AU2007338485A1

AU2007338485A1 - Process and plant for the thermal treatment of particulate solids, in particular for producing metal oxide from metal hydroxide

Info

Publication number: AU2007338485A1
Application number: AU2007338485A
Authority: AU
Inventors: Michael Missalla; Guenter Schneider; Werner Stockhausen; Michael Stroeder
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2006-12-22
Filing date: 2007-12-07
Publication date: 2008-07-03
Anticipated expiration: 2027-12-07
Also published as: EA200900866A1; DE102006062151A1; UA100498C2; WO2008077462A2; EA016147B1; BRPI0722087B1; BRPI0722087A2; WO2008077462A3; AU2007338485B2

Description

WO 2008/077462 PCT/EP2007/010680 Process and Plant for the Thermal Treatment of Particulate Solids, in particular for Producing Metal Oxide from Metal Hydroxide 5 This invention relates to a process and a plant for the thermal treatment of particu late solids, such as the production of burnt gypsum or the dehydration of other salts, combustion of residues with organic pollutants (e.g. sewage sludge), calci nation of refractory ores (e.g. gold ores), calcination of CaC0 3 and other carbon ates, breakdown of CaSO 4 , other sulfates or other salts such as nitrates, but in 10 particular the production of preferably anhydrous metal oxide from metal hydrox ide. In doing so, the metal hydroxide is at least partly dehydrated and preheated, before the metal hydroxide is introduced into a fluidized-bed reactor, in which the metal hydroxide is heated to a temperature of about 650 to about 1250*C by com bustion of fuel and metal oxide is generated, wherein primary air, which is en 15 riched with oxygen to an oxygen content of about 22 to about 99.9 % (in the con text of the present application the cited oxygen content always is considered as % per volume), and/or secondary air, which is enriched with oxygen to an oxygen content of about 30 to about 99.9 %, is supplied to the fluidized-bed reactor. 20 The production of metal oxide from metal hydroxide in a circulating fluidized bed is known for instance from DE 198 05 897 C1. In DE 197 22 382 Al it is proposed to enrich the gas supplied to a fluidized-bed reactor with stationary fluidized bed with oxygen. Via Laval nozzles, the oxygen should be introduced into the reactor with supersonic speed above the distributor plate. In the case of a stationary fluidized 25 bed this is necessary, because with lower gas velocities the oxygen supply noz zles will scale due to the high temperature in the fluidized bed and the high ther mal transfer between fluidized bed and nozzles and therefore will achieve an only short service life. On the other hand, the very high velocity of the oxygen-enriched gas in the fluidized bed leads to a strong load of the granular solids, similar to the 30 load in a jet mill, whereby the solids can disintegrate to a strong to very strong WO 2008/077462 PCT/EP2007/010680 -2 extent depending on the strength thereof. In most cases, such grain disintegration is undesirable. In the process according to DE 198 05 897 C1, the solids throughput can be in 5 creased, when correspondingly more heat is provided via the fuel. As long as only air not enriched with oxygen is used for combustion, the gas velocities in an exist ing plant will be increased, which can lead to an increased emission of dust and also to an increased grain disintegration of the fine-grained solids. Similarly, in creased gas velocities can occur when an existing plant is switched over from a 10 fuel of high calorific value to a fuel of low calorific value with the same solids throughput, as long as air is supplied for combustion. Both the increased dust emission occurring with higher gas velocities and the increasing grain disintegration and the resulting deterioration of the product quality 15 are regarded as unsatisfactory. Therefore, it is the object of the present invention to provide a process as men tioned above by using a circulating fluidized bed, which leads to an increase of performance with existing plants or to an improvement of existing plants for use of 20 fuels of lower calorific value, without an increase in dust emission and a decrease in product quality. In accordance with the invention, this object substantially is solved in that the secondary air enriched with oxygen is introduced into the fluidized-bed reactor of 25 the circulating fluidized bed with a gas velocity between about 5 and about 300 m/s, in particular less than 250 m/s, preferably less than 200 m/s, particularly preferably less than 150 m/s. This introduction of oxygen or gas enriched with oxygen with a particularly low flow rate provides for a procedure particularly gentle for the product obtained, e.g. metal oxide, by reducing gas velocities in the units of WO 2008/077462 PCT/EP2007/010680 -3 the plant, which leads to a significant reduction of grain disintegration. Product quality is considerably improved thereby. At the same time, the gas velocity in the dust filter of the process is decreased as 5 a result of the oxygen enrichment of the secondary air, whereby dust emission can also be reduced. It was noted that the supply of oxygen-enriched secondary air into the fluidized-bed reactor also leads to a distinct increase in performance during calcination. This provides