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Article

Application of Waste Tire Carbon for Iron-Containing Dust Reduction in Industrial Processes

by
Menglan Zeng
,
Chujun Luan
and
Fawei Lin
*
Tianjin Key Lab of Biomass, Waste Utilization, School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(12), 6504; https://doi.org/10.3390/app15126504
Submission received: 18 April 2025 / Revised: 1 June 2025 / Accepted: 2 June 2025 / Published: 9 June 2025
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Abstract

The iron and steel industry generates large quantities of iron-bearing dust (IBD), contributing to resource inefficiency and environmental concerns. This study investigates heating methods and the use of organic solid waste, specifically waste tire carbon (WTC), as a reductant for the recovery of Fe from sintering machine tail dust (SMTD) and steelmaking gravity dust. The results indicate that the optimal reduction conditions occurred at 1000 °C, with a 2:1 ratio of SMTD to WTC, and 0% O2 holding for 45 min. WTC is the best material, and heating methods affect it limitedly. The leaching behavior of seven metals was measured, showing an increase in the leaching of Ca and Al compared to the raw materials. The study shows that WTC provides a promising alternative reductant for IBD reduction, offering an energy-saving and low-carbon alternative to conventional fossil fuel injections in blast furnaces. The risk of Cr leaching should be paid attention to while enhancing Fe recovery.

1. Introduction

IBD is a solid waste generated during the steel production process. It is produced by collecting particulate emissions from exhaust gases through dry dust collectors. Depending on the point of origin, it can be classified into various types, such as feedstock dust, hot gravity dust, and furnace dust [1]. The TFe content in it is relatively high, ranging from 30% to 70%. Additionally, the production yield is significant, accounting for approximately 10% of crude steel output [2]. Based on China’s crude steel production of 1.029 billion tons in 2023, the annual generation of IBD exceeds 100 million tons, with an estimated recoverable iron content of 30 to 70 million tons. This represents economic benefits worth billions. Besides its high TFe content, IBD also contains hazardous elements such as Cr and Pb, typically present in the form of oxides or sulfides. If not treated promptly, the large-scale accumulation of it poses significant environmental risks, including occupying valuable space, generating fugitive dust, and causing the leaching of harmful elements, all of which can result in severe environmental pollution.
The treatment of IBD aims primarily at removing harmful elements and extracting valuable metals. The treatment methods can be divided into three types: physical methods, hydrometallurgical methods, and pyrometallurgical methods. Physical methods use magnetic separation or mechanical separation to sort different elements such as Fe, Zn, and C [3]. However, the treatment efficiency is relatively low [4]. Hydrometallurgical methods mainly target dust with a high Zn content, using acidic or alkaline leaching agents to extract Zn, with a relatively low utilization efficiency for Fe [2]. Pyrometallurgical methods involve directly processing the raw material or pelletizing it before placing it in furnaces such as blast furnaces, converters, or rotary kilns. Under high temperature and the action of reducing agents, valuable elements are reduced. This method is widely used due to its broad applicability to raw materials and high metal recovery rates [2].
In pyrometallurgical reduction, reaction temperature and duration are the most critical factors influencing metal recovery efficiency. Experimental results vary widely across studies. For example, Wang S. et al. achieved an Mn reduction rate of 96.8% using corn straw to reduce soft manganese ore at 600 °C for 90 min [5]. Zhou, T. et al. reached 95.26% Fe reduction using hydrogen on high-purity iron concentrate at 575 °C for 60 min [6], with both demonstrating high efficiency at relatively low temperatures. In contrast, Zheng H. et al. recovered 86% Cu from copper slag using rubber seed oil at 1350 °C within 4 min, indicating high efficiency at high temperature over a short duration [7]. Wei R. et al. studied the temperature characteristics of hydrogen and the carbon reduction of iron oxide with three biomass types, identifying 806–822 K as the characteristic temperature for gas-based reduction and 1116–1132 K for carbon reduction [8].
In the selection of furnace types, the blast furnace has a relatively higher furnace start-up rate in domestic enterprises, at 73.44%, and does not require the pelletizing step for charging materials as in the case of the converter [9]. There has been extensive research on the reduction of IBD in blast furnaces. Researchers typically use vertical furnaces to simulate blast furnaces and study the effects of reduction temperature, time, types of reducing agents, and their mixing ratios on the iron reduction rate.
However, air or oxygen is injected into blast furnaces to maintain temperature, with the oxygen content ranging from 21% (air) to 100% (pure oxygen), though the average O2 level inside the furnace remains below 10%. Increasing O2 content enhances fuel utilization. Industrial data from a modified blast furnace by Chaofeng Y. shows that raising the enrichment level from 2.24% to 3.75% reduced the energy consumption per ton of pig iron by 2.85% [10]. However, further research beyond numerical simulation is needed to clarify how the oxygen content affects ore reduction [11,12]. Additionally, microwave heating, a new heating method following traditional electric heating, can enhance the reduction of Fe through the interaction between the H-field and Fe3O4 [13].
Using WTC as a reductant instead of fossil fuels to recover valuable Fe from iron-rich dust presents considerable economic and environmental benefits. In the context of vehicle electrification and scrappage reforms, China produces over 10 million tons of WT annually, ranking first globally. Like IBD, WT represents a critical industrial solid waste. Extensive studies have been conducted on the pyrolysis of WT, showing that the resulting oil can be used as fuel, while its rubber and carbon black components give a resulting char energy content comparable to pulverized coal, with a carbon content above 70%, making it a promising reductant [14,15,16].
Currently, WTC is primarily used in asphalt modification, rubber reinforcement, adsorptive activated carbon, and electrode materials, with limited research on its application as a reductant. Chengkang Y. et al. found that WTC initiates the reduction of iron–zinc dust at 407 °C, doing so earlier and more effectively than coke powder [17].
This study uses SMTD and SGD with a high Fe content as examples and applies WTC derived from waste tires as the reductant to investigates the basic conditions for Fe reduction while considering the environmental benefits of the progressively developed microwave heating method and reduction residue. After determining the optimal iron reduction conditions, different controls, including microwave reduction, WT reduction, and CC reduction, were added. The leaching rates of heavy metals before and after reduction for each group were compared, with the aim of drawing relatively universal conclusions and providing a reference for the green application of IBD.

