WO2024165393A1 - Process for producing an iron melt and molten slag in an electric smelter - Google Patents

Abstract

The invention relates to a process for producing an iron melt and molten slag in an electric smelter (10) having an angular horizontal main structure.

Description

Process for producing molten iron and liquid slag in an electric melter

The invention relates to a method for producing an iron melt in an electric smelter.

The energy provided by the electrodes is used to melt the feedstocks introduced into electric melters. A large part of the (total) energy is converted into thermal energy, which leads to the melting of the feedstocks, and another part leads to the heating of the melter lining. In order to protect the lining from thermal stress, the wall of the melter is (actively) cooled. As a result, part of the introduced energy is lost again through cooling. In order to operate an electric melter economically, the melting process is carried out in the so-called sump mode, which is well known in the professional world. This means that when the electric melter is emptied, the liquid slag and the molten iron underneath are "tapped off" into different containers and a (bottom-covering) residual molten iron remains in the melter, onto which the new feedstocks to be melted are introduced and the process starts again by applying energy to the electrodes. As a result, the processes and devices for producing iron melts in electric melters are state of the art.

Furthermore, electric melters with a square horizontal basic structure are also known, see for example EP 2 270 239 Bl and EP 3 542 595 Bl . Due to the well-known sump operation, even with a square horizontal basic structure, the lower part of the electric melter is covered with residual iron melt before the melter is filled with new feedstocks. In order to melt the feedstocks over the entire square horizontal basic structure, a high energy input via the electrodes is required, since the distance to the wall is higher compared to a melter with a circular horizontal basic structure, depending on the design; number of electrodes and size of the basic structure, and as a result the energy input is higher in order to create a complete liquid phase over the entire area/volume.

In addition to the energy-intensive operation, this procedure also has a detrimental effect on the lining of the electric melter. The object of the present invention is to further develop this method in such a way that it enables a reduction in the energy required for melting in the electric melter and also reduces wear on the lining in the melter.

This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

The invention relates to a method for producing an iron melt and a liquid slag in an electric melter, which has a square horizontal basic structure with a surrounding wall, wherein iron-containing feedstocks are introduced and melted, wherein the electric melter comprises several electrodes which provide the energy required for melting in order to convert the feedstocks into a liquid phase, comprising the iron melt and the liquid slag arranged on the iron melt. The electrodes are controlled in such a way that the liquid phase only contacts the surrounding wall in certain areas.

In comparison to the prior art, the aim is to avoid complete melting of the feedstock and thus also complete contact of the liquid phase produced with the wall of the electric melter. The invention takes advantage of the fact that less energy has to be provided compared to complete melting of the feedstock introduced in the electric melter, since only a portion of the introduced feedstock has to be melted and therefore only partial contact with the wall is permitted or present. This also has the advantage that only a small proportion of the energy introduced for melting is (directly) dissipated from the liquid phase through partial contact with the cooled wall of the electric melter. This also means that the lining of the wall, at least a large part of it, is less stressed compared to complete melting. The liquid phase generally has a more aggressive and/or abrasive effect on the wall or lining compared to the introduced and thermally activated feedstock. This means that wear on the lining can be minimized.

The lining of metallurgical vessels which come into contact with liquid iron, including melters, as well as the corresponding material, so-called refractory material, are state of the art. The angular horizontal basic structure refers to the basic shape of the interior of the electric melter for melting the charge materials in horizontal (cross) section, whereby the corner areas do not necessarily have to correspond to a right angle due to the infeed.

In other words, with a square, horizontal basic structure of the electric smelter, the feed materials in the corners are not melted, so that columns of feed materials remain distributed in the corners. The columns do shrink due to the thermal stress from the adjacent liquid phase, as parts of the column pass into the liquid phase. Since the feed materials are introduced continuously or, preferably, discontinuously, depending on the operating mode, until the desired fill or output level of the molten iron is reached, the corners are also "filled" again and again. During tapping, the columns can gradually collapse as the liquid phase sinks in the middle area of the electric smelter. This has the advantage that the feed materials which were exposed to a certain temperature in the zones/corners in the previous melting cycle and were not transferred to the liquid phase can fall into the area to be melted in the new melting cycle, and therefore no longer require the full energy to melt like the newly introduced (cold) feed materials. A portion of the collapsing columns enter the sump, so that this portion can reach the liquidus temperature more quickly and also becomes molten.

