Influence of alkaline (Na, K) vapors on carbon and mineral behavior in blast furnace cokes

Fuel 145 (2015) 202–213

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Influence of alkaline (Na, K) vapors on carbon and mineral behavior in blast furnace cokes Kejiang Li a,b, Jianliang Zhang a,b,⇑, Mansoor Barati d, Rita Khanna c, Zhengjian Liu a,b, Jianbo Zhong a,b, Xiaojun Ning a,b, Shan Ren e, Tianjun Yang a,b, Veena Sahajwalla c a

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China Centre for Sustainable Materials Research & Technology (SMaRT), School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia d Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada e College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China b c

h i g h l i g h t s  Alkalization behavior of coke and corresponding influence were investigated.  Potassium and sodium destruct coke texture with different degrees.  Alkalization leads to the formation of additional alkalis-bearing phases.  The destruction mechanism of alkalis vapors on coke was proposed.

a r t i c l e

i n f o

Article history: Received 22 October 2014 Received in revised form 26 December 2014 Accepted 27 December 2014 Available online 5 January 2015 Keywords: Coke Mineral Alkalization Degradation

a b s t r a c t A series of adsorption–alkalization experiments were conducted in a muffle furnace on two types of blast furnace cokes at 1300 °C in the presence of alkali vapors. Coke textures were found to peel off layer by layer after the alkalization process by potassium vapor, and macro fissures were observed for K/Coke ratios higher than 3/100. This phenomenon was not observed in the coke samples alkalized by sodium vapor. A number of additional potassium-bearing and sodium-bearing phases were detected with scanning electron microscope and energy dispersive spectrometer after the alkalization process. The formation of kalsilite or potassium aluminum silicate (KAlSiO4) and sodium alumina silicates (Na6Al4Si4O17) was confirmed through X-ray diffraction, however the formation of intercalation compounds that were expected to form in the alkalized coke samples could not be confirmed. The catalytic effect of sodium and potassium-bearing minerals appeared to be quite similar; the degradation of coke strength by sodium was however found to be stronger than that caused by potassium. The severe degradation of coke quality caused by alkali vapors was attributed to their strong influence on the coke carbon matrix, coke minerals, as well as their catalytic effect on the carbon gasification reaction. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction A reduction in coke consumption is one of the key approaches being used towards minimizing greenhouse gas emissions and energy consumption in the current blast furnace (BF) ironmaking process [1]. Due to the twin pressures of resource shortage of coking coals and continuous requirements for improved coke quality, an in-depth understanding of coke behavior in the BF has become ⇑ Corresponding author at: No. 30 Xueyuan Rd, Haidian District, Beijing, China. Tel.: +86 10 62332364, +86 13910019986. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.fuel.2014.12.086 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

increasingly important [2,3]. As coke traverses through the blast furnace, various changes occurring in coke is poorly understood [3,4]. While the alkalization of quartz has been investigated through studies on mineral grains in tuyere cokes exposed to the blast furnace gas [4] and the catalytic effect of iron on coke reactivity is well known [5], there are several knowledge gaps in our understanding regarding the influence of alkali-bearing minerals on various coke characteristics [6]. Because of the volatilization and condensation of alkalis in different thermal zones, these tend to cycle and remain within the BF leading to their accumulation and interactions with other feed materials. Even small amounts of alkalis in the charge could have a significant impact on the

