SILICA GRANULES FOR THERMAL TREATMENT

Abstract

The invention provides fumed silica granules having a BET surface area of 20 m.sup.2/g to 500 m.sup.2/g; a number average particle size d.sub.50 of 350 μm to 2000 μm; a span (d.sub.90−d.sub.10)/d.sub.50 of particle size distribution of 0.8-3.0; a bulk density of more than 0.35 g/mL; a pore volume for pores >4 nm of not more than 1.5 cm.sup.3/g, process for its preparation and use thereof as a catalyst carrier, a carrier for liquid substances, in cosmetic applications, for thermal insulation, as pharmaceutical excipient, in producing thermally treated silica granules, as an abrasive, as a component of a silicone rubber.

Claims

1-15. (canceled)

16. Fumed silica granules comprising: a BET surface area of 20 m.sup.2/g to 500 m.sup.2/g; a number average particle size d.sub.50 of 350 μm to 2000 μm, as determined by laser diffraction; a span (d.sub.90−d.sub.10)/d.sub.50 of particle size distribution of 0.8-3.0, as determined by laser diffraction; a bulk density of more than 0.35 g/mL, as determined by mercury intrusion; a pore volume for pores >4 nm of not more than 1.5 cm.sup.3/g, as determined by mercury intrusion.

17. The fumed silica granules of claim 16, wherein the d.sub.10 of the granules is from 100 μm to 1000 μm, as determined by laser diffraction.

18. The fumed silica granules of claim 16, wherein the percentage of particles with a particle size of not more than 100 μm is less than 20% by weight of the granules.

19. The fumed silica granules of claim 16, wherein the span (d.sub.90−d.sub.10)/d.sub.50 of particle size distribution of the granules is 0.9-2.0.

20. The fumed silica granules of claim 16, wherein the tamped density of the granules is 300 g/L-600 g/L.

21. The fumed silica granules of claim 16, wherein the granules have a porosity of less than 77%, as determined by mercury intrusion.

22. The fumed silica granules of claim 17, wherein the particles with a particle size of not more than 100 μm is less than 20% by weight of the granules.

23. The fumed silica granules of claim 22, wherein the span (d.sub.90−d.sub.10)/d.sub.50 of particle size distribution of the granules is 0.9-2.0.

24. The fumed silica granules of claim 23, wherein the tamped density of the granules is 300 g/L-600 g/L.

25. The fumed silica granules of claim 24, wherein the granules have a porosity of less than 77%, as determined by mercury intrusion.

26. A process for preparing the fumed silica granules of claim 16, comprising the following steps: a) compacting fumed silica with a water content of 0.1%-10% by weight to obtain compacted silica fragments with a tamped density of at least 200 g/L; b) crushing the compacted silica fragments obtained is step a) and isolating crushed fragments with a size of not more than 2000 μm using a sieve with a mesh size of 1000 μm-2000 μm; c) separating fine particles from the crushed fragments with a size of not more than 2000 μm obtained in step b) using a sieve with a mesh size of 200 μm-600 μm; d) optionally repeating step a) with the sieved fine particles obtained in step c).

27. The process of claim 26, wherein the process is carried out continuously.

28. The process of claim 26, wherein, in step a), a fumed silica with a water content of 0.5%-5.0% by weight is used.

29. The process of claim 26, wherein the mesh size of the sieve used in step b) of the process is 1000 μm-1500 μm.

30. The process of claim 26, wherein the mesh size of the sieve used in step c) of the process is 400 μm-600 μm.

31. The process of claim 26, further comprising step e), wherein the granules obtained in step c) are exposed at a temperature of 400° C. to 1100° C., to an atmosphere which comprises one or more reactive compounds selected from the group consisting of: chlorine; hydrochloric acid; sulphur halides; sulphur oxide halides; hydrogen; and mixtures thereof.

32. The process of claim 26, wherein step a) is performed by means of two compacting rollers and the specific pressure applied between the two compacting rollers is more than 12 kN/cm.

33. The process of claim 28, wherein the mesh size of the sieve used in step b) is 1000 μm-1500 μm.

34. The process of claim 33, wherein the mesh size of the sieve used in step c) of the process is 400 μm-600 μm.

35. The process of claim 34, further comprising step e), wherein the granules obtained in step c) of the process are exposed at a temperature of 400° C. to 1100° C., to an atmosphere which comprises one or more reactive compounds selected from the group consisting of: chlorine; hydrochloric acid; sulphur halides; sulphur oxide halides; hydrogen; and mixtures thereof.

