Background

Galvanization is the process of coating steel with a thin zinc layer for improved corrosion resistance. The Zn coating is a thin layer of metallic zinc that may also include some dopants, e.g. Al, Sn, Ni, Co, Fe. Galvanization can be performed by electrolytic deposition (electrogalvanization) or by dipping the steel body into a liquid of molten zinc. Since galvanization generally improves the steel properties, for example with respect to corrosion resistance, galvanized steel, with a zinc layer making up 3 to up to 40 kg per ton of steel, is a common industrial product.

Generally, the term galvanized steel or galvanized steel scrap as used herein is to be understood in a broad sense: Galvanized steel or galvanized steel scrap includes any sheets or substrates of steel that are covered by a zinc layer. The zinc layer may be applied by galvanic or electrolytic deposition, or the zinc layer may be applied by hot dipping of the steel substrate into molten zinc, or by zinc vapor deposition or by any other method of coating a steel substrate with a zinc layer.

Additionally, the term galvanized steel or galvanized steel scrap as used herein includes any material resulting from a subsequent treatment, e.g., a subsequent annealing of the zinc coated steel substrate for better adhesion or performance of the zinc layer.

However, for the recycling of steel, the Zn coating is undesired because the Zn coating evaporates at higher temperatures resulting in significant amounts of Zn in the flue gas. Scrap comprising Zn coating (galvanized steel scrap or galvanized scrap) has thus a lower value compared to Zn-free scrap, and there exist many attempts in the art to de-zinc the galvanized steel scrap.

There are different options for depletion of Zn from the galvanized steel scrap:

• Pyrometallurgical

• Hydrometallurgical with alkaline leaching

• Hydrometallurgical with acidic leaching

While the performance of acidic leaching is generally better than the one of alkaline leaching, it entails a significant disadvantage, namely that the acidic solution not only leaches Zn, but also Fe. Consequently, the resulting material is not suitable forelectrowinning (pure) Zn metal, unless an additional separation step to purify Zn is done. Such an additional separation step is required even if means for minimizing Fe leaching are implemented, e.g. by monitoring the electric potential by placing a reference electrode in the leach solution, measuring the corrosion potential and stopping the reaction when the measured potential arrives at a plateau, as taught in US2015/0259766.

Consequently, acidic de-zincing of galvanized steel scrap for electrowinning of metallic zinc is considered to be economically unfavorable in the art (see, for example, DEZINCING OF GALVANIZED STEEL by J. Grogan, Colorado Scholl of Mines: https://pdfcoffee.com/qdownload/dezincinc-of-galvanized-stee l-pdf-pdf-free.html, p. 75, lines 15-16).

WO 2023/089234 relates to a process for recovering zinc and iron products from zinc- containing waste materials.

Summary of the invention

In view of this prior art, the present invention addresses the need to provide an efficient and economic process for recycling Zn-coated Fe such as galvanized steel scrap.

The inventor found that the use of an acidic solution comprising Fe ³⁺ for leaching combines technical and economic advantages in a way which, surprisingly, overcomes the previously known drawbacks of acidic leaching: Fe ³⁺ is not only an oxidation agent resulting in an acceleration of Zn leaching, but also generates Fe^ ⁺ in the leaching step from an otherwise useless waste material, making the separation step suitable for the production of Fe^ ⁺ salt and thus industrially feasible.

The present invention thus provides a process for depleting metallic Zn from Zn-coated Fe (such as zinc-coated scrap or galvanized steel scrap, respectively), comprising the following steps: a) Subjecting the Zn-coated Fe to a leaching step in an acidic solution comprising Fe ³⁺ to provide an acidic solution containing Zn^ ⁺ and Fe^ ⁺ ; b) Obtaining a metallic Fe substrate depleted from zinc; and c) Separating Fe^ ⁺ from Zn^ ⁺ to provide a Zn2 ⁺ -containing product and a Fe^ ⁺ salt.

The acidic solution preferably contains Fe ³⁺ in a concentration of 1.5 to 60 g Fe ³⁺ per kg of solution at the initiation of the leaching step. Such an acidic solution may for example be obtained from treatment of materials comprising ferric Fe with acid. In a preferred embodiment, the acidic solution used in step a) is obtained by subjecting electric arc furnace dust (EAFD) to an acid treatment. Moreover, during leaching there is preferably an excess of Fe^ ⁺ with respect to metallic Zn with an atomic ratio Fe^ ⁺ /Zn^ >2.

