WO2004097055A2

WO2004097055A2 - Fluidized bed for treating iron oxide

Info

Publication number: WO2004097055A2
Application number: PCT/EP2004/004462
Authority: WO
Inventors: Franz Reisser; Klaus Steuer; Martin Hirsch
Original assignee: Outokumpu Oyj; Outokumpu Technology Oyj
Current assignee: Outokumpu Oyj; Metso Corp
Priority date: 2003-05-02
Filing date: 2004-04-28
Publication date: 2004-11-11
Anticipated expiration: 2005-11-02
Also published as: WO2004097055A3; DE10319625B3

Abstract

This invention relates to a method and a plant for the heat treatment of solids, in particular for reducing solids containing iron oxide, in a fluidized-bed reactor (5, 30). For fluidizing the solids, fluidizing gas is introduced into the fluidized-bed reactor (5, 30) via distributor grates (7, 32). Between a solids feed pipe (37) and a solids discharge pipe (38) of the fluidized-bed reactor (5, 30), the solids pass through multiple chambers (8, 34) each with a distributor grate (7, 32), which are at least partly separated from each other by weirs (6, 33). The pressure of the fluidizing gas introduced into the individual chambers through the distributor grates (7, 32), which are offset with respect to each other in vertical direction, substantially is the same. The delivery of the solids from the solids feed inlet 15 (37) to the solids discharge outlet (38) is promoted by the inclination of the fluidized-bed reactor (5, 30) with respect to the horizontal.

Description

IVIethod and Plant for the Heat Treatment of Solids Containing Iron Oxide

The present invention relates to a method for the heat treatment of solids, in particular for reducing solids containing iron oxide, in a fluidized-bed reactor, into which fluidizing gas is introduced through a distributor grate or the like for fluidizing the solids, where between an end of the fluidized-bed reactor provided with a solids feed pipe and an end of the fluidized-bed reactor provided with a solids discharge pipe the solids pass through multiple chambers each with a distributor grate, which are at least partly separated from each other by weirs or the like, and to a corresponding plant.

Such method and a plant for reducing solids containing iron oxide, such as iron ores, iron ore concentrates or the like, are known from DE 44 10 093 C1. For this purpose, ore containing iron oxide is charged into a first fluidized-bed reactor and fluidized with heated reduction gas. From this first fluidized-bed reactor with circulating fluidized bed, the solids are supplied to a second reduction stage in a so-called classical fluidized-bed reactor. To keep the mean retention time in this second fluidized-bed reactor almost the same for the solids, several sections or chambers are formed in the second fluidized-bed reactor by means of weirs. However, this division of the fluidized-bed reactor into severalsections or chambers requires a slope between the input end and the discharge end of the reactor, which is usually achieved by reducing the weir heights towards the dis- charge end. The disadvantage is, however, that the bed height in the first chamber is about 1.5 times the bed height in the last chamber, so that a different pressure is required in each chamber for fluidizing the solids. Since the amount of gas supplied to the reactor for fluidization is passed through a single compressor, the compression pressure must be designed to comply with the largest, i.e. the first chamber. As a result, the energy required for the compression of the fluidizing gas is comparatively high, and additional investment costs are incurred for reducing the supply of fluidizing gas to the remaining chambers.

Therefore, it is the object of the present invention to provide a method and a plant for the heat treatment in particular of solids containing iron oxide, which method and plant provide for a rather uniform retention time of the solids in the reactor without increasing the expenditure of energy and the control effort.

In accordance with the invention, this object is solved by a method as mentioned above, in which the pressure of the fluidizing gas introduced into the individual chambers through the distributor grates vertically offset with respect to each other substantially is the same, the transport of the solids from the solids feed pipe to the solids discharge pipe also being effected by an inclination of the fluidized-bed reactor with respect to the horizontal.

