Characterization of Physical and Chemical Properties of Multi-Source Metallurgical Dust and Analysis of Resource Utilization Pathways

Abstract

Steel metallurgical dust, characterized by a substantial output, minute particle size, and intricate composition, poses a considerable risk of environmental contamination while simultaneously embodying an exceptionally high potential for recycling. To achieve its resource utilization, chemical analysis, particle size analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, and water leaching methods were employed to investigate the chemical compositions, particle size distributions, phase compositions, and microscopic morphologies of blast furnace bag dust, sintering dust, converter fine dust, and electric arc furnace dust from steel plants. The results indicate that the four types of dust have extremely fine particle sizes, with the main distribution range of particle size being less than 100 μm. The main constituent element is Fe (19–56%), and it also contains Zn (1.4–33.5%), Pb, K, C, and other valuable elements. Alkali metals in blast furnace bag dust and sintering machine head dust existed mainly in the form of chloride. The zinc phases in sintering machine head dust and converter fine dust were ZnFe₂O₄, and the zinc phases in blast furnace bag dust were ZnCl₂ and ZnFe₂O₄. Zinc in electric furnace dust was composed of ZnO and ZnFe₂O₄, accounting for 70.31% and 23.12%, respectively. There are significant differences in the types and contents of valuable elements among various dusts, making it difficult to achieve full-scale recovery through a single process. In view of this, a process of “in-plant recycling of harmless dusts—collaborative treatment of harmful dusts” has been proposed. Based on the characteristics of metallurgical dusts, multiple processes are used for collaborative treatment (using hydrometallurgical and pyrometallurgical methods), which can not only directly recover iron resources from dusts within the plant, but also avoid the waste of valuable elements such as Zn, Pb, K, Na, etc. It is hoped that the above work can provide a reference for steel enterprises to achieve full-scale and high value-added treatment of metallurgical dusts.

1. Introduction

China’s large- and medium-sized steel enterprises adopt the traditional long-process production process of “raw material—ironmaking—steelmaking—rolling”, supplemented by the modernized short-process production process of “electric furnace-continuous casting-rolling” [1]. The long-process steelmaking technology boasts high production efficiency and large output, but the long production process with resource- and energy-intensive operations results in a huge amount of solid waste generation. Metallurgical dust is a typical iron-bearing solid waste in steel enterprises, mainly including sintering dust, blast furnace dust, converter dust, and electric arc furnace dust. The amount of metallurgical dust generated is 8–12% of the crude steel output [2]. In 2023, China’s crude steel production was 1.019 billion tons, resulting in approximately 100 million tons of dust generated.

The choice of metallurgical dust resource utilization methods is generally determined by the characteristics of the dust being processed. The main constituent elements of sintering dust are Fe, K, Cl, and Ca, with small amounts of Na, Zn, Pb, and other elements [3]. Most of the Fe exists in the form of oxides, while K and Na are present as chlorides. Therefore, a water washing process can be used to extract KCl and NaCl from the dust, and then the iron-containing filter residue after washing can be directly recycled into the sintering production process. This method enables the separation of iron oxides and alkali metal chlorides in sintering dust and is currently the best process for the resource utilization of sintering dust [4].

Blast furnace dust is mainly composed of Fe and C, accounting for approximately 50% of its total composition [5]. In addition, it also contains small amounts of K, Na, Zn, and Pb, and trace amounts of Bi, In, Sn, and other elements [6]. The primary methods for resource utilization of blast furnace dust include mineral processing, pyrometallurgy, and hydrometallurgy. The iron-containing substances in the dust are magnetic, and the carbon has good floatability, allowing for the enrichment of iron and carbon resources in the dust through magnetic separation and flotation. However, it is worth noting that the enriched ore obtained through mineral processing has a relatively low grade and still contains harmful elements such as K, Na, and Zn, making mineral processing not the optimal method for blast furnace dust resource utilization [7]. The content of non-ferrous metals in blast furnace dust is generally low, making hydrometallurgical processes economically unfeasible. Blast furnace dust can utilize its own carbon as a heating and reducing agent in pyrometallurgical processes to separate Fe from non-ferrous metal elements such as Zn and Pb. Unfortunately, the Zn content in blast furnace dust is typically below 10% [8,9,10], and the use of blast furnace dust as the primary raw material for pyrometallurgical processes alone also poses economic challenges.