2. Materials and Methods

2.1. Materials

The two kinds of IBD used in this experiment, separately, were SMTD and SDG, sourced from a steel plant. The three kinds of reductant used in this experiment separately were WT, WTC, and CC. WT was sourced from a recycling facility with 1–3 mm particle size, partially made into WTC. CC was standard commercial coke. All samples were dried, and all except WC were sieved through a 40-mesh screen. Ultimate analysis (C, H, N, and S) of the samples was measured with an elemental analyzer (Thermo Scientific Flash 2000, Waltham, MA, USA), and the chemical composition of the IBD was determined by X-ray fluorescence spectroscopy (XRF) analysis tested by Beijing Chenfei Technologies (Beijing, China), with a titration method tested by Changsha Research Institute of Mining and Metallurgy Co., Ltd. (Changsha, China) to measure the various valence state Fe contents.
As illustrated in Table 1 and Table 2, samples contained few harmful nonmetallic elements (N, S, Cl, P). The C content of CC was the highest among the reductants, ca. 88.53%, while WTC had the lowest, ca. 67.87%. The TFe content in IBD was abundant with varying valence states, and in SDG, iron predominantly existed in the divalent form. ca. 27.88%.
X-ray diffraction (XRD) was used in this study to analyze phase changes and the crystallinity of the samples before and after reaction. Measurements were performed using an X’Pert Pro MPD diffractometer (Malvern PANalytical, Malvern, UK) with a voltage of 40 kV, current of 40 mA, scanning range from 10 to 90°, and a scan rate of 8° per minute. The analysis of Figure 1 indicates that iron in SMTD and SGD primarily exists in the forms of Fe2O3 and Fe3O4, with no significant peaks of MFe observed, consistent with the results shown in Table 2. SMGD contains a strongly magnetic spinel phase MgFe2O4 [18], while the iron in SDG is primarily in the form of Fe3O4, with a relatively high Fe²⁺ content, aligning with the 27.88% reported in Table 2. SGD is produced from the basic oxygen steelmaking process, resulting in relatively high contents of FeO and CaO, with CaO primarily existing in the forms of lime, calcite, and dolomite [19,20]. The strongest diffraction peak of free calcium oxide (f-CaO) is observed at 2θ = 38° [21].

2.2. Experimental Equipment and Methods

The expected blast furnace feed location for the reduction of IBD using organic solid waste is shown in Figure 2a. The high-temperature reduction experiments were conducted in a homemade vertical furnace, as shown in Figure 2b, and a microwave furnace, as shown in Figure 2c. During the experiments in the vertical furnace, the temperature was climbed up to the set temperature (700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C), with the material temperature monitored by a thermocouple (temperature deviation less than 3 °C) under mixed N2 and O2 flow (50 mL/min), and the O2 proportion was adjusted (0%, 2%, 4%, 6%, 8%). A total of 6 g of material was used, comprising a mixture of IBD and reductant in different ratios (1:1, 2:1, 4:1, 9:1, 14:1), and placed in a crucible suspended in the furnace (or preloaded in the furnace and heated at 10 °C/min to the target temperature) for different holding times (15 min, 30 min, 45 min, 60 min, 90 min). The reacted gas was discharged from the bottom of the furnace, condensed with −10 °C anhydrous ethanol, and collected in a gas bag. After the reaction, the system cooled naturally under nitrogen protection. The heating rate under this rapid heating scenario reached 111.5 to 687.1 °C/min. The optimal group was conducted in the microwave furnace as comparation, with the same conditions but placed in a SiC crucible and heated by microwave. The temperature was increased at a rate of 10 °C/min to the target value with less than 1 °C deviation.

