Method for operating a DC electric furnace to produce iron smelt and molten slag

Abstract

The present invention relates to a method for operating a DC electric furnace (10) having at least one upper electrode (11) and at least one lower electrode (11.1) for producing an iron smelt (1) and molten slag (2).

Description

[Technical Field]

[0001] The present invention relates to a method for operating a DC electric furnace for producing an iron melt and a liquid slag. [Background technology]

[0002] The production of liquid iron through the fossil-derived blast furnace route generates a large amount of CO2.

[0003] Direct reduction plants using natural gas result in a solid iron sponge, although CO2 emissions are significantly reduced. The direct reduction route also offers the possibility of operating with higher hydrogen concentrations in the process gas, which could potentially reduce CO2 emissions. The type of direct reduction process used, whether vertical shaft or rotary kiln, does not play an essential role in the process, as long as a solid iron sponge is produced as a product. Full reduction of the iron sponge is not necessary, as higher production yields mean that such systems are more likely to operate in an economically optimal manner.

[0004] The solid iron sponge must be melted in a melting unit, which can be done using a variety of techniques, including induction or electric arc furnaces, or smelting reduction furnaces. During the melting process, the iron sponge is melted and the already reduced parts of the iron sponge are separated from the unreduced parts to form the metallic phase (molten iron). The unreduced parts are then dumped into the liquid slag in a typical melting process, resulting in a suboptimal application and an increased amount of liquid slag with a higher iron content.

[0005] This drawback does not occur in production processes using direct reduction processes in combination with smelting reduction furnaces, which necessarily rely on a reducing agent in the form of carbon in the smelting furnace. The iron smelt thus produced ("electric furnace iron") must then be further processed, at least in subsequent refining steps, to form, for example, a steel smelt. To achieve CO2-neutrality or CO2 reduction, the added carbon must at least be fossil-free, and ideally, the CO2 generated during the process should not leave the process (carbon looping).

[0006] The use of DC electric furnaces to produce an iron melt and liquid slag from a solid charge containing iron and slag-forming agents is prior art, see for example EP 0657549, EP 1124995, DE 19744151. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent No. 0657549 [Patent Document 2] European Patent No. 1124995 [Patent Document 3] German Patent No. 19744151 Summary of the Invention

[0008] The object of the present invention is to identify for operating an electric melter that an optimum yield and amount of reduced liquid slag with a lower iron content compared to the prior art can be achieved up to a substantially complete removal of the iron content.

[0009] The object is achieved by a method having the features of claim 1. Further configurations are set out in the dependent claims.

[0010] The present invention relates to a method for operating a DC electric furnace having at least one upper electrode and at least one lower electrode for producing an iron smelt and a liquid slag, the method comprising the following steps: charging solids including an iron-containing material and a slag-forming agent into the blast furnace; melting the solids to produce an iron smelt and a liquid slag disposed on the iron smelt, wherein the upper electrode functions as a cathode and the lower electrode functions as an anode while the solids are melted; tapping the liquid slag and the iron smelt, wherein before the liquid slag and the iron smelt are tapped, the upper electrode is brought into contact with the liquid slag and a polarity reversal is performed, so that the upper electrode functions as an anode and the lower electrode functions as a cathode.

[0011] In its simplest form, a DC electric furnace has an upper electrode and a base electrode. However, depending on the geometry and implementation of the electric furnace, two or three additional upper electrodes and / or two or three additional lower electrodes may be provided. In the following text, references to upper and lower electrodes may be understood as a plural number. The operating principle / operation of a DC electric furnace is well known to those skilled in the art. After a solid containing an iron-containing material and a slag-forming agent is charged into a blast furnace, the process of melting the solid to produce a molten iron and a liquid slag disposed above the molten iron begins. During the melting process, the upper electrode functions as a cathode and the lower electrode functions as an anode. In particular, a stable, particularly linear, plasma arc from the cathode can impinge on the solid to be melted. In other words, a DC electric furnace is preferably operated as an arc furnace, with the upper electrode positioned at a distance from the solid to be melted. Alternatively, the upper electrode can be positioned in contact with the solid to be melted, in which case the solid is heated by the Joule effect. The principles of operation of both are well known to those skilled in the art.

[0012] When all solids have been substantially melted, according to the present invention, the upper electrode is brought into contact with the liquid slag and a polarity reversal is carried out so that the upper electrode acts as the anode and the lower electrode acts as the cathode before the liquid slag and the iron melt are tapped.

[0013] Contact between the upper electrode and the liquid slag and polarity reversal causes the liquid iron to react with Fe2O3+e - This means that the oxygen still bound to the unreduced parts of the iron-containing material transferred to the liquid slag during melting can be separated, so as to penetrate into the iron melt via →2Fe+3 / 2O2 and thus provide a liquid slag with a particularly low or substantially iron-free content, which can then be tapped. The iron oxide is essentially electrochemically deoxidized by applying a voltage according to the electrochemical series known to those skilled in the art.