The electrodes can be controlled in such a way that the temperature of the feed materials in the zones in which there is or should be no (pure) liquid phase is less than T _L , where T _L corresponds to the liguidus temperature. The zones which, depending on the temperature, form a mushy phase, i.e. a mixed phase of solid and liquid, thus corresponding to a temperature of T _s and T _L , for example, or which retain their solid phase, particularly if the temperature can be set at or below the solidus temperature T _s , thus form in the corners of the electric melter. The formation of the zones depends on the control, for example on the local arrangement of the electrodes within the electric melter, and also on the cooling capacity of the wall. In addition, the temperature control within the electric melter, particularly in the corners, can be influenced by introducing the cold feed materials. The temperature in the liquid phase is more than T _L. The temperatures T _s and T _L can be determined by knowing the composition of the iron melt to be produced, which depends in particular on the iron-containing The temperature in the corresponding zones can be set, for example, between T _s - 300 K and less than T _L , in particular less than T _L - 5 K, preferably less than T _L - 20 K. The electrodes are controlled in such a way that a liquid phase with a temperature of more than T _L , for example of at least T _L + 30 K, in particular at least T _L + 50 K, is established around the electrodes. The liguidus temperature of pure iron is theoretically approx. 1538 °C. The temperature essentially refers to the temperature of the molten iron, which can be measured using known means. Known measuring devices include, for example, thermocouples which are immersed directly in the molten iron or contactless pyrometers. It may also be common practice to make verified assumptions that allow conclusions to be drawn about the temperature in the melter, such as optically tracking the melt pool and detecting different levels of radiation, different flow behavior of the liquid slag/molten iron, changes in morphology and melting of any charge material cones, or measuring the temperature(s) of the wall.

The melting process (cycle) can be monitored using imaging devices, such as a camera or cameras, for example in the form of a temperature profile in plan view. For example, light areas can identify hot and liquid phases and dark, slightly cooler, mixed phases or solid phases. During the melting and transfer of the feedstock and optional additives, a liquid phase is formed which includes the molten iron and the liquid slag. Due to the lower density compared to the molten iron, the liquid slag forms on the molten iron. The temperature of the liquid phase recorded in plan view therefore does not correspond to the molten iron, but to the liquid slag formed on it, which can deviate by up to 200 K (plus) from the actual temperature of the molten iron, particularly if the energy input in the melter occurs via the liquid slag. The temperature profile can therefore be provided with a correction factor in the image processing software in the area of the liquid phase in order to be able to display the approximate temperature of the molten iron in light/dark.

The temperature profile in the interior of the electric melter can be as follows: from the outside to the inside, based on the basic structure, at least in the corners, zones are set completely preferably below T _s , then a mixing zone T _s to T _L , then a liquid zone with a liquid phase above T _L . How these are dimensioned, in particular to influence The gradient of the temperature profile, so that the liquid phase only contacts the surrounding wall in certain areas, is not only controlled via the electrodes, but can also be controlled via a cooling capacity of the wall and/or by introducing the cold feedstocks.

The preferred iron-containing feedstocks are reduced iron ore in the form of sponge iron pieces or sponge iron pellets with a carbon content of between 0 and 4.5 wt.%, in particular > 0 wt.%, and a degree of metallization of at least 85%. The degree of metallization reflects the ratio of the metallic iron content to the total iron content in the sponge iron. The carbon content also influences T _s and T _L , so that these decrease as the carbon content increases, which is advantageous because the energy required for melting can also be reduced. If the iron melt cannot have a defined carbon content, which can be between 0, in particular > 0, and 4.5 wt.%, via the iron-containing feedstocks, carbon-containing additives must be taken into account in quantities that ensure that the desired carbon content in the iron melt can be achieved.

