Method of treatment of sponge iron and sponge iron

Abstract

The invention relates to a method for treating sponge iron, wherein, following the production of sponge iron, the sponge iron is coated with a coating to protect same against reoxidation and / or corrosion, the sponge iron as a core being brought into contact with a carbon-based liquid that forms a coating on the core.

Description

[0001] The invention relates to a method for treating iron sponge and sponge iron, wherein, following the production of the sponge iron, the sponge iron is coated with a coating to protect against reoxidation and / or corrosion.

[0002] To prevent reoxidation of sponge iron produced from iron ore by direct reduction, it is known to coat the sponge iron with at least one coating after production; see, for example, DE 28 29 924 A1, EP 0 041 490 A1, EP 0 042 236 A1, and GB 2 003 057 A1. These documents essentially deal with a lime-based coating applied to the sponge iron. Further prior art references are US 4 376 139 A, US 3 556 838 A, US 2002 / 178864 A1, US 3 628 986 A, US 4 329 168 A, GB 1 471 599 A, and US 3 833 343 A.

[0003] Further methods for treating iron ore or sponge iron or other iron carriers are known from the documents US 4 376 139 A, US 3 556 836 A, US 2002 / 178864 A1, US 3 628 986 A, US 4 329 168 A, GB 1 471 599 A and US 3 833 343 A.

[0004] Although lime-based coatings show good results in the so-called RUL (Refractoriness Under Load) test, they require a comparatively long time to harden after application. Furthermore, these coatings are particularly sensitive to impact and can crumble quickly under unfavorable dynamic loads.

[0005] The object of the present invention is to provide a method for treating sponge iron and a sponge iron coated with a reoxidation and / or corrosion-resistant coating, which can substantially eliminate the aforementioned disadvantages.

[0006] According to a first teaching, this problem is solved by a method having the features of claim 1.

[0007] The invention relates to a method for treating sponge iron, wherein, following the production of sponge iron, the sponge iron is coated with a coating to protect against reoxidation and / or corrosion, wherein the sponge iron as a core is brought into contact with a carbon-based liquid which forms a coating on the core.

[0008] According to the invention, care should be taken to ensure that a substantially closed coating can form on the core, so that the core is essentially completely surrounded by the liquid, or rather, that the coating completely encloses the core. This ensures that the core (iron sponge), which comprises metallic iron, has no contact with the environment and therefore does not reoxidize, or at least not rapidly.

[0009] The use of a carbon-based coating offers no disadvantages when using coated sponge iron, neither in electrically powered smelters nor in blast furnaces. On the contrary, it can increase the carbon content of both the pig iron produced from the sponge iron and the slag, thereby positively impacting downstream processing stages of both the pig iron and the slag.

[0010] According to one embodiment of the process, carbon or graphite is used as the carbon base, which is dispersed and / or dissolved in the liquid before application. For example, a carbon base in particle form is used with a particle size > 0 mm and up to 2 mm, in particular up to 1.5 mm, preferably up to 1 mm, so that it can be easily dispersed and / or dissolved in the liquid, for example by stirring.

[0011] The coating can be applied by soaking / diving, spraying, brushing, or pouring the liquid over the iron sponge. After application, excess liquid can be allowed to drain off, for example, by passing and / or holding the iron sponge over a sieve. This excess liquid is then collected, circulated, and returned to the application process, thus conserving resources. Alternatively, a rolling drum or similar process can be used, in which the iron sponge is coated in a rotating drum or plate.

[0012] According to one embodiment of the process, an aqueous solution is used as the liquid. In particular, distilled water or demineralized water is used. If necessary, "normal" water, for example tap water, can also be used. In particular, surfactants and / or suspension aids can also be added to the aqueous solution, especially to reduce the surface tension and / or to promote the formation of a preferably well-balanced suspension.

[0013] According to an alternative embodiment of the process, an aqueous-alcoholic solution is used as the liquid. Alcohol is particularly well-suited as a solvent because it can dissolve, among other things, hydrophobic, and therefore water-insoluble, substances. Thus, isopropanol or ethanol, for example, are suitable solvents. Other aqueous-alcoholic solutions not mentioned here are also applicable.

[0014] Alternatively, an aqueous, organic, and especially non-alcoholic solution, such as polyacrylic acid, can also be used as the liquid.

[0015] According to one embodiment of the process, the iron sponge coated with the liquid can be dried after application, in particular actively by supplying heat. This means that either the iron sponge coated with the liquid is at a temperature above ambient temperature up to, for example, a maximum of 150°C, in particular a maximum of 120°C, preferably a maximum of 90°C, or the iron sponge is at a temperature above ambient temperature up to, for example, a maximum of 150°C, in particular a maximum of 120°C, preferably a maximum of 90°C, before application, in order to drive off the volatile components as quickly as possible and to achieve rapid hardening of the coating.

