Boron oxide-silicon dioxide mixed oxide

Abstract

A boron oxide-silicon dioxide mixed oxide which has a BET surface of less than 100 m2g, and optionally containing oxides of aluminium, titanium or zirconium, is prepared pyrogenically by flame hydrolysis. The mixed oxide is used in glass making.

Description

This application is based on application Ser. No. 19624392.0 filed in Germany on Jun. 19, 1996 and provisional application Ser. No. 60/029,845 filed in the United States on Oct. 29, 1996, the content of which are incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a boron oxide-silicon dioxide mixed oxide, a process for its preparation, and its use.

2. Prior Art

Document DE-A 21 22 066 describes the use or pyrogenically produced mixed oxides of boron with silicon, aluminum, titanium and/or iron having a boron content of 2 to 20 wt. %, calculated as elementary boron, as a filler in organopolysiloxane compositions (for example bouncing putty). In particular these mixed oxides contain 5 to 10 wt. % of boron, calculated as elementary boron. These mixed oxides can be prepared by using metals and/or metal compounds according to Ullmanns Enzyclopadie der technischen Chemie, (Ullmanns Encyclopedia of Industrial Chemistry), Vol. 15 (1964), p.726. They are preferably prepared from volatile compounds of boron and silicon, aluminum, titanium and/or iron, especially the chlorides of the aforementioned elements, in the presence of water formed in situ at temperatures above 800° C., i.e. by flame hydrolysis. The mixed oxides obtained have a BET surface of 100 to 400 m 2 /g.

According to the example given in document DE-A 21 22 066, 4.8 kg/h of SiCl 4 and 1.2 kg/h of boron trichloride are vaporised and burnt together with 1.5 m 3 /h of hydrogen in a combustion chamber, under the addition of 4.2 m 3 /h of air. The temperature in the combustion chamber is more than 800° C. The resultant mixed oxide contains 82.5 wt. % of SiO2 and 17.5 wt. % of B 2 O3 (=5 wt. % of boron) calculated as boron and has a BET surface of 180 m 2 /g.

The known boron oxide-silicon dioxide mixed oxide has the disadvantage that it occurs in very finely divided form having a BET surface greater than 100 m 2 /g. It is unsuitable for use as a raw material in glass-making.

The known process has the disadvantage that boron trichloride is used as starting material for the boron oxide. Boron trichloride is a poisonous compound with a boiling point of 12.5° C. When used at room temperature special safety measures are therefore necessary. The commercially available boron trichloride may contain noticeable amounts of poisonous phosgene as impurity.

The object of the invention is therefore to prepare a boron oxide-silicon dioxide mixed oxide that does not have these disadvantages.

SUMMARY OF THE INVENTION

The invention provides a boron oxide-silicon dioxide mixed oxide, which is characterised in that it has a BET surface of less than 100 m 2 /g, preferably 10 to 80 m 2 /g, in particular 25 to 50 m 2 /g.

The proportion of boron oxide-may be from 0.01 to 40 wt. %.

In a further development of the invention the boron oxide-silicon dioxide mixed oxide may contain a further oxide or further oxides of metals and/or metalloids as a constituent of the mixed oxide.

Such metals or metalloids may be aluminum, titanium or zirconium.

In a preferred embodiment the boron oxide-silicon dioxide mixed oxides according to the invention may be prepared pyrogenically.

The known process of flame hydrolysis may preferably be employed for this purpose. This process is described in Ullmanns Enzyclopadie der technischen Chemie, 4th Ed., Vol. 21 (1982), pp. 464 and 465.

The boron oxide-silicon dioxide mixed oxide according to the invention having a BET surface of less than 100 m 2 /g, preferably 10 to 80 mg 2 /g, in particular 25 to 50 m 2 /g, can be prepared by vaporising silicon halides and/or organosilicon halides, for example methyl trichlorosilane, preferably the chloride SiCl 4 with a vaporisable boron compound, for example trimethyl borate, separately or together, optionally with the addition of vaporisable compounds of the metalloids and/or metals aluminum, titanium, zirconium, for example AlCl 3 , ZrCl 4 , TiCl 4 or the like, mixing the vapours together with a carrier gas, for example air and/or nitrogen, in a mixer unit, preferably in a burner of known construction, with hydrogen as well as air and optionally further gases such as oxygen and nitrogen, reacting the gases in a flame, then cooling the hot gases and the solid, separating the gases from the solid, and optionally removing halide or raw material residues adhering to the product by a heat treatment with moist air.

