WO2008113553A1

WO2008113553A1 - Process and plant for producing metal oxide from metal salts

Info

Publication number: WO2008113553A1
Application number: PCT/EP2008/002145
Authority: WO
Inventors: Michael Missalla; Günter Schneider; Cornelis Klett; Erwin Schmidbauer
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2007-03-22
Filing date: 2008-03-18
Publication date: 2008-09-25
Anticipated expiration: 2009-09-22
Also published as: AU2008228481B2; UA101804C2; AU2008228481A1; EA200901271A1; BRPI0809403B1; DE102007014435A1; EA016961B1; DE102007014435B4; BRPI0809403A2; BRPI0809403B8

Abstract

When producing metal oxide from metal salts, the metal salt, in particular aluminum hydroxide, is dried and preheated in a first preheater (2) at a temperature of 100 to 200°C, precalcined in a second preheater (5) at a temperature of 300 to 400°C and then calcined in a reactor (8) at a temperature of 850 to 1100°C to obtain metal oxide, in particular alumina. After precalcination in the second preheater (5), a partial stream of the metal salt is branched off and supplied to a mixing tank (14), in which it is mixed with the metal oxide withdrawn from the reactor (8).

Description

Process and Plant for Producing Metal Oxide from Metal Salts

The present invention relates to a process for producing metal oxide from metal hydroxide or other metal salts, in particular from aluminum hydroxide, wherein the metal salt is dried and preheated in at least one first preheating stage at a temperature of 100 to 200⁰C, precalcined in a second preheating stage at a temperature of 200 to 500⁰C and then calcined in a reactor at a temperature of 850 to 1100⁰C to obtain metal oxide, wherein a partial stream of the metal salts is not introduced into the reactor and mixed with metal oxide withdrawn from the reactor, and wherein the product obtained then is cooled.

Such process for producing alumina (AI₂O₃) from aluminum trihydroxide (AI(OH)₃) is known for instance from DE 195 42 309 A1. Here, the humid aluminum trihydroxide first is dried in a first suspension preheater with waste gas having a temperature of about 300⁰C₁ which is supplied from a cyclone separator, and preheated to a temperature of about 160⁰C. Upon separation in a cyclone separator, the solids are supplied to a second suspension preheater, in which they are further dried with waste gas from the recirculation cyclone of a circulating fluidized bed, and upon passing through a cyclone separator then are charged to a fluidized-bed reactor of the circulating fluidized bed, in which the aluminum hydroxide is calcined at temperatures of about 950⁰C to obtain alumina. Before the second suspension preheater, a partial stream of the aluminum trihydroxide preheated in the first sus- pension preheater is branched off and mixed with hot alumina withdrawn from the recirculation cyclone of the circulating fluidized bed. Here, a mixing time of at least two minutes is provided. Subsequently, the hot product mixture is cooled in a multi-stage suspension cooler in direct contact with air and is then supplied to a fluidized-bed cooler for final cooling. Although alumina of increased quality can be produced with the process known from DE 195 42 309 A1 , this process still has some disadvantages. The aluminum trihydroxide branched off from the first suspension preheater has a temperature of about 160⁰C and is mixed with alumina withdrawn from the fluidized-bed furnace at a temperature of about 1000⁰C. Due to the low temperature of the dehydrated aluminum hydroxide and the high expenditure of energy for calcination, merely a relatively small amount of aluminum hydroxide can be branched off as partial stream and be admixed to the alumina. In practice, it was found that the amount of this partial stream is about 10%, in order to ensure that the product mixture is op- timally calcined in the mixing tank. Due to the low content of aluminum hydroxide guided around the reactor, a high expenditure is required to achieve a good mixture with a uniform distribution of the aluminum hydroxide in the mixing tank. In addition, the mixture is impaired in that very much steam is generated by dehydrating the aluminum trihydroxide. This generation of steam leads to local differ- ences in temperature (local subcooling due to heat required for evaporation). The steam generated also drives the reacting particles away from the alumina particles and leads to the particles floating on the hot alumina, so that they cannot be incorporated. With a non-optimized mixture, however, energy efficiency is endangered. Furthermore, both lead to a prolongation of the retention time, in order to ensure a sufficient calcination. In addition, the high temperature difference between the hot alumina of about 1000⁰C and the warm aluminum hydroxide of about 160⁰C leads to a thermal shock of the aluminum hydroxide particles guided around the reactor. This thermal shock can lead to weaker particles breaking apart and to an increased formation of dust.

