Method for producing silicic acid with variable thickening

Abstract

Process for production of silica having variable thickening wherein a) a product stream I comprising at least a vaporous, hydrolysable and/or oxidizable silicon compound, b) a product stream II comprising oxygen and c) a product stream III comprising at least a combustible gas are made to react, characterized in that d) a feed port in a pipepiece A, said pipepiece A comprising one or more static mixing elements, is used to import product stream I into product stream II, or vice versa and thereby create product stream IV, then e) a feed port in a pipepiece B, said pipepiece B comprising one or more static mixing elements, is used to import product stream III in product stream IV and thereby create product stream V, f) product stream V leaving pipepiece B is imported into a reaction chamber, ignited therein and reacted in a flame, and g) the resultant solid material is separated off.

Claims

1. A process for producing silica having variable thickening, the process comprising reacting: a) a product stream I comprising at least one silicon compound selected from the group consisting of a vaporous silicon compound, a hydrolysable silicon compound, and an oxidizable silicon compound, b) a product stream II comprising oxygen and c) a product stream III comprising a combustible gas, wherein the process comprises: importing product stream I into product stream II, or importing product stream II into product stream I, through a feed port in a first pipe piece A to create product stream IV, wherein the first pipe piece A comprises at least one static mixing element; importing product stream III in product stream IV, through a feed port in a second pipe piece B to create product stream V, wherein the second pipe piece B comprises at least one static mixing element; importing product stream V leaving the second pipe piece B into a reaction chamber, igniting the reaction chamber, and reacting the product stream V in a flame; and separating solids resulting from the ignition and the reaction, wherein V.sub.B is a velocity with which the product stream III comprising a combustible gas is imported into the product stream IV, V.sub.A is a velocity of the product stream IV at a point of importing the product stream III comprising a combustible gas, and V.sub.B/V.sub.A4, and wherein V.sub.A is at least 15 Nm/s.

2. The process according to claim 1, wherein the first and second pipe pieces comprising static mixing elements are designed as a flange mixer.

3. The process according to claim 2, wherein the flange mixer comprises a single punctiform feed port.

4. The process according to claim 1, wherein L.sub.A/D.sub.A is 2-20, where L.sub.A is the length of the first pipe piece, D.sub.A is the internal diameter of the first pipe piece.

5. The process according to claim 1, wherein L.sub.B/D.sub.B is 2-20, where L.sub.B is the length of the second pipe piece, D.sub.B is the internal diameter of the second pipe piece.

6. The process according to claim 1, wherein an amount of oxygen is at least sufficient to convert the at least one silicon compound and the combustible gas.

7. The process according to claim 1, wherein the at least one silicon compound is selected from the group consisting of SiCl.sub.4, CH.sub.3SiCl.sub.3, (CH.sub.3).sub.2SiCl.sub.2, (CH.sub.3).sub.3SiCl, (CH.sub.3).sub.4Si, HSiCl.sub.3, (CH.sub.3).sub.2HSiCl, CH.sub.3CH.sub.2CH.sub.2SiCl.sub.3, Si(OC.sub.2H.sub.5).sub.4, Si(OCH.sub.3).sub.4 and a mixture thereof.

8. The process according to claim 1, further comprising introducing a stream comprising oxygen and/or steam into the reaction chamber.

9. The process according to claim 1, further comprising cooling a reaction mixture leaving the reaction chamber to form a cooled reaction mixture and treating the cooled reaction mixture with steam, before separating the solids.

Description

(1) FIG. 1 is a schematic illustration of one embodiment of the invention. Key: A, B=respectively pipepiece A and pipepiece B having the respective length L and the internal diameter D; C=reaction chamber ; I-VII=product streams; product stream I comprising at least a vaporous, hydrolysable and/or oxidizable silicon compound, product stream II comprising molecular oxygen, product stream III comprising at least a combustible gas, product stream IV produced by importing product stream I into product stream II, or vice versa, via a feed port in a pipepiece A comprising one or more static mixing elements, product stream V produced by importing product stream III into product stream IV via a feed port in a pipepiece B comprising one or more static mixing elements, product stream VI comprising oxygen and/or steam, product stream VII comprising at least the reaction products silica and water, optionally carbon dioxide and hydrochloric acid.

