WO2005005318A2

WO2005005318A2 - Process and plant for producing metal oxide from metal hydroxide

Info

Publication number: WO2005005318A2
Application number: PCT/EP2004/006870
Authority: WO
Inventors: Michael Missalla; Martin Hirsch; Michael STRÖDER; Karsten Hoffhaus
Original assignee: Outokumpu Technology Oy
Priority date: 2003-07-11
Filing date: 2004-06-25
Publication date: 2005-01-20
Also published as: DE10331364B3; WO2005005318A3

Abstract

This invention relates to a process for producing metal oxide from metal hydroxide, in which the preheated metal hydroxide is at least partly supplied via at least one pneumatic delivery conduit and via at least one downstream separating cyclone (8, 11) to a fluidized-bed reactor (12), where it is heated to a temperature of 650 to 1250°C by combustion of fuel, and metal oxide is produced. In a mixing vessel (19), the metal oxide is mixed with a partial stream of preheated metal hydroxide such that the preheated metal hydroxide is further heated and at least partly calcined. Then, the metal oxide is supplied from the mixing vessel (19) to at least one cooling stage (27, 28, 30, 31). The energy utilization of the process is increased in that the waste gas of the at least one separating cyclone (8, 11) is introduced into the mixing vessel (19) as conveying air such that the metal oxide from the mixing vessel (19) is pneumatically delivered into the at least one cooling stage or a de livery conduit (24) leading to the same. Furthermore, the invention relates to a corresponding plant.

Description

PROCESS AND PLANT FOR PRODUCING METAL OXIDE FROM METAL HYDROXIDE

The present invention relates to a process for producing metal oxide from metal hydroxide, in which the preheated metal oxide is at least partly supplied via at least one pneumatic delivery conduit and via at least one downstream separating cyclone to a fluidized-bed reactor, where it is heated to a temperature of 650 to 1250°C by combustion of fuel, and metal oxide is produced, which in a mixing vessel is mixed with a partial stream of preheated metal hydroxide such that the preheated metal hydroxide is further heated and at least partly calcined, the metal oxide from the mixing vessel being supplied to at least one cooling stage. The present invention furthermore relates to a corresponding plant.

From EP 0 861 208 B1 , there is known a process of producing anhydrous alumina (alum earth) from aluminium hydroxide (hydrate) in a fluidized-bed reactor, in which the aluminium hydroxide is first dried in a plurality of preheating stages, partly calcined and introduced into the reactor in the preheated condition. In this reactor, alumina is produced by combustion of fuel at temperatures of 800 to 1000°C. The reactor waste gas obtained during the combustion is utilized ener- getically in that it is supplied to the preheating stages. For cooling purposes, the alumina withdrawn from the reactor is passed through a direct cooling stage, in which the alumina is pneumatically transported upwards into a cyclone via a delivery conduit. In this known process, however, the energy utilization of the calcining step is regarded as needing improvement.

From DE 101 40 261 A1 there is furthermore known a similar process for producing anhydrous alumina from aluminium hydroxide, in which the preheated aluminium hydroxide is supplied to the reactor via a first pneumatic delivery conduit (Airlift), which opens into a separating cyclone. From the reactor, alumina together with partly dried aluminium hydroxide from the separating cyclone of the first pneumatic delivery conduit is then introduced into a mixing vessel (Mixing Pot), in which the aluminium hydroxide is heated and calcined by the hot alumina. In this way, an improved energy utilization of the calcining step is achieved, as the thermal energy of the alumina discharged from the reactor is utilized for calcining the preheated aluminium hydroxide in the mixing vessel. From this mixing vessel, the alumina falls into another pneumatic delivery conduit (Lift Duct), which delivers the alumina to various cooling stages. This gravimetric delivery of the alumina from the mixing vessel into the pneumatic delivery conduit requires that the reactor with the mixing vessel be arranged vertically above the delivery conduit. As a result, the overall height of such a plant is increasing such that the assembly of this plant also is quite expensive.

The waste air of the separating cyclone, which is provided downstream of the first pneumatic delivery conduit, is introduced into the pneumatic delivery conduit downstream of the mixing vessel at the end thereof, the waste air being heated, in order to be supplied to the reactor as combustion air later on. Since the aluminium hydroxide in the separating cyclone cannot be separated completely from the conveying air of the pneumatic delivery conduit, an amount of non-calcined aluminium hydroxide will always be introduced into the cooling stages via the pneumatic delivery conduit and be mixed with the alumina, which thereby is contaminated.

