An Innovative Approach Toward Enhancing the Environmental and Economic Sustainability of Resource Recovery from Hazardous Zn-Bearing Dusts from Electric Arc Furnace Steelmaking

Abstract

An innovative approach is reported for recovering Fe and Zn resources from hazardous zinc-bearing electric arc furnace dusts (ZBDs) in a sustainable manner. A combination of carbothermal and H₂ reduction were used to overcome challenges associated with the high temperatures of carbothermal reduction and the high costs/limited supplies of hydrogen. In-depth reduction studies were carried out using zinc-rich (17 wt.%), iron-poor (35 wt.%) ZBD; coke oven battery dry quenching dust (CDQD) was used as reductant. Briquettes were prepared by mixing ZBD and CDQD powders in a range of proportions; heat treatments were carried out in flowing H₂ gas at 700 °C–900 °C for 4 h. The reduced products were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and inductively coupled plasma (ICP). The Fe content of the reduced briquettes showed increases between 50 and 150%, depending on composition and reduction temperature; Zn, Pb, Cl, Na, K and S were completely absent. The gaseous elements were collected in cooled traps at the furnace outlet to recover metallic zinc and other phases. The volatile products collected at the outlet (900 °C) contained more than 70% zinc and 6% lead; small amounts of zinc were also present in the metallic phase. The processing temperatures were significantly lower in the combined approach as compared to 100% carbothermal reduction. While reducing energy consumption and limiting the generation of greenhouse gases, this approach has the potential for enhancing the reutilization of hazardous industrial wastes, resource recovery, and economic and environmental sustainability.

1. Introduction

With persistent high demands and continuous developments in the iron and steelmaking sector, the global crude steel production exceeded 1.9 billion tons in 2023 [1]. China, India, Japan, the USA and Russia were among the leading steel producers in the world. The production of steel is, however, accompanied by the generation of large amounts of secondary wastes in the form of coal and coke dusts, hazardous steelmaking dusts, flue dusts, sludges, slags, wastewater, exhaust gases, etc., which can be sources of environmental contamination and degradation, affecting the health and wellbeing of humans and other living creatures [2,3]. The total waste output of the steel industry is estimated to be typically ~8–12% of crude steel produced [4,5,6]; an estimated 152–228 million tons of steel dusts and sludges were produced in 2023. Electric arc furnace dust (EAFD) is one of the significant wastes generated by the steelmaking industry [7]; typically, 15–25 kg of EAFD is generated per ton of steel produced [8,9]. With ~8 million tons/annum of EAFD generated currently, annual EAFD volumes are predicted to reach 18 million tons globally by 2050 [10]. In the Russian Federation, PJSC “Severstal”, a large enterprise of steel and mining industries, has an annual EAFD generation of about 300 thousand tons. During the high temperature processing of galvanized scrap metals, the dust was found to be contaminated with zinc; similarly, lead and chlorine contaminants were observed for painted and PVC laminated sheet metals [7].

Even though these wastes are rich in elements such as iron, carbon, and zinc, their reutilization in iron- and steelmaking, especially in the blast furnace, can be very challenging [11,12,13]. Recycling ferrous metal scrap in electric arc furnaces can lead to the contamination of both the metal and associated products with several toxic elements; EAFD is a hazardous waste containing not only Zn and Pb, but also Cr, Ni, Cu, Cd, Cl and even As [14,15,16]. While EAFD contaminated with zinc and lead is an important issue in EAF steel mills, it is also a promising secondary material resource containing iron (17–45 wt.%), zinc (2–46 wt.%) and lead (0.15–6 wt.%) [15,17]. If it were possible to extract Zn, Pb, Cl and some other toxic elements, the reutilization of EAFDs in iron making processes would enhance overall resource recovery, with significant contributions to the circular economy and environmental sustainability.