(2) FIGS. 2A and 2B show possible embodiments of the invention as regards product streams I to III. FIG. 2A shows the feeding of the SiCl.sub.4-containing product stream I into pipepiece A comprising one or more static mixing elements. FIG. 2B shows the feeding of the molecular oxygen-obtaining product stream II into pipepiece A comprising one or more static mixing elements. In both cases, product stream III is fed into pipepiece B comprising one or more static mixing elements.

(3) In the process according to the invention, the velocity v.sub.B at which product stream III is imported into product stream IV is greater than the velocity v.sub.A of product stream IV at the time of importation of product stream III. Preference is given to an embodiment wherein v.sub.B/v.sub.A4. The velocity of product stream I and of product stream II is not critical as long as care is taken to ensure that product stream I remains in the vaporous state. The measures for this are known to a person skilled in the art. A general rule for the present invention is that the velocity of the product stream imported via the feed port of pipepiece A should be at least double the velocity of the other product stream.

(4) v.sub.A is preferably at least 15 Nm/s. A range from 20 to 100 Nm/s has proved particularly suitable. v.sub.B is preferably at least 50 Nm/s. Particular preference is given to 100v.sub.B1500 Nm/s. To produce high-thickening silica, the process according to the invention is preferably carried out as an embodiment wherein hydrogen is used as combustible gas and 100v.sub.B1500 Nm/s and 20v.sub.A50 Nm/s. The stated velocities are standardized velocities. v.sub.A is the result of dividing the volume flow in Nm.sup.3/h by the cross-sectional area of pipepiece A. v.sub.B is the result of dividing the volume flow in Nm.sup.3/h by the cross-sectional area of the feed port.

(5) It is known from the prior art that enhanced thickening can be achieved by increasing the imported quantity of combustible gas. It has now been found that thickening can be varied solely by varying the velocities v.sub.B at constant v.sub.A. An increase in v.sub.B, i.e. the velocity at which the combustible gas stream is imported into the product stream comprising the silicon compound and the molecular oxygen can lead to enhanced thickening.

(6) In principle, the process is not limited to the production of particular silicas. It is preferably suitable for production of silicas having a BET surface area of 50-500 m.sup.2/g, more preferably 100-400 m.sup.2/g, even more preferably 150-350 m.sup.2/g and especially 180-280 m.sup.2/g.

(7) Variable thickening is to be understood as meaning that for substantially the same BET surface area it is possible to obtain silicas differing in thickening effect. Substantially the same BET surface area is any one specified BET surface area 25 m.sup.2/g, generally 15 m.sup.2/g. This means, for example, that silicas are obtainable with a BET surface area of 20010 m.sup.2/g and a thickening effect of 3000-3700 mPas.

(8) It is similarly possible to obtain silicas having substantially the same thickening effect coupled with significantly differing BET surface area. This means, for example, that silicas having a thickening effect of 3500100 mPas are obtainable with a BET surface area of 11010 m.sup.2/g and 20010 m.sup.2/g.

(9) Thickening, in mPas, is determined in a dispersion of a silica in an unsaturated polyester resin having a viscosity of 1300100 mPas at a temperature of 22 C. and measured with a rotary viscometer at a shear rate of 2.7 s and a temperature of 25 C. Suitable unsaturated polyester resins comprise cocondensates of ortho- and meta-phthalic acid and maleic or fumaric acid, or anhydrides thereof, and a low molecular weight diol, for example ethylene glycol, 1,2- or 1,3-propanediol, 1,2- or 1,3- or 1,4-butanediol or neo-pentyl glycol ((CH.sub.3).sub.2C(CH.sub.2OH).sub.2), or polyols, such as pentaerythritol, preferably dissolved at 30 to 80 wt %, preferably 60 to 70 wt %, in an olefinic reactive diluent as solvent, for example monostyrene. The viscosity of the polyester resin is 1300+/100 mPas at a temperature of 22 C. 7.5 g of silica are imported into 142.5 g of polyester resin at a temperature of 22 C. and dispersed therein with a dissolver at 3000 min.sup.1. 60 g of this dispersion are admixed with a further 90 g of the unsaturated polyester resin and dispersal is repeated. An example of a useful unsaturated polyester resin is Ludopal P6, BASF.