2.3. Analytical Method

2.3.1. Thermal Analysis

The thermal analysis included TG-DTG experiments and pyrolysis kinetic analysis. The former were conducted using a thermogravimetric analyzer (STA 499F3, Netzsch, Selb, Germany) and a mass spectrometer (Netzsch QMS 403C Aeolos) to reveal the weight loss of samples. Approximately 50 mg of the raw material was placed in an alumina crucible on the reaction platform. The reactor was programmed to ramp from 50 °C to 1100 °C at a rate of 10 °C/min and was held for 20 min with Ar as the background gas at 30 mL/min.
The pyrolysis kinetic analysis was based on the OFW mode [22,23,24]. The reaction order was n = 1. The activation energy Ea and pre-exponential factor A were calculated from the slope and intercept of the fitted line obtained by plotting lnβ against 1000/T.
ln β = ln AE a RF α − 5 . 331 − 1 . 052 E a RT

2.3.2. GC Experiments

The GC experiments were conducted using a GC9790II Plus (Zhejiang Fuli Analytical Instrument Co., Ltd., Taizhou, China) gas chromatograph equipped with a hydrogen flame detector (FID), a thermal conductivity detector (TCD), a 5A molecular sieve, and a Porapak Q column. Based on the different ion numbers and thermal conductivity properties of the various gas components during combustion, the detectors convert the concentration and mass changes in each component gas into electrical signals. The generated chromatographic profile allows for qualitative and quantitative evaluations based on retention time, peak height, and peak area.

2.3.3. TCLP Experiments

The leaching rate of heavy metals in the samples was measured by TCLP. An extraction fluid of acetic acid with a pH value of 2.88 ± 0.05 was prepared in advance. The tests were conducted at a solid/liquid ratio of 1:20 by wight. Batches were placed in a constant temperature shaker at 300 rpm for 18 h. Afterward, it was centrifuged at 4000× g rpm for 30 min, and the supernatant was collected through filtration using a 45 μm water membrane filter into a centrifuge tube. A suitable amount of filtrate was transferred into a digestion tube and heated at a rate of 10 °C/min from 20 °C to 180 °C, maintaining for 30 min. Nitric acid was continuously added until the filtrate became colorless and transparent. After filtration through a 45 μm organic membrane, the sample was diluted for analysis. The leaching concentrations of seven heavy metal elements (Fe, Ca, Mg, Al, Cu, Zn, Cr) in the sample were measured using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS; Optima 7000D, Perkin Elmer, Waltham, MA, USA).

