WO2011136517A2

WO2011136517A2 - Method of preparing silicon tetrafluoride by using crystalline silica

Info

Publication number: WO2011136517A2
Application number: PCT/KR2011/002981
Authority: WO
Inventors: Kyoung Hoon Kang; Yeon Seok Cho; Se Jong Kim; Young Shin Yoo; Moon Gue Jun; Jeong Mo Shim; Hyun Dong Lee; Ji Hye Kim; Min Wook Song
Original assignee: Kcc Corporation
Priority date: 2010-04-27
Filing date: 2011-04-25
Publication date: 2011-11-03
Also published as: WO2011136517A3; TW201204632A; KR20110119267A; KR101168942B1

Abstract

The present invention relates to a method of preparing silicon tetrafluoride (STF, SiF₄) by using crystalline silica (SiO₂). More specifically, the present invention relates to a method of preparing silicon tetrafluoride by reacting finely divided crystalline silica and hydrogen fluoride (HF) in the presence of concentrated sulfuric acid in a continuous manner. According to the present invention, silicon tetrafluoride can be economically prepared with high yield from crystalline silica which exists abundantly in the natural world, and the process productivity, workability and controllability can be improved remarkably. In addition, the problem of corrosion in the recycling process by concentrating diluted sulfuric acid can be solved through controlling the ratio between reactants such that no incorporated hydrogen fluoride remains after the reaction, or on the other hand the difficulty in filtering the product by finely divided particles can be avoided through controlling the ratio between reactants such that the crystalline silica reactant is consumed completely. Furthermore, since the used sulfuric acid can be isolated and recycled, the generation of discarded sulfuric acid can be minimized.

Description

METHOD OF PREPARING SILICON TETRAFLUORIDE BY USING CRYSTALLINE SILICA

The present invention relates to a method of preparing silicon tetrafluoride (STF, SiF₄) by using crystalline silica (SiO₂). More specifically, the present invention relates to a method of preparing silicon tetrafluoride by reacting finely divided crystalline silica and hydrogen fluoride (HF) in the presence of concentrated sulfuric acid in a continuous manner. According to the present invention, silicon tetrafluoride can be economically prepared with high yield from crystalline silica which exists abundantly in the natural world, and the process productivity, workability and controllability can be improved remarkably. In addition, the amount of unreacted hydrogen fluoride can be reduced significantly, and the difficulty in filtering the product by finely divided particles can be avoided since the crystalline silica reactant is consumed completely. Furthermore, since the used sulfuric acid can be isolated and recycled, the generation of discarded sulfuric acid can be minimized.

Silicon tetrafluoride (STF, SiF₄) gas is used in a dry etching process in manufacturing semiconductors and as a raw material for the preparation of wiring for optical fibers and the preparation of amorphous silicon thin films. It is also used as a precursor of monosilane (SiH₄) gas for manufacturing silicon wafers for solar cells.

Known methods of preparing SiF₄ include a method employing a dehydration-thermal decomposition reaction of a concentrated liquid of hexafluorosilicic acid (H₂SiF₆) generated as a byproduct in phosphate fertilizer production with sulfuric acid (International Publication No. WO 2005/030642) and a method employing a thermal decomposition reaction of solid M₂SiF₆ (M = Na, K) prepared from hexafluorosilicic acid (US Patent No. 2,615,872).

However, the process of preparing SiF₄ by thermally decomposing hexafluorosilicic acid generated as a byproduct in phosphate fertilizer production requires use of a large amount of sulfuric acid to remove water which exists together with the hexafluorosilicic acid, and thus has a difficulty that the excessive diluted sulfuric acid generated thereby should be further treated for its recovery. Furthermore, since the precursor hexafluorosilicic acid is a byproduct generated in the production of phosphate fertilizer, the process of preparing SiF₄ depends on the phosphate fertilizer production process, and thus in order to enable the scale-up of SiF₄ production, the scale-up of the phosphate fertilizer production process must accompany it.