(10) In general, the process according to the invention is carried out such that the amount of oxygen is at least sufficient to convert the silicon compound and the combustible gas. In this case, lambda is not less than 1. Lambda describes the ratio of feed oxygen to stoichiometrically needed oxygen. The stoichiometrically needed amount of oxygen when the reaction of silicon tetrachloride and hydrogen as combustible gas is used as an example is attributable to the two reaction equations
2H.sub.2+O.sub.2->2H.sub.2O and SiCl.sub.4+2H.sub.2O->SiO.sub.2+4HCl.

(11) When other combustible gases and silicon compounds are used, corresponding equations must be drawn up. Gamma describes the ratio of feed hydrogen to stoichiometrically needed hydrogen.

(12) Product stream I according to the present invention comprises at least a vaporous, hydrolysable and/or oxidizable silicon compound. Said product stream generally further comprises a carrier gas, for example air or an inert gas such as nitrogen. The vaporous, hydrolysable and/or oxidizable silicon compound used may be with particular preference SiCl.sub.4, CH.sub.3SiCl.sub.3, (CH.sub.3).sub.2SiCl.sub.2, (CH.sub.3).sub.3SiCl, (CH.sub.3).sub.4Si, HSiCl.sub.3, (CH.sub.3).sub.2HSiCl, CH.sub.3CH.sub.2CH.sub.2SiCl.sub.3, Si(OC.sub.2H.sub.5).sub.4, Si(OCH.sub.3).sub.4, or mixtures thereof.

(13) Preference is given to using SiCl.sub.4 alone or as constituent of mixtures. Suitable mixtures are in particular mixtures which in addition to SiCl.sub.4 further comprise (CH.sub.3).sub.3SiCl and/or HSiCl.sub.3. It is immaterial in this connection whether SiCl.sub.4 is the main or a secondary component in these mixtures. Mixtures can for instance be used which in addition to SiCl.sub.4 comprise 70 wt % or more of CH.sub.3SiCl.sub.3 or 10 wt % or more of HSiCl.sub.3.

(14) In one particular embodiment of the invention, the vaporous, hydrolysable and/or oxidizable silicon compound comprises the secondary product generated in the production of polycrystalline silicon from HSiCl.sub.3 and comprising 60-99 wt % of SiCl.sub.4 and 0.5-40% of HSiCl.sub.3 as main constituents and H.sub.2SiCl.sub.2, SiH.sub.4 and H.sub.3SiCl and HCl as secondary constituents at less than 5 wt %, each based on the secondary product.

(15) Product stream II comprises oxygen. This will generally be air or oxygen-enriched air.

(16) Product stream III comprises or consists essentially of one or more combustible gases. These are preferably selected from the group consisting of hydrogen, methane, ethane and/or propane. Hydrogen is particularly preferred.

(17) Product stream IV comprises the vaporous, hydrolysable and/or oxidizable silicon compound and oxygen.

(18) Product stream V comprises the vaporous, hydrolysable and/or oxidizable silicon compound, oxygen and combustible gas.

(19) Product stream VI is optional and can be used to import oxygen, combustible gas and/or steam into the reaction chamber. This can be used to influence the flame and if necessary stabilize it.

(20) Product stream VII comprises at least the reaction products silica and water. Depending on the structure of the silicon compounds used and of the combustible gas, carbon dioxide and hydrochloric acid may also be present.

(21) In a further possible embodiment of the process according to the invention, the reaction mixture leaves the reaction chamber and is then cooled, preferably to temperatures of 80 to 250 C., and subsequently treated with steam, preferably at temperatures of 350 to 750 C., before the solid material is separated off.

EXAMPLES

(22) CompaX mixers from Sulzer are used as pipepieces A and B comprising static mixing elements.

