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Variations in the True Density and Sulfur Removal Forms of Petroleum Coke during an Ultrahigh-Temperature Desulfurization Process Tao Liu, Mujun Long,* Wenxiang Jiang, Dengfu Chen,* Sheng Yu, Huamei Duan, Junhao Sheng, and Chunmei Chen Laboratory of Metallurgy and Materials, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China ABSTRACT: To investigate the variation of true density and the sulfur removal forms during the ultrahigh-thermal desulfurization process, two types of delayed petroleum cokes with different sulfur contents were calcined at temperatures up to 2500 °C at two distinct heating rates. The influence of the desulfurization coefficient on the true density growth was quantitatively discussed. Additionally, the removal forms of sulfur in petroleum cokes during thermal desulfurization were also investigated through a series of thermodynamic calculations. The results revealed a continuous increase in the true density with the uninterrupted release of sulfur during thermal desulfurization, and a maximum true density of 2.3 g/cm3 was obtained at a temperature of 2500 °C. The growth rate of the true density was dependent on the desulfurization coefficient of the petroleum cokes. The relationship between the true density growth rate and the desulfurization coefficient during thermal desulfurization was an exponential function, which can be presented as γTD = −117 × 10−5 exp(−104βD−S/1.45) + 9.07 × 10−5. Furthermore, the calculated results indicated that gaseous elemental sulfur (S2) was released by the pyrogenic decomposition of ferrous disulfide into inorganic sulfur. Organic sulfur was initially degraded into H2S and SO2 through thiophene decomposition, and then free COS, S2, and CS2 were released via a carbon reduction reaction.

1. INTRODUCTION As a cheap carbon source, green petroleum coke is produced from the heavy residual fractions of petroleum or crude oil using a process known as delayed coking.1−3 Calcined petroleum coke, which is the main raw material for carbon products, is produced from green petroleum coke using the calcination process.4−6 Currently, crude oil sources that contain higher sulfur contents and metal impurity levels compared with previous sources are being used frequently to produce green petroleum coke, which gives final green petroleum coke with higher sulfur content and metal impurities.7,8 Previous studies9−11 have shown that additional sulfur removal during calcination leads to an increase in microporosity, which usually has a negative influence on the true density of coke. Therefore, higher sulfur levels in petroleum cokes have a greater effect on the quality of carbon products. Additionally, the formation of H2S, COS, CS2, and SO2 during calcination and electrolysis increases air pollutant emissions.12 Hence, the desulfurization of high-sulfur green petroleum cokes has great economic and environmental significance.13 Generally, there are several common ways to desulfurize for petroleum coke, which are solvent extraction, thermal desulfurization, and hydrodesulfurization.14 Solvent extraction uses chemical solvents to reduce the sulfur content in petroleum coke. This method is unsuitable for the production of petroleum coke since it is prone to contaminate the coke. Furthermore, the method’s complexity and impact on the environment are also unacceptable.15,16 Hydrodesulfurization is a thermal treatment process for forming H2S under an atmosphere of hydrogen or steam. This method is not to contaminate the coke. But the method’s hydrogen consumption © 2017 American Chemical Society

is massive and causes large coke loss. This and the potential safety hazard result in the method not being widely applied at industrial scale.13,16 Thermal desulfurization is a high-temperature calcination treatment without any desulfurizer addition. This method seems to be a promising means of coke desulfurization with a simpler technological procedure. Additionally, previous studies17,18 indicate thermal desulfurization has an added benefit on the graphitization degree. Since the C− S bond in petroleum coke is not prone to cleavage because of the high binding energy, the most direct and effective method for petroleum coke desulfurization is performed at 1400 °C or greater.9,10,19 Hussein et al.20 investigated the thermal desulfurization of petroleum coke and obtained 80% sulfur removal efficiency at 1500 °C. Xiao et al.21 found that the sulfur content in the coke decreased with increasing temperature, and a desulfurization efficiency of 85% was achieved when the temperature reached 1600 °C. Paul et al.22 obtained approximately 91% desulfurization efficiency when the sponge coke was calcined at 1649 °C. Ibrahim et al.23 indicated that the desulfurization efficiency of petroleum coke reached 80% at 1650 °C and further increased to 88% at 1700 °C. Additionally, the results revealed that increasing the holding time also facilitated sulfur removal, but compared to extending the holding time, the advantage of increasing temperature for sulfur removal is more remarkable.23 However, note that insufficient research has been reported on the effect of ultrahigh temperature on the desulfurization degree of petroleum coke, Received: April 17, 2017 Revised: June 5, 2017 Published: June 19, 2017 7693

