AU747716B2

AU747716B2 - Method for final cooling anhydrous aluminum oxide

Info

Publication number: AU747716B2
Application number: AU32524/99A
Authority: AU
Inventors: Klaus Janssen; Martin Rahn; Hans-Werner Schmidt
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-02-13
Filing date: 1999-02-02
Publication date: 2002-05-23
Anticipated expiration: 2019-02-02
Also published as: EP1053211A1; AU3252499A; CN1290234A; WO1999041199A1; DE19805897C1; BR9907869A

Description

METHOD FOR THE FINAL COOLING OF ANHYDROUS ALUMINA

DESCRIPTION

This invention relates to a method for the final cooling of anhydrous alumina.

Methods for the final cooling of anhydrous alumina are known. The DE- OS 195 42 309 discloses a method of producing anhydrous alumina from aluminum hydroxide in a circulating fluidized bed formed of a fluidized-bed reactor, a separator and a return line, where the aluminum hydroxide is introduced into the second stage on the gas side of a two-stage suspension preheater operated with the exhaust gases of the fluidized-bed reactor of the circulating fluidized bed and is at least partly dehydrated, dehydrated aluminum hydroxide from the second stage of the suspension preheater is introduced into the first stage on the gas side of a suspension preheater operated with the exhaust gases of the fluidized-bed reactor of the circulating fluidized bed and is furthermore dehydrated and subsequently supplied to the circulating fluidized oooooe bed, which is operated with oxygen-containing fluidizing gas indirectly heated in a subsequent cooling stage by the alumina produced and with indirectly heated, oxygen-containing secondary gas supplied at a higher level, where the indirect heating of the fluidizing gas is effected in a fluidized-bed cooler. Upon leaving the last suspension cooler, the anhydrous alumina obtained undergoes a final cooling 0: in a fluidized-bed cooler equipped with three cooling chambers. In the first chamber thereof, the fluidizing gas supplied to the fluidized-bed reactor is heated, in the two subsequent chambers the same is cooled against a heat-transfer medium, preferably water, which is guided in a countercurrent flow.

25 The disadvantage of this method consists in that the temperature at which the product enters the water-cooled portion is relatively high, which leads to the fact that a relatively large amount of thermal energy of the product gets into the cooling circuit of the water and cannot be recirculated to the process.

It is therefore an object underlying the present invention to create a method for the final cooling of anhydrous alumina, where the thermal energy withdrawn from the anhydrous alumina is recovered almost completely and can be reused in technical processes. Retrofitting the process into already existing rALi, plants should preferably be possible in a relatively easy way.

According to the present invention, there is provided a method for the final cooling of anhydrous alumina which was prepared from aluminum hydroxide in a fluidized-bed reactor designed as circulating fluidized bed with separator and return line for solids, where the hot, anhydrous alumina coming from the circulating fluidized bed is charged into a fluidized-bed cooler for final cooling, which fluidized-bed cooler consists of two cooling stages arranged in series, where each cooling stage has several successive cooling chambers disposed one beside the other and in each cooling chamber secondary air is introduced for fluidizing the alumina, where primary air is passed through the first cooling stage of the fluidized-bed cooler to effect an indirect cooling, and in the second cooling stage an indirect cooling of the alumina is effected against a liquid heat transfer medium guided countercurrently to the alumina, wherein the partly cooled alumina coming from the first cooling stage together with secondary air is introduced as a dispersion into a delivery line connected with a cyclone, where carrier gas is also supplied to the delivery line directly from a blower and the alumina is introduced through the delivery line into the cyclone in which gases and solids are separated, and wherein the anhydrous alumina withdrawn from the lower portion of the cyclone is directly introduced into the second cooling stage.

Preferably, the anhydrous alumina is supplied to the first cooling stage comes from a cooling cyclone belonging to a precooling system, and the gases discharged from the cyclone are introduced into the cooling cyclone.

Preferably, exhaust gas is withdrawn from the cooling cyclone, mixed with secondary air from the fluidized-bed cooler, and introduced into the fluidized-bed reactor.

