J. Cent. South Univ. (2017) 24: 1736-1744

DOI: https://doi.org/10.1007/s11771-017-3581-y

Inhibition mechanism between sodium (Na₃AlF₆) and sulfur on coke reactivity

ZHONG Qi-fan(仲奇凡)¹, XIAO Jin(肖劲)^{1, 2}, LI Fa-chuang(李发闯)¹, LI Jie(李劼)^{1, 2}

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China;

2. National Engineering Laboratory of Efficient Utilization of Refractory Nonferrous Metal Resources

(Central South University), Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany 2017

Abstract: Inhibition mechanism between sodium (Na₃AlF₆) and sulfur on coke reactivity was investigated by simulating petroleum coke with low-impurity pitch coke and by impurity doping. The mechanism was discussed by scanning electron microscopy, energy-dispersive spectrometry, and X-ray powder diffraction. Results show that Na effectively inhibited S catalysis during carbon–air/CO₂ reactions, and S inhibited the catalysis of Na during carbon– air reaction to a certain extent. A stable structure with a Na-to-S atomic ratio of 1.4 and a cyclic reaction system of “Na₂SO₃→ Na₂S→ Na₂CO₃→ Na₂SO₃” were likely the keys to producing this mutual inhibition.

Key words: mutual inhibition; sulfur; sodium; coke reactivity

1 Introduction

The content of impurity elements, which are the main raw materials of carbon anode, is an influential factor affecting the quality of carbon anode reactivity. Petroleum coke is a major component of the main structure of carbon anode (about 90% of anode quality) and also a main impurity element that is an anode source [1]. Impurity elements affect the carbon anode reaction essentially by affecting the petroleum coke reactivity. Impurity elements, including S, Ca, and V, exist in petroleum coke. S, the main impurity element for coke, is a strong catalytic impurity element for air and CO₂ reactivity of coke. The impurity element S existing in petroleum coke is mainly in the forms of thiophene and thiophene derivatives, whereas the inorganic sulfur in sulfate or pyrite form is generally ≤10% [2]. It is pointed out that the sulfur content in petroleum coke is closely linked with the dibenzothiophene (DBT) content in crude coke [3, 4]. Therefore, most of the organic sulfur in petroleum coke probably comes from DBT. And when studying the effect of other impurity elements on petroleum coke without considering S element, as the main impurity element in coke (1%-10% in mass fraction, usually), S will influence the research results in many aspects.

During aluminum electrolysis process, a mass of cryolite (Na₃AlF₆) will contact with the carbon anode directly, which brings the Na with strong catalysis to coke [5, 6]. It has been proved that it is the Na bringing the catalysis, no Al or F. However, no clear explanation is available for Na catalytic reaction mechanism. MEIJER et al [7] proposed that if the source of Na is different, reactive catalysis strength would change. They suspected that the catalytic mechanism of Na to coke is different because of the different Na sources. KAPTEIJN et al [8] speculated that Na catalysis is probably by increasing the affinity of O₂/CO₂ to carbon matrix. However, as the main element of molten salt (Na₃AlF₆) in the process of aluminum electrolysis, the catalytic effect mechanism of Na element and the mutual inhibition mechanism between S element and it on carbon anode reactivity couldn’t be ignored and need to be studied deeply.

Based on choosing the method of using pitch coke instead of petroleum coke, by adding various sorts of chemical synthesis containing Na and S to low-impurity pitch coke and by combining several testing devices such as scanning electron microscopy (SEM), energy- dispersive spectrometry (EDS), and X-ray powder diffraction (XRD), we aimed to conduct a profound and systematic exploration into the mutual inhibition mechanism of sodium (Na₃AlF₆) on coke reactivity with sulfur existing.

2 Experimental section

2.1 Materials

In the experiment, a kind of low-impurity coal tar pitch was chosen as the raw material which was used in producing impurity-added coke samples. Table 1 shows some main impurity elements content of the pitch coke sample without any impurity adding.

