Recovery of Fe, Pb and Zn from Blast Furnace Gas Ash by Intensive Calcination and Magnetic Separation Techniques

Abstract

Intensive calcination, selection and metallurgical joint comprehensive utilization of solid waste blast furnace gas ash generated by a Chinese iron and steel plant. The main valuable elements in the gas ash are Fe, Pb, Zn, and C, with contents of 22.46%, 3.22%, 10.57%, and 27.02%, respectively. The iron minerals are mainly magnetite and hematite/limonite. Lead exists primarily in the form of lead vanadate and basic lead chloride. Zinc is associated with oxygen, sulfur, and iron in the form of zinc ferrite crystals. The effects of calcination temperature, calcination time, and reducing agent dosage on gasification and reduction indices were investigated. Results showed that using a gasification and reduction calcination–magnetic separation process with weak magnetism, at a calcination temperature of 1150 °C, with 20% anthracite as the reducing agent and a calcination time of 2 h, the volatilization rates of lead and zinc reached 96.70% and 98.26%, respectively. When the roasted ore was ground to a particle size of D₉₀ = 0.085 mm, high-quality iron concentrate with 65.61% iron grade and low lead and zinc contents of 0.08% and 0.17% was obtained, meeting the quality requirements for iron concentrate. The tailings from iron selection can be used as additives in cement and other construction materials. This integrated process combining pyrometallurgy and mineral processing enables the efficient and comprehensive utilization of blast furnace gas dust.

1. Introduction

As the world’s largest steel producer, China annually produces approximately 1 billion tons of crude steel. The steel industry is a typical energy-intensive and resource-intensive sector, requiring high energy consumption and generating significant emissions of solid pollutants. During the production process, the generation rate of steel industry dust and sludge typically ranges from 8% to 12% of the steel output [1,2]. These solid wastes primarily include steel slag, water slag, and iron-containing dust and sludge. Currently, China faces a massive stockpile of steel solid waste, which not only causes environmental pollution and land occupation but also poses threats to ecological and human health. Against this backdrop, there are significant challenges in achieving green and low-carbon development in the steel industry. Resource utilization of steel solid waste has become a top priority for sustainable development. To address these issues, the steel industry should enhance technological innovation and research and development, promote low-carbon production, reduce pollutant emissions, and vigorously advance the development of comprehensive utilization industries for steel solid waste.

Blast furnace gas dust is a by-product generated during the iron-making process in blast furnaces. After primary gravity dust removal, the gas undergoes secondary baghouse filtration, producing fine particles known as gas dust [3,4]. This dust contains valuable elements like iron, carbon, zinc, and lead, with the main minerals being magnetite, hematite, and lead vanadate, and gangue minerals such as feldspar, quartz, dolomite, and carbon black. The iron minerals are often intergrown with gangue, making liberation difficult. The dust also contains harmful elements with chemical toxicity [5,6,7,8]. Direct stockpiling of blast furnace dust not only wastes land and pollutes the environment but also poses health risks due to its fine particle size. Thus, effective and eco-friendly utilization of blast furnace gas dust is crucial. Blast furnace gas dust, rich in diverse valuable elements and with a complex mineral composition, cannot be efficiently recovered through physical ore dressing alone. This study employs a combined dressing and metallurgical process to treat the dust. High-purity iron concentrate is produced for ironmaking, with lead and zinc concentrated in the residue for lead–zinc smelting. The tailings are utilized in construction material production, such as for unburn bricks and cement, achieving comprehensive waste utilization [9,10]. Various countries have explored numerous methods for its treatment and utilization [11,12,13,14,15,16]. Current processes include direct sintering return, physical ore dressing, pyrolysis calcination, and hydrometallurgical leaching [17]. For example, Li et al. used a combined flotation, magnetic, and gravity separation process to obtain carbon, iron, and zinc concentrates [18]. Yi et al. achieved magnetic calcination and weak magnetic separation to produce iron concentrate [19]. Liu employed ammonia leaching and zinc extraction for low-zinc dust, achieving high zinc extraction rates [20]. At present, direct recycling or simple physical separation struggles with the fine intergrowth and complex chemical phases, leading to low-grade products or poor recovery rates for specific elements. Conventional pyrometallurgical processes may require higher temperatures or longer times, or fail to achieve satisfactory separation between iron and volatile metals, resulting in iron concentrate contaminated with Zn/Pb or incomplete volatilization.