The process according to the invention has the advantage that the trimethyl borate that is used is miscible in all proportions with SiCl 4 , and it is therefore possible to adjust the ratio exactly. Since the boiling point of trimethyl borate of 68° C. is sufficiently close to the boiling point of silicon tetrachloride, it can be vaporised jointly with silicon tetrachloride from a vaporisation unit.

The boron oxide-silicon dioxide mixed oxide according to the invention, which may optionally also contain TiO 2 , Al 2 O 3 and/or ZrO 2 , can be used to make high-purity glasses. The high purity and the adjusted particle fineness of the mixed oxide according to the invention are of particular advantage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention now will be described with respect to the accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of a burner arrangement used for Examples 1-4 of the invention;

FIG. 2 is a diagrammatic illustration of a burner arrangement used for Examples 5-9 of the invention;

FIG. 3 is an electron microscope photograph of the powder of Example 2;

FIG. 4 is an electron microscope photograph of the powder of Example 9;

FIG. 5 is a summation curve of the number distribution of Example 2;

FIG. 6 is a differential curve of the number distribution of Example 2;

FIG. 7 is a class frequency representation of the number distribution of Example 2;

FIG. 8 is a summation curve of the weight distribution of Example 2;

FIG. 9 is a differential curve of the weight distribution of Example 2;

FIG. 10 is a class frequency representation of the weight distribution of Example 2;

FIG. 11 is a summation curve of the number distribution of Example 9;

FIG. 12 is a differential curve of the number distribution of Example 9

FIG. 13 is a class frequency representation of the number distribution of Example 9;

FIG. 14 is a summation curve of the weight distribution of Example 9;

FIG. 15 is a differential curve of the weight distribution of Example 9; and

FIG. 16 is a class frequency representation of the weight distribution of Example 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

EXAMPLES

Preparation of Boron oxide-silicon Dioxide Mixed Oxide

Examples 1 to 4

The burner arrangement for examples 1 to 4 is shown diagrammatically in FIG. 1 .

The burner is housed in a heated flirnace part, the temperature of the furnace being roughly the same as the outflow temperature of the gas mixture. The burner consists of a mixing chamber to which is connected a nozzle from which the gas mixture flows, as well as a second (jacket) nozzle annularly surrounding the first nozzle, from which in addition jacket) hydrogen is blown into the flame in order to avoid agglomerations (in the nozzle region).

Example 1

1.0796 kg/h of SiCl4 are vaporised at ca. 130° C. and conveyed together with 0.1 Nm 3 /h of nitrogen as carrier gas into the mixing chamber of the burner. 0.51 Nm 3 /h of air as well as 0.107 Nm 3 /h of oxygen and 0.3 Nm 3 /h of (core) hydrogen are) fed into the same mixing chamber.

0.0643 kg/h of trimethyl borate are vaporised at ca. 120° C. in a separate vaporisation device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm 3 /h). The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) from the nozzle opening of the burner and bums in a reaction chamber (flame pipe).

The temperature of the gas mixture (SiCl 4 )-air-hydrogen, oxygen, trimethyl borate) is measured at the mouth of the burner, and is found to be 128° C. 0.05 Nm 3 /h of hydrogen (jacket hydrogen, room temperature) are fed into the annular nozzle surrounding the mouth of the burner.

After the flame hydrolysis the reaction gases and the boron oxide-silicon dioxide mixed oxide are sucked through a cooling system by applying a vacuum and are cooled to ca. 100 to 160° C. The solid is separated from the waste gas stream in a filter. The boron oxide-silicon dioxide mixed oxide occurs as a white, finely particulate powder.

The specific surface is 45 m 2 /g, and the analytically determined B 2 O 3 content is 4.2 wt. % (=1.3 wt. % of boron, calculated as boron).

The adjustment parameters are summarised in Table 1.

Example 2

The procedure described in Example 1 is employed and the following amounts are used:

0.9387 kg/h of SiCl 4 with 0.1 Nm 3 /h of nitrogen as carrier gas, air 0.76 Nm 3 /h, oxygen 0.159 Nm 3 /h (core) hydrogen 0.268 Nm 3 /h.