Table 1Main impurity elements content of pitch coke

The existing form of S element in petroleum coke is mainly classed as organic S as thiophenes [2]. Meanwhile, DBT (dibenzothiophene) is deduced as organic S impurities. And this deduction was investigated firstly by ENGVOLL [9] several years ago. Engvoll is the person who firstly chose the method of using pitch coke instead of petroleum coke to study the reactivity of coke with DBT-added. Considering that Na impurities in the carbon anode mostly come from cryolite (Na₃AlF₆), and the elements of F and Al in Na₃AlF₆ can be seen as harmless and inactive during coke–air/CO₂ reactions, Na₃AlF₆ is selected as the only source of Na for the study on the mechanism of mutual inhibition between S and Na. The Na₃AlF₆ used in the experiment belonged to reagent grade.

2.2 Samples preparation

The samples were prepared by a few steps. Firstly, the coal tar pitch was melted under the temperature of 200 °C. During pitch melted, the reagents which were chose as the dopants, were added in and mixed well. Then the samples were moved in to furnace reactor and calcined for one hour in advance under the temperature of 550 °C. This step was aimed to prevent the sample from melting again and then polluted. The surfaces of samples were cut and threw away, and the leaving parts were crushed into particles. Finally, the samples are coked under the temperature of 1100 °C. After coked by one hour, the samples can be obtained successfully.

2.3 Samples analysis

2.3.1 Reactivity analysis

In this work, reactivity referred to the chemical reactivity between carbon-air and carbon-CO₂. Reactivity of the coke samples were tested on the condition that the equipment was under the circumstance of constant heat with 50 L/h air volume or CO₂ volume as shown in Fig. 1. In the experiment, the testing samples with a mass of 5 g and a width of 1-1.4 mm were placed in reactors with temperatures of 600 °C (for air reactivity test) or 1000 °C (for CO₂ reactivity test) for 1 h. The detail description of the reactivity testing analyzer and the detail testing method could be found in our previous works [10, 11]. Their reactivity can be tested by high or low change of the mass-loss rate of the coke samples after carbon-air/CO₂ reaction.

Fig. 1 Detailed description of reactivity testing analyzer

2.3.2 Other analysis

The sulfur content in coke was measured by the means of Coulomb with the aid of HDS3000 model AI Sulfur Detector. And the detector was made by Huade Electronic Ltd (Hunan Province, China).

To test SEM and EDS, the kind of FEI Quanta 200 scanning electronic microscope equipped with an accelerated voltage of 20 kV and an X-ray energy dispersive spectrometer was resorted.

To measure XRD, a D/Max 2500 diffractometer using Cu K_α line radiation (Rigaku produced) was employed.

3 Results and discussion

3.1 Effects of S-added and Na-added Na₃AlF₆ on coke reactivity

As shown in Table 2, by doping different amounts of Na₃AlF₆ and DBT into the pitch, the low-sulfur (Na₃AlF₆-added, named NA) samples and high-sulfur (Na₃AlF₆ + DBT-added, named NAS) ones were prepared. It can be seen clearly that with the quantity of DBT added was kept constant, the final S content in the pitch coke slightly increased with increased amount of added Na₃AlF₆ during preparation. The specific effect of “sulfur-fixed” is proved produced by Na-added. In addition, F in Na₃AlF₆ will volatilize during heating, which can be ignored and has no effect to coke.

Table 2 Basic parameters of samples with different Na contents

Figure 2 shows the test results of the samples after carbon-air reaction. With further increase, but it still cannot reach the air reaction residue rate of low-sulfur sample. With increased Na content (<230×10^-6), the S is speculated to still have catalysis effect on coke. However, with increased Na content, the inhibition of Na to the catalysis of S on coke begins. When the Na content exceeded 230×10^-6, the air reaction residue rate of the high-sulfur sample is reduced as well and is still above the air reaction residue rate of the low-sulfur sample. Nevertheless, Na clearly has an inhibition effect on the catalysis of S to coke. The result is similar to the experimental result of HUME et al [12], who had mentioned that S could weaken the sensitivity of coke to Na.