From WO 2006/106443 A2 it is known that in the production of alumina from aluminum trihydroxide a partial stream of the alumina is branched off after the calcining furnace before introduction into the cooling stages and is charged to a reactor, in which it is mixed with filter dust obtained from the waste gas of the preheating stage. The mixture is adjusted such that a temperature of 310 to 325⁰C is obtained in the reactor. The product mixture then is charged to a second cooling stage and mixed with the already precooled alumina from the calcining furnace. At the temperatures of maximally 325°C existing in the reactor, a complete dehydration of the aluminum hydroxide dust supplied to the filter unit can, however, not be achieved without an extremely long retention time of several hours, so that the product quality or energy efficiency is impaired.

In the prior art (cf. for instance DE 31 07 711 A1 ), a so-called aperture blocker frequently is used for dividing streams of solids, which is a mechanical solids valve in the form of a lance with a cone-shaped tip which fits into a corresponding cone- shaped opening of the tank wall. By withdrawing or inserting the lance into the opening, the cross-section is increased or reduced, so that the outflow can be stopped. Problematic at using this aperture blocker is the fact that the control aperture blocker includes mechanically moving parts which are in contact with the hot solids. Therefore, it must be cooled by water cooling.

In the process known from WO 2006/106443 A2, the separation of the partial stream of alumina after the calcining furnace is effected by means of a slide valve. In the course of time, however, the hot temperatures of the calcined alumina lead to a wear of the slide valve and hence to a deterioration of the control quality.

In the process known from DE 195 42 309 A1 , a control means in which mechanically moving parts are in contact with the solids having a temperature of only about 160⁰C can be used without major problems. However, if the division of the stream of solids should be effected at a much higher temperature, another solution will have to be found.

Therefore, it is the object of the invention to further improve the product quality and energy efficiency when producing metal oxides, in particular alumina. In a process as mentioned above, this object substantially is solved with the invention in that the partial stream of the metal salt is branched off after the, at least partial, precalcination in the second preheating stage and supplied to a mixing tank, in which it is mixed with the metal oxide withdrawn from the reactor.

In accordance with the present invention, precalcination is understood to be the partial dehydration or removal of compounds, e.g. HCI and NOx. Calcination, however, refers to the complete dehydration or removal of compounds, e.g. SO₂. Metal salts in accordance with the invention preferably are metal hydroxide or metal carbonate, in particular aluminum hydroxide.

When using aluminum trihydroxide as feedstock, the aluminum trihydroxide is pre- calcined by the elevated temperature in the second preheating stage and at least partly converted to aluminum monohydrate (AIOOH). If this aluminum monohy- drate is admixed to the alumina withdrawn from the reactor, a lower specific formation of steam is obtained as compared to the admixture of aluminum trihydroxide provided in the prior art. As a result, the precalcined aluminum hydroxide can more easily be mixed with the alumina from the reactor. This leads to a more uniform mixing in the mixing tank, lower local temperature differences and a reduced for- mation and circulation of dust. Moreover, the energy demand of the process and the retention time in the mixing tank can be reduced further. As in accordance with the invention the partial stream of aluminum monohydrate branched off has a temperature of 200 to 500⁰C₁ preferably about 300 to 400⁰C, considerably warmer material is mixed with the hot alumina of about 1000⁰C from the reactor, whereby the thermal shock is decreased and the disintegration of particles is reduced. At the same time, a greater amount of aluminum hydroxide can be guided around the reactor due to the higher temperature and the reduced energy demand for the further calcination of the aluminum monohydrate. In accordance with a development of the invention it is therefore provided that about 10 to 40%, preferably 11 to 25%, in particular about 15 to 20% of the pre- calcined metal hydroxide is not introduced into the reactor.