DOI: 10.1021/acs.energyfuels.7b01085 Energy Fuels 2017, 31, 7693−7699

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Energy & Fuels especially at desulfurization temperatures up to 2500 °C. Additionally, detailed research regarding the effect of the sulfur removal on the true density of petroleum coke has not been conducted. In the current paper, the objective of this study was to investigate the effects of the thermal desulfurization temperature and sulfur content on the desulfurization degree. Because the removal of volatile matter and sulfur from the petroleum coke is accompanied by a significant change in its microstructure, the variations in the true density with the temperature and sulfur removal were investigated. The effects of various experimental conditions (calcining temperature, heating rate) on the sulfur removal and true density variation have also been studied. Furthermore, thermodynamic calculations were performed to estimate the relationship between the Gibbs free energy and temperature of the desulfurization reactions, and possible sulfur removal forms were discussed.

Figure 1. Schematic diagram of the experiment.

2. EXPERIMENTAL PROCEDURES

after thermal desulfurization treatment with various preset temperatures were measured. Variations in impurity element contents for coke A with desulfurization temperatures and heating rates are shown in Figure 2. Figure 2(a) indicates that the sulfur was continuously released with increasing temperature, and the sulfur content in the coke dramatically decreased in the temperature range of 1500−2200 °C. Additionally, various heating rates had no obvious effect on the degree of desulfurization. Figure 2(b) reveals that nitrogen was slightly resistant to desulfurization temperatures up to 1300 °C and then rapidly decreased when the temperature surpassed 1500 °C, but all the nitrogen was basically removed from the coke matrix at 1900 °C. In Figure 2(c), the removal of iron shows a similar decreasing trend with that of sulfur removal at high desulfurization temperatures. However, the influence of thermal desulfurization temperature on silicon removal was minimal (Figure 2(d)), and the silicon content after thermal desulfurization was basically constant. Furthermore, the removal curves of the desulfurized samples at various heating rates remained basically parallel and exhibited a similar release rate. The larger heating rate exhibited slightly faster removal rates of sulfur, nitrogen, and iron in cokes compared with those at the lower heating rate; however, the difference between the two heating rates was so small that it could be ignored. Overall, an increase in the desulfurization temperature was beneficial to sulfur, nitrogen, and iron removal but had no obvious effect on silicon removal. 3.2. Effect of Thermal Desulfurization Treatment on Desulfurization Degree. The effect of desulfurization temperature on the change in sulfur content from 1100 to 2500 °C for the two types of petroleum cokes is shown in Figure 3. The two petroleum cokes (0.49 and 1.08 wt % sulfur in the raw coke) both exhibited typical sulfur removal curves with an increase in desulfurization temperature. Additionally, the desulfurization efficiencies of these two petroleum cokes continuously increased with increasing temperatures. Significant sulfur release occurred when the temperature surpassed 1500 °C. For cokes A and B, the sulfur weight losses due to sulfur release at 1900 °C were approximately 0.28 and 0.67 wt %, respectively. In the temperature range of 1900−2500 °C, the sulfur weight losses after sulfur removal were approximately 0.19 and 0.38 wt % for cokes A and B, respectively. These data indicate that thermal desulfurization mainly occurred at 1500−2200 °C for both low- and high-sulfur cokes, and the desulfurization efficiencies for these two petroleum cokes are basically the same at all desulfurization temperatures. The difference in the sulfur weight loss of the two types of petroleum cokes during thermal desulfurization is attributed to the original sulfur content in the cokes. 3.3. Effect of Thermal Desulfurization Treatment on the True Density. The influence of calcined temperature on the true density of petroleum coke is depicted in Figure 4. In the two petroleum cokes, a continuous increase in the true density was observed with an increase in treatment temperature and a sharp increase in the true density was observed in the range of 1500−2200

In this study, two types of industrial petroleum cokes from the same coking plant with different sulfur contents have been used as the raw material for desulfurization. Two types of petroleum cokes were initially calcined at 1100 °C and then cooled to room temperature before the thermal desulfurization experiments. The chemical compositions of two types of precalcined petroleum cokes are listed in Table 1.

