WO2011112097A1

WO2011112097A1 - Method for production of trichlorosilane from silicon, hydrogen and silicon tetrachloride

Info

Publication number: WO2011112097A1
Application number: PCT/NO2011/000075
Authority: WO
Inventors: Harry Morten Rong; Torbjørn RØE; Jan-Otto Hoel; Henning KJØNLI; Bodil Eik-Nes
Original assignee: Elkem ASA
Current assignee: Elkem ASA
Priority date: 2010-03-12
Filing date: 2011-03-04
Publication date: 2011-09-15
Anticipated expiration: 2012-09-12
Also published as: NO20100358A1

Abstract

The present invention relates to a method for the production of trichlorosilane by reaction of silicon with silicon tetrachloride gas and hydrogen gas at a temperature between 300°C and 8000C, and an absolute pressure of 0.5 - 40 atm in a fluidized bed reactor, in a stirred bed reactor or a solid bed reactor, where the silicon supplied to the reactor contains between 50 and 10 000 ppm chromium. The invention further relates to a method for the production of trichlorosilane by reaction of silicon with silicon tetrachloride gas and hydrogen gas, where the chromium content in the reactor is maintained between 100 and 50 000 ppm.

Description

Title of Invention

Method for production of trichlorosilane from silicon, hydrogen and silicon tetrachloride

Field of Invention

The present invention relates to a method for the production of trichlorosilane by reaction of silicon by a mixture of silicon tetrachloride gas and hydrogen gas, and to production of silicon for the use in production of trichlorosilane.

Background Art

In the method of production of trichlorosilane (TCS), metallurgical grade silicon is reacted with silicon tetrachloride (STC) gas and hydrogen gas in a fluidized bed reactor, solid bed reactor or in a stirred bed reactor (US patent 4676967). The process is generally carried out at a temperature between 400 and 600°C.

Si + 3SiCI₄ + 2H₂ = 4HSiCI₃

TCS can also be produced by reacting metallurgical grade with hydrogen chloride (HCI) gas in a fluidized bed reactor, solid bed reactor or in a stirred bed reactor. This process is generally carried out at a temperature between 250 and 1100°C. In the reaction, other volatile silanes than TCS are also formed, mainly STC. The amount of STC is typically about 10-85% depending on the reactor temperature, impurity content in the metallurgical silicon and any catalysts added to the reactor.

Si + 3HCI = HSiCI₃ + H₂

Si + 4HCI = SiCI₄ + 2 H₂

STC is also produced in large amounts during decomposition of TCS to pure silicon and in the redistribution reactions to produce silane (SiH₄) from TCS. 4HSiCI₃ = Si + 3SiCI₄ + 2H₂

4HSiCI₃ = SiH₄ + 3SiCI₄

STC is mainly used to produce silicon dioxide (fumed silica) by reacting STC with hydrogen and oxygen. Some STC is also used to produce optical fibres or sold to other applications. In general too much STC is available in the market and recycling is required to balance the production.

The advantage of reacting STC with metallurgical silicon and hydrogen is that the by-product from the decomposition or redistribution step is recycled to TCS. Recycling will reduce the amount of silicon required to produce polysilicon since the silicon that otherwise produces STC will be converted to polysilicon. Only part of the STC can be converted to TCS in one reactor pass and maximum conversion of STC is given by the equilibrium composition. Complete conversion of the STC requires multiple reactor passes and subsequent distillations to remove TCS. Normally, the conversion of STC in one reactor pass is lower than predicted from the equilibrium composition due to the fact that reaction kinetics are important at these temperatures. Additions of catalysts to the reactor will increase the conversion (closer to the equilibrium composition), and well known catalysts for this process is copper (any copper source will work) and/or iron (any iron source will work). Iron is always present in metallurgical silicon and will act as a catalyst to increase conversion.

Increased temperature will reduce the effect of reaction kinetics, but also reduce the equilibrium composition of TCS. A minimum temperature is required in order to make the reaction possible. Higher pressure will favor a higher TCS formation. Diagrams showing the equilibrium conversion of STC as a function of temperature and pressure are shown in Figures 1 and 2.

Metallurgical grade silicon contains a number of contaminating elements like Fe, Ca, Al, Mn, Ni, Zr, Cr, O, C, Zn, Ti, B, P and others. Some contaminants will either be inert to STC, or form solid, stable chlorides. The stable metal chlorides will, depending on their size, either be blown out of the reactor with the product gas or be accumulated in the reactor. Other contaminants like Al, Zn, Ti, B and P normally form volatile metal chlorides, which leave the reactor together with the silanes produced.

O and C are enriched in slag particles in the silicon that do not react or react very slowly with STC and tend to accumulate in the reactor. The smallest slag particles can be blown out of the reactor and trapped in the filter systems. Species that accumulate will take up space in the reactor, leaving less space for silicon and thereby reducing the effective surface area of silicon. This gives a less effective reaction. In such a way, a chemically inert compound may actually influence the reaction.

