Carbon deposition on iron surfaces in CO

Abstract

Thermodynamic calculations and thermogravimetric (TG) analysis were performed in order to understand the mechanism of carbon deposition on the surfaces of iron particles during the reduction of iron ore in a CO–CO₂ atmosphere. The results of the thermodynamic equilibrium phase analysis indicate that the phases of the carbon deposition process can be predicted on the basis of the carbon potential, reaction temperature and gas pressure. The optimal thermodynamic conditions for carburisation are a low temperature (T < T_m) and a high carbon potential (α_c>1). TG analysis is performed in a gas mixture of 65 vol.-% CO and 35 vol.-% CO₂ at 650, 706 and 750°C. Cementite (Fe₃C) is generated as an intermediate product, which acts as a catalyst for carbon deposition. Carbon deposition is inhibited at high temperatures (T>791°C) owing to the high stability of Fe₃C. When the reaction temperature is higher than the thermodynamic limit for the formation of Fe₃C, carbon deposition cannot occur. A mechanism for carbon deposition is proposed based on the experimental results.

Introduction

Gas based direct reduction of iron ore is a promising route to decrease the carbon dioxide emissions in the iron and steel industry. The results of theoretical calculation and production practice prove that carbon dioxide emission of a shaft furnace is far lower than that of the blast furnace.^1–2 In the shaft furnace reduction process, CO decomposes as (2CO = C + CO₂) on the surfaces of reduced iron at temperatures of 400–900°C and at high carbon activities (α_c>1).^3–6 The deposited carbon can fill in the pores and voids of the iron particles, generating internal stress and decreasing their strength. As a result, the iron particles are broken down and the furnace gas channels can become blocked.

Numerous efforts have been made to understand the mechanism of carbon deposition on the metallic iron particle surface.^7–15 Snoeck et al. ⁷ proposed that the driving force of carbon diffusion is the difference of carbon solubility between the gas/metal interface and the metal/carbon interface. Zhang et al. ^8,9 studied this process with iron in a CO–H₂–H₂O gas mixture at 700°C and found that when a gas mixture with low (e.g. 5%) CO content is used, both α-Fe and Fe₃C are detected. When the CO content increases to more than 30%, Fe₃C is the only iron containing phase observed. Turkdogan and Olsson¹⁰ and Fruehan¹¹ claimed that metallic iron is the active phase during CO decomposition. However, some researchers have suggested that Fe₃C acts as the catalyst for the dissociation of CO.^12–14 Therefore, whether Fe or Fe₃C serves as the catalyst for carbon deposition remains to be determined.

In the present study, the effects of carbon potential and reaction temperature on carbon deposition were investigated both thermodynamically and experimentally. The equilibriums at the metal/gas and metal/carbon interfaces were taken into account. Based on this, a mechanism of carbon deposition on iron surfaces is proposed.

Thermodynamic analysis

In a CO/CO₂ gas mixture, the main carburisation reactions are expressed as reactions (1) and (2): (1) (2)

Using graphite as the standard state ( = 1), and can be defined as equations (3) and (4): (3) where is the carbon potential of the gas phase, K _CO is the equilibrium constant for reaction (1), P _CO and are the partial pressures of CO and CO₂ respectively, and and are the mole fractions of CO and CO₂ respectively. (4) where is the carbon potential of the Fe₃C phase, is the equilibrium constant for reaction (2), and and are the activity of Fe₃C and Fe respectively.

The thermodynamic calculations were performed using the software programme HSC Chemistry 6.0. The carbon potential in the CO/CO₂ gas mixture was determined as a function of the gas composition, reaction temperature and total pressure, as shown in Fig. 1. The dotted lines represent the gas mixture with different CO/CO₂ ratio and total pressure. Defining T ₀ as the temperature where , and T _m as the temperature where , then T ₀ and T _m represent the thermodynamic limit temperatures for the formation of Fe₃C and graphite respectively. The curve shifts to the right for high CO concentrations or high pressure, indicating an increase in the temperature range for carbon deposition. The curves for the potentials of Fe₃C and graphite intersect at 791°C, implying that Fe₃C coexists with graphite at this temperature. Fe₃C is stable ( ) in the high temperature region (>791°C), while graphite is stable in the low temperature region ( < 791°C). If the carbon potential of the gas mixture is less than unity, carbon deposition cannot occur. Therefore, thermodynamically speaking, carbon deposition is most likely to occur at high CO concentrations, low temperatures and high total pressures.

