JP4588396B2 - Method for producing tetrafluorosilane - Google Patents

Description

  The present invention relates to a production method and use of tetrafluorosilane.

Tetrafluorosilane (SiF ₄ ) is required to be a high-purity product, for example, as an optical fiber raw material, a semiconductor raw material, or a solar cell raw material.
Various methods for producing SiF ₄ are known.
For example, as a method that has been known for a long time, there is a method of thermally decomposing hexafluorosilicate.
Na ₂ SiF ₆ → SiF ₄ + 2NaF (1)

However, silicofluorinated metal salts such as hexafluorosilicate contain oxygen-containing silicic acid compounds (for example, SiO ₂ ) as impurities, though they are H ₂ O and trace amounts. Therefore, unless sufficient pretreatment is performed, when pyrolysis is performed, the impurities and SiF ₄ react to produce hexafluorodisiloxane ((SiF ₃ ) ₂ O) (formula (3) described later). reference).

Also known is a method for producing SiF ₄ by reacting SiO ₂ and HF in the presence of concentrated sulfuric acid (Patent Document 1: Japanese Patent Laid-Open No. 57-135711).
4HF + SiO ₂ → SiF ₄ + 2H ₂ O (2)

However, this method has a problem that when the reaction molar ratio of SiO ₂ and HF approaches the theoretical molar ratio, the produced SiF ₄ and SiO ₂ react to produce hexafluorodisiloxane (SiF ₃ ) ₂ O. .

Another method for producing SiF _4, hexafluorosilicic acid (H ₂ SiF ₆₎ solution, a method of manufacturing a SiF ₄ was dehydrated decomposed with concentrated sulfuric acid (Patent Document 2: JP-A-9-183608) is known It has been. However, in this method, hydrogen fluoride (HF) is by-produced as in the thermal decomposition reaction. In the method of the above document, the raw material H ₂ SiF ₆ is taken out as a by-product of the phosphoric acid production process, and the HF produced as a by-product is returned to the phosphoric acid production process and processed, but since the phosphoric acid production process is a premise, Difficult to handle various raw materials.

As another production method, there is known a method of producing SiF ₄ by supplying H ₂ SiF ₆ into a vertical tower and decomposing it with sulfuric acid (Patent Document 3: Japanese Patent Application Laid-Open No. 60-11217). This method also has a problem that hydrogen fluoride (HF) is produced as a by-product and is recovered in a state in which HF is contained in sulfuric acid, as described above. Further, a method is described in which SiO ₂ is suspended in H ₂ SiF ₆ and reacted with HF. However, when SiO ₂ equivalent to HF is supplied, (SiF ₃ ) ₂ O is generated as a by-product. There is a problem.

3SiF ₄ + SiO ₂ → 2SiF ₃ OSiF ₃ (3)
Further, in _{SiF 4 (SiF 3) 2 O} , CO 2, if the impurity gas such as O ₂ is contained, since the use of SiF ₄ as the raw material of the silicon thin film caused the oxygen contamination, semiconductor and fiber Adversely affects properties. Accordingly, there is an increasing demand for high-purity SiF ₄ with less impurities.

As a method for purifying SiF ₄ containing (SiF ₃ ) ₂ O, CO ₂ or HF, for example, a method is known in which SiF ₄ containing (SiF ₃ ) ₂ O is treated with an adsorbent (Patent Document 4: Japanese Patent Application Laid-Open No. H10-260260) Sho-57-156317). However, when the adsorbent after use is heated and regenerated, the initial adsorption capacity may not be exhibited. Although this cause is not certain, it is thought that the adsorbed (SiF ₃ ) ₂ O is decomposed in the adsorbent pores. The SiO ₂ produced by the decomposition clogs the adsorbing pores and makes it difficult to recycle the adsorbent, so that there is a problem that the adsorbent must be treated as waste. Furthermore, if the adsorbent firing before the gas flow is not sufficient, it will cause (SiF ₃ ) ₂ O to be generated due to a side reaction with moisture.

JP-A-57-135711 JP-A-9-183608 Japanese Patent Laid-Open No. 60-11217 JP-A-57-156317

  The present invention has been made under such a background, and in a method for producing tetrafluorosilane using hexafluorosilicic acid as a raw material, impurities (problems which are problematic in conventional pyrolysis and sulfuric acid decomposition methods) In particular, it is an object of the present invention to provide a method for producing high-purity tetrafluorosilane and its use by efficiently reducing hexafluorodisiloxane) and solving the problem of by-product HF.

