Method for compacting pyrogenically prepared oxides

The invention relates to a method for compacting pyrogenically prepared oxides.

Pyrogenically prepared oxides, such as, for example, pyrogenically prepared silica, are known from Ullmanns Enzyklopadie der technischen Chemie [Ullmann' s Encyclopaedia of Industrial Chemistry] , 4th edition, volume 21 pages 464 et seq.

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

It is known that pyrogenically prepared silica can be compacted by mechanically moving it in the presence of an aqueous liquid compacting agent.

Bulk densities of up to 1200 kg/m ³ are achieved thereby (US 2005/0161167 Al) .

The known method has the disadvantage that the pyrogenically prepared silica is changed in an undesired manner in its performance characteristics by the addition of the compacting auxiliary.

It is known that pulverulent substances can be compacted by applying reduced pressure and mechanical pressure to rotating gas-permeable surfaces. The entire filter surface which does not serve for mechanical compaction or is covered with mechanically compacted material moves within a closed housing in the material

to be compacted and transports the material to the compaction site. For the compaction, the layer thickness of the uncompacted material on the roll is adjusted by means of a scraper.

The compacted material is removed from the rolls by means of stripping apparatuses and falls downwards towards the packaging (DE-B 11 29 459) .

In the case of rolls having the woven fabric covering, the scraper must maintain a minimum distance from the roll surface and therefore cannot completely strip off the compacted silica.

The residue layer resulting therefrom proves to be a considerable disadvantage. Firstly, a not inconsiderable transported quantity is held thereby and secondly the pressure difference at the rolls is reduced to such an extent that the degree of compaction decreases.

The known method has the disadvantage that very strong compaction is necessary in order to compensate for the loss of compaction due to the loosening of the product on the way from the compaction apparatus to the packing machine. For achieving a certain degree of compaction in the packaging, it is therefore necessary to establish substantially higher compaction at the compaction apparatus. This gives rise to the danger that, owing to the high compaction, the performance characteristics of the pyrogenically prepared silica are lost.

It is furthermore known that pyrogenically prepared silica can be compacted by compacting the pyrogenically prepared silica by means of a rotary vacuum filter which is equipped with a press belt, the pyrogenically prepared silica being initially introduced into a

container, the rotary vacuum filter being arranged so as to be movable within this container, and the layer of the compacted product being removed from the filter drum by interrupting the vacuum (EP 0 280 851 Al) .

The known methods have firstly the disadvantage that they are relatively complicated and expensive and secondly the disadvantage that compaction of, for example, pyrogenically prepared silica is irreversible.

It was therefore the object to develop a method by means of which the compaction of pyrogenically prepared metal oxides and/or metalloid oxides can be carried out economically. Furthermore, the compaction is to be effected in such a way that it is reversible without adversely affecting the properties of the oxides.

Furthermore, no compaction auxiliary is to be used.

The invention relates to a method for compacting pyrogenically prepared metal oxides and/or metalloid oxides, which is characterized in that the pulverulent pyrogenically prepared metal oxides and/or metalloid oxides are introduced into a stirred container, stirred by means of a stirring member and discharged from the stirred container.

According to the invention, no mass transport should take place. The desired measure is mechanical breaking up of solid bridges, it being possible for a reorientation of the particles or a discharge to take place .

Channels are created through which the air can escape from the heap of particles.

A rabble stirrer, a helical stirrer, a stirrer in the form of a rake, an anchor stirrer or the like can be used as the stirring member.

The stirred container can be subjected to reduced pressure to promote the de-aeration.

In a preferred embodiment of the invention, a helical stirrer can be used as the stirring member.

The stirred container may consist of a cylindrical upper part and a conical lower part. The stirring member can be adapted to the inner surface of the stirred container.

The compacted metal oxide and/or metalloid oxide can be removed at the lower end of the conical part of the stirred container via a discharge member.

The stirred container may furthermore be cylindrical and have a flat base.

In another embodiment of the invention, the stirred container may have a flat base and may have a negatively conical shape (trapezoidal longitudinal cross section) .

The compacted oxide is discharged, optionally via a discharge member, likewise at the base of the container.

In one embodiment of the invention, the stirred container can be subjected to at least one compressed gas pulse before, during and/or after the stirring and then let down before the discharge of the oxide from the stirred container.

