Method for the environmentally friendly production of steel and foundry iron

Abstract

The invention relates to a method for producing steel and foundry iron, wherein zinc-containing scrap is dezincified with an acidic solution and a zinc-containing solution is obtained, and the resulting zinc-free or low-zinc scrap is used to produce steel and foundry iron and the hydrogen formed during the dezincification is used in the steel works or in other industrial processes.

Description

[0001] Processes for the environmentally friendly production of steel and foundry iron

[0002] background

[0003] The European steel industry emits 4% of total CO2 emissions in the EU. It is under pressure from the public, industry, and legislators to become carbon neutral by 2050, which is in line with EU targets. About 60% of European steel is produced via the so-called primary route, an efficient but highly carbon-intensive production method.

[0004] The development and implementation of new technologies is well underway. Direct H2 reduction in shaft furnaces is ready for use and can be gradually introduced into brownfield plants. This ensures operational continuity and reduced emissions during the transition from conventional steelmaking.

[0005] Galvanizing involves coating steel with a thin layer of zinc to improve corrosion resistance. The zinc coating is a thin layer of metallic zinc, which may also contain some dopants, such as Al, Mg, Sn, Ni, Co, or Fe. Zinc plating can be done by electrolytic deposition (electrogalvanizing) or by immersing the steel body in a liquid of molten zinc. Since galvanizing generally improves steel properties, for example, with regard to corrosion resistance, galvanized steel with a zinc layer of 3 to 40 kg per ton of steel is a common industrial product.

[0006] Galvanized steel scrap therefore consists of Fe substrate coated with a zinc layer.

[0007] In general, the term "galvanized steel" or "galvanized steel scrap" used here should be understood in a broad sense: "Galvanized steel" or "galvanized steel scrap" includes all steel sheets or substrates covered with a zinc layer. The zinc layer may be applied by galvanic or electrolytic deposition, or the zinc layer may be applied by hot-dip dipping the steel substrate into molten zinc, by zinc vapor deposition, or by any other method for coating a steel substrate with a zinc layer.

[0008] Furthermore, the term "galvanized steel" or "galvanized steel scrap," as used here, includes any material resulting from subsequent treatment, such as subsequent annealing of the galvanized steel substrate for improved adhesion or performance of the zinc layer. However, the Zn coating is unfavorable for steel recycling because the Zn coating evaporates at process temperatures, resulting in significant amounts of Zn in the flue gas. The resulting material, electric furnace dust (EAFD), requires complex, particularly energy-intensive, processing. Some of the EAFD is even landfilled.

[0009] The use of the metal as cooling scrap in converters is also not possible in all plants because the filter systems are damaged and fail due to excessive zinc content.

[0010] Scrap with a Zn coating (galvanized steel scrap or galvanized scrap) therefore has a lower value compared to Zn-free scrap, and there are many attempts in technology to dezincify the galvanized steel scrap.

[0011] There are various options for the depletion of Zn from galvanized steel scrap:

[0012] • Pyrometallurgical

[0013] • Hydrometallurgical with alkaline leaching

[0014] • Hydrometallurgical with acid leaching

[0015] The process previously used to treat zinc-containing dusts (e.g., EAFD) is the Waelz process, a metallurgical process in a rotary kiln. The Waelz process is currently used primarily for the processing of zinc-containing residues (mainly zinc-containing steel mill dust). The main product of the process is Waelz oxide, which consists essentially of zinc oxide (ZnO) and is further used to extract zinc.

[0016] However, one critical parameter of this process is its high energy consumption. This can be easily explained by the composition of the rolling slag, which contains a high proportion of residual coke and metallic iron. Zinc and lead contents are typically up to 2% each. Since landfilling the slag is associated with high costs and may soon be prohibited, attempts are being made to sell it as a construction material (e.g., for road construction). The elutable components should be as low as possible and therefore play a central role in the slag.

[0017] The exhaust gas purification systems of Wälz plants must meet significantly higher standards than those of the rest of the metallurgical industry. This is because the actual product, namely Wälz oxide, is also removed along with the exhaust gas.

[0018] In practice, a zinc content of around 20% in the forerun represents the economic lower limit in most cases. A zinc content of around 50% represents the upper limit.

[0019] If steel production is to be CO2-free in the medium term, the Waelz process no longer has a future due to its high energy requirements in the form of fossil fuels and reducing agents. Both the slag and filter dust are highly polluting.

[0020] Direct dezincification of steel is not possible with the rolling tube process.