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention have made Step 1 in which H ₂ SiF ₆ is decomposed with sulfuric acid to produce SiF _4, and HF dissolved in sulfuric acid in Step 1 is reacted with SiO _2. and step 2 to produce SiF _4, produced in step 2 returns a (SiF ₃₎ SiF ₄ containing ₂ O in step 1, the (SiF _{3) 2} SiF ₄ and O and HF are reacted with step 3 of the water be obtained to produce SiF ₄ by a process which comprises, further found that high purity SiF ₄ can be obtained by performing the step of contacting further obtained step a molecular sieving carbon is contacted with SiF ₄ in concentrated sulfuric acid, The present invention has been completed.

That is, the present invention relates to the following [1] preparation and use thereof SiF ₄ as shown in to [14].
[1] In a method for producing tetrafluorosilane by decomposing hexafluorosilicic acid with sulfuric acid,
A step of decomposing hexafluorosilicic acid in concentrated sulfuric acid in a first reactor to form tetrafluorosilane and hydrogen fluoride and taking out the produced tetrafluorosilane (step 1);
A part of the concentrated sulfuric acid solution obtained in Step 1 containing hydrogen fluoride is transferred to the second reactor, and the hydrogen fluoride and silicon dioxide supplied to the second reactor are reacted to form hexafluorodisiloxane. A step of producing a tetrafluorosilane containing step (step 2), and a reaction product containing hexafluorodisiloxane and tetrafluorosilane obtained in step 2 is transferred to a first reactor, and hexafluorodisiloxane in the reaction product is removed. A process for producing tetrafluorosilane, comprising a step (step 3) of reacting with hydrogen fluoride to convert to tetrafluorosilane and taking out tetrafluorosilane together with tetrafluorosilane in step 1.
[2] A hexafluorosilicic acid aqueous solution and concentrated sulfuric acid are continuously and intermittently supplied to the first reactor and silicon dioxide to the second reactor, respectively, and continuously or intermittently from the first reactor. The method for producing tetrafluorosilane according to [1], wherein tetrafluorosilane is taken out.
[3] The method for producing tetrafluorosilane according to [1] or [2], wherein the sulfuric acid concentrations in the first reactor and the second reactor are maintained at 70% by mass or more.
[4] The method for producing tetrafluorosilane according to any one of [1] to [3], wherein the reaction temperature of the first reactor and the second reactor is 60 ° C. or higher.
[5] The method for producing tetrafluorosilane according to [1] or [2], wherein the silicon dioxide supplied to the second reactor has a particle size of 30 μm or less.
[6] The method according to [1] or [2], including a step of bringing tetrafluorosilane taken out from the first reactor into contact with concentrated sulfuric acid at 50 ° C. or less and absorbing and removing hydrogen fluoride contained in tetrafluorosilane. Process for producing tetrafluorosilane.
[7] The method for producing tetrafluorosilane according to [6], wherein the tetrafluorosilane taken out from the first reactor is brought into countercurrent contact with concentrated sulfuric acid supplied to the sulfuric acid supply path to the first reactor.
[8] The method for producing tetrafluorosilane according to [1] or [2], further including a purification step in which impurities in tetrafluorosilane taken out from the first reactor are removed by contacting with molecular sieve carbon.
[9] The method for producing tetrafluorosilane according to [8], wherein the impurities to be removed are one or more selected from the group consisting of hydrogen fluoride, hydrogen chloride, sulfur dioxide, hydrogen sulfide, and carbon dioxide.
[10] The method for producing tetrafluorosilane according to the above [8] or [9], wherein a molecular sieving carbon having a pore size smaller than that of tetrafluorosilane is used.
[11] The method for producing tetrafluorosilane as described in [10] above, wherein molecular sieve carbon baked in an inert gas atmosphere and then pretreated through high-purity tetrafluorosilane is used.
[12] An optical fiber production comprising the tetrafluorosilane gas obtained by the production method according to any one of [1] to [11], wherein the contents of transition metal, phosphorus and boron are each 100 ppb or less. Gas.
[13] A semiconductor production comprising a tetrafluorosilane gas obtained by the production method according to any one of [1] to [11], wherein the contents of transition metal, phosphorus and boron are each 100 ppb or less. Gas.
[14] A solar cell comprising a tetrafluorosilane gas obtained by the production method according to any one of [1] to [11], wherein the contents of transition metal, phosphorus and boron are each 100 ppb or less. Production gas.

