Inhomogeneous silicas as carrier material
The invention relates to silicas having an inhomogeneous structure or composition, to processes for preparing them, and to their use as carrier material.
Readily dispersible silicas are prepared, for example, in accordance with EP 0 901 986 or EP 0 647 591 by precipitating waterglass with sulfuric acid, followed by drying. The dried products are subsequently ground and/or granulated.
By means of mechanical granulation, any silica can be prepared in dust-free form; however, this additional process step generally brings about a deterioration in the dispersibility.
In another process, silicas are prepared, likewise by acid precipitation, but are dried by spraying with hot air and at the same time are shaped into beads, which are easily destroyed. Thus EP 018 866 describes the preparation of spray-dried silica having an average particle diameter of more than 80 jam, the particles being solid and possessing a homogeneous structure.
Spray-dried silicas in accordance with EP 0 018 866 are particularly suitable as carrier material since they are dust-free and possess a high sorbency. Dust freedom is an important criterion for the processing of the silica, since simple processing of the silicas without corresponding suction exhaust units is of great economic importance. Besides freedom from dust, the specific surface areas (BET, CTAB) and the oil absorption capacity (DBP) are important for the carrier material utility.











of the apparatus are knocked off by careful tapping on the butter, of the sieve cover. The sieving operation genera J Ly lasts 5 minutes. It is s: an end when the residue remains constant;, generally evident from the free-flowing appearance. Sieving 13 then continued for one more minute in order 10 be on '-he aa i'e side.
If any agglomerates form, the sieving operation 13 briefly interrupted and tha agglomerates are broken down under gentle pressure using a brusn. After sieving, the sieve residue is carefully knocked from Che c-ir jar sieve and reweighed. Ih? sieve residue is expressed in percent, always in conjunction with the mooh oiue of "he sieve.


Example 1
The precipitation suspensions of the silica fractions A and B were itiixed in a 50:50 ratio. This was done by mixing 80 kg of the precipitated silica A (solids content approximately 46 g/i) with 80 kg of the precipitated silica B (solids content approxiinatGly 64 q/1) in a stirred vessel. The resulting mixture was filtered and the filtercake was licruefied with a small amount of acid and sprayed in a jet to-wei: drier. The analytical data are compiled in Table 1.
Example 2
The precipitation suspensions of the silica fractions A and B were mixed in a 70:30 ratio. This was done by mixing 112 kg of the precipitated silica A (solids content approximaiely 4 6 g/1) with 48 kg of the precipitated silica B (solids content approximately 64 g/1) in a stirred vessel. The resulting mixture was filtered and the filtercake was liquefied with a small amount of acid and sprayed in a jst tower drier. The analytical data are compiled in Table 1.
Example 3
The precipitation suspensions of the silica fractions A and B were nixed in a 30:70 ratio. This was done' by mixing 43.8 kg of the precipitated silica A (solids content approximately 46 g/1) with 102.2 kg of the precipitated silica E (solids content approximately 64 g/1) in a stirred Vessel. The resulting irlxture was filtered and the filtercake was liquefied with a small amount of acid and sprayed in a jet tower drier. The analytical data are compiled i:i Tab_e 1.
Example 4
A mixture of the dried silica fractions (50:50) was Piepared.



Performance properties of the silica of the invention
Flow properties of the silica
The products prepared in accordance with the invention, of Examples 1-3, have very good intrinsic flowability.

The maximum choline chloride absorption provides important information on the absorption capacity of a silica. Sine more highly concentrated adsorbates are of advantage, the desire is for as high an absorption capacity as possible. The maximum choline chloride absorption of the inhomogeneous silicas is much higher than in the case of prior art products.

In addition to a high absorption capacity for liquids, it is necessary that the resulting adsorbates are also readily flowable. As an example, a 50% adsorbate of

choline chloride on the corresponding silica was prepared from 66.6 g of a 75% strength aqueous choline chloride solution and 33.3 g of the respective silica, and the flowability was assessed by means of glass efflux vessels and the conical bed height. The inhomogeneous silicas DTT 3120 and DTT 3140 give advantages over standard silicas here (Hubersil 5170).

The agglomerate content gives important information on whether a silica is suitable for use as a carrier substance: a high agglomerate content is undesirable, since it leads to an adsorbate which is difficult to process. The agglomerate content of a 50% choline chloride concentrate prepared from 100 g of the corresponding silica and 200 g of a 75% strength aqueous choline chloride solution, is very low, at 0.3 - 2.1%, for the inhomogeneous silicas investigated. The comparative silicas have much higher agglomerate contents.


Another important parameter for the application is the sorption rate, since in the industrial production of adsorbates the aim is for high throughputs and thus short residence times in the mixer. In the case of the inhomogeneous silicas investigated, the sorption rate for vitamin E acetate is better than that of the comparative products Sipernat 2200 and Hubersil 5170.

