Method for recovering chlorosilanes

Abstract

To develop a method of further suppressing deposition of aluminum chloride scale in a thin film evaporation device, upon recovering chlorosilanes by feeding an aluminum chloride-containing distillation residual liquid of liquid chlorosilanes.SOLUTION: The method of recovering chlorosilanes includes cooling distillation residual liquid obtained by distillating liquid chlorosilanes containing aluminum chloride, with a liquid temperature of 50°C or higher to a liquid temperature of 40°C or lower, and subsequently feeding the cooled distillation residual liquid to a thin film evaporator such as an agitation-type thin film evaporator to evaporate and recover chlorosilanes.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 reacting a gas containing chlorine or hydrogen chloride or silicon tetrachloride with hydrogen, with metallurgical grade silicon or the like.

Background Art

[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 usually contained in metallurgical grade silicon in an amount of about 0.01 to 10% by weight. Thus, since aluminum chloride becomes an undesirable impurity in the manufacturing process of semiconductor grade high-purity silicon and solar power generation grade silicon, it is necessary to separate and remove it from chlorosilanes.

[0003] Since 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), conventionally, chlorosilanes are generally separated as a distillation residue by distilling and recovering chlorosilanes from a chlorosilane liquid containing aluminum using a distillation column. This distillation residue is appropriately withdrawn from the bottom of the distillation column and discarded.

[0004] However, the distillation residue to be discarded still contains useful chlorosilanes such as TCS and STC, usually about 95% by weight. Therefore, it is preferable to evaporate and recover the useful chlorosilanes again, and it has been proposed to use thin-film evaporation as an effective method (see Patent Document 1). Here, in the method using this thin-film evaporation, the supply of the chlorosilane liquid to the thin-film evaporation apparatus is carried out at a high liquid temperature of 55°C because the distillation residue discharged from the distillation column of the chlorosilanes is at a high temperature and is transferred as it is.

Prior Art Documents

Patent Document

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, aluminum chloride contained in chlorosilanes liquid has a property of being easily precipitated due to concentration fluctuations. In particular, when the chlorosilanes are recovered by distillation, in the bottom liquid of the column, the aluminum chloride becomes highly concentrated due to concentration, and thus the precipitation of the aluminum chloride in the distillation residue withdrawn therefrom becomes particularly intense. For this reason, in the recovery by evaporating chlorosilanes from the distillation residue, the deposition of scale on the inner wall of the apparatus becomes a problem. However, if this is carried out by thin-film evaporation proposed in the above Patent Document 1, the deposition of scale in the apparatus can be considerably reduced.

[0007] This utilizes the phenomenon that even when the aluminum chloride concentration in the chlorosilanes liquid exceeds 2% by weight of the precipitation limit, there exists a state where it is difficult for aluminum chloride to precipitate as a solid for about 10 minutes. That is, if the evaporation of chlorosilanes is thin-film evaporation, the residence time of the chlorosilanes liquid in the apparatus can be shortened, and thus the effect of suppressing the deposition of the aluminum chloride as scale is exerted (Patent Document 1

[0015]

[0037] ).

[0008] However, even in the method of applying the thin-film evaporation with such a high effect of suppressing the deposition of aluminum chloride scale, the effect of suppressing the deposition was still not sufficient from the perspective of implementing it at an industrial level. That is, according to the above method, although the deposition of aluminum chloride scale is lowly suppressed on the inner wall heating surface where the evaporation action of chlorosilanes occurs violently in the chlorosilanes liquid, or on the rotating blade surface to which centrifugal force is applied when the thin-film evaporation device is of the stirring type, etc., on the adjacent lower surface of the heating region where such thin-film evaporation is performed, still, the deposition of the above scale could not be sufficiently suppressed. In particular, when the thin-film evaporation device is of the stirring type, in the bearing portion that pivotally supports the lower end of the central rotating shaft to which the rotating blades are connected, a plurality of reinforcing ribs are often provided in the radial direction. Since centrifugal force is not applied to this rib surface, the scale is more likely to deposit and grow. When the operation of thin-film evaporation is prolonged, the inner space of such a device is blocked.

