EP0062605B1 - Device for the portion-like dosage of fluidisable bulk material - Google Patents

Abstract

A silo for storing alumina having a conical outlet at the bottom thereof is fitted with an exchangeable feeding unit mounted over the conical outlet opening. The feeding unit is adapted to close off the outlet opening when in the non-operative position. The feeding unit can take various forms and includes at least one injection nozzle for fluidizing the particulate material.

Description

The invention relates to a device for the portionwise supply of a fluidizable bulk material from a tapered bottom silo provided with a closable outlet opening to a reaction vessel, in particular alumina from a day silo to a crust breakthrough of a melt flow electrolysis cell for the production of aluminum, with a directly above the Outlet opening of the silo, which, in a resting phase, closes, interchangeably arranged and contains at least one siphon (12) with a siphon partition, which has a charging.

For the production of aluminum by melt flow electrolysis of aluminum oxide, this is dissolved in a fluoride melt, which consists largely of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon bottom of the cell, the surface of the liquid aluminum forming the cathode. Anodes which consist of amorphous carbon in conventional processes are immersed in the melt. At the carbon anodes, the electrolytic decomposition of the aluminum oxide produces oxygen, which combines with the carbon of the anodes to form CO ₂ and CO. The electrolysis takes place in a temperature range of approximately 940-970 ° C.

In the course of electrolysis, the electrolyte becomes poor in aluminum oxide. At a lower concentration of 1-2% by weight aluminum oxide in the electrolyte, there is an anode effect, which results in an increase in the voltage from, for example, 4-5 V to 30 V and above. Then, at the latest, the crust made of solid electrolyte material must be hammered in and the aluminum oxide concentration increased by adding new alumina.

The cell is usually operated periodically in normal operation, even if there is no anode effect. In addition, the crust has to be hammered in for each anode effect and the alumina concentration has to be increased by adding new aluminum oxide, which corresponds to cell operation.

To operate the cell, the crust between the anodes and the side board of the electrolytic cell has been hammered in for many years and then new aluminum oxide has been added. This practice is receiving increasing criticism due to the pollution of the air in the electrolysis hall and the outside atmosphere. In the case of encapsulated electrolysis cells, however, maximum restraint of the process gases can only be guaranteed if the operation takes place automatically. After the crust has been hammered in, the alumina is either supplied locally and continuously according to the "point feeder" principle or not continuously over the entire longitudinal or transverse cell axis.

The known storage bunkers or alumina silos arranged on the electrolysis cells are designed in the form of funnels or coolers with a funnel-shaped or conically tapering lower part. The content of the silos arranged on the cell generally covers one to two times a day, so they are also called day silos.

The supply of alumina from the silo to a breakthrough in the crust covering the molten electrolyte takes place in known devices by opening a flap which is pivoted for charging, or according to other systems with metering screws, metering cylinders or the like.

These metering devices have the disadvantage that mechanically movable parts must be installed in the electrolytic cell. As a result, they are subject to the effects of the furnace atmosphere with its heat and dust pollution, which requires more or less extensive maintenance. In many embodiments, there is also the risk of mechanical damage, especially when changing the anode.

DE-AS 1 010 014 relates to a device for the pneumatic emptying of a container for powdered material, a siphon-like part being arranged under the container, which forms both the end of the container and serves to further convey the material. For the most part it is openwork. Finely distributed compressed air is introduced into the material through this pore-like surface. An additional compressed air line opens into the descending branch of the siphon-like part above the lower edge of the siphon partition.

DE-AS 24 33 598 describes a device for the continuous discharge of solids from a treatment container. A collecting room is arranged below a treatment room, which has a funnel-like bottom directed downwards with a central outlet. A liquid introduced from below into a siphon-like part with a certain flow rate conveys the solids continuously through the siphon.

Neither of the two devices disclosed in the above documents is suitable for being used as a metering device that works in portions. They are only suitable for continuous conveying and are not designed to deliver precisely metered portions of bulk goods.

The inventors have therefore set themselves the task of creating a device for portionwise metering of alumina, which works with high accuracy, has no mechanically movable elements and which can be installed as a compact, robust unit in a silo, the simple structure of which is one to ensure cost-effective production and extensive freedom from maintenance.

According to the invention, the object is achieved in that the charging unit operates in two stages and is provided with a ventable metering volume, the nozzle outlet opening of at least one blowing nozzle per siphon opening from the free lower edge of the cutting wall.

