Process for compacting pyrogenic oxides

The invention relates to a process for compacting pyrogenic oxides.

Pyrogenic oxides, for example fumed silica, are known from Ullmanns Enzyklopadie der technischen Chemie, 4th Edition, Volume 21, pages 464 ff.

These pyrogenic oxides have a low bulk density. Owing to their particular agglomerate structure, they are present in a mixture with a large amount of air. For instance, a pyrogenic silica may have a tamped density of only 20 g/1. Owing to this low tamped density, it is necessary to compact the pyrogenic oxides in order, for example, to save packaging material or other transport costs .

It is known that pulverulent substances can be compacted by applying reduced pressure and mechanical pressure to rotating gas-pervious surfaces. At the same time, the entire filter surface which does not serve for mechanical compaction or is covered with mechanical compacted material moves within a closed casing in the material to be compacted and conveys the material to the compaction site. Before the compaction, the layer thickness of the uncompacted material on the roller is adjusted by means of a scraper.

The compacted material is removed from the rollers by means of stripping devices and falls downward toward the packaging (DE-B 11 29 459) .

DE 38 03 049 discloses filling silo vehicles with high- dispersity pulverulent substances without impairing the performance properties of the filled substance, by filling the silo vessels with the high-dispersity pulverulent substance, for example, by means of a

membrane pump and then charging the interior of the silo vessel with an inert gas, for example with compressed air, and then releasing the elevated pressure. For example, in the case of pyrogenic silica, a pressure difference of 2 bar with a rate of pressure change of 0.2 bar/min is employed in order to achieve an increase in the bulk density from 38 g/1 to 80 g/1.

According to DE 38 03 049, the supply of the compressed air and the subsequent pressure release are effected through the same outlet orifice.

The known process has the disadvantage that, owing to the large volume of the silo vehicles, the pressure buildup is not possible instantaneously. Tests with pyrogenic oxides have shown that the more rapidly the compressed air blast can be applied, the greater the compaction can be achieved. The more rapidly the compressed air blast can be applied, the greater is the momentum which acts on the particles and which brings about the compaction. Moreover, especially fumed silica is readily refluidizable . Patent DE 38 03 049 does not describe a strategy for how the pressure is released without the reversibly compacted powder being refluidized again. In the case of a very rapid pressure release, there would be a very great upward gas flow which loosens the compacted bed again and thus at least partly reverses the compaction result. Tests with fumed silica have shown the following. In the course of pressure release, the compaction can be maintained only when the pressure is released very slowly. At the same time, the upward flow of the expanding gas which is enclosed in the powder bed has to be sufficiently low that no particles are entrained or fluidized.

US 2004/0112456 Al discloses compacting inorganic free- flowing metal oxide powders which, owing to the high proportion of air incorporated in the particle

structure, have a relatively low bulk density, for example titanium dioxide, in order to save transport costs .

In this case, a gas, for example air, is introduced rapidly into the container in which the powder is present, which compacts the powder.

In this case, the gas pressure at one end of the container can be introduced against the second end of the container. As soon as the pressure has built up, the second end is opened and the compacted powder is driven out of the container when the pressure is released if it is to be treated further. This procedure has the disadvantage that it is suitable only for powders in which high interparticulate forces prevent refluidization of the powder. Pyrogenic oxides, for example fumed silicas, can be refluidized readily. The process described in US 2004/0112456 Al is therefore unsuitable for sufficient compaction of a fumed silica.

In one variant of the process according to US 2004/0112456 Al, the powder to be compacted is introduced into a reactor, which is subsequently closed. Subsequently, via a valve arranged in the lid of the reactor, compressed air is introduced, which compacts the powder. The elevated pressure is then released through a second valve arranged in the lid of the reactor.

For example, the pressure was built up within 3 to 5 sec, while the release was effected over a period of 10 to 15 sec.

After the pressure release, the compacted powder is removed again from the reactor. It is found that the rapid pressurization is crucial for the significant compaction of the pigment and the bulk density increases with the increase in the pressure. Whether

the pressure is built up rapidly or slowly is unimportant for the degree of compaction according to US 2004/0112456 Al.

