WO2009026989A1

WO2009026989A1 - Process and plant for the thermal treatment of granular solids

Info

Publication number: WO2009026989A1
Application number: PCT/EP2008/005619
Authority: WO
Inventors: Pekka Hiltunen; Günter Schneider; Michael Missalla
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2007-09-01
Filing date: 2008-07-10
Publication date: 2009-03-05
Anticipated expiration: 2010-03-01
Also published as: EA201000417A1; EA016569B1; BRPI0816090A2; DE102007041586A1; AU2008291392B2; AU2008291392A1; DE102007041586B4; UA100992C2; BRPI0816090B1

Abstract

In the production of alumina from aluminium hydroxide, the aluminium hydroxide is calcined in a fluidized-bed reactor upon preheating in at least one preheating stage and is then supplied to at least one fluidized-bed cooler, in which the calcined solids are cooled by means of fluidizing air, wherein the fluidizing air is withdrawn from the cooler and introduced into the fluidized-bed reactor as secondary air. In order to minimize the specific energy consumption in partial-load operation of the plant, the secondary air stream is divided and a bypass stream is guided past the fluidized-bed reactor and introduced into a delivery conduit for the solids.

Description

Process and Plant for the Thermal Treatment of Granular Solids

This invention relates to the thermal treatment of granular solids, in particular the production of alumina from aluminium hydroxide, wherein upon preheating in at least one preheating stage the solids are heated, in particular calcined, in a fluid- ized-bed reactor and are then supplied to at least one fluidized-bed cooler, in which the thermally treated solids are cooled by means of fluidizing gas, wherein the fluidizing gas is withdrawn from the cooler and introduced into the fluidized-bed reactor as secondary gas.

Such process for producing anhydrous alumina (AI₂O₃) from aluminium hydroxide (AI(OH)₃) is known from EP 0 861 208 B1. Here, the aluminium hydroxide is cal- cined in a circulating fluidized bed upon traversing two preheating stages. Preheating the aluminium hydroxide is effected by means of waste gas from a separator provided downstream of the fluidized-bed reactor. From the return conduit of the separator, anhydrous hot alumina is branched off and directly and indirectly cooled with air in a fluidized-bed cooler. The air indirectly heated thereby is introduced into the fluidized-bed reactor as fluidizing air, whereas the air introduced into the fluidized-bed cooler as fluidizing air for direct cooling is withdrawn from the cooler and likewise introduced into the fluidized-bed reactor as secondary air.

It was found out that in partial-load operation of such calcining plant the specific energy consumption (kJ/t AI₂O₃) is greater than in full-load operation. This is due to the fact that in partial-load operation a highly hyperstoichiometric combustion occurs (combustion-air/fuel ratio λ » 1 ), which leads to higher waste gas temperatures than in full-load operation. In partial-load operation, the combustion air cannot be reduced as one likes, in order to adapt the λ ratio, as the combustion air is at the same time also used as conveying gas in the delivery conduits and thus a minimum velocity or minimum quantity of conveying air should be maintained.

It is the object of the invention to reduce the specific energy consumption of a plant also in partial-load operation.

In a process as mentioned above, this object substantially is solved with the invention in that the secondary gas stream is divided in a controlled way at one or more points of the stream, and one bypass stream or several bypass streams is/are guided past the fluidized-bed reactor into a delivery conduit for the solids. The transport and combustion tasks of the air thus are decoupled. The part of the secondary gas stream introduced into the fluidized-bed reactor as combustion air can be controlled corresponding to the fuel supply sufficient for calcining the solids supplied. The remaining part of the secondary gas stream is guided past the fluid- ized-bed reactor as a bypass stream directly into a delivery conduit for the solids, so that a sufficient amount of conveying gas is ensured there.

In accordance with a preferred aspect of the invention, the size of the bypass stream is variable, and is controlled together with the fuel supply in dependence on the supply rate of solids into the fluidized-bed reactor. As a result, the λ ratio in the fluidized-bed reactor can be adjusted optimally for different load conditions of the reactor.