[0021] Subject of the invention The subject of the invention is an environmentally friendly process for the production of steel and foundry iron, wherein zinc-containing scrap is dezincified with an acidic solution, the zinc-free or low-zinc scrap obtained is used for the production of steel and foundry iron, and wherein the zinc-containing solution obtained can be further processed to metallic zinc or zinc compounds and additionally the hydrogen produced during dezincification is utilized in the steelworks.

[0022] The process is particularly environmentally friendly. Firstly, the resulting zinc-free scrap can be used in steel production without generating zinc-containing dust, which would then need to be separated and processed. Secondly, the zinc can be fully recycled, unlike the rolling process, where significant amounts of zinc end up in the rolling slag. Finally, the resulting green hydrogen can be used in steel production or other industrial processes.

[0023] This creates a closed and virtually loss-free material cycle for both galvanized scrap and zinc.

[0024] The hydrogen obtained can be used, for example, for the direct reduction of iron oxides. This way, less hydrogen needs to be produced or sourced elsewhere.

[0025] A particular advantage is that the hydrogen can be used directly at the point of production and does not have to be transported to other locations where it is needed.

[0026] The process is also particularly environmentally friendly because the hydrogen produced during dezincification is green hydrogen and is produced without the use of energy.

[0027] In contrast, grey hydrogen, which is produced from fossil energy sources, currently dominates the conversion of the steel industry.

[0028] It is still possible to use the hydrogen produced during the dezincification of scrap in thermal processes, thus fully or partially replacing natural gas. It is also expedient to use the hydrogen produced during the dezincification of scrap in other industrial processes. Thus, the thermal use of green hydrogen also represents a realistic alternative to fossil fuels in the field of industrial heat generation.

[0029] One possible embodiment of the invention consists in not only using the zinc and the iron that is inevitably dissolved with it to generate hydrogen, but also in deliberately dissolving a larger proportion of the scrap in the acid, even to the point of completely dissolving the entire scrap. In this way, hydrogen production is significantly increased at the expense of usable scrap. This can be advantageous if the hydrogen is (temporarily) more urgently needed than the zinc-free scrap, or if the costs of another type of hydrogen production are temporarily particularly high (e.g. if electricity from wind or solar generation is temporarily particularly scarce). Even if the value of the resulting iron sulfate is temporarily or locally particularly high, dissolving the scrap can be economically viable. Or if waste acid, e.g. from pickling steel, is available at low cost or expensive disposal can be avoided.Or if the scrap is a mixture of normal steel and stainless steel or other metals and the dissolution of the normal steel scrap allows for enrichment or selection of stainless steel or other metals.

[0030] Waste prevention is considered the most important goal of the circular economy, even before recycling, conserving resources and protecting people and the environment.

[0031] The acidic solution used to dezincify the zinc-containing scrap contains H2SO4 and H2O, with the H2SO4 concentration being between 5 and 500 g / l, preferably between 50 and 250 g / l. The H2SO4 concentration is particularly preferably between 100 and 200 g / l.

[0032] The zinc-containing solution obtained by leaching Zn-coated steel can be used for the electrolytic production of zinc.

[0033] The Zn content of the steel before dezincification is typically 0.3 to 3 wt.%. This determination is performed by dissolving the entire steel sample in acid and determining the Zn and Fe content in the resulting solution using ICP. The Zn / Fe mass ratio before dezincification is preferably 0.3 to 3 wt.%.

[0034] During dezincification, >90%, preferably more than 98%, and particularly preferably more than 99% of the zinc is removed from the scrap surface. The zinc content of the steel surface after dezincification, determined by XRF (X-ray fluorescence spectroscopy), is preferably <1 wt.%, preferably <0.5 wt.%, and particularly preferably <0.2 wt.%.

[0035] It may be advantageous to subject the zinc-containing solution obtained during the dezincification of scrap to a separation step for Zn2+ and Fe2+, preferably solvent extraction or ZnS precipitation.

[0036] However, it is also possible to use the zinc-containing solution obtained from the dezincification of scrap as a Zn-Fe fertilizer or to produce a Zn-Fe fertilizer, preferably for foliar application. This avoids the process of separating Zn and Fe.