Step c) provides two different products, a Zn2 ⁺ -containing product and a Fe2 ⁺ -salt.

The Zn2+-containing product obtained in step c) is ZnS obtained from precipitation and is used after conversion into ZnSC>4 to producing metallic Zn by electrowinning. Alternatively, the Zn2+-containing product is a ZnSC>4 or ZnCl2 solution that is obtained by solvent extraction. The Fe^ ⁺ salt is preferably FeCI ₂ or FeSO ₄ , which, after optional purification, can be sold for wastewater treatment, as chromate reducing agent for cement or as animal food additive.

Brief description of the figure

Figure 1 provides a bar chart depicting residual Zn on a steel surface as obtained in Example

4.

Figure 2 provides a bar chart depicting residual Zn on a steel surface as obtained in Example

5.

Detailed description

The acidic solution and the leaching step

The acidic solution can be obtained by reacting waste products with fresh or waste acids and is thus preferably a waste-derived solution. The waste products or waste acids may be any waste H2SO4 (or spent H2SO4 or thin H2SO4), e.g. from TiC>2 production using the sulfate process, waste acidic solutions from manufacturing of zinc, any waste HCI, steel pickling solutions comprising FeCl2 or FeSO4, waste metal chlorides from TiC>2 chlorination of TiC>2 manufacturing using the chloride process, iron chloride solutions from production of synthetic rutile, or any waste acids comprising Fe^ ⁺ , or waste acids used for dissolving of Fe oxides or hydroxides, or combinations of the waste products listed above. The pH of the acidic solution is preferably 1.5 or lower.

Preferably the following industrial residues comprising Fe can be dissolved in or leached with acids:

• EAFD (typically 20-50% Fe; 10-40% Zn)

• EAFD from stainless steel production (comprising Cr and Ni)

• Slags from lead industry • Jarosite or Goethite as waste from zinc industry; Jarosite = MFe3(OH)6(SO ₄ )2 with M = Na, K or NH ₄

• Millscale (preferably after elimination of adherent oil)

• Dusts from cupola furnaces

• Stainless steel production dust

• Blast furnace dust and sludge (typically 30-40% Fe; 1-8% Zn)

• Basic oxygen furnace (BOF) dust and sludge (up to 70% Fe; <5% Zn)

• Spent H2S absorbers comprising EAFD, or ZnO as absorbent

Also, the following industrial residues comprising Zn can be dissolved in or leached with acids, given Fe is adjoined into the acidic solution by using acids comprising Fe or by combination with one of the residues listed above:

• Zinc ash

• Zinc skimmings

• Brass ash

Jarosite is obtained in primary zinc manufacturing by precipitation at 95°C and pH 3.5. As Jarosite mainly is landfilled any use of Jarosite generates additional value due to avoiding landfilling cost. Same applies for Goethite that is obtained as waste in primary zinc manufacturing. For properties and hydrometallurgic treatment of Jarosite see WO 2018/109283.

A further option is leaching of Fe oxides or compositions comprising Fe oxides and/or metallic Fe. It is also possible to using spent acid comprising Fe^ ⁺ and/or Fe3+ as leaching agent. Any of these options can be combined resulting in a high degree of freedom for choosing the best scenario from technical, economical and logistic point of view.

Preferably the acidic solution comprising Fe^ ⁺ is based on H2SO4 or HCI. Depending on waste materials used the acid base can be selected accordingly. For example, recycling steel pickling solutions comprising FeCl2 should prefer a HCI based matrix. Solutions comprising FeSO ₄ should prefer a H2SO ₄ based matrix. If there is free choice, e.g., when dissolving EAFD in acid, the sulfate-based matrix is most preferred, as the FeSO ₄ * 7 H2O crystals take a significant amount of water out of the system.

Preferably the acidic solution used in the process of the present invention comprises 1.5 - 60 g, preferably 4 - 50 g, more preferably 10- 40 g, and most preferably 14 to 28 g Fe^ ⁺ per kg of solution. Determination of Fe^ ⁺ is made by analyzing total dissolved Fe by ICP and Fe^ ⁺ by titration with permanganate; with Fe^ ⁺ being the difference between total dissolved Fe and

Preferably the acidic solution used in the process of the present invention comprises Zn^ ⁺ obtained from leaching industrial waste materials comprising Zn (see above) with acids, most preferably H2SO4. Most preferably, the acidic solution is prepared by leaching of electric arc furnace dust (EAFD) with sulfuric acid (World of Metallurgy - ERZMETALL 75 (2022) No.l, 28- 35).