3. Results

3.1. Characteristics of Raw Materials

Through TG-DTG experiments, both types of IBD are relatively stable at high temperatures, showing two-stage weight loss, with only 3–4% total weight loss at 1100 °C. As shown in Figure 3a, for SMTD, the main phase is α-Fe2O3, which can stably exist below 1200 °C [25]. Stage I (50–196.2 °C), with a weight loss of 0.83%, occurs primarily due to the volatilization of SO3 and free water. The S in SMTD can be incorporated into liquid-phase or solid-phase particles, forming stable substances, partly in the form of SO3 [26], and the MgFe2O4 it contains can adsorb some moisture from the environment, with both substances volatilizing at low temperatures. Stage II (569.5–750.5 °C), peaking at 733.5 °C, with a weight loss of 1.81%, is consistent with the decomposition of CaCO3 [27,28]. The peak may not appear in the spectrum due to its content being below the XRD detection limit [29]. For SGD, Stage I (357.9–461.7 °C), with a weight loss of 1.78%, corresponds to the decomposition of a small amount of Ca(OH)2 formed by the absorption of water by CaO [21]. Stage II (571.4–722.1 °C) is essentially consistent with SMTD, with the peak weight loss rate temperature at 672.7 °C and a weight loss of 1.02%. As for the reductant in Figure 3b, WT starts to lose weight at the lowest temperature, beginning at 170.46 °C. This process can be divided into two types: the volatilization of oil content and the subsequent decomposition of the elastomer [30]. This weight loss occurs in three stages: Stage I (170.46–298.60 °C), peaking at 265.39 °C with a total weight loss of 6.58%, corresponds to the volatilization of moisture and light volatile substances [31]. Stage II (298.60–474.92 °C), peaking at 415.94 °C with a cumulative weight loss of 56.72%, corresponds to the decomposition of natural rubber and partial synthetic rubber (SBR cracking and dehydrogenation to form styrene and butadiene monomers) [32,33]. Stage III (474.92–603.49 °C), characterized by two smaller peaks, resulting in a cumulative weight loss of 88.11%, corresponds to the decomposition of synthetic rubbers such as styrene–butadiene rubber and butyl rubber [32]. Due to the incorporation of additives (such as Zn, Fe, Si, S, etc.) during the tire manufacturing process, a residual ash content of approximately 10% is typically observed [34]. Since the yield of carbon and gases from waste tires remains relatively constant at temperatures above 500 °C, and the carbon conversion rate peaks at 600 °C [35], WTC is produced by calcining WT at 600 °C for 30 min. During the formation process of WTC, some pyrolytic tar adheres to the surface, leading to secondary reactions such as tar conversion to carbon during thermal degradation [36,37]. As a result, a 10.0% weight loss is observed between 600 and 1000 °C. The C/H ratio for CC is 79.04, significantly higher than WTC’s 41.90, indicating a greater degree of unsaturation and more complete reactions with a cumulative weight loss of 86.47% [38].
As shown in Figure 4, neglecting the volatilization of simple components below 200 °C, the correlation coefficients between the fitted curves and the actual calculated values for each reaction stage are generally above 0.96. The R2 value for WTC is above 0.632, which also satisfies the condition that the confidence level for the univariate linear regression is greater than 95% when the number of data points N = 10 [39]. Therefore, assuming a reaction order of n = 1 is feasible.
The corresponding values of A and Ea for each stage were calculated, with the results shown in Table 3. In this study, the Ea of IBD is relatively smaller than in other literature, which may be attributed to the fact that it has been pretreated and sieved, resulting in a higher fine particle fraction and larger specific surface area [40]. Different WT, due to varying component ratios, may also exhibit different Ea [22]. For WTC, since the reacting substance is surface tar and there are no rate-controlling steps such as diffusion or desorption, the Ea is the smallest [41].
Based on the SEM results, the particle size, shape, and dimensions of the IBD are quite similar, both consisting of irregular block-like particles with lengths under 20 μm. SMTD exhibits more pronounced edges, and as shown in Figure 5(a2), the surface is covered with smaller particles of a uniform size, ranging from 20 to 30 nm, consistent with MgFe2O4 [42]. SGD appears looser in structure with spherical particles of less than 250 nm in diameter, possibly f-CaO, consistent with the results shown in Table 2 and Figure 1.
WTC and CC exhibit similar morphologies, with a relatively uniform distribution and few distinct block-like particles. CC is denser, exhibiting a congealed-like structure. From Figure 5(c2), blocky particles with a width of up to 100 nm and dense surface striations are visible. WTC consists of irregular spherical particles with sizes smaller than 50 nm that aggregate to form clusters, with noticeable gaps extending inward, characteristic of carbon black. This structure corresponds to carbon black, which then adheres to form a more three-dimensional carbon black structure [43].

