JP7706386B2 - Method for recovering chlorosilanes - Google Patents

Description

The present invention relates to a method for recovering chlorosilanes, more specifically, 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 and hydrogen with metallurgical grade silicon.

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 at about 0.01 to 10% by weight. Therefore, aluminum chloride becomes an undesirable impurity in the manufacturing process of semiconductor grade high purity silicon and photovoltaic grade silicon, and therefore needs to be separated and removed from the chlorosilanes.

Since aluminum chloride has a higher boiling point than useful chlorosilanes such as trichlorosilane (TCS) and silicon tetrachloride (STC), it has been common practice to use a distillation tower to distill and recover chlorosilanes from the aluminum-containing chlorosilane liquid, separating it as distillation residue. This distillation residue is appropriately extracted from the bottom of the distillation tower and disposed of.

However, the distillation residue liquid that is disposed of still contains useful chlorosilanes such as TCS and STC, usually at around 95% by weight. Therefore, it is preferable to recover the useful chlorosilanes by re-evaporation, and the use of thin-film evaporation has been proposed as an effective method for this (see Patent Document 1). Here, in this method that uses thin-film evaporation, the supply of the chlorosilanes liquid to the thin-film evaporator is carried out at a high liquid temperature of 55°C, because the distillation residue liquid discharged from the chlorosilanes distillation column is at a high temperature and is transported as is.

JP 2001-261324 A

The aluminum chloride contained in the chlorosilanes liquid has a tendency to precipitate due to concentration fluctuations. In particular, when the chlorosilanes are recovered by distillation, the aluminum chloride becomes highly concentrated in the bottom liquid of the column due to concentration, and the precipitation of the aluminum chloride is particularly severe in the distillation residual liquid extracted from this liquid. For this reason, when recovering chlorosilanes from the distillation residual liquid by evaporation, the deposition of scale on the inner walls of the device becomes an issue, but if this is carried out by the thin film evaporation proposed in Patent Document 1, the deposition of scale inside the device can be significantly reduced.

This utilizes the phenomenon that even if the aluminum chloride concentration in the chlorosilane liquid exceeds the precipitation limit of 2% by weight, aluminum chloride is unlikely to precipitate as a solid for approximately 10 minutes. In other words, if thin-film evaporation is used to evaporate chlorosilanes, the residence time of the chlorosilane liquid in the device can be shortened, thereby suppressing the deposition of the aluminum chloride as scale (Patent Document 1 [0015] [0037]).

Even in the case of the method of applying the thin film evaporation, which has a high effect of suppressing the deposition of aluminum chloride scale, the effect of suppressing the deposition is still insufficient from the viewpoint of carrying out the method on an industrial level. That is, according to the above method, the deposition of aluminum chloride scale is suppressed to a low level on the inner wall heating surface where the evaporation action of chlorosilanes occurs strongly in the chlorosilanes liquid, and on the rotating blade surface to which centrifugal force is applied in the case of an agitating type thin film evaporation device, but the deposition of the above scale is still not sufficiently suppressed on the adjacent lower surface of the heating area where such thin film evaporation takes place. In particular, in the case of an agitating type thin film evaporation device, the bearing part that supports the lower end of the central rotating shaft to which the rotating blades are connected is often provided with multiple reinforcing ribs in the radial direction, and since no centrifugal force is applied on the rib surface, scale is more likely to deposit and grow, and when the thin film evaporation is operated for a long time, the internal space of the device is blocked.

In light of the above, the present invention aims to develop a method for more effectively suppressing the deposition of aluminum chloride scale in a thin-film evaporator when the distillation residual liquid is fed to the thin-film evaporator to recover chlorosilanes.

In view of the above problems, the inventors have conducted intensive research. As a result, they have discovered that the above problems can be solved by cooling the distillation residue to a liquid temperature of 40°C or less when feeding it to a thin-film evaporator, and have completed the present invention.