[0014] Contact with the liquid slag can be understood not only as superficially positioning the upper electrode on the surface of the liquid slag, but also as immersion in the liquid slag. However, to prevent a direct short circuit between the anode and cathode, contact with the molten iron located below the liquid slag must be avoided. A short circuit caused by direct contact with the molten iron would result in a significant decrease in efficiency, so the immersion depth is limited to less than the thickness of the liquid slag.

[0015] The present invention takes advantage of the fact that, in this configuration, molten salt electrolysis is performed by reversing the polarity before tapping. The principle of molten salt electrolysis is described by way of example in EP 2609231. In comparison with the present invention, the teachings of this document are implemented with other parameters, namely, the production of carbon dioxide-free liquid metal using natural (unreduced) ore. This transient procedure has advantages over conventional methods for producing iron smelt and liquid slag, particularly a higher iron smelt yield with comparable or even reduced energy requirements. In addition, since there are no drawbacks to melting pre-reduced iron and only the unreduced iron oxide in the liquid slag is reduced, the overall iron production is significantly higher than in the case of exclusive production using molten salt electrolysis as described by way of example in EP 2609231. Compared to the molten salt electrolysis described by way of example, this also has the advantage of achieving significantly shorter process times.

[0016] According to one embodiment, the iron-containing material comprises or consists of iron sponge. The iron sponge obtained from the direct reduction process does not necessarily have to be fully reduced, and therefore the metallization rate of the iron sponge used may be between 20 and 95%. In particular, the metallization rate may be 90% or less, preferably 80% or less, preferably 70% or less. The determination of the metallization rate is known to those skilled in the art, and in particular for iron sponge (Fe 元素 / Fe 全体 )*100.

[0017] According to one embodiment, a carbonaceous mixture can be added. The additional carbonaceous mixture contributes to an increase in the carbon content of the iron melt. By adding the carbonaceous mixture, the carbon content of the iron melt can be increased to at least 2.50% by weight, in particular at least 3.00% by weight, preferably at least 3.50% by weight, and preferably at least 4.00% by weight. Setting the carbon content of the iron melt to a value above 5.5% by weight is technically unsuitable. The additional carbonaceous mixture can be added in solid, gaseous, and / or liquid form. This can be done in solid form, such as biocoke, coke, or charcoal; in gaseous form as a carbonaceous or hydrocarbonaceous gas, including foundry gases, especially coke oven gas, converter gas, and electric blast furnace top gas; or in liquid form, such as alcohols, hydrocarbons, or biofuels.

[0018] According to one embodiment, scrap can be further charged to improve or increase the recycling rate, for example, >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 charged per tonne of produced iron smelt.

[0019] According to one embodiment, a slag-forming agent can be added to form a liquid slag with a basicity B3 of 0.9 to 1.8. B3 can be, in particular, at least 1.0, preferably at least 1.1, and in particular at most 1.7, preferably at most 1.6. The basicity B3 corresponds to the ratio of (CaO + MgO) to (SiO2 + Al2O3), and the determination of these characteristics in solid-state slag is well known to those skilled in the art. The slag-forming agent contains at least one element from the group (CaO, Mg2O, SiO2, Al2O3). When using iron sponge obtained from iron ore, the iron ore preferably consists of oxidic iron (iron oxide) and gangue, the proportion of which can vary depending on the mining site, in addition to gangue resulting from direct reduction to metallic iron. If there is insufficient gangue, a slag-forming agent is added to set the basicity B3 to be achieved.

[0020] According to one embodiment, the energy required for melting is provided by renewable energies (solar, wind, hydro, biomass), which allows the DC electric furnace to operate more climate-neutral.

[0021] The DC electric furnace may preferably be an open slag bath furnace type furnace. These include electric reduction furnaces, in particular DC arc reduction furnaces (DC submerged arc furnaces), which operate using arc resistance heating by generating an arc between an electrode and solid and / or liquid slag or by heating the solid and / or slag by the Joule effect. Alternatively, melting furnaces using direct arc action may also be used, in particular DC arc melting furnaces (DC electric arc furnaces) or DC ladle furnaces (DC ladle furnaces).

[0022] The invention is explained in detail by the following examples in conjunction with the drawings. [Brief explanation of the drawings]

[0023] [Figure 1] The invention will be explained in a schematic side view using the example of a DC electric furnace having at least one upper electrode and at least one lower electrode. [Figure 2] The invention will be explained in a schematic side view using the example of a DC electric furnace having at least one upper electrode and at least one lower electrode. DETAILED DESCRIPTION OF THE INVENTION

[0024] 1 and 2 illustrate the present invention in a schematic side view using the example of a DC electric furnace (10) having at least one upper electrode (11) and at least one lower electrode (11.1). The DC electric furnace (10) comprises a vessel (15) and at least one lower electrode (11.1), preferably arranged as a base electrode in the base of the vessel (15). The DC electric furnace (10) may also comprise a lid (16) that can close the vessel (15) at its top, thereby allowing for the creation of a defined or controlled, preferably reducing, atmosphere within the DC electric furnace (10). The lid (16) may be essentially vertically movable, as seen by the double arrow. When the lid (16) is provided, a charging point (12) in the form of an opening in the lid (16) is provided with an appropriate supply line, by which the DC electric furnace (10) is charged. A DC electric furnace (10) is charged with solids (17) including iron-bearing material and slag-forming agents.