The present invention will be described in detail below.
The method for producing SiF _{4 according to} the present invention includes Step 1 in which H ₂ SiF ₆ is decomposed with sulfuric acid in the first reactor to produce hydrogen fluoride and SiF ₄ , and the hydrogen fluoride obtained in Step 1 is included. Part of the sulfuric acid solution is introduced into the second reactor, and hydrogen fluoride dissolved in sulfuric acid reacts with SiO ₂ to produce SiF ₄ , and SiF containing (SiF ₃ ) ₂ O produced in step 2 ₄ is a method for producing SiF ₄ , which includes step 3 in which ₄ is returned to the first reactor to react SiH ₄ with hydrogen fluoride by-produced from (SiF ₃ ) ₂ O and H ₂ SiF ₆ . Specifically, as shown in FIG. 1, H ₂ SiF ₆ is decomposed with sulfuric acid (step 1) in the first reactor, and at least a part of the sulfuric acid containing by-product HF is transferred to the second reactor. as an impurity is reacted with SiO ₂ Te (SiF ₃₎ generate SiF ₄ containing ₂ O (step 2), which is the impurity returning the SiF ₄ obtained in the second reactor to the first reactor (SiF ₃ ) ₂ O and HF present in the reactor are reacted to convert to SiF ₄ (step 3), which is taken out as high-purity SiF ₄ and appropriately subjected to purification treatment (purification step) as necessary.

By taking such a process, step 1:
H ₂ SiF ₆ → SiF ₄ + 2HF (4)
Most of the HF produced in step 2 is step 2:
4HF + SiO ₂ → SiF ₄ + 2H ₂ O (2)
And step 3:
(SiF ₃ ) ₂ O + 2HF → 2SiF ₄ + H ₂ O (5)
HF is efficiently processed. Moreover, since the by-product (SiF ₃ ) ₂ O in Step ₂ is converted to SiF ₄ in Step 3, high-purity SiF ₄ can be efficiently obtained as the entire process.

Hereinafter, each step will be described.
H ₂ SiF ₆ used in step 1 can be used without any problem even if it is manufactured by any process. For example, like H ₂ SiF ₆ where HF and H ₂ SiF ₆ and SiF ₄ produced by the reaction between SiO ₂ and HF is produced by the reaction can be utilized. For example, in the phosphoric acid production process, H ₂ SiF ₆ produced as a by-product in large quantities when Si and F contained in the raw phosphate ore are decomposed by H ₂ SO ₄ can be used at low cost.

The reaction of step 1 is the following reaction:
H ₂ SiF ₆ → SiF ₄ + 2HF (4)
And sulfuric acid is used as the (dehydration) decomposition agent. However, when the sulfuric acid concentration is low, H ₂ SiF ₆ is stably present in the sulfuric acid and the decomposition reaction does not proceed. Therefore, the sulfuric acid concentration is preferably 70% by mass or more after mixing, more preferably 75% by mass or more, and most preferably 80% by mass or more. When the reaction temperature is low, the decomposition reaction proceeds so slowly that it is not practical. By performing the decomposition reaction at 60 ° C. or higher, SiF ₄ can be efficiently obtained. However, although the decomposition reaction rate increases when the temperature is increased, it is not preferable because HF as a decomposed by-product or moisture in sulfuric acid is volatilized from the sulfuric acid aqueous solution. Therefore, the reaction temperature is preferably 60 to 120 ° C, more preferably 80 to 100 ° C.

The shape of the first reactor is not particularly limited, as long as the contact time between concentrated sulfuric acid and H ₂ SiF ₆ necessary for the breakdown of H ₂ SiF ₆ is obtained. Since the decomposition reaction ends very quickly and instantaneously, a contact time of about 0.1 to 10 seconds is sufficient.

In step 1 the HF generated along with production of SiF ₄ in sulfuric acid in a state dissolved in sulfuric acid, by reacting with SiO ₂ and fed to the second reactor can be a SiF ₄ (Step 2 ).