Example 1

(23) Product stream I consists of 8 kg/h of vaporous silicon tetrachloride, has a velocity v.sub.I of 1.9 Nm/s and is mixed via the punctiform feed port, 3 mm in diameter, of a CompaX mixer A with L.sub.A/D.sub.A=5 into a product stream II, which consists of 11.9 Nm.sup.3/h air and has a velocity v.sub.II of 467.6 Nm/s. The velocity v.sub.A of resultant product stream IV is 23.4 Nm/s. A CompaX mixer B, which has the dimensions L.sub.B/D.sub.B=5 and whose punctiform feed port has a diameter of 1 mm, is used to import a product stream III in the form of 3.9 Nm.sup.3/h hydrogen into said product stream IV. The speed v.sub.B at which product stream III leaves the punctiform feed port is 1379.3 Nm/s. The way the feedstocks are added is shown in FIG. 2A. Resultant product stream V is imported into a reaction chamber via a nozzle 15 mm in internal diameter, resulting in a velocity v.sub.RC of 26.5 Nm/s, and ignited therein. Additionally imported into the reaction chamber is product stream VI consisting of 18 Nm.sup.3/h air. Resultant product stream VII, now comprising silica particles, hydrochloric acid, steam and air, is initially cooled down to a temperature of 120 to 150 C. The silica is subsequently filtered off and steam treated at a temperature of 400 to 500 C.

(24) The silica has a BET surface area of 186 m.sup.2/g and a thickening effect of 3550 mPas at 22 C.

(25) Examples 2 to 8 are carried out in a similar manner, the operational settings are reported in the table. It is evident that the different diameters of the feed port of Compax mixer B and the associated velocities v.sub.B produce for the same dimensions L.sub.A, D.sub.A, L.sub.B and D.sub.B and substantially the same v.sub.A, silicas having substantially the same BET surface area, 186 to 218 m.sup.2/g, and significantly differing thickening, 3575 to 4250 mPas.

(26) Example 9 is a repeat of Example 1 except that product stream II is introduced into product stream I via the punctiform feed port of CompaX mixer A. L.sub.A/D.sub.A is further=3. The way the feedstocks are added is shown in FIG. 2B. Operational parameters and product properties are reported in the table.

(27) Examples 10 and 11 have the same operational parameters as Example 9 except that the length of pipepiece A comprising static mixing elements was varied. L.sub.A/D.sub.A is 3 in Example 9, 6 in Example 10 and 15 in Example 11. While there is substantially the same BET surface area for the silicas obtained, thickening varies distinctly, from 3085 to 3495 mPas.

(28) A mixture of methyltrichlorosilane and silicon tetrachloride is used in Example 12. For substantially the same BET surface area as in Examples 1 to 11, the silica obtained has very low thickening.

(29) TABLE-US-00001 TABLE Operational parameters and product properties Example 1 2 3 4 5 6 7 8 9 10 11 12 Product stream I SiCl.sub.4 kg/h 8 8 8 8 8 8 8 8 8 8 8 .sup.6.sup.a) v.sub.I Nm/s 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.6 Product stream II.sup.b) Air Nm.sup.3/h 11.9 12.3 12.1 12 11.9 11.95 12.05 12.05 13.2 13.2 13.2 12.9 v.sub.II Nm/s 467.6 483.4 475.5 471.6 467.6 469.6 473.5 473.5 518.7 518.7 518.7 506.9 v.sub.A Nm/s 23.4 24.1 23.7 23.6 23.4 23.5 23.6 23.6 25.7 25.7 25.7 24.9 Product stream III H.sub.2 Nm.sup.3/h 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 1.5 hole Mm 1.0 1.2 1.4 1.6 1.9 2.2 2.6 3.6 3 3 3 3 v.sub.B Nm/s 1379.3 957.9 703.7 538.8 382.1 285.0 204.0 106.4 153.3 153.3 153.3 516.4 v.sub.B/v.sub.A 59.0 39.7 29.6 22.9 16.3 12.1 8.6 4.5 6.0 6.0 6.0 20.8 Product stream V.sup.c) lambda.sup.d) 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.4 1.4 1.4 1.3 v.sub.RC.sup.e) Nm/s 26.5 27.1 26.8 26.7 26.5 26.6 26.7 26.7 28.5 28.5 28.5 Product stream VI Air Nm.sup.3/h 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 19.0 19.0 19.0 18 Silica BET m.sup.2/g 186 214 218 199 200 204 206 208 207 204 203 205 Thickening mPas 3550 4250 3955 3625 3600 3575 3770 4070 3340 3085 3495 2580 .sup.a)75:25 parts by weight CH.sub.3SiCl.sub.3/SiCl.sub.4; .sup.b)diameter of hole for feed port of product stream II into product stream I: 3 mm; .sup.c)Diameter of feed port of product stream V in reaction chamber: 15 mm; .sup.d)gamma equals 1.9 in all examples; .sup.e)velocity of feed into reaction chamber