In Figure 1, a known distillation column can be used as the distillation column (1). That is, any commonly used distillation column tray can be used without any restrictions. Examples include packed types filled with structured packing, random packing, bubble cap types, and perforated plate types.

[0024] The distillation column (1) may also have a reboiler. The reboiler may be a jacket type reboiler that directly heats the bottom of the distillation column, or a reboiler that is installed outside the bottom of the distillation column. Alternatively, a reboiler may be installed inside the bottom of the distillation column.

[0025] Generally, a shell-and-tube type heat exchanger is preferably used to increase the heat transfer area, but a coiled tube type or an electric heater can also be used. If the chlorosilane liquid accumulates in the heat exchanger that applies the distillation energy and the aluminum chloride becomes highly concentrated, scaling can occur. Therefore, it is preferable that the heat exchanger has a structure that makes it difficult for the liquid to accumulate. Methods that make it difficult for the liquid to accumulate include a method that uses convection due to heating, and a method that uses a pump or the like to force the liquid to flow.

[0026] A supply pipe (2) for the chlorosilane liquid containing aluminum chloride, which is the liquid to be treated in the present invention, is connected to the distillation column (1). The supply point to the distillation column (1) may be any part, but it is more preferable to supply the liquid directly to the bottom of the distillation column to prevent fouling of the trays.

[0027] Here, since there is a considerable difference in boiling point between the chlorosilanes to be purified and the impurities such as aluminum chloride to be separated and removed, distillation can be carried out satisfactorily without particularly sophisticated rectification, and the reflux ratio may be in a range within which the distillation operation can be maintained, and may be about 0.1 to 1. The purified chlorosilanes are discharged from a purified chlorosilane gas distillation pipe (3) connected to the top of the distillation column (1).

[0028] In the present invention, the distillation is carried out at a column bottoms temperature of 50°C or higher, more preferably 70 to 150°C, and most preferably 80 to 120°C. By setting the column bottoms temperature to 50°C or higher in this manner, the distillation of chlorosilanes can be performed with high efficiency. Since the column bottoms temperature is 50°C or higher, the distillation bottoms withdrawn from the bottom of the distillation column (1) through the distillation bottoms discharge pipe (4) are also at the same high liquid temperature at the beginning of discharge.

[0029] The concentration of aluminum chloride dissolved in the bottom liquid of the distillation column (1) is preferably adjusted to less than the saturated solubility at the temperature of the bottom liquid in order to prevent aluminum chloride from depositing in the bottom of the column and to continue stable operation for a long period of time. For example, when the temperature of the bottom liquid is 50°C or higher, the aluminum chloride concentration in the bottom liquid is preferably maintained in the range of 0.1 to 5.0 mass%, preferably 0.5 to 1.2 mass%.

[0030] In the flow chart of Figure 1, the chlorosilane liquid containing aluminum chloride obtained as the distillation residue is subjected to thin-film evaporation. The liquid temperature is initially as high as 50°C or higher when it is extracted from the bottom of the distillation column (1) into the distillation residue discharge pipe (4). Therefore, when the liquid is subjected to thin-film evaporation at such a high temperature, aluminum chloride tends to precipitate on the inner walls of the thin-film evaporator. In contrast, if the distillation residue is first cooled, preferably to a temperature of 40°C or lower, thereby precipitating the bulk of the dissolved aluminum chloride and increasing the dispersed concentration of the aluminum chloride precipitate, the problem of aluminum chloride scale deposition can be significantly alleviated.

[0031] For this reason, the distillation residue flowing through the distillation residue discharge pipe (4) is preferably sent to a cooling tank (5) and cooled while stored therein before being supplied to the thin-film evaporator (7). Here, the cooling means for the distillation residue in the cooling tank (5) may be air-cooling, but in consideration of the cooling efficiency and ease of control of the cooling temperature, it is preferable to use an external cooler, specifically, forced cooling by surrounding the outer wall of the cooling tank with a cooling jacket or by installing a refrigerant pipe inside the cooling tank.