Therefore, it is the object of the present invention to further improve the utilization of the thermal energy used when calcining metal hydroxide and at the same time increase the purity of the product.

In accordance with the invention, this object is solved by a process as mentioned above, in which the waste gas of the at least one separating cyclone is introduced into the mixing vessel as conveying air such that metal oxide from the mixing ves- sel is pneumatically delivered into the at least one cooling stage and/or a delivery conduit leading to the same. The conveying air of the first pneumatic delivery conduit (Airlift) is utilized for an intermediate delivery, by means of which the metal oxide from the mixing vessel can be transported into the cooling stage. Due to this intermediate delivery, a level difference between the mixing vessel and the further pneumatic delivery conduit (Lift Duct) leading to the cooling stage, which was necessary in the known process, now can be omitted, so that the mixing vessel can, for instance, be installed at ground level. This reduces the overall height of a calcining plant for operating the process of the invention by about 6 to 7 m and thereby also simplifies the assembly. This reduction of the overall height of a plant for performing the process of the invention can in particular be achieved when the metal oxide from the mixing vessel is transported via an at least partly ascending pneumatic intermediate delivery conduit into the at least one cooling stage or a delivery conduit leading to the same. For this purpose, the waste gas of the at least one separating cyclone is introduced as conveying air into the mixing vessel, preferably with a pressure of at least 150 mbar. Preferably, the pressure is above 200 mbar, in particular about 350 mbar. This minimum pressure depends on the difference in height.

In accordance with a particularly preferred embodiment, metal hydroxide contained in the waste gas of the at least one separating cyclone is heated in the mixing vessel by the metal oxide and calcined at the same time. The hot metal oxide, which is withdrawn from the reactor, is mixed with the conveying air and the metal hydrate dust contained therein, so that a calcination of the metal hydrate dust is effected in the mixing vessel or the intermediate delivery conduit leading out of the same. By mixing the metal hydroxide with the hot metal oxide at an elevated temperature already in the mixing vessel, a considerably improved calcination is achieved as compared to the prior art, in which the metal hydroxide is only intro- duced at the end of the pneumatic delivery conduit downstream of the mixing vessel and thus at a lower temperature, so that the metal oxide is not contaminated by only partly calcined metal hydroxide.

The amount of heat required by the process of the invention can further be irn- proved in that from the at least one separating cyclone metal hydroxide is introduced into the mixing chamber via a bypass conduit. In the mixing chamber, this metal hydroxide is likewise heated and calcined by the hot metal oxide, so that the thermal energy which is contained in the metal oxide discharged from the reactor can be utilized for calcining additional metal hydroxide. Preferably, the metal hy- droxide from the separating cyclone directly downstream of the first pneumatic delivery conduit is introduced into the mixing chamber via the bypass conduit. In principle, however, uncalcined or precalcined metal hydroxide can also be withdrawn from any other part of the plant or be introduced into the mixing chamber via a bypass conduit. In accordance with a development of the invention it is pro- vided to control the amount of metal hydroxide supplied to the mixing chamber from the separating cyclone by means of a dosing device, for instance a star feeder, so that an adjustment to other process parameters can be effected.

To achieve a sufficient calcination of the metal hydroxide supplied to the mixing vessel from the separating cyclone, the metal hydroxide should preferably be suspended, dried, preheated to a temperature of at least 75°C, preferably between 100 and 200°C, in particular to about 150°C, and/or partly calcined before the separating cyclone in at least one preheating stage. Such preheating stage com- prises for instance a heat exchanger and a downstream separator.

An approximately complete calcination of the metal hydroxide in the mixing vessel can be achieved when the temperature of the metal oxide in the mixing vessel is between 550 and 900°C, preferably between 600 and 850°C, in particular about 750°C. The temperature in the pneumatic intermediate delivery conduit, in which metal oxide from the mixing vessel is delivered into the at least one cooling stage or a delivery conduit leading to the same, then is between 500 and 850°C, preferably between 550 and 800°C, in particular about 680°C.

The process in accordance with the invention can be used in particular for calcining aluminium hydroxide, which is supplied as starting material for instance with a grain size of less than 100 μm on average.

