FIELD OF THE INVENTION

The present invention generally relates to the field of iron making. More particularly, it relates to the agglomeration of iron ore by pelletization and specifically to a method of operating a pelletizing plant.

BACKGROUND OF THE INVENTION

As it is well known in the art, the blast furnace is a countercurrent gas-solid reactor in which the solid charge materials move downward while the hot reducing gases flow upward. The best possible contact between the solids and the reducing gas is obtained with a permeable burden, which permits not only a high rate of gas flow but also a uniform gas flow, with a minimum of channeling of the gas.

In this context, agglomeration processes have been developed in order to enable the use of fine material, improve the burden permeability and thereby reduce blast furnace coke rates and increase the rate of reduction. They also permit reducing of the amount of fine material blown out of the blast furnace into the gas recovery system. Furthermore, in ironmaking furnaces, agglomerated materials, when they have the proper chemical composition, can substitute for lump ores used as charge ores.

Sintering and pelletizing processes are today the two major agglomeration processes, especially for the blast furnace. Sintered ore is made by partially melting and sintering coarse iron ore 0.5 to 3 mm in size into products having a size of 5 to 50 mm. The sintering process uses the combustion heat of coke breeze (fuel).

Pellets are made from iron ore that is finer than that used for sintered ore. The fine-grained ore can be relatively easily formed into spheroids, called green balls, typically with a diameter ranging from 9 to 18 mm. The green balls are fired into hard product pellets, so-called indurated pellets or simply pellets. These pellets have attracted great interest since they are well adapted for use as raw materials for blast furnaces, but also for direct reduction furnaces.

US 3,864,1 19 discloses a method and apparatus for simultaneously producing heat hardened pellets of two size ranges from finely divided mineral ore. Smaller size green pellets are produced in a balling drum and sent to an induration unit comprising a grate and a rotary kiln. A series of decks collect hardened balls with a predetermined size after the rotary kiln and these are sent to a feedback loop. The feedback loop comprises a balling pan to form larger balls. The larger balls are then placed upon the bed of the smaller size green pellets and fed into the induration unit. In accordance with the conventional pelletizing approach, both the smaller and larger balls are made from iron ore.

US 3,333,951 discloses a process for producing metallized iron ore pellets. The method comprises the steps of blending with the iron ore a solid carbonaceous material and moisture, forming pellets of the moist blend, indurating the pellets, coating the indurated pellets with moistened ore blend, and re-indurating the pellets. Similarly to US 3,864,1 19, pellets from a predetermined size are collected after the indurating process and passed through an agglomeration process with iron ore, before charging them together with green pellets into the indurating apparatus.

US 2015/128766 discloses a pelletizing process having two distinct serial stages for producing green pellets from iron ore. The document particularly discloses a feeding installation for an indurating installation having two pelletizing discs.

In general, the pelletizing process is desirable for agglomeration of finely divided iron ore concentrates because they are normally of such fine size that they will form into a green ball with little difficulty. Concentrates and high grade ores that are not suitable in size for pelletizing are in some cases ground to the required size.

The interest for pellets has increased in modern countries, where pelletizing plants have been built with the goal of producing material for blast furnace under strong environmental regulations. This being said, sinter remains an attractive process since it allows the flexibility to process ores as well as by-products of iron making and steel making industries (oxide and metallic dusts), commonly refer to as "revert materials".

OBJECT OF THE INVENTION

The object of the present invention is to provide an improved concept of pelletizing plant, which can more flexibly handle iron ores and by-products produced either by an integrated plant, or a direct reduction unit of a pellet plant.

This object is achieved by a method as claimed in claim 1 . SUMMARY OF THE INVENTION

With this objective in mind, the present invention proposes a method of operating a pelletizing plant with traditional ore feed and some by-products. The pelletizing plant comprises an indurating furnace in which green pellets are charged and fired to produce hard pellets. The method comprises the steps of preparing iron ore based green pellets; and charging and firing the green pellets in the indurating furnace. The pellets are prepared by conventional pelletizing equipment, e.g. a balling drum and/or a disc pelletizer, so that the pellets are formed by rolling the raw material into a ball.

According to an important aspect of the invention, by-products previously agglomerated are charged and fired with the green pellets in the indurating furnace, whereby hardened pellets and hardened agglomerates of by-products are produced in the indurating furnace.

A remarkable aspect of the present invention is thus the processing of agglomerates of by-products in the indurating furnace, together with green pellets. In other words, agglomerates of by-products and green pellets undergo concurrently/simultaneously the same thermal treatment for their hardening in the indurating furnace. The term 'by-products' is herein used to generally designate the ironmaking, the steelmaking, the peptization and the direct reduction waste materials, and may typically include one or more of: steelmaking slag, mill scale, scrap fines, oxide fines, dust and sludges collected in offgas lines, fines fractions from the screening units, sludges iron- or flux-bearing blast furnace by-products. These by-products, also known as 'reverts', may typically have a size in the range of 0.045 to 5 mm.