US Patent No. 6,770,253 suggests a method of preparing SiF₄ by reacting elemental silicon (Si) with HF at a high temperature condition of 300℃ or more. US Patent No. 4,382,071 suggests a method of preparing SiF₄ by reacting HF dissolved in sulfuric acid with amorphous silica. However, this method has a demerit in that expensive amorphous silica with high purity should be used in order to obtain SiF₄ with a high yield of 70% or more. In addition, if such expensive amorphous silica with high purity is used, the reaction becomes too vigorous and so its control is difficult. On the contrary, if relatively cheap amorphous silica with low purity (e.g., fly ash or the like) is used, there is a problem of impurities. Furthermore, when the concentration of sulfuric acid in the reactor falls below 80% as the reaction proceeds, the reactivity becomes lowered and thus it is necessary to discharge the reaction mixture and feed fresh concentrated sulfuric acid, by which there is a limitation in continuously preparing the product.

[PRIOR ART PUBLICATIONS]

International Publication No. WO 2005/030642

US Patent No. 2,615,872

US Patent No. 6,770,253

US Patent No. 4,382,071

To resolve the problems of prior arts as explained above, the present invention has an object of providing a method of preparing silicon tetrafluoride, according to which silicon tetrafluoride can be economically prepared with high yield from crystalline silica which exists abundantly in the natural world, and the process productivity, workability and controllability can be improved remarkably, the amount of unreacted hydrogen fluoride can be reduced significantly, and the difficulty in filtering the product by finely divided particles can be avoided since the crystalline silica reactant is consumed completely, and since the used sulfuric acid can be isolated and recycled, the generation of discarded sulfuric acid can be minimized.

To achieve the object as explained above, the present invention provides a method of preparing silicon tetrafluoride (SiF₄) comprising: reacting crystalline silica having an average particle size of 60 micrometers or less and hydrogen fluoride (HF) in the presence of concentrated sulfuric acid at 80~140℃ to obtain silicon tetrafluoride.

By utilizing the method of the present invention, silicon tetrafluoride can be economically prepared with high yield from crystalline silica which exists abundantly in the natural world, instead of expensive amorphous silica material, and the process productivity, workability and controllability can be improved remarkably. In addition, the amount of unreacted hydrogen fluoride can be reduced significantly, and the difficulty in filtering the product by finely divided particles can be avoided since the crystalline silica reactant is consumed completely. Furthermore, since the used sulfuric acid can be isolated and recycled, the generation of discarded sulfuric acid can be minimized.

Figure 1 schematically represents an embodiment of the reaction device for performing the method of preparing silicon tetrafluoride according to the present invention in a continuous manner.

The present invention is explained in detail below.

In the method of preparing silicon tetrafluoride of the present invention, crystalline silica existing abundantly in the natural world is used. Examples of source materials of such crystalline silica include sand, quartzite and the like. Such source materials are pulverized and sorted to be used in the method of preparing silicon tetrafluoride of the present invention.

The average particle size of the crystalline silica used in the method of preparing silicon tetrafluoride of the present invention is 60 micrometers or less (for example, from 5 to 60 micrometers), and preferably 50 micrometers or less (for example, from 5 to 50 micrometers). If the average particle size of the crystalline silica is greater than 60 micrometers, the reactivity is insufficient and thus the intended yield (for example, a total yield of 70% or more) cannot be obtained. There is no particular limitation in the lower limit of the average particle size of the crystalline silica. However, if crystalline silica having too small average particle size (for example, less than 5 micrometers) is used, the costs for pulverizing and sorting the source materials in the natural world such as sand having mm size or bulky quartzite becomes too much, and there may be a problem in handling and transferring excessively fine particles. It was conventionally perceived that the use of excessively fine particles of silica would result in too vigorous a reaction and generation of bubbles on the surface of the reactant which causes the reactivity to decrease. However, in the present invention, because of the use of crystalline silica which has a mild reactivity as compared with amorphous silica, the reaction may be controlled even if the particle size becomes small (for example, 5 micrometers or less). In addition, the reactivity can be controlled through adjustment of various process parameters such as the reactant feeding rate and the like. Furthermore, if the preparation process is designed in a continuous manner as in the embodiment of the present invention, even though the reaction rate in the first reactor is somewhat low, the further reaction in the subsequent reactor can increase the total yield to 70% or more, and preferably 90% or more.