For calcining the metal hydroxide in the reactor, a preheated oxygen-containing gas as well as gaseous and/or liquid fuel is supplied thereto. The utilization of energy can be increased further, when gas is heated in the at least one cooling stage and is supplied to an upstream cooling stage, a preheating stage and/or the reactor.

In accordance with a preferred embodiment of the invention it is possible to supply partly dust-laden waste gases from the mixing vessel into a preheating stage upstream of the reactor. The waste gases with possibly not completely calcined metal hydroxide are thereby discharged through the cooling stages separate from the metal oxide. At the same time, the thermal energy contained in the waste gas is thereby utilized for preheating the metal hydroxide. The preheating stage, into which the waste gas of the mixing vessel is introduced, preferably is arranged between the separating cyclone and the reactor.

A plant for producing metal oxide from metal hydroxide in accordance with the invention, which can be used in particular for performing the process described above, comprises a reactor constituting a fluidized-bed reactor, in which metal hydroxide is heated by combustion of fuel and metal oxide is produced. Upstream of the reactor, at least one preheating stage and at least one first pneumatic delivery conduit with at least one downstream separating cyclone is provided, and downstream of the reactor a mixing vessel is provided, which is connected with at least one cooling stage. The at least one separating cyclone has a waste gas conduit leading into the mixing vessel, which via another at least partly ascending pneumatic intermediate delivery conduit is connected with the at least one cooling stage or a delivery conduit leading to the same. In this way, a contamination of metal oxide by metal hydroxide contained in the waste gas of the separating cyclone can be avoided in accordance with the invention, which metal hydroxide can be calcined by the hot metal oxide from the reactor. Due to the intermediate delivery conduit it is also no longer necessary to arrange the mixing vessel and the re- actor several meters above the delivery conduit leading to the cooling stage. As a result, the overall height of the plant in accordance with the invention can be kept distinctly smaller, so that the assembly is also simplified. Therefore, the mixing vessel is arranged below or substantially at the level of the vertically lower end of the delivery conduit which leads to the cooling stage downstream of the mixing vessel.

The retention time of the metal oxide and the metal hydroxide possibly admixed thereto in the mixing vessel can be increased in that in the mixing vessel between a solids feed opening and an opposed solids discharge opening a plurality of weirs are arranged, which are overflown and/or underflown by the metal oxide and/or the metal hydroxide. In this way, a thorough mixing of the metal oxide with the metal hydroxide is effected. To this end, at least one conduit for fluidizing gas may be provided, which opens in particular into the bottom of the mixing vessel. ln accordance with a preferred embodiment of the invention, the intermediate delivery conduit has a delivery tube immersed into the metal oxide and/or the metal hydroxide. The waste gas conduit of the separating cyclone opens into the mixing vessel preferably vertically below the lower edge of this delivery tube. In this way, the metal hydroxide possibly contained in the waste gas from the separating cyclone is mixed with the metal oxide in the mixing vessel and discharged from the mixing vessel via the delivery tube. Calcining the metal hydroxide contained in the waste gas of the separating cyclone is effected in the delivery tube or the intermediate delivery conduit.

In addition to the delivery tube, another waste gas conduit may be provided, which connects the mixing vessel with a preheating stage upstream of the reactor or with the delivery conduit leading to the at least one cooling stage. The hot waste gas of the mixing vessel, which is possibly obtained through the fluidizing gas, thus can either be utilized for preheating the metal hydroxide or for delivering the metal oxide into the cooling stage.

In accordance with a preferred embodiment of the invention, the separating cyclone is connected with the mixing vessel via a bypass conduit, so that metal hy- droxide from the separating cyclone can continuously be delivered directly into the mixing vessel. Due to the high temperature of the metal oxide withdrawn from the reactor, the metal hydroxide from the separating cyclone is calcined in the mixing chamber. For controlling the amount of metal hydroxide supplied to the mixing vessel for the bypass conduit, a star feeder can for instance be provided.

The energy utilization of the plant in accordance with the invention can further be improved in that downstream of the mixing vessel at least two cooling stages are provided, in which the metal oxide is fluidized and at least one of which has a conduit through which heated fluidizing gas is supplied to an upstream cooling stage, a preheating stage and/or the reactor.

The invention will subsequently be explained by means of an embodiment and with reference to the attached drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

In the drawings:

Fig. 1 shows a process diagram of a process in a plant in accordance with the present invention; and

Fig. 2 shows a schematic sectional view of the mixing vessel.