[0032] Furthermore, cooling in the cooling tank (5) is preferably carried out under stirring, from the viewpoint of enhancing the homogeneity of the aluminum chloride solid content dispersed in the distillation residue cooling liquid. Stirring may be carried out using a magnetic stirrer or the like, but stirring with a stirring blade is preferred. Specific examples of stirring blades include paddle-type, ribbon-type, anchor-type, propeller-type, turbine-type, swept-back blade-type, and gate-type. The number of stirring blades attached to the stirring shaft cannot be determined in general depending on the shape and size of the reactor, but generally, 1 to 4 blades are sufficient on the rotating shaft. The peripheral speed of the stirring blade tip is preferably 0.1 to 10 m / s, and the rotation speed is preferably 100 to 300 rpm.

[0033] In the flow chart of Fig. 1, the distillation residue cooling liquid discharged from the cooling tank (5) is supplied to a thin-film evaporator (7) via a distillation residue cooling liquid flow pipe (6). Any known thin-film evaporator that can form a thin film of the liquid to be treated on a heated surface and evaporate the liquid components can be used without limitation. Specific examples include an agitation-type thin-film evaporator in which internal blades rotate relative to a fixed heated surface (evaporation surface) to forcibly agitate the liquid to be treated and form a thin film, and a centrifugal thin-film evaporator in which the heated surface rotates. Agitation-type thin-film evaporators are particularly preferred.

[0034] The above-mentioned agitated thin film evaporators are classified into vertical and horizontal types depending on the direction of the rotation axis, fixed blade and movable blade types depending on the type of blade attachment to the rotation axis, and contact blade and non-contact blade types depending on whether the blades contact the evaporation surface. These types can be used in combination as appropriate. Among these, the vertical contact movable blade type and the vertical non-contact blade type are more preferred, and these thin film evaporators are relatively effective in suppressing scale deposition. In particular, the vertical contact movable blade type is preferred because the blades contact the evaporation surface, thereby providing the highest effect in suppressing scale deposition.

[0035] In the thin-film evaporator (7), if droplets are entrained in the evaporated gas components depending on the operating conditions, the removal rate of aluminum chloride may decrease. In such cases, installing a droplet collector (9) midway through the recovery pipe (8) for the purified chlorosilanes discharged from the thin-film evaporator is an effective means for preventing droplets from being entrained in the recovered chlorosilanes. The droplet collector (9) may be of any type as long as it is capable of collecting and removing droplets. Examples include a baffle (impingement plate) type, a sintered type, and a centrifugal separator type.

[0036] It is also effective to install such a droplet collector (9) in the distillation residue cooling liquid flow pipe (6) connecting the cooling tank (5) and the thin film evaporator (7). The distillation residue cooling liquid discharged from the cooling tank (5) may also contain aluminum chloride droplets, and removing these droplets before supplying the distillation residue cooling liquid to the thin film evaporator (7) is effective in preventing deposition of aluminum chloride inside the evaporator.

[0037] When the thin-film evaporator (7) is of a type having contact movable blades, the peripheral speed of the blade tips is preferably 0.1 m / s or more, more preferably 0.5 m / s or more, and even more preferably 1 m / s or more. In order to exert a sufficient scale-inhibiting effect, it is preferable that the frequency of the blade movement at a point on the evaporation surface is once per second or more, preferably once per 0.5 seconds or more, and even more preferably once per 0.3 seconds or more.

[0038] On the other hand, in the case of a vertical non-contact blade type, the blades do not come into contact with the evaporation surface, so they must be rotated at a higher speed than in a contact type, with the peripheral speed of the blade tip being 1 m / s or more, preferably 3 m / s or more, and more preferably 5 m / s or more. Also, the frequency of the blade movement at a point on the evaporation surface is preferably once per 0.5 seconds or more, preferably once per 0.3 seconds or more, and more preferably once per 0.1 seconds or more. Suitable conditions can be selected depending on the heat transfer area, blade type, etc.