Based on the physicochemical properties and calcination reaction phase transformation mechanism of blast furnace gas dust, this study proposes an integrated calcination and mineral process. The core of the process involves an intensified gasification reduction calcination step, designed to accelerate the transformation of iron, zinc, and lead phases. Under optimized conditions and with the aid of a reductant, this step promotes solid–gas phase transformations and crystalline conversions between valuable and gangue minerals within a single furnace. This enables multiple rapid reactions, particularly the reduction and volatilization of lead/zinc compounds alongside reduction of iron oxides. During this process, lead and zinc are volatilized and enriched in the collected dust, while iron is converted predominantly into magnetic iron within the calcination residue, achieving effective iron–lead–zinc separation. Finally, the roasted residue is subjected to grinding and weak magnetic separation to recover a high-grade iron concentrate. This combined process effectively recovers iron, lead, and zinc from the dust, and the findings offer a reference for the efficient and eco-friendly utilization of similar ironmaking dust, aligning with sustainable development goals and resource conservation principles.

2. Materials and Methods

2.1. Materials

The sample of blast furnace gas dust used in this study was collected from a domestic steel company. The chemical multi-element analysis results are presented in

Table 1. Chemical analysis of the gas ash (wt.%).

Component	Fe	Zn	Pb	SiO2	CaO	MgO	Al2O3	S	P
Content	22.46	10.57	3.22	6.51	3.22	1.13	2.32	1.22	0.038
Component	C	MnO	CuO	NiO	Cr2O3	TiO2	Na2O	K2O	burning loss
Content	27.02	0.23	0.01	0.031	0.038	0.22	0.71	1.43	21.88

, and the energy spectrum diagram from scanning electron microscope and energy dispersive spectrometer (SEM-EDS, FEI Company, Hillsboro, OR, USA) is shown in Figure 1. The results indicate that the main valuable elements in the gas dust are iron, lead, zinc, and carbon. X-ray diffraction (Rigaku D/Max −1200 X-ray diffractometer, Japan) analysis of the gas dust was performed. The test conditions were as follows: copper target, power of 50 kV and 150 mA, step length of 0.02°, and a scanning range of 5–80°. The analysis results are shown in Figure 2. The main minerals identified were hematite, magnetite, and quartz, along with some spectral lines of basic lead chloride (Pb(OH)Cl). The spectra lines of goethite, siderite, and minor silicate minerals overlapped with those of other minerals and were not labeled. Figure 3a,b, based on microscope and electron scanning, reveal that Pb primarily exists in the form of independent minerals, vanadinite, and basic lead chloride. SEM analysis in Figure 3c,d indicates that Zn coexists with O, S, and Fe and is present as zinc ferrite crystals.

Anthracite from Shanxi Province was used as the reducing agent in this study. The industrial analysis indicators and chemical composition analysis results of the coal are presented in

Table 2. Industrial analysis and chemical composition results of anthracite coal (wt.%).

Fixed Carbon	Ash Content	Volatile Matter	Moisture Content	S	P
79.88	12.10	7.52	0.35	0.022	0.003

and

Table 3. Results of the analysis of the main chemical composition of coal ash content (wt.%).

Component	SiO2	Al2O3	CaO	MgO	Fe
Content	44.22	20.36	19.23	1.22	5.56

. The results indicate that the anthracite has a high fixed carbon content of 79.88%, low ash content of 12.10%, and low volatile matter content of 7.52%. Additionally, the sulfur and phosphorus contents are relatively low. The coal ash mainly consists of SiO₂, Al₂O₃, CaO, and minor iron components. These characteristics suggest that this anthracite is suitable for use as a calcination reducing agent.

