METHOD FOR PROCESSING FLY ASH AND PLANT AND METHOD FOR PRODUCTION OF CEMENT

Description

[0001] The invention relates to a process for processing fly ash.

[0002] Fly ash is generated in large quantities in combustion plants, such as waste incineration plants. For example, the estimated fly ash production in India alone in 2010 / 11 was approximately 190 million tons. This amount is expected to more than double in the next ten years. To address the disposal problems of fly ash, great efforts are being made to find ways to use it. Incorporating fly ash into cement is already common practice. Operators of Indian cement plants are even legally required to accept fly ash provided by the market and incorporate it into their production process.

[0003] However, fly ash exhibits highly fluctuating densities, which can lead to problems in processing behavior. The glassy, ​​spherical, fine fly ash particles can appear as solid spheres, hollow spheres (cenospheres), and particle-filled hollow spheres (plerospheres), and vary from fly ash to fly ash. Cement plant operators typically have to process fly ash from different coal-fired power plants and are therefore confronted with highly fluctuating qualities. Furthermore, cement plant operators rarely predict when and which fly ash will be available from the market.

[0004] A method for pulverization is known from JP 2005 246204.

[0005] The invention is therefore based on the object of providing a method for processing fly ash, whereby the fly ash can be more easily integrated into the cement production process.

[0006] According to the invention, this object is achieved by the features of claim 1.

[0007] The process according to the invention for processing fly ash is characterized by grinding the fly ash in a dry-operated agitator ball mill.

[0008] A plant for producing cement for applying the method according to the invention has in particular at least one first grinding stage consisting of a first mill and a first classifier for grinding cement clinker and further provides a dry-operated agitator ball mill for grinding fly ash and at least one discharge device for the joint removal and / or joint storage of the ground cement clinker and the ground fly ash.

[0009] The mechanical activation of fly ash through grinding results in ash with significantly improved lime reactivity, fineness, glass content, and other properties. Portland pozzolana cements produced with this activated ash can therefore meet the specifications of cement standards. The article by SK Chaturvedi, D. Yadav, VP Chatterjee, and A. Pahuja, National Council for Cement and Building Materials, Ballabgarh, India, "Activation of Fly Ash to Improve Performance Properties for Cement Production," Cement International 5 / 2015, Vol. 13, pages 62 to 67, describes the experiments conducted on activating fly ash using a ball mill. However, conventional ball mills require considerable energy for grinding fly ash, which is even higher than that required for grinding cement clinker.Sufficient activation of the fly ash to avoid problems in the process behavior has therefore not been possible in an economically viable manner to date.

[0010] The inventive use of an agitated ball mill to activate fly ash led to a significant reduction in the energy required for activation, so that the activation of fly ash is now economically feasible and the fly ash can be processed to a quality that largely avoids problems in process behavior caused by the incorporation of the fly ash. In contrast to ball mills, agitated ball mills have a stationary housing. In agitated ball mill, too, the fly ash is activated by the action of the balls located inside the mill on the fly ash. While the balls in a conventional ball mill are essentially only accelerated by gravity, the balls in an agitated ball mill receive an additional impulse from the rotating agitator, which results in a higher force input.

[0011] Further energy savings during fly ash activation are achieved by operating the agitated ball mill dry, as this eliminates the need for subsequent drying of the material. Fly ash is already present as a starting material in a relatively fine size (< 500 µm). To increase efficiency, the agitated ball mill can therefore be equipped with correspondingly small grinding media (e.g., in the range of 2 mm to 15 mm, preferably 4 mm to 6 mm). Furthermore, agitated ball mills are significantly more compact than conventional ball mills, as a large portion of the grinding energy is generated by the agitator.

[0012] According to a preferred embodiment, at least 95% of the fly ash is ground in the agitated ball mill to a final fineness of at least 45 µm. Furthermore, it is expedient if the fly ash is ground to the desired final fineness in a single pass through the agitated ball mill. In this way, the equipment required is reduced accordingly by avoiding a classifier and associated conveying equipment. For problem-free incorporation of the fly ash into the cement to be produced, the fly ash has, for example, a reactive silicon dioxide content of at least 25 wt.%. According to the invention, the fly ash in the cement to be produced has a reactive calcium oxide content of less than 15 wt.%.

