TECHNICAL FIELD

The invention relates to sintering equipment and to a plant for exploiting dusts and waste from iron production, as well as to the use thereof.

BACKGROUND

In connection with iron production, enormous amounts of different grades of dusts and waste materi ^¬ al are produced both in metallurgical plants and at pellet loading and unloading stations. The inventors have thus recognized the need for an improved appa- ratus for utilizing dusts and waste material to en ^¬ hance the process economy of such plants.

SUMMARY

The purpose of the present disclosure is to eliminate or at least reduce problems related to prior art. Specifically, it is the purpose of the current disclosure to provide a new type of sintering equip ^¬ ment and a plant for exploiting dusts and waste mate ^¬ rial from iron production.

The sintering equipment according to the pre ^¬ sent disclosure is characterized by what is presented in claim 1.

The plant according to the present disclosure is characterized by what is presented in claim 12.

The use according to the current disclosure is characterized by what is presented in claim 19.

In one aspect, sintering equipment for con ^¬ tinuous sintering of pelletized mineral material is disclosed. The sintering equipment comprises a sinter- ing furnace, comprising sequential process zones, the temperature conditions in each process zone being in ^¬ dependently controllable; the process zones comprising at least one drying zone, at least one heating zone, at least one sintering zone and at least one cooling zone; a gas-permeable conveyor belt for conveying the mineral material through the process zones; a first circulation gas duct for conducting gas from at least one cooling zone to at least one drying zone; a second circulation gas duct for conducting gas from at least one cooling zone to at least one heating zone; and a third circulation gas duct for conducting gas from at least one cooling zone to at least one sintering zone. The sintering equipment is characterized in that the first circulation gas duct comprises at least one burner for controlling the gas temperature in the at least one drying zone.

In an embodiment, the first circulation gas duct comprises one burner. In an embodiment, the first circulation gas duct comprises two burners. In one em ^¬ bodiment of the sintering equipment, each circulation gas duct comprising at least one burner comprises two burners. It is possible that a circulation gas duct comprises more than two burners. For example, the num- ber of the burners may be three. In such embodiments, the third burner may be positioned in the middle of the length of the circulation gas duct. Alternatively, the number of the burners may be four. In such embodi ^¬ ments, the third and fourth burner could be positioned so that the distance between the burners is the same or approximately the same. Further, embodiments may be envisaged in the number of the burners is five. In such embodiments, burners may be placed in the circu ^¬ lation gas duct at regular intervals.

In one embodiment of the sintering equipment, each circulation gas duct comprises at least one burn ^¬ er for controlling the gas temperature in the process zone to which the gas circulation gas duct conducts gas .

The presence of burners in the first circula ^¬ tion gas duct allows the temperature of the at least one drying zone to be regulated, so that even chal ^¬ lenging pellet material may be successfully dried.

Additional regulation may be provided by di ^¬ viding the at least one drying zone into two or more compartments. The lower exhaust gas ducts leading away from the at least one drying zone may be equipped with a blowers. Typically there is one blower in each lower exhaust gas duct. However, not all lower exhaust gas ducts are necessarily equipped with a blower. If the at least one drying zone is divided into two or more compartments, each compartment may have its own lower exhaust gas duct. Each of the lower exhaust gas ducts may have its own blower.

In one embodiment of the sintering equipment, blowers are arranged below the drying zone for regu- lating gas flow and/or gas distribution in the drying zone. The blowers in the current sintering equipment are typically independently controllable. This allows the regulation of gas flow in each zone, or compartment, connected to a blower. Without limiting the cur- rent disclosure to any specific theory, the division of gas flow in different compartments might affect the thermal exchange reactions of the pellet material and gas. This, again, allows the regulation of the temper ^¬ ature gradient within a process zone, and therefore, allows the optimization of the process within a pro ^¬ cess zone.

