The Effect of Coke Powder on the Iron Ore Fines Reduction by Carbon Monoxide

Abstract

Iron ore fines and coke powder are synergistically reduced in a drop-tube reactor. The effects of temperature, CO concentration, and the blending ratio (BR) of coke powder are investigated. The results show that increasing temperature promotes reduction within the range of 800–1200 °C, while CO concentration has a lesser effect. The kinetic mechanism can be explained by a nucleation-and-growth model, and the activation energies for iron ore fines, the mixture with fine coke powder at BR = 0.4, and the mixture with the same particle size coke powder at BR = 0.4 are 19.09 kJ/mol, 50.61 kJ/mol, and 47.80 kJ/mol, respectively. Coke powder causes the outlet CO concentration to be equal to or greater than the inlet CO concentration and the outlet CO₂ concentration decreases. Increasing BR from 0.4 to 0.8 and 0.8 to 1.2 mainly promoted the conversion of Fe₃O₄→FeO and FeO→Fe, respectively. The sticking index reached a minimum of 0.86% when BR = 0.4. However, overall sticking inhibition was more effective when BR = 1.2. The mechanism of sticking inhibition is that the coke powder acts as a physical barrier, reducing the collision and contact probability between the iron ore fines, thereby preventing iron whiskers from entangling or bridging.

1. Introduction

Traditional ironmaking technology is a blast furnace process that includes coking, sintering, and pelletizing, which is under great pressure to reduce CO₂ emissions. , To shorten the production process and mitigate CO₂ emissions, many new ironmaking processes have been researched and optimized in recent decades. , Among available technologies, direct reduction has been identified as an advanced iron-making process. Direct reduction produces direct reduced iron (DRI) by using low-temperature reducing gas rather than high-temperature smelting reduction, thereby reducing CO₂ emissions while avoiding procedures such as coking, sintering, and pelletizing. This aligns with the green, low-carbon, and environmentally friendly development of the iron-steel industry.

Various reactors were used in the studies of the direct reduction reaction. The most studied reactors are thermogravimetric analyzer, , fluidized bed, − and drop-tube reactors. , However, the reaction time of iron ore fines in thermogravimetric analyzer and fluidized bed lasts for several minutes or even tens of minutes. Consequently, the direct reduction of iron ore fines in a drop-tube reactor is an efficient method. The drop-tube reactor is widely applied in research of combustion of coal and biomass, pollutant formation characteristics, and reaction mechanisms. − The heating rate of the drop-tube reactor is extremely fast, allowing the material to reach the reaction temperature almost instantly and saving time on preheating the iron ore fines. The gas flow rate in the drop-tube reactor can be controlled by adjusting the reactor length or the gas flow volume, making it easier to accurately calculate the residence time of particles and reaction kinetic parameters. In addition, the drop-tube reactor has the advantage of providing a large contact area between reducing gas and solid phase. After the reaction, the solid samples settle directly to the bottom for collection, and the exhaust gas is collected after filtration. A high gas–solid separation efficiency is beneficial for subsequent component analysis. Therefore, the direct reduction of iron ore fines in a drop-tube reactor holds significant research value.

The direct reduction process of iron oxides typically exhibits overlapping stages from Fe₂O₃ to Fe₃O₄, FeO, and Fe, which often occur simultaneously. , Reactions involving multiple intermediates are complex. Many previous studies on the reduction of iron ores have focused mainly on the reaction mechanism and kinetic parameters, as shown in Table . The reducing gases used in the literature were CO and H₂. Although the diffusion and reaction rates of H₂ are fast, the reduction of H₂ still faces the challenges of H₂ storage and transportation. , Currently, reducing iron ore with CO has research value and practical significance. The kinetic model for CO reducing iron ore includes the nucleation-and-growth model, the unreacted core model, the phase boundary, etc. The experimental parameters used for model fitting are closely related to factors such as the reaction temperature, CO concentration, and iron ore particle size. For CO reduction, Guo et al. and Sadeghi et al. found that adding biomass and CaO can improve the reduction extent and accelerate the reduction of FeO, respectively. Meanwhile, CO reduction tends to form iron whiskers and dense layered iron structures, which cause sticking and agglomeration. Researchers have adopted methods of applying a cement coating and a conical fluidized bed to prevent sticking, , but the above methods are complicated to operate and hard to use in industry. Coke powder enhances the reduction degree and has no stickiness, which also obviously affects the kinetic parameters. However, few studies have focused on the role of coke powder in CO reduction. There is no literature reporting the applicable kinetic model and parameters after adding coke powder.