The crystalline silica used in the present invention is a cheap material obtainable from sand or quartzite existing abundantly in the natural world, whereas most amorphous silica materials used in conventional processes except for diatomite are artificial products rather than those existing in the natural world and thus are expensive.

In the method of preparing silicon tetrafluoride of the present invention, crystalline silica reacts with hydrogen fluoride (HF) in the presence of concentrated sulfuric acid. This reaction can be represented by the following reaction scheme:

SiO₂ + 4HF/H₂SO₄ → SiF₄ + 2H₂O/H₂SO₄

In the method of preparing silicon tetrafluoride of the present invention, there is no particular limitation to the use amount ratio of crystalline silica and hydrogen fluoride, and thus the use amount ratio may be selected appropriately according to need. In theory, the reaction may be conducted with the equivalent amount ratio as shown in the above reaction scheme. In order that no unreacted crystalline silica exists, crystalline silica may be used in an amount less than the theoretical equivalent amount of the above reaction scheme―for example, 85~95% of the theoretical equivalent amount of the above reaction scheme. On the contrary, in order that no unreacted hydrogen fluoride exists, hydrogen fluoride may be used in an amount less than the theoretical equivalent amount of the above reaction scheme―for example, 85~95% of the theoretical equivalent amount of the above reaction scheme.

More concretely, in the present invention, if crystalline silica is used in an amount corresponding to 85~95% of the theoretical equivalent amount of the hydrogen fluoride, it is possible that no solid silica remains in the reactor after the reaction, and in this case a filtering procedure may be omitted in a process of distilling and recycling diluted sulfuric acid after the reaction.

On the other hand, in the present invention, if hydrogen fluoride is used in an amount corresponding to 85~95% of the theoretical equivalent amount of the crystalline silica, it is possible that no hydrogen fluoride remains in the reactor after the reaction, and in this case a difficulty of reactor material selection due to HF may be overcome in the process of distilling and recycling diluted sulfuric acid after the reaction. That is, according to how the recycling process of sulfuric acid is performed, one of the above two modes may be taken selectively.

According to a preferable embodiment of the present invention, a difficulty in filtering of product due to fine particles may be avoided by using crystalline silica in an amount of 90% of the theoretical equivalent amount of the above reaction scheme. By adjusting the feeding amounts of silica and hydrogen fluoride as such, the diluted sulfuric acid finally generated may not contain the solid silica component or hydrogen fluoride, and thus the process of re-treating sulfuric acid may be performed conveniently.

In the method of preparing silicon tetrafluoride of the present invention, sulfuric acid is used to take up water generated as a result of the reaction between crystalline silica and hydrogen fluoride. A concentrated sulfuric acid having a concentration of 95% or more (for example, 98%) is used. There is no particular limitation to its use amount. Considering the productivity of the entire process and sulfuric acid re-treatment, the concentrated sulfuric acid is used in an amount that makes the concentration of diluted sulfuric acid discharged after the reaction become preferably 60% or more, and more preferably 75% or more. In addition, if sulfuric acid is introduced into a reactor through a generated gas outlet, since the unreacted HF, which is discharged along with SiF₄, is dissolved in sulfuric acid and reintroduced into the reactor, the loss of raw materials can be preferably prevented and a side-reaction of converting SiF₄ to hexafluorosilicic acid can be preferably suppressed by preventing the incorporation of moisture into SiF₄ gas.

