METHOD OF OPERATING A PELLETIZING PLANT

Abstract

A method of operating a pelletizing plant for additional treatment of iron making, steelmaking and pellet plant residues, the plant including an indurating furnace in which green pellets are charged and fired to produce hard pellets, where the method includes: preparing iron ore green pellets; and charging and firing said green pellets in an indurating furnace, such that agglomerates of by-products are charged and fired with the green pellets in the indurating furnace, thereby producing hardened pellets and hardened agglomerates of by-products; a pelletizing plant is also presented including a pelletizing section, an indurating furnace, an agglomerating section for producing agglomerates of by-products, where a feeding arrangement is configured to charge agglomerates of by-products into said indurating furnace, together with the green pellets.

Claims

1. A method of operating a pelletizing plant for iron making furnaces, said pelletizing plant comprising an indurating furnace in which green pellets are charged and fired to produce hard pellets, said method comprising: preparing iron ore green pellets using a balling drum and/or a disc pelletizer; charging and firing said green pellets in an indurating furnace; wherein agglomerates of by-products are prepared by compacting to form lumpy products; and wherein said lumpy agglomerates of by-products are charged and fired with said green pellets in said indurating furnace, thereby producing hardened pellets and hardened agglomerates of by-products.

2. The method according to claim 1, wherein said agglomerates of by-products and said green pellets are loaded on a charge conveyor of said indurating furnace in two superposed layers, the upper layer mainly comprising the agglomerates of by-products and the lower layer mainly comprising said green pellets.

3. The method according to claim 2, wherein charging of said charge conveyor is carried out by means of a segregating unit receiving said green pellets and agglomerates of by-products and distributing the latter into said upper and lower layers.

4. The method according to claim 1, wherein the heat treatment in said indurating furnace is carried out in an oxidizing atmosphere, whereby the obtained hardened pellets and hardened, lumpy agglomerates of by-products have a metallization below 50%.

5. The method according to claim 1, wherein said method comprises a step of preparing agglomerates of by-products, which includes processing by-products, in particular coarse-grained, with additives and optionally fines or other iron bearing material, in a compacting apparatus to form said lumpy agglomerates of by-products.

6. The method according to claim 5, wherein said agglomerates of by-products are prepared from a mixture comprising: by-products including iron and/or steelmaking waste materials; and fines collected from said segregating unit.

7. The method according to claim 6, wherein said by-products are reverts from iron making plants selected from the list comprising: steelmaking slag, mill scale, scrap fines, oxide fines, dust and sludges collected in offgas lines, fines fractions from screening units, sludges iron- or flux-bearing blast furnace by-products, and their combinations.

8. The method according to claim 7, wherein said reverts, during the compacting stage, have a particle size in the range of 0.045 to 5 mm.

9. The method according to claim 1, wherein said agglomerates of by-products contain less than 50 wt. % iron ore.

10. The method according to claim 1, wherein said agglomerates of by-products are formed as lumps or briquettes with a size above 16 mm.

11. The method according to claim 1, wherein said step of preparing green pellets comprises preparing a mixture of raw materials including: fine grained iron ore and possible additives, in particular a binder, and processing said mixture to form substantially ball-shaped pellets.

12. The method according to claim 11, wherein said possible additives include fine by-products with 70 wt. % of the particles having a minus 352 mesh.

13. The method according to claim 1, wherein said pellets consist of more than 61 wt. % fine grained iron bearing particles, in particular particles of iron ore.

14. The method according to claim 1, wherein said pellets have a diameter in the range of 9 to 18 mm.

15. A pelletizing plant comprising: a pelletizing section for producing green pellets comprising fine iron ore by processing a fine iron ore containing mixture in a balling drum and/or a disc pelletizer; an indurating furnace for firing said green pellets into hardened pellets; a feeding arrangement configured to charge said green pellets onto a charge conveyor of said indurating furnace; and an agglomerating section for producing lumpy agglomerates of by-products and/or of coarse iron ore, and in that said feeding arrangement is further configured to charge said lumpy agglomerates of by-products into said indurating furnace, together with said green pellets.