Fig. 1 shows a plant for the heat treatment (calcination) of metal hydroxide, e.g. aluminium hydroxide (hydrate), to obtain metal oxide, e.g. alumina (alum earth), in which metal hydroxide is introduced via a screw conveyor 1 into a Venturi prehea- ter 2 serving as heat exchanger of a first preheating stage. In the Venturi prehea- ter 2, the fine-grained solids, for instance with a grain size substantially less than 5 mm, gets in direct contact with hot gas, so that the solids are dried and preheated. The metal hydroxide can already be partly calcined. To this end, a hot gas mixture (waste gas) with temperatures of e.g. 200 to 500°C is supplied to the Venturi pre- heater 2 of the first preheating stage through a waste gas conduit 3, which hot gas mixture delivers the fine-grained solids via a solids conduit 4 into a filtration means constituting a separator 5, e.g. a bag filter or electrostatic filter. In this filter, the waste gas is separated from the solids and is discharged in a waste gas conduit 6 through a chimney or the like.

From the separator 5, the solids are supplied via a first pneumatic delivery conduit (Airlift) 7 to a separating cyclone 8, in which the metal hydroxide is separated from the conveying air of the pneumatic delivery conduit 7. Via a solids conduit 9, the metal hydroxide is supplied to another Venturi preheater 10 of a second preheating stage and a separating cyclone 11 downstream of the Venturi preheater 10. In the separating cyclone 11 , the solids introduced into the reactor 12 are separated from the waste gas, which via the waste gas conduit 3 is supplied to the Venturi preheater 2 of the first preheating stage. The reactor 12 is a fluidized-bed reactor, for instance with a circulating fluidized bed, to which liquid and/or gaseous fuel is supplied via conduit 13 as well as fluidizing gas via conduit 14. Due to the supply of fluidizing gas, the metal hydroxide in the reactor 12 is fluidized, so that a fluidized bed is formed, in which an intensive mixing of the solids with the fuel and the fluidizing air is effected. The fuel introduced into the reactor 12 is heated by the preheated solids and ignited thereby. By supplying more air through conduit 15, a complete combustion of the fuel in the reactor 12 is achieved, so that the required calcination temperature of about 650 to 1250°C is achieved. In the reactor 12, there are mostly temperatures of about 900 to 1000°C.

The hot exhaust gas of the reactor 12, in which metal oxide is contained, flows through a passage 16 into a recirculating cyclone 17. In the same, the metal oxide is separated from the waste gas, which for preheating the metal hydroxide is intro- duced into the Venturi preheater 10 of the second preheating stage. A part of the hot solids is recirculated into the fluidized bed of the reactor 12 via a return conduit 18a, whereas the rest of the hot metal oxide is recirculated to a mixing vessel 19 via conduits 18b.

Furthermore, a bypass conduit 20 opens into the mixing vessel 19, through which metal hydroxide from the separating cyclone 8 is introduced, which has not traversed the reactor 12. The orifice of the bypass conduit 20 into the mixing vessel 19 is designed such that the metal hydroxide from the bypass conduit 20 mixes with the metal oxide from conduit 18b. By means of the hot metal oxide, a temperature of e.g. about 750°C is obtained in the mixing vessel 19, so that the metal hydroxide is heated and calcined by the hot metal oxide. The amount of metal hydroxide supplied to the mixing vessel 19 through the bypass conduit 20 can be controlled for instance by a dosing device schematically illustrated in Fig. 1. Alternatively or in addition to the bypass conduit 20 shown in Fig. 1, metal hydroxide can also be supplied to the mixing vessel 19 via a bypass conduit from the separating cyclone 11 of the second preheating stage. Due to the further heating in the second preheating stage, the metal hydroxide possibly has already been calcined in part, when it is introduced into the mixing vessel 19. Via a waste gas conduit 21 , the waste gas from the separating cyclone 8, which has been separated from the metal hydroxide, preferably is injected from below into the mixing vessel 19, so that it can deliver the metal oxide from the mixing vessel 19 into an intermediate delivery conduit 23. Since the waste gas from con- duit 21 is heated by the contact with the hot metal oxide from the mixing vessel 19, particles of metal hydroxide, which are contained in the waste gas, can still at least partly be calcined. The waste gas from the separating cyclone 8 is introduced into the mixing vessel 19 with such a pressure that the metal oxide and the possibly present metal hydroxide are discharged from the mixing vessel 19 via the interme- diate delivery conduit 23, which opens into a pneumatic delivery conduit 24. The waste gas of the mixing vessel 19 can either likewise be introduced into the pneumatic delivery conduit 24 via a conduit 25 and/or be supplied to the Venturi preheater 10 via conduit 26. In addition, a fluidizing gas can be introduced into the mixing vessel 19 via a fluidizing conduit 22.