The at least one burner may be positioned in various directions relative to the flow of the gas in the circulation gas duct. In some embodiments, the di- rection of the flame might be perpendicular to that of the gas flow. Alternatively, the flame may be directed along the direction of gas flow. In many embodiments, the angle of the flame relative to the direction of the gas flow is less than 90 °C, i.e. approaching the direction of gas flow. It may, for example be 20° or 30° or 45°. The orientation of each burner relative to the gas flow may be individually selected. Thus varia ^¬ ble angles may be present in a given circulation gas duct .

In one embodiment of the sintering equipment, the sintering furnace comprises three cooling zones (V, VI, VII) . The cooling zones may be termed fist cooling zone (V), second cooling zone (VI) and third cooling zone (VII) in the order that the sintered pel ^¬ lets are conveyed through the zones. Thus, a first circulation gas duct conducts gas from the third cool- ing zone (VII) to at least one drying zone (I) . A sec ^¬ ond circulation gas duct conducts gas from the second cooling zone (VI) to at least one heating zone (II) . A third circulation gas duct conducts gas from the first cooling zone to at least one sintering zone (III) .

In one embodiment of the sintering equipment, the temperature of the gas conducted into the drying zone is regulated to be 200-250 °C, preferably 220-230 °C by the at least one burner in the third circulation gas duct.

In one embodiment of the sintering equipment, the drying zone comprises two compartments.

In one embodiment of the sintering equipment, temperature of the heating gas conducted into the heating zone is regulated to be 1, 100-1, 230 °C, pref ^¬ erably 1, 150-1, 180 °C by the at least one burner in the second circulation gas duct.

In one embodiment of the sintering equipment, temperature of the gas conducted into the sintering zone is regulated to be 1, 150-1, 300 °C, preferably 1,200-1,250 °C by the at least one burner in the first circulation gas duct. In one embodiment of the sintering equipment, product bed temperature at the end of the sintering zone is 1,220-1,410 °C, preferably 1,270-1,390 °C. The product bed temperature may be different from the gas temperature. In addition to the gas temperature, the composition of the pellets affects the product bed temperature, as heating of the pellets may trigger various reactions in the pellet material. These reac ^¬ tions may be exothermic or they may be endothermic, or there may be both. Therefore, the temperature of the product bed is an independent parameter that can be followed when optimizing the sintering equipment ac ^¬ cording to the current disclosure.

In one embodiment of the sintering equipment, exhaust gas from the sintering equipment is cleaned to remove detrimental substances.

In one aspect, a plant for exploiting dusts and waste materials from iron production is disclosed. The plant comprises a pelletizing feed production sta ^¬ tion for mixing dusts, waste materials and binding agent for producing pelletizing feed; a pelletizing apparatus for pelletizing the pelletizing feed to produce green pellets; the sintering equipment according to the present disclosure for producing sintered pel ^¬ lets; and optionally a smelting furnace, such as a blast furnace, or an electric furnace, for smelting the sintered pellets. The smelting furnace may be con ^¬ structed to be a part of the plant according to the current disclosure. Alternatively, the plant may be used for producing sintered pellets, and they can be smelted at another location.

In one embodiment, the plant further compris ^¬ es a device for adding pellet fines from a direct re- duction process into the pelletizing feed. In one embodiment, the plant further compris ^¬ es a device for adding mill scale from a rolling mill into the pelletizing feed.

In one embodiment, the plant further compris- es a device for adding finely divided sludgy dusts from a direct reduction process into the pelletizing feed .

In one embodiment, the plant further compris ^¬ es a device for adding finely divided dry dusts sepa- rated by a bag filter from the exhaust gas of an elec ^¬ tric arc furnace into the pelletizing feed.

In one embodiment, the plant further compris ^¬ es a device for adding underflow of pellets feed to sintering into the pelletizing feed.

In one embodiment, the plant further compris ^¬ es a device for milling coarse raw materials to a de ^¬ sired fineness before adding to the pelletizing feed.

In one aspect, use of the sintering equipment according to the current disclosure for the production of sintered pellets is disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illus ^¬ trate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Fig. 1 is a schematic presentation of a plant according to the present disclosure.