By means of the method in accordance with the invention, particularly uniform retention times of the solids in the reactor can be achieved by dividing the fluidized-bed reactor into multiple chambers separated from each other by weirs. Since the reactor itself is inclined with respect to the horizontal, the solids fluid- ized in the fluidized-bed reactor are transported continuously from the feed inlet of the reactor to its discharge outlet. It is not necessary to vary the height of the chambers by graduating the height of the upper edges of the weirs above the distributor grates, so that for all chambers the same pressure of the fluidizing gas can be used.

Consequently, expensive control devices for adjusting different pressures in the individual chambers can be omitted. By offsetting the distributor grates of the individual chambers with respect to each other in vertical direction, there can nevertheless be produced a slope between the chambers of substantially the same height for transporting the solids from the input end towards the discharge end of the reactor.

The generation of the amount of heat necessary for the operation of the reactor can be effected in every way known to the skilled person for this purpose. Usually, the heat treatment is effected at a temperature of about 450 to 950°C. In accordance with a particular embodiment of the present invention it is provided to supply preheated reduction gas to the fluidized-bed reactor for fluidization, which gas reduces the possibly likewise preheated solids. The reactor tempera- ture, for instance, is below the temperature of the streams of material entering the reactor. As reduction gas, there can in particular be used gas with a hydrogen content of at least 80%, preferably above 90%.

The consumption of fresh reduction gas can be decreased considerably by cleaning the reduction gas in a regeneration stage downstream of the reactor and subsequently recirculating the same to the reactor. During regeneration, the gas is first of all separated from solids, possibly passed through a scrubber and cooled down below the dew point of steam, so that the steam content can be reduced, then compressed, and enriched with fresh hydrogen.

With the method in accordance with the invention, all kinds of ores containing iron oxide, in particular iron ores or iron ore concentrates, can be heat-treated effectively.

To ensure a particularly effective heat transfer and a sufficient retention time of the solids in the fluidized-bed reactor, the pressure and hence the gas velocity of the fluidizing gas supplied to the fluidized-bed reactor via the distributor grates preferably is adjusted such that the dimensionless Particie-Froude- Number (Fr_P) in the fluidized-bed reactor is about 0.02 to 2, preferably 0.05 to 0.5, in particular about 0.15. The Particle-Froude-Numbers are each defined by the following equation:

wherein

u = effective velocity of the gas flow in m/s p_s = density of a solid particle in kg/m³ p_f = effective density of the fluidizing gas in kg/m³ dp = mean diameter in m of the particles of the reactor inventory (or the particles formed) during operation of the reactor g = gravitational constant in m/s².

When using this equation it should be considered that d_p does not designate the mean diameter (d₅o) of the material used, but the mean diameter of the reactor inventory formed during operation of the reactor, which can differ significantly from the mean diameter of the material used (primary particles). Even from very fine-grained material with a mean diameter of e.g. 3 to 10 μm, particles (secondary particles) with a mean diameter of 20 to 30 μm can for instance be formed during the heat treatment. On the other hand, some materials, for instance ores, are decrepitated during the heat treatment.

A plant in accordance with the invention, which is in particular suited for performing the method described above, has a fluidized-bed reactor with a solids feed pipe and a solids discharge pipe, which is disposed at an angle of about 0.5 to 5°, preferably 1 to 2°, in particular about 1.3° with respect to the horizontal, between which multiple chambers are disposed one beside the other in horizontal direction, each with a distributor grate or the like, which are at least partly separated from each other by weirs or the like, and through which fluidizing gas is introduced for fluidizing the solids.

In accordance with a preferred embodiment of the present invention, the weirs and the distributor grates of the individual chambers are arranged downwardly offset with respect to each other in vertical direction from the solids feed pipe towards the solids discharge pipe. Independent of the inclination of the reactor itself, there is thus obtained a slope, by means of which the solids are transported from the input end to the discharge end of the fluidized-bed reactor. The distributor grates or the like preferably are offset with respect to each other in the manner of steps with identical step heights.

In accordance with a development of the idea of this invention it is provided to design the height of the upper edges of the weirs above the distributor grates in each chamber substantially identical. For fluidizing the solids in the individual chambers it is therefore possible to keep the pressure of the fluidizing gas in all chambers substantially the same.

