Method for operating an electric smelter

Abstract

The invention relates to a method for operating an electric smelter according to claim 1.

Description

[0001] Method for operating an electric melter

[0002] The invention relates to a method for operating an electric melter in which iron carriers and carbon carriers are introduced and melted to form molten iron and liquid slag.

[0003] Carbon carriers are deliberately added to increase the carbon content in the molten iron. A high carbon content in the molten iron is necessary, among other things, to preferentially produce the desired material properties of the (liquid) slag produced during smelting, with a low iron oxide content as a result of a desired reducing condition, and to be able to preferentially refine the molten iron or pig iron produced in the further course of the process, particularly with regard to the sulfur content and / or phosphorus content, if, viewed in the direction of the process, other existing units in a metallurgical complex, for example in the converter of a steelworks, can continue to be used economically. Thus, depending on the requirements for the pig iron quality to be produced or downstream steel production, it is expedient to carburize the molten iron or the resulting pig iron in an electric smelter.

[0004] DE 600 04 062 T2 discloses a process for melting sponge iron in an electric melter. In the electric melter, molten iron and liquid slag are produced from the charge introduced. Carbon is additionally injected near the electrodes of the electric melter immersed in the slag with the aim of foaming the liquid slag around the electrodes, thus promoting the production of CO and, associated with it, the formation of a plasma arc, and thus increasing the melter's productivity. Further addition of oxygen promotes post-combustion of CO into CO2, thus releasing more energy in the form of heat. Therefore, the carbon should be injected into the lower third of the liquid slag layer, preferably into the melt-slag boundary zone when injecting carbon-containing solids.The injected carbon added to foam the slag near the electrodes must be added in addition to the carbon required for reducing the charge and carburizing the metal.

[0005] From EP 3 992 309 A1 it is also known to use sponge iron with a high degree of metallization and little gangue as an iron carrier in an electric melter in order to produce less slag, so that less iron in the form of iron oxide can be removed into the slag and with the slag, thus keeping iron loss to a minimum.

[0006] Furthermore, the Richardson-Ellingham diagram is well known in expert circles, in which the oxygen potentials of metal oxides are plotted as a function of temperature.

[0007] When iron and carbon carriers are melted, reduction processes occur at high temperatures. Solid components, some of which still need to be reduced, such as oxides FeO and SiO2, are formed or remain, but mostly molten and gaseous compounds, such as CO and CO2, etc. For a reduction reaction to take place, the free reaction enthalpy AG must be re d of the reduction must be negative. According to Hess's theorem, the free reaction enthalpy AG re d is equal to the difference between the free enthalpies of formation of the compounds, where in general the reduction of a metal oxide MeO2 by a reducing agent R is: Me + 02<==> MeO2with AG Me o2; R + 02<==> R02mit AG R02 ; AG r ed = AGRO2 - AG Me 02- This relationship allows us to compare the free enthalpies of formation of compounds and determine which substances are suitable as reducing agents. Since AG red must be negative and the free enthalpy of formation of stable compounds is always negative, AG R0 2 for the oxidation of the reducing agent must be greater than AG Meo2 for the oxidation of the metal, so that at equilibrium the free standard reaction enthalpy AG° is determined: AG° = AH° - T * AS°, where H° is the standard reaction enthalpy, T is the temperature, and S° is the standard reaction entropy. Under the "idealized" assumption that AH° and AS° are independent of temperature, the equation for AG° allows a graphical plot against the temperature as a straight line, whereby the free enthalpies of formation of different compounds can be compared. For metal oxides, this can be represented in the so-called Richardson-Ellingham diagram. In the Richardson-Ellingham diagram, ideal conditions are assumed, i.e., that all liquid and solid phases exist as pure phases, so that their activity is one. The free standard reaction enthalpy is thus simplified to the so-called oxygen potential: AG° = - R * T * In k = - R * T * In [(a M eo) / (a M e * a 02 )] = R * T * In p 02, with R as the general gas constant (8.314 J mol -1 K" 1 ); T: temperature (K); k: equilibrium constant; a Me o: activity of the metal oxide; a Me : activity of the metal; p 02 : partial pressure of oxygen.