[0039] By using a thin-film evaporator in this manner, the residence time of the distillation residue cooling liquid within the apparatus can be shortened, and the effect of inhibiting the deposition of aluminum chloride scale within the apparatus can be enhanced. At the same time, this effect can be further improved by selecting the structure or operating conditions of the thin-film evaporator described above.

[0040] In the present invention, when the distillation residue cooling liquid is supplied to the thin-film evaporator (7), the treated liquid is heated while forming a liquid film on the evaporation surface, and the chlorosilanes evaporate and the aluminum chloride is concentrated. In this manner, the chlorosilanes are discharged in a gaseous state from the upper outlet of the evaporator (7) into the purified chlorosilanes recovery pipe (8), and the concentrate of the distillation residue cooling liquid stored in the concentrate reservoir (22) is discharged from the lower outlet into the aluminum chloride concentrated slurry discharge pipe (10).

[0041] In this case, the residence time of the distillation residue cooled liquid in the thin film evaporator (7) from when it is supplied until it is taken out as a concentrated aluminum chloride slurry is usually within about 1 minute, or at most within a few minutes, in a vertical thin film evaporator. In the case of a horizontal type thin film evaporator, the residence time can be adjusted to some extent, but it is preferable to adjust it to within the above-mentioned 10 minutes, preferably within 5 minutes.

[0042] Operating conditions for the thin-film evaporation include the heat medium temperature, operating pressure, etc., and as with conventional evaporation operations, appropriate operating conditions can be selected depending on the composition of the distillation residue cooling liquid to be treated, the feed rate, the heat transfer area, the concentration ratio, etc. In this case, sublimation of aluminum chloride from the distillation residue cooling liquid becomes intense at temperatures higher than about 180°C, and on the other hand, the aluminum chloride precipitate formed here has low resolubility in the chlorosilane liquid, as will be described later. Therefore, from the viewpoint of effectively preventing scale deposition in the thin-film evaporator, it is preferable to set the evaporation conditions so that the temperature of the concentrated aluminum chloride slurry at the lower outlet of the thin-film evaporator (7) is 180°C or less, preferably 150°C or less.

[0043] In the present invention, the aluminum chloride concentrated slurry is allowed to contain aluminum chloride at a fairly high concentration, but if the concentration is too high, scale deposition increases, which may make continuous treatment difficult. Furthermore, the fluidity of the slurry decreases, which deteriorates the workability of transferring the aluminum chloride concentrated slurry to an abatement pit or the like, and further leads to the risk of impairing the smoothness of the operation of returning a portion of the aluminum chloride concentrated slurry into the thin-film evaporator, which is a feature of the present invention described below.

[0044] Therefore, the upper limit of the concentration of aluminum chloride precipitate in the concentrated aluminum chloride slurry (solid concentration of aluminum chloride precipitate) is preferably 50 mass%, more preferably 40 mass%, and particularly preferably 30 mass%. The lower limit of the aluminum chloride concentration is preferably 3 mass%, which has been difficult to concentrate using conventional techniques, and further preferably 10 mass%, because the effects of the present invention can be fully exhibited.

[0045] The greatest feature of the recovery method of the present invention is that a part of the concentrated aluminum chloride slurry discharged from the thin-film evaporator is continuously or intermittently returned to the thin-film evaporator during operation of the thin-film evaporator, and discharged into the thin-film evaporator. As a result, the wall surfaces of the thin-film evaporator that come into contact with the concentrated aluminum chloride slurry are washed and wetted, thereby providing a significant effect of effectively preventing adhesion of aluminum chloride precipitates and crystal growth.

[0046] If the idea is that the inside of the thin-film evaporator needs only to be cleaned, those skilled in the art would consider it more reasonable to use the clean chlorosilanes liquid recovered from the distillation column (1) or the thin-film evaporator (7) rather than returning the concentrated aluminum chloride slurry containing a high concentration of aluminum chloride precipitates to be cleaned back into the apparatus. However, in this case, the aluminum chloride scale deposition suppression effect as excellent as that achieved by the configuration of the present invention cannot be obtained.