A slope which is suitable for transporting the solids in the fluidized-bed reactor from the solids feed pipe to the solids discharge pipe is obtained for instance, when the vertical distance of the distributor grate located closest to the solids feed pipe from the distributor grate located closest to the solids discharge pipe approximately corresponds to half the height of the upper edges of the weirs above the distributor grates.

In accordance with a preferred embodiment of the invention, wind boxes defined by the weirs and the distributor grates are formed vertically below the chambers, into each of which opens at least one gas supply duct connected with a common compressor. Due to the inclination of the fluidized-bed reactor, the height of the wind boxes substantially remains the same, although the distributor grates are offset with respect to each other in the manner of steps.

The invention will subsequently be described in detail with reference to preferred 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 drawings:

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

Fig. 2 shows a fluidized-bed reactor in accordance with a second embodi- ment of the present invention;

Fig. 3 shows a section through the fluidized-bed reactor as shown in Fig. 2 along line Ill-Ill, and

Fig. 4 shows a magnification of the detail IV of Fig. 2.

In the method shown in Fig. 1 , which is suited in particular for the heat treatment of solids containing iron oxide, solids are charged into a first reactor 1 via a supply conveyor 2. The, for instance, cylindrical reactor 1 has a supply duct 3 for fluidizing gas at its lower end. The solids pretreated in the reactor 1 are supplied to a second fluidized-bed reactor 5 via a solids supply pipe 4.

The fluidized-bed reactor 5, which for instance constitutes a lying cylindrical tube, is slightly inclined with respect to the horizontal, so that the end of the flu- idized-bed reactor 5, into which opens the supply conduit 4 for solids, is slightly elevated with respect to the opposite end. In the fluidized-bed reactor 5, several weirs 6.1 to 6.3 are disposed, which together with distributor grates 7 form mixing chambers 8.1 to 8.4 and wind boxes 9 located below the distributor grates 7. Both the height of the upper edges of the weirs 6 and the height of the distribu- tor grates 7 is gradually decreasing in vertical direction from the end of the supply pipe 4 to the opposite end of the fluidized-bed reactor 5. Supply ducts for fluidizing gas each open into the wind boxes 9 located below the chambers 8.

For reducing for instance solids containing iron oxide, iron ores are first of all supplied via a conveyor 11 to a Venturi drier 12, in which the solids are dried. In a cyclone 13 downstream of the Venturi drier 12, the dried solids are separated from the exhaust gas, which is cleaned in a scrubber 14. For preheating, the solids separated in the cyclone 13 are then supplied to a combustion chamber 15, into which air and a fuel are introduced via ducts 16 and 17, respectively. Upon heating, the solids are separated from the exhaust gases in the cyclone 18, which exhaust gases are introduced into the Venturi drier 12 for preheating. The preheated and dried solids then are charged into the reactor 1 via conveyor 2.

Due to the supply of fluidizing gas through the supply duct 3, a circulating fluidized bed is formed in the reactor 1 , through which the fluidized solids together with the fluidizing gas are discharged from the reactor 1 and supplied to a cyclone 19. In the same, the solids are separated from the exhaust gas, which via duct 20 is supplied to a heat exchanger 21 and a regeneration stage 22. The solids separated from the exhaust gas are recirculated from the cyclone 19 to the reactor 1 via pipe 23. Furthermore, exhaust gas from the second reactor 5 is supplied to the reactor 1 via duct 24.

In the second fluidized-bed reactor 5, the solids withdrawn from the first reactor 1 via feed pipe4 are first of all supplied to the first chamber 8.1 in flow direction, the left one in the Figure, in which the solids are fluidized by the fluidizing gas streaming through the distributor grate 7. Due to the inclination of the fluidized- bed reactor 5 and as a result of the fluidization, part of the solids is transported over the first weir 6.1 into the second chamber 8.2, in which the solids are like- wise fluidized. In this way, there is obtained a solids flow from the first chamber 8.1 to the opposed chamber 8.4, the right one in the figure, from which the solids are discharged from the fluidized-bed reactor 5 via pipe 25.