either for an increased production of metal oxide from metal hydroxide under the same basic conditions or for the use of fuels of 10 lower calorific value without a reduction of the plant performance or for a combina tion of both modifications. In particular when using fuels of lower calorific value, which have a high content of inert constituents, larger volume flow rates would be obtained in the plant for producing metal oxide without oxygen enrichment of the secondary air, which also leads to higher gas velocities. At the same time, the air 15 requirement for combustion is reduced for instance in the case of fuel gas from an air gasification of bituminous coal, whereby the cooling of the metal oxide pro duced becomes less efficient and the solids temperature is increased. As a result, the specific heat requirement is also increased. In accordance with the invention, these disadvantages are compensated in that the gas supplied to the fluidized-bed 20 reactor as primary air and/or secondary air with a low gas velocity is enriched with oxygen, as in this way not only the effective volume flow rate, but also the gas velocities can be decreased. When for instance the ports of the secondary air supply into the reactor are lined 25 with refractory bricks, refractory concrete or a similar material, there will be no scaling even with an increased oxygen concentration and low gas velocities. In known processes, a refractory lining is not possible with the nozzles of the distribu tor plate.

WO 2008/077462 PCT/EP2007/010680 -4 In accordance with a particularly preferred embodiment of the invention, the gas enriched with oxygen is introduced into the circulating fluidized bed of the fluid ized-bed reactor with a gas velocity of less than 100 m/s, in particular between about 10 and about 100 m/s. 5 For producing metal oxide, for instance for the production of alumina, it turned out to be particularly advantageous when the calcination in the fluidized-bed reactor is effected at a temperature of about 850 to about 10500C. 10 In accordance with the invention, the reduction of grain disintegration, which can for instance be more than 15%, is greater than the smallest reduction of the gas velocity in one of the units of the plant. The reduction of the gas velocity can vary in the different units of the plant and lie for instance between about 15% and about 30%. 15 A particularly distinct reduction of grain disintegration and dust emission can be achieved in that the primary air supplied to the fluidized-bed reactor with a low flow rate is enriched with oxygen to an oxygen content of 22% to 49% and/or the sec ondary air is enriched with oxygen to an oxygen content of about 90 to about 20 99.5%. Preferably, the gases supplied to the fluidized-bed reactor are indirectly and/or directly preheated with process heat to a temperature between about 100 and about 8000C, in particular between about 150 and about 7500C. By preheating the 25 primary gas, secondary gas and/or e.g. gaseous fuel supplied to the fluidized-bed reactor with process heat, the total energy demand of the process of the invention can further be decreased. As an alternative, it is also possible to preheat the gases supplied to the fluidized-bed reactor with foreign heat.

WO 2008/077462 PCT/EP2007/010680 -5 The product temperature of the metal oxide produced with the process of the invention usually should be not more than about 800C. For this purpose, the metal oxide withdrawn from the fluidized-bed reactor can be cooled indirectly in at least one first cooling stage by direct contact with air and/or oxygen or a mixture of the 5 two and in at least one further cooling stage provided downstream of the at least one direct cooling stage, which constitutes a fluidized-bed cooler. It is particularly preferred when the secondary gas supplied to the fluidized-bed reactor is pre heated in at least one of the first and/or second cooling stages. Thus, the oxygen for enrichment of the secondary air can already be added before the cooling 10 stages for the metal oxide, so that the oxygen also contributes to cooling the metal oxide and is preheated at the same time, before it reaches the fluidized-bed reac tor. In accordance with a preferred embodiment of the invention, at least one of the 15 direct cooling stages includes a delivery conduit, which pneumatically conveys the metal oxide in upward direction, and a separating cyclone. Thus, the solids are at the same time cooled and conveyed to a higher position, which possibly provides for further transport by the action of gravity. 20 When a fuel of comparatively low calorific value of below 7500 kJ/kg is used for producing alumina from aluminum hydroxide in accordance with the process of the invention, about 1.5 to 3 Nm 3 /h, preferably between about 2 and 3 Nm 3 /h and particularly preferably between about 2.3 and about 2.5 Nm 3 /h of oxygen (95%) can be admixed to the fluidized-bed reactor per 1 t/d of alumina produced together 25 with the secondary air. By means of the process of the invention, a plant operated for instance with fuel oil as fuel thus can be switched over to a heating gas with a lower calorific value of e.g. about 4000 to about 5500 kJ/kg, without reducing the production or deteriorating the product quality.

