Preparation of trichlorosilane

Abstract

The selectivity of a process for preparing trichlorosilane (TCS) by reaction of metallurgical silicon (mg-Si) and HCl, is improved by utilizing mg-Si having a titanium content greater than 0.06 wt %.

Claims

1. A process for preparing trichlorosilane (TCS), comprising reacting metallurgical silicon (mg-Si) having a titanium content of 0.08 wt % to 0.12 wt. %, and a phosphorus content 30 ppmw with HCl.

2. The process of claim 1, wherein the mg-Si has a phosphorus content between 30 ppmw and 50 ppmw.

3. The process of claim 1, wherein the mg-Si has a phosphorus content greater than or equal to 30 ppmw and less than or equal to 41 ppmw.

4. The process of claim 1, wherein the mg-Si is prepared by solidification from a melt of mg-Si, and exhibits a maximum mean thickness of 30 mm or a maximum mean diameter of 15 mm immediately following solidification.

5. The process of claim 2, wherein the mg-Si is prepared by solidification from a melt of mg-Si, and exhibits a maximum mean thickness of 30 mm or a maximum mean diameter of 15 mm immediately following solidification.

6. The process of claim 3, wherein the mg-Si is prepared by solidification from a melt of mg-Si, and exhibits a maximum mean thickness of 30 mm or a maximum mean diameter of 15 mm immediately following solidification.

7. The process of claim 4, wherein the mg-Si has been prepared by water granulation.

8. The process as claimed of claim 1, wherein the mg-Si has an Si content greater than 98 wt %.

9. The process of claim 1, wherein reacting is effected at a temperature of from 280 to 400 C.

10. The process of claim 1, wherein reacting is effected at a temperature of from 320 to 380 C.

11. The process of claim 1, wherein reacting is effected at a pressure of from 0.1 to 30 bar.

12. The process of claim 9, wherein reacting is effected at a pressure of from 0.1 to 30 bar.

13. The process of claim 1, wherein reacting is effected at a pressure of from 1 to 4 bar.

14. The process of claim 9, wherein reacting is effected at a pressure of from 1 to 4 bar.

15. A process for preparing trichlorosilane (TCS), comprising reacting metallurgical silicon (mg-Si) having a titanium content of 0.06 wt % to 0.12 wt. %, and a phosphorus content 30 ppmw with HCl, wherein the mg-Si is prepared by solidification from a melt of mg-Si, and exhibits a maximum mean thickness of 30 mm or a maximum mean diameter of 15 mm immediately following solidification.

16. The process of claim 15, wherein the mg-Si has been prepared by water granulation.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) It is preferable to use mg-Si having a titanium content greater than or equal to 0.08 wt % and less than or equal to 0.12 wt %. It is particularly preferable to use mg-Si having a phosphorus content greater than or equal to 30 ppmw.

(2) It is preferable to use mg-Si which exhibits a maximum mean thickness of 30 mm or a maximum mean diameter of 15 mm during solidification.

(3) It is preferable to use mg-Si which has been solidified at a rapid cooling rate. Rapidly solidified mg-Si is to be understood as meaning mg-Si which, in particular, has been prepared by thin layer casting (TL), multilayer casting, water granulation (WG) and gas atomization and which exhibits a maximum mean thickness of 30 mm (thin layer Si, multilayer cast Si) or a maximum mean diameter of 15 mm (water granulation, gas atomization) during solidification. It is particularly preferable to use mg-Si which has been prepared by water granulation.

(4) mg-Si is to be understood as meaning silicon having a silicon content of between 95 wt % and 99.8 wt %. The mg-Si used preferably has an Si content greater than 98 wt % and less than or equal to 99.8 wt %.

(5) The reaction is preferably effected at a temperature of from 280 to 400 C., a temperature of from 320 to 380 C. being particularly preferred, and at a pressure of from 0.1 to 30 bar, a pressure of from 1 to 4 bar being particularly preferred.

(6) The inventors have been able to show that P contamination of mg-Si even in the mid double-digit ppmw range (from about 30 ppmw) has a negative influence on TCS selectivity. It is possible to prepare mg-Si having a low phosphorus content (<25 ppmw) in accordance with the state of the art. However, this is associated with additional expense and inconvenience.

(7) It has been found that, surprisingly, the negative influence of phosphorus on TCS selectivity is reduced by using mg-Si having a titanium content greater than 0.06 wt %, preferably greater than or equal to 0.08 wt % and less than or equal to 0.12% and, consequently, high TCS selectivity is achieved with acceptable raw material costs (mg-Si).

(8) mg-Si having a Ti content greater than 0.06 wt % can be prepared by using Ti-rich quartz sand or by addition of Ti-containing compounds into the furnace.

(9) TCS selectivity is further increased when the Ti-rich mg-Si employed has been solidified at a rapid cooling rate.

(10) Preparation of TCS by reaction of HCl and mg-Si having a titanium content greater than 0.06 wt % is preferably effected in a fluidized-bed, stirred-bed or fixed-bed reactor.

(11) The advantages of the invention are more particularly described hereinafter with reference to examples and comparative examples.

Examples and Comparative Examples

(12) The experiments in the following examples were carried out in a laboratory fluidized-bed reactor made of stainless steel.