Converter dust and electric arc furnace dust are both generated during the steelmaking process, with their primary components including Fe, Zn, Ca, and other elements [11]. The Zn content in converter dust and electric arc furnace dust depends on the Zn content in the scrap steel added. The goal of electric arc furnace dust resource utilization is the separation and extraction of Fe and Zn. The choice of resource utilization path for converter dust depends on its Zn content. When the Zn content is less than 1%, it is directly recovered by sintering and pelletizing processes. When the Zn content is high, it needs to be treated with hydrometallurgical and pyrometallurgical methods [12]. The current process for the separation and extraction of Fe and Zn from converter dust and electric arc furnace dust primarily involves a combined pyrometallurgical–hydrometallurgical treatment process. In this process, pyrometallurgy is used to remove zinc from the dust and enrich it in the secondary dust [13,14]. Subsequently, hydrometallurgical leaching is employed to extract Zn from the secondary dust. The combined pyrometallurgical–hydrometallurgical treatment process is the optimal method for handling such dust, although it has the drawback of high energy consumption during the pyrometallurgical stage.

Currently, most Chinese iron and steel companies recover dust with low K and Zn content through the sintering process and sell dust with high K and Zn content to companies that produce K and Zn, respectively. This situation increases transportation costs and the risk of environmental pollution, while the iron in the dust is not effectively used for iron and steel smelting. In 2021~2022, China’s Development and Reform Commission (NDRC), Ministry of Industry and Information Technology (MIIT), Ministry of Science and Technology (MOST), and other departments have jointly issued the “Guiding Opinions on the Comprehensive Utilization of Bulky Solid Wastes in the 14th Five-Year Plan” and the “Implementation Plan for Accelerating the Promotion of Comprehensive Utilization of Industrial Resources”. It is clearly pointed out that in the iron and steel metallurgy industry, the “solid waste does not leave the factory” guideline should be promoted to strengthen the full quantitative utilization requirements. In addition, a single dust treatment process is only applicable to the recovery of a certain type of dust and cannot realize the full quantitative utilization of multiple types of dust. Therefore, iron and steel enterprises need to study the physical and chemical properties of multiple types of dust, and on this basis, through synergizing multiple dust recycling processes, realize the full quantitative utilization of multi-source metallurgical dust in the case of “solid waste not leaving the factory”.

This paper investigates the physicochemical properties of blast furnace bag dust, converter fine dust, sintering machine head dust, and electric arc furnace dust from a steel enterprise in northern China, including their chemical compositions, microscopic morphologies, and phase compositions. Based on this, this paper explores feasible technical solutions for the coordinated treatment of multiple types of dust in this company, aiming to provide a foundation for the comprehensive in-plant resource utilization of multi-source metallurgical dust in steel enterprises.

2. Materials and Methods

2.1. Materials

The experimental raw materials were obtained from a steel enterprise in northern China. The types of dust generated by the enterprise and their utilization pathways are shown in Figure 1. The sintering machine tail dust, pellet dust, blast furnace gravity dust, converter coarse dust, and rolling mill scale produced by this company contain relatively low levels of harmful elements and are directly recycled for use in the sintering process.

However, sintering machine head dust, blast furnace bag dust, converter fine dust, and electric arc furnace dust generally contain harmful elements and cannot be recycled internally within the enterprise, thus being directly sold to other enterprises. Currently, many regions advocate for the resource utilization of such fine-grained dust containing heavy metals within steel enterprises. Therefore, it is necessary to conduct research on the physicochemical properties of these difficult-to-treat dust types.

This paper sampled the sintering machine head dust, blast furnace bag dust, converter fine dust, and electric arc furnace dust from the company. The four types of dust were mixed separately, and 1 kg samples were taken from the mixed dust using the quartering method. All samples were then dried at a temperature of 378 K, sealed, and stored for future use. The main chemical reagents used for dust composition testing are listed in

Table 1. Main chemical reagents used for testing.