Table 1. Chemical Compositions of the Petroleum Cokes, wt % Cokes

C

S

N

Si

Fe

Coke A Coke B

98.99 98.29

0.49 1.08

0.48 0.47

0.01 0.07

0.03 0.09

The desulfurized experiments were conducted in the GSL-3000 high-temperature furnace under normal atmospheric pressure with argon gas as a protective environment. The samples were heated from room temperature up to various target thermal desulfurization temperatures at set heating rates of 5 °C/min and 10 °C/min. Additionally, the target temperatures were 1100 °C, 1300 °C, 1500 °C, 1900 °C, 2200 °C, and 2500 °C. Next, the petroleum cokes were maintained at the target temperature for 30 min because most desulfurization occurred within 30 min of the thermal desulfurization treatment at high temperatures.3,11 Finally, the argon gas flow was interrupted when the coke samples in the furnace were cooled to room temperature in the inert atmosphere. A schematic diagram of the experiment is shown in Figure 1. The sulfur content of the calcined petroleum coke was characterized using C−S analysis equipment (which type is CS-444LS). Sulfur and carbon content analyses were performed based on infrared light absorption during combustion in the oxygen flow. The mixed gas was carried to the SO2 infrared detector to measure the sulfur content. Subsequently, the remaining gas was transported to the CO2 infrared detector to measure the carbon content. The nitrogen content in the samples was tested with TC436 O−N analysis equipment. Furthermore, the ICP-MS was used to analyze the impurity element content in the coke samples, such as iron and silicon. The true density of calcined petroleum cokes was measured in a helium gas medium that was based on the ASTM standard test method. The method for determining the true density of the calcined petroleum coke utilized helium gas as the medium to accurately measure the volume of the coke sample. The ratio of the mass to volume is referred to as the true density of the coke sample.

3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1. Removal of Impurity Elements during Thermal Desulfurization. Impurity element contents in petroleum cokes 7694

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Figure 2. Variations in impurity elements for coke A during the desulfurization process: (a) sulfur content, (b) nitrogen content, (c) iron content, and (d) silicon content. °C. Interestingly, the measurable sulfur, nitrogen, and iron contents also dramatically decreased in that temperature range. The treatment of petroleum coke samples can lead to volatile matter and sulfide release from the carbon matrix, which results in an increase in the true density. Because most volatile matter has been evaporated in the precalcined process for the cokes in this study, there was no extra volatile matter to release. Nevertheless, sulfur removal was likely the leading factor for the increase in true density because maximal sulfur removal was observed in the same temperature range. Moreover, a certain level of structural rearrangement, which was accompanied by desulfurization, also contributed to the increase in true density. Furthermore, the true density of coke A with a low sulfur content was larger than that of coke B with a high sulfur content for all treatment temperatures. The possible reason is that more sulfur removal during thermal desulfurization led to an increase in the microporosity of the coke matrix. Several studies9,10,24 indicated that the true density initially increased with temperature up to a critical temperature, which was generally 1400 °C, and then true density began to decrease when the temperature was greater than the critical temperature. The puffing phenomenon was attributed to the increase in the microporosity of the petroleum coke with sulfur release during thermal desulfurization. However, in this study, the measured results indicated that the true density continuously increased in the temperature range of 1100− 2500 °C and the detrimental effect of sulfur removal on the true density was not determined during desulfurization. The reason may be attributed to the microporosity density during thermal desulfurization, which depends on the total sulfur content in the coke. When the initial sulfur content was low, minimal microporosity was obtained during high-temperature decomposition;11,25 therefore, the sulfur in the coke was effectively removed without deterioration of the microstructure of the petroleum coke during the ultrahigh-temperature thermal desulfurization process.

Figure 3. Variation of the sulfur content and desulfurization efficiency of petroleum coke as a function of calcining temperature at a heating rate of 5 °C/min.

