EP0618065A1 - Process for compacting powdery materials - Google Patents

Abstract

Powdered materials are compacted by enclosing the powdered material in a flexible container, enclosing the container in an autoclave and subjecting the space between the container wall and the wall of the autoclave to compressed gas. <IMAGE>

Description

The invention relates to a method for compacting powdery substances to a predetermined range of bulk density, the powder structure of the powder being retained.

Commercially available steam-jet or air-jet-milled synthetic silicas, such as, for example, precipitated silicas, have, depending on the production or storage conditions, bulk densities of 50 to 90 g / l and a drying loss of 2 to 8% by weight. For some applications, it is necessary to reduce the water content to below 1% by weight using known drying processes. However, some of these processes have a loosening effect on the silica powder, i. that is, the bulk density is reduced to between 30 and 40 g / l during drying. Due to the greatly increased volume of the precipitated silicas, subsequent dosing and packaging is only possible with difficulty. The dried silicas should therefore be compressed to a higher bulk density.

It is known to compact powdery materials, such as synthetic silicas, by means of roller compactors, compacting screws, press belt filters and / or other devices.

These devices have the disadvantage that bulk densities in the range from 50 to 100 g / l cannot be set or cannot be set reproducibly. The compacted powders generally have undesirable inhomogeneities, such as nodules or similar undesirable constituents. In many cases it is not possible to loosen up the compacted powder, which is then present in flakes, clods or lumps. In addition, the known devices are expensive and wear-prone devices.

The object was to find a method and a device for compacting powdery substances to a predetermined range of the bulk density, the powder structure of the powder being retained and shaped bodies, such as clods, lumps or the like, formed during the compaction by the powder coalescing. , be avoided or disintegrate again without significant mechanical influence.

The invention relates to a method for compacting powdery substances to a predetermined range of bulk density, the powder structure of the powder being retained, which is characterized in that the powdery substance is hermetically sealed in a container provided with a flexible, gas-impermeable wall, this Contains container in a closed pressure vessel, the space between the outer wall of the pressure vessel and the container acted on by pressure gas, the pressure for a certain time maintained, relaxed and optionally removed the powdery substance with the container from the pressure vessel.

The container, which has a flexible gas-impermeable wall, can be a bag, a tube welded at the ends, a sack, a packet or the like. The external form is of minor importance. It is essential that its wall does not allow gas to pass through.

In the method according to the invention, when the pressure in the pressure vessel rises, the container containing the pulverulent substance is compressed from all sides (quasi isostatically) until the pressures in the pressure vessel and container are the same, with no gas exchange between the bag and the pressure vessel. The container also compresses the powdery substance to a smaller volume. When the compressed air is released, the bag inflates again to the initial volume, but the powdery substance maintains the smaller volume. The compression processes are shown schematically in FIG. 1 (phases 1 to 3).

The method according to the invention can be applied to all known powdery substances that can be pressed. It can advantageously be used to compress synthetic silicas, such as precipitated silicas or pyrogenic silicas and / or carbon blacks. In particular, it can be used to compact ground, such as Use air-blasted and / or steam-blasted precipitated silicas.

The process according to the invention has the advantage that a very homogeneously compacted powder is obtained. The degree of compaction can be set to a specific range of bulk density. In particular, the bulk density can be set in a targeted manner in the range from 50 to 95 g / l.

Another object of the invention is a device for compacting powdery substances to a predetermined range of bulk density, whereby the powder structure of the powder is preserved, which is characterized in that it consists of a preferably vertically arranged pressure vessel, which has any, preferably a circular cross-section has, on the upper and on the lower cross-sectional side each has a hermetically closable opening, is equipped on the inside with a flexible, gas-impermeable material, also open at the top and bottom, preferably tubular container.

In a preferred embodiment of the invention, the device can be arranged in a pipeline which carries the powdery substance. The compacted powder, which is present directly after the compacting process as a compacted molding or shaped body, and its shape may also form after the pressure has been released Maintaining an inelastic deformation can disintegrate into powder again without any significant mechanical effort, the actual powder structure being unchanged except for the bulk density.