Reagent Name	Analytical Grade	Manufacturer
Hydrochloric acid	AR	Yongda Chemical (Tianjin, China)
Sulfuric acid	AR	Yongda Chemical (Tianjin, China)
Phosphoric acid	AR	Yongda Chemical (Tianjin, China)
Nitric acid	AR	Yongda Chemical (Tianjin, China)
Hydrofluoric acid	AR	Yongda Chemical (Tianjin, China)
Titanium trichloride solution	AR	Yongda Chemical (Tianjin, China)
Sodium fluoride	AR	Yongda Chemical (Tianjin, China)
Sodium hydrogen carbonate	AR	Aladdin Bio-Chem Technology (Shanghai, China)
Iron chloride	AR	Aladdin Bio-Chem Technology (Shanghai, China)
Sodium hydroxide	AR	Aladdin Bio-Chem Technology (Shanghai, China)
Xylenol orange	AR	Yongda Chemical (Tianjin, China)
Potassium dichromate	AR	Yongda Chemical (Tianjin, China)
Sodium diphenylamine sulponate	AR	Yongda Chemical (Tianjin, China)

.

2.2. Methods

The carbon content in the sintering machine head dust, converter fine dust, and electric arc furnace dust was measured using a carbon-sulfur analyzer (CS844, LECO, St. Joseph, MI, USA). The carbon content in the blast furnace bag dust was measured using an industrial analyzer (SDTGA6000A, Sundy, Changsha, China). Fe, Ca, Al, and Si were detected using chemical titration methods. K, Na, Pb, Zn, and Mg were detected using atomic absorption spectrometry (AAS). For chemical composition testing, each component is tested twice, and if the absolute value of the difference between the two measurements is less than the permitted difference, the result is taken as the average of the two results. On the contrary, it is necessary to measure again, and if the difference between the maximum and minimum values of the three measurements is less than 1.2 times the permissible difference, the average result of the three measurements will be taken.

The particle size distribution of the four types of dust was analyzed using a laser particle size analyzer (Mastersizer2000, Malvern Panalytical, Malvern, UK). Ethanol was used as the dispersant during testing, and the measurement range was 0.02 to 2000 μm.

The phase composition of the metallurgical dust was analyzed using a small-angle X-ray diffractometer (Empyrean 03030502, Malvern Panalytical, Malvern, UK). The morphology of the dust particles was analyzed using a scanning electron microscope (Sigma 300, ZEISS, Oberkochen, Germany). The distribution of major elements in the dust was detected using energy dispersive X-ray spectroscopy (Xplore30, Oxford Instruments, Oxford, UK).

The content of ZnFe₂O₄ in the electric arc furnace dust was determined using a Mössbauer spectrometer (MR-2500, Maschinenfabrik Reinhausen GmbH, Regensburg, Germany). The radiation source was ⁵⁷Co(Rh), and the detector was a proportional counter. The velocity was calibrated using an α-Fe foil, and the absorption thickness of the sample was adjusted to 10 mg Fe/cm². The ⁵⁷Fe Mössbauer spectrum was obtained at room temperature. The spectrum was fitted using the least squares method, and the qualitative analysis of the iron-containing phases in the electric arc furnace dust was performed based on the fitting parameters, including the isomer shift (IS), quadrupole splitting energy (QS), hyperfine magnetic field H(T), and absorption linewidth (Γ).

The experimental procedure for the water leaching characteristics of blast furnace bag dust and sintering machine head dust is shown in Figure 2. Separately, 50 g of blast furnace bag dust and sintering machine head dust were mixed with deionized water at a solid-to-liquid ratio (mass ratio) of 1:4, with a rotation speed of 200 r/min, a leaching temperature of 50 °C, and a leaching time of 30 min. After leaching, the mixed slurry was separated into solid and liquid phases using a vacuum filtration machine, and the filter residue was dried at 105 °C. The leachate was crystallized at 60 °C, and then the temperature was adjusted to 150 °C for drying. The dried filter residue and the crystallization products of the leachate were manually ground to a particle size of less than 200 mesh using a mortar, and the composition and phase of the samples were analyzed using XRF and XRD. Three points were taken from each sample for compositional testing, and the average of the three results was taken as the final test result.

3. Results

3.1. Chemical Composition

The main chemical compositions of blast furnace bag dust, sintering machine head dust, converter fine dust, and electric arc furnace dust are shown in

Table 2. Main chemical components of several types of dust/wt%.