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overall process [2,7]. In the 1970s, over ten blast furnaces were successfully dissected in Japan [8,9]. These studies were focused on the variation of physical properties (mean size, drum index, coke-strength) and chemical properties (reactivity with CO2) of coke along with temperature or blast furnace height. From the Japanese dissection results [10], the distribution of alkalis in a blast furnace became better understood, and it was confirmed that the high temperature zones (above 1000 °C) in a blast furnace contained highest levels of alkalis. While industrial dissection results clarified the degradation of cokes by alkalis, detailed mechanism was still not well understood. Since 1980’s, a number of researchers have sought to establish the mechanism of coke degradation by alkalis through controlled laboratory investigations [11,12]. The degradation of raw material quality is an unavoidable new trend for the iron and steel industry. The concentrations of harmful elements especially alkalis in raw materials has kept on increasing in recent years affecting the efficiency of blast furnace operation. Large-scale, medium and small scale enterprises are facing the challenge of harmful elements especially alkalis poor quality raw materials. This previously hot topic of 1980s has once again become a hot topic of research towards the utilization of low quality raw materials and deceasing associated production costs. A recent study has shown that the apparent reaction rate of tuyere level cokes can be up to ten times that of the feed coke reaction rate, and was strongly related to the total amount of potassium species present [13,14]. The presence of recirculating potassium was found to be one of the most distinctive features of tuyere cokes especially outside the raceway [13,15,16]. Previous research [7] in this area usually soaked cokes in solutions with different concentration of alkalis carbonates for a range of times, and then the soaked cokes were dried and tested to analyze the variation of properties. This method can only investigate the influence of alkalis as carbonates on the coke properties. As alkalis are primarily present as a gaseous phase in the high temperature zone of a BF, the influence of alkali vapors on the alkalization of coke minerals and coke characteristics is a subject of key interest and further research. Few results have been reported regarding the interaction of alkali vapors with coke. In this article, we report an in-depth investigation at high temperature (1300 °C) on the alkalization behavior of both carbon and various mineral phases present in two blast furnace cokes and their corresponding influence on the coke behavior. In order to understand the true alkalization process of coke minerals by alkalis vapor in a BF, this reaction needs to be isolated from other reactions that could occur simultaneously in an industrial BF, e.g., coke gasification and/or the reduction of minerals by carbon in coke. The final goal of this research is to establish a detailed mechanism about the alkalization process of coke by alkalis vapor and its corresponding influence on coke properties. Based on a novel approach, this study investigates the adsorption and the alkalization of two BF cokes by Na and K vapor at 1300 °C. The interaction of alkalis vapor with coke carbon and coke minerals were analyzed with scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS) examinations, Xray diffraction and Raman spectroscopy. Key coke characteristics, namely the coke reactivity index (CRI) and the coke strength after reaction (CSR), were measured according to Chinese National Standards (GB/T 4000–2008; equivalent to ASTM Standard D 5341); the correlation between CRI and CSR values and the amount of alkalis incorporated into the coke structure was also established along with possible reaction mechanisms.

2. Experimental Two types of feed cokes with different reactivities, labeled here as Coke 1 and Coke 2, were obtained from a large (bigger than

Table 1 CRI and CSR values, proximate and ultimate analysis, and ash composition of original coke samples. Items

Coke 1

Coke 2

CRI CSR

33.43 60.15

21.80 70.96

Proximate analysis/mass% Moisture, ad Ash, ad Volatile matter, ad Fixed carbon, ad

0.20 12.07 1.54 86.19

0.15 11.80 0.15 87.90

Ultimate analysis/mass% C, ad H, ad N, ad O, ad S, ad

85.68 0.18 0.96 0.27 0.64

86.47 0.10 0.92 0.02 0.54

Ash composition/mass% SiO2 Al2O3 Fe2O3 CaO TiO2 MgO Na2O K2O SO3 P2O5 Other

34.11 19.88 22.06 5.83 2.68 0.55 0.25 0.82 9.94 0.97 2.91

31.25 16.87 22.22 9.83 3.62 0.92 0.48 0.91 8.50 1.00 4.40

4000 m3) blast furnace. CRI and CSR values, proximate and ultimate analysis, and ash compositions of original cokes are shown in Table 1. The feed coke samples were crushed with a jaw crusher and sieved using a 25.0 mm sieve placed on top of a 23 mm sieve to yield sufficient quantities of 23–25 mm size specimens. Discarding flake-like and strip-like pieces, the edges of the remaining coke lumps were manually ground to make their shapes nearly spherical. These lumps were sieved again using a 23 mm sieve to remove

Fig. 1. Fragments and macro fissures formed in Coke 2 after alkalization by potassium vapor: (a) coke lump left after alkalization (Coke 2-K1); (b) coke fragment formed after alkalization (Coke 2-K1); (c) the evolution of mass fraction of peeling fragments with the increase of K/Coke.