1. Comparison of the Present Study with the Previous Study.

Reactor	Particle	Condition	Reduction mechanism	E a (kJ/mol)
Micro fluidized bed	Hematite ore (45–88 μm)	700–850 °C	3D diffusion	23.76–54.89
		25%-75% H2–Ar	3D phase boundary reaction
		30 min	1D formation and growth of nuclei
Conical fluidized bed	Ferric oxide (86.9 μm)	775 °C	internal diffusion control	75.95–131.46
		60%, 30% H2–N2
		5 min
Drop-tube reactor	Hematite (21 μm)	1200–1350 °C	nucleation and growth model	231
		35%-100% CO-N2
		2.1–9.1 s
Thermo-gravimetric analysis reactor	Iron ore pellets (10–13 mm)	800–1100 °C	-	-
		H2, CO
		150 min
Fluidized Bed	Hematite (1–3 mm)	750–850 °C	-	59.11
		50% CO-N2
		10–30 min
Microfluidized Bed	Hematite (100–150 μm)	750–850 °C	contacting volume (R3)	15.288–53.520
		50% CO-N2	second-order (F2)
		5–10 min	first-order (F1)
Tube furnace	Iron oxide pellets (10 mm)	850–1000 °C	unreacted core model	54.0
		H2–CO
		60 min
Thermo-gravimetry	Hematite (<140 μm)	750–900 °C	zero-order contradiction model (F0)	40–65
		10%-50% CO-N2	two-dimensional contradiction (R2)
		20–70 min	unimolecular decay (F1) model
Thermo-gravimetric analysis reactor	Hematite (100–300 μm)	750–900 °C	phase boundary	19.0 ±0.14
		5%-20% CO-N2
		45 min
This study Drop-tube Reactor	Iron ore fines (58–106 μm)	800–1200 °C	nucleation and growth model	19.09–50.61
		30%-70% CO-N2
		2.14–2.47 s

In this study, the effect of coke powder on the reduction of iron ore fines by CO was investigated by using a drop-tube reactor. On the basis of the factors selected by response surface methodology in previous studies, three factors (reduction temperature, CO concentration, and coke content) were the experimental variables. We studied how reaction conditions affect the reduction and reaction rates by evaluating the reduction degree and phase composition. A kinetic model was established to analyze the kinetic parameters of iron ore fines reduction before and after the addition of coke powder. Furthermore, we investigated the sticking behavior of iron ore fines by calculating the sticking index, observing changes in surface morphology, and characterizing porosity. The properties of the solid samples were determined using X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Gas compositions were also tested. Adding coke powder is a simple approach to operate and industrialize, providing valuable insights into iron ore fine reduction in a drop-tube reactor, revealing the effects of CO and coke powder on the reduction kinetics of iron ore fines.

2. Materials and Methods

2.1. Experimental Materials

In this study, Miaoling Gou iron ore fines and a commercial Fugu coke were used as experimental materials, and their chemical compositions and the proximate and ultimate analyses are listed in Tables and , respectively. The preparation process of the materials is that the iron ore fines dried at 105 °C for 4 h and then sieved to approximately 58–106 μm. Fugu coke was crushed and sieved to obtain particle sizes smaller than 58 μm and 58–106 μm, with average diameters of 14.097 and 66.385 μm, respectively. The particle size distributions of Miaoling Gou iron ore fines and Fugu coke powder are shown in Figure . The purities of CO and N₂ were 99.95%.

2. Chemical Compositions of Iron Ore Fines.

Component	Total Fe	FeO	SiO2	Al2O3	MgO	CaO	K2O	Na2O	P	S
Wt. %	65.39	27.75	5.58	0.92	0.62	0.46	0.09	0.05	0.02	0.2

3. Proximate and Ultimate Analyses of Fugu Coke Powder .

Proximate analysis (wt %)				Ultimate analysis (wt %)
Mad	Vad	Aad	FCad	Cdaf	Hdaf	Odaf	Ndaf
5.26	3.60	8.08	83.06	82.33	1.90	15.20	0.57

^aNote: ad, air-dry basis; daf, dry ash-free basis.

1.

Particle size distribution of iron ore fines and Fugu coke powder.

2.2. Experimental System

Figure illustrates the experimental system for the synergistic reduction of iron ore fines and coke powder. The system consisted of a powder feeder, a gas supply system (including primary and secondary gases), a sample receiver, an exhaust gas detection device, a water-cooling device (including feed cooling and receiving cooling), and a drop-tube reactor. The reactor was a heat-resistant aluminum tube with an inner diameter of 120 mm and a length of 3000 mm. To provide the high temperatures required by the experiment, a silicon–molybdenum rod was adopted around the reactor. The entire alumina tube served as the heating zone, with the middle 1500 mm as the constant-temperature zone.

2.

Experimental system for iron ore fines reduction.

It is beneficial to maintain stable and continuous feed and gas supplies to obtain solid and gaseous products. Therefore, a microscrew-type feeder was selected for the experimental system. The iron ore fines and Fugu coke powder were evenly mixed and placed in the feed bin, and the feed rate was calibrated. The gas inlet was controlled by using mass flow meters. The primary gas carries the materials into the reactor, whereas the secondary gas provides a reducing atmosphere and regulates the reaction conditions. The exhaust gas generation and gas analyzer were connected via a three-stage filter for dust and water removal, enabling online monitoring.

The comprehensive factors of the gas flow rate, particle properties, and the thermostatic zone length determine the particle residence time. Assuming that the particles fall at a constant velocity in the thermostatic zone, the terminal falling velocity of the particles u _t can be expressed by eq ,

$$u_t=\frac{d_p^{2}g({\rho }_p-{\rho }_g)}{18\mu }$$ 1

where d _p refers to the particle diameter, m; ρ _p and ρ _g stand for the particle and gas density, kg/m³; and μ is the gas viscosity, m/s.

The residence time t is calculated according to eq :

$$t=\frac{L}{u_g+u_t}$$ 2

where L denotes the length of the thermostatic zone, m; u _g represents the centerline velocity of the gas at the furnace temperature, m/s.