Types of Dust	TFe 1	Fe2O3	FeO	MFe 2	SiO2	Al2O3	CaO
Blast furnace bag dust	24.38	30.88	3.15	0.26	5.04	2.86	2.92
Sintering machine head dust	32.35	43.36	2.6	-	3.72	1.56	10.12
Converter fine dust	55.79	49.75	16.54	8.13	1.31	0.15	10.87
Electric arc furnace dust	19.05	25.36	1.48	0.16	3.14	0.65	8.47
Types of Dust	MgO	Pb	Zn	K	Na	Cl	C
Blast furnace bag dust	3.41	0.32	2.86	5.42	0.59	3.44	30.64
Sintering machine head dust	2.62	0.63	1.41	12.18	1.44	11.13	1.9
Converter fine dust	2.52	0.059	5.56	0.42	0.24	0.39	0.67
Electric arc furnace dust	2.78	0.59	33.48	1.51	2.38	-	1.32

¹ Refers to total iron content; ² refers to the amount of iron monomers.

. As can be seen from Table 2, the fixed carbon mass fraction in the blast furnace bag dust is 35.95%, and the Fe mass fraction is 24.38%. The combined mass proportion of Fe and C exceeds 60%, indicating a high recycling value. The main constituent elements of sintering machine head dust are Fe, K, and Cl, with mass fractions of 32.35%, 12.18%, and 11.13%, respectively. In the converter fine dust, the Fe mass fraction is 55.79%, with an iron grade higher than that of ordinary iron ore. However, due to the presence of zinc, it cannot be directly used as an iron-bearing mineral. Zn and Fe are the main components in electric arc furnace dust, with Zn accounting for 33.48% of the mass fraction, making it a high-quality secondary zinc resource.

In summary, Fe is a common primary component among the various dust types, especially in converter fine dust where the Fe mass fraction exceeds 55%. Therefore, iron recovery should be considered a critical indicator in the dust resourcification process. The sintering process is the primary means for metallurgical enterprises to recover iron-containing solid waste, enabling efficient recovery of iron elements from dust. However, when the total alkali metal (K, Na) content in the dust exceeds 2.5%, or the Zn content exceeds 1%, it cannot be directly used as sintering mix materials [15]. Based on this assessment, none of the four types of dust can be directly recycled into the sintering process.

On the one hand, K, Na, and Zn hinder the direct reuse of iron-containing dust in the pre-ironmaking process, but on the other hand, these elements also possess economic value, especially since K and Zn are scarce resources in China, with the degree of foreign dependence increasing year by year. Therefore, the primary goal of the dust resourcification process should be to fully separate and recover valuable elements such as Fe, Zn, and K.

3.2. Granulometry Analysis

The particle size distribution of three kinds of dust is shown in Figure 3. The particle size of blast furnace bag dust is mainly distributed in two ranges of 0.63~29 μm and 29~375 μm, among which the coarse particle range of 29~375 μm accounts for the main part. The particle size distribution of sintering machine head dust and converter fine ash is relatively uniform, and their main particle size distribution ranges are 2.7~40.4 μm and 1.95~33.6 μm, respectively. The particle size of electric arc furnace dust is distributed in four ranges of 0.1~0.4 μm, 0.4~1.3 μm, 1.3~7.6 μm, and 7.6~52.5 μm, most of which are distributed in the range of 0.1~0.4 μm.

It can be seen that among the four dust samples, the particle size of blast furnace bag dust is the coarsest, and that of electric furnace dust is the finest, which is also confirmed by the subsequent scanning electron microscope images. At present, most iron and steel enterprises adopt pyrometallurgical process to treat zinc-containing dust and sludge. Generally, the pyrometallurgical process requires the granulation of raw materials [12]. Several kinds of dust particles are fine and do not need to be ground in the granulation process. At the same time, the use of dust particles with different particle sizes for granulation will make the pellets dense and increase the pellet strength [16].

In addition, fine dust particles are prone to cause air pollution during handling. Therefore, in order to reduce the risk of environmental pollution, the resource recovery process of metallurgical dust should be carried out within the steel enterprises as much as possible.

3.3. Microscopic Morphology and Element Distribution

The microscopic morphology and main element distribution of blast furnace bag dust, sintering machine head dust, converter fine dust, and electric arc furnace dust particles are shown in Figure 4, Figure 5, Figure 6 and Figure 7.

The particle sizes of blast furnace bag dust and sintering machine head dust are relatively large, while the particle sizes of converter fine dust and electric arc furnace dust are smaller in comparison. The morphology of blast furnace bag dust is dominated by irregular blocky particles with different sizes. The surfaces of smaller particles are rougher, while the surfaces of larger particles are relatively smoother. Some particles have rough, tiny powders attached to their surfaces. Compared with blast furnace bag dust, sintering machine head dust has slightly smaller particle sizes, most of which are fluffy clusters with rough surfaces, and there are also larger and irregular strip-block particles. The particle morphology of converter fine dust and electric arc furnace dust is similar, with smaller particle sizes and relatively uniform particle size distribution. The morphology is mostly spherical, and a few of them aggregate into larger lump particles.