16. The pelletizing plant according to claim 15, wherein green pellets formed in said pelletizing section and agglomerates of by-products formed in said agglomerating section are discharged onto a common conveyor, transferring the green pellets and agglomerates of by-products to said feeding arrangement.

17. The pelletizing plant according to claim 16, comprising a first return route for undersized and oversized pellets, collected during transfer thereof on said common conveyor.

18. The pelletizing plant according to claim 15, wherein said feeding arrangement is configured to charge the green pellets and agglomerates of by-products in two layers, with said agglomerates of by-products on top.

19. The pelletizing plant according to claim 15, wherein said feeding arrangement comprises a set of superposed roller screens.

20. The pelletizing plant according to claim 15, comprising a second return route for collecting undersized material and fines from said feeding arrangement and transferring the latter to said agglomerating section.

21. A method of operating a pelletizing plant for iron making furnaces, said pelletizing plant comprising an indurating furnace in which green pellets are charged and fired to produce hard pellets, said method comprising: preparing iron ore green pellets using a balling drum and/or a disc pelletizer; charging and firing said green pellets in an indurating furnace; wherein agglomerates of coarse iron ore are prepared by compacting to form lumpy products; and in that said lumpy agglomerates of by-products are charged and fired with said green pellets in said indurating furnace, thereby producing hardened pellets and hardened agglomerates of by-products.

22. The method according to claim 4, wherein the heat treatment in said indurating furnace is carried out in an oxidizing atmosphere, whereby the obtained hardened pellets and hardened, lumpy agglomerates of by-products have a metallization below 10% or 5%.

23. The method according to claim 10, wherein said agglomerates of by-products contain less than 20 wt. % iron ore.

24. The method according to claim 10, wherein said lumps or briquettes have a size above 20 or 24 mm.

25. The method according to claim 13, wherein and preferably said pellets consist of at least 80 wt. % of particles having a minus 325 mesh.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0035] The present disclosure will now be described, by way of example, with reference to the accompanying drawing, in which:

[0036] FIG. 1: is a process flow diagram of a pelletizing plant according to an embodiment of the disclosure.

DETAILED DESCRIPTION

[0037] As it is well known, pelletizing plants for the iron making industry involve generally the following processes: [0038] material pre-processing, in particular grinding; [0039] balling; [0040] indurating.

[0041] The pelletizing plant according to the present disclosure, an embodiment of which is shown in the figure, also relies on these three processes, which are briefly described below.

[0042] In the material pre-processing stage, the iron ore is ground into fines having qualities required for the subsequent balling process. The pretreatment may typically include concentrating, dewatering, grinding, drying and prewetting. In general, low-grade iron ore is ground into fines to upgrade the quality of the iron ore, remove gangues containing sulfur and phosphorus, and control the size of the grains.

[0043] Conventional grinding methods are roughly categorized as to the following three aspects:

[0044] 1) wet grindingdry grinding;

[0045] 2) open-circuit grindingclosed circuit grinding;

[0046] 3) single stage grindingmultiple stage grinding.

[0047] These methods are used in combination depending on the types and characteristics of the ore and the mixing ratio, taking into account the economic feasibility.

[0048] Next, the goal of the balling process is to produce spheroid-type green pellets (also referred to as green balls) from the pre-wetted material prepared in the previous process. The balling equipment may comprise a balling drum or a disc pelletizer, which are the most widely used for forming green balls.

[0049] In the indurating process, the green pellets are charged into a furnace, in which they are fired, to increase their hardness. The following furnace systems may be used for indurating pellets: a traveling grate (straight grate) system and a grate-kiln-cooler system. The traveling grate system consists of a single unit which moves a static layer of pellets. The system has a simple structure for drying, preheating, firing and cooling pellets. Due to its relative ease of operation, along with ease of scaling-up, makes the system one used by many plants. The grate-kiln-cooler system consists mainly of a grate, a kiln and a cooler, respectively designed for drying/preheating, firing, and cooling the pellets. This system allows controlling individually the residence time in each zone of the furnace.

[0050] FIG. 1 shows a diagram according to one embodiment of the present pelletizing plant 10. The material pre-processing section may use the above-described conventional material for grinding and handling bulk materials, or any other appropriate equipment.