[0047] The inventors speculate that this is due to the following reasons. Specifically, the aluminum chloride precipitate deposited during thin-film evaporation is concentrated all at once and grows into unstable crystals. In this case, the precipitate exhibits high adhesiveness at the time of deposition and for a while thereafter. However, after this period has passed, the adhesiveness becomes less pronounced, and furthermore, due to the unstable crystal growth of the precipitate, the resolubility of chlorosilanes in the liquid becomes low. For this reason, in the concentrated aluminum chloride slurry discharged from the thin-film evaporation apparatus, the dispersed aluminum chloride precipitate becomes stable granules. When this is returned to the apparatus as a cleaning liquid, it is thought that the precipitate acts like an abrasive, more effectively suppressing the deposition of aluminum chloride.

[0048] The operation of returning the concentrated aluminum chloride slurry to the thin-film evaporation apparatus can be carried out by any known method of discharging the concentrated aluminum chloride slurry toward the inner space of the apparatus, without any restrictions. The discharge may be carried out by discharging the concentrated aluminum chloride slurry into the inner space of the apparatus from a slurry discharge port provided on the inner wall of the apparatus with a force equal to or greater than a certain value, or by allowing the slurry to flow down along the inner wall surface of the apparatus. It is preferable to carry out the discharge so that the concentrated aluminum chloride slurry flow hits, as much as possible, portions of the inner wall surface of the apparatus where aluminum chloride deposition is severe. Specifically, since aluminum chloride scale deposition is severe in a region below the heating section where thin-film evaporation takes place, it is more effective to discharge the slurry into this region.

[0049] In particular, when the thin-film evaporator is an agitator type, discharging the aluminum chloride concentrated slurry below the heating section where thin-film evaporation occurs is preferable because it exhibits a particularly significant effect of inhibiting scale deposition. This is because, as described above, agitator type thin-film evaporators typically have a bearing supporting the lower end of the central rotating shaft of the rotating blades below the heating section where thin-film evaporation occurs, and the central rotating shaft bearing has a structure in which multiple reinforcing ribs extend radially. In this case, the centrifugal force of the rotating shaft is not applied to the plate surfaces of the reinforcing ribs, making it particularly easy for scale to deposit in the aluminum chloride concentrate. In other words, discharging the aluminum chloride concentrated slurry below the heating section of the device more effectively inhibits this severe scale deposition, making this a particularly preferred embodiment of the present invention. To maximize the deposition inhibition effect, it is desirable to have the discharge flow of the aluminum chloride concentrated slurry strike the outer ends of the reinforcing ribs rather than the inner wall of the thin-film evaporator.

[0050] The operation of returning a portion of the concentrated aluminum chloride slurry into the thin-film evaporator will be explained using the flow chart in Figure 1. The concentrated aluminum chloride slurry taken out from the concentrated aluminum chloride slurry discharge pipe (10) connected to the bottom of the thin-film evaporator (7) is temporarily stored in the concentrated aluminum chloride slurry storage tank (11). The aluminum chloride is then sufficiently stabilized and homogenized by being stored here, and then discharged into the concentrated aluminum chloride slurry disposal pipe (12).

[0051] The storage time of the concentrated aluminum chloride slurry in the concentrated aluminum chloride slurry storage tank (11) is preferably 1 hour or more, more preferably 2 to 5 hours. Furthermore, the storage is preferably carried out under conditions of 30°C or higher by keeping the temperature in a jacket.

[0052] The aluminum chloride concentrated slurry flowing through the aluminum chloride concentrated slurry waste pipe (12) is sent to an abatement pit (13) for disposal, but the aluminum chloride concentrated slurry waste pipe (12) branches into an aluminum chloride concentrated slurry circulation line pipe (14) midway through its flow path. Therefore, a part of the aluminum chloride concentrated slurry flows through the aluminum chloride concentrated slurry circulation line pipe (14) and is returned to the thin film evaporator (7) and discharged into the apparatus.