After a further heating in a heater (Venturi preheater) 26, the solids discharged from the fluidized-bed reactor 5 are separated from exhaust gases in a cyclone 27 and supplied for instance to a hot briquetting plant 28 for further processing. The gases supplied to the reactor 5 can be heated in a gas heater 29.

Fig. 2 shows a second embodiment of a fluidized-bed reactor 30, which has a lying, substantially cylindrical shell 31. This shell of the fluidized-bed reactor 30 is inclined with respect to the horizontal by about 1 to 2°, so that its left end in the Figure is elevated with respect to the right end in the Figure. In the shell 31 of the reactor 30, multiple distributor grates 32, ten in the drawing, as well as multiple weirs 33.1 to 33.10 are provided. The distributor grates lie in a horizon- tal plane or parallel to the longitudinal axis of the shell 31 of the reactor 30. As can in particular be taken from the enlarged representation of Fig. 4, the adjacent distributor grates 32a, 32b are each offset with respect to each other in vertical direction, so that the height of the distributor grates 32 in the reactor 30 is gradually decreasing from the left side of the reactor 30 towards the right side of the reactor 30 as shown in the Figure. The height of the upper edges of the weirs 33 likewise is gradually decreasing from the left to the right in the Figure.

Above the distributor grates 32, the weirs 33 form chambers 34.1 to 34.11 open at their upper ends, which substantially have the same size. The chambers 34 communicate with each other through the open space located above the weirs 33. Below the chambers 34, wind boxes 35 are formed, which are defined by the distributor grates 32 as well as the weirs 33 and are each connected with a supply duct 36 for fluidizing gas.

On the left side of the reactor 30 as shown in the Figure, a solids feed pipe 37 is provided, whereas on the opposite side of the reactor 30 a solids discharge pipe 38 is positioned directly above a distributor grate 32.

For reducing solids containing iron oxide in the fluidized-bed reactor 30, the possibly pretreated solids are first of all charged into the reactor 30 via the solids feed pipe 37. By means of the fluidizing gas streaming through the supply duct 36, the wind box 35, and the distributor grate 32 into the chamber 34.1 , the solids in the chamber 34.1 located closest to the solids feed conduit 37 are fluidized. Due to the inclination of the fluidized-bed reactor 30, part of the solids in the chamber 34.1 continuously passes the weir 33.1 and is charged from the chamber 34.1 into the chamber 34.2. As a result, the solids in the chamber 34.1 have a retention time which is adjustable by the pressure loss in the chamber. In the chamber 34.2, too, the solids are fluidized by the fluidizing gas and thus supplied to the solids discharge pipe 38 step by step. The retention times of the solids in the fluidized-bed reactor 30 can be kept constant by the weirs 33, which prevent the solids from being transported through the reactor too rapidly.

The supply ducts 36 for fluidizing gas, which open into the wind boxes 35, are fed via a common register, so that the pressure of the fluidizing gas substantially is the same in all supply ducts 36. Since the size of the chambers 34 approximately is the same due to the step-like graduation of the distributor grates 32 as well as the height of the weirs 33 decreasing from the solids feed conduit 37 towards the solids discharge pipe 38, a good fluidization of the solids is achieved in each chamber 34. For reducing solids containing iron oxide in the fluidized-bed reactor 30, a gas containing hydrogen is for instance used as fluidizing gas, which is heated to a temperature of about 720°C. The pressure of the fluidizing gas preferably is chosen such that the gas velocity of the fluidizing gas in the chambers 34 of the fluidized-bed reactor 30 is adjusted such that the Particle-Froude-Number in the fluidized-bed reactor 30 is about 0.15.