The known process has the disadvantage that, if it is applied to pyrogenic oxides, it would lead to a refluidization of the compacted bed. Beds of pyrogenic oxides are readily refluidizable . For this reason, it very probably makes a difference for this product whether the pressure is built up rapidly or slowly. In the systems described in US 2004/0112456 Al, it is stated explicitly that it has no effect on the compac ^¬ tion if the pressure is built up rapidly or slowly.

It is thus an object of the invention to develop a process for compacting pyrogenic oxides which does not have these disadvantages.

The invention provides a process for compacting pyrogenic oxides in a vessel, the compaction being effected by means of pressurization, which is characterized in that the pressure is released while preventing refluidization of the compacted oxide.

The pressure surge for compacting a pyrogenic oxide should be at a speed of greater than 1 bar/min, preferably greater than 5 bar/min, more preferably greater than 10 bar/min.

The absolute pressure required for the compaction should be greater than 0.3 bar, preferably greater than 0.5 bar, more preferably greater than 1 bar.

Conversely, the pressure should be released more slowly than 10 bar/min, preferably more slowly than 6 bar/min, more preferably more slowly than 2 bar/min.

In order to prevent refluidization of the compacted

bed, the compacted bed, after the compaction and the pressure release, should ideally be discharged by means of pneumatic suction or in a dense stream delivery system.

The process according to the invention is illustrated in detail with reference to drawings.

The drawings show:

Figure 1 the spherical pressure vessel 53. Compressed air is applied and released from the top via valves 54 and 55. First, the vessel is filled by opening the lid 56. The lid 56 and is then closed gas-tight. Via the valve 54, a compressed air blast is applied to the bed which is compacted. The valve 54 is then closed and the valve is opened slowly in order to release the elevated pressure. On completion of the pressure release, the compacted bed can be

Figure 2 the product line 47 which is segmented by gas-tight flaps 48 and 49. Opening the upper flap 48 fills the segment with uncompacted powder. Subsequently, the two flaps 48 and 49 are closed gas-tight and a pressure surge is applied to the segment. Pressure application and release are carried out via the three valves 50, 51 and 52.

Figure 3 the negative conical vessel 42. It features a trapezoidal cross section. The advantage of this vessel geometry is that fewer solid bridges form therein

and thus higher bulk densities can be achieved. Via the flap 46a and the fill ^¬ ing nozzle 46, vessel 42 is filled with uncompacted powder. Thereafter, the nozzle 46 is closed gas-tight via the flap 46a. The flap 43 is likewise closed gas-tight. A compressed air blast is then applied to the bed via the valve

44. Before the valve 45 is opened, the valve 44 has to be closed. The pressure is slowly released again via the valve

45. Opening of the gas-tight flap 43 allows the compacted powder to be removed from the vessel 42.

Figure 4 and 5 a product line 41 segmented by the gas- tight flaps 39 and 40 before and after the compaction of the solid bed 60 by a pressure surge. First, the product line is filled with uncompacted powder by opening the flap 39. The flaps 39 and 40 are then closed gas-tight. Pressure application and pressure release are effected by virtue of the upper flap 39 having a fluidiza- tion tray 39a in the downward direction. In other words, it is provided with a porous sintered material or air-pervious fabric. This allows the compressed air to be applied uniformly over the entire tube cross section. Pressure application and pressure release are controlled, for example, by means of a three-way valve.

Figure 6 the combination of the segmented product line 32 with the vacuum packer 33. In this combination, the segment is first filled with uncompacted powder by

opening of the upper flap 34. The two flaps 34 and 35 are then closed gas- tight. A pressure surge through the valve 36 compacts the enclosed bed. The valve 36 is then closed again and the elevated pressure is released slowly through the valve 37. After the pressure has been released, the two flaps 34 and 35 open and a vacuum packer 33 customary on the market draws the compacted bed from the product line into the paper sack 38 by means of a reduced pressure. The advantage of this combination lies in the lower filling times, since less air per filtration has to be drawn out of the bed at the sack wall.