Preferably the reaction of EAFD with sulfuric acid is made using sulfuric acid of >90% H2SO4.

In a preferred embodiment EAFD and sulfuric acid are fed simultaneously into a mixing area and then are displaced from the mixing area. Thus, the reaction of EAFD and sulfuric acid can be performed in a continuous process. For example, EAFD and sulfuric acid are fed simultaneously into a screw reactor and are conveyed inside the screw reactor to the opposite end. Preferably the screw reactor is double walled and can be heated or cooled, depending on the residence time in the reactor and the amount of heat generated by the reaction. Thus, the temperature of the mixture can be maintained for a longer period at temperatures >200°C.

Preferably the acidic solution comprising Fe ³⁺ has more than 25%, more preferably more than 50%, most preferably more than 75% of Fe in oxidation state Fe ³⁺ .

A preferred embodiment of the invention is

• leaching a Zn containing waste material, preferably EAFD, with H2SO4

• separating undissolved material and obtaining an acidic solution comprising H ⁺ , Zn^ ⁺ , and

• using the acidic solution for depletion of zinc from galvanized steel scrap or a composition comprising galvanized steel scrap

• obtaining scrap depleted from zinc and a solution comprising ZnSC>4 and FeSO4

• separating Fe^ ⁺ from Zn^ ⁺ to provide a Zn2 ⁺ -containing product and a Fe^ ⁺ salt.

Dissolving EAFD in H2SO4 can be done in a single step reaction with concentrated H2SO4 or in two-step dissolution with diluted H2SO4 in step 1 and concentrated H2SO4 in step 2. The diluted H2SO4 can be any waste H2SO4 or the solution obtained after crystallization and separation of FeSO4 * 7 H2O according to the inventive process. The acidic solution resulting from EAFD dissolving comprises H ⁺ , Fe^ ⁺ , and Zn^ ⁺ , and can comprise Fe^ ⁺ . After reaction of the waste material with acid the dissolution in water or diluted acid can be done using reducing agents. Preferably, metallic Zn or Fe from scrap or H2 generated during Zn leaching can be used.

In state of the art acidic leaching Zn is oxidized by H ⁺ , which is known to lead to kinetic problems due to overpotential associated with H2 generation. In contrast, according to the present invention, Fe^ ⁺ is an oxidation agent for metallic Zn. This avoids or at least reduces the kinetic problems with overpotential for H2 generation and thus can result in an accelerated Zn leaching. Moreover, H2 generation can be reduced or even avoided completely, and the reducing power of Zn is not useless but is highly valuable for reduction of Fe^ ⁺ instead of H2 being blown off.

Preferably there are more than 1 leaching step (with at least 1 leaching step being done with a stoichiometric excess of metallic Zn in relation to Fe^ ⁺ , and at least 1 leaching step being done with a stoichiometric excess of H ⁺ and Fe^ ⁺ in relation to metallic Zn). For the leaching step with stoichiometric excess of metallic Zn in relation to Fe^ ⁺ the concentration of H2SO4 preferably is <10%, most preferably <4%. Thus, it is possible to obtain steel scrap with <0.5%, preferably, < 0.2%, most preferably < 0.1% of residual Zn and a solution free from Fe3+ with <2%, preferably <1.5%, most preferably <1% of H2SO4. Any H2SO4 concentration herein means free H2SO4.

Subject of the invention is also Zn-coated Fe (or galvanized steel) that is covered by an acidic solution comprising 1.5 - 60 g, preferably 4 - 50 g, most preferably 10- 40 g Fe^ ⁺ per kg of solution.

Preferably, the acidic solution comprising Fe^ ⁺ is used in an amount so that there is temporarily an excess of Fe^ ⁺ with respect to metallic Zn provided as coating. Preferably, this excess is present at the beginning of the process. However, it could also be present at the end of the leaching step. More preferably, the atomic ratio Fe^ ⁺ /Zn° is > 2, even more preferably >4, most preferably >10. Resulting from this some metallic Fe may be consumed for reduction of Fe^ ⁺ .