Dependence of carbon potential on gas composition, temperature and total pressure

As shown in Fig. 2, the T ₀ and T _m values of the gas mixture (65 vol.-% CO and 35 vol.-% CO₂) are 698 and 709°C respectively. The equilibrium temperature for Fe/FeO in the gas mixture is 831°C; the hatched areas represent the stable temperature regions for Fe and FeO. Three zones (A, B and C) can be differentiated on the basis of the T ₀ and T _m values. Various combinations of the carbon potential and temperature can be predicted thermodynamically, as shown in Table 1. In zone A, the carbon potential of the gas phase is higher than the thermodynamic limit for the formation of Fe₃C. The gas phase provides adequate carbon for iron to form Fe₃C. Consequently, the stable product of iron is Fe₃C, and the excessive carbon is present in the form of graphite. In zone B, the carbon potential of the gas phase is lower than the thermodynamic limit for the formation of Fe₃C. The iron phase is saturated with the dissolved carbon. In zone C, the carbon potential of the gas phase is less than unity, and the carbon dissolves in iron, forming an Fe–C solid solution.

Different temperature zones for carburisation in 65 vol.-% CO and 35 vol.-% CO₂ gas mixture

Table 1

Thermodynamic prediction of carbon deposition based on Fig. 2

Zone	T	Thermodynamic stable phases
A	T < T ₀ < T _m	Fe₃C, graphite
B	T ₀ < T < T _m	[C]_sat, graphite
C	T>T _m>T ₀	[C]

Experimental

Instruments

A thermogravimetric (TG) analyser (Cahn TherMax 700, Thermo Cahn Inc.) was employed as a flow microreactor for the in situ monitoring of weight changes in the test samples. The test sample was placed in an alumina crucible, which was hung in the TG analyser with silica hooks. The gas flowrate was fixed at 100 mL min^− 1; the gas was purged into the TG analyser from the bottom of the furnace.

Materials

The composition of the iron ore sample used in the experiments is given in Table 2. After the milling and screening of the ore, a fine powder with particles smaller than the mesh size of a #80 mesh sieve was obtained and used. The weight of the sample was ∼150 mg. This amount of sample can cover the bottom of the crucible with a thin layer during TG experiments, thus ensuring good mass and heat transfers of gas–solid reactions in the experiments.

Table 2

Composition of iron ore powder used in this experiment/wt-%

TFe	FeO	CaO	MgO	Al₂O₃	S	P	SiO₂	MnO
62.57	0.56	2.65	0.78	1.25	0.02	0.02	5.73	0.21

Pre-reduction

During the CO reduction process, carburisation may occur simultaneously with reduction.¹⁶ Therefore, in order to clarify the mechanism of carbon deposition, the iron ore powder was pre-reduced in pure hydrogen at 800°C for 30 min. The reduced iron was cooled to room temperature and purged with argon gas for 1 h to remove the hydrogen gas adsorbed on the surfaces of the ore particles. Then, the completely reduced iron was used in the carburisation experiments.

Carbon deposition

The reduced iron was heated to the target temperature in argon gas at a rate of 10°C min^− 1. Then, the gas mixture (65 vol.-% CO and 35 vol.-% CO₂) was introduced into the reaction area. Based on the results of the thermodynamic analysis, the temperatures were chosen to be 650, 706 and 750°C. After the sample was held at the selected temperatures for 1 h, the fluidising gas was switched to argon, and the furnace was cooled to room temperature.

Characterisation

The crystal phases of the samples were determined by X-ray diffraction (XRD) analysis, performed on a Rigaku D/Max-2550 diffractometer using Cu K _α (λ = 0.15418 nm) radiation operated at a voltage of 40 kV and a current of 40 mA. The morphologies of the samples were examined by scanning electron microscopy (SEM) performed on a HITACHI SU-1510 microscope and by transmission electron microscopy (TEM) on a JEM-2010F microscope.