0.05 Nm 3 /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner. The gas mixture flows at a rate of 14 m/sec (referred to standard conditions) from the nozzle opening of the burner.

0.1777 kg/h of trimethyl borate are vaporized in a separate vaporization device and likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm 3 /h).

The specific surface of the boron oxide-silicon dioxide mixed oxide is 34 m 2 /g, and the analytically determined B 2 O 3 content is 13.3 wt. % (=4.1 wt. % of boron, calculated as boron).

The adjustment parameters are summarised again in Table 1.

The boron oxide-silicon dioxide obtained has the following particle size distribution:

Total number of particles (N ) 3223.

Particle diameter, arithmetic mean (DN) 45.72 (NM)

Particle diameter, averaged over (DA) 60.81 (NM)

surface

Percentage distribution

			Sum	Sum
Diameter	Number	Number	Number %	Wt. %
D (NM)	N	%	SN %	SND %
7.450	40.	1.241	1.241	0.003
10.210	123.	3.816	5.057	0.030
12.970	155.	4.809	9.867	0.099
15.730	105.	3.258	13.124	0.182
18.490	56.	1.738	14.862	0.254
21.250	64.	1.986	16.848	0.380
24.010	64.	1.986	18.833	0.560
26.770	78.	2.420	21.253	0.865
29.530	98.	3.041	24.294	1.379
32.290	86.	2.668	26.962	1.970
35.050	109.	3.382	30.344	2.926
37.810	127.	3.940	34.285	4.325
40.570	173.	5.368	39.652	6.680
43.330	198.	6.143	45.796	9.963
46.090	193.	5.988	51.784	13.814
48.850	191.	5.926	57.710	18.352
51.610	178.	5.523	63.233	23.339
54.370	160.	4.964	68.197	28.580
57.130	153.	4.747	72.944	34.395
59.890	135.	4.189	77.133	40.306
62.650	140.	4.344	81.477	47.322
65.410	113.	3.506	84.983	53.767
68.170	109.	3.382	88.365	60.805
70.930	86.	2.668	91.033	67.060
73.690	64.	1.986	93.019	72.280
76.450	52.	1.613	94.632	77.015
79.210	43.	1.334	95.966	81.371
81.970	26.	0.807	96.773	84.289
84.730	28.	0.869	97.642	87.761
87.490	26.	0.807	98.449	91.310
90.250	22.	0.683	99.131	94.606
93.010	10.	0.310	99.442	96.246
95.770	3.	0.093	99.535	96.783
98.530	5.	0.155	99.690	97.757
101.290	3.	0.093	99.783	98.393
104.050	7.	0.217	100.000	100.000

DN=45./2

DA=60.81

D50=45.27

K=4

A(K)=0.1247

FQS=1.7246

DN/D50=1.0100

DA/D50=1.3434

DA/DN=1.3301

These data are illustrated graphically in FIGS. 5 to 10 .

Example 3

The procedure described in Example 1 is employed, the following amount being used:

00.5005 kg/h of SiCl 4 with 0.1 Nm 3 /h of nitrogen as carrier gas, air 0.726 Nm 3 /h, oxygen 0.152 Nm 3 /h, (core) hydrogen 0.14 Nm 3 /h.

0.05 Nm 3 /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner. The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) and at a temperature of 124° C. from the nozzle opening of the burner.

0.225 kg/h of trimethyl borate are vaporized in a separate vaporization device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm 3 /h).

The specific surface of the boron oxide-silicon dioxide mixed oxide is 39 m 2 /g, and the analytically determined B 2 O 3 content is 23.0 wt. % (=7.1 wt. % of boron, calculated as boron).

The adjustment parameters are again summarized in Table 1.

Example 4

The procedure described in Example 1 is adopted, the following amounts being used:

0.365 kg/h of SiCl 4 , with 0.1 Nm 3 /h of nitrogen as carrier gas, air 0. 759 Nm 3 /h, oxygen 0.159 Nm 3 Ih, hydrogen 0.103 Nm 3 /h.

0.05 Nm 3 /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner.

0.2575 kg/h of trimethyl borate are vaporized in a separate vaporization device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0. 02 Nm 3 /h).

The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) and at a temperature of 125° C. from the nozzle opening of the burner.