The compressed gas used may be any gas. In a preferred embodiment of the invention, air can be used as the compressed gas.

Pyrogenically prepared oxides are understood as meaning oxides of metals and/or metalloids which were prepared by means of flame or high-temperature hydrolysis. Such oxides may be silica, alumina, titanium oxide or iron oxide and mixed oxides thereof. This method is known from Ullmanns Enzyklopadie der technischen Chemie [Ullmann' s Encyclopaedia of Industrial Chemistry] , 4th edition, volume 21, page 464 et seq. For example, silicon tetrachloride vapour, hydrogen and air are combusted in a cooled combustion chamber.

For this purpose, the oxyhydrogen flame provides both the energy and the amount of water required for the hydrolysis of the silicon tetrachloride. Instead of silicon tetrachloride, other vaporizable silicon compounds may be used.

For the preparation of the mixed oxides, mixtures of different metal chlorides can be hydrolysed together in the flame. The pyrogenically prepared metal oxide and/or metalloid oxide used may be, for example, pyrogenically prepared silica.

If it is taken, for example, from a silo, it may have a bulk density of 20 g/1 prior to compaction if it is hydrophilic.

By means of the method according to the invention, a bulk density of more than 50 g/1, preferably of more than 90 g/1, can be achieved.

By means of the method according to the invention, a hydrophobic, pyrogenically prepared silica which has a bulk density of 20 to 30 g/1 can be compacted to a bulk

density of more than 40 g/1, preferably of more than 50 g/1.

According to the invention, a bed having a high air retention power and low bulk density is gently compacted by slow stirring. No additive, such as, for example, water or an oil, is added.

Thus, for example, a pyrogenic silica from a silo and having a BET surface area of 150 or 200 m ² /g can be compacted by slow stirring. The initial bulk density of the material from the silo is between 20 and 30 g/1. The stirring brings about de-aeration of the bed, with the result that the bulk density increases to about 40 g/1. The circumferential speeds of the stirrer may be, for example, less than 1 m/s, ideally less than 0.5 m/s. The stirrer speeds are, for example, less than 15 rpm, ideally less than 8 rpm.

The aim of the stirring is to break up solid bridges in order to bring about the sedimentation of the particles in the heap. At the same time, the stirring produces channels in the heap which improve the de-aeration. Furthermore, the stirring produces a slight reorientation of the particles in the heap. However, stirring must be effected so slowly that the heap is not fluidized by the stirring.

The stirrer may have the form of a spiral or of a rabble. The stirring should not produce any mass flow in the container but only "plough" through the heap.

The compaction or de-aeration of the bed which was brought about in this manner is gentle. It is distinguished from other compaction methods, such as compactor, vacuum press belt filter, etc., in that it is completely reversible owing to fluidization, i.e. the particle structure is not changed. This is evident

from the fact that the thickening effect, the sieve residue mocker 45 μm and the grindometer value do not change in comparison with the material from the silo.

Because the de-aeration is reversible, the discharge of the compacted heap from the stirred container is not trivial. The heap must not be refluidized thereby. One possibility is to carry out the transport from the stirred container by means of a vacuum or reduced pressure transport, i.e. to carry out pneumatic transport with suction. Dense-stream transport is also conceivable for discharging the pre-deaerated bed from the stirred container without refluidizing it in the process .

A possible field of use for this precompaction is to increase the bagging performance of packing machines. The packer and stirred container are connected to one another by a product line. The product is removed from the stirred container ideally at the lowest point of the stirred container. Here, the packing machine (vacuum packer) generates a reduced pressure which sucks the compacted heap out of the container. By applying a strong reduced pressure, re-aeration of the heap is suppressed.

The advantage for bagging is that the de-aeration of the powder bed to be filled need no longer be effected completely by the vacuum packer but that a large part of the air is removed in the powder bed by the stirring. The de-aeration of the heap in the container

(paper bag) is in principle a filtration. The filter medium is the container, for example the paper bag.

This gives rise to the disadvantage that the powder bed in the container is not present with a homogeneous density. In the region of the bag walls, there is high compaction. This decreases to an increasing extent with increasing distance from the container wall. As a

result of this inhomogeneity, problems may arise with respect to performance characteristics. It has been found that bags which were filled with pyrogenic silica which was pre-deaerated by stirring had a significantly smaller density gradient than the reference bags. The reference bags were filled with material from the silo which had not been pre-de-aerated. A further advantage was significant reduction in the bagging time.