Landfilling zinc-bearing dusts (ZBDs) and sludges is not a desirable option as it would lead to the accumulation of heavy metals in the soil/ground water, contaminating waterways, with a detrimental influence on the environment as well as the loss of valuable Fe and Zn resources [18]. A specialized treatment is therefore required for recycling ZBDs [19]. Several techniques such as hydrometallurgical, pyrometallurgical and beneficiation/chemical stabilization have been used for processing these wastes [10,20,21]. There are a number of technological challenges during the separation of iron and zinc (the key constituents of ZBDs); these two metals are present jointly in the form of zinc ferrite (franklinite, ZnFe₂O₄) phase [22]. Thermal and/or chemical methods are used for the decomposition of this phase prior to zinc removal. In the hydrometallurgical approach, the classical ZINCEX process [23] and EZINEX process [24] have used acidic [25] or alkaline [26] lixiviants for leaching ZnO and/or PbO from ZBDs. While acidic lixiviants (e.g., H₂SO₄) show good recovery of Zn, they can cause the depletion of iron and the additional generation of secondary wastes [27,28]. Alkaline lixiviants (e.g., NaOH), on the other hand, are more selective and achieve better results both technically and economically [29]. However, due to the strong PH dependence of Zn and Pb leaching, multiple process steps and complex operations could become necessary in this approach [30,31].

The chemical stabilization of EAFDs (e.g., the Oregon process) is carried out by molding EAFDs into lumps with additives (CaCO₃, Na₂CO₃, SiO₂, etc.), using sintering and/or stabilization with lime [32,33]. As this method focusses primarily on generating impermeable substrates encapsulating EAFD particles and not on the recovery of valuable metals, it results in the wastage of several key metallic resources [34]. Pyrometallurgical processes are an effective route for recovering valuable elements such as iron, zinc and lead from zinc-bearing wastes. These processes, which offer high potential for metal recovery, an easy treatment of residues and a small number of process steps, are a preferred choice for recycling EAFDs and have even reached the commercialization stage in some cases [35].

The carbothermic reduction of EAFDs using carbons and/or carbon monoxide as reductants (e.g., Waelz process, [36,37]) can, however, be an additional source of greenhouse gas emissions during resource recovery [38,39,40]. The iron- and steelmaking sector is the single largest industrial CO₂ emitter, accounting for 6.5% of all CO₂ emissions on the planet [41]; about 2.1 tons of CO₂ are produced per ton of crude steel [42]. Serious efforts are being made towards carbon-lean steel production and to align with the requirement of a drastic reduction of 80% in all CO₂ emissions by 2050 [43,44,45]. Hydrogen-based steel production, i.e., replacing carbon-based reducing agents with hydrogen, offers enormous potential for reducing CO₂ emissions. Basic technologies for using H₂ as a reductant have been successfully demonstrated with no adverse effects on steel quality [46,47,48]. However, these have yet to reach commercial-scale processing [49,50]. There have been a few studies on the hydrogen reduction of EAFDs, wherein a high degree of Zn removal was achieved at relatively low temperatures [51,52]. Nevertheless, the practical realization of direct hydrogen reduction processes is limited by the high cost of hydrogen, its limited supply and poorly developed technological modes of operation.

Aims of the Investigation

This study presents a novel strategy for resource recovery from hazardous zinc-bearing EAF dusts while overcoming the following challenges:

▪

Key metallic resources (Fe and Zn) are present jointly as ZnFe₂O₄ and are difficult to separate in a simple manner.

▪

While H₂ reduction could be used in principle to extract metallic values, with the advantages of lower operating temperatures and zero GHG emissions, there are supply issues with H₂ and higher operating costs. Due to the large volumes of industrial wastes involved and the relatively low-grade quality of the resources recovered, the H₂-based reduction of ZBDs may not be a cost-effective and economically viable approach.

▪

The carbothermal reduction of ZBDs is associated with high levels of energy consumption and GHG emissions and is clearly not a desirable option as the steel industry is seriously trying to limit and reduce emissions from the sector.

This study presents a novel strategy, namely, an intermediate approach using a combination of carbothermal and hydrogen reduction, which will reduce the amounts of H₂ required due to partial utilization but will use the benefits of lower operating temperatures to reduce energy consumption and GHG emissions during carbothermal reduction. The environmental sustainability of the process will be further enhanced by utilizing carbonaceous waste from the ironmaking sector as the reductant instead of typical carbons such as coal, cokes, or graphite.

