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. 2020 Oct 30;5(44):28541–28546. doi: 10.1021/acsomega.0c03183

Effect of Calcined Coke Size on Pitch Pyrolysis Behavior

Nianbing Zhang †,‡,*, Yahong Yu †,‡, Zhiying Li §, Zhen Yao †,‡, Wei Liu †,‡
PMCID: PMC7658939  PMID: 33195904

Abstract

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Pitch coking determines the quality of pitch coke, which ultimately affects the quality of a carbon anode. In this work, green carbon anodes were studied by scanning electron microscopy (SEM), and the pitch pyrolysis process was tested using a custom-built pyrolysis device. The influence of the coke size on pitch pyrolysis was examined and the action law was analyzed. The results show that the outermost layer of the large size coke has a certain pitch thickness, the subouter layer is filled with a “mixture of fine particles and the pitch,” and the internal area is not soaked by the pitch. Meanwhile, the small particles are soaked and wrapped by the pitch. The pyrolysis dynamics analysis shows that with the increase in particle size, the activation energy gradually increases to 70.00 kJ/mol for 1–2 mm, then rapidly decreases to 31.88 kJ/mol for 3–4 mm, and finally slowly increases to 50.56 kJ/mol for 6–9 mm. When the particle size increases, the coke size <0.5 mm area is dominated by a specific surface area, the 0.5–2 mm area is mainly regulated by a combination of a specific surface area and a porous structure, and the >2–3 mm area is dominated by the porous structure.

1. Introduction

The carbon anodes used in aluminum production are manufactured by carbonizing the petroleum coke blended with a small proportion of coal tar binder pitch.1,2 As a carbon anode binder, pitch preheating melts the coal pitch and soaks the coke particles, thus providing the anode paste with good plasticity to ensure green carbon anode formation. Meanwhile, the high temperature during roasting allows pitch coking, fills the pores, and closely combines these materials with the coke. Thus, the carbon anode gains a certain mechanical strength to prevent breakage during electrolysis. However, the selective oxidation of the pitch coke during electrolysis causes the coking particles to fall off. Hence, the quality of the pitch coke is important in improving the carbon anode performance, and the process determines the quality of the pitch coke.3−5 The factors affecting the properties of the pitch coke are pitch quality,6−9 trace elements,10,11 additives,12−16 sulfur content in coke,17−19 heating process,20−23 and coke particles.24 To date, the effects of coke particles on pitch pyrolysis have never been reported. In this work, scanning electron microscopy (SEM) analysis was conducted for green carbon anodes with different coke sizes soaked by pitch. The pitch pyrolysis of different coke sizes after pitch immersion and green carbon anodes were tested using a custom-built pyrolysis device. The influence of coke size on pitch pyrolysis was examined and the action law was analyzed. These results can serve as a basis for carbon anode preparation.

2. Materials and Methods

2.1. Materials

The chemical composition of the coal tar pitch obtained from the carbon factory of Chalco Guizhou Branch is shown in Table 1. The coke was also obtained from the same factory, and its composition is shown in Table 2. Coke particles of different sizes were obtained through screening.

Table 1. Proximate Analysis of the Coal Tar Pitch.

softening point (°C) quinoline insoluble (%) toluene insoluble (%) β-resin insoluble (%) coking value (%) ash content (%)
112 8.8 28.4 19.6 57.8 57.8
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Table 2. Chemical Composition of Coke Used for Carbon Anodes.

%S Na (mg/kg) Ca (mg/kg) Mg (mg/kg) V (mg/kg)
2.98 370 570 100 480
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2.2. Methods

2.2.1. Soaking the Coke Particles with Pitch

Coke particles of different sizes were sieved and selected. Fifty grams of coke was placed on the screen in a blow-air drying box for heating at 170 °C for 5 min. The heated coke particles were placed in a powdered pitch to ensure that the surface of the coke particles had adhered to and wrapped a certain amount of pitch. The excess powdered pitch was sieved. The coke particles were placed in a blow-air drying box again for heating at 170 °C for 5 min to melt the powdered pitch and immersed in the coke particles. The excess pitch melted and dripped through the screen into the lower disk. This process was repeated three times to fully soak the coke particles with the pitch and ensured that no excess pitch wrap was formed. The coke particles were weighed; the added mass is the saturated pitch mass (SPM), as shown in Table 4.