That is, the present invention is a method for recovering chlorosilanes, which comprises cooling a distillation residue liquid obtained by distilling a chlorosilane liquid containing aluminum chloride at a liquid temperature of 50°C or higher to a liquid temperature of 40°C or lower, and then feeding the resulting distillation residue cooled liquid to a thin-film evaporator to evaporate and recover the chlorosilanes.

The present invention also provides a method for producing chlorosilanes, which involves distilling 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, and which is characterized in that the distillation residual liquid, which has a liquid temperature of 50°C or higher and is obtained by the distillation, is subjected to a method for recovering chlorosilanes.

According to the method of the present invention, it is possible to continue thin-film evaporation of the distillation residue obtained by distilling a chlorosilane liquid containing aluminum chloride while highly suppressing the deposition of scale inside the equipment. As a result, it is possible to stably recover chlorosilanes for a long period of time.

FIG. 1 is a process diagram showing a typical embodiment of the method for recovering chlorosilanes of the present invention.

The chlorosilanes liquid used in 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 chlorosilanes mixed gas produced by reacting metallurgical grade silicon with a purity of about 90 to 99% with chlorine, or hydrogen chloride, or hydrogen and silicon tetrachloride, etc.

The types and proportions of chlorosilanes produced in the above chlorosilane synthesis reaction vary depending on the reaction raw materials and reaction conditions, but the representative chloromonosilanes are TCS (SiHCl ₃ ), STC (SiCl ₄ ), and dichlorosilane (SiH ₂ Cl ₂ ), etc. In addition to these, dimeric chlorosilanes such as pentachlorodisilane (Si ₂ HCl ₅ ) and hexachlorodisilane (Si ₂ Cl ₆ ), and even polymeric chlorosilanes may also be contained.

As mentioned above, metallurgical grade silicon usually contains about 0.01 to 10% by weight of aluminum, and when this 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.

The method for producing the chlorosilane liquid containing aluminum chloride used in the present invention can be described in more detail as follows. For example, a method of subjecting metallurgical grade silicon powder and a chlorine-containing gas to a chlorination reaction at 100°C or higher, generally 100 to 500°C, a method of subjecting metallurgical grade silicon powder and a hydrogen chloride-containing gas to a chlorination reaction at 250°C or higher, generally 250 to 450°C, or a method of subjecting metallurgical grade silicon powder and a mixed gas of STC and hydrogen to a hydrochlorination reaction at 400°C or higher, generally 400 to 600°C, are commonly known.

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.

The chlorosilanes liquid obtained by condensing the chlorosilanes gas thus obtained typically contains 0.01 to 2% by weight of aluminum chloride. Note that metallurgical grade silicon, which is the raw material used in producing the chlorosilanes liquid, typically contains metal impurity components such as iron and titanium in addition to aluminum, and since these are also chlorinated in the production of chlorosilanes, the chlorosilanes liquid may contain chlorides of these (iron chloride, titanium chloride, etc.). The chlorosilanes liquid may also contain silicon powder, etc.

The details of the recovery method of the present invention, in which the treated liquid is a chlorosilane liquid containing aluminum chloride, will be explained using the flow chart in Figure 1, which shows a representative embodiment.

In FIG. 1, the distillation column (1) can be a known distillation column. In other words, any commonly used distillation column tray can be used without any restrictions. Examples include packed types filled with regular packing or irregular packing, bubble cap types, and perforated plate types.

The distillation column (1) may also have a reboiler. The reboiler may be a type in which the circumference of the distillation column bottom is jacketed to directly heat the column, or a type in which a heat exchanger is installed outside the distillation column bottom. A type in which a heat exchanger is installed inside the distillation column bottom may also be used.

Generally, a shell-and-tube type is preferably used as a heat exchanger to increase the heat transfer area, but a coiled tube type or an electric heater can also be used. If chlorosilane liquid accumulates in the heat exchanger that applies the distillation energy and the aluminum chloride becomes highly concentrated, this can cause scaling, so it is preferable for the structure to be such that liquid does not easily accumulate. Methods that do not easily accumulate liquid include a method that uses convection caused by heating, and a method that uses a pump or the like to force the liquid to flow can also be preferably used.