[0009] In view of the above background, the present invention aims to develop a method for more highly suppressing the deposition of aluminum chloride scale into the above device when the distillation residue is supplied to a thin-film evaporation device to recover chlorosilanes.

Means for Solving the Problems

[0010] In view of the above problems, the present inventors have continued intensive studies. As a result, it has been found that when the distillation residue is supplied to a thin-film evaporation device, by performing an operation of cooling it to a liquid temperature of 40°C or lower, the above problems can be solved, and the present invention has been completed.

[0011] That is, the present invention is a method for recovering chlorosilanes, characterized in that a distillation residue having a liquid temperature of 50°C or higher obtained by distilling a chlorosilanes liquid containing aluminum chloride is cooled to a liquid temperature of 40°C or lower, and then the obtained distillation residue cooling liquid is supplied to a thin-film evaporation device to evaporate and recover chlorosilanes.

[0012] The present invention also provides a method for producing chlorosilanes, which comprises distilling a chlorosilanes liquid containing aluminum chloride produced by reacting a gas containing chlorine, hydrogen chloride, or silicon tetrachloride with hydrogen with metallurgical grade silicon, and subjecting the distillation residue having a liquid temperature of 50°C or higher obtained by the above distillation to the method for recovering the chlorosilanes.

Advantages of the Invention

[0013] According to the method of the present invention, for the distillation residue obtained by distilling the chlorosilanes liquid containing aluminum chloride, the scale deposition in the apparatus can be highly suppressed, and the thin film evaporation can be continued. As a result, the chlorosilanes can be stably recovered for a long time.

Brief Description of the Drawings

[0014]

Figure 1

Embodiments for Carrying Out the Invention

[0015] The chlorosilanes liquid to be used in the recovery method of the present invention is not particularly limited as long as it contains aluminum chloride. Generally, a chlorosilanes liquid obtained by condensing a chlorosilanes mixed gas produced by reacting metallurgical grade silicon having a purity of about 90 to 99% with chlorine, hydrogen chloride, or a mixture of hydrogen and silicon tetrachloride is typical.

[0016] In the above chlorosilanes synthesis reaction, the types and production ratios of the chlorosilanes produced vary depending on the reaction raw materials and reaction conditions. However, chloromonosilanes represented by TCS (SiHCl3), STC (SiCl4), and dichlorosilane (SiH2Cl2) are typical. In addition to these, dimer chlorosilanes such as pentachlorodisilane (Si2HCl5) and hexachlorodisilane (Si2Cl6), and even higher polymer chlorosilanes may also be contained.

[0017] As described above, metallurgical grade silicon usually contains about 0.01 to 10% by weight of aluminum. When this is chlorinated or hydro-chlorinated, aluminum chloride is contained in the chlorosilane liquid. The content of aluminum chloride varies depending on the aluminum content in the used metallurgical grade silicon and the reaction conditions for the production of chlorosilanes, etc.

[0018] The method for producing a chlorosilane liquid containing aluminum chloride according to the present invention will be more specifically described as follows. For example, a method of chlorinating a metallurgical grade silicon powder and a chlorine-containing gas at 100°C or higher, generally 100 to 500°C; a method of chlorinating a metallurgical grade silicon powder and a hydrogen chloride-containing gas at 250°C or higher, generally 250 to 450°C; or a method of hydro-chlorinating a metallurgical grade silicon powder and a mixed gas of STC and hydrogen at 400°C or higher, generally at a temperature of 400 to 600°C, etc. are generally known.

[0019] In these reactions, the gases reacted with the metallurgical grade silicon can be reacted by mixing them with each other, or can also be reacted by mixing with a reaction-inert gas such as hydrogen or nitrogen.

