Patent Application
20110305621
The present invention relates to a method of continuously producing tetrafluorosilane (SiF4) by using various fluorinated materials, amorphous silica (SiO2) and sulfuric acid (H2SO4). More specifically, the present invention relates to a method of producing tetrafluorosilane by reacting in a single reactor (i) a fluoride source material capable of reacting with sulfuric acid to generate hydrogen fluoride (HF), (ii) amorphous silica and (iii) sulfuric acid; and then passing the obtained gaseous product through an H2SO4 scrubber. According to the present invention, the yield of tetrafluorosilane can increase and it can be continuously produced in an environmentally friendly manner with low cost by using various fluorinated materials other than hydrogen fluoride. In addition, the amount of hydrogen fluoride (i.e., hydrofluoric acid) generated during the reaction is minimized and thus the corrosion of devices can be minimized, and the pipeline blockage and yield decrease of SiF4 can be prevented by passing the reaction product which is a mixture gas of tetrafluorosilane (SiF4) and water through an H2SO4 scrubber at a high temperature to remove water, which can inhibit the generation of silica gel and hexafluorosilicic acid (H2SiF6) by the side-reaction of condensed water and SiF4.
Tetrafluorosilane (SiF4) has been used in the semiconductor manufacturing industry as a fluorine-doping agent for optical fibers based on quartz, a raw material of photomask for semiconductor lithography and in chemical vapor deposition (CVD) for semiconductor manufacturing. Its purity has grown in importance as the integration level of electronic devices and their performance have gotten higher. Recently, tetrafluorosilane has become a very important basic material, increasingly needed as a precursor of monosilane (SiH4), which is a raw material for preparing polysilicon for solar cells, or the like.
Tetrafluorosilane is known to be prepared by the thermal dehydration reaction of hexafluorosilicic acid (H2SiF6) concentrate, which is conventionally generated as a by-product in the production of phosphate fertilizer, with sulfuric acid (WO 2005/030642) or by the thermal decomposition reaction of the solid salt of hexafluorosilicic acid (M2SiF6, wherein M=Na, K) prepared from hexafluorosilicic acid (U.S. Pat. No. 2,615,872). However, since these conventional methods thermally decompose hexafluorosilicic acid generated as a by-product of phosphate fertilizer production, for the scale-up of tetrafluorosilane production, the process for phosphate fertilizer production has to be modified or scaled up.
U.S. Pat. No. 4,382,071 suggests a method for producing tetrafluorosilane by reacting hydrogen fluoride dissolved in sulfuric acid with silica. However, this method has a problem in that hydrogen fluoride should be preliminarily prepared since it is used as a starting material.
U.S. Pat. No. 6,770,253 suggests a method for producing tetrafluorosilane by reacting metallurgical silicon (Si) with hydrogen fluoride (HF) at a high-temperature condition such as 300° C. or higher. However, this method has a problem in that the production cost is high because the metallurgical silicon (Si) powder as used is very expensive.
To resolve the problems of prior arts as explained above, the present invention has an object of providing a method of producing tetrafluorosilane wherein hydrogen fluoride and tetrafluorosilane can be produced continuously in a single reactor, by which the yield of tetrafluorosilane can increase and thus the amount of unreacted hydrogen fluoride is minimized and accordingly the corrosion of devices can be minimized, and thus tetrafluorosilane can be produced at low cost in an environmentally friendly manner, and in addition, the pipeline blockage and yield decrease of SiF4 can be prevented by inhibiting the generation of silica gel and hexafluorosilicic acid (H2SiF6) by the side-reaction of water and SiF4.
To achieve the object as explained above, the present invention provides a method of producing tetrafluorosilane comprising the steps of: (1) reacting in a single reactor (i) a fluoride source material capable of reacting with sulfuric acid to generate hydrogen fluoride (HF), (ii) amorphous silica and (iii) sulfuric acid; and (2) passing the obtained gaseous product of step (1) through an H2SO4 scrubber.
According to the present method, hydrogen fluoride generated by the reaction of the fluoride source material and sulfuric acid can effectively react with the amorphous silica, and thus the amount converted to tetrafluorosilane can be maximized. Also, the effective removal of water from the reaction product inhibits the generation of silica gel and hexafluorosilicic acid (H2SiF6), which is a by-product of the reaction of water and SiF4, and thus the pipeline blockage problem in the process and yield decrease of SiF4 do not occur. Furthermore, the excellent reaction efficiency minimizes the amount of unreacted hydrogen fluoride, and thus the process can be operated safely from the corrosion of devices thereby. In addition, sodium aluminum tetrafluoride (NaAlF4), which is a by-product generated in monosilane production, or the like can be used as the fluoride source material in the present method, and for the silica, by-products of other industrial processes such as silica fume, cullet, Diatomaceous earth, kaolin, fumed silica, fly ash, slag, activated clay, silica gel, etc. can be used as raw materials. Accordingly, the present method can be operated under more preferred conditions in terms of environmental friendliness and economy.