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Fig. 2. (a) SEM images of coke samples illustrating the damage to coke texture by potassium vapor: (A) original Coke 1; (B) original Coke 2; (C) EDS of position at P1; (D and E) Coke 1-K7; (F–I) Coke 2-K1. (b) Occurrence of potassium in coke carbon matrix: (A–D) SEM images of Coke 1-K7; (E and F) EDS mapping of element C and K of image D; (G–I) EDS spectra at P1, P2 and P3.

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those below this size; the sieved top fraction was used as the test material for this investigation. All the analytic reagent powders, including Na2CO3 (purity P 99.8%), K2CO3 (purity P 99.0%) and activated carbon (analytically pure, burned residue 6 2.0%) were obtained from Sinopharm Chemical Regent Co., Ltd. Activated carbon was used to reduce Na2CO3 and K2CO3 to produce sodium and potassium vapors respectively. A series of adsorption–alkalization experiments were carried out. Approximately 300 g ± 0.5 g of coke specimens were placed atop mixtures of activated carbon and alkali carbonates in a corundum crucible with a corundum lid. A perforated corundum spacer separated coke samples and alkali/carbon powder mixtures. The covered crucible was heated in a muffle furnace to 1300 °C at a heating rate of 10 °C/min. This temperature was chosen to replicate temperatures in the lower part of the BF cohesive zone, where alkalis are known to accumulate as vapors [7]. The furnace was held at 1300 °C for 90 min (10 min for thermal stabilization time and 80 min for experimental time) and then cooled down slowly to room temperature at a cooling rate of about 5 °C/min. Both Na2CO3 and K2CO3 were expected to get reduced by carbon at 1300 °C due to the highly negative Gibbs free energy of reduction reactions, and to produce sodium and potassium vapor [17]. On the other hand, the reduction of coke minerals by carbon was not favorable at this temperature [18–20]. The term K/Coke (or Na/coke) was defined as the mass ratio between alkali vapors and cokes. Numerical values of K/Coke and Na/Coke were set to be 1/100, 3/100, 5/100 and 7/100 to simulate the alkali content in the high temperature zone of a blast furnace. This range was estimated from the highest amounts of alkali enrichment, alkali contents of tuyere cokes and of cokes excavated during the dissection of blast furnaces [7,13,21]. The amount of activated carbon was determined from the stoichiometry of alkali carbonate reductions. Additional carbon (up to 50%) was added to ensure a complete reduction of alkalis carbonates and the removal of oxygen from the sealed crucible. Under these conditions, the reduction reactions for Na2CO3 and K2CO3 can be expressed in the general form:

M2 CO3ðlÞ þ 2CðsÞ ¼ 2MðgÞ þ 3COðgÞ

ð1Þ

where M stands for either Na or K. The coke samples after the adsorption and alkalization by different amounts of potassium or sodium vapors were suitably labeled with different ID numbers. For instance, Coke 1-K1 means the coke 1 samples after the penetration process of K1. After the alkalization treatment, reacted cokes were removed for a detailed characterization. The values of CRI and CSR of the lumps within 23–25 mm were determined as per the Chinese National Standards (GB/T 4000–2008). X-ray diffraction (XRD) investigations were carried out on cokes using a MAC Science Diffractometer (M21XVHF22, MAC Science Co., Ltd. Japan) with Copper Ka radiation (30 kV, 30 mA). These were carried out before and after the alkalization treatment. For scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS) examinations, samples were prepared as per the following procedure. Coke samples before and after the alkalization process were cut and then mounted in an epoxy resin in plastic moulds (25 mm diameter). Surfaces were ground on six different grades of silicon carbide paper under running distilled water and polished with pads covered in diamond paste with particle sizes of 15 lm, 9 lm, 3 lm, and 1 lm. Here it is necessary to indicate that anhydrous alcohol is preferred if condition permits considering the possibility of washing out some alkalis due to the use of water in sample preparation. The dry mounted material was then carbon coated for SEM/EDS studies, and examined under a Quanta 250 Environmental SEM equipped with EDS for chemical