[0020] The chlorosilane liquid obtained by condensing the chlorosilane gas thus obtained usually contains 0.01 to 2% by weight of aluminum chloride. In addition, the metallurgical grade silicon used as a raw material when producing the chlorosilane liquid usually contains metal impurity components such as iron and titanium in addition to aluminum. In the production of chlorosilanes, these are also chlorinated, so the chlorosilane liquid may contain these chlorides (iron chloride, titanium chloride, etc.). Further, the chlorosilane liquid may contain fine silicon powder, etc.

[0021] The details of the recovery method of the present invention using such a chlorosilane liquid containing aluminum chloride as the liquid to be treated will be described using the flow of FIG. 1 which is a representative embodiment.

[0022] In FIG. 1, as the distillation column (1), a known distillation column can be used. That is, any commonly used distillation column trays can be used without limitation. For example, packed columns filled with regular packings, irregular packings, etc., bubble cap columns, perforated plate columns, and the like can be mentioned.

[0023] Further, the distillation column (1) may have a reboiler. The reboiler may be of a direct heating type with a jacket around the bottom of the distillation column, or may be of a type with a heat exchanger installed outside the bottom of the distillation column. Also, a method of installing a heat exchanger inside the bottom of the distillation column can also be adopted.

[0024] As the heat exchanger, generally, the shell and tube method is preferably adopted to obtain a heat transfer area, but a coil type, an electric heater, etc. can also be adopted. In the heat exchanger to which the energy for distillation is applied, if chlorosilanes liquid stays and aluminum chloride is highly concentrated, it will cause scaling, so it is preferable to have a structure in which the liquid hardly stays. As a method in which the liquid hardly stays, a method using convection by heating may be used, or a method of forcibly flowing the liquid using a pump or the like can also be preferably adopted.

[0025] A supply pipe (2) for the chlorosilanes liquid containing the aluminum chloride, which is the liquid to be treated in the present invention, is connected to the distillation column (1). The supply location to the distillation column (1) can be any part, but it is more preferable to supply it directly to the bottom of the distillation column in order to prevent fouling of the trays.

[0026] Here, since the boiling point difference between the chlorosilanes to be purified and impurities such as aluminum chloride to be separated and removed is quite large, the distillation can be well carried out without particularly performing high-precision rectification, and it is sufficient as long as the distillation operation can be maintained. The reflux ratio may be about 0.1 to 1. The purified chlorosilanes are discharged from a purified chlorosilanes gas distillate pipe (3) connected to the top of the distillation column (1).

[0027] In the present invention, the distillation is carried out at a temperature of the bottom liquid of 50° C. or higher, more preferably 70 to 150° C., and most preferably 80 to 120° C. By setting the temperature of the bottom liquid to 50° C. or higher in this manner, the distillation of chlorosilanes can be made highly efficient. Since the temperature of the bottom liquid is 50° C. or higher, the distillation bottom liquid discharged from the bottom of the distillation column (1) through the distillation bottom liquid discharge pipe (4) also has the same high liquid temperature at the beginning of discharge.

[0028] In order to prevent the precipitation of aluminum chloride in the bottom of the distillation column (1) and to continue stable operation for a long period of time, it is preferable to adjust the concentration of aluminum chloride dissolved in the bottom liquid of the distillation column (1) to less than the saturated solubility at the temperature of the bottom liquid. For example, when the temperature of the bottom liquid is 50° C. or higher, it is preferable to maintain the aluminum chloride concentration in the bottom liquid in the range of 0.1 to 5.0 mass%, preferably 0.5 to 1.2 mass%.

[0029] The greatest feature of the recovery method of the present invention is that the distillation residue liquid of 50°C or higher that is extracted into the distillation residue discharge pipe (4) is not fed to the thin film evaporator in such a high temperature state, but is cooled to a liquid temperature of 40°C or lower to increase the dispersion concentration of the aluminum chloride precipitate before being fed. By feeding the distillation residue liquid to thin film evaporation in this way as a distillation residue cooling liquid, it is possible to highly suppress the deposition of scale in the thin film evaporator. The reason for this is not entirely clear, but the inventors speculate that the following action is involved.