[0051] Reference sign 12 designates an indurating furnace, which comprises conveyor means 14 for transferring the material throughout the furnace 10. The indurating furnace 12 is preferably of the traveling grate type. Conventionally the furnace 12 comprises zones for drying, preheating, firing and cooling (not shown). The four zones are arranged one after another. Each zone is held at a predetermined temperature, and heat exchange occurs via hot air and/or combustion gas to fire the pellets. In the travel grate furnace 12, the charge conveyor comprises an endless grate car consisting of grate bars with side walls, in which pellets are charged (typically over a layer of already fired material). The green pellets on the grate travel through the zones for drying, preheating, firing and cooling.

[0052] At the furnace's outlet, the hardened pellets and hardened agglomerates of by-products are discharged together, and may be screened to separate them by particle size.

[0053] It shall be appreciated that in the present pelletizing plant 10, two types of materials are fed into the indurating furnace: green pellets and briquette-like agglomerates of by-products.

[0054] In FIG. 1, one will recognize the conventional balling section 16. It comprises storage bins 18 containing fine grained material ready for balling into associated balling equipment 20. The storage bins are located downstream of a set of raw materials storage bins 17a and mixer equipment 17b of the pre-processing section. A desired recipe of raw materials is prepared on a conveyor belt 17c and conveyed to the mixer equipment 17b. The mixture is then forwarded to the respective bins 18. The balling equipment 20 may be a balling drum or a disc pelletizer. In the balling equipment the raw material is rolled into balls, called green balls. As is known, under the rolling movement of the drum/disc, the raw materials agglomerate to form a ball. This typically occurs without any molding or external member applying compacting or compression forces. Green balls formed in the balling equipment 20 are discharged on a main collecting conveyor 22, typically a belt conveyor, to carry the on-size green pellets towards the furnace 12. The transfer of the green pellets onto the conveyor 22 is done by a roller screen 24. The rolls of the roller screen 24 are configured such that green pellets with desired size proceed to the main collecting conveyor 22, whereas undersized and oversized pellets as well as fines are separated and collected to be recycled back to the bins 18. The recycling route is indicated 26. Typically, the collected material is recycled back to the bins 18 or can also be conveyed to a mixing system to properly homogenize all raw materials as appropriate for balling.

[0055] The storage bins 18 contain a mixture of ready to mix materials, which depends on the desired composition for the green pellets. Typically the mixture comprises at least one type of fine grained iron ore, typically with a mesh minus 320, and at least one additive, e.g. bentonite or limestone. Annexed Tables 1 and 2 list possible materials for the preparation of green pellets and their properties.

[0056] It shall be appreciated that the present pelletizing plant 10 further comprises an agglomerating section 40, for forming agglomerates of reverts-products. As can be seen, the agglomerates formed in this section are also transferred towards the indurating furnace 12. In the shown embodiment, the agglomerates of by-products are discharged on the main collecting conveyor belt 22, with the green pellets.

[0057] The agglomerating section 40 comprises a storage bin 42 with the by-products to be agglomerated. The by-products may be stored in bins 41a upstream of the storage bin 42. One or more types of by-products are discharged on a conveyor belt and forwarded to the storage bin 42, optionally via a mixing/grinding equipment 41c. These materials are then compacted, typically via a pressurized rollers device 44, to form lumpy products of predetermined size. The formed aggregates are discharged from device 44 in a screen 46, where undersized material is recycled to the storage bin 42, as indicated by return line 47, while the qualified agglomerates are transferred onto the main collecting conveyor 22.

[0058] Table 3, in annex, gives an exemplary list of by-products that can be used for preparing agglomerates in the context of the present process, together with typical grain sizes.

[0059] As can be seen in the Figure, the green pellets and agglomerates of by-products are preferably charged into the furnace 12 in two superposed layers. The lower layer 50 on the traveling grate 14 comprises mainly the green pellets. The top layer 52 is mainly formed by the agglomerates of by-products.

