AU2008291392B2

AU2008291392B2 - Process and plant for the thermal treatment of granular solids

Info

Publication number: AU2008291392B2
Application number: AU2008291392A
Authority: AU
Inventors: Pekka Hiltunen; Michael Missalla; Gunter Schneider
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2007-09-01
Filing date: 2008-07-10
Publication date: 2013-05-23
Anticipated expiration: 2028-07-10
Also published as: AU2008291392A1; UA100992C2; EA016569B1; WO2009026989A1; DE102007041586B4; EA201000417A1; BRPI0816090A2; BRPI0816090B1; DE102007041586A1

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fluidized-bed cooler, in which the calcined solids are cooled by means of fluidizing air, wherein the fluidizing air is withdrawn from the cooler and introduced into the fluidized-bed reactor as secondary air.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

WO 2009/026989 PCT/EP2008/005619 1 Process and Plant for the Thermal Treatment of Granular Solids 5 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cooler, in which the thermally treated solids are cooled by means of fluidizing gas, wherein 10 the fluidizing gas is withdrawn from the cooler and introduced into the fluidized-bed reactor as secondary gas. Such process for producing anhydrous alumina (A1 2 0 3 ) from aluminium hydroxide

(AI(OH)

3 ) is known from EP 0 861 208 B1. Here, the aluminium hydroxide is cal 15 cined in a circulating fluidized bed upon traversing two preheating stages. Preheat ing 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 20 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 25 energy consumption (kJ/t A1 2 0 3 ) 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 X >> 1), which leads to higher waste gas tem peratures than in full-load operation. In partial-load operation, the combustion air cannot be reduced as one likes, in order to adapt the k ratio, as the combustion air 2 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. 5 The present invention relates to a process for the thermal treatment of granular solids, in particular for producing 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, 10 wherein the fluidizing air is withdrawn from the cooler and introduced into the fluidized-bed reactor as secondary gas, the secondary gas stream is divided and a corresponding bypass stream is guided past the fluidized-bed reactor and introduced into a delivery conduit for the solids, characterized in that the secondary gas stream is divided based on the ratio of the plant load, that the 15 size of the bypass stream is variable and that the size of the bypass stream is controlled in dependence on the supply rate of the solids into the fluidized-bed reactor. It was found out that even with a very low plant load, in which up to 70% of the 20 secondary gas stream may be 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 may be 25 introduced into the delivery conduit before a preheating stage for the solids, in order to 3593816_1 (GHMatters) P83107.AU 13/08/12 WO 2009/026989 PCT/EP2008/005619 3 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 5 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 by pass stream is fed into the second delivery conduit in accordance with a particu larly preferred aspect of the invention. When calcining aluminium hydroxide, fresh 10 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 15 downstream of the fluidized-bed reactor is introduced into the suspension prehea ter. In another preferred aspect of the invention, the gas/solids mixture from the sus pension preheater is supplied to a second separator, whose waste gas is supplied 20 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 con duit. In principle, the secondary gas stream can be divided at every point of the process 25 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 reac tor).

WO 2009/026989 PCT/EP2008/005619 4 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 5 bed cooler, in which the solids withdrawn from the fluidized-bed reactor via a dis charge 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 sec ondary gas conduit. In accordance with the invention, one bypass conduit or sev eral bypass conduits is/are branched off from the secondary gas conduit, which 10 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 15 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 re maining secondary air conduit, in accordance with a development of the invention. To optimally utilize the heat contained in the secondary gas stream, the bypass 20 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 flu idized-bed reactor, whose waste gas is introduced into a preheater, in particular a 25 suspension preheater, via a first delivery conduit, wherein the suspension prehea ter 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 30 delivery conduit.

WO 2009/026989 PCT/EP2008/005619 5 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. 5 The feed points for the bypass conduits as described above can be provided alter natively or cumulatively in dependence on the plant conditions, wherein the re spective amounts are controlled individually by means of the control valves pro vided in the bypass conduits. 10 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, break down of magnesium sulfate, calcination of ores or preheating of iron ore. 15 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. 20 In the drawing: Fig. 1 schematically shows a plant for performing the process of the inven tion in accordance with a first embodiment of the invention, 25 Fig. 2 shows a plant for performing the process of the invention in accor dance with a second embodiment, Fig. 3 schematically shows a plant for performing the process of the inven tion in accordance with a third embodiment, 30 WO 2009/026989 PCT/EP2008/005619 6 Fig. 4 schematically shows a plant for performing the process of the inven tion in accordance with a fourth embodiment, Fig. 5 schematically shows a plant for performing the process of the inven 5 tion in accordance with a fifth embodiment, Fig. 6 schematically shows a plant for performing the process of the inven tion in accordance with a sixth embodiment, 10 Fig. 7 schematically shows a plant for performing the process of the inven tion in accordance with a seventh embodiment, Fig. 8 schematically shows a plant for performing the process of the inven tion in accordance with an eighth embodiment, and 15 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. 20 Each of the Figures only shows the preferred configuration, in which by way of ex ample 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 25 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 pre heating 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 30 ing and finally to a non-illustrated chimney.

WO 2009/026989 PCT/EP2008/005619 7 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 5 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 10 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 15 temperature in the range from 850 to 1000 0 C by means of fuel supplied via a fuel conduit 14. The oxygen-containing gas streams, e.g. air or air enriched with oxy gen, required for combustion are supplied as fluidizing gas via a primary gas con duit 15 and as secondary gas via a secondary gas conduit 16. 20 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 sepa rated. Via a discharge conduit 18, the solids emerging from the recirculation cy clone 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 25 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 30 in a fluidized-bed cooler 24 equipped with four cooling chambers. In its first cham- WO 2009/026989 PCT/EP2008/005619 8 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. 5 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 10 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 15 and is available for combustion. However, when the plant only operates under par tial 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 re actor 13 via conduit 16 can be reduced, in order to adapt the X ratio to the lower fuel supply required for calcining smaller amounts of aluminium hydroxide and 20 avoid an increase in the combustion and hence waste gas temperatures. The re maining 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 25 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 30 also be branched off from the conduit 23 or from the cooler 29.