3.2. Optimal Reaction Conditions for Iron Reduction

Under the same conditions, the reduction pathway of iron oxides with carbon as the temperature increases is as follows: Fe2O3 → Fe3O4 → FeO → MFe [44]. From Figure 6a, it can be seen that when the SMTD /WTC is 1:1, with a protective N2 flow rate of 50 mL/min and a reduction temperature of 700 °C, iron oxide is completely reduced to Fe3O4 after 60 min of reaction. When the reduction temperature is raised to 800 °C, FeO starts to appear. As the temperature increases, the sample is further reduced. At 900 °C, the FeO content increases and is further reduced to MFe, accompanied by the appearance of sulfides such as FeS and ZnS. According to Table 2, the Ca content in SMTD is 11.75%. At this temperature, CaCO3 has already decomposed to form CaO, and above 935 °C, the reaction is represented by Equation (2) [45], consistent with Figure 6(a5), where ZnS is converted to CaS, and the iron phase exists entirely as MFe. The FeS-CaS reaction process is shown in Equations (3)–(5).
ZnS + CaO + C = Zn + CaS + CO
FeO + C = Fe + CO
FeS + CO = Fe + COS
CaO + COS = CaS + CO2
Due to the stability of CaS as a sulfide [46], it remains present even when the reaction temperature increases to 1100 °C. Additionally, the Al0.5Fe3Si0.5 phase (labeled as AlFe6Si) found in the reduction residue belongs to the τ-phase of the Al-Fe-Si ternary system and is Fe-rich. This ternary phase forms during natural cooling following the reduction reaction. Similarly, Guojun M.A. et al. [47] also identified this compound with 52% Fe content during the carbothermal reduction of fly ash. The natural cooling process, categorized as a semi-rapid solidification (cooling rate between 100–103 K/s), produces dispersed AlFe6Si metastable phase particles, which are unstable at high temperatures [48]. Thus, as the reaction temperature increases to 1100 °C, this phase disappears. Considering both the energy loss and Fe reduction efficiency, 1000 °C is identified as the optimal reaction temperature.
From Figure 6b, it can be seen that when the SMTD/WTC is 1:1 and 2:1, there is little difference in material morphology, with MFe primarily present, along with CaS and AlFe6Si. When the mixing ratio is 4:1, unreacted FeO begins to appear. As the ratio increases to 9:1 and 14:1, the FeO phase becomes more pronounced, while the Fe phase weakens. Due to the insufficient amount of C, there is not enough COS to react with MFe, resulting in the disappearance of the CaS and AlFe6Si phases. Considering both the reduction effectiveness and resource usage, 2:1 is identified as the optimal mixing ratio.
As shown in Figure 6c, it is evident that when the O2 content is between 0 and 8%, under the same conditions (reaction temperature 900 °C, reaction time 60 min, mixing ratio 2:1), the XRD patterns of the reaction residues are almost identical, showing no significant differences. The dominant phase is iron, with a small amount of incompletely reduced FeO, accompanied by CaS and AlFe6Si phases. This suggests that a low-O2 environment (<8%) in an operational blast furnace will not significantly affect iron reduction. For large-scale reaction systems, as the O2 content increases, the reducing atmosphere is enhanced. This leads to a decrease in the amount of carbon used in direct reduction reactions, while the carbon combustion rate increases. These two effects counterbalance each other, resulting in a maximum carbon utilization efficiency at an O2 supply rate of 19 Nm3/kg [49]. Additionally, Figure 6e shows that during the redox reaction between SMTD and WTC, the generated gases are primarily CO, CO2, and minor H2. When the O2 content is between 2 and 6%, as the O2 content increases, more carbon participates in the reduction reaction, leading to more CO2 production and an overall increase in the total gas volume. The highest amount of H2 is observed at 4% O2. When the O2 content further increases to 8%, the reducing atmosphere is slightly disrupted, reducing the production of CO and CO2, and thus the total gas volume decreases. However, this phenomenon is not significantly reflected in the XRD patterns. The O2 content (0–8%) does not significantly impact the reduction effectiveness of the iron-bearing dust.
From Figure 6d, it can be seen that under suitable conditions, when the reaction time is 15 min, significant MFe formation is already observed in the reduced residues. When the reaction time is 45 min, the iron phase in SMTD can be almost completely reduced to MFe. A similar study shows that extending the reaction time by 30 min can increase the iron reduction rate by more than 30% [50]. Considering the reduction effectiveness, reduction efficiency, and energy consumption, 45 min is identified as the optimal reaction time.
In summary, the optimal reaction conditions for reducing SMTD with WTC are a temperature of 1000 °C, a mixing ratio of SMTD to WTC of 2:1, 0% O2 content, and a reduction time of 45 min. The reduction process is illustrated in Figure 7.

3.3. Comparison of Reduction Methods

The XRD spectrum of the reduction residues obtained under different heating methods shown in Figure 8a indicate that there is little difference in the reduction effects of the heating methods. The degree of reduction in all groups is high, with no iron–oxygen phases detected. The rapid reaction at high temperatures (1000 °C) diminishes the influence of the heating method on the reduction process [51]. From Figure 8b, the raw materials affect the phase composition of the residue. The reduction effects of different reductants vary, with the reduction effect having the following order: WTC > WT > CC. The WTC-reduced residue contains no iron–oxygen phases, which is due to the small weight loss during the pyrolysis of WTC, allowing the remaining carbon to fully participate in the redox reaction with Fe. WT and CC exhibit a weight loss of over 85% during pyrolysis, resulting in incomplete reduction reactions, and the residues contain relatively high amounts of FeO. When IBD is replaced by SDG, the lower Si and Al contents in the SDG lead to the substitution of AlFe6Si in the reduced residue by CaS and CaO. Additionally, several other minor peaks are observed, but the iron phase is fully reduced to MFe, further supporting the reduction effectiveness of WTC.
From the SEM images, the metal substances encapsulated in the slag phase can be observed. As shown in Figure 9a–c, the metal appears as spherical particles, which correspond to the MFe formed after reduction [52,53]. The particle size of MFe follows the order MH > FH > PH, with a size difference of approximately 200 nm. The higher heating rate of FH leads to a higher carbon gasification rate, resulting in a looser slag phase. MH exhibits a “lens effect”, which promotes the directional migration of MFe, forming larger metal particles, and this can be seen as significant cavities formed by WTC consumption [54].
In Figure 9d–f, the morphology of the SGD-A reduced residue is similar to that in Figure 9a–c, with the best reduction effect, consistent with the results in Figure 8b. The morphologies of CC-A and WT-A residues are more irregular, indicating the incomplete reduction of wüstite, with the wüstite formed by WT-A being larger. When the metal phase surfaces in Figure 9 are magnified, fine striations can be observed, which are dendritic structures formed during the cooling process [55].
Overall, the reduction process of SMTD using WTC, WT, and CC can be divided into three stages: decomposition, reduction, and cooling. With increasing temperature, SMTD undergoes dehydration and SO3 release, and CaCO3 decomposes to CaO. The three reductants decompose into various hydrocarbons to varying degrees. As the temperature continues to rise, low-melting phases become molten, and solid C along with organic compounds (primarily in WT and CC) begin to reduce Fe3O4 from Fe2O3, accompanied by CO generation. These reductive agents (solid C, CO, and organics) together facilitate further reduction to FeO. At this stage, carbon from WT and CC is largely consumed or volatilized, while WTC retains residual carbon, allowing the further reduction of Fe2⁺ to Mfe. Meanwhile, reactive metals such as Mg and Zn are reduced to their elemental forms, and sufficient reduction may lead to CaS formation. Due to uniform material distribution, Fe phases in the residue are evenly dispersed. During rapid cooling, part of the residue forms AlFe6Si, and dendritic cracks appear in metallic particles. A similar process occurs in the reduction of SGD.