Table 4. Linear Fit of Different Coke Sizes by Pitch Infiltrated, Activation Energy, and CV.
sample a b R2 N Ea/(kJ mol) CV/(%) SPM/(g) PPM/(g)
<0.15 mm + pitch –9.291 –5902.96 0.96 0 49.08 56.21 14.25 8.01
0.15–0.25 mm + pitch –9.435 –5779.58 0.93 0 48.05 56.88 14.1 8.02
0.25–0.5 mm + pitch –9.242 –5894.77 0.96 0 49.01 57.99 17.00 9.86
0.5–1.0 mm + pitch –7.809 –7247.58 0.96 0 60.26 57.94 12.30 7.13
1.0–2.0 mm + pitch –6.490 –8419.74 0.97 0 70.00 53.72 12.10 6.50
2.0–3.0 mm + pitch –9.314 –5372.84 0.99 0 44.67 58.45 4.6 2.69
3.0–4.0 mm + pitch –11.095 –3835.01 0.99 0 31.88 54.23 5.55 3.01
4.0–5.0 mm + pitch –10.449 –4518.90 0.99 0 37.57 52.41 5.40 2.83
5.0–6.0 mm + pitch –9.961 –4767.50 0.99 0 39.64 52.41 5.80 3.04
6.0–9.0 mm + pitch –8.790 –6081.66 0.99 0 50.56 54.07 8.60 4.65
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2.2.2. Construction of the Pitch Pyrolysis Device

Traditional thermal analysis can only be conducted on milligram samples. The quality of experimental samples must be greatly improved for gram samples for studying the pitch pyrolysis process of different coke sizes and green carbon anodes. Hence, a pitch pyrolysis device was constructed and is shown in Figure 1. The test sample was placed in a corundum crucible and suspended on the hook of the measurement mass at the bottom of the electronic balance. Mass data were transmitted to the “mass collect system” in real time via an electronic balance using the software. The temperature of the test sample was measured by a thermocouple, transferred to the “temperature control system”, and collected by the temperature-controlled software from the heating equipment. Temperature and quality data were synchronized for collection to the “pyrolysis control system”. Pitch pyrolysis evaporation was also conducted under a certain negative pressure environment to simulate the roasting of the green carbon anode. The sample mass was adjusted by selecting different volumes of the crucible. In this work, a 200 mL crucible was used to test the maximum 50 g of the pitch specimen.

Figure 1.

Figure 1

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Schematic drawing of the pitch pyrolysis process.

2.2.3. Pyrolysis Analysis of the Pitch

The pyrolysis reactions of the pitch are highly complex. For a simplified modeling, the overall reaction was decomposed into volatiles and residual solid. The reaction rate can be represented as13,23

2.2.3. 1

where α is the rate of weight loss, t is the reaction time, n is the reaction order, and K is the reaction rate constant. The rate constant can be expressed in the Arrhenius form

2.2.3. 2

where A is the pre-exponential factor, E is the activation energy, R is the universal gas constant, and T is the absolute temperature of the sample.

Under a constant heating rate (ϕ = dT/dt), the substitution of eq 2 into 1 gives the following expression for the rate of the reaction

2.2.3. 3

Equation 3 can be written as

2.2.3. 4

Equation 4 points can be obtained

2.2.3. 5

When the reaction order is not 1, Formula 5 can be approximated as

2.2.3. 6

The logarithmic treatment can be obtained as

2.2.3. 7

Formula 7 can also be represented as

2.2.3. 8

where a is ln [AR(1 – 2RT/E)/ϕE] and b is −E/R.

2.2.4. SEM Imaging of the Green Carbon Anode

Calcined coke of different particle sizes and asphalt were mixed, pressed into a mold to prepare a green carbon anode, and ground for SEM analysis using JSM-6490LV SEM. The sample was not coated.

2.2.5. Specific Surface Areas of Different Coke Particle Sizes

The selected samples with different coke particle sizes in Section 2.1 were subjected to surface studies using the surface area and porosity analyzer Tristar 3020. Experimental adsorption and desorption isotherms were measured at −196 °C using analytical gas nitrogen 5 N. Specific surface areas (S-BET, m2/g) of the investigated samples were evaluated by applying the standard multipoint Brunauer–Emmett–Teller (BET) model to the adsorption data. Coke particles >3 mm cannot be measured due to equipment limitations, and the results are shown in Figure 6.

Figure 6.

Figure 6

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Effect of different coke sizes on BET, SPM, pyrolysis pitch mass (PPM), coke value (CV), and Ea.