4HF + SiO ₂ → SiF ₄ + 2H ₂ O (2)
SiO ₂ may be reacted in a solid state, but it is preferable to use powdered SiO ₂ because it can be dispersed in the solution and an efficient reaction can be performed. In addition, although the SiO ₂ powder may be charged into the reactor as it is, the method of supplying SiO ₂ dispersed in sulfuric acid is advantageous for continuous charging. The average particle diameter of SiO ₂ used here is preferably 30 μm or less, more preferably 10 μm or less, and most preferably 5 μm or less, because finer dispersion results in better dispersion.

The concentration of SiO ₂ dispersed in sulfuric acid can be selected as appropriate depending on the physical properties (particle size, density, etc.) of the powder used. If the concentration is too low, the amount of sulfuric acid supplied will increase. If the concentration is too high, the slurry will solidify. Since liquid separation occurs, the supply concentration is preferably 0.1 to 30% by mass. The purity of SiO ₂ used is preferably 90% or more, and more preferably 99% or more.

The reaction temperature is preferably 60 ° C or higher, more preferably 80 to 100 ° C. Further, the amount of SiO ₂ to be added may be a theoretical molar amount (1/4 times mole) with respect to HF, but the SiO ₂ is discharged in step 2 by reacting with a larger or smaller amount than the theoretical molar ratio. The concentration of HF and SiO ₂ contained in sulfuric acid (waste sulfuric acid) can be controlled. Here, in view of the reuse of waste sulfuric acid for other purposes, for example, in the phosphoric acid decomposition step, and the analytical method for control, it is preferable that HF reacts under slightly excessive conditions.

Here, when the amount of SiO ₂ is lowered from the low molar ratio to the theoretical molar ratio with respect to HF, (SiF ₃ ) ₂ O is generated as a byproduct. This is presumably because the produced SiF ₄ and SiO ₂ react.
3SiF ₄ + SiO ₂ → 2SiF ₃ OSiF ₃ (3)
When this (SiF ₃ ) ₂ O is contained in SiF ₄ , it has an adverse effect on semiconductors and optical fibers, so it must be removed.
Therefore, SiF ₄ containing (SiF ₃ ) ₂ O produced in step 2 is returned to the first reactor, and HF and (SiF ₃ ) ₂ O contained in sulfuric acid in the first reactor are reacted to form SiF _4. (SiF ₃ ) ₂ O can be removed by using water and water (step 3).

(SiF ₃ ) ₂ O + 2HF → 2SiF ₄ + H ₂ O (5)
The reaction can be performed under the same reaction conditions as in step 1. The reaction between (SiF ₃ ) ₂ O and HF proceeds both in the gas phase and in the sulfuric acid solution. When the production ratio of (SiF ₃ ) ₂ O is large, it is preferable to blow into the sulfuric acid solution so as to increase the contact time of HF with (SiF ₃ ) ₂ O.

Steps 1 to 3 can be performed batchwise, but are preferably performed continuously. The final product SiF ₄ is taken out from the gas phase of the first reactor.
As can be seen from the above description and FIG. 1, SiF ₄ produced in Step 1, Step 2 and Step 3 contains HF and H ₂ O. Therefore, normally, SiF ₄ taken out from the first reactor is purified in a purification step.

As the purification step, first, a sulfuric acid washing step is exemplified. HF and H ₂ O are removed by washing with sulfuric acid. As a washing method using sulfuric acid, concentrated sulfuric acid may be put into a container and SiF ₄ produced in steps 1 to 3 may be blown into the container. Preferably, sulfuric acid is circulated in the column, and the direction opposite to the sulfuric acid circulation direction is used. Therefore, it is more effective to distribute SiF ₄ . The tower is preferably packed with a packing to increase the contact efficiency. A higher sulfuric acid concentration is preferable because of higher removal efficiency. The sulfuric acid concentration is preferably 90% by mass or more, more preferably 95% by mass or more, and most preferably 98% by mass or more. Moreover, although the one where the sulfuric acid temperature in an absorption tower is low is preferable since there are few amounts of evaporation of HF and water, when it cools too much, a viscosity will become high and handling will become difficult. Therefore, it is preferable to operate the absorption tower at a temperature of 10 to 50 ° C. Here sulfuric acid used is subjected to N ₂ bubbling, the use of pre-was degassed CO ₂ sulfate, CO ₂ contained in SiF ₄ produced in step 1-3 also absorbed in sulfuric acid decreases Can be made.