Many of the contaminants in metallurgical grade silicon influence the performance of the silicon in the process of producing trichlorosilane by reaction of silicon with STC and hydrogen.

Disclosure of Invention

It has now been found that silicon having increased chromium content gives a higher amount of TCS in the product gas during the reaction of silicon with silicon tetrachloride gas and hydrogen gas, hence increasing the value of the silicon in this process. It has further been found that if the chromium content in the reactor is controlled within certain limits an increased amount of TCS in the product gas is obtained.

According to a first aspect, the present invention relates to a method for the production of trichlorosilane by reaction of silicon with silicon tetrachloride gas and hydrogen gas at a temperature between 300° and 800°C and an absolute pressure of 0.5 - 40 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, which method is characterised in that the silicon supplied to the reactor contains between 50 and 10 000 ppm of chromium. Preferably the silicon supplied to the reactor contains between 75 and 1500 ppm chromium and more preferably between 100 and 1000 ppm.

The chromium is then alloyed with the silicon, is mechanically mixed with the silicon or is added to the reactor separately.

The chromium can be alloyed to the silicon in the furnace process for production of metallurgical grade silicon, in the refining ladle or in the casting step. Adding chromium to the furnace can be done in several ways. For instance by addition of chromium containing raw materials to the furnace, using electrodes or electrode casing/ribs containing chromium or any other addition of chromium to the furnace. Chromium can also be added to the silicon during tapping of the furnace for instance by using chromium containing tapping tools or chromium containing materials in the tapping of the silicon from the furnace into the refining ladle.

Chromium can also be added to the silicon in the refining ladle. Any chromium compound added will be reduced by silicon to metallic chromium that will form different intermetallic phases when the silicon solidifies. Different ratios of the main impurities like iron, aluminium, calcium and iron can form different intermetallic phases with chromium.

Chromium can also be added to the silicon in the casting step, for instance by adding a chromium compound to the molten silicon, by using chromium compounds or chromium containing silicon in the casting moulds or by casting the silicon on a surface of a material containing chromium.

Chromium can also be mechanically mixed with silicon. One preferred way of mechanically mixing chromium with the silicon is to subject the silicon to grinding using chromium containing grinding bodies, such as for example chromium containing steel balls.

The silicon used according to the present invention is produced in conventional way in carbothermic reduction furnaces. The chromium content in the silicon can either be regulated and controlled by selection of raw materials, adding chromium to the furnace, using electrodes or electrode casings containing chromium or chromium may be added to molten silicon in the ladle after the silicon has been tapped from the reduction furnace.

It has surprisingly been found that the addition of chromium to silicon improves the conversion of silicon tetrachloride in the process of producing trichlorosilane by reaction of silicon with silicon tetrachloride gas and hydrogen gas.

According to a second aspect the present invention relates to a method for the producing of trichlorosilane by reaction of silicon with silicon tetrachloride gas and hydrogen gas at a temperature between 300° and 800° C and an absolute pressure of 0.5 - 40 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, which method is characterised in that chromium or chromium containing species is added to the reactor in an amount necessary to control a chromium content in the reactor of between 100 and 50 000 ppm based on the weight of silicon in the reactor.

Preferably chromium is supplied to the reactor in an amount necessary to control the chromium content in the reactor to between 250 and 25 000 ppm.

Chromium can be supplied to the reactor in any suitable way . The chromium can be supplied to the reactor alloyed with silicon, mechanically mixed with silicon or added separately to the reactor. According to one embodiment the chromium compounds are added to the reactor with reactant gases. Chromium can also be added to the reactor together with compounds having another or no effect on the trichlorosilane process.

Short description of the drawings

Figure 3 shows a diagram for the conversion of STC to TCS in a quartz reactor at 550°C and 10 bar (absolute). The silicon sample was mixed with 10 000 ppm (1% by weight) chromium (metallic) according to the present invention and compared with the STC conversion according to prior art.

Figure 4 shows a diagram for the conversion of STC to TCS in a quartz reactor at 550°C and 10 bar (absolute). The silicon sample was pure silicon alloyed with 471 ppm Cr according to the present invention and compared with the STC conversion according to prior art.

Detailed description of the invention

The following example 1 was carried out in a laboratory fluid bed reactor made from quartz and embedded in a heated iron block. The temperature of the heating block was kept at 550°C. Due to the low ΔΗ of the reaction, the temperature of the reaction mass is assumed to also be 550°C. The pressure of the reactor was kept at 10 bara. 10 gram of commercially available silicon having a particle size of between 125 and 180 μπτ» was mixed with copper chloride (0.16g) and chromium metal (0.1g) and added to the quartz reactor. A mixture of silicon tetrachloride gas, hydrogen gas and argon gas (inert, reference for GC) in amounts of 252 Nl/min STC, 504 Nml/min H₂ and 84 Nl/min Ar was supplied to the reactor. The composition of the product gas from the reactor was measured with a GC. STC conversion was measured as % of the supplied STC being converted to TCS in the reactor. The experiment was run continous, and silicon reacting was replaced by additions of fresh silicon, also containing 10 000 ppm (1% by weight) chromium, admixed. The conversion of silicon tetrachloride of the sample was measured as first pass conversion. The experiment is compared to experiments with STC conversion according to prior art.