3.4. Leaching of Heavy Metals

The TCLP results are shown in Figure 10, where labels 1 and 2 represent the raw materials and the residues after the reaction, respectively. The effect of different heating methods and raw materials on the leaching levels of various metals varies. The heating method has little effect on the leaching levels of six metals except for Al. After the reduction reaction, the leaching amount of Ca significantly increases, while those of Mg and Zn significantly decrease. The IBD raw materials contain a certain amount of CaCO3, which exhibits lower leachability under acidic conditions compared to CaO [56]. During the high-temperature carbothermal reduction process, CaCO3 partially decomposes into more leachable CaO, leading to an overall increase in Ca leaching from the residue. Notably, the SGD-A1 group, with a higher f-CaO content, showed greater Ca leaching than other materials. Mg and Zn can volatilize in metal form during the carbothermic reduction process [57,58]. A higher reduction degree promotes more complete conversion to their elemental forms, resulting in lower leaching concentrations. Given its higher activity, Mg shows a more pronounced difference in leaching before and after the reaction. For Al, it forms AlFe₆Si with Fe and Si. Since Fe-Al-Si is a negative deviation system with Fe having the largest negative deviation, it can increase the activity of Al [59]. When the reduction reaction is relatively complete and the Mfe content is high, the activity of Al in the multicomponent system approaches that of metallic Al. However, since the temperature is below 1600 °C, most of the aluminum phases remain inert, mainly existing in an amorphous form, which has higher solubility than crystalline phases [60,61]. Consequently, the higher Mfe content in the residue corresponds to increased Al leachability. Figure 10a shows the Al leaching differences before and after reduction and indicates the order of reduction degree as FH > MH > PH. In Figure 10b, the FH2 group, which has the highest Mfe content, exhibits the highest leaching of Al, further confirming this point.
The leaching level of MFe is very low, while the leaching amount of FeO increases over time [62]. In Figure 10b, since Fe in CC-A and SGD-A is mostly reduced only to FeO, the Fe leaching amount is higher than that in FH2, with the order being CC-A > SGD-A > FH2, which is inverse to the reduction degree of Fe.
As shown in Table 4, it is worth noting that although the leaching amounts of Cr and Cu in the raw materials are relatively low, their leaching levels in the reduced residues still show a slight increase. Furthermore, the leaching amount of Cr in all experimental groups exceeds the standards [63], with an average post-reduction increase of 117.60%. In contrast, Cu and Zn are within the permissible limits. Therefore, the residues after the reaction still require a further removal of Cr before they can be safely utilized. Given the presence of Al, Si, and Ca in the residue, geopolymerization can be considered after the magnetic separation of Fe. This allows Cr ions to undergo ion exchange with Na⁺ and Ca2⁺, leading to their immobilization [64].