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

Here, the difference in boiling point between the chlorosilanes to be purified and the impurities such as aluminum chloride to be separated and removed is quite large, so distillation can be carried out well without particularly advanced rectification, and the reflux ratio may be about 0.1 to 1 as long as the distillation operation can be maintained. The purified chlorosilanes are discharged from the purified chlorosilanes gas distillation tube (3) connected to the top of the distillation column (1).

In the present invention, the distillation is carried out at a bottom liquid temperature 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 at 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 residual liquid withdrawn from the bottom of the distillation column (1) through the distillation residual liquid discharge pipe (4) is also in a high liquid temperature state at the beginning of the discharge.

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 precipitation 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, 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%.

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 effects are involved.

That is, the Al-Cl bond is substantially polarized as Al ⁺ -Cl ^- , and thus in the solid aluminum chloride, it becomes a pseudo-ionic bond of Al-Cl-Al-Cl. Thus, if the pseudo-ionic bond is stably grown as a crystal, it will exhibit a certain degree of resolubility in the chlorosilane liquid when the liquid temperature rises. However, if the pseudo-ionic bond is grown as a crystal under unstable conditions, that is, if the chlorosilane liquid is distilled explosively in the thin film evaporation and the precipitate is generated under conditions in which impurity metal elements such as Fe and Ti are easily incorporated, the ionicity between Al and Cl will be broken, and the resolubility in the chlorosilane liquid will be greatly impaired. Thus, it is believed that this causes a large amount of aluminum chloride precipitate to be generated all at once with the explosive distillation of the chlorosilane liquid in the thin film evaporation of the distillation residual liquid, which leads to the problem of scale deposition.

For this reason, 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 the dissolved aluminum chloride is contained as a precipitate that is capable of redissolving by firmly growing crystals under the above-mentioned stable conditions. In other words, when the distillation residue cooled liquid obtained in this way is thin-film evaporated, when the liquid temperature of the treatment liquid rises on the heating surface of the device, the evaporation of chlorosilanes proceeds at a rapid rate, while the aluminum chloride precipitate also redissolves in the chlorosilanes liquid residue. Due to this offset by redissolution, the generation of the aluminum chloride precipitate is greatly suppressed, and as a result, it is believed that the problem of scale deposition in the device is significantly reduced.

Furthermore, according to the present invention, if the dispersion concentration of aluminum chloride precipitates in the distillation residue is increased by the above-mentioned advance cooling and then subjected to thin-film evaporation, the aluminum chloride precipitates will act as abrasive grains on the inner walls of the device with which they come into contact from the beginning of supply, and will also have a scraping effect on deposits on the walls. In particular, this scraping effect is more effective when the thin-film evaporation device is of the stirring type, since pressure is applied to the treatment liquid by stirring. Thus, in the present invention, it is presumed that the scraping effect of the aluminum chloride precipitates will also be added, greatly suppressing the deposition of scale within the device.

In the present invention, in order to more effectively suppress the amount of aluminum chloride scale deposition, it is more preferable that the distillation residue is cooled to a liquid temperature of 30°C or less. On the other hand, if the liquid is cooled to a temperature that is too low, it will be inefficient because it will need to be heated before being fed to the thin-film evaporator, so it is preferable that the liquid temperature be 5°C or higher.

By cooling the distillation residue, the aluminum chloride dissolved therein precipitates in its crude form. Therefore, the solids concentration of aluminum chloride precipitates in the resulting 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 solids concentration in the distillation residue cooling liquid can 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 solids concentration of the aluminum chloride precipitates in the distillation residue cooling liquid becomes high, it takes on a slurry-like form, but its fluidity is well maintained.