The bottom plate of the charging unit lies over the entire surface on the conical lower part of the silo and is welded, riveted or preferably screwed to it. A screwed bottom plate has the advantage that the fasteners can be loosened at any time with a few hand movements, and the charging unit can be removed upwards through the emptied silo.

The charging shaft and thus the outlet opening of the silo is closed with a cover plate. Its side walls protrude into the trough formed by the base plate and charging shaft and form the siphon partition. The fluidizable bulk material solidifies in the lower area of the siphon due to the silo pressure. In order to get into the charging shaft, the bulk material would have to flow upwards around the siphon partition. The height of the charging shaft is selected so that the bulk material cannot flow to the upper edge of the shaft due to the static pressure when the silo level is sufficient.

The geometric shape of the outlet opening and the charging shaft is adapted to the impact device used. When adding punctiform alumina, the opening is expediently round, square or rectangular. The inner wall of the charging shaft preferably corresponds exactly to the outlet opening.

The part of the cover plate which covers the inlet opening of the charging shaft is preferably flat or slightly concave for production-related and economic reasons, although it has any practical geometric shape, such as e.g. a cone, pyramid or saddle roof.

The inner diameter of the blow nozzles can be, for example, 4-10 mm, with an outlet opening that has the same diameter or is narrowed down to 1 mm. In the case of round or square silo outlet openings, which are preferably used for the punctiform alumina addition, the number of blowing nozzles is expediently 3 to 6. With elongated outlet openings for central or transverse operation, e.g. 3 nozzles each arranged on the long sides, while the end faces are without nozzles.

The charging quantity can be kept constant with great accuracy if two single-stage charging devices are connected in series one above the other and thus form a dosing volume. In the working phase, with the first blowing nozzles switched on, the bulk material initially flows from the silo into a metering space located immediately below the upper first siphon. In relation to the lower, second charging device, this dosing room is the silo for the bulk material. If the first blowing nozzles are switched off and the second blowing nozzles are switched on, the bulk material in the dosing chamber can flow out through the outlet opening.

The ventilation when filling or the ventilation when emptying the dosing space can be of essential importance.

With two batching devices with dosing volume connected in series, the fluctuations in the quantity of bulk material charged can be reduced to approximately 1%, assuming a homogeneous quality of the bulk material.

A further improvement of the device according to the invention, in particular with regard to the structural simplification, brings about the formation of a metering space which is constantly open at the top and merges into a siphon at the bottom. A little below the inlet opening preferably at least one blowing nozzle opens into the metering space.

In the resting phase, the dosing room is completely filled with bulk goods. In the working phase, air is blown in through the blow nozzles for a certain time and with a certain pressure. The bulk material flows through the siphon into the charging shaft. Although bulk material flows from the silo into the charging room during the entire working phase, the charging accuracy is surprisingly below 1%. For this reason, it is normally not necessary in practice to provide the inflow opening to the charging space with a closure system that can be activated during the working phase.

In all embodiments of the exchangeable charging unit, care must be taken to ensure that the angle of repose of the material which is most difficult to fluidize is smaller than the slope of the walls of the metering space or of the conical lower part of the silo. Otherwise the requirements regarding charging accuracy cannot be met. Therefore, the slope of the corresponding walls is at least 45 °.

The alumina flowing into the charging shaft is fed to the crust breakthrough in free fall. This inflow can be made more precise if an outflow pipe is arranged below the charging shaft. However, this means that increased susceptibility to mechanical damage is accepted. All of the devices according to the invention are distinguished by the following advantages:

• - Vor Hitze und mechanischer Beschädigung weitgehend geschützt, weil im Silo eingebaut.
• - Kompakte, robuste und wartungsfreie Einheit, im Reparaturfall durch den entleerten Silo nach oben leicht ausbaubar.
• - Einfacher, kostengünstig herzustellender Aufbau.
• - Automatisierungsfreundlich.
• - Unabhängig von den Fliesseigenschaften des eingesetzten Schüttgutes.
• - Keine Undichtigkeitsprobleme in Ruhestellung.

- No mechanically or otherwise moving elements, therefore insensitive to wear in dusty environments.

• - From heat and mechanical damage largely protected because it is installed in the silo.
• - Compact, robust and maintenance-free unit, easily removable upwards in the event of a repair due to the emptied silo.
• - Simple, inexpensive to manufacture.
• - Automation friendly.
• - Regardless of the flow properties of the bulk material used.
• - No leakage problems at rest.