It was found out that even with a very low plant load, in which up to 70% of the secondary gas stream are guided past the fluidized-bed reactor as bypass stream, a distinct reduction of the specific energy consumption can be achieved.

In accordance with a development of the invention, the bypass stream is introduced into the delivery conduit before a preheating stage for the solids, in order to ensure a sufficient fluidization therein. As a result, the thermal energy contained in the secondary gas can also be utilized for preheating the solids.

If the solids are supplied to a preheater, in particular a suspension preheater, in which they are preheated with waste gas from a separator provided downstream of the fluidized-bed reactor, wherein the gas/solids mixture from the suspension preheater is supplied to a second separator via a second delivery conduit, the bypass stream is fed into the second delivery conduit in accordance with a particularly preferred aspect of the invention. When calcining aluminium hydroxide, fresh hydrate is added in the suspension preheater, which should react directly with the hot furnace gases withdrawn from the first separator.

In accordance with another preferred aspect of the invention, the bypass stream is fed into a first delivery conduit, through which waste gas from the first separator downstream of the fluidized-bed reactor is introduced into the suspension preheater.

In another preferred aspect of the invention, the gas/solids mixture from the suspension preheater is supplied to a second separator, whose waste gas is supplied to a first preheating stage via a third delivery conduit for preheating and delivering fresh solids, wherein the bypass stream is fed directly into the third delivery conduit.

In principle, the secondary gas stream can be divided at every point of the process into one or more bypass streams. In the preferred embodiment of the invention, however, the secondary gas stream is divided upon traversing the preheating stages of the secondary gas stream (cooling stages of the material from the reactor). A plant for the thermal treatment of granular solids, which is suitable for performing the process of the invention, comprises a fluidized-bed reactor in which the solids are heated, in particular calcined, at least one preheating stage for preheating the solids before introduction into the fluidized-bed reactor, and at least one fluidized- bed cooler, in which the solids withdrawn from the fluidized-bed reactor via a discharge conduit are cooled by means of fluidizing gas, wherein the fluidizing gas is withdrawn from the cooler and introduced into the fluidized-bed reactor via a secondary gas conduit. In accordance with the invention, one bypass conduit or several bypass conduits is/are branched off from the secondary gas conduit, which is/are guided past the fluidized-bed reactor and is/are connected with a delivery conduit for the solids.

In the bypass conduit, a control valve, gate or the like, including preferably a measuring device for the volumetric flow rate, is provided in accordance with a development of the invention. Furthermore, a means for controlling the pressure and/or the pressure loss can be provided in the bypass conduit and/or in the remaining secondary air conduit, in accordance with a development of the invention.

To optimally utilize the heat contained in the secondary gas stream, the bypass conduit is connected with a delivery conduit leading to at least one preheating stage.

In one aspect of the invention, a first separator is provided downstream of the fluidized-bed reactor, whose waste gas is introduced into a preheater, in particular a suspension preheater, via a first delivery conduit, wherein the suspension preheater is connected with a second separator via a second delivery conduit and the bypass conduit opens into the second delivery conduit.

In another aspect of the invention, the bypass conduit is connected with the first delivery conduit. In yet another aspect, the second separator is connected with a first preheating stage for fresh solids via a third delivery conduit, and the bypass conduit opens into the third delivery conduit.

The feed points for the bypass conduits as described above can be provided alternatively or cumulatively in dependence on the plant conditions, wherein the respective amounts are controlled individually by means of the control valves provided in the bypass conduits.

The procedure in accordance with the invention can be employed for all processes that require a delivery of solids, e.g. calcination of magnesium carbonate, breakdown of magnesium sulfate, calcination of ores or preheating of iron ore.