The method for cooling the high-temperature distillation residue to a liquid temperature of 40°C or less is not particularly limited, and may be performed 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, in terms of the reliability of cooling and the homogeneity of the resulting distillation residue cooled liquid, it is preferable to store the distillation residue in a cooling tank (5).

The distillation residue in the cooling tank (5) may be cooled by air cooling, but considering the efficiency of cooling and the ease of controlling the cooling temperature, it is preferable to use an external cooler, specifically, to forcibly cool the liquid by surrounding the outer wall of the cooling tank with a cooling jacket or by installing a refrigerant pipe inside the cooling tank.

Furthermore, it is preferable to carry out the cooling in the cooling tank (5) under stirring in order to improve the homogeneity of the aluminum chloride solids 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 preferable. 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, size of the reactor, etc., 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.

In the present invention, the distillation residue cooling liquid formed by the cooling is subjected to thin film evaporation. As shown in the flow chart of FIG. 1, the distillation residue cooling liquid discharged from the cooling tank (5) is supplied to the thin film evaporator (7) through the distillation residue cooling liquid flow pipe (6). Here, the thin film evaporator can be any known thin film evaporation method that can form a thin film of the liquid to be treated on a heated surface and evaporate the liquid components, without any restrictions. Specifically, examples include an agitation type thin film evaporator in which an internal blade rotates against a fixed heated surface (evaporation surface) to forcibly agitate the treatment liquid and form a thin film, and a centrifugal type thin film evaporator in which the heated surface rotates, and in particular, an agitation type thin film evaporator is preferred.

The above-mentioned agitated thin-film evaporators are classified into vertical and horizontal types according to the direction of the rotating shaft, fixed blade and movable blade types according to the type of blade attachment to the rotating shaft, and contact blade and non-contact blade types according to whether the blades contact the evaporation surface, but these types can be used in appropriate combinations. Among these, the vertical contact movable blade type and the vertical non-contact blade type are examples of more preferred types, and these thin-film evaporators are relatively effective in suppressing scale deposition. In particular, the vertical contact movable blade type is preferred because it is most effective in suppressing scale deposition because the blades contact the evaporation surface.

In the thin-film evaporator (7), if droplets are entrained in the evaporated gas components due to the operating conditions, the removal rate of aluminum chloride may decrease. In such a case, it is effective to install a droplet collector (9) in the recovery pipe (8) for the purified chlorosilanes discharged from the thin-film evaporator as a means of preventing droplets from being entrained in the recovered chlorosilanes. The droplet collector (9) may be of any type as long as it is a device that can collect and remove droplets. Examples include a baffle plate (impingement plate) type, a sintered type, and a centrifugal separation type.

It is also effective to install such a droplet collector (9) in the middle of the distillation residue cooling liquid flow pipe (6) that connects 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 liquid to the thin film evaporator (7) is effective in preventing deposition of aluminum chloride inside the device.

When the thin-film evaporator (7) has contact movable blades, the peripheral speed of the blade tip 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 addition, the frequency of the blade movement at a point on the evaporation surface is preferably at least once per second, more preferably at least once per 0.5 seconds, and even more preferably at least once per 0.3 seconds, in order to achieve a sufficient scale suppression effect.

On the other hand, in the case of the vertical non-contact blade type, since the blades do not come into contact with the evaporation surface, they must be rotated at a higher speed than the type in which the blades come into contact, and the peripheral speed of the blade tip is 1 m/s or more, preferably 3 m/s or more, and more preferably 5 m/s or more. In addition, the frequency of the blade movement at a point on the evaporation surface is preferably at least once per 0.5 seconds, preferably at least once per 0.3 seconds, and more preferably at least once per 0.1 seconds, and suitable conditions may be adopted depending on the heat transfer area, blade type, etc.

By using a thin-film evaporator in this manner, the time that the distillation residue cooling liquid remains in the device can be shortened, and the effect of suppressing the deposition of aluminum chloride scale in the device can be increased. At the same time, this effect can be further improved by selecting the structure or operating conditions of the thin-film evaporator described above.