Results and discussion

Carbon deposition in CO/CO₂ gas mixture

The weight changes during carburisation at different temperatures are shown in Fig. 3.The results show that carbon deposition occurs at 650°C, with the weight gain being 7.85%. The weight gain is higher than the weight change corresponding to the transformation of Fe into Fe₃C (7.14%), which means that the deposited carbon coexists in the obtained product. The result is further confirmed by the XRD analysis, as shown in Fig. 4. Diffraction peaks related to Fe₃C and graphite are detected when the reaction temperature is 650°C. When the carburisation temperatures are 706 and 750°C, no obvious weight changes are observed from the TG curves. Correspondingly, peaks related to only α-Fe are detected in the XRD pattern.

TG curves of carburisation processes at different temperatures with gas mixture of 65 vol.-% CO and 35 vol.-% CO₂

XRD patterns of pre-reduced iron and carburisation products obtained at different temperatures

The iron particles obtained at 706 and 750°C are well shaped with distinct boundaries, as shown in Fig. 5. In contrast, the sizes of the Fe₃C particles generated at 650°C are smaller. The breaking process can be observed at the outer layer of the iron particles (Fig. 5b ). Furthermore, nanometre sized Fe₃C particles occur on the particle surfaces, and some of the Fe₃C particles are wrapped in the fluffy graphite, as shown in Fig. 6.

SEM images of carburisation products obtained at different temperatures: a 650°C; b high magnification image of area selected in (a); c 706°C; d 750°C

TEM images of carburisation products obtained at different temperatures: a 650°C (inset shows diffraction patterns of marked Fe₃C particles); b 706°C; c 750°C

Effects of carbon potential on carbon deposition

The driving force for carbon diffusion is the differences in the carbon solubility at the gas/metal and metal/carbon interfaces. The solubility of carbon in iron changes with the carbon potential of the gas mixture used and the reaction temperature. The Fe–C phase diagram (Fig. 7) shows the phases corresponding to the different combinations of the carbon content and reaction temperature. The solid lines represent equilibrium with graphite, while the dotted lines represent equilibrium with Fe₃C. At 727°C, the highest solubilities of carbon in ferrite (α-Fe, which is body centred cubic) and austenite (γ-Fe, which is face centred cubic) are 0.022% and 0.77% respectively. At 750°C, the carbon potential increases with the increase of the solubility of carbon in the α-Fe phase. In the α+γ zone, α-Fe and γ-Fe are in equilibrium, with α_A1 = α_B1 = 0.80 [calculated using Pandat 7.0 (demo version)]. At the intersection of the boundary line (point C), the carbon potential of γ-Fe is in equilibrium with graphite (α_C1 = 1). In the γ-Fe + C/Fe₃C zone, the carbon potential is equal to unity.

Schematic diagram showing carbon potential change of iron at 750°C

The phases of the carbon deposition process can be predicted under the equilibrium conditions on the basis of the carbon potential of the gas mixture and the reaction temperature. The carbon potential of the gas mixture (65 vol.-% CO and 35 vol.-% CO₂) and Fe₃C can be calculated using equations (3) and (4) respectively. The carbon potential of gas ( ) and Fe₃C ( ) at 650, 706 and 750°C are calculated to be = 3.79, 1.06 and 0.43 and = 1.45, 1.21 and 1.09 respectively. As shown in Fig. 3, the weight change is clearly evident during the TG experiment at 650°C, while the weights of the samples are relatively constant at temperatures of 706 and 750°C. These results are consistent with the thermodynamic analysis.

Mechanism of carbon deposition on iron surfaces

The results of the thermodynamic calculations and the TG analyses show that the phases of the carbon deposition in the CO/CO₂ gas mixture change with the carbon potential of the gas mixture and the reaction temperature. The carbon deposition process on an iron surface includes the following steps: (i)

Adsorption of gas on the iron surfaces.