List of Reference numerals:

1 (first) fluidized-bed reactor

2 supply conveyor

3 supply duct

4 solids feed pipe

5 (second) fluidized-bed reactor

6 weir

7 distributor grate

8 chamber

9 wind box

10 supply duct

11 screw conveyor

12 venturi drier

13 cyclone

14 gas scrubber

15 combustion chamber

16 supply duct

17 supply duct

18 cyclone

19 cyclone

20 duct

21 heat exchanger

22 gas cleaning stage

23 solids pipe

24 gas duct

25 solids discharge pipe

26 venturi preheater

27 cyclone

28 briquetting plant gas heater fluidized-bed reactor shell distributor grate weir chamber wind box supply duct solids feed inlet solids discharge outlet

Claims

Claims:

1. A method for the heat treatment of solids in a fluidized-bed reactor (5, 30), into which fluidizing gas is introduced through a distributor grate (7, 32) or the like for fluidizing the solids, where between an end of the fluidized-bed reactor provided with a solids feed pipe (4, 37) and an end of the fluidized-bed reactor provided with a solids discharge pipe (25, 38) the solids pass through several chambers (8, 34) each with a distributor grate (7, 32), which chambers are at least partly separated from each other by weirs (6, 33) or the like, character- ized in that the pressure of the fluidizing gas introduced into the individual chambers (8, 34) through the distributor grates (7, 32) offset with respect to each other in particular in vertical direction substantially is the same, and that the solids are delivered from the solids feed pipe to the solids discharge pipe also due to an inclination of the fluidized-bed reactor (5, 30) with respect to the horizontal.

2. The method as claimed in claim 1 , characterized in that reducing gas, in particular hot gas containing hydrogen, is supplied to the fluidized-bed reactor (5, 30) as fluidizing gas.

3. The method as claimed in claim 1 or 2, characterized in that in particular fine-grained solids containing iron oxides are supplied to the fluidized-bed reactor (5, 30).

4. The method as claimed in any of claims 1 to 3, characterized in that the gas velocity of the fluidizing gas supplied to the fluidized-bed reactor (5, 30) via the distributor grates (7, 32) is chosen such that the Particle-Froude-Number in the fluidized-bed reactor (5, 30) is 0.02 to 2, preferably 0.05 to 5, in particular about 0.15.

5. A plant for the heat treatment of solids, in particular for performing a method as claimed in any of the preceding claims, comprising a fluidized-bed reactor (5, 30), in which multiple chambers (8, 34) each with a distributor grate (7, 32) or the like, which are at least partly separated from each other by weirs (6, 33), are arranged one beside the other in horizontal direction, through which chambers fluidizing gas is introduced for fluidizing the solids, wherein two chambers disposed opposite each other in the fluidized-bed reactor (5, 30) are connected with a solids feed pipe (4, 37) and a solids discharge pipe (25, 38), respectively, characterized in that the fluidized-bed reactor (5, 30) is arranged with an inclination of about 0.5 to 5°, preferably 1 to 2°, with respect to the horizontal.

6. The plant as claimed in claim 5, characterized in that the upper edges of the weirs (6, 33) and the distributor grates (7, 32) of the individual chambers (8, 34) are downwardly offset with respect to each other in vertical direction from the solids feed pipe (4, 37) towards the solids discharge pipe (25, 38).

7. The plant as claimed in claim 6, characterized in that the distributor grates (7, 32) are offset with respect to each other in the manner of steps with the same step height.

8. The plant as claimed in any of claims 5 to 7, characterized in that the height of the upper edges of the weirs (6, 33) above the distributor grates (7, 32) substantially is the same in each chamber.

9. The plant as claimed in any of claims 5 to 8, characterized in that the vertical distance of the distributor grate (7, 32) located closest to the solids feed pipe (4, 37) from the distributor grate (7, 32) located closest to the solids discharge pipe (25, 38) approximately corresponds to half the height of the upper edges of the weirs (6, 33) above the distributor grates (7, 32).

10. The plant as claimed in any of claims 5 to 9, characterized in that vertically below the chambers (8, 34) wind boxes (9, 35) defined by the weirs (6, 33) and the distributor grates (7, 32) are formed, into each of which open gas sup- ply ducts (10, 36) connected with a common compressor.