[0053] If the temperature of the concentrated aluminum chloride slurry flowing through the concentrated aluminum chloride slurry circulation pipe (14) drops too much, aluminum chloride may precipitate and condense excessively, possibly causing blockage of the pipe and the slurry discharge port (15) on the inner wall of the thin-film evaporator, so it is preferable to provide an area surrounded by a heater (16) in the pipe. By such heating, the concentrated aluminum chloride slurry is preferably returned to the thin-film evaporator (7) at a temperature of 20 to 60°C, more preferably 25 to 50°C.

[0054] The concentrated aluminum chloride slurry thus returned to the thin-film evaporator (7) through the concentrated aluminum chloride slurry circulation pipe (14) is discharged into the evaporator through a slurry discharge port (15) provided on the inner wall of the thin-film evaporator. The discharge pressure is preferably 0.01 to 1.0 MPa, more preferably 0.02 to 0.5 MPa. The flow rate of the concentrated aluminum chloride slurry discharged into the evaporator through each slurry discharge port (15) is 6 to 600 kg / m with respect to the inner area of ​​the thin-film evaporator at the location where the slurry discharge port (15) is provided. 2 H is preferable, and 60 to 480 kg / m 2 The flow rate of the concentrated aluminum chloride slurry returned is preferably 1 to 100 kg / H, more preferably 10 to 80 kg / H.

[0055] In the flow chart of Fig. 1, the thin-film evaporation device (7) is of an agitation type, and as shown in Fig. 2, which is a cross-sectional view of the device taken along line AA of Fig. 1, a heating section (17) at the upper part of the interior for thin-film evaporation is provided with rotating blades (18) extending from a central rotating shaft (19). A bearing (20) for supporting the lower end of the central rotating shaft (19) is installed below the heating section, and a plurality of (usually 3 to 6) reinforcing ribs (21) extend radially from the central rotating shaft bearing (20).

[0056] The slurry discharge port (15) is preferably provided on the inner wall of the agitated thin-film evaporator (7) in a region below the heating section in the inner cavity so that the concentrated aluminum chloride slurry is discharged therefrom. It is more preferable that the position and discharge angle of the discharge port be adjusted so that the discharged concentrated aluminum chloride slurry strikes the reinforcing ribs (21). As mentioned above, aluminum chloride precipitates from the evaporated thin-film concentrate tend to deposit particularly heavily on the surfaces of the reinforcing ribs (21). In this configuration, even if such deposition were to occur, it would be washed away by the discharged flow of the concentrated aluminum chloride slurry, and the formation of scale is highly suppressed.

[0057] In the flow chart of Fig. 1, the aluminum chloride concentrated slurry circulation line pipe (14) is shown branching off from the aluminum chloride concentrated slurry waste pipe (12), but the invention is not limited to this configuration and may have another circulation configuration. For example, the aluminum chloride concentrated slurry circulation line pipe (14) may be independent of the aluminum chloride concentrated slurry waste pipe (12) and directly connected to the aluminum chloride concentrated slurry storage tank (11). Furthermore, without providing such an aluminum chloride concentrated slurry storage tank (11), the aluminum chloride concentrated slurry discharge pipe (10) connected to the bottom of the thin-film evaporator (7) may directly serve as the aluminum chloride concentrated slurry circulation line pipe (14) (optionally branching off midway to form the aluminum chloride concentrated slurry waste pipe (12)).

[0058] In the above recovery method, the chlorosilanes extracted from the purified chlorosilanes recovery pipe (8) can be effectively used as raw materials for the production of semiconductor-grade high-purity silicon or photovoltaic-grade silicon, etc. On the other hand, the remaining concentrated aluminum chloride slurry sent to the abatement pit (13) can be appropriately disposed of. [Example]

[0059] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The measurements and evaluations carried out in the examples and comparative examples were carried out by the following methods.