The invention can also be used in electric smelters that operate with scrap and/or crude steel as iron-containing feedstocks.

The preferred use of sponge iron as an iron-containing feedstock also brings with it slag-forming components which are naturally contained in iron ore and cannot be expelled in a preceding reduction process and are referred to as gangue. If the gangue provided by the sponge iron is not sufficient, further slag-forming agents can be introduced as additives if required in order to produce a liquid slag which can be further processed. Slag-forming agents are preferably added so that a basicity B3 in the liquid slag of between 0.9 and 1.8 is established. B3 can in particular be at least 1.0, preferably at least 1.1 and in particular a maximum of 1.7, preferably a maximum of 1.6. The basicity B3 corresponds to the ratio (CaO+MgO) to (SiO ₂ +AI ₂ O ₃ ), whereby the determination of the characteristic quantities in the slag in the solid state is familiar to the person skilled in the art. The slag former comprises at least one or more of the elements from the group (CaO, MgO, SiO ₂ , Al ₂ O ₃ ).

In order to improve or increase the recycling rate, scrap can be recycled in addition to the iron-containing input materials, preferably to the sponge iron pieces or sponge iron pellets This can be done, for example, in such a way that > 0 kg, in particular at least 20 kg, preferably at least 50 kg, preferably at least 80 kg up to 200 kg of scrap can be added per ton of molten iron produced.

To melt the iron-containing solids, the electric melter has several electrodes that can be charged with an electric current and thus provide the energy required to convert the solids into a liquid phase comprising an iron melt and a liquid slag. Depending on the size/dimensions of the electric melter, three, four, five, six or more than six electrodes can be used. The energy required for melting is preferably provided by renewable energy (sun, wind, water, biomass). This means that the electric melter can be operated in a more environmentally friendly way.

Depending on the design of the angular basic shape of the electric melter and the arrangement of the electrodes in the electric melter, the electrodes can be controlled differently so that the iron-containing feedstocks in the zones are not completely converted into a liquid phase and the wall is only partially contacted or touched by the liquid phase. For example, the corresponding openings for tapping the liquid slag and the molten iron are located in the wall in the area of regional contact with the liquid phase. The opening for tapping the liquid slag is arranged somewhat higher in height than the opening for tapping the molten iron. Both openings can therefore be arranged one above the other in the area of regional contact. To enable accessibility for sequential tapping, the openings for tapping can be arranged on different sides of the wall, for example opposite each other. On an electric melter with a square horizontal basic structure, the openings are essentially arranged in the middle of one side of the wall. The basic structure can be essentially square, with four sides of the wall of equal length, in which case the electrodes, for example three or more, are controlled in such a way that essentially a circular liquid phase is formed and thus only a partial contact of the wall takes place in four discrete areas, here in the middle areas of the respective four sides.

The basic structure can alternatively be essentially rectangular with two equally long opposite sides of the wall, in which case the electrodes, for example four or more, preferably arranged in a row, are controlled in such a way that essentially an elliptical liquid phase is formed and thus only a partial contact of the wall takes place in four discrete areas, here in the middle areas of the respective four sides.

The electric smelter can preferably be a furnace of the OSBF (Open Slag Bath Furnace) type. These include electric arc furnaces, especially SAF (Submerged Electric Arc Furnace), which are melting furnaces with arc resistance heating that form arcs between the electrode and the feed material and/or the liquid phase or which heat the feed material and/or the liquid phase using the Joule effect. In SAF, the electrodes are immersed in the feed material and/or the liquid phase, in particular in the liquid slag. Depending on the functional principle/mode of operation, the electric arc furnaces can be designed as alternating current arc reduction furnaces (SAFac) or direct current arc reduction furnaces (SAFdc). Alternatively, melting furnaces with direct arc exposure, which deviate from the functional principle/mode of operation described above, so-called EAF (Electric Arc Furnace), can be used, which form arcs between the electrode and the liquid phase. This includes the alternating current arc melting furnace (EAFac), the direct current arc melting furnace (EAFdc) and the ladle furnace (LF).