(13) The titanium and/or phosphorus content of the mg-Si samples tested was determined by X-ray fluorescence analysis (XRFA).

(14) To this end, mg-Si was finely milled (milling time 60 s at 1500 revolutions) in a semi-automatic disk mill (HPM 100 from Herzog).

(15) 7 g of finely milled mg-Si and 1.4 g of carnauba wax (binder) were mixed, dried for 3 min at 150 C., comminuted for about 30 s in an agate mortar and pressed into a tablet for 20 s using 35 t of pressure.

(16) The pressed tablet was analyzed in an XRF (X-ray fluorescence) spectrometer.

(17) The XRF spectrometer used was an Axios.sup.mAX from PANalytical B.V. having an output of 4 kW, an Rh tube, a 300 m collimator, an LiF (200) crystal (for Ti analysis) and a Ge(111)c crystal (for P analysis) and a flow detector.

(18) Ti and P contents were measured at the strongest K line at a voltage of 40 and 32 Kv respectively and a current of 100 and/or 125 mA respectively.

(19) XRFA results were calibrated by comparison with an independent reference method, namely inductively coupled plasma atomic emission spectroscopy (ICP-OES). The XRFA calibration function was established using a set of 10 Si samples having various Ti and P contents (0.02-0.12 wt % and 23-86 ppm respectively).

(20) ICP-OES was calibrated using commercial standard solutions. The experimental uncertainty (4 ) associated with the XRFA is 30 ppmw for titanium and 3 ppmw for phosphorus.

(21) Each experiment was conducted by charging 10 g of mg-Si (commercially available, cooled form: fragmentary (FR), thin layer (TL), water granulate (WG), milled and screened to 0-355 m, Si content 98.4-99.2 wt %, Fe content 0.34-0.75 wt %, Al content 0.19-0.36 wt %, Ca content 0.02-0.13 wt %, Ti content 0.02-0.10 wt %) into the reactor and reacting same with 100 sccm of HCl at a pressure of 1.5 bar (absolute).

(22) The reactor housing was heated to 323 C. and the temperature in the reaction zone was about 360 C. owing to the strong exothermicity of the reaction.

(23) The reaction products were analyzed by mass spectrometry (online gas stream analysis) and gas chromatography (offline condensate analysis).

(24) Mean TCS selectivity was determined at a silicon conversion of 13-38% for each experiment.

Example 1

Sample A, Inventive

(25) An mg-Si sample prepared by rapid cooling (TL) and having a high Ti content (0.062 wt %) and a normal P content (30 ppmw) was reacted with HCl in accordance with the description.

(26) A TCS selectivity of 95.6% was achieved using this sample (A) (see Table 1).

Example 2

Sample B, Inventive

(27) An mg-Si sample prepared by rapid cooling (WG) and having a high Ti content (0.10 wt %) and a high P content (40 ppmw) was reacted with HCl in accordance with the description.

(28) A TCS selectivity of 95.6% was achieved using this sample (B) (see Table 1).

Example 2

Samples C and D, Inventive

(29) 2 mg-Si samples prepared by normal cooling (FR) and having a high Ti content (0.082 and 0.10 wt % respectively) and a high P content (39 and 41 ppmw respectively) were reacted with HCl in accordance with the description.

(30) TCS selectivities of 94.5% and 94.0% respectively were achieved using these samples (C and D) (see Table 1).

(31) TABLE-US-00001 TABLE 1 examples (inventive): Ti P Si TCS Cooling rate content content content selectivity Sample (form) (%) (ppm) (%) (%) A Rapid (TL) 0.062 30 98.4 95.6 B Rapid (WG) 0.096 40 98.8 95.6 C Normal (FR) 0.082 39 99.0 94.5 D Normal (FR) 0.100 41 98.7 94.0

Example 3

Comparative Example

Noninventive (Samples E, F, G and H)

(32) 4 mg-Si samples prepared by normal cooling (FR) and having a normal Ti content (0.023-0.032 wt %) were reacted with HCl in accordance with the description. These samples illustrate firstly that, when Ti content is normal, TCS selectivity falls with increasing P content (see Table 2).

(33) Secondly, sample G serves as a comparative example for Ti-rich sample A (normal P content & rapid cooling rate) and sample H serves as a comparative example for Ti-rich sample B (high P content & rapid cooling rate) and for the Ti-rich samples C and D (high P content and normal cooling rate).

(34) It can be seen from this comparison (A vs. G, and B to D vs. H respectively) that (when P contents are similar) the samples having a high Ti content in accordance with the invention (>0.06 wt %) show TCS selectivity 1.8-3.9% higher than the samples having a normal Ti content (<0.04 wt %).

(35) Finally, the samples having a high Ti content, similar (high) P contents and different cooling rates are compared with one another (B vs. C and D).

(36) This shows that the rapidly cooled sample (B) exhibits TCS selectivity 1.1-1.6% higher than the normally cooled samples (C and D).

(37) TABLE-US-00002 TABLE 2 comparative examples (noninventive): Ti P Si TCS content content content selectivity Sample Cooling rate (%) (ppm) (%) (%) E Normal (FR) 0.023 15 99.0 96.1 F Normal (FR) 0.030 21 99.1 95.4 G Normal (FR) 0.029 29 98.7 93.8 H Normal (FR) 0.032 40 99.2 91.7