This allows a more thorough leaching for better Zn removal. If scrap is in the form of pressed packages, some surface area coated with Zn is not directly accessible for leaching agents. Consequently, a certain degree of "overleaching" can help approaching (close to) 100% Zn depletion; "hot dipped" Zn layers comprise Zn-Fe interlayers that can result in co-leaching of metallic Fe. This "overleaching" (meaning dissolving some metallic Fe from the scrap) can be done to variable extent depending on chemical and economic conditions: A slight "overleaching" saves metallic Fe for further use (e.g. in steelmaking), a moderate "overleaching" can be reasonable or necessary for maximum Zn depletion (especially in case of galvanealed Zn layers), and a more thorough "overleaching" can be promising for maximizing output of Fe salt, especially if additional waste streams comprising Fe(lll) can be converted into valuable products (e.g. 1 metallic Fe converts 2 Fe ³⁺ from Jarosite or Goethite into 3 valuable Fe ²⁺ species).

Preferably the leaching rate (or Zn removal rate) of metallic Zn is >90% or >98% or >99%, more preferably >99.5%; most preferably >99.8%.

Accepting some "overleaching" of Fe can also help for more efficient Zn removal in case of polymer, painting or lacquer on top of the Zn coated steel substrate.

Another embodiment is the dissolution of Fe(lll) compounds such as Fe oxides, Goethite or

Jarosite in sulfuric acid or in the acidic solution comprising H ⁺ , Zn ²⁺ , and Fe ³⁺ . The Fe ³⁺ is reduced to Fe ²⁺ in the subsequent reaction with galvanized steel scrap.

Taking Zn from galvanized steel scrap as reducing agent for Fe ³⁺ obtained after dissolution of EAFD saves the life for Fe as metal; Fe has more value as metal in scrap than as Fe ²⁺ .

Preferably, the reaction of the acidic solution with the galvanized steel scrap is continued until all or nearly all Zn has been dissolved (reaction A) and (subsequently) all Fe3+ has been reduced (reaction B):

Reaction A: Zn + 2 Fe ³⁺ => Zn ²⁺ + 2 Fe ²⁺

Reaction B: 2 Fe ³⁺ + Fe => 3 Fe ²⁺

The leaching of metallic Zn can be done in a 2-step or multi-step process: Fresh galvanized steel scrap can be subjected to a first leaching step with an acidic solution obtained from previous leaching steps, or with a recycled solution from another process step, e.g. solution obtained from first step ZnS precipitation. And further leaching steps can follow using other solutions with higher acidity or higher Fe ³⁺ concentration for improved leaching of the very last fraction of the metallic Zn. This can be done e.g. by dipping baskets with galvanized steel scrap into the corresponding solutions or by passing different leaching solutions one after another through a reactor filled with the scrap.

Partial recycling of solutions can help optimizing the acidity for leaching steps, adjusting Fe ³⁺ level for optimized Zn leaching, increasing ion concentration for more efficient separation steps, or fine-tuning pH for ZnS precipitation. For example, the solution obtained after crystallization and separation of FeSO4 * 7 H2O can be used due to its acidity in the process step of manufacturing the acidic solution comprising Fe ³⁺ . If required for the subsequent step of ZnS precipitation or solvent extraction of Zn ²⁺ , the pH can be raised. This can be achieved by supply of new Zn coated material (reaction C) or continuing leaching metallic Fe (overleaching; reaction D), or by alkaline materials (reaction E) or by reaction with new waste materials (reaction F):

Reaction C: Zn + H2SO4 => ZnSC>4 + H2

Reaction D: Fe + H2SO4 => FeSC>4 + H2

Reaction E: 2 NaOH + H2SO4 => Na2SO4 + H2O

Reaction F: Fe2C>3 + 3 H2SO4 => Fe2(SC>4)3 + 3 H2O

Reaction E is least favorable due to the generation of Na2SO4 that has to be separated or ends in a wastewater stream. Reaction C and D has the disadvantage of generating hydrogen, but the advantage of simply continuing the leaching step and no additional treatment step being necessary. Reaction C has the advantage over reaction D that no metallic Fe is lost.

The preferred option is reaction F. Raising the pH can be achieved by dissolution of iron oxide (e.g. from wastewater treatment after crystallization of FeSC>4 * 7 H2O), dissolution of EAFD or other waste materials. In a preferred embodiment reaction F is not done after the reaction A or B, but after part of ZnS having been precipitated and separated. However, reaction F requires another treatment according to reaction A or B for reduction of Fe^ ⁺ to Fe^ ⁺ .