Figure 7 the combination of the pressure surge compaction with the screw packer. The outlet of the filling nozzle can be closed by a gas-tight flap 1. First, the vessel 27 is filled with uncompacted powder through the filling nozzle 28. This nozzle is then closed by means of the gas-tight flap 28a. The flap 1 on the filling nozzle is likewise closed gas-tight. The powder is then compacted by a pressure surge by opening the valve 29. The valve 29 is then closed and the elevated pressure is released through the valve 30 without the powder being refluidized. After the pressure has been released, the flap 1 is opened and the compacted powder is conveyed into the sack 31 via the screw. The precompaction of powder has an effect especially toward the end of the sack filling when the sack is already almost completely

filled, since less air has to escape through the sack wall during the sack filling .

Figure the pressure vessel 23 in combination with the vacuum packer 22 customary on the market .

The outlet of the pressure vessel 23 is closable by the gas-tight flap 2. Uncom- pacted powder is introduced via the product line 24 into the pressure vessel 23. After the vessel 23 has been filled, this inlet is closed with the gas-tight flap 24a. Opening of the valve 25 imparts an abrupt pressure surge to the bed in the vessel 23 and compacts the powder. The valve 25 is then closed and the pressure is released slowly via the valve 26. After pressure buildup, the flap 2 is opened. A reduced pressure sucks the precompacted bed into the paper sack in the vacuum packer. The advantage of the precompaction can be considered to be that of shorter filling times, since less air has to be with ^¬ drawn from the powder during the sack filling by filtration at the sack wall.

Figure 9 the combination of a pressure vessel with a filling station for containers (FIBC) .

The apparatus 4 is known from WO 03/006314 Al.

The apparatus 4 consists of the filling apparatus, by means of which the compacted fumed silica is filled into a container. The container is held in a cage apparatus which consists of two

parts which can be folded out, and filled, for example, by means of a membrane pump. The pressure vessel 8 can be closed gas-tight with the flap 3 at its outlet. Uncompacted powder is filled into the pressure vessel 8 via the product line 5. After the filling of the vessel 8, this inlet is closed with the gas-tight flap 9. Opening of the valve 6 imparts an abrupt pressure surge to the bed in the vessel 8 and compacts the powder. The valve 6 is then closed and the pressure is released slowly via the valve 7. After the pressure has been released, the flap 3 is opened and the container is filled with the compacted powder in apparatus 4.

Figure 10 the combination of the vacuum compactor roller 11, customary on the market, with the pressure surge vessel 15. The outlet of the pressure vessel can be closed gas-tight with the flap 10. The vessel 15 is filled via the inlet orifice 12, which can likewise be closed gas-tight by means of a flap 12a. The vessel is first filled with uncompacted powder. The flaps 10 and 12a are then closed gas-tight. The valve 14 is used to impart a rapid pressure surge to the bed and to compact the powder. The valve 14 is then closed and the pressure is slowly released again via the valve 13. The compacted bed is then introduced onto the vacuum compactor roller 11 by opening the flap 10. The advantage of the precompaction can be considered that of a greater capacity of the vacuum

roller press.

Figure 11 the combination of the pressure surge vessel 21 with the compactor 17 custo- mary on the market .

The outlet of the vessel 21 is closable by means of the gas-tight flap 16. Simi ^¬ larly to Figures 8, 9, 10, the bed in the vessel 21 is first compacted by a pressure surge. The vessel 21 is filled by means of the inlet orifice 18, which can likewise be closed gas-tight by means of a flap 18a. The vessel is first filled with uncompacted powder. The flaps 16 and 18a are then closed gas- tight. A rapid pressure surge is imparted to the bed via the valve 19 and the powder is compacted. The valve 19 is then closed and the pressure is slowly released again via the valve 20. By opening the flap 16, the compactor is charged with precompacted powder. The advantage of the precompaction lies in a higher capacity of the compactor 17.