Figure 3 shows the CO₂ reactivity of each sample. With increased Na content, the CO₂ reaction residue of low-sulfur coke showed a slight declining trend. Thecatalysis of Na to coke is smooth. However, with increased Na content, the residual rate of high-sulfur coke after carbon–CO₂ reaction initially increased and then decreased. With increased Na content from 0 to 230×10^-6, the CO₂ residue of high-sulfur coke sharply increased. However, when Na content exceeded 360×10^-6, the CO₂ residue of high-sulfur coke slightly decreased, the same as low-sulfur coke. Although the Na content increased from 0 to 1000×10^-6, the CO₂ reaction residue rate of high-sulfur coke is lower than that of low-sulfur coke. Clearly, Na has an inhibition catalysis effect on carbon–CO₂ reaction. However, the inhibition effect of Na reaches its limit at about 300×10^-6. With further increased Na content, the newly added Na resumed its reaction catalytic ability again. Nevertheless, with 2% added in the coke, S has no obvious inhibition effect on the catalytic ability of Na.

Fig. 2 Residue rate of low- and high-sulfur Na₃AlF₆-added cokes after carbon-air reaction

Fig. 3 Residue rate of low- and high-sulfur Na₃AlF₆-added cokes after carbon-CO₂ reaction

According to the above experimental results, in the process of coke reaction with air and CO₂, two different types of interaction behaviors maybe exist between Na and S impurities. During the carbon–air reaction, a mutual inhibition effect between S and Na occurs, whereas during the carbon–CO₂ reaction, only Na has an inhibition effect on the catalysis of S unilaterally. Above all, both the two kinds of interaction behaviors are seemed to have “poor efficiency”. In both carbon–air and carbon–CO₂ reactions, when the Na content in the coke exceeds 300×10^-6, the residue of high-sulfur coke decreased with increased Na content. Hence, if Na content in coke is high, not all Na would be involved in the interaction behavior of Na and S, and part of Na is likely to be formed into a relatively independent free form and keep their original carbon–air/CO₂ reaction catalytic ability. The experiment of Section 3.3 was carried out as followed for verifying the speculation which stated above.

3.2 Mechanism analysis of mutual inhibition between Na and S

Combining XRD, SEM, and EDS, an experiment was conducted to analyze the surface formations of the pitch coke samples, including NA6 (Na₃AlF₆-doped) and NAS6 (Na₃AlF₆+DBT-doped), by comparing them before and after air/CO₂ reactivity test.

Figure 4 shows the SEM images of the two pitch coke samples before the carbon-gas reaction, whereas Table 3 details the EDS analysis results of the surface formations of the samples. As shown in Figs. 4(a) and (b), many block white bodies were detected on the surface of Na₃AlF₆-doped coke. The EDS analysis results show that elements C, Na, Al, F, and O can be found in these block, white body places (e.g., A1, A2, and A3), and the atomic ratio of Na-to-Al is approximately 3. Eliminating the carbon base, the block white bodies are speculated to be Na₃AlF₆ doped in the experiment. To verify the stated speculation, a pitch coke sample doped with 10% Na₃AlF₆ was analyzed by XRD, and the result is shown in Fig. 5. Only Na₃AlF₆ characteristic peaks existed, which means that the Na₃AlF₆ doped in the coke does not change during pitch coking. Therefore, the block white bodies in the sample should be Na₃AlF₆. As shown in Figs. 4(c) and (d), only few white needle crystal structures exist on the plane surface of NAS6 sample, and no obvious foreign matter has been found on the fracture surface. EDS analysis results show that except for C, elements Na, Al, and O can be found in the white needle crystal structures (region A4 and A5), and the atomic ratios of Na-to-Al and Na-to-O are approximately 1 and 1.5–2, respectively. No other polluting reagent is observed, thus it is speculated to be other forms, which were transformed from Na₃AlF₆ and named as NaAlO_1.5–2.