• - Entfernung der Tonerdedosierklappen,
• - Verschliessen der Siloöffnungen bis auf die Austrittsöffnungen,
• - Einbau der Chargiereinheiten und Verlegen der Druckluftleitungen,
• - Umbau der Brechbalken zu punktförmig arbeitenden Meisselbrechern,
• - Anpassung der Pneumatiksteuerung,
• - Anschluss an Prozessrechner.

Another advantage is that existing center control cells can be converted to point control, preferably with two units, without high costs. It is not necessary to replace the entire anodic part of the cell with considerable investment. Rather, only the following measures need to be taken:

• - removal of the alumina dosing flaps,
• - closing the silo openings except for the outlet openings,
• - Installation of the charging units and laying of the compressed air lines,
• - Conversion of the crushing beams into punctiform chisel breakers,
• - adjustment of the pneumatic control,
• - Connection to process computer.

The invention is explained in more detail with reference to the drawing.

• - Fig. 1 einen Vertikalschnitt- durch zwei übereinander in Reihe geschaltete Chargiereinheiten;
• - Fig. 2 eine aufgeschnittene Ansicht einer zweistufigen Chargiereinheit, oben offen, unten mit einem Siphon;
• - Fig. 3 eine Draufsicht der Chargiereinheit von Fig. 2.
• Fig. 1 zeigt eine zweistufige auswechselbare Chargiereinheit, die auf den Chargierschacht 26 "aufgesteckt" ist. Dieser Schacht 26 ist also nicht Bestandteil der Chargiereinheit, sondern ist entlang der Austrittsöffnung 20 mit dem untersten Teil 18 des Silos verschweisst oder einstückig mit diesem ausgebildet.

They show schematically:

• 1 shows a vertical section through two charging units connected in series one above the other;
• 2 shows a cut-away view of a two-stage charging unit, open at the top, with a siphon at the bottom;
• 3 is a top view of the charging unit of FIG. 2.
• 1 shows a two-stage interchangeable charging unit which is “plugged” onto the charging shaft 26. This shaft 26 is therefore not part of the charging unit, but is welded along the outlet opening 20 to the lowermost part 18 of the silo or is formed in one piece therewith.

• - Ein horizontales, auf der linken Seite zur oberen Siphonwand 30' abgewinkeltes Deckblech 14',
• - eine im Abstand a' von der Siphonscheidewand 30' vertikal angeordnete Seitenwand 46, welche im unteren Teil in die U-förmige Aussenwand 48 des unteren Siphons 12" übergeht, und
• - eine steiler als der grösste Schüttwinkel der dosierten Materialien angeordnete Seitenwand 52.

The metering chamber 44, which is essentially prismatic and has a rectangular cross section, has the following side surfaces:

• - A horizontal cover plate 14 ', angled on the left side to the upper siphon wall 30',
• a vertically arranged side wall 46 at a distance a 'from the siphon partition 30', which merges in the lower part into the U-shaped outer wall 48 of the lower siphon 12 ", and
• a side wall 52 arranged steeper than the largest angle of repose of the metered materials.

Immediately outside the upper siphon separating wall 30 'are three upper first blowing nozzles 16' which are fed by a common gas pipe 54. The blow nozzle openings 40 'are in the region of the edge 32' of the siphon partition 30 '.

The lowermost horizontal region of the prism-shaped metering space 44 merges into a part of the lower second siphon 12 "with a U-shaped outer wall 48. The siphon 12" is delimited on the inside by the lower siphon partition 30 ", formed by the vertical lower extension of the oblique side wall 52 The lower three second blowing nozzles 16 ", fed by a common gas dividing pipe 56 still arranged in the metering space 44, protrude into the lower siphon 12". Depending on the flow properties of the metered material, the lower nozzle outlet openings 40 "may be higher than lower.

When the dosing chamber 44 is filled, the exhaust air escapes through an opening 58 into a dust separator 60 and from there via a gas channel 62 into the space under the cell encapsulation. From there, the exhaust air, together with the furnace gases, is extracted and cleaned. On the other hand, when the dosing chamber 44 is emptied, ventilation takes place in the opposite direction.

In the resting phase, the dosing room is completely filled with bulk goods. Compared to the dust separator 60, the opening 58 is the pouring limit, in the lower siphon 12 "the pouring cone 64.