[0013] According to a preferred mode of operation, the agitator ball mill is operated such that the fly ash is ground to a fineness with a Blaine specific surface area of ​​at least 5,000 cm 2 / g.

[0014] The cement production plant may further comprise a second grinding stage comprising at least one second mill that interacts with a second classifier or with the first classifier. The agitated ball mill has an inlet opening for feeding the fly ash and an outlet opening for discharging the fly ash, wherein the outlet opening is preferably connected to the discharge device. Furthermore, a material divider having an inlet, a first outlet, and a second outlet may be provided between the outlet opening of the agitated ball mill and the discharge device. The outlet opening of the agitated ball mill is connected to the inlet of the material divider, the first outlet is connected to the discharge device, and the second outlet is connected to the second mill of the second grinding stage.In this way, either part or all of the processed fly ash can be ground together with the cement clinker in the second grinding stage and thus thoroughly mixed.

[0015] The invention further relates to a method for grinding cement, wherein cement clinker and at least one further additive (such as gypsum, granulated blast furnace slag, etc.) are ground in at least a first grinding stage and fly ash is ground in a dry-operated agitator ball mill, wherein the ground cement clinker is mixed with the at least one further additive and the ground fly ash.

[0016] In the drawing show Fig. 1 is a schematic representation of a plant according to the invention for producing cement with a dry-operated agitator ball mill for grinding fly ash according to a first embodiment, Fig. 2 is a schematic representation of a plant according to the invention for producing cement with a dry-operated agitator ball mill for grinding fly ash according to a second embodiment, and Fig. 3 is a schematic representation of the dry-operated agitator ball mill for grinding the fly ash.

[0017] The Fig. 1 The schematically illustrated plant for the production of cement essentially consists of a first grinding stage 1, a second grinding stage 2 for grinding cement clinker and a dry-operated agitator ball mill 3 for grinding fly ash.

[0018] The first grinding stage 1 provides a first mill 10, here designed as a material-bed roller mill, a first classifier 11, and a cyclone separator 12. The first classifier 11 is designed as a static-dynamic classifier and provides a static classifier 11a and a dynamic classifier 11b. The material to be classified is first fed via an inlet 11c to the static classifier 11a, which, for example, has a slanted or stepped aeration base through which classifying air 4 flows transversely. The coarse material 5 from the static classifier 11a enters the first mill 10, is crushed there, and returned to the inlet 11c of the static classifier 11a by means of a conveyor device 13, designed, for example, as a bucket elevator.

[0019] The fines from the static classifier 11a, together with the classifying air 4, enter the dynamic classifier 11b, which is typically operated with a classifying rotor. The grit 6 from the dynamic classifier 11b enters a second mill 20, designed, for example, as a ball mill, of the second grinding stage 2. The fines from the dynamic classifier 11b are discharged together with the classifying air 4 and fed to the cyclone separator 12. There, the fines 7 are discharged as fully ground cement clinker via an extraction device 14. The separated classifying air 4 is partially recirculated to the static classifier 11a or, alternatively, is freed from residual dust 8 in a dust filter 15, which is then discharged together with the ground fines 7 via the extraction device 14. The material to be ground in the first grinding stage 1 consists of cement clinker 40 and at least one further additive 41, such as granulated blast furnace slag or gypsum.

[0020] In the illustrated embodiment, the material to be crushed is first fed to the static classifier 11a via the first conveyor 13. However, it would also be conceivable for the material to be fed directly to the first mill 10.

[0021] The grits 6 are further crushed in the second mill 20 and, via a second conveyor 21, are fed into the dynamic classifier 11b via a second inlet 11d of the first classifier 11. Alternatively, the material milled in the second mill could also be recirculated to the first inlet 11c of the first classifier. Furthermore, the second mill 20 could interact with a separate classifier.