In the method of preparing silicon tetrafluoride of the present invention, the reaction of crystalline silica and hydrogen fluoride is conducted at a temperature of 80~140℃, preferably 90~130℃ and more preferably 100~120℃. If the reaction temperature is lower than 80℃, the reaction becomes slow and thus the productivity decreases. If the reaction temperature is higher than 140℃, anhydrous hydrogen fluoride is not dissolved in sulfuric acid but is changed in gas phase and the reaction becomes difficult, resulting in the lowering of SiF₄ yield or the problem of escape from the reactor.

The method of preparing silicon tetrafluoride of the present invention may preferably be performed in a continuous manner by using plural reactors connected in series, by which a higher yield may be obtained.

Referring to Figure 1, the continuous process of preparing silicon tetrafluoride according to the present invention is explained more concretely below.

In Figure 1, crystalline silica having an average particle size of 60 micrometers or less is introduced into the first reactor A via line 1, and at the same time anhydrous hydrogen fluoride and concentrated sulfuric acid are introduced into the reactor via lines 2 and 3, respectively. Reactor A is maintained at 80~140℃, and the reactants are agitated by a mechanical stirrer. After a given time (for example, 20 minutes ~ 1 hour), SiF₄ gas generation begins and the generated SiF₄ gas is then collected via line 4. When reactor A is filled with the reaction mixture to a predetermined level (for example, about 80%, L_HA level) after a given time, the unreacted slurry mixture is transferred to the second reactor B in which further reaction is conducted to complete the reaction of the unreacted mixture. If necessary, a third reactor, a fourth reactor and the like may be further connected in series. For efficient operation of the present continuous process, the time for the further reaction in the second reactor may be set to the time for re-filling the reactants in the first reactor to a predetermined level (i.e., the time for the reaction mixture level in the first reactor to rise from L_LA to L_HA), and the time for the further reaction in the second reactor may be adjusted by controlling the feeding rate of reactants into the first reactor A. SiF₄ gas generated in the second reactor B is discharged out of the second reactor via line 6 and then collected via line 4. In a preferred embodiment for the continuous process of the present invention, by controlling the feeding rate of the reactants, 60 to 70 mole% of the total yield of silicon tetrafluoride (STF) was prepared in the first reactor and the further reaction was conducted in the second reactor, by which the total yield could be increased to 95 mole% or more.

The present invention is explained in more detail by the following Examples and Comparative Example. However, the scope of the present invention is not limited by them.

[Examples 1 and 2]

Silicon tetrafluoride was prepared by using the continuous-type reaction device as shown in Figure 1.

Into the Teflon-lined first reactor A of 1L volume, crystalline silica having an average particle size of 20 micrometers (SiO₂) was incorporated via line 1 at a feeding rate of 15g per hour (0.25mol/hr). At the same time, anhydrous hydrogen fluoride (HF) liquid was incorporated via line 2 at a feeding rate of 20g per hour (1mol/hr) and 38g of 98%-concentrated, cold sulfuric acid (~10℃) was incorporated via line 3 into reactor A, respectively.

The temperature inside reactor A was maintained at 120℃ by heating wire and the incorporated reactants were agitated by a mechanical stirrer. After 30 minutes from the start of the reactant feeding, SiF₄ gas began to generate and the generated SiF₄ gas was collected via line 4. Cold sulfuric acid was introduced through SiF₄ gas outlet line 4 in order to prevent unreacted hydrogen fluoride from escaping from the reactor along with the generated SiF₄ gas.

The gas discharged via line 4 was quenched for ingredient analysis by subsequently passing it through a trap containing sulfuric acid and a trap containing 5% aqueous solution of hydrogen fluoride. Through the sulfuric acid trap, the generation of water and unreacted HF escaping outside were checked. Through the hydrogen fluoride trap, the generated SiF₄ was converted to hexafluorosilicic acid which was then titrated to check the amount of generated SiF₄.

When the first reactor A was filled to L_HA level (80v/v% level) by incorporating the reactants for 16 hours, the transfer pump was operated to transfer the reaction mixture in reactor A to reactor B. In the second reactor B, the reaction was further conducted to be completion. The further reaction in reactor B was conducted at 120℃ for 15 hours in order that the unreacted hydrogen fluoride could be consumed completely. The SiF₄ gas generated at that time was discharged out of the second reactor via line 6 and was then analyzed via line 4 as explained above.