Even after passing through the absorption tower, SiF ₄ may contain hydrogen chloride, hydrogen sulfide, sulfur dioxide, nitrogen, oxygen, hydrogen, carbon monoxide, carbon dioxide, and HF as impurities. Among these impurities, impurities other than low-boiling components such as nitrogen, oxygen, hydrogen and carbon monoxide can be removed with molecular sieve carbon.

By using here as molecular sieving carbon used is smaller than the molecular diameter of SiF ₄ pore size, suction without adsorb SiF _4, HCl, H ₂ S, the impurity of CO _2, HF, etc. can do. Preferably, those having a pore diameter of 5 mm or less, such as Molseabon 4A (manufactured by Takeda Pharmaceutical Company Limited) can be used. The molecular sieving carbon used here is preferably calcined in the temperature range of 100 to 350 ° C. in advance under the flow of an inert gas such as N ₂ in order to remove moisture, CO _{2 and the} like. The drying N ₂ uses a dew point of −70 ° C. or less, and can be completed when the dew points at the firing inlet and outlet become the same value. The molecular sieve carbon that has been baked here has completely removed moisture, but a small amount of hydroxyl groups and oxides remain on the surface of the adsorbent. Therefore, when SiF ₄ is circulated, it reacts with the hydroxyl groups and oxides on the activated carbon surface. Thus, (SiF ₃ ) ₂ O and HF are generated.

SiF ₄ + (C—OH) + (C−H) → (SiF ₃ ) ₂ O + 2HF + (2C−F)

Therefore, when using the adsorbent after firing, using the by contacting the adsorbent with SiF _4, by reacting sites and SiF ₄ to produce an impurity reacts advance the adsorbent surface and SiF ₄ The production of by-product (SiF ₃ ) ₂ O and HF can be suppressed. Examples of the contact method include a method in which SiF ₄ is reacted while flowing to analyze the impurities at the outlet (for example, SiF ₃ OSiF ₃ ) to confirm the end point, a method in which pressure is accumulated and the reaction is performed for a predetermined time. If the contact reaction temperature is equal to or higher than the temperature at which impurities are adsorbed, the reaction proceeds without problems, and after completion, the impurities can be adsorbed without problems. The reaction pressure is preferably below the pressure at which SiF ₄ liquefies. From the viewpoint of reducing the amount of SiF ₄ used for the treatment, it is more preferable to carry out at about atmospheric pressure. Further, the purity of SiF ₄ to be used is not particularly limited, when using SiF ₄ in which impurities contained in a large amount, since the pre-treatment is sometimes undesirable welcomed adsorbent breakthrough before exiting, as much as possible It is preferable to use high-purity SiF ₄ . Furthermore, SiF ₄ containing (SiF ₃ ) ₂ O and HF produced in the pretreatment can be returned to the reaction step 3 and purified.

Here, when SiF ₄ contains low-boiling components such as N ₂ , O ₂ , H ₂ , and CO after adsorbing and removing impurities by molecular sieve carbon, a general method such as distillation can be used. SiF ₄ can be further purified.

Next, the use of high-purity SiF ₄ obtained by the method of the present invention will be described.
Increasing the degree of transistor integration with the miniaturization of semiconductor devices has the advantage of increasing the density or increasing the switching speed of individual transistors. However, the propagation delay due to the wiring offsets the speed improvement merit of the transistor. In generations with a line width of 0.25 μm or more, wiring delay becomes a big problem. In order to solve this problem, a copper wiring instead of aluminum is adopted as a low resistance wiring, and a low dielectric constant interlayer insulating film is adopted to reduce the capacitance between wirings. A typical low dielectric constant material employed in the line width 0.25 to 0.18 or 0.13 μm generation is SiOF (fluorine-doped oxide film ε = around 3.5) by HDP (high density) plasma CVD. Adoption of a process using SiOF as an interlayer insulating film and wiring as an aluminum alloy is in progress. In such SiF ₄ for producing SiOF, it is preferable that the content of impurities such as transition metals such as iron, nickel, and copper, and impurities such as phosphorus and boron is low. Specifically, the contents of transition metal, phosphorus and boron are each preferably 100 ppb or less, more preferably 50 ppb or less, and particularly preferably 10 ppb or less. The high-purity SiF ₄ of the present invention that satisfies such conditions can be used as this dope material.