The larger particles in blast furnace bag dust have a higher content of fixed carbon, which should be unburned coal powder or coke. The smaller particles have a higher distribution of Fe, O, K, Na, and Zn. Overall, the distributions of Fe, O, and K, Cl are relatively consistent, while Zn overlaps with the distributions of Cl, Fe, and O. Therefore, iron and potassium in blast furnace bag dust exist as FeO_x and KCl, while the occurrence state of zinc may be ZnCl₂, ZnO, or ZnFe₂O₄. In sintering machine head dust, the distributions of K and Cl are consistent, both of which are distributed in particles with relatively rough surfaces, and the corresponding substance should be KCl. The larger, blocky particles with smoother surfaces are mostly iron and calcium compounds. The distributions of Fe, O, and Zn in converter fine dust are relatively uniform, and the particles are mostly iron oxides with oxides or compounds of zinc attached to their surfaces. Zn is the main element in electric arc furnace dust, and the distributions of Zn, Fe, and O are similar. The corresponding substances should be mainly ZnO and FeO_x, with a high possibility of ZnFe₂O₄ existing as well.

Based on the distribution of major elements in the dust, Fe and C in blast furnace bag dust are interactively adhered to elements such as K and Zn and are not completely separated. The use of physical mineral processing techniques (such as magnetic separation, flotation) poses difficulties in achieving separation. In sintering machine head dust, K and Cl are attached to the surface of iron-bearing materials or exist independently as small fluffy particles, making it likely that KCl can be separated from the dust through water washing processes. Conversely, in converter fine dust and electric arc furnace dust, Fe, Zn, and O are uniformly distributed, necessitating the adoption of pyrometallurgical or hydrometallurgical methods to separate Zn from Fe. Furthermore, it is crucial to determine the phase compositions of the several major elements to facilitate the formulation of an accurate resource recovery plan.

3.4. Phase Composition

The X-ray diffraction analysis results of blast furnace bag dust, sintering machine head dust, converter fine dust, and electric arc furnace dust are shown in Figure 8. The zinc-containing phases in blast furnace bag dust are ZnFe₂O₄ and ZnCl₂, and the iron-containing phases are Fe₂O₃ and Fe₃O₄. In addition, K and Na exist in the forms of KCl and NaCl.

The iron-containing phases in the sintering machine head dust are primarily Fe₂O₃ and Fe₃O₄, with potassium and sodium existing in the form of chlorides, and zinc existing in the form of ZnFe₂O₄.

The iron-containing phases in the converter fine dust are Fe₃O₄, FeO, and a small amount of MFe, while the zinc-containing phases are primarily ZnFe₂O₄, with the presence of CaO, Ca(OH)₂, and SiO₂. In the electric arc furnace dust, the zinc phases, in addition to ZnFe₂O₄, also include ZnO [17].

From the perspective of phase composition, the blast furnace bag dust and sintering machine head dust are suitable for utilizing a water washing salt extraction process to recover KCl and NaCl from the dust. The converter fine dust has a relatively low zinc content, with the zinc-containing phase being ZnFe₂O₄, which can be enriched in secondary dust through a pyrometallurgical process for centralized extraction in later stages. The zinc content in electric arc furnace dust is high, and the phases of zinc are ZnFe₂O₄ and ZnO. The zinc can be separated from the iron-containing materials by the fire process, and the zinc can also be extracted directly by the wet process. The leaching conditions applicable to the two phases of ZnFe₂O₄ and ZnO are different. Therefore, it is necessary to further determine the content of ZnFe₂O₄ and ZnO in the dust.

3.5. Mössbauer Spectrum

Composition analysis revealed a zinc content of 33.48% in electric arc furnace dust, enabling Zn recovery from dust via wet methods. However, XRD diffraction showed that Zn phases in electric arc furnace dust were predominantly ZnFe₂O₄ and ZnO. As ZnFe₂O₄ has a stable molecular structure, its content significantly impacts Zn leaching efficiency. Accurate quantification of iron-bearing phases can be achieved using Mössbauer spectroscopy, thus this study employed the technique to determine ZnFe₂O₄ content in electric arc furnace dust.