Description

EXAMPLES

[0085] Particles size of the particles (d.sub.10, d.sub.50, d.sub.50) was measured using a laser diffraction analyzer Beckman Coulter LS in a dry state.

[0086] Bulk density, porosity and the cumulative pore volume for pores larger than 4 nm were determined by the mercury intrusion method according to DIN ISO 15901-1 using AutoPore V 9600 device (Micomeritics). Only the pore volume of pores into which mercury can penetrate, i.e. the pores with a pore diameter of >4 nm, at the maximal pressure applied (417 MPa) was detected.

[0087] Specific BET surface area [m.sup.2/g] was determined according to DIN 9277:2014 by nitrogen adsorption in accordance with the Brunauer-Emmett-Teller method.

[0088] Preparation of Silica Granules

Example 1 (According to the Invention)

[0089] Fumed silica powder AEROSIL©90 (BET=90 m.sup.2/g, manufacturer: Evonik Resource Efficiency GmbH) is placed in a storage tank and treated with the demineralized water in a mixing unit (target value 1.5 wt % H.sub.2O). In this unit, the fines resulting from screening in one of the following process steps are then added and homogenized. From there, the material flows unfed, i.e. only due to its mass, into a hopper in which a stuffing screw rotates. The hopper is subjected to negative pressure from outside. Its walls consist of a cloth-covered sintered metal. While the material is vented by the vacuum, the stuffing screw transports the fumed silica powder to the rolls. Between the rolls, which have a corrugated profile (6 mm), the material is compressed with a specific pressure of more than 12 kN/cm and less than 18 kN/cm. Due to the corrugated profile, “rods” of compressed, compacted fumed silica are formed. These rods are then crushed in a screen granulator. The mesh size in the screen granulator is 1250 μm. The mesh size in the screen granulator limits the upper grain size. The lower size is defined in the subsequent screening.

[0090] In a screen with ultrasonic cleaning, the material broken up in the screen granulator is screened and the undersize is separated. The mesh size is 500 μm. The fines are returned to the storage container by a vacuum cycle conveyor.

Comparative Example 1

[0091] Conducted as example 1 but using a sieve with a mesh size of 100 μm instead of a sieve with a 500 μm mesh size used in example 1, in a subsequent screening.

Comparative Example 2

[0092] Silica granules were prepared from an aqueous dispersion containing 20 wt % AEROSIL© 90 by spray drying techniques (atomization by nozzle, pressure of dispersion: 8 bar) at an inlet temperature of 350° C. and outlet temperature of the spray-drier of 100° C.). Drying took place in a counterflow mode. The product was post treated in a fluidized bed to further increase agglomerate size. The finished product was separated by a filter.

[0093] Silica granules of example 1 and comparative examples 1 and 2 have the physico-chemical properties summarized in Table 1.

[0094] Thermal Treatment of Silica Granules

[0095] Silica granules of example 1 and comparative examples 1 and 2 were thermally treated continuously in a rotary kiln of ca. 140 mm diameter and 2 m length under identical conditions (maximal temperature=1350° C.). The feed rate of silica granules was continuously increased in each case until the first signs of overload and congestion were apparent. Thus, the maximal sintering performance [in kg/h] was determined and compared for different granules (Table 1).

[0096] The silica granules prepared in example 1 turned out to have a much higher maximal throughput rate without any congestion, than the silicas from comparative examples 1 and 2 (Table 1).

[0097] Granules of all three types have similar BET surface areas. Due to their preparation, the granules of example 1 have higher average particle size, higher bulk density, lower porosity and pore volume for pores >4 nm, than other granules (Table 1). Silica granules from comparative example 2 have a much higher flowability (data not shown in Table 1) and a narrower particle size distribution than the granules from example 1. Nevertheless, silica granules from example 1 achieve a higher maximal sintering performance, which cannot be explained purely by a particle form or size, but by a particularly suitable combination of relatively large average particle size, relatively low porosity and high bulk density of such granules.

TABLE-US-00001 TABLE 1 Properties of silica granules Bulk pore density volume > Porosity Ratio of by 4 nm by by Maximal particles < (d.sub.90- Hg- Hg- Hg- sintering d.sub.50 100 μm, d.sub.10)/ BET intrusion intrusion intrusion performance Example [μm] wt. % d.sub.50 [m.sup.2/g] [g/mL] [mL/g] [%] [kg/h] Example 1 920 5 1.40 93 0.665 0.88 58.4 6 Comparative 252 30 4.15 92 0.469 1.67 78.0 2 Example 1 Comparative 148 27 1.19 83 0.335 2.38 79.4 0.5 Example 2