[0008] In the Richardson-Ellingham diagram, curves for the more noble metals lie above those for the less noble ones, with the less noble metals having a correspondingly higher affinity for oxygen and thus, in principle, being suitable as reducing agents, while they themselves are sometimes difficult to reduce. The Richardson-Ellingham diagram shows that a phase change (from solid to liquid metal) has a major influence on the temperature dependence of the oxygen potential (phase changes lead to a change in the slope of the straight line, since the entropy increases from solid to liquid to gaseous). It must be noted, however, that the Richardson-Ellingham diagram assumes ideal conditions, so that, for example, an activity of the metal and the oxide different from one, e.g. due to the formation of mixed phases, leads to a deviation from the described relationships.A further disadvantage is that only statements about the theoretically achievable equilibrium states are possible, while the path and time to reach equilibrium, i.e. the crucial area of ​​reaction kinetics, are ignored, cf. https: / / application.wilev-vch.de / books / sample / 3527315802 cOl.pdf, page 37 ff.

[0009] Until now, a statement regarding the quality of the produced liquid slag and also of the iron melt could only be made through sampling during the respective tapping process, so that the desired qualities could only be reconditioned in subsequent process steps if necessary. In-situ monitoring, particularly of the quality of the liquid slag and / or the iron melt, has not been considered to date.

[0010] The object of the present invention is to further develop this process in such a way that a desired quality of the liquid slag and / or the iron melt can be adjusted in the melter.

[0011] This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

[0012] According to the invention, the redox potential of the liquid slag is adjusted during the melting operation such that iron oxide in the liquid slag is substantially completely reduced to iron, wherein the adjustment of the redox potential during the melting operation is carried out by adding iron carriers and / or carbon carriers.

[0013] In particular, a high-quality liquid slag essentially contains little to no iron, or a low-iron liquid slag. The redox potential required for the desired, essentially complete reduction of the slag (components) can be easily determined as follows via the free enthalpy - R * T In p 02 , which, depending on the temperature, ideally lies between -290 and -620 kJ / mol O2. According to one embodiment, the redox potential can be determined during melting operation by an actual analysis.

[0014] The actual analysis can be determined, in particular, from continuous oxygen activity measurements. It is well known in expert circles to carry out oxygen activity measurements to estimate the current oxygen concentration in iron melts, since different oxygen concentrations are required or should be adjusted in the various metallurgical treatment processes and also during steel production. For example, in a steel mill during the refining of pig iron, where the oxygen concentration is actively increased in order to reduce the carbon content in the melt. Disposable probes from Heraeus, known under the trade name CELOX, are well known for their ability to measure the current oxygen activity a(02) of the melt across the entire technically possible concentration range, for example, during desulfurization, in the furnace, in the ladle, or in the continuous casting tundish.For example, a correlation of the measured oxygen to sulfur and silicon in pig iron, to carbon and aluminum content in steel can be implemented in a firmware of the respective measuring instrument.

[0015] The actual analysis can be determined alternatively or additionally from a continuous thermal radiation measurement. For example, by using pyrometers, especially two-color pyrometers or two-color pyrometers, whose functionality for measuring emissivity of liquids (measurement objects) and the resulting temperature is known.

[0016] The actual analysis can be determined alternatively or additionally from a particularly continuous spectroscopic measurement. The use of spectroscopy to determine chemical relationships in liquids and / or solids is well known; see, for example, Chapters 4.2 and / or 4.3 in "Understanding the Structure and Structural Effects on the Properties of Blast Furnace Slag (BFS)," Sajid et al., ISIJ International, Vol. 59 (2019), No. 7, pp. 1153-1166.

[0017] For example, using LIBS (Laser Induced Breakdown Spectroscopy), a laser can generate a plasma that emits characteristic radiation, which can then be analyzed based on the spectral values ​​to determine the chemical composition of the material being examined. Instead of a laser, the plasma can also be generated by an arc between the electrode and the (liquid) slag and used for measurement.

[0018] The actual analysis can be determined alternatively or additionally by a continuous density measurement of the (liquid) slag. For example, this can be done according to the Archimedes principle or using X-rays or gamma radiation. The principle of density measurement is well known.

[0019] The actual analysis can be determined alternatively or additionally by a continuous viscosity measurement of the (liquid) slag. This is based on the fact that the oxides present in a (liquid) slag can be divided into network-forming oxides, network-transforming oxides, and those that can have both properties depending on their composition (more network-forming than network-transforming oxides, or vice versa). A high proportion of network-forming oxides increases the viscosity, while an increasing proportion of network-transforming oxides decreases the viscosity; see, for example, Chapter 3 ff. in "Influence of Basicity and FeO Content on Viscosity of Blast Furnace Type Slags Containing FeO," Lee et al., ISIJ International, Vol. 44 (2004), No. 8, pp. 1283-1290.