WO 2008/077462 PCT/EP2007/010680 -6 In accordance with a further embodiment, about 23 to about 25 Nm 3 /h of additional air is supplied to the fluidized-bed reactor per 1 t/d of alumina produced, to which additional air about 2 to 4 Nm 3 /h, preferably between about 2.5 and 3.5, particu larly preferably between 2.9 and about 3.1 N/m 3 of oxygen (95%) is admixed. The 5 amount of additional air supplied to the fluidized-bed reactor thereby is reduced significantly as compared to conventional processes, so that the effective volume flow rate and thus to the same extent also the velocity is reduced. This leads to an unexpectedly high reduction of the grain disintegration and thus to an improve ment of the product quality. 10 The object underlying the invention is furthermore solved by a plant for the thermal treatment of particulate solids, which includes at least one preheating stage, at least one fluidized-bed reactor, a means for supplying fuel gas into the fluidized bed reactor, and at least one cooling stage which consists of at least three (partial) 15 coolers, at least one of which is arranged and connected with the means for sup plying fuel gas such that the fuel gas is passed through the at least one cooler for preheating the fuel gas before entrance into the fluidized-bed reactor. Preferably, two of the (partial) coolers constitute fluidized-bed coolers. The same 20 can each consist of a plurality of chambers. In accordance with a preferred em bodiment, the further (partial) cooler provided for heating the fuel gas can be a cooling cyclone. In accordance with the invention, a pneumatic conveyor for supplying solids into 25 the fluidized bed reactor can be provided upstream of the fluidized-bed reactor, which pneumatic conveyor preferably is connected with a delivery conduit for hot solids from the fluidized-bed reactor via a conduit. For instance, a cyclone pro vided upstream of the reactor thereby is connected with a cyclone provided down stream of the reactor such that the gas from the cyclone provided upstream of the WO 2008/077462 PCT/EP2007/010680 -7 reactor can mix with the solids from the cyclone provided downstream of the reac tor. The invention will subsequently be explained in detail by means of embodiments 5 and with reference to the drawing, in which: Fig. 1 schematically shows a flow diagram in accordance with a first em bodiment of the invention, 10 Fig. 2 schematically shows a flow diagram in accordance with a second embodiment of the invention, and Fig. 3 schematically shows a flow diagram in accordance with a third em bodiment of the invention. 15 In the process shown in Fig. 1 to 3, the metal hydroxide to be dehydrated is sup plied through a conveyor screw 1 or the like and introduced into a preheating stage, which can be formed by an entrained-bed preheater 2. A hot gas mixture with temperatures between about 200 and about 5000C is supplied to the en 20 trained-bed preheater 2 through a conduit 3. Via a conduit 4, the solids-gas mix ture is supplied to a filter means 5, which can constitute for instance a bag filter, a cyclone or an electrostatic precipitator. The waste gas of the filter means 5 es capes via a conduit 6. Alternatively, a means for the further waste gas treatment (waste gas scrubber, method for water condensation, etc.) can be provided down 25 stream of the filter means 5. The metal hydroxide dried in this way is delivered through a conduit 7 to the base of a pneumatic conveyor 8, in which solids are supplied to a separating cyclone 10 by supplying air from a conduit 9. The waste gas of the separating cyclone 10 flows through a conduit 11 to a further cyclone 12. 