4. DISCUSSION 4.1. Sulfur Removal Coefficient during Thermal Desulfurization Treatment. Based on the measured sulfur removal curves for the two types of petroleum cokes during continuous thermal desulfurization, the thermal desulfurization coefficient of the coke was obtained. The coefficient is

Figure 4. Variation in the true density with calcining temperature.

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Figure 6 shows the true density growth rate in cokes A and B at a heating rate of 5 °C/min. The results suggest that the

represented by the desulfurization speed as a function of temperature in the continuous desulfurization process, and it is calculated by comparing the sulfur content in the samples before and after thermal desulfurization. The expression is described as f (T0) − f (T0 + ΔT ) (1) ΔT where βD−S is the thermal desulfurization coefficient of the coke, wt %·°C −1; ΔT is the corresponding thermal desulfurization temperature interval, °C; f(T0) and f(T0 + ΔT) are the sulfur contents in coke at thermal desulfurization temperatures T0 and T0 + ΔT, respectively, wt %. Figure 5 shows the thermal desulfurization coefficients of cokes A and B at a heating rate of 5 °C/min. The thermal βD − S = limΔT → 0

Figure 6. Growth rates of the true density at a heating rate of 5 °C/ min.

calcined temperature significantly influenced the growth rate of the true density in cokes. However, the growth rate was different in various temperature ranges and two stages of growth change were apparent. In the temperature range of 1100−1900 °C, the true density growth rates steadily increased with increasing temperature. Additionally, the true density growth rate of coke A with a low sulfur content was faster than that of coke B with a high sulfur content. These results are confirmed by the results in Figure 3, which exhibit abundant sulfur removal in coke B in the temperature range of 1100− 1900 °C. At temperatures above 1900 °C, the true density growth rates in the cokes decreased with increasing thermal desulfurization temperature, which is caused by a decrease in the thermal desulfurization coefficient and a reduction in the sulfur content. Figures 5 and 6 indicate that the true density is highly correlated with variations in the sulfur removal rate. To further analyze the relationship between the growth rate of the true density and the desulfurization coefficient in petroleum cokes during ultrahigh-temperature desulfurization, a predictive model of the true density growth rate as a function of the desulfurization coefficient was established in combination with the experimental results. The equation is shown as follows.

Figure 5. Thermal desulfurization coefficients of the cokes at a heating rate of 5 °C/min.

desulfurization coefficient and the relative change in sulfur removal are a function of temperature. In the temperature range of 1100−2500 °C, the thermal desulfurization coefficients of cokes first steadily increased with temperature up to a critical temperature and then gradually decreased when the thermal desulfurization temperature surpassed the critical temperature. The critical temperature of thermal desulfurization was approximately 1900 °C, which resulted in the maximal thermal desulfurization coefficient during thermal desulfurization. The difference in the two thermal desulfurization coefficient curves resulted from different original sulfur contents between the cokes and sulfur release during thermal desulfurization. Meanwhile, the variation in thermal desulfurization coefficients affected the microstructure and especially the true density variations in petroleum coke. 4.2. Effect of Desulfurization Coefficient on the True Density Growth. Using the same data processing method, which is based on the measured true density curves of the two petroleum cokes, the growth rate of the true density in cokes can be obtained. The expression is described as

⎛ 104 × β ⎞ D−S ⎟ −5 γTD = −117 × 10−5 × exp⎜⎜ − ⎟ + 9.07 × 10 1.45 ⎝ ⎠ (3)

where γTD is the growth rate of the true density (TD) in cokes, g·(cm−3·°C−1); βD−S is the thermal desulfurization coefficient of the cokes, wt %·°C−1. Figure 7 illustrates the variation in the true density growth rate at high temperature with the thermal desulfurization coefficient. The results demonstrate that the true density in coke significantly increased with continuous sulfur removal, which is caused by pyrogenic decomposition and structural rearrangement. Note that the growth rate of the true density first increased and then maintained a constant value with a continuous increase in the thermal desulfurization coefficient. The rapid removal of large quantities of sulfur during thermal desulfurization led to considerable microporosity formation inside the coke and at the surface, and it is responsible for the