Through the pneumatic delivery conduit 24, the metal oxide is introduced into a first cooling cyclone 27, in which the conveying air is separated from the waste gas. For combustion purposes, the conveying air is supplied to the reactor 12 via the waste gas conduit 15, whereas the solids from the first cooling cyclone 27 are supplied to a first fluidized-bed cooler 28. In this fluidized-bed cooler 28, cooling coils are provided, in which the fluidizing air supplied to the reactor 12 is preheated. In addition, fluidizing gas, e.g. ambient air, is introduced into the fluidized- bed cooler 28. Via conduit 29, the solids from the fluidized-bed cooler 28 are supplied to a second cooling cyclone 30, from which the solids separated from the waste gas are introduced into a second fluidized-bed cooler 31. In the second fluidized-bed cooler 31 , a coolant circuit 32 circulates a cooling medium, e.g. water, so that the metal oxide is further cooled. In addition, fluidizing gas is also introduced into the second fluidized-bed cooler 31. Via conduit 33, the cooled product, e.g. alumina, leaves the second fluidized-bed cooler 31.

With reference to Fig. 2, the design of the mixing vessel 19 will now be explained in detail. On the left in the Figure, the solids conduit 18b from the reactor 12 as well as the bypass conduit 20 from the separating cyclone 8 open into a solids feed opening 34 of the mixing vessel 19. In the mixing vessel 19, a plurality of weirs 35 are arranged such that the metal oxide introduced into the mixing vessel 19 and the metal hydroxide overflow or underflow the weir 35 and are intermixed at the same time. Furthermore, a plurality of fluidizing conduits 22 open into the mixing vessel 19, so that the metal oxide is further intermixed with the metal hy- droxide.

On the right of the mixing vessel 19 as shown in the Figure, a pot-like region 36 is defined, in which flow the metal oxide and the metal hydroxide which likewise was calcined by the intimate contact with the hot metal oxide. The waste gas conduit 21, from which the waste gas of the separating cyclone 8, which possibly contains dust-like metal hydroxide, is introduced under pressure, opens into the bottom of this pot-like region 36. Opposite the waste gas conduit 21 , a delivery tube 37 of the pneumatic intermediate delivery conduit 23 protrudes into the pot-like region of the mixing vessel 19, so that the level of metal oxide in the pot-like region 36, which is schematically indicated in the Figure, lies above the lower orifice of the delivery tube 37.

The waste gas introduced under pressure from conduit 21 , which possibly contains dust-like metal hydroxide, entrains the metal oxide from the pot-like region 36 and supplies the same via the delivery tube 37 through the pneumatic intermediate delivery conduit 23 to the pneumatic delivery conduit 24, which opens into the first cooling cyclone 27. Due to the hot metal oxide, the temperature in the delivery tube 37 or the pneumatic intermediate delivery conduit 23 is so high, e.g. about 680°C, that the metal hydroxide, which in the case of an incomplete separation of waste gas and solids in the separating cyclone 8 is possibly contained in the waste gas introduced into the mixing vessel 19 via conduit 21 , is heated and calcined in the intermediate delivery conduit 23. Therefore, substantially pure metal oxide is introduced into the pneumatic delivery conduit 24, which metal oxide is not contaminated by metal hydroxide.

The waste air of the mixing vessel 19 can likewise be supplied to the pneumatic delivery conduit 24 via conduit 25. Alternatively or in addition, it is also possible to supply waste gas from the mixing vessel 19 through conduit 26, which is indicated in Fig. 2, to the Venturi preheater 10. Example (Production of Alumina)

In the plant as shown in Fig. 1 , about 223,000 kg/h of aluminium hydroxide with a temperature of 70°C and a surface moisture of 6 wt-% are supplied via the screw conveyor 1. The aluminium hydroxide pretreated in the first preheating stage is introduced from the separating cyclone 8 for one part into the mixing vessel 19 and for seven parts into the second preheating stage. Behind the second preheating stage, the solids have a temperature of 340°C and are preheated to such an extent that about 65% of the hydrate water is removed. As fuel, about 9,500 Nm³/h of cold natural gas are supplied to the fluidized-bed reactor 12 through the fuel conduit 13, so that a temperature of about 950°C is obtained during the combustion in the reactor 12.