30 WO 2008/077462 PCT/EP2007/010680 -8 The solids obtained in the separating cyclone 10 are delivered through a conduit 13 to a further entrained-bed preheater 14, in which the at least partly dehydrated solids get in direct contact with hot waste gas from a conduit 15 and subsequently are supplied through a conduit 16 into a separating cyclone 17, whose waste gas 5 is supplied to the first entrained-bed preheater 2 via conduit 3. The solids sepa rated in the further separating cyclone 17 are supplied via a conduit 18 to a fluid ized-bed reactor 19 in which temperatures of about 850 to 1050*C exist. In its lower portion, the fluidized-bed reactor 19 includes a relatively dense fluid 10 ized bed 20 of metal oxide particles. The fluidization of this fluidized bed is ef fected by primary air from a conduit 21, which is delivered through a distributor 22 into the lower portion of the fluidized bed 20. In doing so, the primary air is pre heated in an air preheater 23 explained in detail below. 15 In addition, gaseous and/or liquid fuel is introduced into the fluidized bed 20 from outside through one or more lances 24, the fuel being heated and ignited by the hot metal oxide particles of the fluidized bed 20. The complete combustion is effected in the reactor 19 together with the preheated secondary air supplied through a conduit 25. The desired calcination temperature is achieved by means 20 of this combustion. The fluidized-bed reactor 19 can also constitute an annular fluidized-bed reactor in accordance with DE 102 60 739. In this case, the supply of secondary air can be effected through the central tube of the annular fluidized-bed reactor. However, it 25 is also possible to divide the supply of secondary air and introduce the same both through a conduit above the distributor plate and through the central tube. The hot waste gas of the fluidized-bed reactor 19, which includes metal oxide, is delivered through a passage 26 into a recirculation cyclone 27. The waste gas of 30 the recirculation cyclone 27 is supplied to the second entrained-bed preheater 14 WO 2008/077462 PCT/EP2007/010680 -9 via conduit 15. Part of the hot solids separated in the recirculation cyclone 27 are returned via a conduit 28 into the fluidized-bed reactor 19, whereas the remaining part of the hot solids is supplied to a first cooling stage through a conduit 29. This first cooling stage is configured such that via a conduit 30 additional air, via a 5 further conduit 31 preheated fluidizing air, and via a third conduit 33 technical oxygen are mixed with each other and supplied to a pneumatic delivery conduit 34 via a further conduit 33. The hot solids from conduit 29 are introduced into this delivery conduit 34, so that the hot solids mix with the mixture of air and oxygen from conduit 33, whereby the solids are cooled, whereas the mixture of air and 10 oxygen is heated. The waste gas of the separating cyclone 10 is admixed to this solids-gas mixture via conduit 11 and then introduced into the cooling cyclone 12 via a conduit 35. In said cooling cyclone, gas and solids are separated from each other, the gas flow being supplied as preheated, oxygen-enriched secondary air to the fluidized-bed reactor 19 via conduit 25. Via conduit 36, the solids are supplied 15 to the fluidized-bed cooler 23, which at the same time constitutes the air preheater for the primary air. The solids are cooled further in the fluidized-bed cooler 23, whereas the primary air is heated in the tube bundles. The primary air heated in this way then is delivered via conduit 21 into the fluidized-bed reactor 19. 20 Fluidizing air is supplied into the fluidized-bed cooler 23 via conduit 37, which is also connected with a further fluidized-bed cooler 38. In the second fluidized-bed cooler 38, the solids are cooled to the desired final temperature by means of one or more liquid cooling media 39. The fluidizing air introduced into the two fluidized bed coolers via conduits 37 is supplied by a blower 41 via a conduit 40. The pri 25 mary air, which is heated in the tube bundles of the first fluidized-bed cooler, is supplied via a further blower 42. Alternatively or in addition to the supply of oxygen via the delivery conduit 34, the technical oxygen can also be admixed to the pri mary air via the blower 42 or to the fluidizing air for the two fluidized-bed coolers 23 and 38 via a conduit 43. 