ρ(T0 + ΔT ) − ρ(T0) (2) ΔT where γTD is the growth rate of the true density (TD) in cokes, g·(cm−3·°C−1); ΔT is the corresponding thermal desulfurization temperature interval, °C; ρ(T0 + ΔT) and ρ(T0) are the true densities in coke at thermal desulfurization temperatures T0 + ΔT and T0, respectively, g·cm−3. γTD = limΔT → 0

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Figure 8. The thermodynamic calculation results show that the ΔG values of the reaction in the selected temperature range are

Figure 7. Relationship between growth rate of true density and desulfurization coefficient. Figure 8. Relationship between ΔG and T for the decomposition reactions of FeS.

flat variation in the true density growth. Additionally, the results clearly indicate that the increase in the true density of the coke depends on both the sulfur removal coefficient and total sulfur content. 4.3. Sulfur Removal Form during Thermal Desulfurization. Sulfur can be present in petroleum coke in various forms. Generally, the forms of sulfur that exist in petroleum coke are divided into organic sulfur and inorganic sulfur. Organic sulfur represents approximately 90% of the sulfur content in petroleum coke, which is bound to the carbon matrix and present in the form of thiophene. Inorganic sulfur can also be found in petroleum coke, which exists as sulfates and pyritic sulfur, and free sulfur may occasionally be present. Thiophene is typically difficult to remove because of its strong hightemperature thermal stability. Even when heated to 1300 °C, minimal sulfur loss occurs since thiophenes are stably attached to the aromatic carbon skeleton.3,13,14 Therefore, the desulfurization of petroleum coke involves the desorption of inorganic sulfur that is present in the coke pores or on the coke surface and the partition of organic sulfur that is attached to the aromatic carbon skeleton. To provide a better understanding of the desulfurization reactions for petroleum coke during thermal desulfurization, a thermodynamic calculation of the desulfurization equilibrium was performed with FactSage 6.3 software (CRCT-ThermFact, Inc., Montreal, Canada) that is based on the detailed chemical composition of selected cokes (given in Table 1) and all the possible generated phases in the databases. First, the pyritic sulfur in petroleum coke was presented as ferrous sulfide (FeS) or ferrous disulfide (FeS2). Hence, the changes of Gibbs free energy (ΔG) of the pyrogenic decomposition reactions for FeS and FeS2 in the temperature range of 1000−3000 °C were calculated using FactSage software, and the decomposition reactions of FeS and FeS2 are shown as follows. 1 FeS(s) = Fe(s) + Sn(g) (4) n FeS2(s) = Fe(s) +

2 Sn(g) n

all positive; therefore, it is infeasible that the ferrous sulfide pyrogenic decomposition reaction occurred under normal atmospheric pressure during the thermal desulfurization, which is because the available energy was insufficiently high for the decomposition. However, ferrous sulfide is apt to decompose under vacuum and high-temperature conditions, and a reduction in system pressure benefits the ferrous sulfide decomposition reaction. Figure 9 depicts the thermodynamic

Figure 9. Relationship between ΔG and T for the decomposition reactions of FeS2.

calculation results of the ferrous disulfide decomposition in the calcined temperature range. When the change of Gibbs free energy ΔG equals zero, the decomposition reaction occurred. In Figure 9, the ΔG of the possible reactions in the selected temperature range has a negative value, which means that the decomposition reactions are feasibly based on the thermodynamics. However, the results suggest that ferrous disulfide is likely to decompose into S2 (gas) and Fe by comparison because it begins to decompose at 1300 °C and fits well with the measured results, in which the sulfur begins to release (Figure 2). Therefore, the calculations demonstrate that gaseous sulfur is released during the high-temperature desulfurization process. Similarly, the calculations also suggest

(5)

where Sn is the existing form of gaseous sulfurs, and n is a value from 1 to 8 in one interval. The relationship between the ΔG of the FeS pyrogenic decomposition and temperature T was obtained, as shown in 7697