Overall set-up of process parameters is heavily influenced by acid consumption for dissolving the waste materials for generation of the acidic solution. High acid ratio means better dissolution rates for Zn and Fe from waste materials, but also means more effort for maintaining acid balance for complete and selective precipitation of ZnS. Depending on relative volume of waste material and galvanized steel scrap, on Fe and Zn content of each material, on price for Zn and FeSC>4 * 7 H2O, or on logistic or quality constraints, the process can be adjusted with a considerable amount of freedom.

Process control of the Zn leaching step can be done by monitoring redox potential. Complete dissolution of Zn and reduction of Fe3+ each is indicated by characteristic feature of redox potential (ref. US2015/0259766). It is possible to recycle any acidic solution from the inventive process partly back into a previous process step, e.g. the solution obtained after Zn leaching, the solution obtained after ZnS precipitation and separation, or the solution obtained after crystallization of FeSC>4 * 7 H2O. This can help improving the acid balance of the inventive process. Preferably, the acid needed for generation of the acidic solution by dissolving a waste product can originate in part from recycling from a later process step. Use of waste materials as source for Fe^ ⁺ can generate another source of revenue due to negative prices for some waste products (e.g. EAFD with low Zn content or Jarosite or Goethite). This applies especially for waste products with low zinc content with no satisfactory way for recycling existing for those materials.

Zn-coated Fe covered by Fe3+-containing acidic solution

In line with the previous description of the acidic solution and the leaching step, the present invention also provides the product that is present at the beginning of the leaching step. The present invention thus also provides Zn-coated Fe that is covered by an acidic solution comprising 1.5 - 60 g, preferably 4 - 50 g, most preferably 10- 40 g Fe^ ⁺ per kg of solution. The Zn-coated Fe as well as the acidic solution are described in detail in the previous section.

The galvanized steel scrap

Galvanized steel scrap consists of or comprises Fe substrate that is coated with a zinc layer. Fe substrates coated with zinc is often used for corrosion resisting products, e.g. in the automotive industry. Sometimes the protective layer is not pure zinc, but can contain other elements like Sn, Al, or Ni.

Galvanized steel scrap can be subjected to shredding or pressed into packages that are easy to handle, e.g. by dipping of a package into the leaching liquid (i.e. the acidic solution comprising Fe ³⁺ ) and taking out again after the Zn leaching step. Another option is filling of a vessel or a column with the galvanized steel scrap and subjecting to the acidic solution in continuous or discontinuous manner. Another option is putting the galvanized steel scrap into a basket and immersing this basket in the leaching liquid., i.e. the acidic solution comprising Fe ³⁺ . Yet another option is shredding the galvanized steel scrap and conveying this material on a moving belt and passing an area of being sprayed by the acidic solution and a subsequent area of being sprayed by water.

After finishing the leaching of Zn, the metallic Fe substrate depleted from zinc can be washed with diluted acid and/or water for removing adhering solution comprising Zn and anions like sulfate or chloride.

The Fe substrate depleted from zinc can be used as a high-quality raw material for iron smelting processes, e.g., for steelmaking using electric arc furnace, for blast oxygen furnace, or for casting.

The separation step Separation of dissolved Zn ²⁺ and Fe ²⁺ can be made by precipitation of ZnS by bubbling H2S into the solution.

A preferred option is:

• precipitation of the obtained Zn^ ⁺ in the solution as ZnS

• separating the ZnS from the solution comprising dissolved Fe^ ⁺

• crystallization of the Fe compound by evaporation of water and/or lowering temperature

• separation of the crystals of the Fe compound and recycling the obtained liquid phase back into one of the previous process steps

During precipitation of ZnS with H2S the pH drops. In a preferred embodiment of the invention the precipitation of ZnS is done in (at least) a 2-step process. In a first step part of the Zn^ ⁺ is precipitated as ZnS using H2S off-gas from second step or H2S obtained from stripping the solution obtained afterthe second step precipitation and separation of ZnS. And in the second step a high concentration of H2S is being used in order to shift the equilibrium towards ZnS precipitation. Another option is precipitation of ZnS using Na2S or (NH^S.