The specific surface of the boron oxide-silicon dioxide mixed content is 30 m 2 /g, and the analytically determined B 2 O 3 content is 29.0 wt. % (=9.0 wt. % or boron calculated as boron).

The adjustment parameters are again summarized in Table 1.

TABLE 1
Experimental conditions and flame parameters calculated therefrom in
the preparation of pyrogenic boron oxide-silicon dioxide mixed oxides
	SiCl 4	TMB	Air	O 2	H 2 core	N 2 core	gamma	lambda	BET	B 2 O 3
No.	[kg/h]	[kg/h]	[Nm 3 /h]	[Nm 3 /h]	[Nm 3 /h]	[Nm 3 /h]	[—]	[—]	[m 2 /g]	[wt. %]
1	1.0796	0.0643	0.510	0.107	0.300	0.12	1.27	1.01	45	4.2
2	0.9387	0.1777	0.760	0.159	0.268	0.12	1.78	1.03	34	13.3
3	0.5005	0.2250	0.726	0.152	0.140	0.12	2.72	1.04	39	23.0
4	0.3651	0.2575	0.759	0.159	0.103	0.12	3.67	1.04	30	29.0

TMB=Trimethiyl borate B (OCH 3 ) 3

Ratio H 2 gamma=Ratio of fed-in hydrogen in the core (taking into account the hydrogen contained in TMB) to stoichiometrically required hydrogen

Ratio O 2 lambda=Ratio of fed-in oxygen (atmospheric oxygen+additionally added O 2 ) in the burner to stoichiometrically required oxygen

Deacidification and Removal of Raw Material Residues

In order to remove raw material residues that have possibly incompletely reacted and are still adhering to the product, and to reduce the chloride content of the samples, the oxides prepared according to Examples 1 to 9 can undergo a finther temperature treatment stage.

For this purpose the powders are treated with moist air in a countercurrent downpipe arrangement at temperatures between 400° and 700° C. (preferably 650° C.). (Deacidification)

The analysis date of the powders prepared according to Examples 1 to 4 before and after deacidification are summarised in Tables 2 and 3.

TABLE 2
Analysis data of the samples obtained according to Examples 1 to 4
(B—Si-mixed oxide)
	Before deacidification	After deacidification
					Cl					Cl
	BET	B 2 O 3	TV	GV	Content	BET	B 2 O 3	TV	GV	Content
No	m 2 /g	Wt. %	Wt. %	Wt. %	Wt. %	m 2 /g	Wt. %	Wt. %	Wt. %	ppm
1	45	4.2	1.5	0.4	200	46	3.6	0.6	0.1	138
2	34	13.3	1.8	2.7	142	37	10.1	0.4	1.5	87
3	39	23.0	0.8	3.1	87	44	22.2	0.5	2.5	18
4	30	29.0	1.0	3.8	48	33	28.6	0.7	1.3	13

TV=Drying loss (2 h at 105° C., according to DIN/ISO 787/II, ASTM D 280,

JIS K 5101/21)

GV=Annealing loss (2 h at 1000° C., according to DIN 55921, ASTM D 1208,

JIS K 5101/23, referred to the substance dried for 2 hours at 105° C.

TABLE 3
Further analysis data of the samples obtained
according to Examples 1 to 4
(B—Si-mixed oxide)
	Before deacidification	After deacidification
			Bulk	Tamped			Bulk	Tamped
	BET		density	density	BET		density	density
No.	m 2 g	pH	g/l	g/l	m 2 /g	pH	g/l	g/l
1	45	3.84	58	69	46	4.09	88	113
2	34	3.89	46	52	37	3.93	91	115
3	39	3.69	39	46	44	3.88	69	87
4	30	3.52	41	48	33	3.88	94	118

pH=pH in 4t aqueous suspension.,

Tamped density according to DIN/ISO 787/XI, JIS K 5101/18 (not screened).

Examples of the Preparation of Boron Oxide-aluminum oxide-silicon Dioxide Mixed Oxide

The burner arrangement for Examples 5 to 9 is illustrated diagrammatically in FIG. 2 .

Example 5

The two liquids SiCl 4 and trimethyl borate are mixed in the desired ratio before being vaporised and are then converted together into the gaseous phase in a vaporiser at 135° C. 0.876 kg/h of SiCl 4 is vaporised together with 0.231 kg/h of trimethyl borate. The gases are conveyed with a stream of nitrogen at a rate of 0.120 Nm 3 /h into the mixing chamber of the burner mentioned in Example 1.