25 Preferably, in each cooling stage the anhydrous alumina is passed through three cooling chambers.

Preferably, the secondary air passed through the second cooling stage is introduced into the delivery line.

In an exemplary embodiment of the invention, the dispersion containing anhydrous alumina, which has been with- 3 drawn from the first cooling stage, is passed through a cyclone, and that the anhydrous alumina withdrawn in the lower portion of the cyclone is subsequently directly introduced into the second cooling stage. Before entering the fluidizedbed cooler, the anhydrous alumina has a temperature of 800 to 1200 0 C and is technically anhydrous, i.e. it has a water content of merely 0.1 to 1 The primary gas introduced into the first cooling stage is used as heat-transfer medium and thus as coolant of the first stage. The cooling stages are designed such that they consist of several cooling chambers, each of which is charged with secondary gas as fluidizing gas, so that in each cooling chamber a fluidized bed is formed. In general, 2 to 6 cooling chambers are provided in each cooling stage. As liquid heat-transfer medium of the second cooling stage water can be used to a particular advantage. The disperse phase of the dispersion is solid and is formed by the anhydrous alumina. The dispersion phase is gaseous and is formed by air. The dispersion itself, which is formed by the anhydrous alumina and air, thus has a dust-like character. The cyclone disposed between the two cooling stages advantageously acts as cooling cyclone for the final cooling, where an indirect heat exchange is effected. Surprisingly, it turned out that with the method for the final cooling of anhydrous alumina it is possible to almost completely reuse the thermal energy withdrawn from the anhydrous alumina in the fluidized-bed cooler in technical processes, so that only very small thermal losses are observed. These thermal losses merely include that thermal energy which in the second cooling stage is transferred into the circuit of the liquid heat-transfer medium. However, these thermal losses are small, as the method for the final cooling provides for reducing the inlet temperature of the anhydrous alumina at the inlet of the fluidized-bed cooler below 450 0

C.

Such reduction of the temperature at the inlet of the fluidized-bed cooler would also be possible when the first cooling stage would accordingly be designed larger. However, it would 4 be disadvantageous that corresponding to the larger design of the first cooling stage larger amounts of primary gas would have to be passed through the same as heat-transfer medium, which would lead to the fact that for the technical processes, to which the thermal energy withdrawn should be supplied advantageously, too large an amount of heated primary gas would be available. For instance, for adjusting the fluidized bed in the reactor of the circulating fluidized bed, much too much fluidizing gas would be present, which is formed by the primary gas. The method for the final cooling thus provides for the advantageous reduction of the temperature of the anhydrous alumina directly at the inlet of the second cooling stage of the fluidized-bed cooler, without having to use major amounts of primary gas as heat-transfer medium of the first cooling stage. Since most of the thermal energy withdrawn from the anhydrous alumina is transferred into gases, it is relatively easily possible to reuse the thermal energy withdrawn in technical processes, by supplying the heated gases to the technical processes. It is, for instance, advantageous to supply the hot gases withdrawn from the cyclone to a technical process and thus utilize the thermal energy of such gas. In terms of construction, the arrangement of the cyclone between the two cooling stages can S be effected relatively easily, so that already existing *ooo plants can be retrofitted relatively easily.

A preferred aspect of the invention consists in that the gases withdrawn from the central tube of the cyclone are recirculated to at least one cooling cyclone of the precooling.

This ensures that a particularly large amount of thermal energy, which is withdrawn from the anhydrous alumina in the cyclone, can for instance advantageously be recirculated to the process in the circulating fluidized bed as technical process.

In accordance with a further embodiment of the invention the ggases are mixed with secondary gas withdrawn from the fluid- 5 ized-bed cooler before being introduced into the fluidizedbed reactor. Thus, the thermal energy of the secondary gas can likewise advantageously be recirculated to the process in the circulating fluidized bed, where it is ensured at the same time that due to the common transport of secondary gas and the gases withdrawn from the central tube of the cyclone tubes can be saved as compared to a separate transport.