A preferred embodiment is reacting the precipitated ZnS with H2SO4 resulting in a ZnSC>4 solution and gaseous H2S and recycling the H2S obtained for precipitation of ZnS again. Preferably the moist ZnS filtercake is reacted with diluted H2SO4. It can be necessary to strip residual H2S by N2 or air for shifting the equilibrium in the desired direction. Or reacting the H2S with NH4OH or NaOH for obtaining Na2S or (1^4)2$.

However, ZnS can be the preferred material in case of transportation over a large distance to a zinc manufacturing location being necessary.

There are synergies for dissolving EAFD with H2SO4 and de-zincing of galvanized steel scrap in purification (e.g., Cu, Cd, Mn, Ni from Zn-Ni coatings) - if purification is done for creating higher value Zn^ ⁺ solution.

H2S can be produced from H2 and S or can be obtained as a by-product from production of coke. H2S may also be obtained from spent absorbers for H2S, e.g. EAFD as absorber for H2S (Energy Fuels 2022, 36, 3695-3703).

There is wide industrial usage of metal oxides as H2S removal adsorbents from industrial gases such as synthesis gas, coke gas from steel production, fuel cell hydrogen, sulfur recovery unit tail gas, and bio/natural gas. These materials can be used for H2S generation and at the same time can be used as a source for Fe and/or Zn in manufacturing the acidic solution comprising Fe ³⁺ .

Another option for separation of dissolved Zn ²⁺ and Fe ²⁺ is solvent or membrane extraction. Preferably Zn ²⁺ is transferred into an organic phase, most preferably D2EHPA, di-(2-ethyl hexyl) phosphoric acid. D2EHPA is a cheap and widely available reagent.

The process steps of Zn ²⁺ extraction are:

D2EHPA is dissolved in an organic solvent, typically an inert hydrocarbon such as kerosene or naphtha. This organic phase is immiscible with the aqueous solution containing Zn ²⁺ . This D2EHPA-containing organic phase is brought into contact with the aqueous solution containing Zn ²⁺ :

Zn ²⁺ (aq) + 2 D2EHPA (org) Zn(D2EHPA)2 (org)

The Zn(D2EHPA)2 complex is more soluble in the organic phase than in the aqueous phase. As a result, the complex transfers from the aqueous phase into the organic phase.

The loaded organic phase, comprising the extracted Zn(D2EHPA)2 complex, is then separated from the aqueous phase by settling, centrifugation, or decantation.

To recover the extracted zinc from the organic phase, H2SO4 is used as stripping agent. After stripping the zinc from the organic phase, the D2EHPA can be regenerated and reused in the extraction process.

Preferably the solvent extraction process involves multiple extraction and stripping stages to increase the overall efficiency of zinc recovery.

Preferably the pH of the solution to be treated by extraction is >0.5, more preferably >1.0, most preferably >1.5.

As Mn ²⁺ is extracted together with Zn ²⁺ there is a need for separating Mn. This can be done by cementation with fine Zn powder: Zn + Mn ²⁺ => Zn ²⁺ + Mn. Mn can be separated by sedimentation, filtration or centrifugation together with unreacted Zn. Another option is oxidation of Mn ²⁺ to MnCh. Preferably this can be done by O2, O3, H2O2, or H2SO5 with subsequent separation of MnCh by sedimentation, filtration or centrifugation.

Production of Fe salt

The solution obtained after ZnS precipitation and separation of the precipitate or removal of Zn^ ⁺ by solvent or membrane extraction is used for reclamation of the Fe compound. Stripping of residual H2S in the solution can be done by N2. Crystallization is the preferred option for obtaining a solid Fe compound from solutions comprising FeSO/ Preferably water is removed from the Fe ²⁺ solution by vacuum evaporation and supplying heat in order to prevent temperature to drop down too quickly. A preferred option is to allow temperature to drop down to +5°C, and then separating the FeSO4 * 7 H2O crystals by filtration or centrifugation. The process of crystallization and separation of FeSC>4 * 7 H2O from acidic solutions is state of the art in the TiO2 production. Specific parameters depend on composition of solution. General principles and considerations can be found in DE 10 2007 032 418.

Preferably the FeSO4 concentration in the solution is >100 g/l. Due to the large amount of crystal water being bound in FeSO4 * 7 H2O water is efficiently removed from the solution resulting in significant drop of pH. Final concentration of saturated solution at +5°C is ca. 35- 40 g/l Fe ²⁺ .