The compacted oxide can be discharged by means of pneumatic transport with suction or dense-stream transport, ensuring that re-aeration does not take place .

The method according to the invention can be combined with known compaction methods and/or packing methods.

The method according to the invention can preferably be combined with a vacuum packer. This combination has the advantage that the vacuum packer builds up a vacuum by means of which the de-aerated metal oxide and/or metalloid oxide can be removed without re-aeration from the apparatus for compaction and can be simultaneously packed.

Furthermore, the method according to the invention can be combined with other known compaction apparatuses.

Thus, for example, the compacted oxide can be further compacted by means of roll compacters . These are disclosed in US 3,742,566, US 3,860,682 and US 3,762,851.

Furthermore, the method according to the invention can be effected upstream of a packing process for containers, such as, for example, FIBC.

Such a method is disclosed in WO 03/006314 Al.

The method according to the invention has the advantage that the pyrogenically prepared metal oxide and/or metalloid oxide can be gently de-aerated in such a way that the compaction is reversible.

At the same time, the filling time for filling the packing in containers, such as, for example, bags or the like, can be substantially reduced. Furthermore, the tamped density of the oxides inside the bag is more uniform. The buildup of electrical charge can be avoided.

The invention is explained in more detail with reference to the drawings:

Figure Ia, show the schematic diagram of the Figure Ib and use of the method according to the Figure Ic inventionin a packing process by means of a vacuum packer

Figure shows the schematic diagram of the use of the method according to the invention in a packing process by means of a vacuum packer and bag press

Figure shows the graph of the degree of compaction of a hydrophobic pyrogenic silica as a function of time

Figure shows the graph of the results of various compaction experiments with a hydrophilic pyrogenic silica

Figure shows the graph of the increase of the bulk density as a function of

time

Figure shows the schematic diagram of a stirred container with a rabble stirrer

Figure shows the schematic diagram of a stirred container having a trapezoidal longitudinal section

Figure shows the schematic diagram of a packing apparatus for valve bags

Figure shows the schematic diagram of a packing apparatus for valve bags by means of a screw conveyor

Figure 10 shows the schematic diagram of the combination of a stirred container with a vacuum roll press

Figure 11 shows the schematic diagram of a stirred container with a packing apparatus for FIBC

Figure 12 shows the schematic diagram of a stirred container which can be subjected to compressed air.

According to Figure Ia, the hydrophilic pyrogenically prepared silica AEROSIL 200 is introduced from the silo 1 by means of the diaphragm pump 2 into the stirred container 3 which is equipped with the helical stirrer 4. During the stirring, the compacted pyrogenically prepared silica is discharged into the vacuum packer 5.

According to Figure Ib, the hydrophilic pyrogenically prepared silica AEROSIL 200 is discharged from the silo 1 by means of gravity into the stirred container 3 which is equipped with the helical stirrer 4, compacted there and at the same time discharged into the vacuum packer 5.

According to Figure Ic, the hydrophilic pyrogenically prepared silica AEROSIL 150 is introduced by means of the diaphragm pump 2 into the stirred container 3 which is equipped with the helical stirrer 4. It is compacted in the stirred container 3 and discharged into the vacuum packer 5.

According to Figure 2, the hydrophilic pyrogenically prepared silica is discharged from the silo 1 by means of gravity into the stirred container 3 which is equipped with the helical stirrer 4.

The pyrogenically prepared silica is compacted in the stirred container 3 by means of a stirrer and discharged in the vacuum packer 6 and packed there in the bag 7. The bag 7 is then pressed by means of the bag press 8 in order to achieve a volume reduction to the bag 9.

According to Figure 3, the greatest compaction speed is achieved during the first minutes of the stirring if hydrophilic pyrogenically prepared silica AEROSIL ^® 200 is compacted by the method according to the invention.

According to Figure 4, strong (about 40%) compaction of the hydrophilic pyrogenically prepared silica is achieved by the method according to the invention.

According to Figure 5, the hydrophobic pyrogenically prepared silica AEROSIL ^® 972 is compacted according to the invention by means of a stirrer. The bulk densities

achieved are plotted as a function of the stirring time .