The inventive method and the inventive device have the advantage that no mechanical parts are used to build up the pressure. Mechanical wear on the device cannot therefore occur.

example

The precipitated silica FK 500 DS, manufactured by Degussa AG, Frankfurt, is used in the implementation of the example. This precipitated silica has the following physico-chemical characteristics: Surface according to BET ¹⁾ m² / g 450 Medium size of the agglomerates µm 3.5 ⁸⁾ Ramming density ²⁾ g / l 70 to 80 Loss of drying when leaving the delivery plant (2 h at 1000 ° C) ³⁾ % 3rd Loss on ignition (2 h at 1000 ° C) ^{4) 9)} % 5 pH value (in 5% aqueous dispersion) ⁵⁾ 6.5 DBP absorption ^{6) 9)} g / 100 g 330 SiO₂ ¹⁰⁾ % 98.5 Na₂O ¹⁰⁾ % 0.6 Fe₂O₃ ¹⁰⁾ % 0.03 SO₃ ¹⁰⁾ % 0.7 Sieve residue (according to Mocker, 45 µm) ⁷⁾ % 0.02 ¹⁾ according to DIN 66 131 ²⁾ according to DIN ISO 787 / XI, JIS K 5101/18 (not screened) ³⁾ according to DIN ISO 787 / II, ASTM D 280, JIS K 5101/21 ⁴⁾ according to DIN 55 921, ASTM D 1208, JIS K 5101/23 ⁵⁾ according to DIN ISO 787 / IX, ASTM D 1208, JIS K 5101/24 ⁶⁾ according to DIN 53 601, ASTM D 2414 ⁷⁾ according to DIN ISO 787 / XVIII, JIS K 5101/20 ⁸⁾ Coulter Counter, 50 µm capillary ⁹⁾ based on the substance dried at 105 ° C. for 2 hours ¹⁰⁾ based on the substance annealed at 1000 πC for 2 hours

A cylindrical blast pressure vessel (autoclave) with a hemispherical bottom and a volume of approx. 50 l is available for the tests (⌀: approx. 300 mm with a length of 700 mm). The pressure vessel can be closed with a removable lid after inserting a rubber seal using 12 screws. A pressure measuring device and a ball valve are flanged to the cover. The autoclave can be completely vented using the ball valve before opening. The connection for the compressed air supply is located on the side of the steel cylinder. The autoclave is designed for a maximum operating pressure of approximately 10 bar; a corresponding pressure relief valve is installed.

All experiments are carried out with the precipitated silica FK 500 DS, which is available as sacks with a bulk density of 60 to 70 g / l. To the compaction attempts under conditions like them after using known drying processes to be able to carry out, the silica is first ground using a pin mill. The experiments are carried out with undried, then with dried silica. Important starting product data can be found in Table 1. Table 1 Precipitated silica property Moisture / drying loss ¹⁾ (% by weight) Bulk density (g / l) Ramming density (g / l) undried approx. 4 40 50 dried 1 30th 35 ¹⁾ Conditions: 105 ° C / 18 h

After the milling and drying of the FK 500 DS, the compaction experiments are started. For this purpose, polyethylene (PE) bags are filled almost completely with the precipitated silica (weight: 1,200 g) and sealed. The bags are dimensioned so that they fill about 80% of the autoclave volume when filled (the distance between the PE bag and the autoclave wall is around 3 to 5 cm). After placing a bag in the autoclave, the autoclave is closed.

The desired test pressure (1 bar to max. 4 bar overpressure) is set by carefully opening and interrupting the compressed air supply in good time. After the selected dwell time (0.5 to 3 min), the autoclave is slowly vented and then opened. The PE bag (polyethylene bag) is only partially filled with precipitated silica after the compression tests, in contrast to before. After removal from the autoclave, the compressed precipitated silica is partly in powder and partly in the form of soft clods. The clods disintegrate into powder even with slight mechanical stress. Samples are taken from the compressed precipitated silica and the bulk, tamped and clod densities are measured immediately.

• a. Bestimmung der Schüttdichte (Meßvolumen: 200 cm³);
• b. Bestimmung der Stampfdichte (Meßvolumen: 200 cm³, Hubzahl: 1250) gemäß DIN ISO 787/XI, JIS K 5101/18;
• c. Bestimmung der Schollendichte;
  Durchführung: Mit Hilfe eines dünnwandigen Metallrohres (Innen-⌀: 35 mm) wird aus einer Scholle geeigneter Größe eine Probe mit definierten Außenabmessungen ausgestochen. Nach dem Auswiegen des Probekörpers kann die Schollendichte näherungsweise errechnet werden.
• d. Bestimmung des Wiederauflockerungsverhaltens der verdichteten Fällungskieselsäure;
  Durchführung 1: Messung der Stampfdichten nach dem freien Fall des Produktes durch ein Rohr (⌀: 7,5 cm; Länge: 80 cm) mit aufgesetztem Trichter in ein Vorlagegefäß.
  Durchführung 2: Messung der Stampfdichte nach dem Durchlauf durch eine Dosierschnecke (Hersteller: Gericke; ⌀: 3,5 cm; Länge: 40 cm) und Fall in einen PE-Beutel (Fallhöhe: 30 bis 40 cm).