The glass for optical fibers is composed of a core part and a clad part. The core part has a higher refractive index than the surrounding clad part in order to facilitate light transmission at the center part. In order to increase the refractive index, Ge, Al, Ti or the like may be added as a dopant. However, there is a side effect that light scattering increases due to the dopant and the light transmission efficiency decreases. When fluorine is added to the cladding part, the refractive index can be made lower than that of pure quartz, so that pure quartz or quartz with less dopant can be used for the core part, and the light transmission efficiency can be increased. In order to add fluorine, the glass fine particles (SiO ₂ ) are heated in a SiF ₄ atmosphere in He. In such a SiF ₄ atmosphere, it is preferable that the content of impurities such as transition metals such as iron, nickel and copper, and impurities such as phosphorus and boron is low. Specifically, the contents of transition metal, phosphorus and boron are each preferably 100 ppb or less, more preferably 50 ppb or less, and particularly preferably 10 ppb or less. The high-purity SiF ₄ of the present invention that satisfies such conditions can be used as an optical fiber gas.

A silicon type solar cell is made of a pin type photovoltaic element. However, when the I type semiconductor layer is formed of SiF ₄ , a trace amount of F atoms is contained in the silicon film. By including fluorine atoms in the silicon thin film in this way, when light is irradiated from the surface of the photovoltaic element, the rearrangement of atoms in the vicinity of the crystal grain boundary proceeds due to the interaction between heat and fluorine atoms. For example, the structural distortion is relieved, or the product produced by the reaction of moisture and fluorine, which is thought to penetrate mainly through the grain boundary from the surface side, binds to silicon atoms with dangling bonds. However, the light conversion efficiency is self-recovered by causing a change in the charged state. In the production gas used under such conditions, it is preferable that the content of impurities such as transition metals such as iron, nickel and copper, and phosphorus, boron and the like deteriorate the characteristics. Specifically, the contents of transition metal, phosphorus and boron are each preferably 100 ppb or less, more preferably 50 ppb or less, and particularly preferably 10 ppb or less. The high-purity SiF ₄ of the present invention that satisfies such conditions can be used for the production of such a solar cell.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

The outline of the manufacturing system used in the present invention will be described with reference to FIG.
In FIG. 2, 1 and 3 are a sulfuric acid tank and an H ₂ SiF ₆ tank, respectively. These sulfuric acid and H ₂ SiF ₆ are respectively supplied to the first reactor (7) by a fixed liquid feeding pump (2, 4). During continuous operation, sulfuric acid also functions as a means for purifying the product gas by entering the system through a sulfuric acid washing tower (5) for washing the product gas. The first reactor (7) is maintained at a predetermined temperature by heating means (8) such as an oil bath. Each solution supplied into the first reactor (7) is uniformly mixed by a stirring motor (6).

In the first reactor, H ₂ SiF ₆ is decomposed into SiF ₄ and HF (step 1). Since most of the SiF ₄ gas appears in the gas phase, it is taken out through a sulfuric acid washing tower (5) that continuously supplies it. The gas phase in the first reactor (7) can be extracted and analyzed from the sampling valve (11), and the SiF ₄ gas passing through the sulfuric acid washing tower (5) is extracted from the sampling valve (23) and analyzed. It is possible.

SiO ₂ dispersed in sulfuric acid is supplied from the tank (15) to the second reactor (17) through the valve (16). Further, when the solution in the first reactor (7) reaches a predetermined level, the sulfuric acid solution (containing a large amount of by-produced HF) in the first reactor (7) is supplied by the stop valve (12). Feed into the second reactor (17). The second reactor (17) is maintained at a predetermined temperature by a heating means (18) such as an oil bath. Each solution supplied into the second reactor (17) is uniformly mixed by the stirring motor (14).

In the second reactor (17), HF and SiO ₂ produced in step 1 are reacted to produce SiF ₄ and H ₂ O containing (SiF ₃ ) ₂ O as impurities (step 2). Since most of the SiF ₄ gas appears in the gas phase, it can be extracted from the sampling valve (10) and analyzed, and this is introduced into the first reactor (7) by opening the stop valve (9), Impurity (SiF ₃ ) ₂ O and HF produced as a by-product in step 1 are reacted to form SiF ₄ (step 3). The sulfuric acid concentration in the second reactor (17) can be adjusted by additional supply from the sulfuric acid tank (20). The additional supply can be performed by any liquid feeding means, for example, a metered liquid feeding pump (19). Further, the sulfuric acid in the second reactor can be taken out into the waste sulfuric acid tank (22) via the valve (21). Therefore, the liquid level in the system can be arbitrarily adjusted.