4. Discussion

This study employed a custom vertical furnace reactor to closely simulate the high-temperature conditions of a blast furnace. It focused on the influence of varying oxygen concentrations caused by blast air on the reduction efficiency of Fe. The optimal conditions for reducing SMTD using WTC were explored, and the reduction performance of different heating methods and feedstocks was compared. Heavy metal leaching from the residues was also examined to assess potential environmental impacts. The conclusions are as follows:
1.
Reduction conditions: The optimal conditions for reducing Fe to MFe are 1000 °C, with a mass ratio of SMTD to WTC of 2:1, and 0% O2, for a reaction time of 45 min. Under these conditions, the replacement of IBD with SGD ensures that all Fe phases detected by XRD are in the MFe form, without affecting Fe reduction under typical blast furnace oxygen injection rates of 0–8%;
2.
Effects of reductants: The reduction efficiency follows the order: WTC > CC > WT. Although WTC has 20.66% lower carbon content than CC, its higher saturation, smaller TG weight loss, and a surface tar Ea of only 5.79 kJ/mol contribute to a faster reaction rate. Additionally, the carbon black structure is more three-dimensional;
3.
Effects of heating methods: The three heating methods (PH, FH, MH) have little impact on Fe reduction. Among them, the higher heating rate of FH results in a faster carbon gasification rate, making the FH residue more uniformly loose. MH exhibits a more concentrated reaction zone, leading to larger metal particle sizes;
4.
TCLP results: Sufficient carbon thermal reduction significantly suppresses the leaching of Mg and Zn while promoting the leaching of Ca and Al. It also promotes the Cr leaching of 117.60%. The reduced residues require further processing to remove Cr to meet environmental safety standards.

Author Contributions

Conceptualization, F.L. and M.Z.; methodology, M.Z.; formal analysis, M.Z.; investigation, C.L.; resources, F.L.; data curation, M.Z.; writing—original draft preparation, M.Z.; writing—review and editing, M.Z.; supervision, F.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (52376205) and Young Scientific and Technological Talents (Level Two) in Tianjin (QN20230228).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
SMTDsintering machine tail dust
SGDsteelmaking gravity dust
IBDiron-bearing dust
CCcarbon coke
WTwaste tire
WTCwaste tire carbon
TFetotal iron
MFemetallic iron
FHflash heating group
PHprogrammed heating group
MHmicrowaved heating group
OOthe optimal group
CC-Areduced group using CC as alternative material
WT-Areduced group using WT as alternative material
SGD-Areduced group using SGD as alternative material