According to Figure 6, pyrogenically prepared silica is introduced from a silo through the inlet 11 into the stirred container 3 which is equipped with the rabble stirrer 10. The stirrer 10 is caused to rotate by means of the motor 12, with the result that the silica is compacted.

The compacted silica is discharged from the stirred container 3 via the outlet 13.

In order to accelerate the compaction process, reduced pressure can be applied at the opening 14.

According to Figure 7, pyrogenically prepared silica is introduced from a silo through the inlet 17 into the stirred container 15 which has a trapezoidal longitudinal section and is equipped with the helical stirrer 16. The stirrer 16, which is adapted in its dimensions to the longitudinal section of the stirred container 15, is caused to rotate by the motor 18, with the result that the silica is compacted.

The trapezoidal cross section has the advantage that there is less possibility of the formation of solid bridges. Consequently, a higher bulk density can be achieved.

The compacted silica is discharged through the outlet 19 by means of the discharge member 20.

In order to accelerate the compaction process, reduced pressure can be applied at the opening 21.

According to Figure 8, the stirred container 3, which is equipped with the helical stirrer 4, is filled with

pyrogenically prepared silica via inlet 22. The stirrer 4 is caused to rotate by means of the motor 23, with the result that the silica is compacted. The compacted silica is fed via the discharge opening 24 to the vacuum packer 25 and packed there in a valve bag. De- aeration, optionally by means of a vacuum or reduced pressure, can be effected via the opening 26.

According to Figure 9, pyrogenically prepared silica is introduced through the inlet 27 into the cylindrical stirred container 28. The stirred container 28 is equipped with the helical stirrer 29, which is adapted to the geometry of the stirred container and is driven by the motor 30.

The silica compacted by the stirring movement of the helical stirrer 29 is transported by means of the screw conveyor 31, which is driven by means of the motor 32, into the valve bag 33.

During the compaction, de-aeration, optionally by means of reduced pressure, can be effected via the outlet opening 34.

According to Figure 10, the pyrogenically prepared silica is introduced via the feed opening 22 into the stirred container 3 and compacted by means of a helical stirrer 4 in the stirred container 3, discharged via the discharge opening 24 and compacted between the compacting rolls 35 and 36. The compacting rolls are ideally covered with sintered metal and subjected to a vacuum from the inside.

Such a compacter is known, for example from US 3,742,566, US 3,860,682 or US 3,762,851.

According to Figure 11, the pyrogenically prepared silica is introduced via the feed opening 22 into the

stirred container 3 and compacted by means of stirring with the stirrer 4, which is in the form of an anchor stirrer, in the stirred container 3, discharged via the discharge opening 24 and packed by means of the apparatus 37 into containers, such as, for example, FIBC. Between the discharge opening 24 and the inlet in the FIBC container, a diaphragm pump can be used. This transports the pre-deaerated silica from the stirred container into the container.

The apparatus 37 is disclosed in WO 03/006314 Al. The apparatus 37 consists of a filling apparatus, via which the compacted pyrogenically prepared silica is introduced into a container. The container is held in a cage apparatus which consists of two hinged parts.

Figure 12 shows the stirred container 3 in which the compaction is promoted by a compressed air pulse. Furthermore, the de-aeration opening 41 by means of which air can be removed from the stirred container 3 is arranged on the stirred container 3. The de-aeration can optionally be effected by application of reduced pressure .

Via the opening 40, a compressed air pulse can be applied to the stirred container 3 before, during and/or after the stirring, a further compaction of the pyrogenically prepared silica taking place thereby.

According to Figure 12, the pyrogenically prepared silica is introduced via the filling opening 37 into the stirred container 3 which is equipped with the anchor stirrer 4, compacted by stirring with the anchor stirrer 4 and discharged via the discharge opening 38, which can be connected gas-tight to the flap 39. The filling opening 37 can likewise be connected gas-tight via a flap. A compressed air pulse can be applied via the valve 40, before, during and/or after the stirring,

with closed flap 39, 41 and 37, in order to achieve further compaction of the pyrogenically prepared silica. After the treatment by means of a compressed air pulse, the system can be let down via the valve 41.

Examples

Three different arrangements according to Figure 1 are used.

1st arrangement

A stirred container 3 having a volume of 860 1 was connected to the vacuum packer 5.