[0060] 1) Measurement of aluminum chloride concentration in chlorosilane liquid The measurement sample, a chlorosilane liquid or slurry, is accurately weighed and placed in a container, and the contained chlorosilanes are thoroughly evaporated and removed while a sufficiently dry inert gas is passed through the headspace of the sample at 50°C or below. Dilute hydrochloric acid is added to the evaporation residue after evaporation to dissolve the aluminum chloride, and the residue is filtered. The aluminum chloride content of the filtrate is quantified using an ICP (inductively coupled plasma) emission spectrometer or the like, and the mass is calculated by dividing the mass by the measurement sample.

[0061] 2) Measurement of aluminum chloride precipitate solids concentration in distillation residue cooling liquid A portion of the distillation residue cooling liquid was extracted from the distillation residue cooling liquid flow pipe (6), and its mass was measured and used as a measurement sample. This measurement sample was filtered by suction filtration using a membrane filter with a mesh size of 5 μm, and the aluminum chloride contained in the evaporation residue obtained by evaporating and removing the chlorosilanes was quantified for the resulting cake in the same manner as in "1) Measurement of aluminum chloride concentration in chlorosilane liquid" above, and the mass was divided by the mass of the measurement sample to obtain the aluminum chloride content.

[0062] 3) Evaluation method for scale adhesion in thin film evaporators After the operation of the thin film evaporator was completed, the device was stopped and opened. The amount of scale adhering to the inside of the device was visually evaluated. The evaluation was made on the following four levels. "Severe": Scale deposition is severe inside the equipment, and a large amount of scale has adhered to the reinforcing ribs in the central rotating shaft bearing of the rotating blades, causing blockages inside the equipment. For this reason, operation cannot continue unless this scale is removed. "Large": There is a large amount of scale deposition inside the equipment, but the amount of adhesion on the reinforcing rib at the central rotating shaft bearing part of the rotating blade has not reached the point where it clogs the inside of the equipment. In order to continue operation, it is desirable to remove the scale. "Slight": A small amount of scale is observed on the reinforcing ribs in the central rotary shaft bearing portion of the rotary blades within the device, but this does not interfere with continued operation. "None": A state in which no scale or only a very small amount of scale is observed inside the device, including the reinforcing ribs in the central rotary shaft bearing portion of the rotary blade.

[0063] Example 1 A chlorosilane liquid containing aluminum chloride was obtained by reacting metallurgical-grade silicon powder with a chlorine-containing gas, and contained 80 mass % of STC, 20 mass % of TCS, and 0.02 mass % of dissolved aluminum chloride. This chlorosilane liquid was treated according to the flow shown in FIG. 1.

[0064] The chlorosilane liquid was fed to the distillation column (1) at a feed rate of 60,000 kg / H through the chlorosilane liquid feed pipe (2), and distillation was carried out in the distillation column (1) at a column bottom temperature of 80°C. As a result, a distillation residue at 70°C was discharged at a rate of 1,200 kg / H to the distillation residue discharge pipe (4). This distillation residue had a composition containing 90% by mass of STC, 10% by mass of TCS, and 0.9% by mass of aluminum chloride (total of dissolved and supersaturated precipitated amounts).

[0065] The distillation residue was sent to a cooling tank (5) and cooled to a cooling temperature of 25°C while stored therein. The cooling tank (5) had a structure in which the outer wall of the tank was surrounded by a cooling jacket and an agitator blade was installed inside the tank. The agitator blade was rotated under conditions of a peripheral speed of the blade tip of 4 m / s and a rotation speed of 200 rpm to cool the distillation residue. This cooling caused the aluminum chloride to precipitate and disperse in the distillation residue to become supersaturated, and a highly dispersed aluminum chloride precipitate at a temperature of 25°C and a solids concentration of 0.9 mass% of the aluminum chloride precipitate flowed through the distillation residue cooling liquid flow pipe (6). This distillation residue cooling liquid was supplied to a thin-film evaporator (7) while being maintained at approximately the above temperature of 70°C.