The advantage of using submerged arc furnaces (SAF) is that they operate with a reducing atmosphere, whereas direct arc furnaces (EAF) operate with an oxidizing atmosphere.

The invention is explained in more detail using the following embodiments in conjunction with the drawing.

The invention is explained in more detail with the aid of Figures 1 to 3 using the example of an electric melter (10) with a square horizontal basic structure. Figures 1 and 2 show a sketch of an example with a substantially square basic structure and Figure 3 shows a sketch of an example with a substantially rectangular basic structure.

The invention provides a method for producing an iron melt and a liquid slag in an electric melter (10) which has a square horizontal basic structure with a circumferential wall (12). Iron-containing feedstocks are introduced and melted. The required feedstocks can be supplied by means not shown. The iron-containing feedstocks comprise or consist of sponge iron pieces or pellets. In addition, other iron-containing feedstocks, such as iron-containing scrap, can also be supplied in order to increase the recycling rate. Other additives, such as slag formers such as lime, silicon dioxide, magnesium oxide and/or aluminum oxide, can also be introduced, in particular if the so-called gangue of the preferably used sponge iron is not sufficient to be able to set the desired basicity of the liquid slag to be tapped off. Setting the desired basicity by appropriate mixing/addition is familiar to the person skilled in the art. The amount of iron-containing feedstocks introduced depends on the desired output of the molten iron. The electric melter (10) comprises several electrodes (11), for example three in Figures 1 and 2 and six, arranged in series, in Figure 3, which provide the energy required for melting in order to convert the feedstocks into a liquid phase (L) comprising the molten iron and the liquid slag arranged on the molten iron. The electrical energy required for melting can preferably be generated from renewable energy (sun, wind, water). Contrary to the prior art, the electrodes (11) are controlled in such a way that the liquid phase (L) only contacts the surrounding wall (12) in certain areas.

The electrodes (11) are controlled in such a way that a temperature of the feedstocks in the zones (S) in which no liquid phase (L) is present of less than T _L - 50 K, where T _L corresponds to the liguidus temperature, and that a liquid phase (L) with a temperature of at least T _L + 30 K is established around the electrodes (11).

Depending on the design of the square basic shape of the electric melter (10) and the arrangement of the electrodes (11) in the electric melter (11), the electrodes can also be controlled differently so that the iron-containing feedstocks in the zones (S) are not completely converted into a liquid phase (L) and the wall (12) is only partially contacted by the liquid phase (L), or is touched. For example, in the wall (12) in the area of the partial contact of the liquid phase (L), there are the corresponding openings (13, 14) for tapping the liquid slag and the molten iron. The opening (13) for tapping the liquid slag is arranged somewhat higher in height than the opening (14) for tapping the molten iron. Therefore Both openings (13, 14) can be arranged one above the other in the area of the area-by-area contacting, as shown by way of example in the figures. In order to enable access for sequential cutting, the openings (13, 14) for cutting can be arranged on different sides of the wall (12), for example opposite each other. On an electric melter (10) with a square horizontal basic structure, the openings (13, 14) are arranged essentially in the middle of one side of the wall (12).

The essentially square basic structures, see Figures 1 and 2, are designed with four equally long sides of the wall (12), whereby, for example, three electrodes (11) are controlled in such a way that essentially a circular liquid phase (L) is formed and thus only a partial contact of the wall (12) can take place in four discrete areas, here in the middle areas of the respective four sides, see Figure 1. The difference in Figure 2 shows that the upper electrode (11) arranged in the sketch is subjected to a higher power and thus the influence zone and thus also the liquid phase (L) extends further than with the two adjacent electrodes (11), which are subjected to a lower power. The liquid phase (L) can have a kind of trefoil shape, whereby a regional contacting of the wall (12) must take place in at least one discrete area, here in the middle area of the upper sides shown, at which the openings (13, 14) for tapping are arranged.