In accordance with a further preferred aspect of the invention the anhydrous alumina is passed through three cooling chambers in each cooling stage. The arrangement of three cooling chambers each provides for a homogeneous cooling of the anhydrous alumina while at the same time optimizing the space required for the fluidized-bed cooler. The arrangement of three cooling chambers each ensures a particularly advantageous release of thermal energy from the anhydrous alumina.

A further preferred aspect of the invention consists in that the secondary gas passed through the second cooling stage together with the dispersion containing anhydrous alumina, which has been withdrawn from the first cooling stage, is passed through the cyclone. It is particularly advantageous that the thermal energy of the secondary gas of the second cooling stage can be supplied particularly easily to the direct heat exchange in the cyclone.

00040: The subject-matter of embodiments of the invention will 0.

subsequently be described in detail and by way of example oo. with reference to the drawing (Fig. 1 to Fig. 3).

Fig. 1 shows a simplified schematic process flow diagram of the method for the final cooling of anhydrous alumina.

Fig. 2 shows a simplified schematic process flow diagram of a variant of the method for the final cooling of anhydrous alumina.

6 Fig. 3 shows a simplified schematic process flow diagram of a known method for the final cooling of anhydrous alumina in accordance with the prior art.

Fig. 1 represents a simplified schematic process flow diagram of the method for the final cooling of anhydrous alumina. The anhydrous alumina withdrawn from the fluidized-bed reactor is supplied in the form of a dispersion via line 1 into a cooling cyclone 2 for the actual cooling. The cooling cyclone 2 may consist of several cooling cyclones arranged in series.

The gas withdrawn from the central tube of the cooling cyclone 2 is discharged via line 3 and possibly introduced into the reactor of the circulating fluidized bed. The cooled anhydrous alumina is delivered via line 4 into the first cooling stage 5a of a fluidized-bed cooler, which consists of three cooling chambers A, B, C. By means of Roots vacuum pumps 11, 12, 13, the individual cooling chambers A, B, C are charged with secondary gas via line 29, which secondary gas is used for the respective formation of a fluidized bed. The secondary gas leaves the first cooling stage 5a via line 14 and can then again be supplied to the fluidized-bed reactor.

It is, however, also possible to again supply the secondary gases carried in line 14 to the cooling cyclone 2 via line 1.

By means of the Roots vacuum pumps 6, 7, primary gas is supplied to the first cooling stage 5a via line 16, which primary gas is passed through the individual cooling chambers C, B, A and serves as heat-transfer medium. The primary gas leaves the first cooling stage 5a via line 15 and is advantageously used as fluidizing gas for the fluidized-bed reactor of the circulating fluidized bed (not represented). The anhydrous alumina withdrawn from the first cooling stage 5a is introduced via line 17 and line 18 into the cyclone 20. By means of the Roots vacuum pumps 8, 9, 10, line 18 is charged with gases, so that the anhydrous alumina is again introduced into the cyclone 20 in the form of a dispersion. The gases withdrawn from the central tube of the cyclone 20 are with- 7 drawn via line 21 and advantageously supplied to the fluidized-bed reactor or the cooling cyclone 2. The anhydrous alumina separated in the cyclone 20 is introduced into the second cooling stage 5b of the fluidized-bed cooler via line 22.

The second cooling stage 5b is likewise divided into three cooling chambers D, E, F. In contrast to the first cooling stage 5a, the cooling chambers are charged with a liquid heat-transfer medium via line 23, where water may advantageously be used as heat-transfer medium. The liquid heattransfer medium is passed through the three cooling chambers F, E, D one after the other and leaves the second cooling stage 5b via line 24. By means of the Roots vacuum pumps 11i, 12, 13, the second cooling stage 5b is also charged with secondary gas via line 29, so that there is also formed a respective fluidized bed. The secondary gas passed through the second cooling stage Sb is introduced into the first cooling stage 5a via line 25 and finally withdrawn from the first cooling stage 5a via line 14. The anhydrous alumina is withdrawn from the second cooling stage 5b via line 26, an outlet sluice 27, and a line 28.