The Mössbauer spectrum of electric arc furnace dust is shown in Figure 9. The iron-bearing phases in the dust used in this study were Fe₃O₄ and ZnFe₂O₄, which align with the findings of Machado et al. [18] and corroborate XRD results. The iron mass ratio of Fe₃O₄ to ZnFe₂O₄ in

Table 3. Mössbauer parameters used in the spectrum fit of the electric arc furnace dust.

Sample	Phase	IS (mm/s)	QS (mm/s)	H (T)	Γ (mm/s)	Area (%)
Electric arc furnace dust	ZnFe2O4	0.22	0.50	—	0.47	70.0
	Fe3O4(A)	0.13	−0.03	49.72	0.65	10.0
	Fe3O4(B)	0.42	−0.01	47.13	1.29	20.0

is 3:7. By combining the mass fractions of Fe and Zn in the dust, the masses of ZnO and ZnFe₂O₄ can be quantitatively calculated using Equations (1)–(4) [18,19].

$$\%\mathrm{Z}nF{e}_2{O}_4={\displaystyle \frac{\%F{e}_{(\mathrm{Z}nF{e}_2{O}_4)}\times M_{(\mathrm{Z}nF{e}_2{O}_4)}}{M{\mathrm{Fe}}_{(\mathrm{Z}nF{e}_2{O}_4)}}}$$

(1)

In the formula, $\%\mathrm{Z}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4$ is the mass fraction of ZnFe₂O₄ in the dust; $\%{\mathrm{F}\mathrm{e}}_{\left(\mathrm{Z}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4\right)}$ is the mass fraction of Fe in ZnFe₂O₄ in dust; $\mathrm{M}{\mathrm{F}\mathrm{e}}_{\left(\mathrm{Z}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4\right)}$ is the molar mass of Fe in ZnFe₂O₄; and ${\mathrm{M}}_{\left(\mathrm{Z}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4\right)}$ is the molar mass of ZnFe₂O₄.

$${\%\mathrm{Zn}}_{(\mathrm{Z}nF{e}_2{O}_4)}={\displaystyle \frac{\%\mathrm{Z}nF{e}_2{O}_4\times {{\rm M}}_{({\rm Z}n)}}{M_{(ZnF{e}_2{O}_4)}}}$$

(2)

In the formula, $\%{\mathrm{Z}\mathrm{n}}_{\left(\mathrm{Z}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4\right)}$ is the mass fraction of Zn in ZnFe₂O₄; ${\mathrm{M}}_{\left(\mathrm{Z}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4\right)}$ is the molar mass of ZnFe₂O₄; and $M_{\left(Zn\right)}$ is molar mass of Zn.

$$\%Z{n}_{(ZnO)}=\%Zn-\%Z{n}_{(ZnF{e}_2{O}_4)}$$

(3)

In the formula, $\%{\mathrm{Z}\mathrm{n}}_{\left(\mathrm{Z}\mathrm{n}\mathrm{O}\right)}$ is the mass fraction of Zn in ZnO and $\%\mathrm{Z}\mathrm{n}$ is the mass fraction of Zn in dust.

$$\%ZnO={\displaystyle \frac{\%Z{n}_{(Zno)}\times M_{(ZnO)}}{M_{(Zn)}}}$$

(4)

In the formula, $\%\mathrm{Z}\mathrm{n}\mathrm{O}$ is the mass fraction of ZnO in dust and $M_{\left(ZnO\right)}$ is the molar mass of ZnO.

Calculations show that ZnFe₂O₄ accounts for 28.69% and ZnO for 32.076% of the dust’s mass. Zn in ZnFe₂O₄ comprises 23.12% of the total Zn content. ZnFe₂O₄ possesses a stable spinel structure, posing significant challenges for Zn leaching, affecting economic efficiency [20,21]. With 70% of iron in ZnFe₂O₄, using concentrated strong acids can enhance Zn leaching but also dissolves iron, complicating impurity removal and hindering iron recovery. It is worth noting that ZnFe₂O₄, although structurally stable, is capable of decomposing to ZnO and Fe₃O₄ in high temperature environments with a volume fraction of CO above 4% [22]. It is capable of being converted to metallic iron and zinc vapor when the CO content is sufficient [23]. Therefore, carbothermal reduction is more attractive in the process of resourcing electric furnace dust.