[0016] According to a second teaching, this problem is solved with the features of claim 6.

[0017] The invention further relates to an iron sponge coated against reoxidation and / or corrosion, comprising a core and a coating, wherein the iron sponge comprises metallic iron as a core and the coating comprises carbon.

[0018] After direct reduction, sponge iron contains metallic iron as its main component, thus comprising more than 70%, particularly more than 80%, preferably more than 90% of its total microstructure, and therefore depends on the composition of the original iron ore used. Sponge iron with less than 70% metallic iron in its microstructure cannot be economically utilized. Depending on the original composition of the iron ore, other components may include silicon dioxide (SiO₂), also known as quartz, and, depending on the degree of reduction, wüstite (FeO), hematite (Fe₂O₃), magnetite (Fe₃O₄), and / or, depending on the composition of the reducing gas during direct reduction, cementite (Fe₃C). Therefore, the components in the microstructure of the sponge iron or core can comprise up to 30%, particularly up to 20%, preferably up to 10%, in addition to unavoidable impurities.Impurities may include, for example, traces of titanite (CaSiTiO 5 ), albite (NaAlSi 3 O 8 ) and / or mixed crystals between diopside (CaMgSi 2 O 6 ) and hedenbergite (CaFe 2+< Si 2 O 6 ).

[0019] According to the invention, the coating comprises carbon as its main component, comprising at least 90%, in particular at least 95%, and preferably at least 98% of the coating composition. Further components may include silicon dioxide (SiO₂), a glassy phase (Si-Fe-Ca-Mg-Al-Na), potassium feldspar (Kaisi₃O₈), wüstite (FeO), hematite (Fe₂O₃), and / or magnetite (Fe₃O₄), one or more of the aforementioned components, such that the coating may contain up to 10%, in particular up to 5%, and preferably up to 2%, of these components, along with unavoidable impurities. Impurities may include, for example, the aforementioned components, individually or in combination, if present in trace amounts, for example, less than 0.1%. Furthermore, lime (CaCO₃), magnesium oxide (MgO), and / or alkalis may also be present as impurities.

[0020] The components of the mixture mentioned in this text can be determined using light microscopy, scanning electron microscopy, and electron back-scattered diffraction (EBSD). To determine the microstructural constituents, samples of the coated iron sponge can be taken, prepared as (cross-)sections, and etched with alcoholic nitric acid containing 3% by volume (also known as "Nital") or sodium disulfite. The respective proportions of the microstructural constituents can then be determined using light or scanning electron microscopy in the usual manner by area analysis.

[0021] According to one embodiment of the coated iron sponge, the coating can include silicon dioxide as a component in addition to carbon, for example > 0% to a maximum of 10%, in particular to a maximum of 5%, preferably to a maximum of 2%.

[0022] According to the invention, a transition area is formed between the core and the coating, which comprises at least one of the components silicon dioxide (SiO 2 ), glass phase (Si-Fe-Ca-Mg-Al-Na ), wüstite (FeO ), hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ).

[0023] The transition region can essentially be reduced to the area containing less than 40% metallic iron, particularly less than 30%, preferably less than 20%, and less than 40% carbon, particularly less than 30%, preferably less than 20%. The remainder of the microstructure in the transition region thus contains one or more of the aforementioned constituents along with impurities. The thickness, or radial thickness, can vary and is therefore not essentially uniform with a constant thickness when considered around the circumference.

[0024] The invention is explained in more detail with reference to the following exemplary embodiments in conjunction with the Figur 1 . This shows Figur 1 An evaluation of a RUL test on four different iron sponge samples.

[0025] Iron sponge in pellet form, originating from a direct reduction plant and with a particle size of 10 to 12.5 mm, was provided. Four different pellet samples, each weighing one kilogram, were used for the subsequent analysis.The first pellet samples (1) were used as uncoated reference samples; the second pellet samples (2) were coated with a lime-based coating, prepared prior to application by mixing lime milk comprising quicklime / whitewash / CaO with water in a ratio of 150–350 grams of CaO per liter of H₂O; the third pellet samples (3) were coated with a carbon-aqueous-alcoholic coating, available as a product from James Durran under the designation RSB 5302-01, which is based on a mixture of carbon sizing (graphite) with isopropanol; and finally, the fourth pellet samples (4) were coated with a carbon-aqueous coating, available as a product from James Durran under the designation RWB 5802-01, which is based on a mixture of carbon sizing (graphite) with distilled water.