2.3. Experimental Conditions

The experimental conditions are important factors affecting the reduction of iron ore fines. The heating temperature range used in this study is 800–1200 °C. Above 800 °C, high internal and external diffusion rates, as well as a high reduction reaction rate, can be ensured. The purpose of maintaining the reduction temperature below 1200 °C is to keep the product in a solid state. In addition to temperature, the blending ratio (BR) of the Fugu coke powder also affects the characteristics of the reduction products. By adjusting the amounts of iron ore fines and Fugu coke powder, four blending conditions (Table ) were obtained to study the effects of the blending ratio on reduction. The blending ratio is defined as follows:

$$BR={\mathrm{C}}_{coke}/{\mathrm{O}}_{IOF}\times 100\%$$ 3

where C _coke is the amount of carbon in the Fugu coke powder and O _IOF is the oxygen content in the iron ore fines.

4. Details of the Four Blending Conditions.

Blending ratio	Proportion of coke powder (wt %)	Feeding rate (g/min)
0	0	7.05
0.4	10.7	7.89
0.8	21.4	8.97
1.2	32.1	10.39

For each blending condition, five CO concentrations (30, 40, 50, 60, and 70%) were selected to study the effect of the gas atmosphere. The CO concentration was controlled by using mass flow meters, and N₂ was used for gas distribution. Among these, 30% and 70% correspond to the minimum and maximum CO concentrations in gasification with air, oxygen-enriched air, and pure oxygen, respectively. Although there are differences between the experimental system and the actual entrained flow, reasonable experimental conditions can still reflect the actual conditions.

2.4. Characterization of Samples

The reduction degree (X) and the metallization rate (M) are important indicators reflecting the reduction level of samples. The reduction degree is the percentage of reduced oxygen in the iron ore to the initial total oxygen content. The metallization rate is the percentage of reduced precipitated metallic iron to the total iron. The contents of TFe, Fe²⁺, and Fe⁰ are measured according to GB/T 6730.65–2009, Iron oresDetermination of total iron content-Titanium­(III) chloride reduction potassium dichromate titration methods (routine methods), GB/T 6730.8–2016, Iron oresDetermination of iron­(II) content-Potassium dichromate titrimetric method, and GB/T 6730.6–2016, Iron oresDetermination of metallic iron content-Ferric chloride-sodium acetate titrimetric method, respectively. The calculation of reduction degree and metallization rate is shown in eqs and , ,

$$X=1-\frac{1.5\times \mathrm{F}{\mathrm{e}}^{3+}\times \mathrm{F}{\mathrm{e}}^{2+}}{1.5\times T\mathrm{F}\mathrm{e}}\times 100\%$$ 4

$$M=\mathrm{F}{\mathrm{e}}^0/T\mathrm{F}\mathrm{e}$$ 5

where TFe is the total iron content in the sample, Fe³⁺ is the ferric iron content, Fe²⁺ is the ferrous content, and Fe⁰ is the metallic iron content.

The mineral compositions of the samples were identified by XRD with an X-ray diffractometer (PANalytical Aeris), using a Cu–Kα radiation. The X-ray diffractometer was operated at 40 kV and 15 mA in a scanning range of 10–90°. The test results were qualitatively analyzed using Jade 6 software, and the mineral phases were quantitatively obtained using the Rietveld refinement method. The surface morphologies of typical samples were observed using SEM (SUPRA 55, Carl Zeiss, Germany), and a gold layer was deposited on the samples before scanning to improve conductivity. The exhaust gas components were monitored in real time using a gas analyzer from Hubei Ruiyi Company, which was calibrated with the refinery gas analysis.

The sticking index (SI) is often used to describe and evaluate sticky behavior in fluidized reduction. , The sticking index refers to the mass percentage of agglomerates formed owing to adhesion during high-temperature reduction to the total sample, which is defined as follows:

$$SI=m_{agglomerates}/m_{total}\times 100\%$$ 6

where m _agglomerates and m _total are the masses of reduced particles larger than the original sample and the entire sample, respectively. Samples with different particle sizes were obtained via sieving. The larger the sticking index, the more severe the sticking.

3. Results and Discussion

3.1. Reduction Degree and Metallization Rate

As shown by the solid line in Figure a, the reduction degree of iron ore fines and the mixture with BR = 0.4 exceeds 15%, and the overall reduction degree increases with temperature. The maximum reduction degree of iron ore fines at 1100 °C is 23.25%. The reduction degree of the mixture with BR = 0.4 is significantly improved at 1100 and 1200 °C, especially at 1200 °C, where it reaches 38.84%. The metallization rate of iron ore fines is little affected by temperature and is highest at 1100 °C at 3.91%. When the mixture with BR = 0.4, the metallization rate increases with the temperature increasing, which goes for a maximum of 7.01% at 1200 °C.

3.

Reduction degree and metallization rate. (a) 30% CO; (b) 1200 °C.

The reduction degree of the iron ore fines increased with increasing CO concentration, as shown in Figure b. When the CO concentration was 30%, the reduction degree was the lowest at 20.37%, whereas the highest reduction degree was 33.20% at 70% CO. The reduction degree of the mixture with BR = 0.4 was higher than that of the iron ore fines. However, it changes little with the CO concentration, averaging approximately 37%. Regarding the metalization rate, the iron ore fines reached a maximum of 9.67% with 70% CO. When BR = 0.4 with fine coke powder, the CO concentration had no significant effect on the metallization rate.