WO 2009/026989 PCT/EP2008/005619 9 The second embodiment of the present invention as shown in Figure 2 substan tially 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 5 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 10 is guided in counterflow. Moreover, the configuration and operation of the plant shown in Figure 2 corre spond to the plant of the first embodiment as shown in Figure 1, so that reference can be made to the above description. 15 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 20 produced in the fluidized-bed reactor 13a. This process is described in detail in EP O 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 25 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 in creased thereby. The size of the bypass stream is controlled by means of the con trol valve 27b.

WO 2009/026989 PCT/EP2008/005619 10 Moreover, the configuration and operation of the plant shown in Figure 3 corre spond to the plant of the first embodiment as shown in Figure 1, so that reference can be made to the above description. 5 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 sus pension 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 10 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 15 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 20 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 25 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. 30 While in the embodiments shown in Figures 1 to 6 a circulating fluidized bed with a WO 2009/026989 PCT/EP2008/005619 11 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 5 102 60 741 Al. 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. 10 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 con duit 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 15 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 cu mulatively. In this case, the division of the bypass stream is effected by corre sponding actuation of the control valves 27a, b, c corresponding to the require ments of the plant. Furthermore, it is of course possible to dispose the branching 20 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 25 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 op eration.

12 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 5 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 X = 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 10 provided, the specific excess consumption is only increased by about 7% (195 kJ/kg). In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or 15 necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 20 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 3593816_1 (CHMatters) P83107.AU 13/08/12 WO 2009/026989 PCT/EP2008/005619 13 List of Reference Numerals: 1 charging station 5 2 flash reactor (first preheating stage) 3 separating means 4 gas cleaning 5 conduit 6 preheater 10 7 first delivery conduit 8 recirculation cyclone (first separator) 9 second delivery conduit 10 separation cyclone (second separator) 11 third delivery conduit 15 12 solids supply conduit 13a fluidized-bed reactor (circulating fluidized bed) 13a' return conduit 13b flash reactor 13c annular fluidized-bed reactor 20 14 fuel conduit 15 primary gas conduit 16 secondary gas conduit 17 connecting conduit 18 discharge conduit 25 19 rising conduit 20 cyclone separator 21 rising conduit 22 cyclone separator 23 conduit 30 24 fluidized-bed cooler WO 2009/026989 PCT/EP2008/005619 14 25 conduit 26a,b,c bypass conduit 27a,b,c control valve 29 first cooling stage 5 30 second cooling stage 31 hydrate bypass 32 mixing chamber 33 hydrate bypass

Claims

1. A process for the thermal treatment of granular solids, in particular for producing alumina from aluminium hydroxide, wherein upon preheating in at 5 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, the secondary gas stream is divided and a corresponding 10 bypass stream is guided past the fluidized-bed reactor and introduced into a delivery conduit for the solids, characterized in that the secondary gas stream is divided based on the ratio of the plant load, that the size of the bypass stream is variable and that the size of the bypass stream is controlled in dependence on the supply rate of the solids into the fluidized-bed reactor. 15

2. The process according to claim 1, characterized in that up to 70% of the secondary gas stream are guided past the fluidized-bed reactor as bypass stream.

3. The process according to any one of the preceding claims, characterized in that the bypass stream is introduced into the delivery conduit 2o before a preheating stage for the solids.

4. The process according to any one 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 25 to a second separator via a second delivery conduit, and that the bypass stream is fed into the second delivery conduit.

5. The process according to any one 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 30 first delivery conduit, and that the bypass stream is fed into the first delivery conduit. 3593816_1 (GHMattere) P83107.AU 13/08/12 16

6. The process according to any one 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 5 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. 10

7. A plant for the thermal treatment of granular solids, in particular for performing a process according to any one of the preceding claims, comprising 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 15 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, wherein from the secondary gas conduit and/or from one of the conduits supplying to the same a bypass conduit is branched off, which leads past the 20 fluidized-bed reactor and is connected with a delivery conduit for the solids, characterized in that in the bypass conduit a control valve is provided.

8. The plant according to claim 7, characterized in that the bypass conduit is connected with a delivery conduit leading to at least one preheating stage.

9. The plant according to any one of claims 7 or 8, characterized in that 25 downstream of the fluidized-bed reactor a first separator is provided, whose waste gas is introduced into a preheater via a first delivery conduit, that the preheater is connected with a second separator via a second delivery conduit, and that the bypass conduit is connected with the second delivery conduit.

10. The plant according to any one of claims 7 to 9, characterized in that 30 downstream of the fluidized-bed reactor a first separator is provided, whose waste gas is introduced into a preheater via a first delivery conduit, and that the bypass conduit is connected with the first delivery conduit. 3593816_1 (GHMatters) P83107.AU 13/08/12 17

11. The plant according to any one of claims 7 to 10, characterized in that a first separator is provided downstream of the fluidized-bed reactor, whose waste gas is introduced into a preheater via a first delivery conduit, that the preheater is connected with a second separator via a second delivery conduit, 5 that the second separator is connected with a first preheating stage for fresh solids via a third delivery conduit, and that the bypass conduit is connected with the third delivery conduit.

12. A process for the thermal treatment of granular solids substantially as hereinbefore described with reference to the accompanying Figures. 10

13. A plant for the thermal treatment of granular solids substantially as hereinbefore described with reference to the accompanying Figures. 3593816_1 (GHMatters) P83107.AU 13/08/12