The temperature in the mixing tank also is more stable due to the smaller temperature difference between the joined streams of material. In particular, a temperature of about 500 to 820⁰C, preferably about 600 to 800⁰C, particularly preferably 700 to 78O⁰C is adjusted in the mixing tank in accordance with the invention for the production of alumina. A complete dehydration of the aluminum monohydrate and hence a complete conversion of the starting product aluminum trihydroxide to alumina can be ensured thereby. At the same time, the retention time in the mixing tank can be reduced. A further temperature increase in the mixing tank to e.g. 820 to 900⁰C is possible and leads to a further reduction of the retention time. However, the quantity delivered in the bypass must then be reduced considerably.

When a suspension preheater is used as second preheating stage, a separator will be provided downstream of the same in accordance with the invention, in which the precalcined metal hydroxide is separated from the gas stream. The separation of the partial stream guided around the reactor then is effected after this separator.

In accordance with a particularly preferred aspect of the invention it is provided that the stream of solids withdrawn after the second preheating stage is at least partly discharged via a downpipe and fluidized at the bottom of the downpipe by supplying a conveying gas, and that at least part of the stream of solids is delivered by the conveying gas via a rising pipe branched off from the first downpipe to a mixing tank. By means of this downpipe/rising pipe arrangement, which is also referred to as seal pot, a division of the stream of solids thus is effected without movable parts of the apparatus getting in direct contact with the hot solids. As the stream of solids is delivered to the top via the rising pipe, the various process stages no longer must be built one on top of the other, but can also be erected one beside the other. Construction height and hence costs will be saved thereby.

In accordance with a particularly preferred aspect of the invention, the supply of the conveying gas at the bottom of the downpipe is varied by a control means. In this way, the quantity of the stream of metal hydroxide branched off before the reactor can be determined particularly easily. ,

Preferably, the temperature in the mixing tank is used as a control variable for supplying the stream of conveying gas, so that suitable process conditions are ensured for the mixture and for the complete dehydration of the metal hydroxide. If the temperature in the mixing tank differs from a specified setpoint, the supply of the fluidizing gas is adapted such that correspondingly more or less solids are delivered through the rising pipe and as a result the temperature in the mixing tank is returned to the desired value. In contrast to the mass flows of the solids, the temperature can be measured very easily, so that a reliable control is easily possible.

In accordance with a preferred embodiment of the invention, the pressure difference between the bottom and the top of the downpipe is kept smaller than the pressure loss corresponding to a fluidized downpipe. If, as likewise provided in accordance with the invention, the pressure at the bottom of the downpipe is kept greater than the pressure at the top of the downpipe, the solids in the downpipe behave like a sinking bed with a porosity close to that of a fixed bed. Thus, a non- fluidized, traversed moving bed is present in the downpipe.

The pressure difference of the downpipe, ΔP_D, here is defined by

AP_D =AP_{R +} P_R}K -P₀ -AP^_B > 0 (1 ) Here, ΔP_R is the pressure loss over the rising pipe, which depends on the conveying gas flow and the solids mass flow. Since the gas supply to the rising pipe is varied, in order to realize a certain solids mass flow, a corresponding pressure loss is obtained here.

PR,K is the pressure at the top of the rising pipe, which in the case of a recirculation of solids into a fluidized bed mostly is equal to the pressure in the fluidized bed at the point where the rising pipe is connected to the fluidized-bed tank. This pressure need not be constant, because it depends for instance on the variable solids inventory of the fluidized-bed tank. The pressure can also be much higher than the ambient pressure. If the rising pipe opens into an expansion tank, ambient pressure will exist there in many cases. The pressure can vary, however, e.g. when the waste air suction of a fluidizing channel is too strong and a negative pressure is produced. If a further process part is provided downstream of the rising pipe, the pressure PR_,« can also be much higher than the ambient pressure, for instance also higher than the pressure Po.