[0066] The thin-film evaporator (7) has a heat transfer area of ​​0.15 m 2 A contact moving blade type vertical thin film evaporator with an inner diameter of 0.15 m was used. Thin film evaporation of the distillation residue cooled liquid using this device was carried out under the following conditions: temperature of the concentrate at the bottom outlet was 50°C, the peripheral speed of the rotating blade tip was 4 m / s, and the blade rotation frequency at one point on the evaporation surface was once every 3.3 seconds.

[0067] After this thin-film evaporation reached steady state operation, concentrated aluminum chloride slurry at 50°C was discharged from the bottom of the thin-film evaporator (7) to the concentrated aluminum chloride slurry discharge pipe (10) at a rate of 80 kg / H. The concentrated aluminum chloride slurry had an aluminum chloride precipitate solids concentration of 6.0 mass%.

[0068] This concentrated aluminum chloride slurry was sent to a concentrated aluminum chloride slurry storage tank (11) and stored there for 4 hours, and then discharged at 35° C. into a concentrated aluminum chloride slurry disposal pipe (12). Then, part of the slurry in this flow path was diverted into a concentrated aluminum chloride slurry circulation pipe (14), where the temperature was raised to 40° C. by a heater (16), and then returned to the thin-film evaporator (7).

[0069] The agitation type thin-film evaporator (7) was configured to have a total of three auxiliary ribs (21). Correspondingly, the same number of slurry discharge ports (15) were provided on the inner wall of the apparatus at positions slightly higher than the height at which the auxiliary ribs (21) were provided, and the parabolic flow of the concentrated aluminum chloride slurry discharged from each port struck the opposing auxiliary rib and flowed downward.

[0070] The flow rate of the concentrated aluminum chloride slurry discharged from each of these slurry discharge ports was 30 kg / H (the flow rate of the concentrated aluminum chloride slurry discharged into the apparatus from each slurry discharge port (15) was 180 kg / m with respect to the internal space area of ​​the thin-film evaporator at the location where the slurry discharge port (15) was provided). 2 The injection pressure of the concentrated aluminum chloride slurry discharged from the slurry discharge port (15) was 0.1 MPa.

[0071] After operating the thin-film evaporation for 24 hours, the aluminum chloride concentration of the recovered liquid obtained by condensing the gas from the purified chlorosilanes recovery pipe (8) was measured and found to be 6.0% by mass. The mass of the recovered liquid was 54 kg / H, and the recovery rate of chlorosilanes in the thin-film evaporation was calculated from this value and found to be 85%.

[0072] After continuing thin film evaporation in this state for another 14 days, the scale adhesion inside the thin film evaporator was evaluated and found to be "not present."

[0073] Comparative Example 1 In Example 1, the recovery of a chlorosilane liquid was carried out in the same manner as in Example 1, except that the clean chlorosilane liquid (STC 99% by mass, TCS 1% by mass) flowing out from the top of the distillation column (1) and recovered was used as the cleaning liquid to be used for cleaning the inside of the thin-film evaporator (7) and to be discharged from the slurry discharge port (15) instead of the concentrated aluminum chloride slurry diverted from the concentrated aluminum chloride slurry waste pipe (12).

[0074] As a result, the state of the liquid recovered from the purified chlorosilanes recovery pipe (8) after operating the thin film evaporation for 24 hours was almost the same as in Example 1. However, when this thin film evaporation was continued for another 14 days, the scale adhesion inside the thin film evaporation apparatus was evaluated and found to be in a "severe" state.