In-depth results are reported on the simultaneous extraction of iron-rich and Zn-rich phases from zinc-bearing EAF dusts, achieving a clear separation between the two metallic phases recovered. A combination of carbothermal and hydrogen reduction processes was used on briquettes prepared with ZBD powders. Coke dry quenching dust (CDQD), another solid waste from the ironmaking sector, was used as the carbonaceous reductant. The in-depth results on the metallic Fe and Zn recovered are presented as a function of briquette composition and reduction temperatures. This recycling approach for hazardous ZBDs is expected to lead to lower energy consumption, enhanced economic sustainability through resource recovery, environmental sustainability through reductions in greenhouse gas emissions and advances in hazardous waste management.

2. Materials and Methods

2.1. Material Characterization

The zinc-bearing EAF dust specimens used in this study were obtained from the electric steelmaking operations at PJSC ‘Severstal’, Russian Federation. As these operations have a high consumption of sheet scraps, the resulting dust had a low iron content and high contents of zinc, lead and chlorine. The carbon-based reductant, coke dry quenching dust, was also sourced from these operations. The particle size distributions and morphologies of ZBD and CDQD powders are shown in Figure 1 and Figure 2, respectively.

These figures indicate that ZBDs had a multimodal size distribution consisting of particles with two types of shapes: spherical and fragmented. The spherical particles of ZBD had sizes ranging between 0.2 and 2 microns. However, some particles with sizes larger than 10 microns were also observed. The fragmented particles typically had smaller sizes and tended to form aggregates in some places. Coke dust powders generally consisted of irregularly shaped particles with a monomodal distribution and sizes ranging from about 1 to 100 microns.

X-ray diffraction investigations with Cr Kα radiation on the phase composition of ZBD powders (Figure 3) show the presence of zinc ferrite (ZnFe₂O₄) phase and ZnO. A small amount (~5 wt.%) of carbon was also detected in the ZBD; the diffraction pattern showed low-intensity peaks characteristic of Fe₂O₃ and carbon. The X-ray diffractogram of CDQD (Figure 3) is typical for predominantly amorphous products; the presence of small amounts of crystalline phases was also detected.

2.2. Preparation of Briquettes

Briquettes were prepared by mixing ZBD and CDQD powders in a range of proportions. Mixing was carried out using an overhead stirrer; water was added in the mixture at a rate of 14 wt.% of the total solids. The briquettes were pressed into molds with a cylindrical shape (diameter: 10 mm; height: 2 mm), followed by pressing under a load of 5 tons. The elemental compositions (wt.%) of ZBD and CDQD, as determined by the EDX analysis method, and the briquettes, as theoretically computed based on the composition of the ZBD and CDQD, are summarized in

Table 1. Elemental compositions (wt.%, ±1) of ZBD, CDQD and blended briquettes.

Element	ZBD	CDQD	Briquette CDQD Content
			6 wt.%	8 wt.%	10 wt.%
Fe	35	5	33	33	32
O	29	25	29	29	29
Zn	18	-	17	16	16
C	6	67	9	10	12
Na	4	-	4	4	3
Mn	3	-	3	3	3
Ca	3	-	3	2	2
Cl	2	-	2	2	2

. Typical error bars of ±1% in the data are also indicated.

2.3. Experiments: Reduction Investigations

The reduction behavior of ZBD-CDQD briquettes was investigated in a hydrogen metallization system consisting of several parts: (1) a Carbolite KZS12-200-600 tubular furnace; (2) a hydrogen drying system based on silica gel and zeolites of various compositions; (3) a zinc capture system (bubbling operating principle with distilled water at room temperature for gas purification before the gas pump); and (4) a hydrogen generator SAM-1 with a capacity of up to 80 L/h. A schematic representation of the hydrogen metallization system is shown in Figure 4. The gas flow rate of the hydrogen generator was set to 80 L/h to carry out the reduction reactions; these studies were performed at atmospheric pressure.