The present invention is explained in detail below.
In the present method of producing tetrafluorosilane, a material which can generate hydrogen fluoride (HF) by the reaction with sulfuric acid is used as the fluoride source material. Examples of such fluoride source material include sodium aluminum tetrafluoride (NaAlF4), chiolite (Na5Al3F14.2AlF3), cryolite (Na3AlF6), calcium fluoride (CaF2), sodium fluoride (NaF), aluminum fluoride (AlF3), etc. and mixtures thereof. Each of the above fluoride source materials reacts with sulfuric acid to generate hydrogen fluoride according to the following reaction formulas, respectively.
NaAlF4+2H2SO4→4HF+NaAl(SO4)2 Reaction Formula 1
CaF2+H2SO4→2HF+CaSO4 Reaction Formula 2
Na5Al3F14·2AlF3+10H2SO4→20HF+5NaAl(SO4)2 Reaction Formula 3
Na3AlF6+3H2SO4→6HF+Na3Al(SO4)3 Reaction Formula 4
2NaF+H2SO4→2HF+Na2SO4 Reaction Formula 5
2AlF3+3H2SO4→6HF+Al2(SO4)3 Reaction Formula 6
That is, when reacting with sulfuric acid, sodium aluminum tetrafluoride is converted to sodium aluminum sulfate (NaAl(SO4)2, Reaction Formula 1), calcium fluoride is converted to calcium sulfate (CaSO4, Reaction Formula 2), chiolite is converted to sodium aluminum sulfate (NaAl(SO4)2, Reaction Formula 3), cryolite known as belonging to the monoclinic system is converted to sodium aluminum sulfate (Na3Al(SO4)6, Reaction Formula 4), sodium fluoride is converted to sodium sulfate (Na2SO4, Reaction Formula 5) and aluminum fluoride is converted to aluminum sulfate (Al2(SO4)3, Reaction Formula 6), generating hydrogen fluoride.
In the present method of producing tetrafluorosilane, the purity of the fluoride source material is 90% or more (e.g., 90 to 99%), preferably 93% or more (e.g., 93 to 99%) and more preferably 95% or more (e.g., 95 to 99%), based on weight. The particle size of the fluoride source material properly ranges from 10 to 2,000 μm, preferably from 40 to 1,700 μm and more preferably from 50 to 1,500 μm.
Among the above fluoride source materials, as the sodium aluminum tetrafluoride compound, the present method can use a by-product generated during the process for preparing monosilane by reacting tetrafluorosilane (SiF4) gas with sodium aluminum tetrahydride (NaAlH4) as a reducing agent as shown in the following Reaction Formula 7, or a product prepared by mechanically milling a mixture of aluminum trifluoride (AlF3) and sodium fluoride (NaF), preferably at high temperature, as shown in the following Reaction Formula 8.
SiF4+NaAlH4→SiH4+NaAlF4 Reaction Formula 7
AlF3+NaF→NaAlF4 Reaction Formula 8
In the present method of producing tetrafluorosilane, silica in amorphous form is used as the silicon source. Examples of such amorphous silica include silica fume, which is a microsilica particle obtained by collecting and filtering the gaseous material generated in the production of ferrosilicon and silicon metal; cullet, which is generated in the glass-production process and conventionally broken or discarded; Diatomaceous earth, which is a kind of soft rock and soil made from diatom's body; kaolin, which mainly consists of kaolinite and halloysite and is formed from feldspar through chemical weathering by carbonic acid and water; fumed silica, which is prepared by the pyrolysis reaction of silicon tetrachloride or the like; fly ash, which is coal cinder collected by using a dust collector from a chimney gas of a boiler for burning pulverized coal; slag, which is the remainder after the abstraction of metal from ore; activated clay which is a porous material and also used as an adsorbent; silica gel; etc. and mixtures thereof.
In the present method of producing tetrafluorosilane, the content of SiO2 in the amorphous silica raw material conventionally ranges from 25 to 100% based on weight, and may vary according to concrete materials.
Silica fume has an SiO2 content ranging conventionally from 80 to 99%, preferably from 85 to 99% and more preferably from 90 to 99% by weight, and a particle size ranging conventionally from 10 to 500 μm, preferably from 20 to 300 μm and more preferably from 50 to 200 μm.