In addition, the pressure P₀ in the head space of the connected fluidized bed must be considered, and the pressure ΔP_WS.B. which is caused by the fluidized bed of the bed height H_WS,_B above the downpipe inlet. Both pressures depend on the plant behavior of the fluidized-bed tank or of possibly further upstream apparatuses. Thus, the pressure difference ΔP_D over the downpipe is obtained automatically corresponding to the adjustment of the conveying gas flow. Moreover, this pressure difference should not become greater than that which would be obtained if the downpipe was fluidized. This would mean that the porosity in the downpipe is reduced and the backpressure from the rising pipe, or also from the fluidized-bed tank, no longer could be sealed off reliably. This is expressed by

ΔP_D < ΔP_Amax = (l -*_m/) -_Λ - g - tf_o (2) wherein

^ε _mf ^~ porosity of the solids in the fixed-bed condition p_s = solids density g = gravitational acceleration

H₀ = height of the rising pipe

Under these conditions, the bulk material in the downpipe acts as a pressure seal, and the pressure at the top of the rising pipe is decoupled from the pressure at the inlet of the downpipe. Furthermore, the solids mass flow now delivered or the bed height and the solids inventory in the fluidized-bed tank can be adjusted or controlled by varying the conveying gas. The conveying gas, for instance air, flows upwards in the rising pipe for the major part and delivers as much solids to the top as corresponds to its load bearing capacity. A minor part of the conveying gas traverses the moving bed in the downpipe and thereby causes the pressure loss in the downpipe.

In principle, a preheating stage consists of at least one, but also several pre- heaters. In accordance with a development of the invention, the first preheating stage consists of a drier, which dries and heats the aluminum hydroxide to about 110⁰C, and a further preheater, which heats the dried aluminum hydroxide to about 150-190⁰C. The second preheating stage only consists of one preheater, which preheats the dried aluminum trihydroxide to about 300-400⁰C and at least partly precalcines the same. In accordance with another development of the inven- tion, the first preheating stage consists of a drier, which dries and heats the aluminum trihydroxide to about 110⁰C, and of a second preheating stage, comprising two preheaters, in which the dried aluminum hydrate is heated and precalcined in a first preheater first to about 210-250°C and then to about 350-400⁰C. It is likewise possible that the two preheating stages each consist of two or more pre- heaters. In accordance with the invention, the precalcined aluminum hydroxide is removed from a preheater of the second preheating stage at a temperature of greater than 160⁰C, preferably greater than 200⁰C, usually greater than 220°C. Such arrangement has advantages because of the lower energy demand for calcination and the higher temperature of the stream of metal hydroxide branched off as compared to the process known from DE 195 42 309 A1. When further preheating stages are arranged, it is of course possible to also perform the division of the stream of metal hydroxide only after these further preheating stages, the efficiency of the process being changed in this case. It is furthermore possible to constructively solve the preheating such that several preheaters operate in parallel one beside the other and heat the divided stream of material to the same temperatures.

This invention also extends to a plant for producing metal oxide from metal hydroxide with the features of claim 11. After a preheater of the second preheating stage, the bypass conduit for the precalcined metal hydroxide here is branched off from a conduit which directly or indirectly supplies the metal hydroxide to the reactor.

In accordance with the invention, a downpipe for delivering the stream of solids withdrawn from the second preheating stage, of which a rising pipe is branched off to the top, is provided after a preheater of the second preheating stage or after a separator provided downstream of the same. Via a conveying gas supply, conveying gas is introduced into the first downpipe below the rising pipe, in order to deliver solids through the rising pipe to the mixing tank.

In accordance with the invention, the variation of the supply of conveying gas is effected via a control valve, wherein a temperature measuring device is provided on the mixing tank in accordance with a preferred aspect of the invention, and wherein the open position of the control valve can be controlled via a control circuit on the basis of the temperature measured with the temperature measuring device. In accordance with a further aspect of the invention, a third preheater is provided behind the second preheater, wherein after the third preheater the bypass conduit is branched off from the conduit supplying the metal hydroxide to the reactor.

Developments, advantages and possible applications of the invention can also be taken from the following description of embodiments and the drawing. All features described and/or illustrated in the drawing 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 drawing

Fig. 1 schematically shows a plant for performing the process of the invention, and

Fig. 2 schematically shows an apparatus for dividing the stream of solids in the plant as shown in Fig. 1.