References

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Applsci 15 06504 g001
Figure 1. XRD spectrum of the IBD.
Figure 1. XRD spectrum of the IBD.
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Figure 2. Material input schematic and experimental instrument: (a) Material input schematic; (b) vertical furnace; (c) microwave furnace.
Figure 2. Material input schematic and experimental instrument: (a) Material input schematic; (b) vertical furnace; (c) microwave furnace.
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Figure 3. TG-DTG of raw materials: (a) IBD; (b) reductant.
Figure 3. TG-DTG of raw materials: (a) IBD; (b) reductant.
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Figure 4. Kinetic fitting curves of raw materials: (a) IBD: (a1) SMTD, (a2) SGD; (b) reductant: (b1) WT, (b2) WTC, (b3) CC.
Figure 4. Kinetic fitting curves of raw materials: (a) IBD: (a1) SMTD, (a2) SGD; (b) reductant: (b1) WT, (b2) WTC, (b3) CC.
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Figure 5. SEM image of raw materials: (a) SMTD: (a1) ×1k, (a2) ×16k; (b) SGD: (b1) ×1k, (b2) ×16k; (c) WTC: (c1) ×5k, (c2) ×100k; (d) CC: (d1) ×5k, (d2) ×100k.
Figure 5. SEM image of raw materials: (a) SMTD: (a1) ×1k, (a2) ×16k; (b) SGD: (b1) ×1k, (b2) ×16k; (c) WTC: (c1) ×5k, (c2) ×100k; (d) CC: (d1) ×5k, (d2) ×100k.
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Figure 6. Reduction of samples under different conditions: (a) XRD spectrum of different temperature: (a1) 700 °C, (a2) 800 °C, (a3) 900 °C, (a4) 1000 °C, (a5) 1100 °C; (b) XRD spectrum of different carbon ratio: (b1) 1:1, (b2) 2:1, (b3) 4:1, (b4) 9:1, (b5) 14:1; (c) XRD spectrum of different oxygen ratio: (c1) 0%, (c2) 2%, (c3) 4%, (c4) 6%, (c5) 8%; (d) XRD spectrum of different reaction time: (d1) 15 min, (d2) 30 min, (d3) 45 min, (d4) 60 min, (d5) 90 min; (e) GC results of different oxygen ratio.
Figure 6. Reduction of samples under different conditions: (a) XRD spectrum of different temperature: (a1) 700 °C, (a2) 800 °C, (a3) 900 °C, (a4) 1000 °C, (a5) 1100 °C; (b) XRD spectrum of different carbon ratio: (b1) 1:1, (b2) 2:1, (b3) 4:1, (b4) 9:1, (b5) 14:1; (c) XRD spectrum of different oxygen ratio: (c1) 0%, (c2) 2%, (c3) 4%, (c4) 6%, (c5) 8%; (d) XRD spectrum of different reaction time: (d1) 15 min, (d2) 30 min, (d3) 45 min, (d4) 60 min, (d5) 90 min; (e) GC results of different oxygen ratio.
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Figure 7. Schematic diagram of the WTC reduction process of SMTD.
Figure 7. Schematic diagram of the WTC reduction process of SMTD.
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Figure 8. XRD spectrum of reduced residue in different conditions: (a) Heating methods: (a1) PH, (a2) FH, (a3) MH; (b) different materials: (b1) OO (FH), (b2) CC-A, (b3) WT-A, (b4) SGD-A.
Figure 8. XRD spectrum of reduced residue in different conditions: (a) Heating methods: (a1) PH, (a2) FH, (a3) MH; (b) different materials: (b1) OO (FH), (b2) CC-A, (b3) WT-A, (b4) SGD-A.
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Figure 9. SEM image of reduced residues in different conditions: (a–c) heating methods: (a) PH: (a1) ×16k, (a2) ×100k; (b) FH: (b1) ×16k, (b2) ×100k; (c) MH: (c1) ×16k, (c2) ×100k; (d–f) different materials: (d) CC-A: (d1) ×5k, (d2) ×50k; (e) SGD-A: (e1) ×5k, (e2) ×50k; (f) WT-A: (f1) ×5k, (f2) ×50k.
Figure 9. SEM image of reduced residues in different conditions: (a–c) heating methods: (a) PH: (a1) ×16k, (a2) ×100k; (b) FH: (b1) ×16k, (b2) ×100k; (c) MH: (c1) ×16k, (c2) ×100k; (d–f) different materials: (d) CC-A: (d1) ×5k, (d2) ×50k; (e) SGD-A: (e1) ×5k, (e2) ×50k; (f) WT-A: (f1) ×5k, (f2) ×50k.
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Figure 10. TCLP leaching level of seven kinds of metals from reduced residues: (a) heating methods; (b) different materials.
Figure 10. TCLP leaching level of seven kinds of metals from reduced residues: (a) heating methods; (b) different materials.
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Table 1. Elemental analysis of raw materials (wt%).
Table 1. Elemental analysis of raw materials (wt%).
MaterialCHNS
WT75.016.370.662.75
WTC67.871.620.603.94
CC88.531.120.880.84
SMTD1.150.050.020.72
SGD1.090.640.140.00
Table 2. Chemical composition analysis of the IBD (wt%).
Table 2. Chemical composition analysis of the IBD (wt%).
MaterialCaAlMgCuCrZnOSi
SMTD11.751.372.19--0.0130.112.87
SGD11.680.163.86--0.1819.580.72
MaterialClPTFeFe2+MFe
SMTD0.140.0849.322.270.34
SGD0.120.0961.7427.8830.91
Table 3. Pyrolysis kinetic parameters of raw materials.
Table 3. Pyrolysis kinetic parameters of raw materials.
MaterialStageFitted CurveEa/(kJ/mol)A/(min−1)R2
SMTD-y = −4.155x + 3.82432.8423.960.9856
SGD1y1 = −6.148x + 13.22148.59195,125.090.9984
2y2 = −2.632x + 3.83020.8038.050.9672
WT2y2 = −2.760x + 6.08221.81344.910.9971
3y3 = −2.670x + 5.52421.10204.070.9658
WTC-y = −0.732x + 1.1215.799.110.8950
CC-y = −6.695x + 7.79652.91789.310.9994
Table 4. Leaching quantity of Zn, Cr, Cu (mg/L).
Table 4. Leaching quantity of Zn, Cr, Cu (mg/L).
Limiting ValueCC-A1CC-A2SGD-A1SGD-A2FH1FH2PH2MH2
Zn1004.05 248.71 261.77 16.08 257.32 3.41 7.57 6.96
Cr11.15 3.45 2.04 4.22 1.27 2.53 2.58 2.25
Cu1000.27 1.44 0.50 1.49 0.30 0.36 0.62 0.43
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Zeng, M.; Luan, C.; Lin, F. Application of Waste Tire Carbon for Iron-Containing Dust Reduction in Industrial Processes. Appl. Sci. 2025, 15, 6504. https://doi.org/10.3390/app15126504

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Zeng M, Luan C, Lin F. Application of Waste Tire Carbon for Iron-Containing Dust Reduction in Industrial Processes. Applied Sciences. 2025; 15(12):6504. https://doi.org/10.3390/app15126504

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Zeng, Menglan, Chujun Luan, and Fawei Lin. 2025. "Application of Waste Tire Carbon for Iron-Containing Dust Reduction in Industrial Processes" Applied Sciences 15, no. 12: 6504. https://doi.org/10.3390/app15126504

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Zeng, M., Luan, C., & Lin, F. (2025). Application of Waste Tire Carbon for Iron-Containing Dust Reduction in Industrial Processes. Applied Sciences, 15(12), 6504. https://doi.org/10.3390/app15126504

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