124 t/h of alumina from the reactor 12 and 30 t/h of aluminium hydroxide from the separating cyclone are introduced into the mixing vessel 19. As a result, a temperature of about 650°C is obtained in the mixing vessel 19.

From the separating cyclone 8, about 9,500 Nm³/h of waste gas containing about 2 t/h of aluminium hydroxide are introduced into the mixing vessel 10 via conduits 21. In this way, 146 t/h of alumina are discharged through the delivery tube 37 at a temperature of about 605°C. The aluminium hydroxide contained in the waste gas of the separating cyclone 8 is calcined at the same time.

87,000 Nm³/h of waste gas from the second cooling cyclone 30 are supplied to the pneumatic delivery conduit 24, in order to deliver the alumina via the pneumatic delivery conduit 24 into the first cooling cyclone 27.

By means of this procedure, the energy of the alumina discharged from the reactor 12 can be utilized for calcining aluminium hydroxide. The total energy demand of the plant is decreased thereby. Furthermore, the aluminium hydroxide contained in the waste gas of the separating cyclone 8 is also at least partly calcined, so that the purity of the alumina discharged from the discharge conduit 33 is improved. List of Reference Numerals:

1 screw conveyor 19 mixing vessel

2 Venturi preheater 20 bypass conduit

3 waste gas conduit 21 waste gas conduit

4 solids conduit 22 fluidizing conduit

5 separator, filtration means 23 intermediate delivery conduit

6 waste gas conduit 24 pneumatic delivery conduit

7 pneumatic delivery conduit 25 conduit

8 separating cyclone 26 conduit

9 solids conduit 27 cooling cyclone

10 Venturi preheater 28 fluidized-bed cooler

11 separating cyclone 29 conduit

12 reactor 30 cooling cyclone

13 fuel conduit 31 fluidized-bed cooler

14 fluidizing conduit 32 coolant circuit

15 waste gas conduit 33 discharge conduit

16 passage 34 solids feed opening

17 recirculating cyclone 35 weir

18a solids return conduit 36 pot-like region

18b solids conduit 37 delivery tube

Claims

Clai s

1. A process for producing metal oxide from metal hydroxide, in which the pre- heated metal oxide is at least partly supplied via at least one first pneumatic delivery conduit (7) and via at least one downstream separating cyclone (8, 11) to a fluidized-bed reactor (12), where it is heated to a temperature of 650 to 1250°C by combustion of fuel, and metal oxide is produced, which in a mixing vessel (19) is mixed with a partial stream of preheated metal hydroxide such that the preheated metal hydroxide is further heated and at least partly calcined, the metal oxide from the mixing vessel (19) being supplied to at least one cooling stage (27, 28, 30, 31), characterized in that the waste gas of the at least one separating cyclone (8, 11) is introduced into the mixing vessel (19) as conveying air such that metal oxide from the mixing vessel (19) is pneumatically delivered into the at least one cooling stage (27, 28, 30, 31 ) and/or a delivery conduit (24) leading to the same.

2. The process as claimed in claim 1 , characterized in that the metal oxide from the mixing vessel (19) is delivered via an at least partly ascending pneumatic intermediate delivery conduit (37, 23) into the at least one cooling stage (27, 28, 30, 31 ) or a delivery conduit (24) leading to the same.

3. The process as claimed in claim 1 or 2, characterised in that the waste gas of the at least one separating cyclone (8, 11) is introduced into the mixing vessel (19) as conveying air with a pressure of at least 150 mbar.

4. The process as claimed in any of the preceding claims, characterised in that metal hydroxide contained in the waste gas of the at least one separating cyclone (8, 11) is heated and calcined by the metal oxide in the mixing vessel (19).

5. The process as claimed in any of the preceding claims, characterized in that from the at least one separating cyclone (8, 11) metal hydroxide is introduced into the mixing vessel (19) via a bypass conduit (20).