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analysis and elemental mapping. The Raman spectra of coke samples before and after the alkalization process was acquired using a laser confocal micro-Raman spectrometer, JY-HR800, manufactured by Jobin Y’von of France (HORIBA Scientific, Edison, NJ). The experiments were carried out at room temperature using excitation wave-length of 532 nm and the light source was a semiconductor laser with power of 2 mW. The frequency band measured in this work ranged from 700 to 2200 cm1.

3. Results and discussion 3.1. The influence of alkalis on coke texture A visual examination of potassium alkalized coke specimens showed expanded cokes with a number of layers peeled off from the coke lump. This phenomenon, referred to as ‘‘pealing effect’’ in this study, was observed in both Coke 1 and Coke 2 (see Fig. 1(a and b)). The extent of these peels was seen to increase with increasing K/Coke ratios. A plot of the mass percentage of the peels against K/Coke ratio is shown in Fig. 1(c), which shows a stronger dependence below a ratio 3.0/100. Another feature of alkalized cokes was the formation of numerous macro crevices that appeared to run deep into the lump. However, the alkalization of cokes by sodium vapor did not show the peeling effect or the formation of macro crevices. Negligible amounts of coke fines were found at the bottom of the crucible after the experiments involving Na vapors. The SEM images of coke samples (Coke 1 and Coke 2) before and after alkalization with potassium are shown in Fig. 2(a). As seen in Fig. 2(a) A–B, the original coke had a compact structure with scattered macro pores; the pore sizes of Coke 1 appeared to be larger than those of Coke 2. Alkalized coke however showed several distinct features: the formation of crevices transformed the solid matrix into a layered structure (Fig. 2(a) D–I). It appears that the carbon structure was split into layers by an internal expansion caused by the penetration of K vapors. Fig. 2(b) shows the occurrence of potassium in coke carbon matrix by SEM/EDS and X-ray mapping. Macro cracks could be observed in Fig. 2(b) A, while further magnification confirmed the destruction of carbon matrix through the penetration of potassium ions in the coke texture (Fig. 2(b) C). The intercalation of potassium into coke carbon matrix was expected to be the major cause for the internal expansion which lead to formation of crack. From Fig. 2(a) F and I, it can

Fig. 3. Height-normalized Raman spectra of Coke 1 samples before and after alkalization by potassium vapor (a) and sodium vapor (b).

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be seen that the local expansion was also caused by minerals located between the layers. The EDS data from these regions indicates the presence of K, Al, Si and O. 3.2. The variation of carbon structure during the alkalization process 3.2.1. The influence of potassium vapor on coke carbon structure Fig. 3 shows the baseline-corrected Raman spectra of Coke 1 samples before and after alkalization by potassium vapor and sodium vapors as per the procedure developed by Dong et al. [22]. Two key Raman spectral parameters, i.e., the intensity ratio (ID/IG) of the D band (1284–1600 cm1) and the G band (ca. 1600 cm1) and the intensity ratio (IV/IG) of the valley between the D and the G band to the G band have been found to be useful in assessing features of the coke carbon structure. A reduction in ID/

IG indicates a growth of basic graphene structural units (BSUs), while a decrease in IV/IG suggests a reduction in the amorphous component or an improved ordering of the overall carbon structure [22]. From Fig. 3(a), it can be seen that the ID/IG ratios of Coke 1 lumps had increased slightly, while the corresponding IV/IG ratio had decreased after the penetration by potassium vapor. However, the Raman spectrum of a fragment of Coke 1 after penetration by potassium vapor was found to be significantly different from that of the original coke samples. While the G⁄ band had become much sharper, the D⁄ band had disappeared to a great extent; this result indicates the preferential alkalization of disordered carbons (represented by D⁄ band) from coke fragments during alkalization. Two new weak peaks were observed at around 800 cm1 and 1100 cm1 in the Raman spectra indicating the formation of new compounds.