Fig. 2 is a schematic presentation of sintering furnace according to the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION Large amounts of different grades of iron- bearing dusts and wastes are available globally. Therefore, it would be economically sensible to con ^¬ vert these dusts and waste materials to a suitable form in which they can be charged into a blast furnace or an electric furnace, depending on the contamination level of the dusts.

The disclosure relates to producing a pelletizing feed from a mixture of dusts, waste mate- rials and binding agent and pelletizing the feed to produce green pellets. Green pellets are sintered in a steel belt sintering furnace which comprises one or more zones for drying, heating, sintering and cooling of the pellets. The sintered pellets are charged into a blast furnace or an electric furnace.

According to the present disclosure, iron- bearing sintered pellets can be produced using a steel belt sintering furnace. The raw material can be milled, if necessary, and fed in a certain grain size to a pelletizing unit before sintering. The need for milling depends on the composition of the starting materials, as the efficiency of the pelletizing process depends on the size distribution of the particles to be pelleted. Another aspect that affects the function- ing of the pelletizing, is the proportion of easily and poorly pelletizable material. High grade green pellets and sintered pellets can be produced. Sintered pellets are hard and abrasion-resistant and their re- ducibility is high due to their highly porous struc- ture . Exhaust gas from the sintering equipment can be cleaned by generally used pro-sustainable methods de ^¬ pending on the content of harmful components in the gas .

Dusts and waste material from iron production can be used as raw material in the sintering equipment and plant according to the present disclosure. Such dusts and waste material comprise, for instance, pel- let fines from direct reduction process, mill scale from a rolling mill, sludgy dusts from a direct reduc ^¬ tion process, dry dusts from a bag filter and pellet underflow .

By pellet fines is herein meant pellets and pellet fragments under 6 mm diameter. By pellet underflow, on the other hand, is herein meant sintered pel ^¬ lets that are too small to be used in a furnace. Thus, pellet underflow has already been sintered. In this respect, it differs from other materials described in the current disclosure.

By mill scale is herein meant scale from a rolling mill. It is typically a waste or side product of steel making. Mill scale may contain magnetite, wustite and metallic iron, can be used in the present sintering equipment and plant.

By sludgy dusts is herein meant finely divid ^¬ ed dusts, also referred to as sludge, from the clean ^¬ ing systems of a direct reduction process. Sludgy dusts originally contain moisture, although they may be dried before actual utilization in the current sys ^¬ tems. Sludgy dusts may contain detrimental substances.

By dry dusts is herein meant dusts separated by a bag filter from exhaust gas of an electric arc furnace. This material is also referred to as baghouse dust. Also dry dusts may contain detrimental substanc ^¬ es .

The above-mentioned detrimental substances can include, for instance, compounds of alkali metals, zinc, chlorine and fluorine.

In figures 1 and 2, the components of the plant 5 and the sintering furnace 1, respectively, are drawn schematically. All design details are omitted for clarity.

FIGURE 1 In the plant 5 according to the present dis ^¬ closure, dusts and waste material are mixed in a pelletizing feed production station 6 to produce a pelletizing feed 14 for use in manufacturing of sin- tered pellets 16, which can be charged into a smelting furnace 8, such as a blast furnace or an electric fur ^¬ nace .

The concept was tested by grinding coarse raw materials in a mill to reach a pelletizing fineness, except for mill scale 19, which is partly metallic ma ^¬ terial. The mixtures to be examined were prepared so that the amount of normal pellet raw material was 50- 70 w-% and the amount of mill scale 19 varied between 10 and 20 w-% of the mixture. The amount of dried sludgy dust 20 was varied between 20 and 30 w-% of the mixture and the amount of dry dusts 21 was varied be ^¬ tween 1 and 10 w-% of the mixture. Bentonite, organic binder, or a mixture thereof was used as a binding agent in pelletizing. The amount of binding agent was kept low, between 0.1 and 1.0 W ~6 ·

In the embodiment of fig. 1, the pelletizing feed production station comprises a device 9 for add ^¬ ing underflow of pellet feed 17 into the pelletizing feed 14. The embodiment also comprises a device 10 for adding pellet fines 18 into the pelletizing feed 14. An additional feature of the embodiment of fig. 1 is a device 11 for adding mill scale 19 from a rolling mill into the pelletizing feed 14. Also a device 12 for adding sludgy dusts 20 into the pelletizing feed 14 is present, as well as a device 13 for adding dry dusts 21 into the pelletizing feed 14.