[0020] The actual analysis can be determined alternatively or additionally by a continuous electrical conductivity measurement of the (liquid) slag. The concentration and mobility of the electrons are influenced by the number of available cations and the size of the cations, respectively; see, for example, Chapter 5.4 in "Understanding the Structure and Structural Effects on the Properties of Blast Furnace Slag (BFS)," Sajid et al., ISIJ International, Vol. 59 (2019), No. 7, pp. 1153-1166

[0021] For example, reduced iron ore carriers in the form of sponge iron pieces or sponge iron pellets with a carbon content between 0 and 4.8 wt.%, in particular > 0 wt.%, and a degree of metallization of at least 85% are preferably used as iron carriers (in the smelter). The degree of metallization reflects the ratio of the metallic iron content based on the total iron content in the sponge iron. It can in particular be at least 88%, preferably at least 90%, preferably at least 92%, and ideally up to 100%, in particular up to 99%. If the iron melt has a defined carbon content, which can be between 0, in particular > 0, preferably > 1.0, preferably > 2.0, particularly preferably > 3.0, and 4.8 wt.%, cannot be provided via the iron carrier alone, carbon carriers must be taken into account in such an amount that the desired carbon content in the iron melt can be achieved.

[0022] The use of the invention is also conceivable in electric melters which are operated (only) with scrap and / or pig iron / steel as iron carriers in combination with carbon carriers.

[0023] The preferred use of sponge iron as an iron carrier also brings with it slag-forming components that are naturally present in the iron ore carrier and cannot be expelled in a prior reduction process, and are referred to as gangue. If the gangue provided by the sponge iron is insufficient, further slag-forming agents can be introduced as additives if necessary to produce a liquid slag that 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 achieved. 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 (SiO2+Al2O3), 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, SiO2, AI2O3).

[0024] In principle, any material in gaseous, liquid, and / or solid form containing reducible free carbon that can be introduced into the electric melter is suitable as a carbon carrier. In solid form, examples include coke dust, coke slaked coal, coke breeze, or coal particles. In liquid form, examples include ethanol, methanol, and other hydrocarbons. In gaseous form, carbon-containing gases such as carbon dioxide, methane (natural gas), carbon monoxide, propane, and butane are suitable.

[0025] In order to improve or increase the recycling rate, scrap can be added in addition to the iron carriers, 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 500 kg, in particular up to 300 kg, preferably up to 200 kg of scrap can be added per ton of molten iron produced. To melt the iron carriers and carbon carriers, 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 molten iron and 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 from renewable energy sources (sun, wind, water, biomass). This allows the electric melter to operate in a more climate-neutral manner.

[0026] The electric melter is preferably an OSBF (Open Slag Bath Furnace) furnace. These include submerged electric arc furnaces, especially SAF (Submerged Electric Arc Furnaces), which are melting furnaces with arc resistance heating, which form arcs between the electrode and the charge material and / or the liquid phase, or which heat the charge material and / or the liquid phase using the Joule effect. In SAF, the electrodes are immersed in the charge material and / or the liquid phase, especially the liquid slag. Depending on the functional principle / mode of operation, the submerged electric arc furnaces can be designed as alternating current submerged arc furnaces (SAFac) or direct current submerged arc furnaces (SAFdc).Alternatively, melting furnaces with direct arc action, which deviate from the functional principle / operation described above, can also be used, so-called EAFs (Electric Arc Furnaces), 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).

[0027] The advantage of using submerged arc furnaces (SAF) is that they operate in a reducing atmosphere, whereas direct arc furnaces (EAF) operate in an oxidizing atmosphere.

[0028] The invention is explained in more detail using the following embodiments in conjunction with the drawing.

[0029] The invention provides a method for operating an electric melter in which iron carriers and carbon carriers are introduced and melted during melting operation to form iron melt and liquid slag, wherein during melting operation the redox potential of the liquid slag is adjusted such that iron oxide in the liquid slag is reduced to iron, wherein the adjustment of the redox potential during melting operation is carried out by adding iron carriers and / or carbon carriers.

[0030] Figure 1 shows an Ellingham diagram in which an ideal process window is spanned with a temperature between 1200 and 1500 °C, a lower curve (boundary) by Si + O 2 → SiO 2 , and an upper curve (boundary) by Fe + V 2 → O 2 → FeO 2 . Thus, the corner points of the ideal process window are at 1200 °C with a minimum free enthalpy of approximately -350 kJ / mol O 2 and a maximum free enthalpy of approximately -620 kJ / mol O 2 , and at 1500 °C with a minimum free enthalpy of approximately -290 kJ / mol O 2 and a maximum free enthalpy of approximately -560 kJ / mol O 2 .

