SPRAY EVAPORATION OF A LIQUID RAW MATERIAL FOR PREPARATION OF SILICON DIOXIDE AND METAL OXIDES

Abstract

The present invention relates to a process for preparing a metal oxide,

comprising a) spraying a liquid raw material comprising at least one metal compound by mixing it with a gas to form an aerosol;
b) forming a gaseous reaction mixture from the aerosol obtained in step a) by complete evaporation thereof;
c) converting the gaseous reaction mixture obtained in step b) to metal oxide in the presence of oxygen.

Claims

1-15. (canceled)

16. A process for preparing silicon dioxide and/or a metal oxide, comprising the following steps: a) spraying a liquid raw material comprising at least one silicon compound and/or a metal compound by mixing it with a gas to form an aerosol; b) forming a gaseous reaction mixture from the aerosol obtained in step a) by complete evaporation thereof; c) converting the gaseous reaction mixture obtained in step b) to silicon dioxide and/or metal oxide in the presence of oxygen.

17. The process of claim 16, wherein the aerosol formed in step a) comprises liquid droplets having a numerical average particle size of not more than 2 mm.

18. The process of claim 16, wherein the ratio of gas volume in standard cubic metres used in total in steps a) and b) to the amount of the liquid raw material used in kilograms is from 0.1 to 100 m.sup.3 (STP)/kg.

19. The process of claim 16, wherein the gas used in step a) and/or b) comprises oxygen.

20. The process of claim 16, wherein the liquid raw material is preheated to a temperature of 50 to 500 C. prior to performance of step a).

21. The process of claim 16, wherein the gas used in step a) and/or b) is preheated to a temperature of 50 to 400 C.

22. The process of claim 16, wherein the liquid raw material used in step a), prior to performance of step a), has a pressure of at least 1.5 bar and the gas mixture obtained in step b) has a pressure of not more than 1.2 bar.

23. The process of claim 16, wherein the gaseous reaction mixture used in step c) has a temperature at least 10 C. higher than the dew point temperature of this mixture.

24. The process of claim 16, wherein the liquid raw material is sprayed through at least one nozzle.

25. The process of claim 16, wherein steps a) and b) take place simultaneously.

26. The process of claim 16, wherein a gaseous fuel is used in at least one of steps a)-c).

27. The process of claim 16, wherein the metal oxide comprises at least one of the elements Al, Ce, Fe, Mg, In, Ti, Sn, Y, Zn and/or Zr as the metal component.

28. The process of claim 16, wherein a silicon compound is used for preparation of silicon dioxide.

29. The process of claim 28, wherein the silicon compound is a non-halogenated compound selected from the group consisting of: tetraalkoxyorthosilicates; silanes; silicone oils; polysiloxanes and cyclic polysiloxanes; silazanes; and mixtures thereof.

30. The process of claim 28, wherein the silicon compound is a chlorinated compound selected from the group consisting of: silicon tetrachloride; dichlorosilane; trichlorosilane; methyltrichlorosilane; dimethyldichlorosilane; methyldichlorosilane; dibutyldichlorosilane; ethyltrichlorosilane; propyltrichlorosilane; and mixtures thereof.

31. The process of claim 17, wherein the ratio of gas volume in standard cubic metres used in total in steps a) and b) to the amount of the liquid raw material used in kilograms is from 0.1 to 100 m.sup.3 (STP)/kg.

32. The process of claim 31, wherein the gas used in step a) and/or b) comprises oxygen.

33. The process of claim 32, wherein the liquid raw material is preheated to a temperature of 50 to 500 C. prior to performance of step a).

34. The process of claim 32, wherein the liquid raw material used in step a), prior to performance of step a), has a pressure of at least 1.5 bar and the gas mixture obtained in step b) has a pressure of not more than 1.2 bar.

35. The process of claim 34, wherein the metal oxide comprises at least one of the elements Al, Ce, Fe, Mg, In, Ti, Sn, Y, Zn and/or Zr as the metal component.