[0030] That is, the Al-Cl bond is essentially Al + -Cl -It is polarized as described above, and thus, in solid aluminum chloride, it forms a pseudo-ion conjugate of Al-Cl-Al-Cl. Moreover, if the pseudo-ion conjugate is crystal-grown under stable conditions, it exhibits a certain degree of re-dissolubility in the chlorosilane liquid as the liquid temperature rises. However, the re-dissolubility in this chlorosilane liquid is significantly impaired when the pseudo-ion conjugate is crystal-grown under unstable conditions, that is, when the evaporation of the chlorosilane liquid during the thin-film evaporation is explosive and impurities such as Fe and Ti are easily incorporated into the precipitate. Thus, this is considered to cause a large amount of aluminum chloride precipitate to be generated all at once with the explosive evaporation of the chlorosilane liquid during the thin-film evaporation of the distillation residue, leading to the problem of scale deposition.

[0031] Therefore, in the present invention, prior to the thin-film evaporation, the distillation residue is cooled to the above temperature so that a certain amount of dissolved aluminum chloride is firmly crystal-grown under the above stable conditions and contained as a precipitate having re-dissolubility. That is, when the distillation residue cooling liquid thus obtained is subjected to thin-film evaporation, as the liquid temperature of the treatment liquid rises on the heating surface of the apparatus, the evaporation of the chlorosilane proceeds at a high speed, while the re-dissolution of the aluminum chloride precipitate into the remaining fraction of the chlorosilane liquid also occurs. Due to the subtraction caused by this re-dissolution, the generation of the aluminum chloride precipitate is significantly suppressed, and as a result, it is considered that the problem of scale deposition in the apparatus is remarkably reduced.

[0032] According to the present invention, if the dispersion concentration of the aluminum chloride precipitate in the distillation residue is increased by the above-mentioned preliminary cooling and then the distillation residue is subjected to thin-film evaporation, the aluminum chloride precipitate acts like an abrasive on the inner wall of the device with which it is in contact from the beginning of the supply, and has a scraping effect on the deposits on the wall. In particular, this scraping effect is more effective when the thin-film evaporation device is of the stirring type, since the stirring loads the treatment liquid with pressure. Thus, in the present invention, it is presumed that the scraping effect of the aluminum chloride precipitate is also added, and the deposition of scale in the device can be significantly suppressed.

[0033] In the present invention, from the viewpoint of suppressing the amount of deposition of aluminum chloride scale to a greater extent, it is more preferable that the distillation residual liquid is cooled to a liquid temperature of 30° C. or less. On the other hand, if the liquid is cooled to too low a temperature, it becomes inefficient because it is necessary to raise the temperature when it is fed to the thin film evaporator, so the liquid temperature is preferably 5° C. or more.

[0034] By cooling the distillation residue, the aluminum chloride dissolved therein is precipitated in its crude form. Therefore, the solid content concentration of aluminum chloride precipitate in the obtained distillation residue cooling liquid is usually 0.1 to 5.0 mass%, preferably 0.5 to 1.2 mass%, the same as the aluminum chloride concentration in the distillation column bottom liquid. Here, the aluminum chloride precipitate solid content concentration in the distillation residue cooling liquid may be measured by a method in which the aluminum chloride is quantified using an ICP emission spectrometer for the evaporation residue after evaporating chlorosilane from a cake obtained by suction filtration using a membrane filter. When the solid content concentration of the aluminum chloride precipitate in the distillation residue cooling liquid becomes high, it takes on a slurry-like form, but its fluidity is well maintained.

[0035] The method of cooling the distillation residue in the high-temperature state to a liquid temperature of 40°C or lower is not particularly limited, and it may be carried out, for example, by passing the distillation residue flowing through the distillation residue discharge pipe (4) through a known liquid cooling device that circulates a refrigerant inside or outside the container. However, from the perspective of cooling certainty and the homogeneity of the resulting cooled distillation residue liquid, it is preferable to carry out the cooling by storing the distillation residue in the cooling tank (5).