3. Results and Discussion

3.1. SEM Imaging of the Green Carbon Anode

The carbon anode has good uniformity for easy formation. During its production, different sizes of coke and pitch are required to mix evenly to obtain a paste with good plasticity. SEM analysis for the green carbon anode was conducted to identify the state in which the different coke sizes are soaked by the pitch (Figure 2). As shown in Figure 2a, area “1” displays small particles covered by the pitch, and its magnification figure is shown in Figure 2d. As shown in Figure 2a, region “3” exhibits the shape of large particles soaked by the pitch, and the outermost layer of large particles has a certain pitch layer thickness (Figure 2a in region 2). The subouter shape of large particles can be seen in Figure 2a in area “4” amplification. Area “5” in Figure 2b,c shows that the large particles of the pores are filled by “the mixture of fine particles and the pitch.” Irregular closed holes are found inside the large particle coke, as shown in area “6” in Figure 2c. This finding indicates that pitch fails to penetrate during immersion, and this part of the pores is retained in the carbon anode product.

Figure 2.

Figure 2

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Pitch infiltration SEM images of different coke sizes in the green carbon anode: (a) pitch infiltration, (b) local amplification of the four regions in (a), (c) local amplification of the four regions in (a), (d) local amplification of the one regions in (a), and (e) pitch-soaked schematics of large particles.

After different particles are charred by the pitch, the distribution of the pitch in the green carbon anode has the following characteristics: for the large particles, the outermost layer has a certain thickness of the pitch layer, the subouter layer is filled by the mixture of fine particles and pitch, and the internal area is not soaked by the pitch (Figure 2e). Meanwhile, the small particles are soaked and wrapped by pitch, as shown in Figure 2d.

3.2. Thermal Dynamics Analysis of the Pitch

Under the heating rate of 10 °C/min, 10 g of the pitch was placed into the crucible and heated to 1050 °C (Figure 1). Pitch quality was measured with the change of temperature during heating, and the test results are shown in Figure 3. The pyrolysis temperature of the pitch is between 500 and 700 °C, and the temperature interval was calculated using formula 8. The best linear relationship was obtained with assumed values of different reaction orders. The fitting linear correlation coefficient (R2) of the pitch is 0.99, indicating that the pyrolysis model is in good agreement with the actual process. On the basis of pyrolysis data for the best linear relationship, the apparent activation energy can be calculated using the line slope of the b-value, as shown in Table 3. According to the actual carbon anode preparation conditions, the coke value (CV) at 1050 °C can be directly read using the test results, as shown in Table 3. Pitch pyrolysis results show that the apparent activation energy is 82.31 kJ/mol and CV is 58.70%.

Figure 3.

Figure 3

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TG and ln {[1–(1 – α)1–n]/(1 – n)T2} – 1/T curves of pitch pyrolysis.

Table 3. Linear Fit, Activation Energy, and CV of Pitch Pyrolysis.

sample A B R2 n Ea/(kJ mol) CV/(%)
pitch/10 g –4.969 –9899.99 0.99 0 82.31 58.70
pitch in the green anode –0.955 –14 000.77 0.99 0 116.40 56.20
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3.3. Effect of Different Coke Sizes on the Pitch Pyrolysis Process

The coke particles soaked in pitch are shown in Section 2.2. Under the heating rate of 10 °C/min, 50 g of coke with different sizes after immersion in pitch were placed in the crucible and heated at 1050 °C. Pitch quality after immersion in the coke particle was measured with the change of temperature during heating, and the results are shown in Figure 4. The pyrolysis temperature of pitch is also between 500 and 700 °C, and the temperature interval was also calculated using formula 8.

Figure 4.

Figure 4

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Thermogravimetric (TG) curves of different coke sizes after pitch soaking under 10 °C/min.

On the basis of the pyrolysis data of the best linear relationship, the apparent activation energy can be calculated by the line slope of the b-value, as shown in Table 4. According to the actual carbon anode preparation conditions, the CV of different coke sizes at 1050 °C can be directly read through the test results, as shown in Table 4. The pyrolysis pitch mass (PPM) shown in Table 4 can be calculated by the difference between the mass after pyrolysis and the initial coke mass.

3.4. Pitch Pyrolysis of the Green Carbon Anodes

The method of determining the pitch pyrolysis of green carbon anodes is discussed in Section 2.2. The experimental green anode mass was 344.6 g, the heating rate was 10 °C/min, and the temperature was up to 1050 °C. Changes in pitch pyrolysis quality were measured, and the results are shown in Figure 5. The quality of the carbon anode after pyrolysis was 322.2 g, the pyrolysis temperature of pitch was between 550 and 750 °C, and the temperature interval was also calculated using formula 8. On the basis of the pyrolysis data of the best linear relationship, the apparent activation energy can be calculated by the line slope of the b-value, and the CV at 1050 °C can be directly read through the test results. The pitch pyrolysis in the green carbon anode results shows that the apparent activation energy is 116.40 kJ/mol and the CV is 56.20% (Table 3).