30 WO 2008/077462 PCT/EP2007/010680 - 10 In the embodiment as shown in Fig. 2, a heating gas is used as fuel. As described above, the same is introduced into the fluidized-bed reactor 19 via the lances 24. The heating gas can be preheated before it is supplied to the fluidized-bed reactor 19. For this purpose, the heating gas is supplied via a conduit 44 to the tube bun 5 dle of a further fluidized-bed cooler 45, in which the solids from the cooling cyclone 12 are cooled. The fluidized-bed cooler 45 thus is provided upstream of the fluid ized-bed cooler 23, so that the solids from the cooling cyclone 12 first pass the fluidized-bed cooler 45, then the fluidized-bed cooler 23 and finally the fluidized bed cooler 38. The fluidized-bed coolers can have a different number of chambers. 10 Another embodiment is shown in Fig. 3. Heating gas is likewise used as fuel in the fluidized-bed reactor 19. For this purpose, the heating gas is introduced via a conduit 46 into a further cooling cyclone 47, whose waste gases are introduced into the fluidized-bed reactor 19 via the lances 24. The solids separated in the first 15 fluidized-bed cyclone 12 are introduced into a conduit 48 which for instance serves as pneumatic delivery conduit, into which also opens the conduit 46 for supplying the heating gas. The solids-gas mixture introduced into the second cooling cyclone 47 is separated in the cooling cyclone 47, the solids being supplied to the first fluidized-bed cooler 23 via a conduit 36'. 20 Example 1: Improvement of a plant for use of a gas of low calorific value An existing plant for producing 3300 t of alumina per day is operated with fuel oil which has a calorific value of 39876 kJ/kg. This plant should be switched over to a 25 heating gas which merely has a calorific value of 4642 kJ/kg. There should still be achieved an alumina production of 3300 t/d. About 8000 Nm 3 /h of oxygen (95%) are admixed to the additional air, and the heating gas is preheated. This preheating of the heating gas is effected either in 30 the fluidized-bed cooler 45 as shown in Fig. 2 to 180 0 C or, if the gas quality per- WO 2008/077462 PCT/EP2007/010680 - 11 mits, directly to 450 0 C by the cooling cyclone 47 in accordance with the embodi ment as shown in Fig. 3. Via the lances 24, the oxygen is introduced into the fluidized-bed reactor 19 together with the heating gas with a gas velocity between about 20 and about 50 m/s. The alumina leaves the fluidized-bed cooler 38 with a 5 product temperature of not more than 80 0 C. Due to the above-described supply of oxygen into the fluidized-bed cooler 19, the aluminum production of 3300 t/d can also be maintained with the fuel of lower calorific value. At the same time, an improvement of the product quality is 10 achieved by a reduction of the grain disintegration. The effective volume flow rates in the plant are smaller or maximally equal to the effective volume flow rates of the heating oil case. Dust emission thereby can be minimized. The oxygen content in the gas outlet of the fluidized-bed reactor 19 is equal to the oxygen content in the heating oil case. 15 Example 2: Quality improvement of the product in terms of grain disintegra tion In a plant in accordance with the embodiment as shown in Fig. 1, in which there 20 are produced 3300 t of alumina per day with an air quantity of 120000 Nm 3 /h, about 10000 Nm 3 /h of oxygen (95%) are admixed via conduit 32 to the additional air supplied via conduit 30, the supply of additional air via conduit 30 being re duced by about 40000 Nm 3 /h. Thus, an oxygen content of about 34.3% is obtained in conduit 33. The effective volume flow rate and hence the gas velocity are re 25 duced thereby in the entrained-bed preheater 14 and the separating cyclone 17 by about 18%, in the cooling cyclone 12 by about 28%, and in the fluidized-bed reac tor 19 and the recirculation cyclone 27 by about 16%. As a result, the grain disin tegration of the alumina can be reduced by more than 16%.

Claims

2. The process according to claim 1, characterized in that the oxygen or the gas enriched with oxygen is introduced into the fluidized-bed reactor (19) with a gas velocity between about 10 and about 100 m/s. 20 3. The process according to any of claims 1 or 2, characterized in that the particulate solids are metal hydroxide which is converted to metal oxide.