[0036] The cooling of the distillation residue in the cooling tank (5) may be carried out by air cooling. However, considering the cooling efficiency and the ease of controlling the cooling temperature, it is preferable to carry out the cooling by using an external cooler, specifically, by forcibly cooling, for example, by surrounding the outer wall of the cooling tank with a cooling jacket or installing a refrigerant pipe inside the cooling tank.

[0037] Furthermore, the 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 cooled distillation residue liquid. The stirring method may be carried out by a magnetic stirrer or the like, but stirring with a stirring blade is preferable. Specific examples of the stirring blade include paddle type, ribbon type, ikari type, propeller type, turbine type, retreat wing type, gate type, etc. The number of stirring blades attached to the stirring shaft cannot be generally determined depending on its shape, the size of the reactor, etc., but generally, 1 to 4 blades on the rotating shaft are sufficient. The peripheral speed of the tip of the stirring blade is preferably 0.1 to 10 m / s, and the rotation speed is preferably 100 to 300 rpm.

[0038] In the present invention, the cooled distillation residue liquid formed by the cooling is subjected to thin-film evaporation. As shown in the flow of FIG. 1, the cooled distillation residue liquid discharged from the cooling tank (5) is supplied to the thin-film evaporation device (7) through the cooled distillation residue liquid circulation pipe (6). Here, the thin-film evaporation device can be applied without limitation by a known thin-film evaporation method capable of forming a thin film of the liquid to be treated on the heating surface and evaporating the liquid components. Specifically, a stirring type thin-film evaporation device in which internal blades rotate to forcibly stir the treatment liquid to form a thin film with respect to a fixed heating surface (evaporation surface), and a centrifugal thin-film evaporation device in which the heating surface rotates can be mentioned. In particular, a stirring type thin-film evaporation device is preferable.

[0039] The above-mentioned stirred thin-film evaporator can be classified into vertical and horizontal types according to the direction of the rotation axis, and into fixed vane type and movable vane type according to the attachment form of the vanes to the rotation axis. Furthermore, it can be classified into contact vane type and non-contact vane type according to whether the vanes contact the evaporation surface or not. These methods can be used in appropriate combinations. Among these, more preferable forms include the vertical contact movable vane type and the vertical non-contact vane type. These thin-film evaporators have a relatively excellent effect of suppressing the deposition of scale. In particular, the vertical contact movable vane type is most preferable because the vanes contact the evaporation surface, resulting in the highest effect of suppressing the deposition of scale.

[0040] In the thin-film evaporator (7), when droplets are entrained in the evaporation gas component depending on its operating conditions, the removal rate of aluminum chloride may be reduced. In such a case, as a means of preventing droplets from being entrained in the recovered chlorosilanes, it is an effective means to install a droplet collector (9) in the middle of the recovery pipe (8) of the purified chlorosilanes discharged from the thin-film evaporator. As the above-mentioned droplet collector (9), any form may be used as long as it can collect and remove droplets. For example, a baffle plate (impact plate) type, a sintered type, and a centrifugal separation type can be mentioned.

[0041] In addition, it is effective to interpose such a droplet collector (9) also in the middle of the distillation residue cooling liquid circulation pipe (6) connecting the cooling tank (5) and the thin-film evaporator (7). There is also a possibility that droplets of aluminum chloride are entrained in the distillation residue cooling liquid discharged from the cooling tank (5). Removing this in advance before supplying it to the thin-film evaporator (7) is effective in suppressing the deposition of aluminum chloride in the apparatus.

[0042] When the thin-film evaporator (7) has a form with contact movable vanes, the peripheral speed at the tip of the vanes 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. Also, the frequency of the vanes' movement around a certain point on the evaporation surface is preferably 1 time or more per second, more preferably 1 time or more per 0.5 seconds, and even more preferably 1 time or more per 0.3 seconds in order to exert a sufficient scale suppression effect.