[0037] The low-Zn scrap obtained according to the invention is preferably used as a feedstock for the production of foundry iron, preferably in induction furnaces, or in steelworks as low-CO2 scrap in arc furnaces. It can also be used as cooling scrap in converters and to adjust the casting temperature in the ladle. To increase hydrogen production, other zinc-containing materials, in addition to zinc-coated steel, can also be reacted with the acidic solution used to dezincify the scrap, for example, zinc slag, drilling and grinding residues from zinc processing, drilling and grinding residues from brass processing, residues or defective batches from the galvanization of sheet steel, or other materials containing a proportion of metallic zinc.

[0038] Another possibility for producing additional hydrogen is the conversion of materials that contain iron in metallic form and preferably in finely divided form.

[0039] The galvanized steel scrap can be crushed or pressed into easily handled packages, e.g., by immersing a package in the leaching liquid (i.e., the acidic solution) and removing it again after the Zn leaching step. Another possibility is to fill a container or column with the galvanized steel scrap and expose it to the acidic solution continuously or intermittently. Another possibility is to place the galvanized steel scrap in a basket and immerse this basket in the leaching liquid, i.e., the acidic solution. Yet another possibility is to crush the galvanized steel scrap and transport this material on a treadmill or conveyor, passing it through an area where it is sprayed with the acidic solution and then through a water spray area.

[0040] After completion of Zn leaching, the zinc-depleted metallic Fe substrate can be washed with dilute acid and / or water to remove adhering solution containing Zn and anions such as sulfate or chloride.

[0041] The dezincified scrap can be used as a high-quality, purified raw material, for example, in foundries with induction furnaces or in steelworks as low-CO2 scrap in electric arc furnaces. It can also be used as cooling scrap in converters and to adjust the pouring temperature in the ladle. A major advantage in all applications is that, thanks to its dezincification treatment, zinc contamination cannot occur. This prevents the re-creation of waste-like substances, especially dust. Zinc exposure in the work environment is also eliminated for humans and the environment.

[0042] The separation of dissolved Zn2+ and Fe2+ in the acidic solution used for dezincification can be achieved by precipitating ZnS by introducing H2S. A preferred option is:

[0043] • Precipitation of the obtained Zn2+ in the solution as ZnS

[0044] • Separation of the ZnS from the solution containing dissolved Fe2+ • Crystallization of the Fe compound by evaporation of water and / or lowering the temperature

[0045] • Separation of the crystals of the Fe compound and return of the resulting liquid phase to one of the previous process steps

[0046] A preferred embodiment is the reaction of the precipitated ZnS with H2SO4, resulting in a ZnSO4 solution and gaseous H2S, and the recycling of the resulting H2S to precipitate ZnS. Preferably, the moist ZnS filter cake is reacted with dilute H2SO4. To shift the equilibrium in the desired direction, it may be necessary to remove residual H2S with N2 or air. Or, the H2S can be reacted with NH4OH or NaOH to obtain Na2S or (NH4)2S.

[0047] However, ZnS may be the preferred material when long-distance transportation to a zinc production site is required.

[0048] H2S can be produced from H2 and S or as a byproduct of coke production. H2S can also be recovered from spent H2S absorbers, e.g., EAFD as an absorber for H2S (Energy Fuels 2022, 36, 3695-3703).

[0049] Another option for separating dissolved Zn2+ and Fe2+ is solvent or membrane extraction. Preferably, Zn2+ is transferred into an organic phase, most preferably D2EHPA, di-(2-ethylhexyl)phosphoric acid. D2EHPA is an inexpensive and widely used reagent.

[0050] In summary, the method according to the invention offers some significant advantages:

[0051] • Significant reduction in the zinc input into the electric furnace, eliminating the need for the costly dezincification of the resulting filter dust

[0052] • Production of green hydrogen without energy consumption and without CO2

[0053] • Hydrogen is produced at the site of steel production; therefore, no complex logistics are required

[0054] • High temporal flexibility: The production of hydrogen can be shifted to times when production from electricity is particularly expensive

[0055] • High technical flexibility: Hydrogen production can be increased at the expense of dezincified scrap by partially dissolving it when the demand for hydrogen is (temporarily) particularly high or when the demand or price for iron sulfate is (temporarily) particularly high

[0056] • The recovered zinc can be reused, creating a closed loop and conserving resources • Increased use of dezincified sheet metal in foundry induction furnaces: Previously only very rarely possible due to the heavy smoke generated during melting by zinc evaporation.

[0057] • Steel production eliminates the possibility of large quantities of waste-like substances, especially dust, being generated. Zinc contamination in the work environment is also eliminated for humans and the environment.