Mixture of the generated SiF ₄ gas and SiF ₄ gas is purified second generated by the reactor of step 2 in the first reactor of step 3 in step 1 of the first reactor, the reaction is steady state for each reactor Then, by opening the stop valve (24), it is introduced into the adsorbent (30) previously fired by the heating means (31) and subjected to adsorption purification. Firing is usually performed while N ₂ gas is introduced from the N ₂ source through the flow meter (25) and the valve (27), and then the flow meter (26) and valve (28) from the high-purity SiF ₄ source. SiF ₄ gas may be introduced through the gas. The purification outlet gas of the adsorption cylinder (29) can be analyzed by the sampling valve (32). The purified gas is taken out of the system through a stop valve (33).

  Specific results of the experimental operation performed based on the above operation are shown below. In the following examples, the dimensions of each element of the system are referred to. However, these can be carried out at any scale, and the reactor and the like can withstand the reaction conditions and can react. Any material can be used as long as it does not become an obstacle.

Examples 1-8
A first reactor 7 (cylindrical polytetrafluoroethylene reactor, φ100 × 260 L, approximately 2 L) was charged with 500 ml of a sulfuric acid solution having a predetermined concentration adjusted in advance. To satisfy the conditions shown in Table 1, the sulfuric acid solution was heated to a temperature, and 20% H ₂ SiF ₆ aqueous solution and 98% H ₂ SO ₄ were added so that the concentration of the sulfuric acid solution was constant. Sulfuric acid was removed from the valve (12) so that the amount of the reaction solution was constant. The product gas extracted from the valve (11) after the reaction reached a steady state was analyzed by FT-IR, and the sulfuric acid solution extracted from the valve (13) was analyzed by ion chromatography. The results are shown in Table 1.

Examples 7-10
The H ₂ SO ₄ solution of Example 1 or 4 was quantitatively supplied to the second reactor (17) (φ100 × 260 L, about 2 L) through the valve (12). Against HF content in H ₂ SO ₄ that supplied, a solution prepared by dispersing SiO ₂ in H ₂ SO _4, was fed while controlling a constant flow rate with flow control valve performs pressurization with nitrogen (16) . The reaction solution temperature was controlled while controlling with an oil bath, and sulfuric acid was quantitatively extracted from the valve (21) so that the reaction solution amount was constant. After the reaction reached a steady state, the product gas extracted from the valve (10) was analyzed by FT-IR. The results are shown in Table 2.

Examples 11 and 12
The SiF ₄ gas obtained from Example 9 was supplied from the valve (9) to the first reactor (7) where the reaction was continued under the same conditions as in Example 4. The product gas extracted from the valve (11) was analyzed by FT-IR. Further, after the product gas passed through the sulfuric acid washing tower (5), the gas extracted from the valve (23) was analyzed by FT-IR. The sulfuric acid washing tower (5) used was a 50 cm ½ inch polytetrafluoroethylene tube filled with a polytetrafluoroethylene packing (120 ml). The results are shown in Table 3.

Examples 13 and 14
The case where the gas of Example 12 was passed through the adsorption tower (29) and calcined with N ₂ alone was compared with the case where N ₂ was followed by pretreatment with SiF ₄ .
The adsorption cylinder used was a 3/4 inch SUS tube, and 100 ml of Molseabon 4A (manufactured by Takeda Pharmaceutical Co., Ltd.) was filled as an adsorbent. Baking was performed until the outlet dew point was −70 ° C. or lower by heating to 300 ° C. while N ₂ was distributed at 400 ml / min. It cooled to room temperature, distribute | circulated the gas similar to Example 12, and analyzed the exit gas.

Also when processing the adsorbent after firing the high purity SiF ₄ is a SiF ₄ at room temperature adsorbent distributed in 100 ml / min, was performed continuously analyzed by FT-IR the outlet gas. Pretreatment was performed until no hexafluorodisiloxane was detected in the outlet gas, and then the same gas as in Example 12 was circulated to analyze the outlet gas. The results are shown in Tables 4 and 5.