[0043] On the other hand, in the case of the vertical non-contact blade type, since the blades do not contact the evaporation surface, it is necessary to rotate them at a higher speed than the type in which the blades contact. The peripheral speed of the blade tips is 1 m / s or more, preferably 3 m / s or more, and more preferably 5 m / s or more. Also, the frequency of blade circulation at a certain point on the evaporation surface is preferably 1 time or more per 0.5 seconds, preferably 1 time or more per 0.3 seconds, and more preferably 1 time or more per 0.1 seconds. Appropriate conditions may be appropriately adopted according to the heat transfer area, blade type, etc.

[0044] By using the thin-film evaporation device in this way, the residence time of the distillation residue cooling liquid in the device can be shortened, and the effect of suppressing the deposition of aluminum chloride scale in the device can be enhanced. At the same time, this effect can be further improved by selecting the structure or operating conditions of the thin-film evaporation device described above.

[0045] In the present invention, when the distillation residue cooling liquid is supplied to the thin-film evaporation device (7), the treatment liquid is heated while forming a liquid film on the evaporation surface, and the evaporation of chlorosilanes and the concentration of aluminum chloride proceed. In this way, chlorosilanes are taken out in a gaseous state from the upper outlet of the evaporation device (7) to the purified chlorosilane recovery pipe (8), and the concentrate of the distillation residue cooling liquid is taken out from the lower outlet to the concentrate discharge pipe (10).

[0046] In this case, the residence time in the thin-film evaporation device (7) from when the distillation residue cooling liquid is supplied until it is taken out as a concentrate is usually within about 1 minute, and at most within several minutes, in a vertical thin-film evaporation device. In the case of a horizontal type, the residence time can be adjusted to some extent, but it is preferably adjusted within 10 minutes, preferably within 5 minutes.

[0047] Examples of the operating conditions in the thin-film evaporation include the temperature of the heating medium, the operating pressure, etc. These can be appropriately set according to the composition, supply amount, heat transfer area, concentration ratio, etc. of the distillation residue cooling liquid to be processed, in the same manner as the evaporation operations of conventional methods. At this time, the precipitation of aluminum chloride from the distillation residue cooling liquid becomes intense at temperatures higher than about 180°C, and the aluminum chloride precipitate generated here has low re-dissolvability in the chlorosilane liquid as described above. From the perspective of highly preventing scale deposition on the thin-film evaporation device, it is preferable to set the evaporation conditions so that the temperature of the concentrate at the lower outlet of the thin-film evaporation device (7) is 180°C or lower, preferably 150°C or lower.

[0048] In the present invention, by utilizing the special behavior of aluminum chloride with respect to the above distillation residue cooling liquid, the concentrate obtained after evaporation is allowed to contain aluminum chloride up to an extremely high concentration. However, if the concentrate dries up, it may partially scale, or lose the function of washing away the solid content originally contained, resulting in intense scaling and difficulty in continuous processing.

[0049] Therefore, in the present invention, it is necessary to perform thin-film evaporation under conditions where the obtained concentrate remains in a liquid state. In particular, the preferable upper limit of the aluminum chloride concentration in the concentrate is 80% by weight, preferably 50% by weight, more preferably 40% by weight. Also, the lower limit is preferably 3% by weight, and even more preferably 10% by weight, as it can fully exhibit the effects of the present invention because concentration has been difficult with conventional techniques.

[0050] In the above recovery method, the chlorosilanes taken out from the purified chlorosilanes recovery pipe (8) may be effectively used as raw materials for the production of semiconductor-grade high-purity silicon, solar power generation-grade silicon, etc. On the other hand, the concentrate discharged into the concentrate discharge pipe (10) may be appropriately discarded after being supplied to a decontamination pit or the like.

Examples

[0051] Hereinafter, examples will be given to explain the present invention in detail, but the present invention is not limited to these examples. In addition, the measurements and evaluations carried out in the examples and comparative examples were determined by the following methods.