Fig. 2 represents a simplified schematic process flow diagram of a variant of the method for the final cooling of anhydrous alumina. The outlet sluice 27 is disposed between the cooling stages 5a, 5b. As desired, the outlet sluice 27 may be charged with gases by means of the Roots vacuum pumps 8, 9, through a bypass line 31. Via line 17, the outlet sluice 27 and line 17' the anhydrous alumina from the first cooling stage 5a is introduced into line 18, in which it is supplied to the cyclone 20. The secondary gases withdrawn from the second cooling stage 5b via line 25 are likewise introduced into line 18 and thus supplied to the cyclone Fig. 3 represents a flow diagram of the known method for the final cooling of anhydrous alumina in accordance with the prior art. In contrast to the inventive method it is provided to connect the two cooling stages 5a, 5b merely by one line 8 through which the alumina withdrawn from the first cooling stage 5a is introduced into the second cooling stage and the secondary gas passed through the second cooling stage is introduced into the first cooling stage 5a. It is, however, disadvantageous that the temperature at the inlet of the second cooling stage 5b is relatively high, so that it is not possible to recover the thermal energy withdrawn from the anhydrous alumina in the second cooling stage 5b and recirculate the same to technical processes, as a relatively large amount of thermal energy is released to the liquid heattransfer medium of the second cooling stage, for instance water, and cannot be recovered. Cooling with liquid heattransfer medium can, however, not be omitted in the second stage, as for instance an alternative air cooling would require a relatively large amount of air, which cannot be utilized expediently in technical processes, as for instance in the preceding circulating fluidized bed. The three cooling chambers D, E, F of the second cooling stage 5b are each charged with secondary gas by means of the Roots vacuum pumps ii', 11ii, 12, so as to form a fluidized bed.

Claims

1. A method for the final cooling of anhydrous alumina which was prepared from aluminum hydroxide in a fluidized-bed reactor designed as circulating fluidized bed with separator and return line for solids, where the hot, anhydrous alumina coming from the circulating fluidized bed is charged into a fluidized-bed cooler for final cooling, which fluidized-bed cooler consists of two cooling stages arranged in series, where each cooling stage has several successive cooling chambers disposed one beside the other and in each cooling chamber secondary air is introduced for fluidizing the alumina, where primary air is passed through the first cooling stage of the fluidized-bed cooler to effect an indirect cooling, and in the second cooling stage an indirect cooling of the alumina is effected against a liquid heat transfer medium guided countercurrently to the alumina, wherein the partly cooled alumina coming from the first cooling stage together with secondary air is introduced as a dispersion into a delivery line connected with a cyclone, where carrier gas is also supplied to the delivery line directly from a blower and the alumina is introduced through the delivery line into the cyclone in which gases o. and solids are separated, and wherein the anhydrous alumina withdrawn from the i o lower portion of the cyclone is directly introduced into the second cooling stage.

2. The method as claimed in claim 1, wherein the anhydrous alumina is supplied to the first cooling stage comes from a cooling cyclone belonging to a precooling system, and the gases discharged from the cyclone are introduced into the cooling cyclone. :gO The method as claimed in claim 1 or 2, wherein exhaust gas is withdrawn from the cooling cyclone, mixed with secondary air from the fluidized-bed cooler, and introduced into the fluidized-bed reactor.

4. The method as claimed in any one of claims 1 to 3, wherein in each cooling stage the anhydrous alumina is passed through three cooling chambers. The method as claimed in claim 1, wherein the secondary air passed (j rough the second cooling stage is introduced into the delivery line.

6. A method for the final cooling of anhydrous alumina substantially as hereinbefore described and illustrated with reference to the accompanying drawings, excluding drawings relating to the prior art. DATED this 30th day of November 2001 METALLGESELLSCHAFT AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA SKP/RJS/MEH P17913AU00.DOC S S.. 555 S S. S S 5555 S S S S S S *.SS