WO2006031120A1

WO2006031120A1 - Method for production of trichlorosilane, method for production of silicon and silicon for use in the production of trichlorosilane

Info

Publication number: WO2006031120A1
Application number: PCT/NO2005/000310
Authority: WO
Inventors: Geir Johan Andersen; Jan-Otto Hoel; Harry Morten Rong; Torbjørn RØE; Harald Arnljot ØYE
Original assignee: Elkem As
Priority date: 2004-09-13
Filing date: 2005-08-29
Publication date: 2006-03-23
Also published as: NO20043828D0; NO20043828L

Abstract

The present invention relates to a method for the production of trichlorosilane by reaction of silicon with HCI gas at a temperature between 250 and 1100°C, and a pressure of 0,1-30 atm in a fluidized bed reactor, in a stirred bed reactor or a solid bed reactor, where the silicon supplied to the reactor contains less than 100 ppm manganese. The invention further relates to a method for producing silicon for use in the production of trichlorosilane by reaction of silicon with HCI gas, said silicon containing less than 100 ppm manganese.

Description

Title of Invention

Method for production of trichlorosilane, method for production of silicon and silicon for use in the production of trichlorosilane.

Field of Invention

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

Background Art In the method of production of trichlorosilane (TCS), metallurgical grade silicon is reacted with HCI gas in a fluidized bed reactor, solid bed reactor or in a stirred bed reactor. The process is generally carried out at a temperature between 250 and 1100⁰C. In the reaction other volatile silanes than TCS are formed, mainly silicon tetrachloride (STC). Since TCS normally is the preferred product, the selectivity of the reaction given as the molar ratio of TCS/(TCS + other silanes) is an important factor. The other important factor is the reactivity of the silicon, normally measured as first pass HCI conversion. Preferably more than 90% of HCI is converted to silanes, but industrially lower reactivity can be observed. Reactivity of a silicon can also be expressed by its ignition temperature; the necessary temperature for the silicon to react with HCI.

The selectivity and reactivity will depend strongly on the process temperature when silicon and HCI is reacted. According to the equilibrium calculation the amount of TCS should be about 20 - 40% (remaining is mainly STC) in the temperature range given above. However, in practical terms a significantly higher TCS selectivity is observed, and at temperatures below 400°C it is possible to observe TCS selectivity of more than 90%. The reason for this big deviation from equilibrium is that the product composition is given by kinetic limitations (formation of active species on the silicon surface). Higher temperature will move the product distribution towards the equilibrium composition and the gap between the observed selectivity and the calculated selectivity will get smaller. Reactivity will increase with higher temperature. Coarser silicon particles (lumps) can therefore be used when the temperature is increased and still maintaining close to 100% HCI conversion.

Higher pressure will favour a higher TCS selectivity.

Metallurgical grade silicon contains a number of contaminating elements like Fe, Ca, Al, Mn, Ni, Zr, O, C, Zn, Ti, B, P and others. Some contaminants will either be inert to HCI, or like Fe and Ca form solid, stable chlorides. The stable metal chlorides will, depending on their size, either be blown out of the reactor with the silane 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 of the silicon that do not react or react very slowly with HCI and tend to accumulate in the reactor. The smallest slag particles can be blown out of the reactor and trapped in the filter systems.

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 HCI gas. Thus both the reactivity of the silicon and the selectivity can be effected both positively and negatively by contaminants.

Disclosure of Invention It has now been found that silicon containing above 100 ppmw manganese content or addition of manganese promoter to the reactor results in a reduced reactivity and lower selectivity when used in the method for the production of trichlorosilane by reaction with HCI.

The present invention thus relates to a method for the production of trichlorosilane by reaction of silicon with HCI gas at a temperature between 250 and 1100⁰C and a pressure of 0,1 - 30 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, which method is characterised in that silicon containing less than 100 ppmw of manganese is added to the reactor. Preferably the silicon added to the reactor contains less than 50 ppmw manganese.

It has surprisingly been found that by keeping the manganese content in the reactor at a low level the reactivity of the silicon is increased.

Further, it has also been found that a low manganese content in the reactor increases the selectivity of the reaction to silanes, that is, increases the ratio TCS/(TCS + Other silanes) by decreasing the amount of STC produced.

The present invention further relates to a method for the production of silicon by carbothermic reduction of quartz for use in the production of trichlorosilane by reaction of silicon with HCI gas as a temperature between 250 and 1100⁰C and at a pressure of 0.1-30 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, which method is characterized in that the silicon produced contain less than 100 ppmw manganese.

Preferably the silicon produced contains less than 50 ppmw manganese.

The manganese content in the silicon produced is controlled by selecting raw materials having a low content of manganese and by using electrodes, electrode paste and electrode casings having a low content of manganese.

In order to further reduce the manganese content in the silicon produced by the present invention the silicon may after solidification, preferably be leached with HF, HCI or FeCI₃ solution.