[0026] The pellet samples (2) - (4) were each immersed in a previously described liquid and then transferred to a sieve, allowing excess liquid to drain off. Pellet samples (2) - (4) were not actively heated, so the liquid on the sponge iron pellets hardened naturally under ambient conditions, forming a coating. Pellet samples (3) and (4) were dry within minutes. For pellet sample (2), this process took a very long time (> 24 h). It is possible that actively applying heat could have accelerated the drying / hardening process. During the drying of pellet sample (2), contact surfaces formed due to the agglomeration of individual, interconnected coated pellets, and the lime milk coating crumbled slightly. This phenomenon also occurred occasionally during the drying of pellet sample (3). Furthermore, the coating occasionally exhibited irregularity.No abnormalities were observed during the processing of the pellet samples (4). All pellet samples (2) - (4) exhibited a high "absorbent" property, resulting in a comparatively high volume of the respective coating solution required. At least the aqueous, water-based solutions could be diluted further.

[0027] The four different pellet samples (1) - (4) were subjected to a so-called RUL test. A gas-fed furnace was used as the apparatus for this test, in which the atmosphere and temperature could be varied over time. Phases 1 to 4, listed in Table 1, were carried out with the corresponding parameters (temperature, heating rate, gas composition, and flow rate totaling 83 l / min in all phases), so that the results shown in Table 1 were obtained. Figur 1 The evaluation shown was achieved. Table 1 Parameter T von [°C] bis [°C] T-Rate [°C / min] T-Dauer [min] % CO2 % CO N2 [I / min] CO2 [I / min] CO [l / min] Phase 1 25 625 2 300 0,5 0,5 50 16,5 16,5 Phase 2 625 725 1,3 77 0,46 0,54 50 15,18 17,82 Phase 3 725 815 2 45 0,44 0,56 50 14,52 18,48 Phase 4 815 910 2 48 0,29 0,61 50 12,87 20,13

[0028] In the temperature range between 25 and 350 °C, pellet samples (1) - (3) behaved almost identically; only pellet sample (4) experienced a significant weight loss up to approximately 350 °C. It was actually expected that pellet sample (2), due to its aqueous solution, would also show a comparable result to pellet sample (4). The reason why, surprisingly, no weight loss occurred in pellet sample (2) within the aforementioned temperature range is explained by the formation of calcium hydroxide.

[0029] In the temperature range between 450 and 650 °C, all pellet samples show an identical trend and increase in weight. Due to the formation of calcium carbonate, the highest weight increase in this temperature range is observed in pellet samples (2).

[0030] In the temperature range between 600 and 710 °C, a significant weight reduction can again be observed in all pellet samples due to a further reduction of the iron sponge and thus of the core of the coated iron sponge.

[0031] A weight increase, which serves as an indicator of reoxidation, is observed in pellet samples (1) and (2) at approximately 720 °C, in (4) at approximately 740 °C, and in (3) only at approximately 820 °C. Overall, pellet samples (3) and (4) thus show a higher effectiveness of the passivation protection compared to the other pellet samples (1) and (2), with the reoxidation of pellet sample (3) being significantly delayed.

Claims

1. A process for treating sponge iron, wherein following to sponge iron production the sponge iron is coated with a coating against reoxidation and / or corrosion, wherein the sponge iron is brought into contact as a core with a carbon-based liquid, wherein the liquid substantially completely surrounds the core and forms a substantially closed coating on the core, wherein the coating comprises carbon as the main component with at least 90% in the composition of the coating and characterized in that a transition region is formed between the core and the coating which comprises at least one of the components silicon dioxide, glass phase, wüstite, hematite and magnetite, wherein the transition region can be reduced to the region which has a proportion of metallic iron of less than 40% and a proportion of carbon of less than 40%.

2. The method according to claim 1, wherein carbon or graphite is used as the carbon base, which is dispersed and / or dissolved in the liquid before application.

3. The method according to claim 1 or 2, wherein an aqueous solution is used as the liquid.

4. The method according to claim 1 or 2, wherein an aqueous-alcoholic solution is used as the liquid.

5. Method according to one of claims 1 to 4, wherein, after application, the iron sponge coated with the liquid is dried, in particular actively with the addition of heat.

6. A sponge iron coated against reoxidation and / or corrosion with a core and a coating, wherein the sponge iron comprises metallic iron as core, wherein the coating comprises carbon as main component with at least 90 % in the composition of the coating and completely envelops the core, characterized in that a transition region is formed between the core and the coating, which comprises at least one of the components silicon dioxide, glass phase, wüstite, hematite and magnetite, wherein the transition region can be reduced to the region which has a proportion of metallic iron of less than 40% and a proportion of carbon of less than 40%.

7. The coated sponge iron according to claim 6, wherein the coating comprises silicon dioxide.

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