The following tests are carried out on the compacted silica FK 500 DS:

• a. Determination of bulk density (measuring volume: 200 cm³);
• b. Determination of the tamped density (measuring volume: 200 cm³, number of strokes: 1250) according to DIN ISO 787 / XI, JIS K 5101/18;
• c. Determination of clod density;
  Implementation: With the help of a thin-walled metal tube (inner ⌀: 35 mm), a sample of a suitable size is cut out with a defined outer dimension. After the test specimen has been weighed out, the clod density can be approximately calculated.
• d. Determination of the loosening behavior of the compressed precipitated silica;
  Procedure 1: Measurement of the tamped densities after the free fall of the product through a tube (⌀: 7.5 cm; length: 80 cm) with a funnel attached to a receptacle.
  Procedure 2: Measurement of the tamped density after passing through a metering screw (manufacturer: Gericke; ⌀: 3.5 cm; length: 40 cm) and dropping into a PE bag (drop height: 30 to 40 cm).

• a. Versuchsreihe A: Verdichtungsgrad als Funktion des Druckes
• b. Versuchsreihe B: Verdichtungsgrad als Funktion der Verweilzeit
• c. Versuchsreihe C: Verdichtungsgrad als Funktion der Einwaage
• d. Versuchsreihe D: Wiederauflockerungsverhalten der verdichteten Fällungskieselsäure

Die Ergebnisse einer Verdichtung von FK 500 DS im Druckbehälter sind in Tabelle 2 als Funktion des Druckes zusammengefaßt, wobei die Ergebnisse für die ungetrocknete und die getrocknete Fällungskieselsäure getrennt aufgeführt sind. Die Ergebnisse sind in Figur 2 graphisch dargestellt.

The following test series are carried out:

• a. Test series A: Degree of compaction as a function of pressure
• b. Test series B: Degree of compaction as a function of the dwell time
• c. Test series C: Degree of compaction as a function of the weight
• d. Test series D: loosening behavior of the compressed precipitated silica

The results of a compression of FK 500 DS in the pressure vessel are summarized in Table 2 as a function of the pressure, the results for the undried and the dried precipitated silica being listed separately. The results are shown graphically in FIG. 2.

The influence of the residence time in the autoclave on the compression of FK 500 DS can be seen in Table 3.

The dwell times are varied with undried precipitated silica, in each case at 1, 1.5 and 2 bar compression overpressure, with dried precipitated silica FK 500 DS the compression behavior is examined at 4 bar. The results are shown graphically in FIG.

The test results on the influence of the weight (autoclave filling) on the compression of FK 500 DS are summarized in Table 4. The compression conditions for undried FK 500 DS are 2 bar overpressure with a residence time of 1.5 minutes, those for dried precipitated silica 4 bar overpressure with a residence time of 0.5 min. The parameters are chosen so that approximately comparable degrees of compaction result for the dried and undried precipitated silica. The results are shown graphically in FIG.

• a. Freier Fall von nicht getrockneter Fällungskieselsäure FK 500 DS durch ein Rohr mit aufgesetztem Trichter (Länge: ca. 80 cm) in ein Vorlagegefäß
• b. Durchlauf von getrockneter Fällungskieselsäure FK 500 DS durch eine Gericke-Schnecke (⌀: 3,5 cm; Länge: 40 cm) und anschließenden Fall in einen PE-Beutel (Fallhöhe: 30 bis 40 cm).

The following tests are carried out to investigate the loosening behavior of compacted precipitated silica FK 500 DS (cf. 3.3):

• a. Free fall of non-dried precipitated silica FK 500 DS through a tube with a funnel (length: approx. 80 cm) into a receptacle
• b. Passing dried precipitated silica FK 500 DS through a Gericke screw (⌀: 3.5 cm; length: 40 cm) and then dropping it into a PE bag (drop height: 30 to 40 cm).

The test results are summarized in Table 5:

• a. Im Stampfdichtebereich bis 90 g/l zerfällt das bei der Verdichtung gebildete schollenförmige Produkt bei nur leichter Berührung zu Pulver; die Schollen haben so gut wie keine mechanische Festigkeit.
• b. Im Stampfdichtebereich bis zur Verdichtungsgrenze von etwa 95 g/l zerfällt das bei der Verdichtung gebildete schollenförmige Produkt bei nur leichter Berührung zunächst in kleinere Schollen, die wiederum leicht zu Pulver zerfallen. Die mechanische Festigkeit der Schollen hat gegenüber a. leicht zugenommen.