In the present invention, when the distillation residue cooling liquid is supplied to the thin-film evaporator (7), the treatment liquid is heated while forming a liquid film on the evaporation surface, and the chlorosilanes evaporate and the aluminum chloride is concentrated. In this way, the chlorosilanes are extracted in gaseous form from the upper outlet of the evaporator (7) into the purified chlorosilanes recovery pipe (8), and the concentrated distillation residue cooling liquid is extracted from the lower outlet into the concentrate discharge pipe (10).

In this case, the residence time of the distillation residue cooling liquid in the thin-film evaporator (7) from when it is supplied until it is taken out as a concentrate 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, the residence time can be adjusted to some extent, but it is preferable to adjust it to within the above 10 minutes, preferably within 5 minutes.

The operating conditions for the thin-film evaporation include the heat transfer medium temperature, operating pressure, etc., and as with conventional evaporation operations, appropriate operating conditions can be adopted according to the composition of the distillation residue cooling liquid to be treated, the supply amount, the heat transfer area, the concentration ratio, etc. At this time, the precipitation of aluminum chloride from the distillation residue cooling liquid becomes intense at temperatures higher than about 180°C, and since the aluminum chloride precipitate formed here has low resolubility in the chlorosilane liquid as described above, from the viewpoint of highly preventing scale deposition in 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 less, preferably 150°C or less.

In the present invention, by utilizing the special behavior of aluminum chloride with respect to the above-mentioned distillation residue cooling liquid, the concentrate obtained after evaporation is allowed to contain aluminum chloride to an extremely high concentration. However, if the concentrate dries up, it will no longer have the function of washing away some of the scaled material or solids that were originally contained therein, and scaling may become severe, making continuous processing difficult.

Therefore, in the present invention, it is necessary to carry out thin-film evaporation under conditions in which the resulting concentrate maintains its liquid state. In particular, the preferred upper limit of the aluminum chloride concentration in the concentrate is 80% by weight, preferably 50% by weight, and more preferably 40% by weight. In addition, the lower limit is preferably 3% by weight, which was difficult to achieve with conventional techniques, and further preferably 10% by weight, because this allows the effects of the present invention to be fully exhibited.

In the above recovery method, the chlorosilanes extracted from the purified chlorosilanes recovery pipe (8) can be effectively used as raw materials for manufacturing high-purity semiconductor-grade silicon or photovoltaic-grade silicon. On the other hand, the concentrate discharged into the concentrate discharge pipe (10) can be supplied to an abatement pit or the like and then appropriately disposed of.

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 performed in the examples and comparative examples were performed by the following methods.

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

2) Measurement of aluminum chloride precipitate solids concentration in distillation residue cooling liquid A part of the distillation residue cooling liquid was extracted from the distillation residue cooling liquid flow pipe (6), and the mass was measured to obtain 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 chlorosilanes was quantified for the resulting cake in the same manner as in "1) Measurement of aluminum chloride concentration in chlorosilanes liquid" above, and the mass was divided by the mass of the measurement sample to obtain the amount.
3) Evaluation method for scale adhesion inside the thin film evaporator After the operation of the thin film evaporator was finished, the device was stopped and opened. The amount of scale adhered inside the device was evaluated visually. 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 at the central rotating shaft bearing of the rotor blades, causing blockage inside the equipment. For this reason, operation cannot be continued unless this scale is removed.
"Large": There is a large amount of scale deposition inside the equipment, but the amount of scale adhering to the reinforcing rib at the central rotating shaft bearing portion of the rotor blade has not yet reached the point of clogging 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 rotating shaft bearing portion of the rotating 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 on the reinforcing ribs in the central rotating shaft bearing portion of the rotating blade.