The reduction was carried out in a flowing hydrogen with temperatures set at 700, 800 and 900 °C. The other experimental parameters were as follows: specimen weight: 10 g; heating rate: 20 °C/min; exposure time: 4 h. The weights of the briquettes were recorded before and after the reduction experiments. The elements evaporating from the briquettes were collected at the furnace outlet in the zinc capture system. The evaporation of Zn occurred through sublimation at these temperatures [53]. The Zn-based powders/products captured at the reactor outlet were later characterized using SEM/EDS and X-ray diffraction.

2.4. Analytical Instruments Used for Characterization

The microstructure and elemental composition were analyzed using an electronic scanning microscope, Tescan Vega 3 (TESCAN, Czech Republic with Oxford instruments EDS detector), equipped with an energy-dispersed X-ray detector (EDX), Oxford Instruments INCA X-Act. The material was applied on carbon tape without additional modification and the sputtering of electrically conductive coating. Data statistics for EDX included at least 10 points analyzed; typical error bars were found within ±1%. The X-ray diffractometer Diffray-401k (Scientific Instruments, St. Petersburg, Russia) was used to study the phase composition. Diffraction patterns were taken with Cr Kα radiation; 25 KV, 4 mA; an angular range of 14–140°; a step size of 0.1° and a time step of 2 s. The elemental composition of various materials was determined using an ARL 9900 WS X-ray fluorescence wave spectrometer (Thermo Fisher Scientific Inc. Waltham, MA, USA). The presence of small quantities of Zn and Pb elements in the briquettes after reduction was additionally verified with high accuracy using the spectral atomic emission method with inductively coupled plasma (ICP) on iCAP 6300 Radial View by Thermo Fisher Scientific Inc. (Waltham, MA, USA)

3. Results

3.1. Weight Loss of Briquettes

Weight loss was observed for all briquettes after reduction in hydrogen atmosphere at a range of temperatures. These results are summarized in

Table 2. Weight loss in briquettes after reduction for a range of compositions and reduction temperatures.

Temperature (°C)	Weight Loss (%, ±0.5) CDQD Content
	6 wt.%	8 wt.%	10 wt.%
700	31.3	31.1	34.9
800	31.4	33.3	38.4
900	40.3	40.4	42.1

. These weight losses are associated with the removal of water, volatiles, sublimation (including reduction and sublimation) of light elements and the reduction of iron and zinc from oxides as well as from zinc ferrite.

3.2. Elemental Composition of Briquettes After Reduction

The elemental compositions of briquettes, as determined using EDX, were characterized following the reduction treatment. The results for 6 wt.% CDQD, 8 wt.% CDQD and 10 wt.% CDQD briquettes are shown in

Table 3. Elemental composition of 6 wt.% CDQD briquettes after combined carbothermal and hydrogen reduction.

Temperature (°C)	Content (wt.%, ±1)
	Initial	700 °C	800 °C	900 °C
Fe	33	55	59	66
O	29	19	18	13
Zn	17	–	–	–
C	9	11	9	5

,

Table 4. Elemental composition of 8 wt.% CDQD briquettes after combined carbothermal and hydrogen reduction.

Temperature, (°C)	Content (wt.%, ±1)
	Initial	700 °C	800 °C	900 °C
Fe	33	49	64	68
O	29	19	12	12
Zn	16	–	–	–
C	10	14	8	5

and

Table 5. Elemental composition of 10 wt. % CDQD briquettes after combined carbothermal and hydrogen reduction.

Temperature, (°C)	Content (wt.%, ±1)
	Initial	700 °C	800 °C	900 °C
Fe	32	49	68	79
O	29	8	6	5
Zn	16	–	–	–
C	12	30	11	4

, respectively, at three temperatures. The elemental data are provided here for the main elements (Fe, O, Zn and C) only; the corresponding data for the remaining elements are given in Supplementary Tables S1–S3.