[0075] Example 2 Chlorosilane liquid recovery was carried out in the same manner as in Example 1, except that the heating of the concentrated aluminum chloride slurry flowing through the concentrated aluminum chloride slurry circulation line pipe (14) by the heater (16) was stopped and the temperature of the concentrated aluminum chloride slurry returned to the thin-film evaporator (7) was set to 15° C. As a result, the state of the liquid recovered from the purified chlorosilanes recovery pipe (8) after operating the thin-film evaporation for 24 hours was almost the same as in Example 1, and when this thin-film evaporation was continued for another 14 days, the scale adhesion inside the thin-film evaporator was evaluated and found to be "low." [Explanation of symbols]

[0076] 1: Distillation tower 2: Chlorosilane liquid supply pipe 3: Purified chlorosilane gas distillation tube 4: Distillation residual liquid discharge pipe 5: Cooling tank 6: Distillation residue cooling liquid flow pipe 7: Thin film evaporation device 8: Purified chlorosilanes recovery pipe 9: Droplet collector 10: Aluminum chloride concentrated slurry discharge pipe 11: Aluminum chloride concentrated slurry storage tank 12: Aluminum chloride concentrated slurry waste pipe 13: Abatement pit 14: Aluminum chloride concentrated slurry circulation line 15: Slurry discharge port 16: Heater 17: Inner space upper heating section 18: Rotating blades 19: Central rotation axis 20: Central rotating shaft bearing 21: Reinforcement rib 22: Concentrate reservoir

Claims

1. A method for recovering chlorosilanes, in which a liquid chlorosilane containing aluminum chloride is supplied to a thin-film evaporator to evaporate and recover chlorosilanes, characterized in that during operation of the thin-film evaporator, a concentrated aluminum chloride slurry discharged from the thin-film evaporator is stored for one hour or more, and then a portion of the concentrated aluminum chloride slurry after storage is returned continuously or intermittently into the thin-film evaporator to be released into the thin-film evaporator.

2. 2. The method for recovering chlorosilanes according to claim 1, wherein the operation of returning a portion of the concentrated aluminum chloride slurry discharged from the thin-film evaporator and discharging the concentrated aluminum chloride slurry into the thin-film evaporator is carried out in a manner such that the concentrated aluminum chloride slurry is discharged into a region below a heating section where thin-film evaporation is carried out.

3. 3. The method for recovering chlorosilanes according to claim 1, wherein the thin-film distillation apparatus is an agitation-type thin-film distillation apparatus equipped with a rotor blade.

4. 4. The method for recovering chlorosilanes according to claim 3, wherein the agitated thin-film evaporator comprises a bearing for supporting the lower end of the central rotating shaft of the rotating blades, disposed below the heating section where thin-film evaporation takes place, and the bearing for supporting the lower end of the central rotating shaft has a structure in which a plurality of reinforcing ribs extend radially.

5. 5. The method for recovering chlorosilanes according to claim 4, wherein the operation of returning a portion of the concentrated aluminum chloride slurry discharged from the agitation-type thin-film evaporator into the thin-film evaporator and discharging the concentrated aluminum chloride slurry into the thin-film evaporator is carried out in such a manner that the discharging flow of the concentrated aluminum chloride slurry hits a reinforcing rib extending from a central rotary shaft bearing of the rotary blade.

6. The method for recovering chlorosilanes according to any one of claims 1 to 5, further comprising a storage tank for the concentrated aluminum chloride slurry discharged from the thin-film distillation apparatus, and a circulation path for the concentrated aluminum chloride slurry, through which the concentrated aluminum chloride slurry is extracted from the storage tank and returned to the thin-film distillation apparatus.

7. A method for producing chlorosilanes by distilling a chlorosilane liquid containing aluminum chloride, the chlorosilane liquid being produced by reacting a gas containing chlorine, hydrogen chloride, or silicon tetrachloride and hydrogen with metallurgical-grade silicon, the method comprising: feeding the chlorosilane liquid containing aluminum chloride obtained as a distillation residue from the distillation to a thin-film evaporator; and applying the method for recovering chlorosilanes according to claim 2 or 4 during the operation of the thin-film evaporator.

Images