[0025] The iron-bearing material may include or consist of iron sponge, and the metallization rate of the iron sponge may be 50-95%. Additionally, scrap may be added as a separate iron-bearing material, e.g., from >0 kg to up to 200 kg of scrap per ton of produced iron smelt (1). The slag-forming agent may include at least one element from the group (CaO, MgO, SiO, AlO) and may be measured to form a liquid slag (2) with a basicity B3 of 0.9-1.8. If necessary, a carbonaceous mixture may be added to increase the carbon content of the iron smelt (1) to at least 2.50% by weight.

[0026] When the DC electric furnace (10) is sufficiently charged with solids (17), operation of the DC electric furnace (10) can be initiated, with at least one upper electrode (11) positioned above the solids (17). An arc is ignited between the solids (17) and the upper electrode (11) to gradually melt the solids (17) and produce a liquid slag (2) disposed above the iron melt (1) and iron melt (2). During the melting process, the upper electrode (11) functions as a cathode (-) and the lower electrode (11.1) functions as an anode (+). Solids (17) can be continuously added to the DC electric furnace (10) until the desired amount of iron melt (1) is achieved. The energy required for melting is preferably provided by renewable energy. When substantially all of the solids (17) have been melted, the liquid slag (2) still contains a portion of unreduced iron-containing material.

[0027] Simultaneously or at different times, the upper electrode (11) is brought into contact with the liquid slag (2) and polarity is reversed, with the upper electrode (11) acting as the anode (+) and the lower electrode (11.1) acting as the cathode (-). In the case of multiple upper electrodes (not shown here), it is also possible to alternately polarize them as anodes and cathodes. The polarity of the contacts is then reversed by immersing the upper electrode (11) in the liquid slag (2), preferably without immersing it in the iron melt (1), as shown by the double arrow. To improve the iron yield from the liquid slag (2), liquid electrolysis can also contribute to slag conditioning until the desired basicity B3 is achieved. When the requirements for the iron melt (1) and liquid slag (2) are met, the iron melt (1) generated in the vessel (15) and the liquid slag (2) disposed above the iron melt (1) are ready for tapping.

[0028] The liquid slag (2) is tapped, for example, via tapping point (13) and the iron melt (1) is tapped via tapping point (14) in a vessel (15).

[0029] Nozzles (not shown) may be arranged in the vessel 15, particularly in the base of the vessel 15, to affect the movement of the smelt 1. The DC electric furnace 10 may also be pivotally mounted to allow tilting and thereby tapping the liquid slag 2 in one direction and the smelt 1 in the other.

[0030] Likewise, it is not shown how the iron melt (1) is removed and fed to further processing steps. Preferably, the iron melt (1) is sent to processing in order to reduce the carbon in the iron melt (1) to a desired level. This is done, for example, with oxygen in the so-called oxygen-blowing process, more preferably in a converter. The tapped liquid melt (2) is also preferably sent to pelletization to produce slag, in particular for the construction industry.

[0031] It is also not shown how the resulting gases are vented and treated at the lid. Preferably, the gases and dust from each of the melting or electrolysis stages can be drawn off separately by suction, since the resulting waste gases contain oxygen.

[0032] The electrodes (top and bottom electrodes) are made of suitable materials, such as WC, Ti-Ir, Pt, C, graphite, or mixtures thereof. Mixtures can also be used as additives for Soderberg electrodes. Coated electrodes can also be used, such as ceramic rods coated or doped with the above-mentioned materials.

[0033] In particular, to generate an overvoltage, the voltage is preferably 3 V. Higher voltages increase the yield, but in turn increase electrode wear and reduce the associated factors.

[0034]

Claims

1. A method for operating a DC electric furnace (10) having at least one upper electrode (11) and at least one lower electrode (11.1) for producing an iron melt (1) and a liquid slag (2), comprising the steps of: - charging solids containing iron-bearing material and slag-forming agents into the electric furnace (10); - melting the solid to produce an iron melt (1) and a liquid slag (2) disposed on the iron melt (1), the upper electrode (11) acting as a cathode and the lower electrode (11.1) acting as an anode while the solid is melted; - tapping the liquid slag (2) and the iron melt (1); In a method comprising: Before the liquid slag (2) and the iron melt (1) are tapped, the upper electrode (11) is brought into contact with the liquid slag (2) and a polarity reversal is carried out, so that the upper electrode (11) acts as an anode and the lower electrode (11.1) acts as a cathode. method.

2. 2. The method of claim 1, wherein the iron-containing material comprises or consists of sponge iron, and the metallization rate of the sponge iron can be from 20 to 95%.

3. The method according to claim 1 or 2, further comprising the step of charging a carbonaceous mixture.

4. 4. The method according to claim 1, wherein scrap is further charged.

5. 5. The method according to claim 1, wherein the slag-forming agent is charged to form a basicity B3 in the liquid slag of 0.9 to 1.

8.

6. 6. The method according to any one of claims 1 to 5, wherein the energy required for melting is provided from renewable energy sources.

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