Fig. 4. (a) SEM/EDS analysis of minerals in original coke samples: (A and B) Coke 1; (C) Coke 2. (b) EDS mapping of Al, Si, K, O and C in Coke after alkalization by potassium vapor: (A) Coke 1-K1; (B) Coke 2-K3. (c) EDS mapping of Al, Si, Na, O and C in Coke 1 after the alkalization by sodium vapor: (A) Coke 1-Na3; (B) Coke 1-Na5.

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3.2.2. Influence of sodium vapor on coke carbon structure While the ‘‘peeling effect’’ was not observed in the coke samples after alkalization by sodium vapor, a small amount of coke fines were formed when the Na/Coke ratio was increased to 3/100. The lump of Coke 1 samples after the alkalization by sodium vapor (Na/Coke = 3/100) was labeled with Coke 1-N3-lump, and the fines were labeled as Coke 1-N3-fine. Fig. 3(b) shows the baseline-corrected Raman spectra of coke samples before and after alkalization by sodium vapor. The Raman spectra of coke samples were not much affected after the alkalization by sodium vapor. For the Coke 1-N3-lump, both the ID/IG ratio and the IV/IG ratio decreased slightly as compared to the original Coke 1 sample. However, the ID/IG ratio of Coke 1-N3-fine increased slightly, while its IV/IG ratio had decreased slightly. It indicates that amorphous content in the coke fines formed after the alkalization by sodium vapor was less than that of original coke and Coke 1-N3-lump samples. 3.3. Transformation of minerals during the alkalization process 3.3.1. Minerals present in original cokes A number of mineral phases of varying sizes and compositions were observed in the coke matrix of samples under investigation. Some of these had one of their sides open to the pores or had extensive cracks, and were consequently accessible for interaction with circulating gases (Fig. 4(a) A–C), while several others were found to be further away from pores (Fig. 4 (a) C). From the EDS results shown in Fig. 4 (a), alumino-silicates constituted the main group of minerals present in cokes; this finding is in good agreement with the Ref. [23]. SEM/EDS could not identify any potassium or sodium bearing minerals probably due to the very small percentage of these elements in the original coke samples. From Table 1, it can be seen that the weight fractions of both K2O and Na2O in coke minerals were less than 1.0 wt%. 3.3.2. Transformation of coke minerals after penetration by potassium vapor In contrast to the original coke samples, potassium bearing minerals were widely detected with SEM/EDS after the alkalization by potassium vapor (Fig. 4(b)). The presence of K bearing minerals can be attributed to the interaction of K vapors with the aluminosilicates present in coke minerals. With EDS, semi-quantitative analysis of the composition (wt.%) of the bright areas of Fig. 4(b) A was obtained as follows: 28.50% Si, 17.69% Al, 17.80% K, 24.05% O and 11.39% C, while that of the bright area of Fig. 4(b) B was

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listed as below: 47.62% Si, 6.41% Al, 3.49% K, 32.27% O, and 9.59% C. XRD analysis confirmed the formation of kalsilite and/or potassium aluminum silicate (KAlSiO4) (Fig. 5(a)). As the characteristic peaks of kalsilite overlap closely with that of potassium aluminum silicate, it is difficult to distinguish between these two phases due to their low concentrations. It is known that kalsilite can be synthesized by dry method with starting materials consisting of a reagent mixture of SiO2, Al2O3 and K2CO3 at about 1000 °C, and the higher the synthesis temperature is, the higher the crystallinity [24]. Thus it can be proposed that the kalsilite was formed following the adsorption of potassium vapor on alumina silicates present in coke ash. The formation of the new potassium-bearing compounds could have occurred through the following reaction pathway:

Al—Si—OðmeltÞ þ OðgasÞ þ KðgasÞ ! Al—Si—K—OðmeltÞ

ð2Þ

Cracks were also observed in the coke minerals penetrated by potassium vapor (Fig. 6). These cracks appear to originate from coke pores indicating potassium vapor accessing minerals through the pores (Fig. 6(a)). These cracks were formed due to a significant volume expansion of coke minerals, according to mechanisms proposed by Van Niekerk et al. [12]. They concluded that potassium facilitated the formation of a liquid phase in coke minerals at high temperatures by lowering their melting point, which was accompanied by their expansion and a reduction in the strength of coke ash. 3.3.3. Transformation of coke minerals after penetration by sodium vapor Zhao and Cheng [7] proposed that sodium vapor were only adsorbed on the coke surface and would not react with coke minerals or coke carbon. In the present study, sodium-bearing phases were observed in coke minerals after penetration by sodium vapor (Fig. 4(c)), a result similar to that observed for potassium vapor. All alumino-silicates present in the original coke samples appear to be alkalized by sodium vapor. In addition, Na2CO3 (as determined from the relative atomic proportions of constituents) was observed near the pore surface when Na/Coke ratio increased to 5/100, as shown in the left side of SEM image of Fig. 4(c) (B). This result suggests the possibility of Na2CO3 forming in the presence of excessive sodium. From the XRD pattern of coke samples before and after the penetration of sodium (Fig. 5(b)), the characteristic peaks of sodium alumino silicate (Na6Al4Si4O17) can be clearly observed, indicating the formation of sodium bearing new phases. The

Fig. 5. (a) X-ray diffraction pattern of Coke before and after the potassium vapor penetration at 1300 °C (Coke 2-K5); (b) X-ray diffraction pattern of Coke before and after the sodium vapor penetration at 1300 °C (Coke 1-Na5). In both (a) and (b): A is normalized XRD pattern; B is un-normalized XRD pattern of graphite peak.

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Fig. 6. (a) Cracks of coke minerals caused by the expansion of minerals after penetration by potassium vapor (Coke 1-K5); (b) cracks of coke minerals caused by the expansion of minerals after penetration by sodium vapor (Coke 1-Na5).

formation of the sodium-bearing phases could occur through the following reaction pathway:

Al—Si—OðmeltÞ þ OðgasÞ þ NaðgasÞ ! Al—Si—a—OðmeltÞ

ð3Þ

Due to the volume expansion on the formation of sodium aluminum silicate, cracks were also observed in the coke minerals after the penetration of sodium vapor (Fig. 6(b)). However, the degree of expansion of sodium alumino silicate was smaller than that of kalsilite. A small amount of Na2SO4 was observed after the alkalization by sodium vapor (Fig. 6(b) A, Point 2), and this phase existed mainly in coke pores and did not cause an expansion of the coke matrix. The alkalization of minerals with Na therefore had a less destructive effect on coke characteristics.

3.4. Effect of alkalization on the high temperature properties of coke Coke samples with required particle size (23–25 mm) after the alkalization process were used to test the CRI and CSR values; coke fines were not included in these measurements. Fig. 7(a) shows the evolution of high temperature characteristics of Coke 1 with increasing K/Coke and Na/Coke ratios. The CRI values were found to increase with increasing alkali concentration, while CSR values showed an opposite trend. In contrast to the Ref. [7], where potassium has been shown to have a stronger catalytic effect on coke gasification and destruction of coke strength than sodium, the catalytic effect of sodium and potassium appeared to be quite similar in this study. The reduction in coke strength by sodium was