In accordance with the flow diagram of the process of the invention shown in Fig. 1 , filter-humid aluminum trihydroxide (AI(OH)₃) is introduced by means of a conveying screw 1 into a first suspension preheater 2 (first preheating stage) and entrained by a waste gas stream coming from a second suspension preheater 5 (second preheating stage). Subsequently, the gas-solids stream is separated in a succeeding cyclone separator 3. For dedusting purposes, the waste gas dis- charged from the cyclone separator 3 is supplied to an electrostatic gas cleaning 4 and finally to a chimney (not shown).

The solids discharged from the cyclone separator 3 and the electrostatic gas cleaning 4 subsequently are introduced into the second suspension preheater 5, in which the solids are entrained by the waste gas discharged from a recirculation cyclone 6 of a circulating fluidized bed and are further dewatered at temperatures of about 35O⁰C and dehydrated to obtain aluminum monohydrate (AIOOH). In the succeeding separating cyclone 7, a separation of the gas-solids stream is effected again, wherein the aluminum monohydrate is supplied downwards and the waste gas is introduced into the first suspension preheater 2.

After the separating cyclone 7 succeeding the second suspension preheater 5, the stream of aluminum monohydrate is divided by means of an apparatus described in detail below (cf. Fig. 2). A main stream containing about 80 to 90% of the stream of solids is supplied via a conduit (conveying means 30) to a fluidized bed reactor 8, in which the aluminum monohydrate is calcined at temperatures of about 1000⁰C and completely dehydrated to obtain alumina (AI₂O₃). The supply of the fuel required for calcination is effected via a fuel conduit 9, which is disposed at a small height above the grid of the fluidized-bed reactor 8. The oxygen- containing gas streams required for combustion are supplied via supply conduit 10 as fluidizing gas and via supply conduit 11 as secondary gas. As a result of the gas supply, a relatively high suspension density is obtained in the bottom region of the reactor between the grid and the secondary gas supply 11 , and a comparatively low suspension density above the secondary gas supply 11.

Via a connecting conduit 12, the gas-solids suspension enters the recirculation cyclone 6 of the circulating fluidized bed, in which another separation of gas and solids is effected. The solids discharged from the recirculation cyclone 6 via conduit 13, which have a temperature of about 1000⁰C, are introduced into a mixing tank 14. The partial stream of the aluminum monohydrate separated below the separating cyclone 7, which has a temperature of about 350⁰C, is also introduced into the mixing tank 14 via a bypass conduit 15. In the mixing tank 14, a mixing temperature of about 750⁰C is adjusted corresponding to the mixing ratio between the hot alumina stream supplied via conduit 13 and the aluminum monohydrate stream supplied via bypass conduit 15. The two product streams are thoroughly mixed in the mixing tank 14, which includes a fluidized bed, in order to also completely calcine the aluminum monohydrate supplied via the bypass conduit 15 to obtain alumina. A very long retention time of up to 30 min or up to 60 min leads to an excellent calcination in the mixing tank. It was noted, however, that in general a retention time of less than two minutes, in particular about one minute, already is sufficient for this purpose. A retention time of less than 45 s, in particular less than 30 s, is preferred quite particularly.

From the mixing tank 14, the product obtained is supplied to a first suspension cooler formed of rising pipe 16 and cyclone separator 17. The waste gas of the cyclone separator 17 flows into the fluidized-bed reactor 9 via conduit 11, the solids are introduced into the second suspension cooler formed of rising pipe 18 and cyclone separator 19 and finally into the third suspension cooler formed of rising pipe 20 and cyclone separator 21. The gas flow through the individual suspension coolers is effected in counterflow to the solids via conduits 22 and 23.

Upon leaving the last suspension cooler, the alumina produced undergoes a final cooling in the fluidized-bed cooler 24 equipped with three cooling chambers. In its first chamber, the fluidizing gas supplied to the fluidized-bed reactor 9 is heated, in the succeeding second chambers it is cooled against a heat transfer medium, preferably water, which is guided in counterflow. The alumina finally is discharged through conduit 25.