2.2. Methods

Weigh a specific quantity of gas dust and mix it with a reducing agent at a specified mass percentage. The mixture was placed in a graphite crucible. The furnace was then heated to the target calcination temperature at a constant heating rate of 10 °C/min. To facilitate lead and zinc volatilization and maintain a reduction atmosphere, the crucible was left uncovered during calcination in a self-made high-temperature box-type resistance furnace equipped with a dust extraction device. Simultaneously, gas extraction and dust collection were initiated. After calcination for a set duration, the crucible was removed from the furnace, and the roasted material was rapidly quenched in water. The quenched product was ground using an XMQ φ240 × 90 mm conical ball mill until 85% of the particles were smaller than 0.074 mm. Subsequently, magnetic separation was performed using a φ400 × 300 mm wet drum magnetic separator with a magnetic field strength of 180 mT to obtain iron concentrate. The reducing agent used was anthracite from a local steel company. The principle process flow for reduction calcination, lead–zinc volatilization, and iron recovery via grinding and magnetic separation is illustrated in Figure 4.

3. Results and Discussion

3.1. Gasification Reduction Calcination

3.1.1. Calcination Temperature

An appropriate temperature is a critical condition for chemical reactions. To investigate the effects of variations in calcination temperature on the volatilization of lead and zinc, as well as the reduction of iron, experiments were conducted under the following fixed conditions: 20% by weight of coal as the reducing agent, a calcination duration of 2 h, and a material thickness of 50 mm. The roasted ore was cooled using water quenching. The experimental results are presented in Figure 5. As indicated by the results, the calcination temperature significantly affects the volatilization of lead and zinc as well as the reduction atmosphere of iron. With the increase in temperature, the grade of Fe gradually increases, the recovery rate of Fe first rises and then declines, and the grades of Pb and Zn gradually decrease, while the recovery rates of Pb and Zn gradually increase. It can be seen that the increase in temperature promotes the transformation of mineral phases. On the one hand, lead and zinc minerals are more rapidly reduced from the solid phase to the gaseous phase of lead and zinc. On the other hand, high temperatures promote the transformation of the iron oxide phase into the magnetite phase and increase the crystalline grain size of the magnetite, which is beneficial for improving the grade and recovery rate of iron. Considering both the product indices and energy consumption, a calcination temperature of 1150 °C was selected. At this temperature, the Fe grade in the iron concentrate was 65.52%, and the grades of Pb and Zn in the volatile matter were 18.68% and 66.12%, respectively.

3.1.2. Calcination Time

Adequate time is crucial for the thorough occurrence of chemical reactions. To examine the impact of variations in calcination time on the volatilization of lead and zinc and the reduction indices of iron, experiments were conducted under the following fixed conditions: 20% by weight of coal as the reducing agent, a calcination temperature of 1150 °C, and a material thickness of 50 mm. The roasted ore was cooled using water quenching. Experiments were carried out with different calcination durations. The experimental results are presented in Figure 6. As indicated by the results, under relatively short calcination durations, the grades of Fe in the iron concentrate and the distribution ratios of Pb and Zn in the volatile matter are low, implying that insufficient reaction time hinders the adequate reduction of lead, zinc, and iron. With the extension of calcination time, the grade of Fe in the iron concentrate shows a gradual increase, the recovery rate of Fe first rises and then declines, the grades of Pb and Zn in the volatile matter decrease, and the recovery rates of Pb and Zn increase. It can be inferred that the reduction and volatilization processes of lead and zinc are accelerated, and the transformation of iron oxide to magnetite is promoted, enhancing the grade and recovery rate of iron. Considering these factors comprehensively, a calcination duration of 2 h is determined to be appropriate.