Fig 4 SEM images of plane surface of NA6 (a), fracture surface of NA6 (b), plane surface of NAS6 (c) and fracture surface of NAS6 (d)

Table 3 Element composition of area A1-A5 in Fig. 4

Fig 5 XRD patterns of coke sample prepared by doping 10% Na₃AlF₆

3.2.1 Mechanism analysis of mutual inhibition between Na and S on air reactivity

Figure 6 presents the SEM images of the two samples, namely, NA6 and NAS6, after the air reactivity tests. Table 4 shows the EDS analysis results of the foreign bodies that exist on the sample plane and fracture surfaces. As shown in Fig. 6(a) and the data of A6 and A7 in Table 4, the Na₃AlF₆ has been replaced by a new form of white foreign bodies after carbon–air reaction. The white foreign bodies have no F elements, but with C, O, Na, and Al, and the atomic ratio of Na-to-Al is approximately 1.6 (named Na_1.6AlO). Na_1.6AlO is speculated to be transformed by Na₃AlF₆ and has close relationship with the catalysis of Na to carbon–air reaction. As shown in Figs. 6(b) and (c), the white needle crystal structure, NaAlO_1.5–2 has disappeared completely, and some white foreign bodies appeared, as shown in A11 region. The white foreign bodies are similar to Na_1.6AlO in the elementary composition, but the atomic ratio of Na-to-Al is small (<0.5). The new white foreign bodies and Na_1.6AlO should be the same series of matter, but in different phase compositions; to show the difference, it is named Na_0.5AlO. The element S is mostly enriched in two kinds of regions: the first kind of region is the corrosion pit and holes, as shown in A9 and A10; the other kind is the corrosion residual areas (A10-A13), and the regions are composed not only by S, but also by Na. The atomic ratio of Na-to-S is approximately 1.4. According to the test result above, most Na breaks away from Na₃AlF₆ and complexes with S by atomic ratio of 1.4, thereby forming into a kind of stable structure during the carbon–air reaction. This stable structure lost the catalysis that they have had before, and the lost Na keeps the catalysis effect in coke in the form of Na_0.5AlO.

According to the analysis of the test and experimental results above, the mechanism analysis of the mutual inhibition between Na and S on air reactivity is as follows.

1) If only Na₃AlF₆ is in coke, Na₃AlF₆ that exists on the surface of coke would transform into Na_1.6AlO during the carbon–air reaction, which would make the catalysis effect on coke, which can be explained by the theory proposed by MOULIJN and KAPTEIJN [13]; Na has a stronger affinity to O₂ in the air than C in the coke during the carbon–air reaction, thus Na_1.6AlO is similar to a bridge that attracts more O₂ to the surface of the coke and speeds up the reaction between C and O₂.

Fig. 6 SEM images after carbon-air reaction:

Table 4 Element composition of areas A6-A13 in Fig. 6

2) When both S and Na exist in the coke, most Na breaks away from Na₃AlF₆ and complexes with S by the atomic ratio of 1.4, thereby forming a kind of stable structure. This stable structure lost the catalysis that they have had before, and the lost S and Na, which is in the form of Na_0.5AlO, keep the catalysis effect in coke.

Under the influence of the two kinds of behavior above, the experiment results are as presented in Fig. 2.

3.2.2 Mechanism analysis of mutual inhibition between Na and S on CO₂ reactivity

Figure 7 illustrates the SEM images and EDS analysis of sample NA6 after the CO₂ reactivity tests. As shown in Fig. 7(b), the Na₃AlF₆ has been replaced by a new form of filamentous structure after carbon–CO₂ reaction. The filamentous structures have C, O, Na, and Al elements, and the atomic ratio of Na-to-Al is approximately 4 (named Na₄AlO). The filamentous structures mostly exist on the bottom and lining wall of the large corrosion holes, which are shown similar to the corrosion holes corroded by the filamentous structures (Fig. 7(c)). Na₄AlO has a close relationship with the catalysis of Na to carbon–CO₂ reaction.