Fig. 2 shows an apparatus for dividing the stream of solids withdrawn from the separating cyclone 7 after the second preheater 5. The aluminum monohydrate discharged from the separating cyclone 7, which has a temperature of about

35O⁰C, is withdrawn from the separating cyclone at about ambient pressure. From the conveying means 30 designed for instance as fluidizing channel, at least part of the aluminum monohydrate flows off via a downpipe 31 , while the other part is moved on in the conveying means 30 and supplied to the fluidized-bed reactor 8 via various non-illustrated process stages. At the bottom 32 of the downpipe 31 , a rising pipe 33 is branched off, which substantially extends vertically to the top. The solids at the bottom of the downpipe 31 are fluidized by means of at least one nozzle 34. There is shown an upwardly directed nozzle 34, but it is also possible to direct the nozzle downwards, so that clogging can be prevented more reliably. One of skill in the art can employ all measures known to him for suitably fluidizing the solids at the bottom of the downpipe 31. It is possible, for instance, to provide a cap nozzle or a nozzle with a porous body provided at its end, which should prevent clogging of the nozzle. It is also possible to supply the conveying gas via a fluidizing cloth or other porous medium, which is disposed at the bottom of the downpipe above a non-illustrated gas distributor.

The solids rise through the rising pipe 33 into an expansion tank 35 and are supplied from the same via a delivery conduit 36 to the mixing tank 14. Instead of the expansion tank 35, a simple elbow can also be provided at the end of the rising pipe 33.

In the mixing tank 14, the aluminum monohydrate is mixed with alumina from the fluidized-bed reactor 8, which is supplied via conduit 13. The alumina has a tem- perature of about 1000⁰C, so that with the mixing ratio provided in the fluidized mixing tank 11 a mixing temperature of about 750⁰C and a retention time of 20 s are obtained. The pressure in the mixing tank 14 is about 1.14 bar (abs), i.e. there is a slight excess pressure with respect to the surroundings. In this embodiment, the mixing tank 14 can be arranged above or below the conveying means 30.

The temperature in the mixing tank 14 depends on the mixing ratio between the aluminum monohydrate supplied via the rising pipe 33 and the alumina supplied via conduit 13 and on the temperatures of these streams of solids. The temperature in the mixing tank 14 is controlled by the amount and the temperature of the solid streams from the furnace and the preheating stage. However, the solids mass flows in the rising pipe 33 and in conduit 13 can be measured only with difficulty. Therefore, it is preferred in accordance with the invention to detect the easily measurable temperature in the mixing tank 14 by means of a temperature measuring device 37 and use it as a control variable for controlling a control valve 38 in the supply conduit 39 to the nozzle 34, by means of which the supply of the conveying gas at the bottom 32 of the downpipe 31 is adjusted. In this way, the mixing ratio and hence the temperature in the mixing tank 14 can be influenced very easily, in that the supply of conveying gas via the nozzle 34 is increased when the actual temperature in the mixing tank 14 exceeds the setpoint and hence a greater amount of colder aluminum monohydrate is introduced into the mixing tank 14. As a result, the temperature in the mixing tank is decreasing again. When the temperature in the mixing tank 14 decreases below the setpoint, the supply of the aluminum monohydrate is reduced by correspondingly closing the control valve 38.

ϋst of Reference Numerals

1 conveying screw

2 first preheating stage

3 cyclone separator

4 gas cleaning

5 second preheating stage

6 recirculation cyclone

7 separating cyclone

8 fluidized-bed reactor

9 fuel conduit

10 supply conduit fluidizing gas

11 supply conduit secondary gas

12 connecting conduit

13 conduit

14 mixing tank

15 bypass conduit

16 rising pipe

17 cyclone separator

18 rising pipe

19 cyclone separator

20 rising pipe

21 cyclone separator

22 conduit

23 conduit

24 fluidized-bed cooler

25 conduit

30 conveying means

31 downpipe bottom rising pipe nozzle expansion tank delivery conduit temperature measuring device control valve supply conduit

Claims

Claims:

1. A process for producing metal oxide from metal salts, in particular from aluminum hydroxide, wherein the metal salt is dried and preheated in at least one first preheating stage at a temperature of 100 to 200⁰C, is precalcined in a further preheating stage at a temperature of 200 to 500⁰C and then calcined in a reactor at a temperature of 850 to 1100⁰C to obtain metal oxide, wherein a partial stream of the metal salts is not introduced into the reactor and mixed with metal oxide withdrawn from the reactor, and wherein the product obtained then is cooled, characterized in that after the precalcination in the further preheating stage the partial stream of the metal salt is branched off and supplied to a mixing tank, in which it is mixed with the metal oxide withdrawn from the reactor.