4. The process according to any of the preceding claims, characterized in that the primary air supplied to the fluidized-bed reactor (19) is enriched with oxy 25 gen to an oxygen content of about 22 to about 49% and/or secondary air enriched with oxygen to an oxygen content of about 90 to about 99.5%.
5. The process according to any of the preceding claims, characterized in that the oxygen-enriched air flows supplied to the fluidized-bed reactor (19) are WO 2008/077462 PCT/EP2007/010680 - 13 indirectly and/or directly preheated with process heat to a temperature between about 100 and about 800*C, in particular between about 150 and about 7500C.
6. The process according to any of the preceding claims, characterized in 5 that the heating gas supplied to the fluidized-bed reactor (19) is indirectly and/or directly preheated with process heat to a temperature between about 100 and about 300 0 C, in particular between about 150 and about 250 0 C.
7. The process according to any of the preceding claims, characterized in 10 that the metal oxide withdrawn from the fluidized-bed reactor (19) is indirectly cooled in at least one first cooling stage (34) by direct contact with air and/or oxy gen or a mixture thereof and in at least one further cooling stage (23, 38, 45), which constitutes a fluidized-bed cooler, downstream of the first cooling stage (34). 15 8. The process according to claims 5 to 7, characterized in that the gases supplied to the fluidized-bed reactor (19) are preheated in at least one of the first and/or second cooling stages (23, 34, 38, 45).
9. The process according to any of claims 7 or 8, characterized in that at 20 least one of the first cooling stages includes a delivery conduit (34), which pneu matically delivers the metal oxide in upward direction, and a separating cyclone (12).
10. The process according to any of the preceding claims, characterized in 25 that the fuel has a calorific value of below 7500 kJ/kg, in particular between about 4000 and about 5500 kJ/kg, wherein additional air, to which between about 1.5 and about 3.5 Nm 3 /h of oxygen (95%) are admixed per 1 t/d of alumina produced, is supplied to the fluidized-bed reactor (19). WO 2008/077462 PCT/EP2007/010680 -14
11. The process according to any of the preceding claims, characterized in that about 23 to about 25 Nm 3 /h of additional air, to which between about 2 and about 4 Nm 3 /h of oxygen (95 %) are admixed, are supplied to the fluidized-bed reactor (19) per 1 t/d of alumina produced. 5
12. The process according to claim 1, characterized in that the metal hydrox ide in the fluidized-bed reactor (19) is heated to a temperature of about 850 to about 1050 0 C by combustion of fuel, and metal oxide is generated. 10 13. The process according to any of the preceding claims, characterized in that the reduction of the grain disintegration is greater than the smallest reduction of the gas velocity in one of the units of the plant.
14. A plant for thermal treatment of particulate solids, in particular for perform 15 ing a process according to any of the preceding claims, comprising at least one preheating stage (2), at least one fluidized-bed reactor (19), a means for supplying fuel gas into the fluidized-bed reactor (19), and at least one cooling stage (23, 34, 38, 45, 47), characterized in that the cooling stage consists of at least three (partial) coolers, wherein at least one of these coolers (23, 34, 38, 45, 47) is ar 20 ranged and connected with the means for supplying fuel gas such that for preheat ing the fuel gas before entrance into the fluidized-bed reactor (19), the fuel gas is passed through the at least one cooler (23, 34, 38, 45, 47).
15. The plant according to claim 14, characterized in that a pneumatic con 25 veyor (8) for supplying solids into the fluidized-bed reactor (19) is provided up stream of the fluidized-bed reactor (19), the pneumatic conveyor (8) being con nected with a delivery conduit (34) for hot solids from the fluidized-bed reactor (19) via a conduit (11). WO 2008/077462 PCT/EP2007/010680 - 15
16. The plant according to any of claims 14 and 15, characterized in that two of the (partial) coolers constitute fluidized-bed coolers (23, 38).
17. The plant according to claim 16, characterized in that the fluidized-bed 5 coolers (23, 38) each consist of a plurality of chambers.
18. The plant according to any of claims 14 to 17, characterized in that the (partial) cooler (47) for heating the fuel gas is a cooling cyclone.