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

The chlorosilanes liquid was fed to the distillation column (1) at a feed rate of 60,000 kg/H from the chlorosilanes liquid supply pipe (2), and distillation was carried out in the distillation column (1) at a column bottom temperature of 80°C. As a result, distillation residual liquid at 70°C was discharged to the distillation residual liquid discharge pipe (4) at a rate of 1,200 kg/H. This distillation residual liquid 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 portions).

The distillation residue was sent to a cooling tank (5) and cooled to a cooling temperature of 25°C while stored there. The cooling tank (5) has a structure in which the outer wall of the tank is surrounded by a cooling jacket and an agitator blade is installed inside the tank. The agitator blade is rotated under the 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 crude to precipitate and disperse in the distillation residue, resulting in a highly dispersed aluminum chloride precipitate at a temperature of 25°C and a solid concentration of aluminum chloride precipitate of 0.9 mass% flowing through the distillation residue cooling liquid flow pipe (6). This distillation residue cooling liquid was supplied to the thin film evaporator (7) while maintaining the temperature of the above 70°C.

The thin-film evaporator (7) used was a contact movable blade type vertical thin-film evaporator with a heat transfer area of 0.15 ^m2 and an inner diameter of 0.15 m. The thin-film evaporation of the distillation residue cooling 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 frequency of the blade movement at one point on the evaporation surface was once every 3.3 seconds.

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%. On the other hand, the mass of the concentrate discharged from the concentrate discharge pipe (910) was measured and found to be 77 kg/H.

After continuing thin-film evaporation in this state for a further 10 days, the scale adhesion inside the thin-film evaporator was evaluated and found to be "non-existent."

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

Example 2
Chlorosilane liquid recovery was carried out in the same manner as in Example 1, except that the cooling temperature of the cooling tank (5) was changed so that the distillation residue cooling liquid flowing through the distillation residue cooling liquid flow pipe (6) had a temperature of 35° C. and an aluminum chloride precipitate solids concentration of 3.0 mass %. As a result, the state of the recovered liquid 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 the scale adhesion inside the thin film evaporator was evaluated after continuing this thin film evaporation for an additional 10 days, it was found to be in a "low" state.

1: Distillation column 2: Chlorosilanes liquid supply pipe 3: Refined chlorosilanes gas distillation pipe 4: Distillation residue discharge pipe 5: Cooling tank 6: Distillation residue cooling liquid flow pipe 7: Thin film evaporator 8: Refined chlorosilanes recovery pipe 9: Droplet collector 10: Concentrate discharge pipe

Claims (6)

A method for recovering chlorosilanes, comprising: cooling a distillation residue obtained by distilling a chlorosilane liquid containing aluminum chloride at a liquid temperature of 50°C or higher to a liquid temperature of 40°C or lower; and then feeding the resulting cooled distillation residue liquid to a thin-film evaporator to evaporate and recover the chlorosilanes. The method for recovering chlorosilanes according to claim 1, wherein the thin-film evaporator is an agitated thin-film evaporator. The method for recovering chlorosilanes according to claim 1 or claim 2, wherein the cooling of the distillation residue is carried out by storing the distillation residue in a cooling tank and lowering the liquid temperature to 40°C or less while stirring. A method for recovering chlorosilanes, comprising circulating and supplying the chlorosilanes recovered by the method according to any one of claims 1 to 3 to the distillation of a chlorosilanes liquid containing aluminum chloride. The method for recovering chlorosilanes according to any one of claims 1 to 4, wherein the chlorosilanes liquid containing aluminum chloride is produced by reacting a gas containing chlorine, hydrogen chloride, or silicon tetrachloride and hydrogen with metallurgical grade silicon. A method for producing chlorosilanes by distilling a chlorosilane liquid containing aluminum chloride, which is produced by reacting a gas containing chlorine or hydrogen chloride or silicon tetrachloride and hydrogen with metallurgical grade silicon, the method for producing chlorosilanes being characterized in that the distillation residual liquid obtained by the distillation, which has a liquid temperature of 50°C or higher, is subjected to the chlorosilanes recovery method described in any one of claims 1 to 4.