In the working phase, the lower blow nozzles 16 "are initially switched on until the metering chamber 44 and the lower second siphon 12" are completely emptied. A pouring cone 66 is formed in the upper first siphon 12 '.

Immediately after the lower second blowing nozzles 16 "have been switched off, the upper first blowing nozzles 16 'are switched on until the metering space 44 is completely filled again.

In the embodiment according to FIGS. 2 and 3, a structurally significantly simplified embodiment of a charging unit is shown.

_ The dosing chamber 44 has the shape of a raised prism with a rhombus-shaped cross section with short, horizontally arranged side edges. Of course, as variants of this embodiment dosing rooms can have a shape of vertically arranged double pyramids or double cones, as well as cuboids or cylinders with pyramid or double sided. have a conical end, etc. As an essential requirement, however, the above explanations regarding the angle of repose must be observed. For this reason, e.g. spherical dosing rooms out of consideration.

The dosing chamber 44 is fed via a pipe socket 68 projecting vertically into the silo filled with the bulk material. This is welded to the charging space by means of a sleeve 70. The inlet opening 72 is dimensioned such that the metering space 44 is filled in about 30-90 seconds. The entry opening can be with be provided with a closure system known per se.

The entire lowest area of the metering chamber 44 is designed as a siphon 12. The lower edge 32 of the siphon partition 30 is so deep that the cone 64 does not reach the upper edge 34 of the U-shaped siphon outer wall 48. The charging shaft is indicated by the baffle 74. The conical lowermost part of the silo with the outlet opening is omitted for the sake of simplicity. They are designed as in the previous exemplary embodiments.

A blowing nozzle 16 with a horizontal action is arranged on an end face of the metering space 44, a little below the inlet opening 72.

In the rest phase, the dosing chamber 44 is completely filled with bulk material, the lower limit is the cone 64.

In the working phase, air is blown through the nozzle 16. The fluidized bulk material flows through the siphon 12 within a few seconds. Bulk material continues to flow during the entire emptying time. After the blower nozzle 16 has been switched off, the metering space 44 is completely filled again.

The charging unit shown in FIGS. 2 and 3 is not only characterized by simple construction, which results in robustness and economy, but also by surprising metering accuracy. Various series of measurements have shown deviations of less than 10 grams with a dosage of 2500 grams of aluminum oxide. The accuracy of the charging unit is well below 1% when dosing aluminum oxide.

The present invention has been illustrated primarily by the supply of aluminum oxide to a melt flow electrolysis cell for the production of aluminum. However, it is not limited to these special embodiments, but can generally be used for metering fluidizable bulk goods with homogeneous quality, such as Cryolite and cement, but also rice, grain and sugar.

Claims (5)

1. A device for feeding a fluidizable bulk material in batches to a reaction vessel from a silo (18) which tapers to a cone at the bottom and is provided with a closable outlet opening (20), in particular for feeding alumina from a day silo to a break through the crust on a fusion electrolysis cell for the production of aluminium, having directly above the outlet opening (20) from the silo a charging unit (10) which closes the latter off in a rest phase and which is arranged to be exchangeable and contains at least one siphon (12) with a siphon partition wall (30) and which has a charging chute (26),

characterized in that

the charging unit (10) works in two stages and is provided with a batch volume (44) capable of being aerated, and the nozzle outlet opening (40) of at least one blast nozzle (16) per siphon opens out above the free bottom edge (32) of the partition wall.

2. A device as in Claim 1, characterized in that in each charging unit (10) 3 to 6 blast nozzles (16) ara provided.

3. A device as in Claim 1 or 2, characterized in that the charging unit (10) consists of an upper first siphon (12') having upper first blast nozzles (16'), of the batch volume (44) capable of being serated, and of a lower second siphon (12") having lower second blast nozzles (16"), where the upper and lower blast nozzles may be actuated one after the other.

4. A device as in Claim 1 or 2, characterized in that the charging unit (10) consists of the batch chamber (44), of an open pipe stub (68) in the uppermost region, which projects vertically into the bulk material in the silo, of the siphon (12) extending over the whole horizontal length in the lowermost region and of at least one blast nozzle (16) arranged a little below the inlet opening (72) from the pipe stub (68).

5. A device as in Claim 4, characterized in that the batch chamber (44) is made in the form of a priam of rhomboidal vertical crosssection placed on edge and having short side edges arranged horizontally, of a double pyramid or a double cone as well as of a right parallelepiped or cylinder having a pyramidal or conical termination on both sides.

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