3.6. Water Leaching Characteristics

When the solid–liquid ratio (mass ratio) is mixed at 1:4, the rotational speed is 200 r/min, the leaching temperature is 50 °C, and the leaching time is 30 min, the major element contents of the sintering machine head dust and blast furnace bag dust before and after leaching and their leachate crystallization products are shown in

Table 4. Composition of two types of dust, leaching residue, and leaching solution crystals/wt%.

Sample	Fe	Cl	K	Ca	S	Na	Zn	Pb	Others
Sintering machine head dust	39.01	18.34	17.50	10.53	3.62	3.34	0.10	—	7.56
Head dust filter residue	72.36	0.20	0.27	13.29	1.88	—	0.18	—	11.82
Crystallization of head dust filtrate	0.02	37.57	43.07	5.36	5.27	6.73	—	—	1.98
Blast furnace bag dust	40.58	22.09	9.483	4.82	2.57	—	9.24	1.73	9.487
Bag dust filter residue	73.05	0.52	0.66	3.73	2.71	—	3.21	2.34	13.78
Crystallization of bag dust filtrate	—	50.20	19.45	8.88	1.46	—	17.77	0.80	1.44

. As can be seen from Table 4, almost all K and Cl elements in sintering machine head dust and blast furnace bag dust enter the filtrate after water leaching, with leaching rates of 98.92% and 94.69%, respectively. In addition, 73.48% of Zn in the blast furnace bag dust enters the filtrate, and a small part remains in the filter residue.

The phase composition of leaching residue and crystallized product in leachate is shown in Figure 10. As can be seen from Figure 10, after water leaching of the two kinds of dust, the main phases of leaching residue are both Fe₂O₃, and the sintering machine head dust filter residue also contains Ca₂Fe₂O₅.

The phases of the crystalline matter in the leachate of sintering machine head dust are KCl and NaCl. In fact, a small amount of Ca and Fe ions also enter the solution during the leaching process, but due to the low content and crystallinity, relevant diffraction peaks do not appear in the X-ray. The crystalline phase of blast furnace bag dust includes KCl, NaCl, CaSO₄, CaCl₂, and K₂(ZnCl₄). The presence of the K₂(ZnCl₄) phase is due to the formation of ZnCl₂ and KCl in the leachate during the crystallization process. This also confirms the presence of ZnCl₂ in the blast furnace bag dust. While Zn did not enter the leachate in the sintering machine head dust, indicating that the phase of Zn in the sintering machine head dust is ZnFe₂O₄.

The sintering machine head dust and blast furnace bag dust can be leached of their chlorides through a water washing process. After leaching, KCl and NaCl products are obtained through evaporation and crystallization processes. The decision on whether to return the filter residue directly to sintering or for further processing is based on the contents of K, Na, and Zn in the residue. It is worth noting that KCl production enterprises are only interested in dust with high K content. Additionally, to reduce production costs, some K and Na remain in the leaching residue, which is still not suitable for returning to the sintering process.

4. Analysis of Resource Utilization Methods

Through physical and chemical characterization, it was found that iron and carbon in blast furnace bag dust, potassium chloride in sintering machine head dust, and zinc in converter fine dust and electric arc furnace dust all have high recovery value. At present, the dust resource utilization processes mainly include the pyrometallurgical process, water washing and salt extraction process, wet zinc extraction process, and pyrometallurgical wet process combined treatment process. The pyrometallurgical process, water washing and salt extraction process, and wet process are aimed at the harmless treatment of dust, the production of potassium chloride, and the extraction of non-ferrous metals, respectively. It is difficult to fully quantify the valuable elements in dust using a single treatment process. The combined pyrometallurgical and hydrometallurgical treatment process includes two parts, harmless dust treatment and valuable element extraction, and is currently the optimal process for metallurgical dust resource utilization [24].

At present, “solid waste not leaving the factory”, “full resource utilization”, and “low-carbon energy conservation” have become the three major labels for solid waste recycling. The requirement of “solid waste not leaving the factory” requires that solid waste generated by enterprises should be treated as much as possible within the factory. The purpose of “full resource utilization” is to recover the main elements Fe, Zn, and K in dust, as well as extract trace elements such as Pb, In, and Bi. The combination of pyrometallurgical and hydrometallurgical processes can perfectly achieve these two goals. China Honghe Zinc Union Technology Development Co., Ltd. has adopted a combined pyrometallurgical and hydrometallurgical treatment process to solve some technical difficulties in pyrometallurgical roasting and hydrometallurgical extraction. At present, the company’s annual processing capacity for metallurgical dust has reached 1.3 million tons.