[0052] 1) Measurement of aluminum chloride concentration in chlorosilanes solution Accurately weigh the chlorosilanes solution or slurry as the measurement sample into a container, and at 50 °C or lower, while flowing an inert gas that has sufficiently dried the upper space of the object to be measured, evaporate and remove the contained chlorosilanes sufficiently. After evaporation and removal, add dilute hydrochloric acid to the evaporation residue to dissolve aluminum chloride, then filter, and quantify aluminum chloride in the filtrate with an ICP (inductively coupled plasma) optical emission spectrometer or the like, and obtain it by dividing by the mass of the measurement sample.

[0053] 2) Measurement of solid content concentration of aluminum chloride precipitate in distillation residue coolant Extract a part of the distillation residue coolant from the distillation residue coolant flow pipe (6) and weigh its mass as the measurement sample. For this measurement sample, filter it by suction filtration using a membrane filter with a pore size of 5 μm, and for the obtained cake, in the same manner as in "1) Measurement of aluminum chloride concentration in chlorosilanes solution", evaporate and remove chlorosilanes, and quantify the aluminum chloride contained in the evaporation residue, and obtain it by dividing by the mass of the measurement sample. 3) Evaluation method for scale adhesion in thin film evaporator 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 levels were set as the following four levels. "Severe": Scale deposition is severe inside the device, and a large amount of scale has adhered to the reinforcing ribs at the central rotation shaft bearing part of the rotating blade, causing the inside of the device to be blocked. Therefore, the operation cannot continue without removing this scale. "Many": Scale deposition is a lot inside the device, but the amount of adhesion to the reinforcing ribs at the central rotation shaft bearing part of the rotating blade has not reached the point of blocking the inside of the device. It is desirable to remove the scale for continuous operation. "Small amount": Although a small amount of scale is observed on the reinforcing ribs etc. in the central rotation shaft bearing portion of the rotating blade inside the device, there is no problem in continuing the operation. "None": Inside the device, no scale is observed, or only a very small amount is observed, including the reinforcing ribs in the central rotation shaft bearing portion of the rotating blade.

[0054] Example 1 As a chlorosilane liquid containing aluminum chloride, a chlorosilane liquid containing 80% by mass of STC, 20% by mass of TCS, and 0.02% by mass of dissolved aluminum chloride, which was obtained by reacting metallurgical grade silicon powder with a chlorine-containing gas, was subjected to the treatment according to the flow shown in Fig. 1.

[0055] The above chlorosilane liquid was fed into the distillation column (1) at a supply rate of 60,000 kg / H from the supply pipe (2) of the chlorosilane liquid, and distillation was carried out in the distillation column (1) at a bottom temperature of 80°C. As a result, a distillation residue was discharged from the distillation residue discharge pipe (4) at a rate of 1,200 kg / H, and the distillation residue was at 70°C. 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 content and supersaturated precipitated content).

[0056] The above distillation residue was fed into the cooling tank (5) and cooled to a cooling temperature of 25°C during storage. The cooling tank (5) had a structure in which the outer wall of the tank was surrounded by a cooling jacket and stirring blades were installed inside the tank. The stirring blades were rotated under the conditions that the peripheral speed at the blade tip was 4 m / s and the rotation speed was 200 rpm to cool the above distillation residue. By this cooling, the crude form of aluminum chloride in the distillation residue was supersaturated and precipitated and dispersed. As a result, a highly dispersed liquid of aluminum chloride precipitate with a temperature of 25°C and a solid content concentration of aluminum chloride precipitate of 0.9% by mass flowed through the distillation residue cooling liquid circulation pipe (6). This distillation residue cooling liquid was supplied to the thin film evaporation device (7) while maintaining the above temperature of 70°C.

[0057] The thin film evaporation device (7) has a heat transfer area of 0.15 m 2, a vertical thin-film evaporator of the contact movable blade type with an inner diameter of 0.15 m was used. The thin-film evaporation of the distillation residue coolant using this device was carried out under the temperature condition that the temperature of the concentrate at the lower outlet was 50 °C, the peripheral speed of the rotating blade tip was 4 m / s, and the revolving frequency of the blade at one point on the evaporation surface was once every 3.3 seconds.