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Fig. 7. Effect of alkalization on CRI and CSR values of coke: (a) variation of CRI and CSR of Coke 1 after the penetration of potassium and sodium vapors at 1300 °C; (b) the effect of potassium vapor on CRI and CSR values of different cokes.

somewhat stronger than that by potassium. The CRI values of Coke 1 alkalized with sodium were slightly higher than that of Coke 1 alkalized with potassium for K/coke ratios of 1/100, 3/100 and 5/ 100. The differences between CRI values became smaller with increasing ratios, and were negligible for K/Coke and Na/Coke ratios of 7/100. However the CSR values of Coke 1 alkalized with potassium were always lower than those alkalized with sodium. In a significant contrast to previous studies [7,21,25] where alkalis vapor was produced and mixed with CO2-CO gases to react with coke at 1100 °C, the alkalization process and the gasification process were separated in the present study. Thus the catalytic effect was caused by the K-bearing or Na-bearing mineral phases formed during the alkalization process, and not just by the alkalis vapor. Fig. 7(b) shows the effect of potassium vapor on high temperature properties (CRI/CSR) of different cokes. It can be seen that the CRI values of Coke 2 became higher than those of Coke 1 for K/Coke ratios above 3/100 even though the original CRI value of Coke 1 was higher than that of Coke 2. The CSR values of Coke 2 were always higher than those of Coke 1 except for K/Coke of 5/100; however the gap became quite small for K/Coke of 7/100. Thus Coke 1 seems to possess a stronger resistance to the attack of alkalis vapor than Coke 2 even though its original properties were worse than that of Coke 2. This result indicates that Coke 1 may have a similar performance in the high temperature zone of a blast furnace. This result is consistent with Babich et al.’s [25] conclusion that the catalytic effect of alkalis appears strongly for low CRI cokes and almost disappears for high reactivity cokes. It should be pointed out that the CRI and CSR values were from the cores (above 23 mm) of coke lumps after the alkalization due to the limitation of standard test procedure for CRI/CSR. The coke fines generated by K penetration may possess higher reactivity than the coke cores because the coke fine structures were severely changed by K vapors (see Fig. 3(a)) and the existence of active element K in the coke fines may promote the reactivity greatly. Further research is required to investigate the reactivity of coke cores and coke fines after the K penetration with non-standard methods such as thermogravimetry. Due to the above limitation, the variation of CRI/CSR caused by the Na penetration tested in this study can indicate the real influence of Na vapor on coke CRI/CSR in a industrial BF, while that caused by the K penetration tested in this study may be not as strong as that in a industrial BF because

the coke fines generated cannot be considered in the standard CRI/CSR test procedure. The correlation of alkalis/coke ratios with CRI/CSR values provides a guidance for the control of alkalis content in the charged materials and the selection of appropriate coke for blast furnace. This study (Fig. 7) has shown that there is a rapid degradation of coke properties when alkalis/coke ratio reaches 5/100. Thus the allowed highest limitation of alkalis in an operation blast furnace can be estimated with the obtained alkalis/coke ratio and the coke ratio of the blast furnace. With the pervious established mathematical model [26–28] for alkalis enrichment in a blast furnace, which is not in the scope of this study due to limited space, the corresponding limitation of input alkalis content of raw materials can be calculated, which can provide significant guidance for the control of alkalis input for different blast furnace with different operation conditions. In addition, the resistance of coke to alkalis attack can be estimated with these correlations. With the current evaluation standard of coke quality according to which low CRI and high CSR are better than high CRI and low CSR, Coke 1 (CRI is 33.43, CSR is 60.15) is not as good as Coke 2 (CRI is 21.80, CSR is 70.96). However, as Coke 1 has a stronger resistance to alkalis attack than Coke 2, it may be a better choice for blast furnace with high input content of alkalis. As different optical components were reported to have different resistance to alkalis attack [2], further research is required regarding a more detailed study on the degradation mechanism of different optical components of coke by alkalis. 3.5. Reaction mechanisms From Fig. 5(a) and (b), it can be seen that the graphite peak became broader and less prominent after the penetration of potassium and sodium vapors, indicating increasing disorder and a possible intercalation of these elements between carbon layers. The intercalation of carbon layers by potassium was proposed by Van Niekerk et al. [12]; Akuzawa et al. reported a similar phenomenon for sodium [29]. From Fig. 4(b) and (c), it can be seen that potassium and sodium can coexist with carbon around potassium and sodium-bearing phases respectively, indicating a possible existence of intercalation compounds. Previous researches on reactions of non-graphitic carbons with potassium suggests that coke degradation is essentially the result of intercalation of potassium into