Developments, advantages and possible applications of the invention can be taken from the following description of embodiments and the drawings. All features described and/or illustrated per se or in any combination form the subject-matter of the invention, 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 in accordance with a first embodiment of the invention,

Fig. 2 shows a plant for performing the process of the invention in accordance with a second embodiment,

Fig. 3 schematically shows a plant for performing the process of the invention in accordance with a third embodiment, Fig. 4 schematically shows a plant for performing the process of the invention in accordance with a fourth embodiment,

Fig. 5 schematically shows a plant for performing the process of the inven- tion in accordance with a fifth embodiment,

Fig. 6 schematically shows a plant for performing the process of the invention in accordance with a sixth embodiment,

Fig. 7 schematically shows a plant for performing the process of the invention in accordance with a seventh embodiment,

Fig. 8 schematically shows a plant for performing the process of the invention in accordance with an eighth embodiment, and

Fig. 9 shows a diagram which illustrates the reduction of the specific energy consumption in dependence on the bypass stream guided past the fluidized-bed reactor.

Each of the Figures only shows the preferred configuration, in which by way of example a bypass conduit is branched off from the secondary air conduit after the last preheating of the secondary air.

In the plant in accordance with the first embodiment of the invention, which is shown in Figure 1 , filter-moist aluminium hydroxide (AI(OH)₃) is introduced at a charging station 1 into a first flash reactor 2 or a suspension preheater (first preheating stage), in which it is entrained by the waste gas stream coming from the plant and supplied to a separating means 3. The waste gas emerging from the separating means 3 is supplied to an e.g. electrostatic gas cleaning 4 for dedust- ing and finally to a non-illustrated chimney. The solids emerging from the separating means 3 are delivered via a conduit 5 into a second preheater, which in particular constitutes a suspension preheater 6 (second preheating stage), in which the solids are entrained by the waste gas emerging from a recirculation cyclone (first separator) 8 of a circulating fluidized bed via a first delivery conduit 7 and are further dewatered or dehydrated. Via a second delivery conduit 9, the gas/solids mixture from the suspension preheater 6 is supplied to a separation cyclone (second separator) 10, in which the solids are separated from the gas. Via a third delivery conduit 11 , the gas is introduced into the flash reactor 2 as conveying gas and conveys the fresh aluminium hydroxide to the separating means 3, preheating the solids at the same time.

Via a solids supply conduit 12, the solids separated in the separation cyclone 10 are introduced into a fluidized-bed reactor 13a, in which they are calcined at a temperature in the range from 850 to 1000⁰C by means of fuel supplied via a fuel conduit 14. The oxygen-containing gas streams, e.g. air or air enriched with oxygen, required for combustion are supplied as fluidizing gas via a primary gas conduit 15 and as secondary gas via a secondary gas conduit 16.

Via a connecting conduit 17, the gas-solids suspension enters the recirculation cyclone 8 of the circulating fluidized bed, in which gas and solids are newly separated. Via a discharge conduit 18, the solids emerging from the recirculation cyclone 8 are supplied to a first suspension cooler formed of rising conduit 19 and cyclone separator 20. Via the secondary gas conduit 16, the waste gas of the cy- clone separator 20 flows into the fluidized-bed reactor 13a, the solids are delivered into a second suspension cooler formed of rising conduit 21 and cyclone separator 22. Via a conduit 23, the waste gas of the second suspension cooler is introduced as conveying gas into the rising conduit 19 of the first suspension cooler. Upon leaving the last suspension cooler, the alumina produced undergoes a final cooling in a fluidized-bed cooler 24 equipped with four cooling chambers. In its first cham- ber, the fluidizing gas (primary gas) supplied to the fluidized-bed reactor 13a is heated, in the downstream chambers it is cooled against a heat-transfer medium, preferably water, which is guided in counterflow. The alumina finally is discharged via conduit 25.

A bypass conduit 26a, which opens into the second delivery conduit 9, is branched off from the secondary gas conduit 16 before the fluidized-bed reactor 13. In the bypass conduit 26a, a control valve 27a or a gate or the like is provided, in order to adjust the amount of the bypass stream branched off from the secondary gas con- duit 16 in correspondence with the plant load.