The temperature of the furnace in which the burner is housed is ca. 240° C. 0.056 kg/h of AlCl 3 is vaporized in a separate vaporiser at temperatures of ca. 230° C. and is similarly conveyed in a stream of nitrogen as carrier gas at a rate of 0.120 Nm 3 /h into the mixing chamber of this burner.

0.938 Nm3/h of air as well as 0.196 Nm 3 /h of oxygen and 0.254 Nm 3 /h of hydrogen are introduced into the burner mixing chamber.

The gas mixture flows at a rate of 17 m/sec (referred to standard conditions) from the nozzle opening of the burner and burns in a reaction chamber (flame pipe).

The temperature of the gas mixture (SiC 4 , AlCl 3 , air, hydrogen, oxygen, and trimethyl borate) is measured at the mouth of the burner and found to be 235° C. 0.05 Nm 3 /h of hydrogen (jacket hydrogen, room temperature) is fed intos the annular nozzle surrounding the mouth of the burner.

After flame hydrolysis the reaction gases and the boron oxide-silicon dioxide-aluminium oxide mixed oxide are sucked through a cooling system by applying a vacuum and cooled to ca. 100° to 160° C. The solid is separated from the waste gas stream in a filter. The solid (boron oxide aluminium oxide-silicon dioxide mixed oxide) is in the form of a white, finely divided powder.

The specific surface of the boron oxide-aluminum oxide silicon dioxide mixed oxide is 28 m 2 /g, the analytically determined B 2 O 3 content is 12.3 wt. % ({tilde over (=)}3.8 wt. % of boron) and the analytically determined Al 2 O 3 content is 3.9 wt. %.

Example 6

The procedure described in Example 5 is adopted, the following amounts being used: 0.83 kg/h of SiCl 4 is vaporised together with 0.219 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm 3 /h into the mixing chamber of the burner mentioned in Example 1. 0.108 kg/h of AlCl 3 is vaporised in a separate vaporiser at a temperature of ca. 240° C. and conveyed in a stream of nitrogen as carrier gas at a rate of 0.120 Nm 3 /h into the mixing chamber of this burner.

0.782 Nm 3 /h of air as well as 0.164 Nm 3 /h of oxygen and 0.255 Nm 3 /h of hydrogen are introduced into the same mixing chamber.

The gas outflow temperature at the mouth or the burner is 235° C.

After flame hydrolysis and oxidation, and separation of the gaseous reaction gases, a boron oxide-aluminium oxide silicon dioxide mixed oxide is obtained having a BET surface of 30 m 2 /g, the analytically determined B 2 O 3 content being 13.9 wt. % ({tilde over (=)}4.3 wt. % of boron) and the analytically determined Al 2 O 3 content being 8.3 wt. %.

Example 7

The procedure described in Example 5 is adopted, the following amounts being used:

0.788 kg/h of SiCl4 is vaporised together with 0. 208 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a i rate of 0.120 Nm 3 /h into the mixing chamber of the burner mentioned in Example 1. 0.151 kg/h of AlCl 3 is vaporised in a separate vaporiser at temperatures of ca. 240° C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm 3 /h into the mixing chamber of this burner. 0.762 Nm3/h of air as well as 0.160 Nm 3 /h of oxygen and 0.257 Nm 3 /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235° C.

After flame hydrolysis and separation ot the gaseous reaction gases, a boron oxide-aluminium oxide-silicon dioxide mixed oxide is obtained having a BET surface of 37 m 2 /g, the analytically determined B 2 O 3 content being 14.3 wt. % (4.4 wt. % of boron) and the analytically determined Al 2 O 3 content being 12.9 wt. %.

Example 8

The procedure described in Example 5 is adopted, the following amounts being used:

0.721 kg/h of SiCl4 is vaporised together with 0.190 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm 3 /h into the mixing chamber of the burner mentioned in Example 1. 0.232 kg/h of AlCl 3 is vaporised in a separate vaporiser at a temperature of ca. 240° C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm 3 /h into the mixing chamber of this burner.

0.842 Nm 3 /h of air as well as 0.176 Nm 3 /h of oxygen and 0.259 Nm 3 /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235° C.