The methods of measuring the flow properties, choline chloride absorption, agglomerate content, and sorption rate are in accordance with the brochure *Synthetische Pigmente als Fliefiihilfsmittel und als Tragersubstanz" [Synthetic pigments as flow aids and carriers]. Pigments Brochure Series No. 31, Degussa AG, 1992, and also Nos. 1 and 30.

The results of the investigation demonstrate that the novel inhomogeneous carrier silicas are suitable for preparing highly concentrated adsorbates, are readily flowable, and produce little dust. This is demonstrated from the example of the absorption of vitamin E acetate and 75% strength aqueous choline chloride solution. Both products are used in the adsorbate form in the feed industry. Also conceivable in practice is the preparation of other highly concentrated adsorbates, such as melamine resins (additive in the rubber industry), acids, e.g., formic or phosphoric acid (feed industry), and pigments, e.g., tagetes extracts (feed industry).

What is claimed is:
1. A silica comprising at least two silica fractions, wherein said at least two silica fractions differ by at least 10% in at least one value for BET surface area, CTAB surface area and DBP absorption.
2. The silica as claimed in claim 1, which is in the form of particles having an average diameter of more than 8 0 pm.
3. The silica as claimed in either of claims 1 and 2, which has the following physicochemical data:
BET surface area 100 - 900 mVg, CTAB surface area 100 - 500 mVg, DBP absorption 150 - 350 g/100 g.
4. The silica as claimed in any of claims 1 to 3, wherein the respective proportion of one silica fraction in the silica is between 5 and 95% by weight.
5. The silica as claimed in any of claims 1 to 4, which is hydrophobicized.
6. The silica as claimed in any of claims 1 to 5, wherein at least one silica fraction is hydrophobicized.
7. The silica as claimed in any of claims 1 to 6, wherein one or more silica fractions comprise a precipitated silica.
8. The silica as claimed in any of claims 1 to 7, wherein the silica fractions are prepared by precipitating silicate with an acid and the resulting precipitation suspensions are mixed.

9. The silica as claimed in any of claims 1 to 7, wherein the silica fractions are prepared by precipitating silicate with an acid, the precipitation suspension is filtered, and the resulting filtercakes are mixed.
10. The silica as claimed in any of claims 1 to 7, wherein the silica fractions are prepared by precipitating silicate with an acid, the filtercakes or ready-dried silica are liquefied, and the resulting suspensions are mixed.
11. The silica as claimed in any of claims 1 to 6, wherein one or more silica fractions comprise a pyrogenic silica.
12. The silica as claimed in any of claims 1 to 7 and 11, wherein the silica fractions are mixed in the dried state.
13. A process for preparing silicas comprising at least two silica fractions, which comprises mixing with one another at least two silica fractions which differ by at least 10% in at least one value for BET surface area, CTAB surface area and DBP absorption.
14. The process as claimed in claim 13, wherein the silica is in the form of particles having an average diameter of more than 80 ]im.
15. The process as claimed in either of claims 13 and 14, wherein the silica has the following physicochemical data:
BET surface area 100 - 900 mVg, CTAB surface area 100 - 500 mVg, DBP absorption 150 - 350 g/100 g.

16. The process as claimed in any of claims 13 to 15, wherein the respective proportion of one silica fraction in the silica is between 5 and 95% by weight.
17. The process as claimed in any of claims 13 to 16, wherein the silica is hydrophobicized.
18. The process as claimed in any of claims 13 to 16, wherein at least one silica fraction is hydrophobici zed.
19. The process as claimed in any of claims 13 to 18, wherein one or more silica fractions comprise a precipitated silica.
20. The process as claimed in any of claims 13 to 19, wherein the silica fractions are prepared by precipitating silicate with an acid and the resulting precipitation suspensions are mixed.
21. The process as claimed in any of claims 13 to 19, wherein the silica fractions are prepared by precipitating silicate with an acid, the precipitation suspension is filtered, and the resulting filtercakes are mixed.
22. The process as claimed in any of claims 13 to 19, wherein the silica fractions are prepared by precipitating silicate with an acid, the filtercakes or ready-dried silica are liquefied, and the resulting suspensions are mixed.
23. The process as claimed in any of claims 13 to 18, wherein one or more silica fractions comprise a pyrogenic silica.

24. The process as claimed in any of claims 13 to 19
and 23, wherein the silica fractions are mixed in
the dried state.
25. The use of the silica as claimed in any of
claims 1 to 12 as carrier or support material.
26. The use of the silica as claimed in any of
claims 1 to 12 as carrier material for vitamins,
vitamin acetates, choline chloride, proteins or
enzymes.
27. The use of the silica as claimed in claim 26 as
support material for catalytically active
substances.