3.2. Compositional Analysis of Samples

The temperature in this study is above 570 °C, so the reduction reactions of iron ore fines proceed in two steps, as shown in (r1) and (r2). To better understand the sample composition, Figure shows the qualitative and quantitative analyses of the reduced samples by XRD. The results show the presence of Fe₃O₄ and FeO compounds at T = 900 °C. In contrast, Fe is observed at temperatures above 1100 °C. As the temperature increased, the Fe₃O₄ content decreased, whereas the FeO and Fe contents increased. This indicates that the conversions of Fe₃O₄→FeO and FeO→Fe occurred simultaneously during the reduction process, and the ferrous valences decreased with increasing temperature.

$${\mathrm{Fe}}_3{\mathrm{O}}_4+\mathrm{CO}\to {\mathrm{3FeO}+\mathrm{CO}}_2$$ r1

$$\mathrm{FeO}+\mathrm{CO}\to {\mathrm{Fe}+\mathrm{CO}}_2$$ r2

4.

Characterization of the reduced iron ore fines samples. (a) XRD patterns of samples versus temperature (30% CO); (b) XRD patterns of samples versus CO concentration (1200 °C); (c) sample compositions versus temperature (30% CO); (d) sample compositions versus CO concentration (1200 °C).

With increasing CO concentration, the overall Fe₃O₄ content decreases, while the FeO and Fe contents increase. At a CO concentration of 70%, the Fe content reached 12.31%.

Because 30% CO is a typical proportion of syngas composition, this concentration was selected for carbon synergistic reduction. As shown in Figure , the Fe₃O₄ content decreased significantly with increasing temperature, whereas the FeO and Fe contents increased significantly. The difference is that FeO is formed at 800 °C and Fe is formed at 1000 °C. This indicates that coke powder reduces the formation temperatures of FeO and Fe and accelerates the reaction rates of the unreacted iron oxides. The C in coke powder reacts with CO₂ produced from the reduction of iron ore fines to regenerate CO, as shown in (r3). This increases the reduction potential and drives the forward progress of (r1) and (r2). In addition, FeO is the main component of the product above 1100 °C.

$${\mathrm{C}+\mathrm{CO}}_2\to \mathrm{2CO}$$ r3

5.

Characterization of the mixture samples versus temperature and blending ratio using 30% CO. XRD patterns of (a) BR = 0.4, (b) BR = 0.8, and (c) BR = 1.2; sample compositions of (d) BR = 0.4, (e) BR = 0.8, and (f) BR = 1.2.

As the temperature increases, the Fe₃O₄ content decreases significantly, the Fe content increases significantly, and the FeO content initially increases and then decreases. This means that the higher the temperature, the lower the valence iron. At 1200 °C, the Fe content reached 31.68%, and the Fe₃O₄ content decreased to 4.7%. FeO, as an intermediate product, decreased at 1200 °C because it partially converted to Fe.

Compared with BR = 0.8, the Fe₃O₄ content was lower and the Fe content was higher under the same conditions when BR = 1.2; however, the range of change narrowed. This shows that increasing the proportion of coke powder above BR = 0.8 results in only a limited improvement in the reduction effect.

The characterization of the mixture samples at 1200 °C with BR = 0.4 is shown in Figure a and d. The main reduction product is FeO, accounting for about 60%. The secondary product was Fe, whose content showed an increasing trend. The Fe₃O₄ content decreased to less than 15%, and the Fe content increased to more than 18% when the inlet CO concentration was above 50%.

6.

Characterization of the mixture samples versus CO concentration and blending ratio at 1200 °C. XRD patterns of (a) BR = 0.4, (b) BR = 0.8, and (c) BR = 1.2; sample compositions of (d) BR = 0.4, (e) BR = 0.8, and (f) BR = 1.2.

The XRD analysis shown in Figure b and e indicates that the proportion of the main product, FeO, increased to approximately 70%. The secondary product, Fe, was approximately 20% after the BR was increased to 0.8. The Fe₃O₄ content is extremely low, at less than 12%. The results suggest that increasing the proportion of coke powder from BR = 0.4 to 0.8 significantly promoted the conversion of Fe₃O₄ to FeO.

The main product of the reaction was still FeO, with an approximately 50% content when BR = 1.2. However, the Fe content of the secondary product increases from 27.54% to 44.86%, as presented in Figure f. The results indicate that Fe₃O₄ was converted into FeO, and increasing the proportion of coke powder promoted the conversion of a considerable amount of FeO into Fe.

3.3. Reaction Rate Model and Activation Energy

Metallurgists have proposed various kinetic equations to match experimental data. ,, Many studies have suggested that the mechanism of iron oxide reduction can be explained by the unreacted core model, which primarily involves secondary processes such as external and internal diffusion, and interfacial chemical reactions. Some researchers also think that the nucleation and growth model can represent the kinetic mechanism of iron ore reduction. , We fitted the experimental data with commonly used gas–solid reaction kinetic models, and the results are shown in Figure . Based on the fitting degree of the linear relationship, the 1D nucleation and growth model can better describe the reaction mechanism of iron ore fines by CO, as shown in Figure a. The entrained drop-tube reactor provides a high terminal falling velocity, facilitating the diffusion of CO and the reaction with the particles. Thus, the reaction process is mainly controlled by nucleation and growth reactions.