In full-load operation of the plant, the control valve 27a will normally be closed, so that the entire secondary gas supplied via the secondary gas conduit 16 and the associated upstream coolers and conduits is fed into the fluidized-bed reactor 13a and is available for combustion. However, when the plant only operates under partial load, i.e. when only a smaller amount of aluminium hydroxide is charged via the charging station 1 , the amount of secondary gas fed into the fluidized-bed reactor 13 via conduit 16 can be reduced, in order to adapt the λ ratio to the lower fuel supply required for calcining smaller amounts of aluminium hydroxide and avoid an increase in the combustion and hence waste gas temperatures. The remaining part of the secondary gas stream is directly fed into the second delivery conduit 9 via the bypass conduit 26a and promotes the delivery of the solids from the suspension preheater 6 through the further parts of the plant. This ensures that a sufficient amount of conveying gas is always available and a deposition and ac- cretion of particles on the walls of the delivery conduits or separators is avoided. At the same time, the thermal energy contained in the bypass stream is utilized for preheating the solids.

Alternatively, the bypass stream from the secondary gas conduit can for instance also be branched off from the conduit 23 or from the cooler 29. The second embodiment of the present invention as shown in Figure 2 substantially has the same configuration as the plant shown in Figure 1. However, in the plant shown in Figure 2, the fluidized-bed cooler is composed of two separate cooling stages 29, 30, wherein the first cooling stage 29 corresponds to the first chamber of the fluidized-bed cooler 24 in accordance with the first embodiment and serves to heat up the fluidizing gas (primary gas) supplied to the fluidized-bed reactor 13a. In the second cooling stage 30, the alumina produced is finally cooled in three cooling chambers against a heat-transfer medium, preferably water, which is guided in counterflow.

Moreover, the configuration and operation of the plant shown in Figure 2 correspond to the plant of the first embodiment as shown in Figure 1 , so that reference can be made to the above description.

In the third embodiment of a plant in accordance with the invention as shown in Figure 3, a partial stream of the moderately warm aluminium hydroxide is branched off before being fed into the suspension preheater 6 and supplied via a hydrate bypass 31 to a mixing chamber 32, in which it is added to the hot alumina produced in the fluidized-bed reactor 13a. This process is described in detail in EP 0 861 208 B1.

In the third embodiment, the bypass conduit 26b branched off from the secondary gas conduit 16 leads into the first delivery conduit 7, through which the hot waste gas from the recirculation cyclone 8 is introduced into the suspension preheater 6. The gas stream available for fluidization in the suspension preheater 6 is increased thereby. The size of the bypass stream is controlled by means of the control valve 27b. Moreover, the configuration and operation of the plant shown in Figure 3 correspond to the plant of the first embodiment as shown in Figure 1 , so that reference can be made to the above description.

The fourth embodiment of a plant in accordance with the invention as shown in Figure 4 only differs from the plant shown in Figure 3 in that the branching point for the hydrate bypass 33 has been shifted. Instead of branching off before the suspension preheater 6 like in Figure 3, the hydrate bypass 33 of the plant shown in Figure 4 is branched off after the second separator 10 before introducing the sol- ids into the fluidized-bed reactor 13a. As a result, a precalcination of the aluminium hydroxide contained in the bypass stream is already achieved, so that the final calcination of these solids in the mixing chamber 32 is accelerated.

The fifth embodiment of the present invention as shown in Figure 5 substantially has the same configuration as the plant shown in Figure 3. However, merely the feed point for the bypass conduit 26c has been shifted. In this embodiment, the bypass conduit 26c branched off from the secondary gas conduit 16 leads into the third delivery conduit 11 , which connects the separation cyclone 10 with the flash reactor 2. In this way, the fresh aluminium hydroxide added via the charging sta- tion 1 is further preheated in the first preheating stage. In addition, similar to the embodiment as shown in Figure 2, the fluidized-bed cooler has been configured with two separate cooling stages 29, 30.

In the sixth embodiment of the present invention as shown in Figure 6, the branch- ing point for the hydrate bypass 33 has merely been shifted with respect to the plant shown in Figure 5, namely in the same way as in the fourth embodiment shown in Figure 4.