The analysis of the elemental composition of the briquettes after reduction (Table 3, Table 4, Table 5 and Table S1–S3) shows that the Fe content of the briquettes showed a significant increase after the reduction reactions. This increase ranged between 50 and 150%, depending on the briquette composition and the reduction temperature. At the same time, Zn, Pb, Cl, Na, K and S were almost completely removed from the briquettes. The concentration of Zn in the briquettes was examined using the ICP-OES method (sensitivity: 0.001 wt.%); dilute (5%) nitric acid was used for digestion. While the amounts of oxygen, Al and P were found to decrease significantly in the briquette composition, the concentrations of Ca, Mn and Cr had increased slightly. Zinc was completely removed from all briquettes after reductions at 700 °C to 900 °C for CDQD concentrations ranging from 6 wt.% to 10 wt.%.

Figure 5 shows changes in briquette compositions in terms of target elements, namely, Fe, O and C. While the iron concentration generally showed a significant increase after reduction, different trends were observed in terms of CDQD contents and temperatures. At 700 °C, a small reduction was observed upon increasing the CDQD content from 6 wt.% to 8 wt.%; no further increases in reduction were observed at CDQD contents of 10 wt.%. On the other hand, iron contents were found to increase continuously with increasing CDQD levels at 800 °C and 900 °C.

While the oxygen content was found to decrease with increasing temperatures and carbon contents (i.e., increases in CDQD proportions), the results did not follow a well-defined trend. The results at 700 °C and 900 °C remained unchanged with CDQD contents increasing from 6 wt.% to 8 wt.%. These results are next interpreted in terms of the carbon content of the briquettes. While the reduction temperature of 700 °C may be insufficient for the onset of carbothermal processes, it would be sufficient for the hydrogen reduction to take place, resulting in oxygen contents decreasing by 30–70% and iron contents increasing by 50–60%. When the process is carried out at 800 °C and 900 °C, a reduction in carbon contents and an increase in iron contents are observed, indicating the onset of the carbothermal reduction of iron and other oxides. Simultaneous carbothermal and hydrogen reductions were observed in these reaction scenarios.

3.3. Recovery of Zn-Based Powders from ZBDs

The gaseous elements escaping during the reduction of briquettes were collected in cooled water traps at the furnace outlet to capture zinc-based phases. The captured powders were separated through sedimentation, discounting and drying at 80 °C. The powders captured at the reactor outlet were analyzed using SEM/EDS and X-ray diffraction. Further details are provided below.

3.3.1. Elemental Composition

The average elemental compositions (wt.%) of the captured powders for 10 wt.% CDQD briquettes, heat treated at 800 °C and 900 °C, are given in

Table 6. Average elemental compositions (wt.%, ±1) of captured powders for 10 wt.% CDQD briquettes, heat treated at 800 °C and 900 °C.

Element		Content (wt.%, ±1)
		800 °C	900 °C
Zn	16		51
C	65		24
O	16		13
Na	2		4
Pb	5		14

. Due to data scatter observed in several measurements, only average values are tabulated as representative trends. Zinc, carbon, lead and other light elements were detected in the captured products. With increasing briquette processing temperatures, the proportion of carbon present in the gaseous phase was found to decrease whereas the captured amounts of zinc and lead showed an increase. This is an important finding and will be discussed later in conjunction with other findings.

3.3.2. Electron Microscopic Investigations

A representative example of the SEM/EDS results on the captured powders after the heat treatment of 10 wt.% CDQD briquettes at 800 °C is shown in Figure 6. While no specific features were detected in the SEM micrograph, there were few outlines of spherical shapes distributed randomly. From the two sets of EDS data presented here, the elemental composition was determined to be as follows: #A: C (93 wt.%); Zn (5.1 wt.%); Na (1.3 wt.%); S (0.4 wt.%); Cl (0.2 wt.%). #B: C (61.3 wt.%); O (21.5 wt.%); Zn (12.3 wt.%); Na (2.1 wt.%); Pb (2.1 wt.%); S (0.4 wt.%); Cl (0.4 wt.%). The elements indicated in red font were determined with high uncertainty due to the limitations of the method and sample preparation.

The SEM/EDS results on the powders captured after the heat treatment of 10 wt.% CDQD briquettes at 900 °C are shown in Figure 7.