7.

Experimental and fitting results of the mechanism functions of different kinetic models versus time for the iron ore fines reduction with 30% CO. (a) 1D formation and growth of nuclei, (b) 2D formation and growth of nuclei, (c) 3D formation and growth of nuclei, (d) 1D diffusion, (e) 2D diffusion, (f) 3D diffusion, (g) 1D phase boundary reaction, (h) 2D phase boundary reaction, (i) 3D phase boundary reaction.

The overall nucleation and growth rate equations are as follows:

$${[-\mathrm{l}\mathrm{n}(1-X)]}^{1/n}=[k·f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)]·t=k_{app}·t$$ 7

where, X represents the reduction degree, n is the Avrami exponent, k represents the reaction rate constant, f _p (pCO, pCO ₂ ) stands for the gas concentration driving force, t is the residence time, and k _app is the apparent reaction rate constant.

Once the temperature and CO concentration are determined, the Avrami exponent n can be obtained from eq . Based on Figure , n is set to 1 in this study.

$$\ln [-\ln (1-X)]=n·(\ln (t)+\ln (k_{app}))=n·\ln (t)+D$$ 8

The apparent reaction rate constant can be derived from eq , as below:

$$k_{app}=k·f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)$$ 9

As k is constant at a fixed temperature, k _app and f _p (pCO, pCO₂) exhibit linear relationships. However, the CO₂ partial pressure increased as the reaction proceeded, and the conversion of FeO→Fe (FeO + CO = Fe + CO₂) was limited by equilibrium. Therefore, the gas concentration driving force gradually decreases with the CO partial pressure decreasing. The chemical equilibrium and the reaction order related to the CO partial pressure together determine the gas pressure exponent. Therefore, the gas concentration driving force can be shown as follows:

$$f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)=p\mathrm{C}{\mathrm{O}}^m-{(p\mathrm{C}{\mathrm{O}}_2/K)}^m$$ 10

where m is the reaction order related to the CO partial pressure, and K is the chemical equilibrium constant for the reaction of FeO and CO, which has different values at different temperatures. HSC Chemistry 9.5 was used to calculate the equilibrium constant K. (pCO₂ / K) ^m indicates a lower gas concentration driving force promoting the forward reduction reaction owing to thermodynamic equilibrium constraints.

The amount of CO was based on the stoichiometric ratio of complete reduction in the chemical equation, and the amount of CO₂ produced was less than 23% of the inlet CO during the experiment. Given the varying CO₂ concentrations produced by the reaction, the average gas concentration driving force in gas inlet and outlet of the reactor was used. Based on the amount of iron ore fines fed and the reduction degree, the molar amounts of CO₂ generated by the reduction reaction $n_{\mathrm{C}{\mathrm{O}}_2}$ and the remaining CO after reduction n _CO can be calculated according to eqs and .

$$n_{\mathrm{C}{\mathrm{O}}_2}=4\times n_{iof}\times X$$ 11

$$n_{\mathrm{C}\mathrm{O}}=n_{\mathrm{C}\mathrm{O},i}-n_{\mathrm{C}{\mathrm{O}}_2}$$ 12

$$p\mathrm{C}{\mathrm{O}}_2=\frac{n_{\mathrm{C}{\mathrm{O}}_2}}{n_{\mathrm{C}{\mathrm{O}}_2}+n_{\mathrm{C}\mathrm{O}}+n_{{\mathrm{N}}_2}}$$ 13

where n _iof is the amount of iron ore fines (mol) and n _CO,i is the initial amount of CO (mol). $p\mathrm{C}{\mathrm{O}}_2$ is the partial pressure of CO₂ (atm), and $n_{{\mathrm{N}}_2}$ is the amount of N₂ (mol).

The partial pressures of the outlet CO₂ under different experimental conditions varies with the inlet CO concentration. Previous studies have shown that either the logarithmic or the arithmetic mean can reasonably represent the average gas concentration driving force, corresponding to a first- or half-order relationship, respectively. Both the above order dependencies must be tested to ultimately determine the order related to CO partial pressure. The equations for the logarithmic and arithmetic means are as follows:

$${[f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)]}_{LM}=p\mathrm{C}\mathrm{O}-p\mathrm{C}{\mathrm{O}}_2/K$$ 14

$${[f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)]}_{AM}=p\mathrm{C}{\mathrm{O}}^{1/2}-{(p\mathrm{C}{\mathrm{O}}_2/K)}^{1/2}$$ 15

The k _app values obtained using eq are plotted against the logarithmic and arithmetic means in Figure using experimental data at 1200 °C. It can be seen that better fitting was achieved with m = 1 for the iron ore fines; however, the fitting performance improved with m = 1/2 after blending fine coke powder. Therefore, the reduction reaction of the iron ore fines related to the CO partial pressure has a first-order relationship. In contrast, the reduction reaction of the mixture with BR = 0.4 fine coke powder has a half-order relationship under the experimental conditions..

8.

Plot of k_app and average gas concentration driving force at 1200 °C. (a) logarithmic mean, (b) arithmetic means.