After the further reaction in the second reactor B, diluted sulfuric acid was discharged via line 7 and analyzed. After make-up to a proper concentration on the basis of the analysis result, it was recycled for use in the reaction again. Meanwhile, silica, hydrogen fluoride and sulfuric acid were continuously incorporated into the first reactor A in order to keep the reaction proceeding.

The following Table 1 shows the generated amounts of SiF₄ when anhydrous hydrogen fluoride and silica were used in the equivalent amount ratio (Example 1) and when the use amount of silica was reduced to 90% of that of anhydrous hydrogen fluoride (Example 2), and the analysis result of the diluted sulfuric acid discharged out of the second reactor B finally.

Table 1

SiF₄ yield: Based on the use amount of HF

ND: Not detected

[Examples 3~5 and Comparative Example]

By the same methods as those of Example 1 with the reaction conditions shown in the following Table 2, SiF₄ was prepared and the products were analyzed. The results are shown in Table 2.

Table 2

SiF₄ yield: Based on the use amount of HF

ND: Not detected

Reactant feeding rates for 16 hours

- Anhydrous hydrogen fluoride liquid: 20g/hr (1mole/hr)

- SiO₂: 15g/hr (0.25mole/hr)

- 98% H₂SO₄: 38g/hr

As can be seen from the above Table 2, when the average particle size of crystalline silica was 60 micrometers or less, a total yield of SiF₄ of 70% or more could be obtained. Accordingly, the amounts of unreacted HF and SiO₂ could be remarkably decreased as compared with the Comparative Example.

[Example 6]

By the same methods as those of Example 1 with the various reactant feeding rates and retention times in each reactor shown in the following Table 3, SiF₄ was prepared and the products were analyzed. The results are shown in Table 3. The raw material feeding rate to the first reactor was 1.5 times greater than that of Example 1, and the reaction time in the first reactor was 11 hours while that in the second reactor was 10 hours.

Table 3

SiF₄ yield: Based on the use amount of HF

[EXPLANATION OF THE SYMBOLS]

A: Teflon-lined 1^st reactor

B: Teflon-lined 2^nd reactor

P: Transfer pump

1: Silica inlet line

2: Anhydrous hydrogen fluoride liquid inlet line

3: Sulfuric acid inlet line (cold, ~10℃)

4: SiF₄ recovery line

5: 1^st reaction mixture transfer line

6: SiF₄ recovery line from 2^nd reactor

7: Reactant discharge line

8: Silica inlet line for controlling unreacted HF (optional)

L_HA - L_LA: Volume of reactant collected in reactor A for a given time

L_HB - L_LB: Volume of reactant transferred and further reacted for a given time

Claims

A method of preparing silicon tetrafluoride (SiF₄) comprising:

reacting crystalline silica having an average particle size of 60 micrometers or less and hydrogen fluoride (HF) in the presence of concentrated sulfuric acid at 80~140℃ to obtain silicon tetrafluoride.
The method of preparing silicon tetrafluoride according to claim 1, wherein the crystalline silica is used in an amount corresponding to 85~95% of the theoretical equivalent amount of the hydrogen fluoride.
The method of preparing silicon tetrafluoride according to claim 1, wherein the hydrogen fluoride is used in an amount corresponding to 85~95% of the theoretical equivalent amount of the crystalline silica.
The method of preparing silicon tetrafluoride according to claim 1, wherein the concentrated sulfuric acid is used in an amount that makes a concentration of diluted sulfuric acid discharged after reaction become 60% or more.
The method of preparing silicon tetrafluoride according to claim 1, wherein the concentrated sulfuric acid is introduced into a reactor through a generated gas outlet.
The method of preparing silicon tetrafluoride according to claim 1, which is performed in a continuous manner by using plural reactors connected in series.
The method of preparing silicon tetrafluoride according to claim 6, wherein two reactors connected in series are used.