Diatomaceous earth has an SiO2 content ranging conventionally from 80 to 99% and preferably from 85 to 99% by weight, and a particle size ranging conventionally from 10 to 2,000 μm, preferably from 30 to 1,700 μm and more preferably from 50 to 1,500 μm.
Cullet has an SiO2 content ranging conventionally from 60 to 90% and preferably from 80 to 90% by weight, and a particle size ranging conventionally from 10 to 2,000 μm, preferably from 10 to 1,500 μm and more preferably from 10 to 1,000 μm.
Kaolin has an SiO2 content ranging conventionally from 60 to 90%, preferably from 70 to 90% and more preferably from 80 to 90% by weight, and a particle size ranging conventionally from 10 to 1,000 μm, preferably from 10 to 800 μm and more preferably from 50 to 700 μm.
Fumed silica has an SiO2 content ranging conventionally from 60 to 100%, preferably from 70 to 100% and more preferably from 80 to 100% by weight, and a particle size ranging conventionally from 10 to 500 μm, preferably from 10 to 300 μm and more preferably from 50 to 200 μm.
Fly ash has an SiO2 content ranging conventionally from 60 to 90%, preferably from 70 to 90% and more preferably from 80 to 90% by weight, and a particle size ranging conventionally from 10 to 500 μm, preferably from 10 to 300 μm and more preferably from 50 to 200 μm.
For the slag, iron slag, copper slag or both may be used. Slag has an SiO2 content ranging conventionally from 20 to 40%, preferably from 25 to 40% and more preferably from 30 to 40% by weight, and a particle size ranging conventionally from 10 to 500 μm, preferably from 10 to 300 μm and more preferably from 50 to 200 μm.
Activated clay has an SiO2 content ranging conventionally from 50 to 90%, preferably from 60 to 90% and more preferably from 65 to 90% by weight, and a particle size ranging conventionally from 10 to 500 μm, preferably from 10 to 300 μm and more preferably from 50 to 200 μm.
Silica gel has an SiO2 content ranging conventionally from 50 to 100%, preferably from 60 to 100% and more preferably from 65 to 100% by weight, and a particle size ranging conventionally from 10 to 2,000 μm, preferably from 30 to 1,700 μm and more preferably from 50 to 1,500 μm.
In the present method of producing tetrafluorosilane, the purity of sulfuric acid used ranges from 80 to 100%. Sulfuric acid is used in an amount of 1 to 5 times, preferably 1 to 3 times and more preferably 1 to 2 times the theoretically equivalent amount required to be reacted with the fluoride source material as explained above.
The present method of producing tetrafluorosilane is typically carried out in a rotary kiln reactor in a continuous manner. To increase the reaction efficiency, a kneader reactor may be added before the kiln reactor, or the inner space of the kiln reactor may be designed to have a dual-tube structure. An inner screw may be placed therein to pulverize and disperse bulky solids, by which the reactivity can increase. The reaction has a two-step mechanism consisting of: the first reaction step of generating hydrogen fluoride by reacting sulfuric acid and the fluoride source material; and the second reaction step of producing tetrafluorosilane by reacting the generated hydrogen fluoride and silica fed thereto continuously. Examples of the first reaction step include reactions using various fluorinated compounds such as Reaction Formulas 1 to 6 above. Examples of the second reaction step include the reaction of HF generated in the kiln and a raw material of silica (SiO2) such as the following Reaction Formula 9.
4HF+SiO2→SiF4+2H2O Reaction Formula 9
HF gas generated in the kiln by the first reaction step should react with a raw material of silica (SiO2) and convert to SiF4 before it is discharged from the kiln. For this, the present invention uses amorphous silica having a good reactivity with HF—i.e., silica fume, cullet, Diatomaceous earth, kaolin, fumed silica, fly ash, slag, activated clay, silica gel and the like.
The reaction temperature inside the kiln reactor is 150 to 800° C., preferably 200 to 700° C. and more preferably 250 to 600° C., and the reaction is carried out under the operation pressure inside the kiln reactor of at least −1,000 mmH2O in order to smoothly transfer the gas generated by the reaction. The upper limit of the operation pressure is not especially limited, and thus the reaction may be carried out under the condition of atmospheric pressure or higher.
After the second reaction step, the generated gaseous product containing SiF4, water and a small amount of HF gas is passed through a sulfuric acid (H2SO4) scrubber in which water and HF are removed, and the purified product of SiF4 is obtained. The purified SiF4 is then transferred to a storage tank and stored therein. According to an embodiment of the present invention, the H2SO4 scrubber is operated at a temperature condition of preferably 10 to 150° C. and more preferably 10 to 100° C. The gaseous product coming out of the reactor, which is a high-temperature mixture gas of SiF4, water vapor (H2O) and a small amount of HF, is preferably transferred from the reactor to the H2SO4 scrubber while maintaining the temperature at its dew point or higher (preferably 100° C. or higher, for example, 100 to 200° C.). If the transfer temperature is lower than the dew point, moisture may be generated by the condensation of water vapor and the moisture may react with SiF4 to form silica gel or H2SiF6, which can cause pipeline blockage and a yield decrease due to loss of tetrafluorosilane (the following Reaction Formulas 10 and 11, respectively).