After establishing the dependency on reduction and CO partial pressure, the reaction rate constants are derived from eq .

$$k=\frac{k_{app}}{{[f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)]}_{LM}}=\frac{k_{app}}{p\mathrm{C}\mathrm{O}-p\mathrm{C}{\mathrm{O}}_2/K}$$ 16

$$k=\frac{k_{app}}{{[f_p(p\mathrm{C}\mathrm{O},p\mathrm{C}{\mathrm{O}}_2)]}_{AM}}=\frac{k_{app}}{p\mathrm{C}{\mathrm{O}}^{1/2}-{(p\mathrm{C}{\mathrm{O}}_2/K)}^{1/2}}$$ 17

$$k=A·\exp (-\frac{E_{\mathrm{a}}}{RT})$$ 18

where A is the pre-exponential factor and E _a is the activation energy. R represents the molar gas constant, with a value of 8.314 J/(mol·K).

Ln k and 1/T for iron ore fines and coke-blended mixtures are linearly related, which can be plotted by combining eqs and , and . According to the parameters of the linear functions shown in Figure , it can be calculated that the activation energy for the reduction of iron ore fines is 19.09 kJ/mol, and the pre-exponential factor is 2.36 atm^–1s^–1 in the temperature range of the experiment. For the reaction of the mixture blended with fine coke powder at BR = 0.4, the activation energy is 50.61 kJ/mol, and the pre-exponential factor is 47.47 atm^–1s^–1. When BR = 0.4 and the iron ore fines are mixed with coke powder of the same particle size, the activation energy and the pre-exponential factor are 47.80 kJ/mol and 37.71 atm^–1s^–1, respectively.

9.

Arrhenius plot at different temperatures.

For the complex reaction of iron ore fines reduction, the activation energy is the algebraic sum of the activation energies of each elementary reaction that constitutes the overall reaction, reflecting the difficulty of the reaction. The greater the activation energy, the more difficult it is for the reaction to proceed. The physical significance of the pre-exponential factor is the total number of collisions of reactant molecules per unit time and per unit volume. The larger the pre-exponential factor, the easier the reaction is. In this study, both the activation energy and the pre-exponential factor increase after blending coke powder, but the increase in the pre-exponential factor has a greater effect than the increase in the activation energy. Therefore, the overall reduction reaction rate of the iron ore fines increases. By combining the kinetic mechanisms of various coke powder gasification reactions, − we found that the activation energy of coke powder gasification mainly ranges from 158.11 to 234.32 kJ/mol. The activation energy for coke powder gasification is obviously higher than that for iron ore fines reduction. The overall reaction after blending coke powder is the sum of reactions (r1) to (r3). Therefore, the occurrence of the coke gasification reaction leads to an increase in the overall reaction activation energy.

3.4. Morphology Analysis

As shown in Figure , a small amount of slender iron whiskers form at 800–1000 °C. However, most of the iron ore fines remain flat on the surface and macroscopically nonsticking. At 1100–1200 °C, a large number of stick-like protrusions appear on the surface of iron ore fines, and extremely uneven iron whiskers are hooked, indicating a clear sticking state. After blending coke powder with BR = 0.4, although many iron whiskers form at 800–1200 °C, due to the physical isolation effect of the coke powder, the nonsticking state is observed at 800–1000 °C, and the sticking phenomenon at 1100–1200 °C is also inhibited..

10.

Surface morphology of iron ore fines (BR = 0) and BR = 0.4 with coke powder, reduced with 30% CO.

Iron whiskers persisted even after increasing the CO concentration, resulting in stick-like or block-like protrusions on the surface of the iron ore fines that were extremely uneven and exhibited pronounced sticking, as shown in Figure . After blending with coke powder, the coke powder not only formed a physical isolation around the iron ore fines but also reduced the collision probability during the falling process. Through the above two mechanisms, coke powder effectively inhibits sticking.

11.

Surface morphology of iron ore fines (BR = 0) and BR = 0.4 with coke powder, reduced at 1200 °C.

3.5. Components of Exhaust Gas

The outlet gas components were affected by temperature and inlet gas components. In the range of 800 to 1200 °C, the outlet CO concentration decreases and the outlet CO₂ concentration increases after the reduction of pure iron ore fines (BR = 0), as shown in Figure a and b. The CO and CO₂ concentrations at the outlet of the mixture with BR = 0.4 remain basically unchanged after the reduction. The difference is that the outlet CO concentration of the mixture with BR = 0.8 and BR = 1.2 is significantly higher, and the CO₂ concentration is significantly lower after reduction. This is because Fe₃O₄ and FeO consume CO to produce CO₂ during the reduction of pure iron ore fines; thus, the CO concentration decreases, and the CO₂ concentration increases. After the coke powder was added, the C in the mixture and the CO₂ generated by the reduction reaction were reformed into CO, increasing the CO concentration at the outlet. When the temperature is above 1000 °C and BR≥ 0.8, the outlet CO concentration is generally equal to or greater than the inlet CO concentration. This is beneficial for further utilization or recycling of the exhaust gas.

12.

Components of exhaust gas. Effect of reduction temperature on: (a) CO concentration, (b) CO₂ concentration; effect of inlet CO concentration on: (c) CO concentration, (d) CO₂ concentration.