2. The process according to claim 1 , characterized in that 10 to 40%, in par- ticular 11 to 25% of the precalcined metal salt are guided around the reactor.

3. The process according to claim 1 or 2, characterized in that the partial stream guided around the reactor has a temperature of 200 to 500⁰C, preferably 300 to 400⁰C.

4. The process according to any of the preceding claims, characterized in that the temperature in the mixing tank is adjusted to about 500 to 820⁰C, preferably 600 to 800°C.

5. The process according to any of the preceding claims, characterized in that subsequent to a preheater of the second preheating stage a separator is provided, in which the precalcined metal hydroxide is separated from the gas stream, and that the separation of the partial stream guided around the reactor is effected after the separator.

6. The process according to any of the preceding claims, characterized in that a partial stream of the stream of solids withdrawn from the second preheating stage is discharged via a downpipe and fluidized at the bottom of the downpipe by supplying a conveying gas, and that via a rising pipe branched off from the down- pipe the partial stream is delivered to the mixing tank by the conveying gas.

7. The process according to claim 6, characterized in that the supply of the conveying gas is varied at the bottom of the downpipe.

8. The process according to claim 7, characterized in that the temperature in the mixing tank is used as a control variable for supplying the stream of conveying gas.

9. The process according to any of claims 6 to 8, characterized in that the pressure difference between the bottom and the top of the downpipe is kept smaller than the pressure loss corresponding to a fluidized downpipe.

10. The process according to any of the preceding claims, characterized in that subsequent to a preheater of the further preheating stage or a separator pro- vided downstream of the same a second further preheater is provided and that the division of the precalcined metal hydroxide is effected after the second further preheater.

11. A plant for producing metal oxide from metal salts, in particular for perform- ing a process according to any of the preceding claims, comprising at least one preheater (2) in a first preheating stage for drying and preheating the metal salt, at least one preheater (5) in a further preheating stage for precalcining the metal salt, a reactor (8) for calcining the metal salt to obtain metal oxide, a bypass conduit (15) for guiding a partial stream of the metal salt or a product of this metal salt around the reactor (8), a mixing tank (14) for mixing the metal salt guided around the reactor (8) via the bypass conduit (15) with metal oxide withdrawn from the reactor (8), and comprising a possibly multi-stage cooler for cooling the product obtained, characterized in that the bypass conduit (15) is branched off from a conduit (30) supplying the metal salt to the reactor (8) after a preheater of the fur- ther preheating stage.

12. The plant according to claim 11 , characterized in that after a preheater of the further preheating stage (5) or a separator (7) provided downstream of the same a downpipe (31 ) is branched off from the conduit (30) leading to the reactor (8), via which downpipe the partial stream of the metal salt can be withdrawn, that a rising pipe (33) is branched off from the downpipe (31 ) to the top, that a conveying gas supply is provided, via which conveying gas is introduced into the down- pipe (31 ) below the rising pipe (33), in order to deliver solids through the rising pipe (33), and that the rising pipe (33) is connected with the mixing tank (14).

13. The plant according to claim 12, characterized by a control valve (38) for varying the supply of conveying gas.

14. The plant according to claim 13, characterized in that on the mixing tank (14) a temperature measuring device (37) is provided, that the supply of the stream of conveying gas is effected via a control valve (38), and that the open position of the control valve (38) can be controlled via a control circuit on the basis of the temperature measured with the temperature measuring device (37).

15. The plant according to any of claims 11 to 14, characterized in that behind a preheater (5) of the further preheating stage a second further preheater is provided and that after this second further preheater the bypass conduit (15) is branched off from the conduit (30) supplying the metal salt to the reactor (8).