The devices 9, 10, 11, 12, 13 are optional, and the current plant 5 may comprise one or more of them, depending on the available raw material and the process economy of the operation in question. The positioning and design of said devices may be selected by the skilled person according to the plant 5 specif- ics. One or more of said devices 9, 10, 11, 12, 13 may be operationally connected to a device for milling coarse raw materials to a desired fineness (not de ^¬ picted in fig. 1) . One milling device may be used for milling one or more of the raw materials.

In the plant 5 according to the current dis ^¬ closure, the pelletizing feed 14 is pelletized in a pelletizing apparatus 7, which may be a pelletizing drum or a disc pelletizer.

The moisture of the green pellets 15 was low ^¬ er than 9% when an organic binder was used and no baghouse dust was included in the mixture. The mois ^¬ ture of the pellets WcLS θνθΓ 9"6 when a mixture of ben- tonite and organic binder was used as binding agent. The moisture of the pellets was between 8 and 8.5% when only bentonite was used as binding agent. In the industrial scale, the moisture of the green pellets 15 can be expected to be somewhat higher.

The wet strength of the green pellets 15 var- ied from 1.8 to 3 kg for a pellet of 12 mm diameter. These are quite satisfactory values. The dry strength of the green pellets 15 was between 3.5 and 15 kg for a pellet of 12 mm diameter. The highest dry strengths could be reached when using from 7.5 to 10 w-% baghouse dust. Addition of finely divided dolomite in ^¬ to the pelletizing mixture had a positive effect on the dry strength of the green pellets 15.

In the plant 5 according to the current dis ^¬ closure, the green pellets 15 are sintered in a sin- tering furnace 1 according to the present disclosure. The details of the sintering furnace 1 are described in connection with fig. 2.

The compression strength of the sintered pel ^¬ lets 16 is high enough for reduction and smelting. The compression strength of sintered pellets 16 produced without baghouse dust was 170-300 kg/pellet. When the sintered pellets 16 contained bag filter dust, the compression strength was 180-260 kg/pellet, depending on the amount of baghouse dust and the type of binding agent used.

The sintered pellets 16 are charged into a smelting furnace 8, which may be a a blast furnace, or an electric furnace.

FIGURE 2

In fig. 2, sintering equipment according to the present disclosure is depicted. The green pellets 15 are fed into the sintering furnace 1, which is a steel belt sintering furnace. The green pellets 15 are lead through sequential process zones of the sintering furnace 1 along a gas-permeable conveyor belt 3. Thus, the sintering is continuous. The process zones com ^¬ prise at least one drying zone I, at least one heating zone II, at least one sintering zone II, IV and at least one cooling zone V, VI, VII. The temperature conditions in each process zone are independently con- trollable by blowers 2 and burners B.

Circulation gas ducts 4a, 4b, 4c are located above the conveyor belt 3 for conducting gas from at least one cooling zone V, VI, VII to the drying zone I, heating zone II and sintering zone III, and onto the bed of pellets. A first circulation gas duct 4a conducts gas from cooling zone VII to the drying zone I . A second circulation gas duct 4b conducts gas from cooling zone VI to the heating zone II. A third circu ^¬ lation gas duct 4c conducts gas from cooling zone V to the sintering zone III.