[0058] When the above thin-film evaporation was operated for 24 hours, the aluminum chloride concentration in the recovered liquid obtained by condensing the gas from the purified chlorosilanes recovery pipe (8) was measured and found to be 6.0 mass%. Also, the mass of the recovered liquid was 54 kg / H, and the recovery rate of chlorosilanes in the thin-film evaporation was determined from this value and found to be 85%. On the other hand, regarding the concentrate discharged from the concentrate discharge pipe (910), its mass was measured and found to be 77 kg / H.

[0059] After continuing this state of thin-film evaporation for another 10 days, the scale adhesion in the thin-film evaporator was evaluated and found to be in a "none" state.

[0060] Comparative Example 1 In Example 1 above, except that the cooling tank (5) was not provided and the evaporation residue discharged from the distillation column (1) flowed through the evaporation residue discharge pipe (4) and was directly supplied to the thin-film evaporator (7) at a temperature of 70 °C, the recovery of the chlorosilane liquid was carried out in the same manner as in Example 1. As a result, the state of the recovered liquid from the purified chlorosilanes recovery pipe (8) when the thin-film evaporation was operated for 24 hours was almost the same as that in Example 1. However, after continuing this thin-film evaporation for another 10 days, the scale adhesion in the thin-film evaporator was evaluated and found to be in a "severe" state.

[0061] Example 2 In Example 1 described above, the cooling temperature of the cooling tank (5) was changed, and the distillation residue coolant flowing through the distillation residue coolant circulation pipe (6) was set to have a temperature of 35°C and a solid content concentration of aluminum chloride precipitate of 3.0% by mass. Except for this, the recovery of the chlorosilane solution was carried out in the same manner as in Example 1. As a result, the state of the recovered liquid from the purified chlorosilanes recovery pipe (8) when the thin-film evaporation was operated for 24 hours was almost the same as that in Example 1. When the scale adhesion in the thin-film evaporation apparatus was evaluated after continuing this thin-film evaporation for another 10 days, it was in the "low" state.

Explanation of symbols

[0062] 1: Distillation column 2: Chlorosilanes liquid supply pipe 3: Purified chlorosilanes gas distillation pipe 4: Distillation residue discharge pipe 5: Cooling tank 6: Distillation residue coolant circulation pipe 7: Thin-film evaporation apparatus 8: Purified chlorosilanes recovery pipe 9: Mist collector 10: Concentrate discharge pipe

Claims

1. The distillation residue liquid with a liquid temperature of 50°C or higher obtained by distilling a chlorosilane liquid containing aluminum chloride is cooled to a liquid temperature of 40°C or lower, and then the obtained distilled residue cooling liquid is fed to a thin-film evaporation device to evaporate and recover the chlorosilanes. A method for recovering chlorosilanes, characterized by this.

2. The method for recovering chlorosilanes according to Claim 1, wherein the thin-film evaporation device is a stirred thin-film evaporation device.

3. The cooling of the distillation residue liquid is carried out by storing the distillation residue liquid in a cooling tank and lowering the temperature to the liquid temperature of 40°C or lower with stirring. The method for recovering chlorosilanes according to Claim 1 or Claim 2.

4. The chlorosilanes recovered by the method according to any one of Claims 1 to 3 are circulated and supplied to the distillation of the chlorosilane liquid containing aluminum chloride. A method for recovering chlorosilanes.

5. The chlorosilane liquid containing aluminum chloride is produced by reacting a gas containing chlorine or hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical-grade silicon. The method for recovering chlorosilanes according to any one of Claims 1 to 4.

6. In a method for producing chlorosilanes in which a chlorosilane liquid containing aluminum chloride produced by reacting a gas containing chlorine or hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical-grade silicon is distilled, for the distillation residue liquid with a liquid temperature of 50°C or higher obtained by the above distillation, the method for recovering chlorosilanes according to any one of Claims 1 to 4 is applied. A method for producing chlorosilanes, characterized by this.

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