The following example 2 was carried out in a laboratory fluid bed reactor made from quartz and embedded in a heated iron block. The temperature of the heating block was kept at 550°C. Due to the low ΔΗ of the reaction, the temperature of the reaction mass is assumed to also be 550°C. The pressure of the reactor was kept at 10 bar (absolute). 10 gram of pure silicon alloyed 471 ppm chromium, having a particle size of between 125 and 180 μιη was mixed with copper chloride (0.16g) and added to the quartz reactor. A mixture of silicon tetrachloride gas, hydrogen gas and argon gas (inert, reference for GC) in amounts of 252 Nl/min STC, 504 Nl/min H₂ and 84 Nl/min Ar was supplied to the reactor. The composition of the product gas from the reactor was measured with a GC. STC conversion was measured as % of the supplied STC being converted to TCS in the reactor. The experiment was run continous, and silicon reacting was replaced by additions of fresh silicon, also pure silicon alloyed 471 ppm chromium. The conversion of silicon tetrachloride of the sample was measured as first pass conversion. The experiment is compared to experiments with STC conversion according to prior art.

Example 1

The chemical analysis of silicon samples A and B are shown in Table 1.

Table 1.

Samples A according to the invention containing 10000 ppm chromium was made by mixing sample B with 10000 ppm chromium. Sample B is Silgrain® silicon, produced by Elkem AS, screened to 125-180 microns. Samples A and B were used to produce trichlorosilane in a laboratory fluid-bed reactor described above. The amount of STC converted to TCS from sampleA and sample B are shown in Figure 3.

As can be seen from Figure 3, the addition of 10 000 ppm (1 % by weight) of chromium to silicon sample B resulted in a substantial increase in STC conversion. Example 2

A high-purity silicon (polysilicon) was melted and alloyed with chromium in an induction furnace and cast in inert atmosphere. The sample was crushed and screened to a particle size between 125 and 180 μιτι and named sample C.

The chemical analysis of silicon sample C is shown in Table 2.

Table 2

Samples C according to the invention and sample B according to the prior art were used to produce trichlorosilane in a laboratory fluid-bed reactor described above. The amount of STC converted to TCS from samples C and B are shown in Figure 4.

As can be seen from Figure 4, alloying chromium to the high purity silicon resulted in a substantial increase in STC conversion to TCS compared to the standard silicon sample B.

Claims

Claims:

1. Method for the production of trichlorosilane by reaction of silicon with silicon tetrachloride gas and hyrogen gas at a temperature between 300°C and 800°C, and an absolute pressure of 0.5 - 40 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, characterized in that the silicon supplied to the reactor contains between 50 and 10000 ppm chromium.

2. Method according to claim ^characterized in that the silicon supplied to the reactor contains between 75 and 1500 ppm chromium.

3. Method according to claim 1 and 2, characterized in that the silicon supplied to the reactor contains between 100 and 1000 ppm chromium.

4. Method according to claim 1 - 3, c h a r a c t e r i z e d i n that chromium is alloyed with the silicon.

5. Method according to claim 1 -3, characterized in that chromium is mechanically mixed with the silicon before the silicon is supplied to the reactor.

6. Method according to claim 5, characterized in that chromium is mechanically mixed with silicon by subjecting the silicon to grinding using chromium-containing grinding bodies.

7. Method according to claim 1 -3, characterized in that chromium is added to the reactor separately from the silicon.

8. Method for the production of trichlorosilane by reaction of silicon with silicon tetrachloride gas and hydrogen gas at a temperature between 300 and 800°C, and an absolute pressure of 0.5 - 40 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, ch a racte rized i n that chromium is supplied to the reactor in an amount necessary to control a chromium content in the reactor of between 100 and 50000 ppm based on the weight of silicon in the reactor.

9. Method according to claim 8, characterized in that chromium is supplied to the reactor in an amount necessary to control the chromium content in the reactor between 200 and 25000 ppm chromium.

10. Method according to claim 8 or 9, c h a r a c t e r i z e d i n that chromium supplied to the reactor is alloyed with the silicon.

11. Method according to claim 8 or 9, c h a r a c t e r i z e d i n that chromium supplied to the reactor is mechanically mixed with the silicon before the mixture is supplied to the reactor.

12. Method according to claim 11, characterized in that chromium is mechanically mixed with silicon by subjecting the silicon to grinding using chromium-containing grinding bodies.

13. Method according to claim 8 or 9, c h a r a c t e r i z e d i n that chromium and silicon are added separately to the reactor.

14. .Method according to claim 13, c h a r a c t e r i z e d i n that the chromium compounds are added to the reactor with the reactant gases.

15. Method according to claim 8 or 9, cha racterized i n that the chromium is added to the reactor together with a compound having another or no effect on the thriclorosilane process.