The inlet CO concentration increased from 30% to 70%, and the outlet CO concentration increased approximately linearly after the reduction reaction, as shown in Figure c and d. The outlet CO concentrations of the mixtures with BR = 0 and BR = 0.4 were similar after reduction. However, both were lower than the inlet CO concentrations. The outlet CO concentrations of the mixtures with BR = 0.8 and BR = 1.2 increased after the reaction, and both were higher than the inlet CO concentration. The outlet CO₂ concentration increased with the inlet CO concentration in pure iron ore fines (BR = 0), indicating that CO was the limiting factor for the reaction. When the BR was 0.4, 0.8, or 1.2, the outlet CO₂ concentration was minimally affected by the inlet CO concentration, indicating that the C in the coke powder reacted with CO₂ generated by the reduction and consumed some CO₂.

3.6. Influence of Coke Powder on Sticking

Figure shows the sticking indices at different temperatures, blending ratios, and coke powder particle sizes. The results indicate that the sticking index decreases with increasing temperature from 800 to 1000 °C, while it increases significantly from 1000 to 1200 °C. The reason is that the gas flows slowly, providing a longer residence time at 800 °C and increasing the probability of particle–particle collisions. At this point, a small number of elongated iron whiskers had already formed on the surface of the iron ore fines, which interlocked during collisions, leading to agglomeration. However, the surface of a single iron ore particle transforms into spherical protrusions at 1000 °C that do not easily stick together. In addition, a shorter residence time reduces the chances of particle collisions, resulting in minimal agglomeration. From 1000 to 1200 °C, the residence time is even shorter. However, on the one hand, low-melting-point compounds are formed at higher temperatures, and on the other hand, rod-like protrusions appear on the surface of iron ore fines. During collisions, the uneven surfaces interlock, forming larger, more robust aggregates.

13.

Sticking index at different blending ratios.

Blending coke powder during the reduction reaction of iron ore fines helps inhibit sticking. When blending fine coke powder with BR = 1.2, the inhibiting effect on sticking is observed throughout the entire experimental temperature range. The greatest improvement in inhibiting sticking occurs at 800 °C, with the sticking index decreasing from 31.03% to 5.37%. This was mainly due to the high blending ratio and the small particle size of the coke powder. A large amount of fine coke powder is dispersed throughout the reactor, providing physical isolation and reducing the collision probability of iron ore fines, as presented in Figure b. The sticking index of fine coke powder mixed with BR = 0.4 and BR = 1.2 are similar in the range of 800–1100 °C, but the coke powder no longer works effectively at 1200 °C. Coke powder with a low blending ratio does not provide sufficient physical isolation at this temperature and may even be encapsulated within the agglomerates, as shown in Figure a.

14.

Schematic diagram of inhibiting sticking. (a) BR = 0.4 with fine coke powder, (b) BR = 1.2 with fine coke powder, and (c) BR = 0.4 with the same particle size coke powder.

The EDS elemental mapping for five samples at 1100 °C are presented in Figure a–e. In the case of iron ore fines reduction, the Fe element is evenly distributed in the product. When fine coke powder is blended with iron ore fines, it can be observed that the Fe element is uniformly divided into several regions, resulting in a decrease in agglomeration performance due to the physical isolation effect of the C element. The distribution of the C element is abundant and uniform, indicating good dispersion in the iron ore fines. Consequently, adding fine coke powder significantly reduces the agglomeration degree of the iron ore fines at higher temperatures. The cross-sectional SEM images of high-temperature agglomerates are shown in Figure f–g. The measuring points are marked on images, and the compositions obtained by EDS analysis are shown in Table . The main component at the particle sticking area is Fe, indicating that the newly formed iron has high surface activity and tends to stick together easily. It is meaningful to add coke powder to prevent sticking.

15.

EDS elemental mapping and cross-sectional SEM images after reduction at 1100 °C with 30% CO. (a)–(e) EDS elemental mapping of different reduced products (wt.); (f)–(g) cross-sectional SEM images of high-temperature agglomerates.

5. EDS Results of the Composition for the Four Measuring Points of Different Agglomerates in Figure f and g.

	Composition (wt %)
Measuring points	Fe	O	C	Si
1	92.60	0.69	6.61	0.09
2	68.22	26.13	5.50	0.15
3	46.82	34.63	6.52	12.03
4	34.31	40.66	6.68	18.34

When fine coke powder was replaced with coke powder of the same particle size as the iron ore fines, the inhibitory effect on sticking worsened. As shown in Figure c ande, the amount of coke powder decreases at the same blending ratio. Iron ore fines cannot be physically isolated evenly, and the collision probability between iron ore fines increases, making it difficult to prevent sticking. The N₂ adsorption–desorption isotherms and pore size distributions of the two coke powders are shown in Figure a and b. The hysteresis loop and rapid increase in adsorption capacity at low relative pressure of the two coke powders indicate the presence of mesopores and a large number of micropores, respectively. In addition, the pore size distributions in Figure b show that the two coke powders are mainly composed of micropores ranging from 0.4 to 2 nm and mesopores ranging from 2 to 10 nm. The detailed surface area and pore structure characteristics of the two coke powders are listed in Table . By comparing the two coke powders, we found that the specific surface area and total pore volume of fine coke powder are slightly smaller than those of the same particle size coke powder. Coke powder of the same particle size has more active sites participating in the gasification reaction, which corresponds to the result that the activation energy for reduction with the same-particle-size coke powder is slightly lower than that with fine coke powder. If two coke powders are simplified into spheres, then the volume specific surface area is inversely proportional to the particle radius. Substituting the average particle size, the ratio of the volume specific surface area of fine coke powder to that of the same-particle-size coke powder is 4.71:1. Therefore, the fine coke powder, with a larger volume specific surface area, can achieve better physical isolation and anti-sticking effect.