Alternative aspects of the fluidized-bed reactor 13 are shown in Figures 7 and 8. While in the embodiments shown in Figures 1 to 6 a circulating fluidized bed with a fluidized-bed reactor 13a and a return conduit 13a' is provided, a flash reactor 13b is provided in the seventh embodiment of the invention as shown in Figure 7. In the eighth embodiment of the invention as shown in Figure 8, however, an annular fluidized-bed reactor 13c is provided, as it is described for instance in detail in DE 102 60 741 A1. Moreover, the mode of function and operation of the plants shown in Figures 7 and 8 corresponds to the first to sixth embodiments, so that in so far reference is made to the above description. It is, however, always possible to also use reactors other than fluidized-bed or flash reactors with separators, such as cyclones, rotary kilns or similar industrial furnaces.

It should be appreciated that the alternative feed points of the bypass conduit 26a, b, c into the second delivery conduit 9, first delivery conduit 7 or third delivery conduit 11 and the configuration of the fluidized-bed reactor 13 or of the fluidized-bed cooler, as they have been described in detail in the individual plants, can also be used in any combination in the respective plants shown in the other Figures. It is also possible to provide all three feed points of the bypass conduits 26a, b, c cumulatively. In this case, the division of the bypass stream is effected by corresponding actuation of the control valves 27a, b, c corresponding to the requirements of the plant. Furthermore, it is of course possible to dispose the branching points at any point of the secondary gas conduit or of the entire upstream supply conduits for secondary gas.

By means of the inventive division of the secondary gas stream into a bypass stream fed into the fluidized-bed reactor 13 as secondary gas and a bypass stream guided past the fluidized-bed reactor 13 and fed directly into the delivery conduits 7, 9, 11 , which serves the delivery and heat recovery, the specific energy consumption of a calcining plant can substantially be reduced in partial-load operation. In Figure 9, the specific energy consumption of a plant shown in Fig. 1 is illustrated as a function of the partial load of 1250 t/d to 2500 t/d. The upper, continuous curve corresponds to the course of the specific energy consumption, when no bypass stream is used. The lower, dashed curve corresponds to the course of the specific energy consumption with bypass stream and the condition that the calcination is operated with an excess of air λ = 1.2. If no bypass stream is provided, a specific excess consumption of 20% (573 kJ/kg) is obtained under a partial load of 1250 t/d as compared to a full-load operation of 3300 t/d. On the other hand, if a bypass stream of 70% is provided, the specific excess consumption is only in- creased by about 7% (195 kJ/kg).

List of Reference Numerals:

I charging station 2 flash reactor (first preheating stage)

3 separating means

4 gas cleaning

5 conduit

6 preheater 7 first delivery conduit

8 recirculation cyclone (first separator)

9 second delivery conduit

10 separation cyclone (second separator)

I 1 third delivery conduit 12 solids supply conduit

13a fluidized-bed reactor (circulating fluidized bed)

13a¹ return conduit

13b flash reactor

13c annular fluidized-bed reactor 14 fuel conduit

15 primary gas conduit

16 secondary gas conduit

17 connecting conduit

18 discharge conduit 19 rising conduit

20 cyclone separator

21 rising conduit

22 cyclone separator

23 conduit 24 fluidized-bed cooler conduit a,b,c bypass conduita,b,c control valve first cooling stage second cooling stage hydrate bypass mixing chamber hydrate bypass

Claims

Claims:

1. A process for the thermal treatment of granular solids, in particular for pro- ducing alumina from aluminium hydroxide, wherein upon preheating in at least one preheating stage, the solids are heated in a fluidized-bed reactor and are then supplied to at least one fluidized-bed cooler, in which the thermally treated solids are cooled by means of fluidizing air, wherein the fluidizing air is withdrawn from the cooler and introduced into the fluidized-bed reactor as secondary gas, charac- terized in that the secondary gas stream is divided based on the ratio of the plant load and a corresponding bypass stream is guided past the fluidized-bed reactor and introduced into a delivery conduit for the solids.