Due to evaporation of water from the FeSO4 solution and water capturing of the heptahydrate the acidity of the solution gets even significantly higher (pH«0.5). Therefore, this solution or part of this solution can be used for closing the loop and recycling back into one of the previous steps: for reaction with EAFD or for Zn leaching from galvanized steel scrap.

A similar situation is found for the crystallization of FeCl2 as FeCl2 * 4 H2O.

At least part of this solution leaves the process as waste water and a sink for some ions, e.g. Na or K. Preferably, Fe is precipitated by alkaline treatment, separated and recycled into one of the preceding steps, e.g. for generation of the acidic solution comprising Fe ³⁺ or for raising the pH according to reaction F.

Electrowinning

Careful precipitation of ZnS and washing the precipitate with diluted H2SO4 (pH ~2) can achieve a nearly complete separation of Fe and other impurities. This ZnS can be fed into the primary Zn production process.

After conversion of ZnS into ZnSC>4 solution by reacting with H2SO4 the solution obtained can be used for electrowinning of metallic zinc; further purification by a cementation step with metallic zinc powder is possible for removing impurities even more, e.g., Ni, Cd, Cu, and Fe. As there are only small concentrations of these impurities present the demand for zinc powder is very low.

More details on cementation with further references can be found in: DEZINCING OF GALVANIZED STEEL by J. Grogan, Colorado Scholl of Mines: https://pdfcoffee.com/qdownload/dezincinc-of-galvanized-stee l-pdf-pdf-free.html, p. 25. In summary, the inventive process offers some striking advantages:

• no major waste streams from EAFD recycling

• Hydrometallurgic processes generally have less energy consumption and lower CO2 footprint compared to thermal processes (e.g. Waelz process)

• Leaching step with max. efficiency for Zn depletion (implies some co-leaching of Fe)

• High flexibility

• Possibility for shifting the process towards more H2 generation for further use

• Separation step: using H2S for ZnS precipitation, if there is cheap H2S available; or using solvent extraction, if there is no H2S or no cheap H2S available

The invention also provides the use of an acidic solution comprising Fe^ ⁺ for use as an oxidizing agent, preferably for de-zincing of galvanized steel, wherein the waste acidic solution is obtained by reacting is EAFD with sulfuric acid.

Example 1: Recycling of electric arc furnace dust (EAFD) and de-zincing of galvanized steel scrap

An acidic solution is prepared by reacting electric arc furnace dust (EAFD) with concentrated sulfuric acid and dissolving the obtained reaction product in water. After separation of the solid residue an acidic solution comprising Fe^ ⁺ and Zn^ ⁺ is obtained.

Galvanized steel scrap is leached with this acidic solution until the Zn coating is dissolved to >99% and the Fe^ ⁺ is reduced to Fe^ ⁺ . This can be made by dipping baskets filled with scrap into acidic solution or by passing acidic solution through a column filled with scrap. The obtained Zn-free metallic Fe substrate can be washed with water, dried and used as Fe resource for steelmaking or in foundries.

A typical mass ratio of EAFD and galvanized steel scrap is 1:8.

The solution obtained from the Zn leaching step comprising Zn^ ⁺ and Fe^ ⁺ is treated with H2S gas for precipitating of ZnS. ZnS is separated by filtration or centrifugation and washed with diluted H2SO4 (pH ~2) to remove FeSO/j.. The ZnS can be used as raw material in primary zinc production or can be converted into a ZnSC>4 solution by dissolving in H2SO4 for electrowinning of zinc.

The FeSC>4 solution obtained after separation of the ZnS precipitate is taken for crystallization of FeSC>4 * 7 H2O (copperas) by vacuum evaporation of water and allowing the system to cool down to 5°C. The FeSC>4 solution obtained after separation of the copperas can be recycled back (completely or in part) into one of the previous process steps.

Products obtained: Zn depleted scrap, ZnS (or ZnSO/i.), and FeSO4 * 7 H2O (copperas)

Example 2: Recycling of Jarosite and de-zincing of galvanized steel scrap

Jarosite is dissolved in diluted H2SO4 (preferably waste H2SO4 from TiO2 production).

Galvanized steel scrap is leached with this acidic solution until the Zn coating is dissolved to >99% and the Fe^ ⁺ is reduced to Fe^ ⁺ . This can be made by dipping baskets filled with scrap into acidic solution or by-passing acidic solution through a column filled with scrap. The obtained Zn-free metallic Fe substrate can be washed with water, dried and used as Fe resource for steelmaking or in foundries.