A distribution of spherical morphologies was observed in the captured powders. High-resolution images of these powders are shown in Figure 8. Most of the larger particulates were found to be spherical in shape, with typical sizes ~2–6 μm or so. The nucleation of a large number of spherical phases that were sub-micron in size can also be seen. These phases were very rich in Zn; a typical composition, as discerned from the EDS data, was as follows: Zn (78.2 wt.%); C (10.0 wt.%); Pb (6.0 wt.%); O (5.8 wt.%).

The capture of particles after reduction with distilled water imposes a number of restrictions on their composition; in particular, some soluble compounds can be washed out and not detected. The key result here is the detection of metallic zinc as well as lead in these powders. It is planned to study quantitative indicators and the material balance of combined restoration using model installations.

3.3.3. X-Ray Diffraction Studies

The X-ray diffraction results on the captured powders are shown in Figure 9. These results indicate the presence of metallic Zn, ZnO and small amounts of KCl. The presence of metallic Zn in a reduced form is an important finding that is significantly different from the traditional Waelz process [38,39]. The generation of valuable metallic raw materials detected here will be more efficient for further purification and resource utilization.

4. Discussion

Based on the ZBD composition data and data from the literature regarding the mechanisms of carbothermic and hydrogen reduction [54,55,56], the following key reactions are likely to occur during carbothermal and hydrogen reduction at high temperatures:

ZnFe₂O₄ + 4H₂(g) → Zn(g) + 2Fe + 4H₂O(g)

(1)

ZnFe₂O₄ + 2C → Zn(g) + 2Fe + 2CO₂(g)

(2)

Fe₂O₃ + 3C → 2Fe + 3CO(g)

(3)

Fe₂O₃ + 3H₂(g) → 2Fe + 3H₂O(g)

(4)

The hydrogen reduction of ZnFe₂O₄ (Equation (1)) is likely to initiate in the temperature range of 400–500 °C [55], but carbothermal processes would initiate at temperatures up to 1000 °C [53]. The carbothermal reduction of ZnFe₂O₄ can proceed in different ways, with the following products being formed: ZnO, FeO, Zn, CO and CO₂ [56].

Based on the calculations of Gibbs free energy (∆G⁰) changes in the reactions (Figure 10), the initiating temperature for this process is above 500 °C, and zinc can be released in the zinc vapor phase, a result which is fully consistent with the observed results (Figure 9). Based on the calculation from general principles, the beginning of the reduction of zinc ferrite with hydrogen should occur at about 500 °C, and above 1370 °C with carbon (Figure 10).

The reduction of Fe₂O₃ with carbon and hydrogen is a complex process involving several stages: Fe₂O₃ → Fe₃O₄ → FeO → Fe [52,56]; these are widely covered in the scientific literature. The reduction of iron (Equations (3) and (4)) in general is expected to initiate at ~1050–1100 °C [54]. The carbothermal processes (Equations (2) and (3)) of zinc ferrite and iron oxide decomposition are generally expected to start at temperatures above 1000 °C [54,57,58]. However, the presence of easily melting salts of alkali and alkaline-earth metals can contribute to the formation of eutectic mixtures, which will exhibit increased reactivity at lower temperatures. The reduction of carbon dioxide to carbon monoxide in the presence of carbon (the well-known Boudouard–Bell reaction) is likely to occur at temperatures above 700 °C [57,58].

The decomposition of zinc oxide and zinc ferrite, as well as higher iron oxides, during reduction was confirmed by X-ray diffraction analysis (Figure 11). After reduction, no zinc-containing phases were observed in the reduction residues for all three temperatures investigated. A reflection peak for carbon was observed at 700 °C, indicating the presence of unconsumed carbon. The carbon peak was no longer present at 800 °C and 900 °C, indicating the onset of carbothermal reactions and the consumption of carbon. Peaks for metallic iron were observed at all three temperatures investigated. This result indicates that at 700 °C, where the carbothermal reduction had yet to fully initiate, metallic iron was produced through the hydrogen-based reduction. The peaks for FeO were also observed at all three temperatures. The observed diffraction data further confirm the dissociation and the absence of zinc ferrite (ZnFe₂O₄) phase, the iron oxide reduction and the complete removal of Zn from the reduction residues.