As shown in the above results, it was possible to continuously produce SiF ₄ gas with the impurity concentration reduced to a level undetectable by FT-IR.

As described above, according to the present invention, it is possible to continuously produce SiF ₄ gas having an impurity concentration reduced to a level that cannot be detected by FT-IR. For this reason, it has become possible to provide high-purity SiF ₄ required by the electronic component manufacturing industry and the like. Moreover, since HF discharged as a by-product in the conventional method is used as SiF ₄ , the utilization efficiency of raw materials is high, and harmful emissions are suppressed.

An outline of the reaction scheme of the present invention is shown. 1 schematically shows a reaction system that can be used in the present invention.

Explanation of symbols

1, 20 Sulfuric acid tanks 2, 4, 19 Metering pump 3 H ₂ SiF ₆ tank 5 Sulfuric acid washing towers 6, 14 Stirring motor 7 First reactors 8, 18, 31 Heating means 9, 12, 24, 33 Stop valve 10, 11, 13, 23, 32 Sampling valve 15 Sulfuric acid tank 17 in which SiO ₂ is dispersed Second reactor 16, 21, 27, 28 Valve 22 Waste sulfuric acid tank 25, 26 Flow meter 29 Adsorption cylinder 30 Adsorbent

Claims (11)

In a method for producing tetrafluorosilane by decomposing hexafluorosilicic acid with sulfuric acid,
A step (step 1) of decomposing hexafluorosilicic acid in sulfuric acid in a first reactor to form tetrafluorosilane and hydrogen fluoride and taking out the produced tetrafluorosilane;
Part of the sulfuric acid solution obtained in Step 1 containing hydrogen fluoride is transferred to the second reactor, and the hydrogen fluoride and silicon dioxide supplied to the second reactor are reacted to contain hexafluorodisiloxane. The step of producing tetrafluorosilane (Step 2), and the reaction product containing hexafluorodisiloxane and tetrafluorosilane obtained in Step 2 are transferred to the first reactor, and the hexafluorodisiloxane in the reaction product is fluorinated. A process for producing tetrafluorosilane, comprising a step (step 3) of reacting with hydrogen fluoride to convert to tetrafluorosilane and taking out tetrafluorosilane together with tetrafluorosilane in step 1.   Aqueous hexafluorosilicic acid and sulfuric acid were fed to the first reactor and silicon dioxide was fed to the second reactor continuously or intermittently, respectively, and tetrafluorosilane was continuously or intermittently fed from the first reactor. The manufacturing method of the tetrafluorosilane of Claim 1 taken out.   The manufacturing method of the tetrafluorosilane of Claim 1 or 2 which maintains the sulfuric acid concentration of a 1st reactor and a 2nd reactor at 70 mass% or more.   The method for producing tetrafluorosilane according to any one of claims 1 to 3, wherein the reaction temperature of the first reactor and the second reactor is 60 ° C or higher.   The method for producing tetrafluorosilane according to claim 1 or 2, wherein an average particle diameter of silicon dioxide supplied to the second reactor is 30 µm or less.   The method for producing tetrafluorosilane according to claim 1, comprising a step of bringing tetrafluorosilane taken out from the first reactor into contact with sulfuric acid at 50 ° C. or less and absorbing and removing hydrogen fluoride contained in tetrafluorosilane. .   The method for producing tetrafluorosilane according to claim 6, wherein the tetrafluorosilane taken out from the first reactor is brought into countercurrent contact with sulfuric acid supplied to the sulfuric acid supply path to the first reactor.   The manufacturing method of the tetrafluorosilane of Claim 1 or 2 including the refinement | purification process which contacts the molecular sieving carbon and removes the impurity in the tetrafluorosilane taken out from the 1st reactor.   The method for producing tetrafluorosilane according to claim 8, wherein the impurities to be removed are at least one selected from the group consisting of hydrogen fluoride, hydrogen chloride, sulfur dioxide, hydrogen sulfide, and carbon dioxide.   The method for producing tetrafluorosilane according to claim 8 or 9, wherein molecular sieve carbon having a pore size smaller than that of tetrafluorosilane is used.   The method for producing tetrafluorosilane according to claim 10, wherein molecular sieve carbon baked in an inert gas atmosphere and then pretreated through high-purity tetrafluorosilane is used.

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