Figure 5.

Figure 5

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TG and ln {[1–(1 – α)1–n]/(1 – n)T2} – 1/T curves of pitch pyrolysis in the green carbon anode.

3.5. Effect of Different Coke Sizes on Pitch Pyrolysis

BET was applied to determine the specific surface areas of different coke sizes, and the results are shown in Figure 6. When the particle size increases, the specific surface area gradually decreases from 3.3521 m2/g (less than 0.15 mm) to 1.4775 m2/g (0.25–0.5 mm) and continuously decreases to 1.3897 m2/g (2–3 mm).

As stated in Section 2.2.1, different grain sizes were soaked with pitch, and SPM was obtained. The results are shown in Table 4 and Figure 6. When the particle size increases, the SPM increases from 14.25 g (less than 0.15 mm) to 17.00 g (0.25–0.5 mm), then decreases to 4.60 g (2–3 mm), and finally slowly increases to 8.60 g (6–9 mm).

The pitch pyrolysis of different coke sizes was detected using the pitch pyrolysis device, and the test results are shown in Figure 6. The apparent activation energy of pitch is shown in Table 4 and Figure 6. When the granularity increases, the activation energy gradually increases to 70.00 kJ/mol (1–2 mm), then rapidly decreases to 31.88 kJ/mol (3–4 mm), and finally slowly increases to 50.56 kJ/mol (6–9 mm).

Under the roasted temperature condition of the green carbon anode, the quality after pitch pyrolysis was selected at 1050 °C for CV; the CV is shown in Table 4 and Figure 2. When the particle size increases, the CV increases to 57.99% (0.25–0.5 mm), then decreases to 53.72% (1–2 mm), and rapidly increases to 58.45% (2–3 mm). The final CV shows a decreasing and subsequent increasing trend. PPM was calculated according to the accumulation of SPM and CV. Numerical changes are not evident because CV is within the range of 53.72–58.45%. These results show that PPM follows the same law as SPM.

When the particle size increases, the coke particle size <0.5 mm area can provide a coking interface for pitch coking, i.e., “A area,” which is dominated by the specific surface area. The coke particle size >2–3 mm area is dominated by the porous structure area, i.e., “C area,” where the effect is related to the pore size, porosity, and pore distribution. The coke particle size of 0.5–2 mm area, i.e., “B area” is mainly controlled by a combination of the specific surface area and the porous structure.

According to SEM micromorphology results, the pitch pyrolysis of the green carbon anode involves the following: the pitch pyrolysis in Figure 2a, region “2”, of the outermost layer of the pitch immersion layer; the pitch pyrolysis of “small particles and pitch mixture”, as shown in Figure 2d; the pitch pyrolysis in Figure 2a, region “4”, of the subouter small particles and pitch mixture layer; and the interaction between different particles. Therefore, the pitch pyrolysis of the green carbon anode is the result of the combination of many factors. This observation can be supported by the following experimental results. The pitch CV of the green carbon anode is 56.2%, which is within the 53.72–58.45% range for coke with different particle sizes. The apparent activation energy of the pitch pyrolysis of the green carbon anode is 116.40 kJ/mol, which is larger than the 31.88–70.00 kJ/mol range for coke with different particle sizes and the 82.31 kJ/mol for large-mass pitch.

4. Conclusions

  • (1)

    Different coke sizes are charred by the pitch. The outermost layer of large coke size has a certain thickness of the pitch layer, the subouter layer is filled by the mixture of fine particles and the pitch, and the internal area is not soaked by the pitch. Meanwhile, the small particles are soaked and wrapped by the pitch.

  • (2)

    Pitch pyrolysis data conformed to the dynamic model of formula 7. Pyrolysis dynamics analysis shows that when the particle size increases, the activation energy gradually increases to 70.00 kJ/mol for 1–2 mm, then rapidly decreases to 31.88 kJ/mol for 3–4 mm, and finally slowly increases to 50.56 kJ/mol for 6–9 mm.

  • (3)

    When the particle size increases, the coke particle size <0.5 mm is dominated by the specific surface, 0.5–2 mm is mainly controlled by the mixing control of the specific surface and the porous structure, and >2–3 mm is dominated by the porous structure.

Acknowledgments

This work was supported by the Top Talent Support Program of Guizhou Education Department (KY[2016]062), the Guizhou Provincial Science and Technology Foundation (No. [2020]1Y224), and the Guizhou Provincial Science and Technology Support Project (No. [2020]4Y036).

The authors declare no competing financial interest.

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