To confirm the complete removal of Zn and Pb during the reduction process, the briquettes were investigated by ICP. According to the ICP data, the content of Zn in the dust samples before reduction was 18.20 ± 0.15 wt.%; after reduction, it ranged between 0.022 ± 0.008 and 0.048 ± 0.013 wt.%, depending on the temperature and composition. The zinc levels in the reduced briquettes were below the typical Zn levels (0.05–0.09 wt.%) in the iron ores used in the primary production of steel.

While Zn was completely removed from the reduced briquettes rich in iron, it was captured in the gaseous exhaust in the form of powders. For the briquettes containing the highest (10 wt.%) CDQD reductant, a few interesting trends were observed. The levels of Zn captured showed an increase from 16 wt% (800 °C) to 51 wt.% (900 °C); however, the amount of carbon captured decreased from 65 wt% (800 °C) to 24 wt.% (900 °C). The presence of carbon among the gaseous products can be attributed to the high vapor pressures of Zn and Pb, which could carry solid carbon particles with Zn/Pb vapors [59,60]. As indicated in Figure 5, there was a significant presence of residual carbon in the briquettes reduced at 800 °C; however, the concentration of carbon was much reduced at 900 °C due to the onset of the carbothermal reactions. Even though there was a significant release of Zn vapors at 900 °C, not much carbon had escaped into the exhaust phase.

The X-ray diffraction studies (Figure 9) on the recovered Zn-based powders showed the presence of metallic Zn phase along with ZnO. Li et al. [61] were able to recover 98% of iron, more than 99% of Zn and 89% Pb from zinc-bearing EAFDs through a smelting reduction at 1450 °C. A complete separation and recovery of both Fe and Zn was achieved at significantly lower temperatures in this study. Yang et al. [62] were able to recover Zn and Pb from EAFDs through a vacuum carbothermal reduction at 1000 °C; however, no iron was recovered in their study. The present study operates at much lower temperatures, using a combination of carbothermal and hydrogen reductions to achieve a high level of Fe and Zn recovery. It therefore presents a significant advance in the field towards enhancing the environmental and economic sustainability of resource recovery from hazardous wastes from the steelmaking sector.

The reduced briquettes can be mixed with other raw materials in steelmaking, e.g., in blast furnaces. The captured evaporated products, thanks to a partially restored form of zinc, can be easily divided into separate elements compared to oxide products. In addition, advances in technology can lead to the recovery of metallic phases at the output, making these more attractive to the market, including the possibility of returning to the process of galvanizing steel. In addition, finely dispersed oxides of zinc and lead can find application in the rubber, paint and varnish industries.

5. Conclusions

This study established the feasibility of a combined carbothermal and hydrogen reduction approach for processing hazardous zinc-bearing EAF dusts (ZBDs) at relatively low temperatures. While the operating temperatures used in this study were somewhat higher than those required for 100% H₂ reduction, these were significantly lower than those required for 100% carbothermal reduction. This approach can make valuable contributions to the field in view of the limited availability of H₂ gas. In addition, by using a carbothermal reduction in conjunction with a H₂ reduction, the generation of greenhouse gases (CO₂) was much reduced as compared to 100% carbothermal reduction. It is important to note that another waste metallurgical product, namely, coke oven battery dry quenching dust (CDQD), was used as a carbonaceous reductant, which allowed us to consider the technology as an integrated approach for ensuring a closed production cycle of a metallurgical enterprise. Zn-based phases could be recovered in a metallic state through capture in the reactor exhaust. A complete separation was also achieved in the iron and zinc metallic phases recovered.

This study has shown that hazardous industrial waste such as EAFDs can be valuable resources for iron and Zn metals. Using a combination of two reduction methods, the iron recovery potential of EAFDs was significantly enhanced along with the removal of hazardous Zn dust. This study has the potential to open new research areas in industrial waste management wherein the use of combined reduction approaches could offer significantly lower operating temperatures and enhance process efficiencies and productivities in a range of applications. This approach would lead to enhanced resource recovery, process sustainability, reductions in greenhouse gas emissions and environmental protection.