[0060] In the embodiment, the furnace feeding unit 54 comprises a double deck roller screen device comprising superposed set of rolls configured to carry the agglomerates toward the furnace. The top roll arrangement 54.sub.1 is configured to transfer the agglomerates of by-products to the furnace, whereas the lower roll arrangement 54.sub.2 is configured to carry the green pellets to the furnace. As it will be understood, the spacing between the top rolls 54.sub.1 is defined so that green pellets fall through the rolls onto the lower rolls 54.sub.2. Likewise, the roll spacing of the lower screen 54.sub.2 is such that green pellets with a given minimum size are carried to the furnace, while undersized green pellets, particles and fines fall through the lower screen 54.sub.2 and are collected.

[0061] The ends of the respective screens 54.sub.1 and 54.sub.2 are positioned so that the green pellets are deposited first on the furnace conveyor 14, forming the first layer 50, and the agglomerates are deposited by the second screen over the first layer 50, to form the second layer 52.

[0062] The ratio between green pellets and agglomerates of by-products, in particular the thickness of each layer 50, 52, can be varied by adjusting the amounts of pellets and briquettes delivered onto the main collecting conveyor 22.

[0063] The undersized green pellets, particles and fines collected at the lower screen 54.sub.2 are returned to storage bin 42 via a return route 56, to be recycled into briquettes. It should be noticed that when the agglomerating section 40 is in operation, all undersized material from the furnace feeding system 54, is preferably conveyed to the storage bin 42.

[0064] Alternatively, the pellets, particles and fines collected at the lower screen 54.sub.2 may be recycled towards the pelletizing section 16, as indicated by return route 58 (dashed line). This is return route is namely used when the agglomerating section 40 is not in operation.

[0065] In other embodiments, the presently described process and plant can be operated with agglomerates consisting mainly of coarse iron ore, i.e. having a grain size over 0.150 mm, typically in the range of 0.150 to 5.0 mm. Such agglomerates are likewise prepared in agglomerating section. In the formed lumpy agglomerates the coarse-grained iron ore content is above 50 wt. % and can go up to 80 wt. % and more. They may also contain a fraction of reverts, i.e. less than 50 wt. % and e.g. less than 20 wt. %. The same protocols and additives can be used as described above with reference to reverts.

TABLE-US-00001 TABLE 1 Iron Ore Concentrate Bentonite Limestone Fe.sub.wt % wt 68 n.d. n.d. FeO % wt 16 n.d. n.d. CaO % wt 0.5 1.4 50 MgO % wt 1.5 2.4 0.8 SiO.sub.2 % wt 1 60 12.3 Al.sub.2O.sub.3 % wt 1 12 1.3

TABLE-US-00002 TABLE 2 Bulk density Bulk density Specific loose compacted Moisture Grain size surface t/m.sup.3 t/m.sup.3 % wt distribution cm.sup.2/g Iron ore 1.8 2.3 9.5 100% < 150 m 1800-2000 concentrate 80% < 45 m Limestone 1.4 1.6 5 100% < 25 mm coarse Limestone 1.2 1.4 <1 100% < 150 m 2000-3000 ground 80% < 45 m Bentonite 0.7 1.0 5 98% < 147 m 2000-3000 95% < 74 m 80% < 45 m Mixed Material 1.8 2.2 9 100% < 150 m 80% < 45 m

TABLE-US-00003 TABLE 3 +1 mm +0.500 mm +0.250 mm +0.100 mm +0.050 mm +0.025 mm 0.025 mm Total Dust- Steelmaking EC [wt %] 5.00 17.00 33.00 25.00 20.00 >1 0.00 100.00 Dust- BOF B/F [wt%] 1.00 6.00 22.00 25.00 45.00 >1 0.00 99.00 Dust- Steelmaking E/P [wt%] 30.00 30.00 20.00 15.00 4.00 >1 0.00 99.00 Sludge- Steelmaking OG [wt%] 52.00 18.00 8.00 6.00 15.00 >1 0.00 99.00 Sludge - BF [wt%] >1 >1 5.00 25.00 25.00 20.00 25.00 100.00 DRI fines [wt%] 0.00 0.00 0.00 >1 >1 70.00 30.00 100.00 DRI Oxide Fines [wt%] 30.00 5.00 5.00 10.00 20.00 15.00 10.00 95.00 DRI - Sludges [wt%] >1 2.50 2.50 10.00 15.00 40.00 30.00 100.00