Figure 8 and Table 5 present the SEM images and EDS analysis of sample NSA6 after the CO₂ reactivity tests. Two different kinds of white foreign particles exist on the surface. As shown in A15–A18 regions, this kind of foreign body has a regular shape structure and has C, O, Na, and S elements, and the atomic ratio of Na-to-S and Na-to-O is approximately 1.2 and 0.5, respectively (named as Na_1.2SCO). As shown in Figs. 8(c)–(e), this kind of white foreign particle generally exists in the small corrosion grooves. Meanwhile, as shown in regions A19 and A20, other kinds of irregularly shaped structures exist, which have no S element, but with C, O, Na, and Al; the atomic ratio of Na-to-Al is <2, and Na-to-O ratio is high. Compared with A8 in Fig. 6(c), the irregularly shaped structures have the same element composition and element proportion with Na_0.5AlO. Thus, the structures are speculated to be the same kind of matter; not similar to sample NAS6 after carbon–air reaction, no region where both S and Na are enriched together is observed. However, as shown in regions A21 and A22, S is enriched at some corrosion regions on the surface, therefore S still has catalysis effect on coke during carbon–CO₂ reaction.

According to the analysis of the test and experimental results above, the inhibition of Na to the catalysis of S can affect the cyclic reaction system of S, in which Na_1.2SCO should to be the key. Na_1.2SCO must be combined with varieties of Na compounds. The S in the Na_1.2SCO cannot have a stable high valence state, such as SO₄^2-, S₂O₃^2-, because of the reducing system of carbon–CO₂ reaction (1000 °C, atmosphere of CO/CO₂, C-rich, less H₂S and SO₂). Therefore, only Na₂SO₃, Na₂S, Na₂S_y, and Na₂CO₃ may exist.

Fig. 7 SEM images and EDS data of NA6 after carbon-CO₂ reaction:

Fig. 8 SEM images and EDS data of NSA6 after carbon-CO₂ reaction:

Table 5 Element composition of areas A15-A22 in Fig. 8

According to Ref. [14], the stability of Na₂SO₃ in the oxidizing atmosphere under high temperature is poor. At >600 °C, Na₂SO₃ decomposes into Na₂S and Na₂SO₄ through reaction 1.

4Na₂SO₃=Na₂S+3Na₂SO₄, △_rG⁰_1000°C=-45.8 kJ/mol  (1)

At >700 °C and carbon-rich condition, Na₂SO₄ transforms into Na₂S via reaction 2. Reaction 2 is one of the main methods of Na₂S preparation in the industry [15, 16].

Na₂SO₄+4C=Na₂S+4CO(g), △_rG⁰_1000°C=-236.2 kJ/mol (2)

Under carbon-rich and CO₂ conditions, Na₂CO₃ rapidly transforms into Na₂S through reaction 3 [17, 18].

Na₂S+8CO₂(g)+4C=Na₂CO₃+SO₂(g)+11CO(g),

△_rG⁰_1000°C=-80.9 kJ/mol                     (3)

In Refs. [19, 20], it was pointed out that Na₂CO₃ has strong adsorption ability to sulfurous gas by reactions 4 and 5, thereby generating Na₂SO₃ and Na₂S.

Na₂CO₃+SO₂(g)=Na₂SO₃+CO₂(g),_rG⁰_1000°C=-11.4 kJ/mol              (4)

Na₂CO₃+H₂S(g)=Na₂S+CO₂(g)+H₂O(g),△_rG⁰_1000°C=-31.2 kJ/mol          (5)

Below 1200 °C by reaction 3 (react quickly below 800 °C), Na₂S_y is generated in the presence of Na₂S and elemental S[21].

Na₂S+S_x→Na₂S_y (x=1, 2, 4, 6, 8; y>1)             (6)

In addition, [Na] can be seen as the active Na atom on coke surface. The initial generation process of Na₂SO₃ and Na₂S cannot be speculated concretely, hence simplified into reactions 7 and 8.