When carrying out the tests, the following properties of the FK 500 DS compressed according to the invention are found:

• a. In the tamped density range up to 90 g / l, the clod-shaped product formed during compaction breaks down into powder with only slight contact; the clods have practically no mechanical strength.
• b. In the tamped density range up to the compression limit of about 95 g / l, the clod-shaped product formed during the compaction initially disintegrates into smaller clods, which in turn easily disintegrate into powder. The mechanical strength of the clods has a. slightly increased.

The results show that undried and dried FK 500 DS can be compacted in a pressure vessel if the precipitated silica is sealed in a plastic bag (e.g. PE) beforehand.

• a. Nicht getrocknete FK 500 DS kann bei niedrigeren Drücken verdichtet werden als die getrocknete Fällungskieselsäure.
• b. Die Kieselsäure-Schüttdichten von 50 bis etwa 95 g/l lassen sich bei getrockneter Fällungskieselsäure durch Variation des Autoklavendruckes im Bereich von 1 bis 4 bar reproduzierbar erreichen.
• c. Bei getrockneter Fällungskieselsäure ist für das Verdichtungsergebnis primär der Verdichtungsdruck entscheidend; verlängerte Verweilzeiten resultieren in einer Zunahme der Schüttdichte von "nur" etwa 3 g/l pro Minute.
• d. Bei hoher Volumenausfüllung (Einwaage) des Druckgefäßes mit Fällungskieselsäure werden höhere Verdichtungsgrade erzielt als bei nur teilweiser Ausfüllung.
• e. Die Auflockerungseigenschaften von ungetrockneter und getrockneter Fällungskieselsäure FK 500 DS sind gleich.
• f. Inhomogenitäten in den Produktdichten können nicht festgestellt werden.

The results can be summarized as follows:

• a. Undried FK 500 DS can be compressed at lower pressures than the dried precipitated silica.
• b. The silica bulk densities of 50 to about 95 g / l can be dried when dry Precipitated silica can be reproducibly achieved by varying the autoclave pressure in the range from 1 to 4 bar.
• c. In the case of dried precipitated silica, the compaction pressure is primarily decisive for the compaction result; extended residence times result in an increase in the bulk density of "only" about 3 g / l per minute.
• d. If the pressure vessel is filled to a high volume (weighed in) with precipitated silica, higher degrees of compaction are achieved than if it was only partially filled.
• e. The loosening properties of undried and dried precipitated silica FK 500 DS are the same.
• f. Inhomogeneities in the product densities cannot be determined.

FIG. 5 shows an embodiment of the method and the device according to the invention. According to FIG. 5, the pulverulent substance is filled through the filling funnel 1. The discharge valve 2 (or flap) is closed. After filling with the powdered substance, the filling slide 3 (or flap) is closed. The powdery substance is in the space made by the filling slide 3, the discharge slide 2 and the compression membrane 4 made of rubber was formed. The compression membrane 4 is tubular and its dimensions match the interior of the pressure vessel 5, which is attached to the frame 6. Compressed air is now introduced via the connection 7 into the space between the compression membrane 4 and the wall of the pressure vessel 5 until a pressure of 0.1 to 8 bar is set. This pressure is maintained over a longer period of time. After a time of 0.1 to 10 minutes, the compressed air is released via the outlet valve 8. The discharge slide 2 is opened and the pulverulent substance is emptied into the filling container. Complete emptying can be achieved by light pressure surges in the space between the pressure vessel wall 5 and the compression membrane 4 with the emptying slide 2 open. When using an elastic compression membrane 4, the adaptation to the inner mass of the pressure vessel 5 is not to be understood in the absolute sense alone. The compression membrane 4 can expand depending on the pressure ratio set in the intermediate space (positive or negative pressure), so that the space enclosed by the compression membrane 4 becomes larger or smaller. When using the expandable compression membrane 4, it is possible to suck the powder to be compressed into the device through the filler opening 1 when the filling slide 3 is open by applying negative pressure in the intermediate space.

Claims (2)

Process for compacting powdery substances to a predetermined range of bulk density, whereby the powder structure of the powder is retained, characterized in that the powdery substance is hermetically sealed in a container provided with a flexible gas-impermeable wall, this container is enclosed in a closed pressure vessel, the Intermediate space between the outer wall of the pressure vessel and the container is acted upon by compressed gas, the pressure is maintained for a certain time, relaxed and, if appropriate, the powdery substance is removed from the pressure vessel with the container. Device for compacting powdery substances to a predetermined range of bulk density, whereby the powder structure of the powder is preserved, characterized in that it consists of a preferably vertically arranged pressure vessel, which has an arbitrary, preferably circular cross-section, on the upper and on the lower cross-sectional side each has a hermetically closable opening, inside is equipped with a flexible container made of gas-impermeable material, also open at the top and bottom.

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