3.1.3. Reductant Agent

The reduction reactions of the oxides of lead, zinc, and iron require an adequate amount of reducing agent under certain temperature conditions. To examine the impact of variations in the reducing agent dosage on the volatilization of lead and zinc and the reduction indices of iron, experiments were conducted under the following fixed conditions: the calcination time was fixed at 2 h, the calcination temperature at 1150 °C, and the material thickness at 50 mm. The roasted ore was cooled using water quenching. Experiments were carried out with different reducing agent dosages. The experimental results are presented in Figure 7. As indicated by the results, in the absence of a reducing agent, the Fe grade in the iron concentrate was 57.30%, the Fe distribution ratio was 51.95%, and the contents of Pb and Zn in the volatile matter were 15.99% and 62.61%, respectively, with distribution ratios of 57.19% and 67.66%. This implies that the carbon content in the gas alone is insufficient to meet the requirements of the reduction reaction. With the increase in the reducing agent dosage, the Fe grade in the iron concentrate and the distribution ratios of Pb and Zn in the volatile matter gradually increase. Considering both the recovery indices and the cost of the reducing agent, a reducing agent dosage of 20% is determined to be appropriate.

3.1.4. Material Thickness

To examine the effects of different material thicknesses on the volatilization of lead and zinc and the reduction indices of iron, experiments were conducted under the following fixed conditions: a calcination time of 2 h, a calcination temperature of 1150 °C, and a reducing agent dosage of 20%. The roasted ore was cooled using water quenching. Experiments were carried out with different material thicknesses. The experimental results are presented in Figure 8. As indicated by the results, different material thicknesses significantly affect the volatilization of lead and zinc and the reduction indices of iron. This implies that the permeability of the calcination material can influence the volatilization of lead and zinc to a certain extent. When the material thickness is small, the permeability is good, which is favorable for the volatilization of lead and zinc. However, the rapid volatilization of reducing gases leads to insufficient reduction of iron. As the material thickness increases, the volatilization of lead and zinc becomes less effective, but the reduction of iron improves, with both the grade and recovery rate of iron in the iron concentrate increasing. Considering the need to maintain a high volatilization rate of lead and zinc while ensuring an iron grade in the iron concentrate above 65%, a material thickness of approximately 50 mm is determined to be optimal.

3.1.5. Cooling Method for Roasted Ore

The cooling method of roasted ore significantly affects the phase transformation of roasted sand. To investigate the impact of cooling methods on the calcination process, experiments were conducted using two cooling methods: furnace cooling and water quenching (cooling the roasted ore by placing it in water). The experiments were carried out under the following fixed conditions: a calcination time of 2 h, a calcination temperature of 1150 °C, a reducing agent dosage of 20%, and a material thickness of 50 mm. The experimental results are presented in

Table 4. Effect of cooling method on test index.

Cooling Method	Product	Yield (%)	Grade (%)			Distribution Rate (%)
			Pb	Zn	Fe	Pb	Zn	Fe
Furnace cooling	Iron concentrate	24.51	0.07	0.17	64.76	0.57	0.39	71.41
	Tailings	59.23	0.11	0.21	10.73	2.15	1.15	28.59
	Volatile matter	16.26	18.12	65.23	/	97.28	98.46	/
	Feed	100.00	3.03	10.77	22.23	100.00	100.00	100.00
Water quenching cooling	Iron concentrate	25.65	0.08	0.18	65.52	0.67	0.43	75.33
	Tailings	58.44	0.13	0.22	9.42	2.48	1.20	24.67
	Volatile matter	15.91	18.68	66.12	/	96.86	98.37	/
	Feed	100.00	3.07	10.69	22.31	100.00	100.00	100.00

. As indicated by the results, the volatilization rates of lead and zinc in the roasted ore cooled in the furnace are slightly higher than those cooled by water quenching. However, the iron concentrate obtained through water quenching has a higher iron grade, 0.76 percentage points higher, and a higher recovery rate, 3.92 percentage points higher. This is attributed to the slow cooling rate and longer cooling time of furnace cooling, during which a small amount of lead and zinc continue to volatilize. Additionally, furnace cooling causes oxidation on the surface of the roasted ore, converting some iron phases into iron oxide, thereby affecting the grade and recovery rate of iron in the iron concentrate. In contrast, water quenching rapidly isolates the roasted sand from air contact, preventing the re-oxidation of magnetite and ensuring efficient iron recovery. It also enhances processing efficiency. Therefore, water quenching is determined to be a more effective cooling method for roasted ore.