[Na]+SO₂(g)→Na₂SO₃                  (7)

[Na]+H₂S(g)→Na₂S                    (8)

Therefore, combining the reactions above, the inhibition of Na to S during carbon–CO₂ reaction is speculated to be generated by a set of reaction systems as followed from reactions 7 and 8 to reactions 1, 2, 3, 4, 5 and 6. With the situation of CO₂ atmosphere and 1000 °C, the structures in coke containing Na and S elements cannot remain stable. Na and S come into contact with the outside atmosphere gradually. S begins to show its catalysis to carbon-CO₂ reaction in the forms of H₂S, SO₂, COS and elemental S. At the same time, the Na element in coke changes into [Na]. [Na] will capture SO₂ and H₂S, and Na₂SO₃ and Na₂S will generate. By reactions 1 and 2, volatile Na₂SO₃ will change into Na₂S. By reaction 3, Na₂S is transformed into Na₂CO₃, with releasing S element in the form of SO₂. However, Na₂CO₃ will capture SO₂ and H₂S again, with generating Na₂SO₃ and Na₂S (reactions 4 and 5). Therefore, the cyclic reaction system “Na₂SO₃→Na₂S→Na₂CO₃→Na₂SO₃” consumes the SO₂ and H₂S continually, which could destroy the original catalytic cycle reaction system of S. When carbon-CO₂ reaction is over, with the temperature reduction, Na₂S_y will be formed by reaction 6. After cooling, the surface of coke sample will leave the mixture of Na₂SO₃, Na₂S, Na₂CO₃ and Na₂SO₃.

However, a certain amount of S could still make catalytic effect on coke during carbon–CO₂ reaction because of the large content difference between S and Na, which is also the reason why the carbon–CO₂ reaction residue of high-sulfur coke cannot reach that of the low-sulfur coke. Furthermore, after carbon–CO₂ reaction, Na_0.5AlO-like analogues are found on the NAS6 surface. Thus, when the Na content exceeds 360×10^-6, the CO₂ residue of high-sulfur coke slightly decreases (Fig. 3), and the inhibition efficiency of Na to S is also limited during the carbon–CO₂ reaction. With increased Na content until the limit, more Na becomes independence of S and keeps their catalysis to coke in the form of Na_0.5AlO.

4 Conclusions

When both Na (Na₃AlF₆) and S impurities exist in coke, mutual inhibition occurs between the individual catalytic activities of S and Na. Na can effectively inhibit the catalysis of S during carbon–air/CO₂ reactions, and S could inhibit the catalysis of Na during carbon–air reaction to a certain extent. Taken together, our results indicate that the mechanism of mutual inhibition between S and Na during carbon–air/CO₂ reactions is as follows:

1) During carbon–air reaction, Na composites with S at 1.4 atomic ratio form a kind of stable structure. At the same time, the inhibition efficiency of Na to S is also limited. With increased Na content, Na partly regains its catalysis to coke through the form of Na_0.5AlO.

2) During carbon–CO₂ reaction, the inhibitive effect of Na on the reactive catalysis of S is most probably related to each other by the cyclic reaction system “Na₂SO₃→Na₂S→Na₂CO₃→Na₂SO₃”, which could destroy the original catalytic cycle reaction system of S.

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(Edited by YANG Hua)

Cite this article as: ZHONG Qi-fan, XIAO Jin, LI Fa-chuang, LI Jie. Inhibition mechanism between sodium (Na₃AlF₆) and sulfur on coke reactivity [J]. Journal of Central South University, 2017, 24(8): 1736-1744. DOI: https://doi.org/ 10.1007/s11771-017-3581-y.

Foundation item: Projects(51374253, 51574289) supported by the National Natural Science Foundation of China

Received date: 2016-02-29; Accepted date: 2016-05-23

Corresponding author: XIAO Jin, Professor, PhD; Tel: +86-731-88830277; E-mail: changshaxiaojin@126.com