Description

EXAMPLE 1

[0054] Octamethylcyclotetrasiloxane (D4) is initially charged in a 200 litre vat and conveyed with a gear pump at a constant conveying rate of 12.5 kg/h to a pipe coil heated with thermal oil (FIG. 1, D), in order to preheat D4 to 150 C. The octamethylcyclotetrasiloxane that has been preheated in this way is guided to a one-phase nozzle (FIG. 1, E) from SCHLICK (Hollow-Cone Mod. 121) with a bore diameter of 0.7 mm, which in this case generates a backpressure of about 2.8 bara. The filter (FIG. 1, C) installed upstream of the nozzle ensures that the nozzle cannot become clogged by any solid particles present. The preheated D4 finely distributed by means of the one-phase nozzle is mixed with an air stream preheated to 295 C. (FIG. 1, 2). The spraying of the liquid octamethylcyclotetrasiloxane into this preheated air results in complete evaporation of D4 in the downstream pipeline (burner tube) (FIG. 1, A) having the diameter of 80 mm and a length of 4.2 metres, and forms a gas mixture. In a downstream static mixer (FIG. 1, F) from Sulzer (Mischer CompaX with metered addition), 6.25 m.sup.3 (STP)/h of hydrogen (FIG. 1, 3) are mixed in (primary H.sub.2). Good mixing of all components promotes complete and homogeneous conversion of the raw materials in the downstream reaction zone (FIG. 1, B). The gas mixture thus produced is fed to the burner and, with a calculated exit velocity of 51 m/s (under standard conditions) or 99 m/s (under operating conditions), exits from the mouth of the burner (FIG. 1, G) having diameter 32 mm into the reaction zone (FIG. 1, B). For stabilization of the flame, what is called a peripheral flame is generated. For this purpose, an additional 3 m.sup.3 (STP)/h of hydrogen flows out of a concentric annular gap with gap width 1.5 mm, and burns in a diffusive pilot flame. The hot reaction products are drawn into the reaction zone with 55 m.sup.3 (STP)/h of externally introduced air (FIG. 1, 6). The gas/solids mixture produced after the reaction is cooled down to <200 C. and then supplied pneumatically to a filter system. The pyrogenic oxide formed (10 kg/h) is separated here from the main gas stream and conveyed into a bunker. The further details for performance of this experiment can be found in Table 2.

[0055] By contrast with the preparation process for pyrogenic silicon dioxide described in EP 0471139 A2, in the process according to the invention, the raw material to be processed (octamethylcyclotetrasiloxane) is not converted to the gas phase in an external evaporator via heating at a hot wall surface, but via spraying, i.e. via direct transfer of a preheated, finely distributed liquid into the gas phase by mixing with the air stream in the reactor zone A intended for the purpose.

[0056] The process described here was operable successfully for several months without clogging of the nozzle or other apparatuses by solid particles or deposits in gel form.

EXAMPLE 2

[0057] Analogously to Example 1, silicon tetrachloride (tetrachlorosilane, SiCl.sub.4) is used as raw material for preparation of pyrogenic silicon dioxide. The details for performance of this experiment can be found in Table 2.

EXAMPLE 3

[0058] Analogously to Example 1, titanium tetrachloride (TiCl.sub.4) is used as raw material for preparation of pyrogenic titanium dioxide. The details for performance of this experiment can be found in Table 2.

TABLE-US-00002 TABLE 2 Example 1 Example 2 Example 3 Raw material D4 SiCl.sub.4 TiCl.sub.4 Raw material boiling point [ C.] 171-175 57 136 Raw material feed temperature 150 30 30 [ C.] Raw material throughput [kg/h] 12.5 80 530 Primary air rate [m.sup.3 (STP)/h] 128 70 1545 Primary air feed temperature [ C.] 295 270 230 Primary H.sub.2 rate [m.sup.3 (STP)/h] 6.3 26 180 Primary H.sub.2 feed temperature [ C.] 35 30 30 Reaction mixture temperature 255 63 138 upstream of the reaction zone [ C.] Pressure upstream of the reactor 995 985 990 zone [mbar] One-phase nozzle diameter [mm] 0.7 (1) 1.4 (1) 2.3 (3) (number of nozzles) Burner tube diameter [mm] 80 80 200 Burner tube length [m] 4.2 4.2 3.0 Burner tube volume [l] 15 15 377