[0031] If the actual analysis shows that the melting operation cannot lead to the desired target operation and thus cannot achieve the target quality, it is preferable to add carbon carriers, which are provided in gaseous, liquid and / or solid form with reducible free carbon and optionally iron carriers to influence the melting operation.

[0032] The adjustment is advantageously carried out in such a way that an oxygen partial pressure of approximately IO -9 up to 10 _ 16 cash, especially from about IO -9 to IO -22 bar, preferably about 10 10 to IO -20 bar, preferably about 10 11 up to 10 18 bar, for example, the CO / CO2 ratio being between 5 and 10 5 , especially 10 and 10 4 , preferably 10 and 10 3 is elected.

[0033] The redox potential is determined during melting operations by an actual analysis, whereby the actual analysis can be determined from an oxygen activity measurement, a thermal radiation measurement, a spectroscopy measurement, a density measurement, a viscosity measurement, and / or a measurement of the electrical conductivity, in particular of the (liquid) slag. The actual analysis is preferably carried out automatically, preferably continuously or discretely at specific times. An automated oxygen activity measurement is particularly preferred. Further preferably, the actual analysis can be used to determine optimal carburization of the molten iron and the degree of iron reduction in the liquid slag.

[0034] A minimal oxygen activity can be found in the liquid slag at AG° S iO2 = - R * T * ln k = - R * T * In [(a S io2) / (asi * a 02 )] and high oxygen activity at AG° Feo = - R * T * ln k = - R * T * In [(a 2 Feo) / (a 2 Fe * a02)]. Furthermore, the oxygen activity in the melter above the liquid slag in the furnace atmosphere can be determined with AG° C o2= - R * T * ln k = - R * T * In [(a C o2) / (aco * a°' 5 O2 )] can be determined.

[0035] In addition, other iron-containing iron carriers, such as iron-containing scrap, can be added 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 added, particularly if the gangue of the preferably used sponge iron is insufficient to adjust the desired basicity of the slag to be tapped. Adjusting the desired basicity through appropriate mixing / addition is familiar to those skilled in the art. The amount of iron carriers added depends on the desired yield of the molten iron.

[0036] Also not shown is how the molten iron is removed and fed to a further processing step. The molten iron is preferably subjected to a treatment 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 to produce slag, particularly for the construction industry.

Claims

Patent claims 1. A method for operating an electric melter in which iron carriers and carbon carriers are introduced and melted during melting to form iron melt and liquid slag, characterized in that during melting the redox potential of the liquid slag is adjusted such that iron oxide in the liquid slag is reduced to iron, wherein the adjustment of the redox potential during melting is effected by adding iron carriers and / or carbon carriers, wherein the redox potential is defined as free enthalpy - R * T In pO2, which, depending on the temperature, ideally lies between - 290 and - 620 kJ / mol O2.

2. The method according to claim 1, wherein the temperature is ideally between 1200 and 1500 °C.

3. Method according to one of the preceding claims, wherein an ideal process window is spanned in an Ellingham diagram with a temperature between 1200 and 1500 °C, a lower curve through Si + O2 -> SiO2 and an upper curve through Fe + V2 O2 -> FeO, so that the corner points of the ideal process window are at 1200 °C with a minimum free enthalpy of - 350 kJ / mol O2 and a maximum free enthalpy of approximately - 620 kJ / mol O2 and at 1500 °C with a minimum free enthalpy of approximately - 290 kJ / mol O2 and a maximum free enthalpy of approximately - 560 kJ / mol O2.

4. Method according to one of the preceding claims, wherein the redox potential is determined during the melting operation by an actual analysis.

5. The method according to claim 4, wherein the actual analysis is determined from an oxygen activity measurement.

6. The method according to claim 4, wherein the actual analysis is determined from a thermal radiation measurement.

7. The method according to claim 4, wherein the actual analysis is determined from a spectroscopy measurement.

8. The method according to claim 4, wherein the actual analysis is determined from a density measurement.

9. The method according to claim 4, wherein the actual analysis is determined from a viscosity measurement.

10. The method according to claim 4, wherein the actual analysis is determined from an electrical conductivity measurement.

11. A process according to any one of the preceding claims, wherein reduced iron ore carriers in the form of sponge iron pieces or sponge iron pellets with a carbon content of between 0 and 4.8 wt.% and a degree of metallization of at least 85% are used as iron carriers.

12. Process according to one of the preceding claims, wherein slag formers are additionally introduced which comprise at least one or more of the elements from the group (CaO, MgO, SiO2, AI2O3) such that a basicity B3 in the liquid slag of between 0.9 and 1.8 is established, wherein the basicity B3 corresponds to the ratio (CaO+MgO) to (SiO2+AI2O3).

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