3.2. Roasted Ore Grinding Magnetic Separation Test

To investigate the impact of grinding fineness on the separation of minerals in roasted ore, grinding and weak magnetic separation tests for iron were conducted. The degree of mineral liberation, which is crucial for mineral separation in the dressing process, is closely related to the grinding fineness. The roasted ore was ground to different particle sizes and subjected to weak magnetic separation tests. The test procedure consisted of one roughing and one cleaning stage, with magnetic field strengths of 200 mT and 180 mT, respectively, and the tailings were combined for analysis. The experimental results are presented in Figure 9. As indicated by the results, when the grinding fineness was D₉₀ = 0.12 mm, the obtained iron concentrate had a low iron grade of only 56.33% due to the coarse particle size and poor liberation of minerals. As the grinding fineness increased, the iron grade in the concentrate gradually increased, while the iron recovery rate decreased. When the grinding fineness reached D₉₀ = 0.085 mm, an iron concentrate with a grade of 65.61% could be obtained. Further grinding did not significantly improve the iron grade of the concentrate.

3.3. Study of Calcination Reaction Mechanism

To investigate the calcination reaction mechanism, an iron phase analysis of the roasted ore was conducted, and the results are presented in Figure 10. As indicated by the results, after calcination, the distribution ratio of iron in hematite (or limonite) decreased from 50.45% to 3.08%, and the distribution ratio of iron in carbonate iron decreased from 12.47% to 7.29%. In contrast, the distribution ratio of iron in magnetite increased from 32.84% to 76.11%. This suggests that most of the iron oxides were reduced to magnetite. Additionally, the lead and zinc in the gas were reduced and volatilized. The potential chemical reactions are shown in Equations (1)–(5).

2C +O₂ = 2CO

(1)

FeO + CO = Fe + CO₂

(2)

3Fe₂O₃ + CO = 2Fe₃O₄ + CO₂

(3)

ZnO + CO = Zn + CO₂

(4)

PbO + CO = Pb + CO₂

(5)

3ZnFe₂O₄ + 4CO = 3Zn + 2Fe₃O₄ + 4CO₂

(6)

4. Conclusions

(1)

Blast furnace gas dust mainly contains valuable elements such as iron, lead, zinc, and carbon. The iron minerals are primarily magnetite and hematite. Lead exists mainly as lead vanadate and basic lead chloride, while zinc is associated with O, S, and Fe in the form of zinc ferrite. Gas dust is an artificial ore with complex components and mineral phases, making it extremely difficult to utilize through conventional mineral processing methods.

(2)

Using a gasification reduction calcination–grinding-weak magnetic separation process, with a calcination temperature of 1150 °C, 20% anthracite as the reducing agent, and a calcination time of 2 h, the distribution ratios of Pb and Zn in the volatile matter reach 96.86% and 98.37%, respectively. When the roasted sand is ground to a particle size of D₉₀ = 0.085 mm, an iron concentrate with an Fe grade of 65.61% can be obtained.

(3)

The key to effective gasification reduction calcination is maintaining the furnace reaction atmosphere. It is essential to reduce lead and zinc oxides to metals for timely volatilization, collection, and to transform iron oxide into magnetite to avoid over-reduction. The final iron-selecting tailings can be used as an additive in cement and other construction materials. The combined selection and metallurgy process enables the efficient comprehensive utilization of blast furnace gas dust.

(4)

This study successfully demonstrated the technical feasibility of the integrated process at the laboratory scale. However, it must be acknowledged that the current work still has certain limitations, which provide clear directions for future research. Firstly, the static roasting method adopted in the experiments differs fundamentally from ideal industrial continuous reactors in terms of mass and heat transfer conditions. Subsequent studies should therefore focus on simulation and scale-up experiments based on dynamic systems to evaluate the engineering performance of the process. Secondly, the proposed reaction mechanism is primarily inferred from macroscopic product analysis. Future work involving systematic microstructural analysis of intermediate phases would offer more direct kinetic evidence for the phase transformation processes, thereby providing deeper insights into the intrinsic mechanisms governing reduction and volatilization behaviors.