The electric melters (10) with a square horizontal base structure are preferably designed to be stationary and not pivotable. The operation of electric melters (10) is also familiar to those skilled in the art.

What is not shown is that in the region of at least partial contact with the liquid phase (L), in which the opening (13, 14) for cutting off is also arranged, the wall (12) or the infeed has a greater thickness than the remaining wall (12) or infeed, at least 10%, preferably at least 20%, preferably at least 25%.

The size ratio of the electrodes to the melter (vessel) is not shown. Furthermore, at least one electrode (in addition) to the (standard) electrodes can be arranged in the bottom of the melter, not shown here, cf. EP 3 542 595 Bl. Also not shown is how the molten iron is removed and fed to a further processing step. The molten iron is preferably fed to a treatment in order to reduce the carbon in the molten iron to a desired level. This is done, for example, using oxygen in a so-called oxygen blowing process, particularly preferably in a converter. The tapped liquid slag is also preferably fed to a granulation process in order to produce slag, in particular for the construction industry.

Claims

Patent claims 1. Method for producing an iron melt and a liquid slag in an electric melter (10) which has a square horizontal basic structure with a circumferential wall (12), wherein iron-containing feedstocks are introduced and melted, wherein the electric melter (10) comprises a plurality of electrodes (11) which provide the energy required for melting in order to convert the feedstocks into a liquid phase (L) comprising the iron melt and the liquid slag arranged on the iron melt, characterized in that the electrodes (11) are controlled in such a way that the liquid phase (L) only contacts the circumferential wall (12) in certain areas. 2. Method according to claim 1, wherein the electrodes (11) are controlled such that a temperature of the feedstocks in the zones (S) in which no liquid phase (L) is present is less than T _L , wherein T _L corresponds to the liguidus temperature. 3. Method according to one of the preceding claims, wherein the electrodes (11) are controlled such that a liquid phase (L) with a temperature of more than T _L is established around the electrodes (11). 4. Method according to one of the preceding claims, wherein additionally a contact of the circumferential wall (12) with the liquid phase (L) can be controlled only in certain regions via a cooling capacity of the wall and/or via introduction of the cold feedstocks. 5. Process according to one of the preceding claims, wherein reduced iron ore in the form of sponge iron pieces or sponge iron pellets with a carbon content between 0 and 4.5 wt.% and a degree of metallization of at least 85% are used as iron-containing feedstocks. 6. Process according to one of the preceding claims, wherein slag formers are added which comprise at least one or more of the elements from the group (CaO, MgO, SiO ₂ , Al ₂ O ₃ ) such that a basicity B3 in the liquid slag is between 0.9 and 1.8. 7. Method according to one of the preceding claims, wherein the energy required for melting is provided from renewable energy. 8. Method according to one of the preceding claims, wherein the electrodes (11) are controlled differently. 9. Method according to one of the preceding claims, wherein the corresponding openings (13, 14) for tapping the liquid slag and the molten iron are arranged in the wall (12) in the region of the region-wise contacting of the liquid phase (L). 10. Method according to one of the preceding claims, wherein the basic structure of the electrical melter (10) is designed to be square with four sides of the wall (12) of equal length, wherein the electrodes (11), three or more, are then controlled in such a way that a substantially circular liquid phase (L) is formed and thus only a partial contact of the wall (12) takes place in four discrete areas, here in the middle areas of the respective four sides. 11. Method according to one of claims 1 to 9, wherein the basic structure of the electrical melter (10) is rectangular with two opposite sides of the wall (12) of equal length, wherein the electrodes (11), four or more, preferably arranged in a row, are then controlled in such a way that an essentially elliptical liquid phase (L) is formed and thus only a partial contact of the wall (12) takes place in four discrete areas, here in the middle areas of the respective four sides.