16.

Pore structure characteristics of two coke powders. (a) N₂ adsorption–desorption isotherms, (b) pore size distributions.

6. Surface Area and Pore Distribution of Two Coke Powders.

Coke	Specific surface area (m2/g)	Micropore specific surface area (m2/g)	Total pore volume (cm3/g)	Micropore volume (cm3/g)	Average pore size (nm)
Fine coke powder	411.456	200.454	0.352	0.084	3.324
Same particle size coke powder	434.610	218.80	0.364	0.092	3.281

4. Conclusion

In this study, the effect of coke powder on the reduction of iron ore fines by CO was elucidated. Moreover, the mechanism preventing the sticking of coke powder was explained at the microscopic level. The specific conclusions are as follows:

Within the range of 800–1200 °C, increasing temperature promotes reduction. The Fe₃O₄ content decreased, whereas the Fe content increased significantly. This is because higher temperatures increase reaction kinetics and promote conversion to FeO and Fe. The reduction reaction is less affected by the CO concentration because the iron ore fines mass was adjusted simultaneously to maintain the same equivalence ratio. The kinetic mechanism could be explained by a nucleation and growth model, and the activation energy for iron ore fines, the mixture blended with fine coke powder at BR = 0.4, and the mixture blended with same-particle-size coke powder at BR = 0.4 were 19.09 kJ/mol, 50.61 kJ/mol, and 47.80 kJ/mol, respectively.

The addition of coke powder promoted the forward progress of iron ore fines reduction. Under reduction with a typical air gasification composition containing 30% CO, the appearance temperature of FeO decreased from 900 to 800 °C, and the appearance temperature of Fe decreased from 1100 to 1000 °C after blending coke powder. The FeO content initially increased and then decreased with increasing temperature, indicating that more FeO was converted to Fe at high temperatures. Increasing the blending ratio from 0.4 to 0.8 primarily promoted the conversion of Fe₃O₄ to FeO, whereas increasing it from 0.8 to 1.2 primarily promoted the conversion of FeO to Fe. The reaction mechanism of the coke-assisted reduction involves increasing the CO concentration and reducing the CO₂ concentration at the outlet. Because the C in the coke powder reacts with the CO₂ produced by the reduction and transforms into CO, the proportion of CO increases as CO₂ is consumed. This is beneficial for further utilization or recycling of the exhaust gas.

The blended coke powder inhibits sticking. The sticking index decreases from 800 to 1000 °C, while the sticking index increases significantly from 1000 to 1200 °C. This indicates that the sticking index is lowest at 1000 °C, regardless of whether coke powder is blended. When blending fine coke powder with BR = 1.2, the inhibiting effect on sticking is observed over the temperature range of 800–1200 °C. Blending fine coke powder is more effective at inhibiting sticking than blending it with the same particle size as iron ore fines. From a microscopic perspective, the main reason is that the coke powder provides physical isolation and reduces the collision and contact probability between a single iron ore fine and other iron ore fines. Consequently, the iron whiskers are not easily entangled or bridged.

Glossary

Abbreviations

BR

blending ratio

SI

sticking index

AM

the arithmetic mean

LM

the logarithmic mean

Glossary

Greek letters

μ

the gas viscosity (m/s)

ρ _g

the gas density (kg/m³)

ρ _p

the particle density (kg/m³)

Glossary

Parameters

A

the pre-exponential factor (atm^–1s^–1)

C _coke

the amount of carbon in the Fugu coke powder

d _p

the particle diameter (m)

E _a

the activation energy (kJ/mol)

Fe ⁰

the metallic iron content

Fe ²⁺

the ferrous content

Fe ³⁺

the ferric iron content

f _p (pCO, pCO ₂)

the gas-concentration driving force

K

the chemical equilibrium constant for the reaction of FeO and CO

k

the reaction rate constant

k _app

the apparent reaction rate constant

L

the length of the thermostatic zone (m)

M

the metallization rate

m

the reaction order related to the CO partial pressure

m _agglomerates

the masses of the reduced particles, larger than the original sample (g)

m _total

the masses of the entire sample (g)

n

the Avrami exponent

n _CO

the remaining CO (mol)

n _CO,i

the initial amount of CO (mol)

$n_{C{O}_2}$

the molar amounts of CO₂ (mol)

n _iof

the amount of iron ore fines (mol)

$n_{N_2}$

the amount of N₂ (mol)

O _IOF

the oxygen content in the iron ore fines

$pC{O}_2$

the partial pressure of CO₂ (atm)

(pCO ₂ / K) ^m

the lower gas-concentration driving force

R

the molar gas constant with a value of 8.314 J/(mol·K)

T

temperature

t

the residence time (s)

TFe

the total iron content

u _g

the centerline velocity of the gas at the furnace temperature m/s

u _t

terminal falling velocity of the particles, m/s

X

the reduction degree

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c05900.

• The Supporting Information provides supplementary explanations for the content in Sections (PDF)

Y.Z.: Writingoriginal draft, Formal analysis, Investigation, Data curation. T.W.: Writing – review and editing, Methodology, Funding acquisition. T.X.: Investigation. B.Z.: Supervision. Y.D.: Writingreview and editing. C.M.: Conceptualization.

The authors declare no competing financial interest.