The vacuum packer 5 was equipped with its own vacuum pump and its own vacuum buffer container. It was thus possible to ensure constant vacuum. The pyrogenically prepared silica AEROSIL ^® 200 was transported from the silo 1 by means of the diaphragm pump 2 into the stirred container 3.

The stirrer 4 is in the form of a spiral (helical stirrer) . The stirrer 4 performs two tasks. Firstly, it accelerates the de-aeration and the compaction of the oxide and secondly it ensures complete discharge of the compacted oxide from the container 3 into the vacuum packer 5.

This arrangement is shown schematically in Figure Ia.

2nd arrangement

A stirred container 3 having a volume of 860 1 was connected to the vacuum packer 5.

The pyrogenically prepared silica AEROSIL ^® 200 flowed directly by means of gravity out of the silo 1 into the

stirred container 3. It was compacted there by means of stirring and at the same time discharged into the vacuum packer 5. This arrangement is shown schematically in Figure Ib.

3rd arrangement

A stirred container 3 having a volume of 860 1 was connected to the vacuum packer 5. The pyrogenically prepared silica AEROSIL ^® 150 was transported by means of a diaphragm pump from the silo into the stirred container .

This arrangement is shown schematically in Figure Ic.

The further packing-related treatment of the AEROSIL ^® 150 by means of shaping of the container/bags under pressure is shown schematically in Figure 2.

Samples were taken from the packed bags, one sample being taken at the edge of the bag and a second sample from the centre. At the same time, the buildup of electrostatic charge was determined at the edge and in the centre of the bag. The EFM 231 device from Kleinwachter was used for this purpose.

Carrying out the experiments

25 kg of AEROSIL ^® were introduced from the silo, in which the pyrogenically prepared silica AEROSIL ^® had a bulk density of less than 35 g/1, into the stirred container and left to rest for 16 hours. At the start of the stirring, the bulk density increases sharply.

After 80 minutes, a bulk density of 50 g/1 was achieved.

In order to avoid long storage times, the stirring experiments were limited to 20 minutes. As shown in

Figure 3, it was possible to achieve the strongest compaction within the first minutes.

Figure 4 shows various compaction experiments with AEROSIL ^® , a stirring speed of 6 to 8 revolutions per minute being maintained. The circumferential speed of the stirrer was between 0.2 and 0.4 m/sec. These experiments were started immediately after the filling of the stirred container.

The method according to the invention produced a phenomenal change in the tamped density in the stirred container .

4th arrangement

The pyrogenically prepared, hydrophobic silica AEROSIL ^® R972 was investigated in an 8 1 stirred container.

The bulk density after the stirring was between 40 and 43 g/1.

Figure 5 shows the increase in the bulk density as a function of the stirring time.

The filling experiments were carried out with a pyrogenically prepared silica which had a bulk density of more than 40 g/1 after the compaction by stirring.

In order to avoid blockage of the lines, the distance between the discharge of the stirred container and the inlet of the packer was kept as short as possible.

By means of the method according to the invention, it was possible substantially to reduce the packing time.

A further advantage is that the flow rate of the pyrogenically prepared silica AEROSIL ^® could be kept

constant by the method according to the invention during the filling of a plurality of bags. A dependence of the flow rate on the level of fill of the silo, which caused an irregular flow rate, is no longer present.

A further advantage of the method according to the invention was found inside the packed bags. In comparison with the bags which had been filled with pyrogenically prepared silica AEROSIL ^® directly from the silo, the bags which had been filled with the pyrogenically prepared silica AEROSIL ^® compacted according to the invention have a uniform tamped density distribution within the amount of pyrogenically prepared silica AEROSIL ^® present in the bag. This gives rise to further advantages in terms of performance characteristics, such as, for example, the better incorporability into silicone rubber or Palatal.

A further advantage was found on determination of the buildup of electrostatic charge of the silica in the filled bags.

The bags which had been filled by means of the method according to the invention showed a buildup of static charge which was lower by a factor of 2 than the bags which had been filled with the pyrogenically prepared silica AEROSIL ^® which was taken directly from the silo.

Examples of industrially produced pyrogenic silicas which can be compacted by this method are the pyrogenic silicas with the brand names AEROSIL ^® , Cab-O-Sil ^® , HDK ^® .