[0058] • The process is particularly environmentally friendly

[0059] • It is also possible to use the hydrogen produced during the dezincification of scrap in thermal processes and thus replace natural gas in whole or in part.

[0060] Waste prevention is considered the most important goal of the circular economy, even ahead of recycling. It conserves resources and protects people and the environment. This process is designed for this purpose, among others.

[0061] Example: Dezincification of galvanized steel scrap and use of the dezincified scrap and hydrogen to produce steel

[0062] 10 g of galvanized steel scrap containing 0.89% Zn (determined by dissolving a sample in acid and determining the Zn and Fe contents in the solution using inductively coupled plasma (ICP) analysis) is treated with an acidic solution consisting of 150 g / l H2SO4 (balance: water) for 10, 20, and 30 s. After specified times, the steel scrap is removed from the acidic solution, washed with deionized water, and dried prior to XRF characterization.

[0063] Zinc dissolution begins immediately with significant evolution of hydrogen. After 30 seconds, zinc dissolution is complete, and hydrogen evolution is very weak.

[0064] Figure 1 shows the surface zinc concentration determined by XRF as a function of treatment time. After 30 s, the dissolution or detachment of zinc from the surface of the steel scrap is complete.

[0065] The value of 45.9% Zn for the untreated steel means that the (calculated) penetration depth of the X-ray beam during XRF is such that both the surface zinc coating and a portion of the underlying steel are included in the measurement result. The value of 0.1% after 30 s means that virtually all of the zinc has been dissolved out and removed in the near-surface area. The amount of hydrogen generated after 30 s is 33 ml (20°C, atmospheric pressure).

[0066] The resulting dezincified scrap can be used in an electric arc furnace to produce steel. The hydrogen generated during the dezincification of the scrap can be used for the direct reduction of iron oxides.

[0067] The resulting zinc-containing solution can be processed into metallic zinc or zinc compounds, thus quantitatively reintroducing the zinc into the material cycle.

Claims

Claims 1. A process for the production of steel and foundry iron, wherein a) zinc-containing scrap is dezincified with an acidic solution, b) the zinc-free or low-zinc scrap obtained is used for the production of steel and foundry iron, c) the hydrogen produced in step a) is used in the steelworks or in other industrial processes, d) and in step a) a zinc-containing solution is obtained.

2. A process according to claim 1, characterized in that the acidic solution used for dezincification of zinc-containing scrap in step a) contains H2SO4 and H2O, the H2SO4 concentration being between 5 and 500 g / l, preferably between 50 and 250 g / l, most preferably between 100 and 200 g / l.

3. A process according to any one of claims 1 to 2, characterized in that the acidic solution obtained during the leaching of Zn-coated steel in step 1 is used for the electrolytic extraction of zinc or for the production of zinc compounds or for a further leaching step.

4. Process according to one of claims 1 to 3, characterized in that the Zn / Fe mass ratio determined after dissolving the steel in acid and determining Zn and Fe by ICP before dezincification is 0.3 to 3 wt.%.

5. Method according to one of claims 1 to 4, characterized in that the Zn content of the steel surface after dezincification, determined by means of XRF, is <1 wt.%, preferably <0.5 wt.%, particularly preferably <0.2 wt.%.

6. Process according to one of claims 1 to 5, characterized in that the zinc-containing solution obtained during the dezincification of scrap is subjected to a separation step for Zn2+ and Fe2+, preferably solvent extraction or ZnS precipitation.

7. Process according to one of claims 1 to 5, characterized in that the zinc-containing solution obtained during the dezincification of scrap is subjected to a separation step for Zn2+ and Fe2+, preferably solvent extraction or ZnS precipitation.

8. Process according to one of claims 1 to 5, characterized in that the solution obtained from the leaching step is used as a Zn-Fe fertilizer or for producing a Zn-Fe fertilizer, preferably for foliar fertilization.

9. Process according to one or more of claims 1 to 7, characterized in that the hydrogen produced during the dezincification of scrap is used for the direct reduction of iron oxides.

10. Process according to one or more of claims 1 to 8, characterized in that the hydrogen produced during the dezincification of scrap completely or partially replaces the natural gas in thermal processes.

11. Process according to one or more of claims 1 to 9, characterized in that the Zn-poor scrap thus obtained is used as a feedstock for the production of foundry iron, preferably in induction furnaces or in steelworks, as CO2-poor scrap in arc furnaces or as cooling scrap in the converter or for adjusting the casting temperature in the ladle.

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