After flame hydrolysis and separation of the gaseous reaction gases, a boron oxide-aluminium oxide-silicon 29 m 2 /g, the analytically determined B 2 O 3 content being 14.6 wt. % ({tilde over (=)}4.5 wt. % of boron) and the analytically determined Al 2 O 3 content being 19.3 wt. %.

Example 9

The procedure described in Example 5 is adopted, the following amounts being used:

0.641 kg/h of SiCl 4 is vaporised together with 0.169 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm 3 /h into the mixing chamber of the burner mentioned in Example 5. 0.321 kg/h of AlCl 3 is vaporised in a separate vaporiser at a temperature of ca. 240° C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm 3 /h into the mixing chamber of this burner.

0.800 Nm 3 /h of air as well as 0.167 Nm 3 /h of oxygen and 0.261 Nm 3 /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235° C.

After flame hydrolysis and separation of the gaseous reaction gases a boron oxide-aluminium oxide-silicon dioxide mixed oxide is obtained having a BET surface of 37 m 2 /g, the analytically determined B 2 O 3 content being 12.9 wt. % ({tilde over (=)}4.0 wt. % of boron) and the analytically determined Al 2 O 3 content being 29.0 wt. %.

The boron oxide-aluminium oxide-silicon dioxide mixed oxide obtained has the following particle size distribution:

Total number of particles (N) 4349

Particle diameter, arithmetic mean (DN) 77.66 (NM)

Particle diameter, averaged over surface (DA) 101.81 (NM)

Percentage distribution

			Sum	Sum
Diameter	Number	Number	Number	Wt. %
D (NM)	N	%	%	%
14.900	367.	8.439	8.439	0.038
20.420	90.	2.069	10.508	0.062
25.940	53.	1.219	11.727	0.092
31.460	47.	1.081	12.808	0.138
36.980	54.	1.242	14.049	0.224
42.500	95.	2.184	16.234	0.454
48.020	143.	3.288	19.522	0.953
53.540	198.	4.553	24.074	1.912
59.060	214.	4.921	28.995	3.302
64.580	255.	5.863	34.859	5.468
70.100	293.	6.737	41.596	8.650
75.620	315.	7.243	48.839	12.946
81.140	297.	6.829	55.668	17.949
86.660	306.	7.036	62.704	24.230
92.180	300.	6.898	69.602	31.640
97.700	278.	6.392	75.994	39.815
103.220	248.	5.702	81.697	48.416
108.740	176.	4.047	85.744	55.553
114.260	123.	2.828	88.572	61.339
119.780	129.	2.966	91.538	68.330
125.300	84.	1.931	93.470	73.541
130.820	86.	1.977	95.447	79.613
136.340	64.	1.472	96.919	84.728
141.860	42.	0.966	97.885	88.509
147.380	29.	0.667	98.551	91.436
152.900	25.	0.575	99.126	94.254
158.420	15.	0.345	99.471	96.135
163.940	8.	0.184	99.655	97.247
169.460	4.	0.092	99.747	97.861
174.980	4.	0.092	99.839	98.536
180.500	3.	0.069	99.908	99.093
186.020	1.	0.023	99.931	99.296
191.540	1.	0.023	99.954	99.517
197.060	2.	0.046	100.000	100.000

DN=77/66

DA=101.81

D50=76.56

K=6

A(K)=0.0991

FQS=1.7952

DN/D50=1. U144

DA/D50=1.3298

DA/DN=1.3110

These data are shown graphically in FIGS. 11 to 16 .

TABLE 4
Experimental conditions and flame parameters calculated therefrom in
the preparation of pyrogenic boron-silicon-aluminium mixed oxides
	SiCl 4	TMB	AlC 3	Air	O 2	H 2	gamma	lambda	BET
No	[kg/h]	[kg/h]	[kg/h]	[Nm 3 /h]	[Nm 3 /h]	[Nm 3 /h]	[—]	[—]	[m 2 /g]
5	0.876	0.231	0.056	0.938	0.196	0.254	1.95	1.10	28
6	0.830	0.219	0.108	0.782	0.164	0.255	1.90	0.97	30
7	0.788	0.208	0.151	0.762	0.160	0.257	1.87	0.97	27
8	0.721	0.190	0.232	0.842	0.176	0.259	1.78	1.10	29
9	0.641	0.169	0.321	0.800	0.167	0.261	1.70	1.11	29