In a CO atmosphere, the species strongly adsorbed on the iron surfaces are O, CO and C. Fruehan¹¹ verified that the rate controlling reaction in the CO–He gas mixture is the formation of the activated complex. The gas adsorption process can be expressed by reactions (5)–(8), where □ is the vacant site where adsorption takes place. (5)

(6)

(7)

(8)

The separation between Fe–Fe atoms (0.287 nm) is greater than that between the C–O atoms (0.116 nm) in CO.^17,18 When CO is adsorbed on the surfaces of iron, the strength of the C–O bond decreases, resulting in the formation of an Fe·CO (ads) complex. When the Fe·CO (ads) complex is struck by another CO molecule, the oxygen atom in the complex moves to the CO molecule, which is then converted into a CO₂ molecule. The residual adsorbed carbon dissolves in iron, forming a solid solution of iron and carbon.

(ii)

Carbon diffusion in iron

Thermodynamically, the driving force for diffusion is the difference in the chemical potentials of the different phases or different positions. In a metal matrix, the carbon potential decreases with an increase in the distance to the reaction interface, as shown in Fig. 8. Baker¹⁹ measured the carbon deposition rates for different metals such as Ni, α-Fe, Co, V, Mo and Cr and suggested that the critical parameters involved in the carburisation process are the solubility and the diffusion rate of carbon in metals.

(iii)

Chemical reaction at the metal/carbon interface

Schematic diagram showing change in carbon potential on iron surface

The diffusion rate of carbon in Fe₃C is low,^19–22 which in turn decreases the carbon potential at the metal/carbon interface. Fe₃C is thermodynamically unstable and can be decomposed through reaction (2) when the carbon potential is reduced to unity. The graphite obtained through the decomposition of Fe₃C deposits on the surfaces of the iron particles. Therefore, the iron atoms diffuse through the graphite to the outer surface due to the concentration gradient and form small clusters. The decomposed iron then acts as a catalyst for further carbon deposition. Fe₃C is thus formed as an intermediate product and accelerates the decomposition of CO on the iron surfaces.

(iv)

Desorption of gas products from the iron surfaces

(9)

The iron particles are broken during the carbon deposition process, as shown in Fig. 5b . The process is described schematically in Fig. 9. The surfaces of the reduced iron particles are porous, and the micropores and cracks formed on the surfaces can act as channels for gas diffusion. The reactant gas diffuses into the iron particles from the cracks and pores on the surfaces, where carbon can be deposited preferentially. When the carbon potential is high enough, Fe₃C is produced. With the continuation of the recycling of Fe₃C, carbon deposition occurs steadily. The continuous deposition of carbon enlarges the cracks, which eventually leads to the breakage of the iron particles.

SEM images of a surface of porous iron reduced at 800°C in H₂; b iron clusters on surface; c broken surface; d schematic showing breaking of iron particles during carbon deposition

Conclusions

Based on the carbon potential equilibrium phase diagram, the carbon deposition process can be predicted from the carbon potential of the gas mixture, the reaction temperature and the total pressure. The TG analysis demonstrates that carbon deposition occurs preferentially at a low temperature (T < T _m) and a high carbon potential (α_c>1). When temperature is higher than 791°C, carbon deposition cannot occur. Fe₃C is formed as intermediate product during carbon deposition and accelerates the rate of carbon precipitation. The recycling reaction of Fe₃C produces a large amount of graphite on the surfaces of the iron particles. The mechanism underlying the breaking of the iron particles during the carbon deposition process is proposed and discussed.

Footnotes

Acknowledgements

The authors thank the National Key Basic Research Program of China (973) (grant no. 2014CB643403) and the National Natural Science Foundation of China (grant nos. 51304132 and 51225401) for financial support.

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Carbon deposition on iron surfaces in CO–CO 2 atmosphere

Abstract

Introduction

Thermodynamic analysis

Thermodynamic prediction of carbon deposition based on Fig. 2

Experimental

Instruments

Materials

Composition of iron ore powder used in this experiment/wt-%

Pre-reduction

Carbon deposition

Characterisation

Results and discussion

Carbon deposition in CO/CO2 gas mixture

Effects of carbon potential on carbon deposition

Mechanism of carbon deposition on iron surfaces

Conclusions

Footnotes

Acknowledgements

References

Carbon deposition in CO/CO₂ gas mixture