The present invention also relates to a method for milling and/or grinding silicon for use in the production of trichlorosilane by reaction of the silicon with HCI gas at a temperature between 250 and 1100⁰C, which method is characterized in that the milling and grinding are carried out using grinding bodies having a low manganese content to provide milled silicon containing less than 100 ppmw manganese and preferably less than 50 ppmw manganese. Finally, the present invention relates to the use of silicon containing less than 100 ppmw manganese and preferably less than 50 ppmw manganese for the production of trichlorosilane by reaction of silicon with HCI-gas.

Short description of the drawings

Figures 1 - 4 show diagrams for reactivity of silicon in a fixed bed reactor, described as first pass HCI, used in the process described in the present invention,

Figure 5 shows a diagram for ignition temperature for two samples of silicon in a fixed bed reactor, and

Figure 6 and 7 shows diagrams for selectivity of silicon in a continuous bed reactor.

Detailed description of the invention

The following examples 1 to 4 were all carried out in a laboratory fixed-bed reactor made from quartz and embedded in a heated aluminium block. The temperature of the heating block was kept at 350°C, 400°C or 500°C (given for each example), which gives a temperature in the reactor of 365°C, 415⁰C and 515°C respectively, due to the exothermic nature of the reaction. For each test 1 gram of silicon having a particle size of between 180 and 250 μm was added to the quartz reactor. A mixture of HCI and argon in an amount of 10 ml/min each was supplied to the reactor. The composition of the product gas from the reactor was measured with a gas chromatograph (GC). The reactivity of the samples were measured as first pass of HCI, how low in Si- amount the sample can go, and still maintain 100% of HCI conversion without increasing temperature.

Another way of measuring reactivity is as mentioned to measure the minimum temperature required to make silicon react with HCI. Since the reaction between silicon and HCI is exothermic, reaction can be detected by measuring the temperature of the silicon mass and surroundings. Without any reaction the two will heat up equally fast. When reaction occurs the temperature of the silicon mass will increase faster than the surroundings. This method is used in Example 5.

For Examples 6 and 7 a continuous fluidized bed reactor made from steel and containing 5 grams of silicon was used. The steel reactor is embedded in an aluminium heating block kept at 325°C or 350°C. 5 grams silicon is maintained in the reactor by continuously replacing reacted silicon with new silicon. A mixture of HCI and Ar (280 and 20 ml/min respectively) was supplied to the reactor, and the product composition was measured with a GC. Selectivity are in these examples measured as TCS/(TCS + Other silanes). Reactivity was measured as the silicon mass ability to maintain 100% HCI conversion.

Example 1

A synthetic sample of highly pure silicon alloyed 0.2% Fe, 0.2% Al and 930 ppmw Mn was produced in an induction furnace, milled and screened to a particle size between 180 and 250 μm (sample A). A silicon with a very low Mn content of 7 ppmw was produced in the same way and is identified as sample B.

The chemical compositions of samples A and B are given in Table 3.

Samples A and B were used to produce trichlorosilane at 415°C in a laboratory fixed-bed reactor as described above. The HCI conversion fractions from samples A and B are shown in Figure 1.

As can be seen from Figure 1 , sample A containing 930 ppmw Mn looses 100% HCI conversion earlier than reference sample B, showing the lower reactivity of sample A compared to B. HCI conversion drop as a function of silicon conversion is also shown in Table 1.

Table 1

Example 2

Silicon, produced by Elkem ASA, Fiskaa Verk, containing 82 ppmw manganese was milled and screened to a particle size between 180 and 250 μm (sample C). 1 % by weight (10 000 ppmw) manganese powder was mechanically mixed with sample C and identified as sample D.

The chemical analysis of samples C and D are shown in Table 3.

Samples C and D were used to produce trichlorosilane at 365⁰C in a laboratory fixed-bed reactor as described above. The HCI conversion fractions from samples C and D are shown in Figure 2.

As can be seen from Figure 2, sample D containing 1 % by weight Mn looses 100% HCI conversion earlier than reference sample C, showing the lower reactivity of sample D compared to C. HCI conversion drop as a function of silicon conversion is also shown in Table 2.

Table 2

Example 3

Highly pure Si was milled and screened to a particle size between 180 and 250 μm (sample E). 1.4% by weight (14 000 ppmw) manganese powder was mechanically mixed with sample E and identified as sample F.

The chemical analyses of samples E and F are shown in Table 3.

Samples E and F were used to produce trichlorosilane at 515°C in a laboratory fixed-bed reactor as described above. The HCI conversion fractions from samples E and F are shown in Figure 3.

As can be seen from Figure 3, sample F containing 1.4% by weight Mn never reaches 100% HCI conversion. The conversion is about 10% during the entire run, even at this elevated temperature. Sample E shows 100% HCI conversion until 75-81% of the original silicon is utilized. This clearly shows the higher reactivity of sample E compared to sample F.

Example 4

Sample E from Example 3 was mechanically mixed with 0.3% by weight (3 000 ppmw) manganese powder to produce sample G.

The chemical analysis of sample G is shown in Table 3.

Samples E and G were used to produce trichlorosilane at 500°C in a laboratory fixed-bed reactor described above. The HCI conversion fractions from samples E and G are shown in Figure 4.