[0022] The device for processing fly ash 9 comprises at least one collection container 30 for the fly ash 9 to be crushed and an agitated ball mill 31. The agitated ball mill 31 has an inlet opening 31a connected to the collection container 30. Furthermore, an outlet opening 31b is provided, which is connected to the discharge device 14 for the joint discharge of the ground cement clinker (fines 7) and the ground fly ash 9. A first material divider 32 can also be provided between the collection container 30 and the inlet opening 31a of the agitated ball mill to bypass the agitated ball mill. In this way, it is possible for some or all of the fly ash to be mixed with the ground cement clinker without further comminution. This is particularly useful when the fly ash is already of sufficient quality and fineness.

[0023] Otherwise, the first material splitter 32 can also be set so that part of the fly ash is ground, while another part is mixed with the ground cement clinker 7 without further activation. A material separator in the form of a filter 36 and a second material splitter 33 are arranged between the outlet opening 31b and the discharge device 14. The fly ash separated in the filter 36 is fed to the inlet 33a of the second material splitter 33, while the conveying air separated in the filter 36 is discharged as exhaust air. The second material splitter further has a first outlet 33b, which is connected to the discharge device 14. The second outlet 33c is connected to an inlet 20a of the second mill 20 via an intermediate container 34.

[0024] Furthermore, a third material divider 35 is provided between the intermediate container 34 and the second mill 20, with which the fly ash from the intermediate container 34 is initially fed to the first classifier 11, bypassing the second mill 20. This has the advantage that fly ash that has already been sufficiently finely processed does not need to be unnecessarily re-comminuted in the second mill 20. The material comminuted in the second mill 20 is discharged via the outlet 20b and passes via the second conveyor device 21 into the first classifier 11, or alternatively into a separate classifier. If the second mill 20 is operated with an air stream, the separation between air and material first takes place in a further dust separator 22, with the separated material passing via the second conveyor device 21 into the first classifier 11.

[0025] In the illustrated embodiment, the ground cement clinker (fines 7 from the first separator 11) is already mixed with the fly ash 9 on the discharge device 14. Both components are also conveyed together into a collection container (not shown in detail). Within the scope of the invention, however, it is also conceivable for the ground cement clinker 7 and the fly ash 9 to be transported to a common storage facility via separate conveyor systems.

[0026] Fig. 2 shows a second embodiment of a cement production plant with a dry-operated agitator ball mill for grinding fly ash. The difference from the first embodiment is that the filter 36 of the Fig. 1 in Fig.2 has been replaced by a cyclone 37. The fly ash separated in the cyclone 37 is again fed into the material divider 33, while the separated air is returned as circulating air to the agitator ball mill 31 and there at least partly as conveying air 42 ( Fig. 3 ) is used.

[0027] Fig. 3 shows an enlarged view of the agitator ball mill 31 according to the Figuren 1 and 2 In contrast to a conventional ball mill, the agitator ball mill 31 has a stationary housing 31c and an agitator 31e inside the housing driven by a motor 31d. Furthermore, the agitator ball mill is operated with grinding media 31f, in particular grinding balls.

[0028] The fly ash 9 to be shredded is metered into the feed opening 31a via a rotary valve 31g. The activation or processing of the fly ash occurs essentially through contact with the grinding media 31f, which are lifted and accelerated by the agitator 31e. A discharge rotor 31h, which rotates with the agitator and separates the processed fly ash from the grinding media 31f, is provided in the area of ​​the outlet opening 31b. Conveying air 42 for discharging the processed fly ash is supplied via a conveying air opening 31i provided in the area of ​​the outlet opening 31b. This conveying air 42 flows into the discharge rotor 31h and discharges the processed fly ash via the outlet opening 31b. To support the conveying effect, at least one further conveying air opening 31j can also be provided in the area of ​​the outlet opening 31b.

Claims

1. A process for processing fly ash (9) by grinding fly ash (9), wherein the fly ash (9) is ground in a dry-operated agitator bead mill (31), characterised in that the fly ash (9) has a content of reactive calcium oxide of less than 15% by weight.

2. Process according to claim 1, characterised in that at least 95% of the fly ash (9) is ground to a final fineness of at least 45 µm.

3. Method according to claim 1, characterised in that the fly ash (9) is ground to a desired final fineness in one pass through the agitator bead mill (31).

4. Process according to claim 1, characterised in that the fly ash (9) is ground to a fineness with a specific surface area according to Blaine of at least 5,000 cm2 / g.