TMB=Trimethyl borate B(OCH 3 ) 3

Ratio H 2 gamma=Ratio of fed-in hydrogen in the core (taking into account the hydrogen contained in TMB) to stoichiometrically required hydrogen

Ratio O 2 lambda=Ratio of fed-in oxygen (atmospheric oxygen+additionally added 02) in the burner to stoichiometrically required oxygen

TABLE 5
Analytical data of the samples (Al—B—Si mixed oxide) obtained
according to Examples 5 to 9
	Before deacidification	After deacidification
	BET	B 2 O 3	Al 2 O 3	TV	GV	BET	B 2 O 3	Al 2 O 3	TV	GV
No.	m 2 /g	Wt. %	Wt. %	Wt. %	Wt. %	m 2 /g	Wt. %	Wt. %	Wt. %	Wt. %
5	28	12.3	3.9	0.5	3.2	27	13.5	4.5	0.2	2.1
6	30	13.9	8.3	1.6	3.3	29	13.9	9.3	0.2	2.2
7	27	14.3	12.9	0.7	3.5	29	13.0	12.9	0.6	1.7
8	29	14.6	19.3	1.9	3.7	32	13.2	19.4	1.1	1.5
9	27	12.9	29.0	0.6	1.9	34	12.4	27.0	0.3	1.5

TV=Drying loss (2 h at 105° C., according to DIN/ISO 787/II, ASTM D 280,

JIS K 5101/21)

GV=Annealing loss (2 h at 1000° C., according to DIN 5bY21, ASTM D

1208, JIS K 5101/23, referred to the substance dried for 2 hours at 105° C.

TABLE 6
Further analytical data of the samples (Al—B—Si-mixed oxide) obtained
according to Examples 5 to 9
	Before deacidification	After deacidification
		Cl		Bulk	Tamped		Cl		Bulk	Tamped
	BET	content		density	density	BET	content		density	density
No	m 2 /g	ppm	pH	g/l	g/l	m 2 g	ppm	pH	g/l	g/l
5	28	150	3.47	65	82	27	130	3.90	175	236
6	30	148	3.79	76	91	29	85	4.06	190	240
7	27	132	3.84	80	97	29	115	4.38	210	270
8	29	223	4.21	74	97	32	310	4.45	202	255
9	27	210	4.33	75	90	34	414	4.55	215	276

pH=pH in 4 percent aqueous suspension.

Tamped density according to DIN/ISO 787/XI, JIS K 5101/18 (not screened).

Claims

1. A process for preparing a boron oxide-silicon dioxide mixed oxide, comprising the steps of:vaporizing at least one member selected from the group consisting of a silicon halide and organosilicon halide in a carrier gas; vaporizing a boron compound in a carrier gas; mixing the halide and boron vapors with hydrogen and air; reacting the mixed vapors and gases in a flame; and cooling the reaction product and separating gas from solid material.

2. A process according to claim 1, wherein;mixing of the halide and boron vapors with hydrogen and air additionally occurs in the presence of at least one member selected from the grout consisting of oxygen and nitrogen.

3. A process according to claim 1, further comprising:heat treating the solid material to remove residue adhering thereto.

4. A process according to claim 2, further comprising:heat treating the solid material to remove residue adhering thereto.

5. A process according to claim 1, further comprising:additionally vaporizing at least one member selected from the group consisting of a metal and a metalloid in a carrier gas.

6. A process according to claim 5, wherein;mixing of the vapors with hydrogen and air additionally occurs in the presence of at least one member selected from the croup consisting of oxygen and nitrogen.

7. A process according to claim 5, further comprising:heat treating the solid material to remove residue adhering thereto.

8. A process according to claim 6, further comprising:heat treating the solid material to remove residue adhering thereto.

9. A process according to claim 1, further comprising:additionally mixing at least one member selected from the group consisting of a metal and a metalloid, with the halide and boron vapors, hydrogen and air.

10. A process according to claim 9, wherein:mixing of the vapors with hydrogen and air additionally occurs in the presence of at least one member selected from the group consisting of oxygen and nitrogen.

11. A process according to claim 9, further comprising:heat treating the solid material to remove residue adhering thereto.

12. A process according to claim 10, further comprising:heat treating the solid material to remove residue adhering thereto.