Burners B are placed in the circulation gas ducts to heat the gas that travels in the circulation gas duct 4a, 4b, 4c. In the sintering equipment ac ^¬ cording to the current disclosure, each circulation gas duct 4a, 4b, 4c comprises at least one burner B for controlling the gas temperature in the process zone I, II, III, V, VI, VII to which the circulation gas duct conducts gas. In the embodiment of fig. 2, there are two burners B in each of the circulation gas ducts 4a, 4b, 4c. In the embodiment of fig. 1, sinter ^¬ ing equipment comprising one burner in each circula- tion gas duct is shown.

Lower exhaust gas ducts 22 are located below the conveyor belt 3 for conducting the gas that exits the drying zone I, heating zone II and sintering zone III and that has been conducted through a pellet bed on the conveyor belt 3 and through the conveyor belt 3. Blowers 2 are arranged in the lower exhaust gas ducts 22 to provide gas movement. In the embodiment of figure 2, a blower 2 is depicted for each lower exhaust gas duct 22. A lower exhaust duct 22 is connect- ed to each compartment of the drying zone (I) and the heating zone (II) . The first sintering zone III com ^¬ prises only one compartment, and the lower exhaust gas duct 22 connected to it is also equipped with a blower 2. The blower configuration in the embodiment of fig. 2 allows the regulation of the temperature gradient in the drying zone I and heating zone II so that the sin ^¬ tering process in the sintering equipment may be ad ^¬ justed according to the pellets fed into the sintering equipment .

Lower inlet gas ducts 23 are located below the conveyor belt 3 for conducting the gas to the cooling zones V, VI, VII. Blowers 2 are arranged in the lower inlet gas ducts 23 to provide gas movement. The green pellets 15 are first led to a dry ^¬ ing zone I comprising two compartments, in which they are dried. The drying time used in the sintering fur ^¬ nace 1 may be 8-13 minutes depending on the composi ^¬ tion of the mixture used. The temperature of the dry- ing gas supplied into the drying zone I is 220-230° C, although it may vary between 200 °C and 250 °C. Such low drying temperatures allows avoiding disintegration of the green pellets 15.

The drying zone I is followed by a heating zone II. Although in the embodiment of fig. 2 there are two heating compartments, embodiments may be en ^¬ visaged where the heating zone II comprises only one compartment. The duration heating is configured to be, for example, 10, or, in the case of two compartments, 5 minutes in each. With such configuration, a suitable temperature rise velocity may be achieved. Drastic ox ^¬ idation of iron and iron oxides could be observed in the heating zone at 900 °C. The gas temperature sup ^¬ plied into the heating zone was 1,150-1,180 °C. After the heating zone(s) II, the pellets are lead to two sintering zones (III, IV) . The retention time in the sintering zones is, for example, 6 minutes. The retention time may be varied depending on the composition of the pelletizing feed 14 and the moisture of the green pellets 15. At the end of the sintering zone III, the temperature in the material bed is between 1, 270 and 1, 390 °C depending on the feed composition, while the gas temperature of the heating gas fed into the sintering zone III is 1,200- 1,250 °C.

The total porosity of sintered pellets 16 containing no bag filter dust may be, for example, 33- 35%. The total porosity may be 25-27% when the sin ^¬ tered pellets 16 contain bag filter dust. These values are high in view of reduction.

Abrasion strengths of the sintered pellets 16 produced in the sintering equipment are high. The abrasion strength may become slightly reduced when baghouse dust was added in the pelletizing feed 14.

Sintered pellets have been tested according to ISO 11257 standard. The metallization degree of iron was found to be 94.4-100%, which is very high. Fines formation remains minimal when the current sin ^¬ tering furnace 1 is used for the production of sintered pellets 16. The strength of the sintered pellets 16 after reduction may be approximately 50 kg, which is a good value.

During sintering, a part of detrimental sub ^¬ stances are vaporized into the dusts. Dusts containing detrimental substances can be recycled. From time to time the detrimental substances must be removed from the process recirculation.

Experiments carried out with iron-containing dusts, waste and materials showed that it is possible to technically upgrade those materials to such a form which is suitable for feeding into a direct reduction furnace, a blast furnace or an electric arc furnace.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead, they may vary within the scope of the claims.