In terms of low-carbon energy conservation, it is necessary to reduce the dependence on coal powder and coke in the dust treatment process, which can be replaced by using secondary carbon resources, biomass charcoal, organic solid waste, and hydrogen as energy raw materials. At present, the use of biomass charcoal and organic solid waste [25,26] as the fuel or reducing agent for metallurgical high-temperature production processes is still in the research stage. Hydrogen has been applied in some direct reduction and steel smelting processes [27,28], but its high production cost restricts its large-scale application in pyrometallurgical processes. It is worth noting that blast furnace bag dust contains abundant secondary carbon resources, with a fixed carbon content of around 20% [29,30]. Using carbon in dust as a heating and reducing agent in pyrometallurgical processes is beneficial for achieving low-carbon energy conservation and is also very wise. Zhang et al. [31] and Li et al. [32] proposed a synergistic combination of various dust and sludge based on their respective component characteristics in product design. By reasonably proportioning blast furnace dust with other dust, the use of new fossil carbon resources can be reduced or eliminated. And through scientific coupling between multiple unit technologies, achieve multi-component cascade separation and full utilization technology.

Steel companies can directly recycle harmless dust in the sintering process, while other dust can be treated using a comprehensive recycling process that combines multiple processing techniques based on the characteristics of the dust. The process route is shown in Figure 11. The process is based on a combination of pyrometallurgical and hydrometallurgical treatment, first using a water washing process to recover KCl from high-K dust. The blast furnace dust is first reasonably mixed with potassium removal filter slag and zinc-containing dust, and the pyrometallurgical process is used to convert the dust into secondary dust enriched with iron-containing roasted ore and non-ferrous metals. Then, the wet leaching process is used to gradually recover non-ferrous metals from the secondary dust. Finally, the iron-containing roasted ore is recovered in the sintering process.

The application of this process solves two technical difficulties, namely the full quantitative recovery of metallurgical dust and the completion of the recovery process entirely within the plant. In addition, the process is conducive to saving the transport cost of dust and also reduces the risk of environmental pollution. It is an important direction for the future recovery of dust in large- and medium-sized iron and steel enterprises in China.

5. Conclusions

This paper investigates the blast furnace dust, converter dust, sintering dust, and electric arc furnace dust of a steel enterprise in northern China. This study reveals that these four types of dust consist of extremely fine particles, with the majority having sizes below 100 μm. The Fe and C in blast furnace dust, Fe and K in sintering dust, and Fe and Zn in converter dust and electric arc furnace dust all possess significant recovery value.

The primary components of blast furnace bag dust and sintering machine head dust are Fe₂O₃, Fe₃O₄, ZnFe₂O₄, KCl, and NaCl. In addition, the blast furnace bag dust also contains ZnCl₂. In terms of leaching efficiency, the respective rates of potassium (K) extraction from blast furnace bag dust and sintering machine head dust stand at 94.69% and 98.93%. Therefore, the two types of dust can be segregated and reclaimed, enabling the recovery of the potassium salts within them through a combined washing and salt extraction procedure.

The zinc content of converter fine dust is 5.56%, which is low zinc dust. It is mainly composed of phases such as Fe₃O₄, FeO, Fe, and ZnFe₂O₄. The zinc content in electric arc furnace dust is as high as 33.48%, and the main phases are Fe₃O₄, ZnFe₂O₄, and ZnO. The mass fractions of ZnFe₂O₄ and ZnO in the electric arc furnace dust were determined to be 28.69% and 32.076%, respectively, by Mössbauer spectroscopy.

By analyzing the physical and chemical characteristics of several difficult to handle dust particles in the factory, a process route for the steel plant to handle these dust particles was proposed. The route integrates the water washing process, pyrometallurgical process, and hydrometallurgical process, extracts chloride and non-ferrous metals such as Zn and Pb from dust, and finally recovers iron-containing tailings through the sintering process, theoretically achieving the full resource utilization of metallurgical dust. In addition, this process helps in the recovery of metallurgical dust without leaving the plant, reducing the risk of environmental contamination.