The by-products are preferably agglomerated by a compacting process via pressurized rollers, as is known in the art, to form the agglomerates of byproducts. The by-products may be processed in an extrusion or roll device, optionally together with an additive (e.g. lime, binder, etc.), to form lumpy products of predetermined shape and size. A so-called briquetting machine can be used for the preparation of briquette-like agglomerates having a non- spherical shape with dimensions greater than pellets. Preferably, the briquettes/agglomerates of by-products have a minimum size of 20 mm (in all dimensions). For example, the briquettes may be 20 x 30 x 30, or 20 x 20 x 40, all expressed in mm, or even larger, e.g. 50 x 40 x 100. The agglomerates are formed as lumpy products, typically non-spherical. They typically have a low sphericity, forming rather oblong products, including cushion- and parallellepiped-like shapes, rectangular and oval shapes, and combinations thereof. In some embodiments, the lumpy products could have a rounded shape however with low sphericity.

The agglomerates of by-products preferably have larger dimensions than the pellets and are placed in the indurating furnace on top of the pellets to allow for a greater firing duration.

It shall be appreciated that, in the process according to the present invention, the briquetting-type agglomerating process is used to produce agglomerates of by-products that are charged together with the green pellets in the indurating furnace. This is to be put in contrast with conventional plants, where briquettes are not thermally hardened and directly charged into the blast furnace or the electric furnace. A great advantage of the present invention is to provide a pelletizing process and plant that is adapted to completely substitute a sinter plant in a coal-based iron making installation. Another great benefit is for gas-based iron making plants, where the present process provides a circular technology. Indeed, as sinter plants are more and more being challenged with higher amounts of finer iron ore, the present invention offers an attractive alternative process route for customers operating blast furnaces. The invention may be of particular interest for gas-based iron making plants operating with iron ore pellets, where the present process makes it possible to produce pellets and treat all by-products with one technology - whereas current gas-based iron making plants transfer such iron waste into other facilities, outside the plant boundaries. The present invention thus constitutes an attractive solution addressing also environmental aspects and namely waste management strategy in a circular technology.

The pellets are conventionally prepared in a balling section of the plant, using balling equipment that produces spheroidal or ball-like aggregates referred to as green pellets or green balls. Any appropriate equipment may be used, in particular a balling drum or a disc pelletizer. Pellets are prepared from a fined grained mixture that includes mainly iron bearing material, in particular iron ore, and some additive(s), in particular a binder (e.g. bentonite). The material ready for balling has a grain size below 0.15 mm with about 80% thereof below 45 μιτι (i.e. minus 325 mesh).

In the present process, the green pellets typically preferably contain at least 61 % iron ore, and have a diameter in the range of 9 to 18 mm, preferably 9- 14 mm, typically with a mean diameter of about 12 mm.

The indurating furnace can be based on any appropriate technology, e.g. traveling grate furnace, grate-kiln-cooler, or circular induration furnace.

Preferably, the agglomerates of by-products and green pellets are loaded on the charge conveyor of the indurating furnace in two superposed layers, the upper layer mainly comprising the agglomerates of by-products and the lower layer mainly comprising the green pellets. The charging is advantageously made by means of a segregating unit receiving the green pellets and agglomerates of by-products and distributing the latter into the upper and lower layers, based on particle size.

Conventionally, the heat treatment in said indurating furnace may be carried out in an oxidizing atmosphere, whereby the obtained hardened pellets and hardened, lumpy agglomerates of by-products have a metallization below 50%, preferably below 10%, more preferably below 5%.

In some embodiments, the agglomerates of by-products may contain coarsegrained iron ore that cannot typically be processed by pelletizing, however no more than 50 wt.%, preferably no more than 20 wt.%.

These and other features of the present process are also recited in the appended dependent claims.

According to another aspect, the invention concerns a pelletizing plant comprising: a pelletizing section for producing green pellets comprising fine iron ore; 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 characterized in that it further comprises an agglomerating section for producing agglomerates of by-products, and in that said feeding arrangement is further configured to charge said agglomerates of by-products into said indurating furnace, together with said green pellets.

The above and other features of the present pelletizing plant are also recited in the appended dependent claims.

According to a further aspect of the invention, the present method may be implemented with agglomerates consisting mainly (above 50 wt.%) of coarse iron ore instead of by-products, or with lower amounts of by-products (less than wt. 50%, namely less than 30 wt.%). BRIEF DESCRIPTION OF THE DRAWING

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

Figure 1 : is a process flow diagram of a pelletizing plant according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As it is well known, pelletizing plants for the iron making industry involve generally the following processes:

- material pre-processing, in particular grinding;

- balling;

- indurating.

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

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.

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

1 ) wet grinding - dry grinding;

2) open-circuit grinding - closed circuit grinding;

3) single stage grinding - multiple stage grinding.

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. 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.

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.

Figure 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.

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

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.

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.

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.

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 byproducts are discharged on the main collecting conveyor belt 22, with the green pellets.

The agglomerating section 40 comprises a storage bin 42 with the by-products to be agglomerated. The by-products may be stored in bins 41 a 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 41 c. 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.

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.

As can be seen in the Figure, the green pellets and agglomerates of byproducts 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.

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 ₁ is configured to transfer the agglomerates of by-products to the furnace, whereas the lower roll arrangement 54 ₂ is configured to carry the green pellets to the furnace. As it will be understood, the spacing between the top rolls 54 ₁ is defined so that green pellets fall through the rolls onto the lower rolls 54 ₂ . Likewise, the roll spacing of the lower screen 54 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 ₂ and are collected.

The ends of the respective screens 54 ₁ and 54 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.

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.

The undersized green pellets, particles and fines collected at the lower screen 54 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.

Alternatively, the pellets, particles and fines collected at the lower screen 54 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.

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.