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occur when the thermal desulfurization temperature was above 1700 °C. The calculated results can be used to clearly describe the decomposition of the refractory thiophenes during the desulfurization process. First, the sulfur that was attached to the carbon skeleton was initially degraded into H2S and SO2. Then, free H2S and SO2 was released from the carbon matrix and bonded with carbon atoms to possibly form COS, S2, and CS2. Free H2S and SO2 may be released when the decomposition reactions occurred in the coke pores or on the coke surface. Note that the values of ΔG of reactions 8 and 11 are both negative and thermodynamically feasible. SO2 was converted to S2 through reaction 8 in the temperature range of 1000−2500 °C, and then S2 combined rapidly with carbon atoms to form CS2 in the high-temperature process. These data suggest that minimal S2 was released from petroleum coke when the decomposition reactions occurred on the coke surface, whereas most S2 that was formed within the coke matrix was converted to CS2 and released. Furthermore, H2S was converted to COS through reaction 10 in the temperature range of 1000−2500 °C.

that ferrous sulfide does not exist in experimental petroleum cokes. Additionally, petroleum coke desulfurization is mainly focused on the decomposition of thiophene during the thermal treatment process. The phenomenon of thiophene decomposition was observed during thermal desulfurization when the temperature surpassed 1400 °C; however, the thermodynamic data for the thermal desulfurization of thiophene is lacking.3,26 Therefore, the previous research results regarding coke were used to qualitatively evaluate the decomposition reaction of thiophene. Thus, it is speculated that the thermal decomposition reactions for thiophene are probably caused by the following reactions.26−30 Thiophene sulfur → H 2S(g)

(6)

Thiophene sulfur → SO2 (g)

(7)

nSO2 (g) + nC(s) = Sn(g) + nCO2 (g)

(n = 2, 4, 6, 8) (8)

H 2S(g) + CO2 (g) = COS(g) + H 2O(g)

(9)

5. SUMMARY (1) With an increase in the desulfurization temperature, the contents of impurity elements, such as sulfur, iron, and nitrogen, in petroleum cokes gradually decreased. However, the influence of the thermal desulfurization temperature on silicon removal was minimal, and the heating rates had a small effect on impurity element removal. (2) The true density during thermal desulfurization was influenced by sulfur removal, and the highest growth rate was obtained at a temperature of 1900 °C. (3) The dependence of the true density growth rate on the sulfur removal coefficient was clearly indicated by the derivation analysis of the curves, and a predictive model of the true density growth as a function of the desulfurization coefficient was established, which is shown as follows.

H 2S(g) + C + 2CO2 (g) = COS(g) + 2CO(g) + H 2O(g) SO2 (g) + 2CO(g) =

1 Sn(g) + 2CO2 (g) n

(n = 2, 4, 6, 8) Sn(g) +

(10)

n n C(s) = CS2 (g) 2 2

(11)

(n = 2, 4, 6, 8)

(12)

Reactions 6 and 7 cannot be evaluated because the exact component of thiophene is unknown; therefore, these reactions were selected to present the conversion of organic sulfur to H2S and SO2 based on the experimental results of previous references. The ΔG values for reactions 8−12 in the temperature range of 1000−2500 °C were calculated using FactSage software, the value of n in reactions 8−12 was set as 2, and the calculated results are shown in Figure 10. In Figure 10, except for reactions 10 and 11, the ΔG values for the other reactions are all negative in the temperature range of 1000−2500 °C. Therefore, reactions 8, 9, and 12 are thermodynamically feasible. Furthermore, reaction 11 did not

⎛ 104 × β ⎞ D−S ⎟ −5 γTD = −117 × 10−5 × exp⎜⎜ − ⎟ + 9.07 × 10 1.45 ⎝ ⎠

(4) The thermodynamic calculations revealed that ferrous disulfide exists in experimental petroleum cokes, and individually gaseous sulfur (S2) is released via the pyrogenic decomposition reaction. (5) Organic sulfur was initially degraded into H2S and SO2 by thiophene decomposition. Next, SO2 was converted to S2 via a carbon reduction reaction, and then most S2 rapidly converted to CS2 under the high-temperature process. COS was formed during the entire desulfurization process because of H2S conversion.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-023-65102467. E-mail: [email protected]. *Phone: +86-023-65102467. E-mail: [email protected]. ORCID

Mujun Long: 0000-0003-1186-0914 Notes

Figure 10. ΔG of reactions 8−10 in the temperature range of 1000− 2500 °C.

The authors declare no competing financial interest. 7698

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (NSFC, project no. 51374260, 51504048, and 51611130062). The authors thank the members of Laboratory of Metallurgy and Materials, Chongqing University, for the support of this work.

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