The solution obtained from the Zn leaching step comprising Zn^ ⁺ and Fe^ ⁺ is treated with H2S gas for precipitating of ZnS. ZnS is separated by filtration or centrifugation and washed with diluted H2SO4 (pH ~2) to remove FeSO4- The ZnS can be used as raw material in primary zinc production or can be converted into a ZnSC>4 solution by dissolving in H2SO4 for electrowinning of zinc.

The FeSO4 solution obtained after separation of the ZnS precipitate is taken for crystallization of FeSO4 * 7 H2O (copperas) by vacuum evaporation of water and allowing the system to cool down to 5°C. The FeSO4 solution obtained after separation of the copperas can be recycled back (completely or in part) into one of the previous process steps.

Products obtained: Zn depleted scrap, ZnS (or ZnSO4), FeSO4 * 7 H2O (copperas); whereby three low-cost waste materials are being recycled and converted into valuable products: Jarosite, waste diluted H2SO4, galvanized steel scrap.

Example 3: Recycling of electric arc furnace dust (EAFD) and de-zincing of galvanized steel scrap

An acidic solution is prepared by reacting electric arc furnace dust (EAFD) with concentrated sulfuric acid and dissolving the obtained reaction product in water. After separation of the solid residue an acidic solution comprising Fe^ ⁺ and Zn^ ⁺ is obtained.

Galvanized steel scrap is leached with this acidic solution until the Zn coating is dissolved to >99% and the Fe^ ⁺ is reduced to Fe^ ⁺ . This can be made by dipping pressed scrap elements or baskets filled with scrap into the acidic solution or by passing the acidic solution through a vessel filled with scrap. The obtained Zn-free metallic Fe substrate is being washed with water, dried and used as Fe resource for steelmaking or in foundries.

The solution obtained from the Zn leaching step comprising Zn^ ⁺ and Fe^ ⁺ is treated by solvent extraction with D2EHPA in kerosene. The loaded organic phase, comprising the extracted Zn(D2EHPA)2 complex, is then separated from the aqueous phase by settling. The organic phase is mixed with H2SO4 as stripping agent. After stripping the zinc from the organic phase the D2EHPA is reused for the extraction process.

Mn ²⁺ is converted into MnCh by oxidation with O3 and subsequently separated by filtration.

The FeSO4 solution obtained after solvent extraction of Zn^ ⁺ is taken for crystallization of FeSO4 * 7 H2O (copperas) by vacuum evaporation of water and allowing the system to cool down to 5°C. The FeSO4 solution obtained after separation of the crystalline copperas can be recycled back (completely or in part) into one of the previous process steps.

Products obtained: Zn depleted scrap, ZnSC>4, MnCh, and FeSO4 * 7 H2O (copperas)

Example 4: De-zincing of galvanized steel scrap

Galvanized steel scrap comprising 0.9% Zn is leached with

(a) an acidic solution comprising 150 g/l H2SO4

(b) an acidic solution comprising 150 g/l H2SO4 and 10 g/l Fe ³⁺ (as Fe2(SO4)s) for 10, 20, and 30 s. After the leaching step the scrap samples are washed with deionized water and dried prior to XRF characterization. The results (in Fig. 1) with the bars indicating the surface Zn concentration as determined by XRF clearly reveal the acceleration of Zn dissolution in presence of Fe ³⁺ .

Example 5: De-zincing of galvanized steel scrap

Galvanized steel scrap comprising 0.9% Zn is leached with

(a) an acidic solution comprising 20 g/l H2SO4

(b) an acidic solution comprising 20 g/l H2SO4 and 28 g/l Fe ³⁺ (as Fe2(SO4)s)

(c) a solution comprising 55 g/l Fe ³⁺ (as Fe2(SO4)s) (pH of solution < 4) for up to 300 s. After the leaching step the scrap samples are washed with deionized water and dried prior to XRF characterization. The results (in Fig. 2) with the bars indicating the surface Zn concentration as determined by XRF clearly reveal the acceleration of Zn dissolution in presence of Fe ³⁺ .

After 300 s the residual Zn on the steel surface is 0.1% and 1.1% for solutions comprising Fe ³⁺ , and 17% for a H2SO4 solution without Fe ³⁺ .