6. The process as claimed in any of the preceding claims, characterized in that before the at least one separating cyclone (8, 11) the metal hydroxide is suspended, dried, preheated to a temperature of at least 75°C, in particular to about 150°C, and/or partly calcined in at least one preheating stage (2, 5) comprising a heat exchanger (2) and a downstream separator (5).

7. The process as claimed in any of the preceding claims, characterized in that the temperature in the mixing vessel (19) is between 550 and 900°C, preferably between 600 and 850°C, in particular about 750°C.

8. The process as claimed in any of the preceding claims, characterized in that the temperature in the pneumatic intermediate delivery conduit (23, 37), in which metal oxide from the mixing vessel (19) is delivered into the at least one cooling stage (27, 28, 30, 31 ) or a delivery conduit (24) leading to the same, is be- tween 500 and 850°C, preferably between 550 and 800°C, in particular about 680°C.

9. The process as claimed in any of the preceding claims, characterized in that aluminium hydroxide with a grain size of less than 100 μm on average is sup- plied as starting material.

10. The process as claimed in any of the preceding claims, characterized in that preheated oxygen-containing gas as well as gaseous and/or liquid fuel is supplied to the reactor (12).

11. The process as claimed in any of the preceding claims, characterised in that gas heated in the at least one cooling stage (30, 31) is supplied to an upstream cooling stage (27, 28), a preheating stage (2, 5, 10, 11) and/or the reactor (12).

12. A plant for producing metal oxide from metal hydroxide, in particular for performing a process as claimed in any of the preceding claims, comprising a reactor (12) constituting a fluidized-bed reactor, in which metal hydroxide is heated by combustion of fuel and metal oxide is produced, at least one preheating stage (2, 5, 10, 11) and at least one pneumatic delivery conduit (7) with at least one downstream separating cyclone (8, 11) being provided upstream of the reactor (12) and a mixing vessel (19), which is connected with at least one cooling stage (27, 28, 30, 31), being provided downstream of the reactor (12), characterized in that the at least one separating cyclone (8, 11) has a waste gas conduit (21) leading into the mixing vessel (19), and that the mixing vessel (19) is connected with the at least one cooling stage (27, 28, 30, 31) or a delivery conduit (24) leading to the same via an at least partly ascending pneumatic intermediate delivery conduit (23, 37).

13. The plant as claimed in claim 12, characterized in that the mixing vessel (19) is arranged below or substantially at the level of the vertically lower end of the delivery conduit (24) which leads to the cooling stage (27, 28, 30, 31) downstream of the mixing vessel (19).

14. The plant as claimed in claim 12 or 13, characterized in that in the mixing vessel (19) a solids feed opening (34) is provided and at an end opposite the same a solids discharge opening (37) is provided, between which a plurality of weirs (35) are arranged, which are overflown and/or underflown by the metal oxide and/or the metal hydroxide.

15. The plant as claimed in any of claims 12 to 14, characterized in that the intermediate delivery conduit (23) has a delivery tube (37) immersed into the metal oxide and/or the metal hydroxide.

16. The plant as claimed in claim 15, characterised in that the waste gas conduit (21) of the at least one separating cyclone (8, 11 ) opens into the mixing vessel (19) vertically below the lower edge of the delivery tube (37).

17. The plant as claimed in any of claims 12 to 16, characterized in that at least one conduit (21) for fluidizing gas opens in particular into the bottom of the mixing vessel (19).

18. The plant as claimed in any of claims 15 to 17, characterized in that in addition to the delivery tube (37) another waste gas conduit (25, 26) is provided, which connects the mixing vessel (19) with one of the preheating stages (2, 5, 10, 11 ) upstream of the reactor (12) or the at least one cooling stage (27, 28, 30, 31) or the delivery conduit (24) leading to the same.

19. The plant as claimed in any of claims 12 to 18, characterized in that the at least one separating cyclone (8, 11) is connected with the mixing vessel (19) via a bypass conduit (18b).

20. The plant as claimed in any of claims 12 to 19, characterized in that at least two cooling stages (27, 28, 30, 31) are provided downstream of the mixing vessel (19), in which the metal oxide is fluidized and at least one of which has a conduit through which heated fluidizing gas is supplied to an upstream cooling stage (27, 28), a preheating stage (2, 5, 10, 11 ) and/or the reactor (12).