As can be seen from Figure 4, sample G containing 0.3% by weight Mn never reaches 100% HCI conversion even at this elevated temperature. The conversion peaks at about 10% of the Si converted, with a conversion factor of about 0.97 (97%) before falling. Sample E shows 100% HCI conversion until 75-81% of the original silicon is utilized. This shows the higher reactivity of sample E compared to sample G.

Example 5 Samples A and B from example 1 were heated at a fixed rate of 2°C/minute in presence of HCI, measuring both the temperature in the silicon mass and in the surroundings (heating block).

In Figure 5 the temperature difference between the silicon and the surroundings for samples A and B are shown against the temperature of the silicon. A positive diversion from a horizontal line indicates reaction.

Sample B, containing 7 ppmw manganese, shows a positive diversion (ignition) at a temperature of 316°C, while sample A, containing 930 ppmw manganese, shows ignition at 326⁰C. This shows that a higher temperature is needed for the reaction between silicon and HCI when Mn is present, and hence that manganese reduces the reactivity of the silicon. Example 6

A metallurgical grade silicon produced by Elkem ASA was milled and screened to a particle size between 180 and 250 μm (sample H).

The chemical analysis of sample H is shown in Table 3.

Table 3

Sample H was used to produce trichlorosilane at 325⁰C in a laboratory fluid- bed reactor described above. The selectivity for TCS produced from sample H is shown in Figure 6. Manganese was added in discrete portions marked in Figure 6. Measurements with lowered reactivity (loss of 100% HCI conversion) are also marked in the figure.

The first addition of manganese (at about 17g Si converted) gives a drop in selectivity of 4 percentage points. The second addition (at about 27g Si converted) eventually leads to loss of 100% HCI conversion. 100% HCI conversion is regained after heating the reactor to 350°C (at about 33g Si converted). The third addition (at about 38g Si converted) seemingly has no influence on the process. Attempting to reduce the reactor temperature back to 325°C (at about 43g Si converted) again leads to loss of 100% HCI conversion, showing that reactivity of the silicon is reduced. Example 7

Metallurgical grade silicon produced by Elkem ASA (sample H) was crushed and milled to a particle size between 180 and 250 μm. The composition of the sample is given in Table 3.

Sample H was used to produce trichlorosilane in a continuous laboratory fluidised-bed reactor described above at a temperature of 350°C. Two parallel runs were made, but in the second run manganese was added in discrete portions marked in the figure 7 which shows the selectivity for TCS produced from sample H. The runs were performed to study the effect of manganese on selectivity without loosing 100% HCI conversion during run.

The first addition of manganese (at about 16g Si converted) gives a selectivity drop of about 2 percentage points. The second addition (at about 28g Si converted) gives a selectivity drop of about 3 percentage points. After this addition the selectivity regains a little, before the third addition addition (at about 39g Si converted) again gives a drop in selectivity of about 2 percentage points. 100% HCI conversion is maintained throughout the experiment.

Claims

Claims:

1. Method for the production of trichlorosilane by reaction of silicon with HCI gas at a temperature between 250 and 1100⁰C, and a pressure of 0.1-30 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, characterized i n that the silicon supplied to the reactor contains less than 100 ppmw manganese.

2. Method according to claim ^cha racte rized i n th at the silicon added to the reactor contains less than 50 ppmw manganese.

3. Method according to claims 1, c h a ra cte ri ze d i n that the absolute pressure is between 1 and 5 bars.

4. Method for production of silicon by carbothermic reduction of quartz for use in production of trichlorosilane by reaction of silicon with HCI gas at a temperature between 250 and 1100⁰C, and at a pressure of 0.1-30 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, characte rized i n that the silicon from the process contains less than 100 ppmw manganese.

5. Method according to claim 4, ch a ra cte rized i n th at the silicon contains less than 50 ppmw manganese.

6. Method according to claim 4 or 5, characterized i n th at the manganese content in the silicon is controlled by selecting raw materials having a low content of manganese.

7. Method according to claim 4 or 5, ch a racterized i n th at the silicon with less than 100 ppmw is produced by using electrodes, electrode paste and electrode casings having a low content of manganese.

8. Method according to claim 4 or 5, cha racterized i n that the silicon after solidification is leached with HF, HCI or FeCI₃ to reduce the manganese content.

9. Method for milling and/or grinding of silicon for use in production of trichlorosilane by reaction of the silicon with HCI gas at a temperature between 250 and 1100⁰C, and a pressure of 0.1-30 atm in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor, characterized i n th at the silicon is milled using grinding bodies having a low content of manganese to obtain milled silicon containing less than 100 ppmw manganese.

10. Method according to claim 9, characterized i n that the milled silicon contains less than 50 ppmw manganese.

11. Use of silicon containing less than 100 ppmw manganese for the production of trichlorosilane by reaction of silicon with HCI gas.

12. Use of silicon containing less than 50 ppmw for the production of trichlorosilane by reaction of silicon with HCI gas.