2. The process according to claim 1 , characterized in that the size of the by- pass stream is variable.

3. The process according to claim 1 or 2, characterized in that the size of the bypass stream is controlled in dependence on the supply rate of the solids into the fluidized-bed reactor.

4. The process according to any of the preceding claims, characterized in that up to 70% of the secondary gas stream are guided past the fluidized-bed reactor as bypass stream.

5. The process according to any of the preceding claims, characterized in that the bypass stream is introduced into the delivery conduit before a preheating stage for the solids.

6. The process according to any of the preceding claims, characterized in that the solids are supplied to a preheater, to which waste gas from a first separator downstream of the fluidized-bed reactor is supplied via a first delivery conduit, that the gas/solids mixture from the preheater is supplied to a second separator via a second delivery conduit, and that the bypass stream is fed into the second delivery conduit.

7. The process according to any of the preceding claims, characterized in that the solids are supplied to a preheater, to which waste gas from a first separator downstream of the fluidized-bed reactor is supplied via a first delivery conduit, and that the bypass stream is fed into the first delivery conduit.

8. The process according to any of the preceding claims, characterized in that the solids are supplied to a preheater, to which waste gas from a first separator downstream of the fluidized-bed reactor is supplied via a first delivery conduit, that the gas/solids mixture from the preheater is supplied via a second delivery conduit to a second separator, in which the waste gas is separated from the solids, that the waste gas of the second separator is supplied via a third delivery conduit to a first preheating stage for preheating and delivering fresh solids, and that the bypass stream is fed into the third delivery conduit.

9. A plant for the thermal treatment of granular solids, in particular for performing a process according to any of the preceding claims, comprising a fluidized-bed reactor (13a, 13b, 13c) in which the solids are heated, in particular calcined, at least one preheating stage (2, 6) for preheating the solids before introduction into the fluidized-bed reactor (13a, 13b, 13c), and at least one fluidized-bed cooler (20, 21 , 22, 23, 24), in which the solids withdrawn from the fluidized-bed reactor (13a, 13b, 13c) via a discharge conduit (18) are cooled by means of fluidizing gas, wherein the fluidizing gas is withdrawn from the cooler (20) and introduced into the fluidized-bed reactor (13a, 13b, 13c) via a secondary gas conduit (16), characterized in that from the secondary gas conduit (16) and/or from one of the conduits (23, 29) supplying to the same a bypass conduit (26a, b, c) is branched off, which leads past the fluidized-bed reactor (13a, 13b, 13c) and is connected with a delivery conduit (7, 9, 11 ) for the solids.

10. The plant according to claim 9, characterized in that in the bypass conduit (26a, b, c) a control valve (27a, b, c) is provided.

11. The plant according to any of claims 9 or 10, characterized in that the bypass conduit (26a, b, c) is connected with a delivery conduit (7, 9, 11 ) leading to at least one preheating stage (2, 6).

12. The plant according to any of claims 9 to 11 , characterized in that downstream of the fluidized-bed reactor (13a, 13b, 13c) a first separator (8) is provided, whose waste gas is introduced into a preheater (6) via a first delivery conduit (7), that the preheater (6) is connected with a second separator (10) via a second de- livery conduit (9), and that the bypass conduit (26a) is connected with the second delivery conduit (9).

13. The plant according to any of claims 9 to 12, characterized in that downstream of the fluidized-bed reactor (13a, 13b, 13c) a first separator (8) is provided, whose waste gas is introduced into a preheater (6) via a first delivery conduit (7), and that the bypass conduit (26b) is connected with the first delivery conduit (7).

14. The plant according to any of claims 9 to 13, characterized in that a first separator (8) is provided downstream of the fluidized-bed reactor (13a, 13b, 13c), whose waste gas is introduced into a preheater (6) via a first delivery conduit (7), that the preheater (6) is connected with a second separator (10) via a second delivery conduit (9), that the second separator (10) is connected with a first preheating stage (2) for fresh solids via a third delivery conduit (11 ), and that the bypass conduit (26c) is connected with the third delivery conduit (11 ).