SiF4(g)+2H2O(l)→SiO2(s, silica gel)+4HF(g) Reaction Formula 10
2HF(aq)+SiF4(g)→H2SiF6(aq) Reaction Formula 11
According to an embodiment of the present invention, a rotary kiln reaction facility as shown in
The present invention is explained in more detail by the following working examples and comparative examples. However, the scope of the present invention is not limited by the working examples.
By using the rotary kiln reactor as shown in
The dried raw materials of sodium aluminum tetrafluoride (6.87 kg/hr) and silica fume having an SiO2 content of 90% (3.66 kg/hr) were fed into the reactor through the line (1) and at the same time, sulfuric acid having a concentration of 98% (10.7 kg/hr) was fed through the line (2).
The reactor was equipped with an inner screw therein for smooth agitation of the raw materials. Tetrafluorosilane gas was generated immediately after the feeding of the reactants. The gas discharged through the line (3) was passed through the H2SO4 scrubber and then collected. After maintaining the reaction for 12 hours, the products sampled from the H2SO4 scrubber and the final storage tank were analyzed. The analysis results are shown in Table 1 below.
In Examples 2 to 5, the fluoride source materials were changed as shown in Table 1 below. As the raw material of silica, silica fume having an SiO2 content of 90% (3.66 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 1 below.
In Examples 6 to 10, the fluoride source materials were changed as shown in Table 2 below. As the raw material of silica, Diatomaceous earth having an SiO2 content of 88% (3.74 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 2 below.
In Examples 11 to 15, the fluoride source materials were changed as shown in Table 3 below. As the raw material of silica, cullet having an SiO2 content of 71% (4.63 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 3 below.
In Examples 16 to 20, the fluoride source materials were changed as shown in Table 4 below. As the raw material of silica, kaolin having an SiO2 content of 80% (4.11 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 4 below.
In Examples 21 to 25, the fluoride source materials were changed as shown in Table 5 below. As the raw material of silica, fumed silica having an SiO2 content of 98% (3.29 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 5 below.
In Examples 26 to 30, the fluoride source materials were changed as shown in Table 6 below. As the raw material of silica, fly ash having an SiO2 content of 54% (6.09 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 6 below.
In Examples 31 to 35, the fluoride source materials were changed as shown in Table 7 below. As the raw material of silica, slag having an SiO2 content of 35% (9.40 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 7 below.
In Examples 36 to 40, the fluoride source materials were changed as shown in Table 8 below. As the raw material of silica, activated clay having an SiO2 content of 75% (4.39 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 8 below.
In Examples 41 to 45, the fluoride source materials were changed as shown in Table 9 below. As the raw material of silica, silica gel having an SiO2 content of 90% (3.66 kg/hr) was used. The device and procedure of the production were the same as those of Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The analysis results are shown in Table 9 below.
Instead of silica fume as in Example 1, crystalline silica having a particle size of about 100 μm and an SiO2 content of 98% or more was used as the raw material of silica. The various fluoride source materials as shown in Table 10 below and the crystalline silica were fed through the line (1) and at the same time, sulfuric acid having concentration of 98% (10.7 kg/hr) was fed through the line (2). After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The yields of tetrafluorosilane gas were 30 to 35% as shown in Table 10 below.
After milling of the crystalline silica having a particle size of about 100 μm in Comparative Example 1 to have a particle size of about 20 μm, the obtained crystalline silica having a particle size of about 20 μm was used as the raw material of silica. The device and procedure of the production were the same as those of Comparative Example 1. After the 12-hour reaction, the generated gas was analyzed in the same manner as Example 1. The yields of tetrafluorosilane gas were 35 to 40% as shown in Table 11 below.
According to Tables 1 to 11 above, it can be known that Examples 1 to 45 of the present invention produced tetrafluorosilane gas with high yields whereas Comparative Examples 1 to 10 produced tetrafluorosilane gas with low yields. In addition, in the examples of the present invention, discarded materials or by-products of other industrial processes such as cullet and silica fume could be used as raw materials to produce tetrafluorosilane by an environmentally friendly and economic process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2010-0055401 | Jun 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR10/06136 | 9/9/2010 | WO | 00 | 10/25/2010 |