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the carbon structure [30–34]. Thermodynamic calculations indicate that the formation of intercalation compounds is likely under reducing environment at high temperatures, i.e. 1127 °C and above [35,36], thus the thermodynamic conditions of this study were suitable for the formation of intercalation compounds. However, the presence of intercalates could not be detected with XRD probably due to their relatively low concentrations and chemical instability. This difficulty has been reported previously as well [16,37]. Volume expansion is an important factor causing coke degradation in the presence of alkali gases. Both the formation of alkalized minerals and that of graphite intercalation compounds will lead to volume expansion. Detailed information regarding the unit cell of quartz [38], mullite [39], kalsilite [40], potassium aluminum silicate [41] and sodium aluminum silicate [42] were obtained from the published results. Based on this data, the crystal structure of minerals can be drawn and the volume of the unit cell calculated, as show in Fig. 8. From Fig. 8(c–e), it can be seen that the atomic radii of potassium and sodium are greater than that of silicon,

aluminum and oxygen, and the introduction of alkali atoms into minerals caused local relaxation and an increase in the volume and complexity of crystal structures. The volume expansion from hexagonal quartz to hexagonal kalsilite, orthorhombic potassium aluminum silicate and cubic sodium aluminum silicate was determined to be 77.50%, 975.84% and 243.67% respectively, whereas the corresponding increase during the transformation of orthorhombic mullite to three alkalized minerals was estimated to be 19.87%, 626.51% and 132.08% respectively. As both kalsilite and potassium aluminum silicate are likely to form after alkalization with K and only sodium aluminum silicate was detected after the alkalization with Na, the expansion of K-bearing minerals was expected to be greater than that by of Na-bearing minerals. The intercalation of donor intercalants will result in lattice expansion of graphite. From the available literature, the unit cell volume of graphite is only 35.54 Å3 [43], whereas the volume of GICs with higher stage than NaC64 has been proposed to be greater than 294.60 Å3 (unit cell volume of NaC64 [44]). Thus the volume

Fig. 8. Two models (ball and stick model and space-filling model) of mineral structures revealing different aspects of the crystal structure evolution as well as the volume expansion after the alkalization process.

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expansion of graphite to GIC could be up to ten times or even higher, which is much greater than that expected from the alkalization of minerals. The GICs would cause expansion of semi-graphitic layers of the coke matrix and result in both chemical and mechanical weakening of the structure.

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The degradation of coke lumps upon exposure to Na and K vapors at 1300 °C has been summarized in Fig. 9(a). The alkalis vapor penetrates in the coke matrix through pores and are adsorbed on the pore walls and on the surface of exposed mineral particles. As the temperature increases in the alkalization process,

Fig. 9. (a) A schematic representation for the degradation mechanisms of alkali vapors on coke; (b) a partially alkalized coke mineral illustrating the alkalization process and the formation mechanisms of cracks at the frontier of alkalization (observed in Coke 1-K1).

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micro cracks are formed that allow alkali vapors to penetrate into the coke carbon matrix and make further contact with minerals. The interaction of alkali vapors with micro graphite crystals of coke leads to the formation of intercalation compounds which can cause an expansion of the coke carbon matrix, and subsequently form more cracks. The reactions of alkalis with coke minerals led to the formation of kalsilite and sodium aluminum silicates which was accompanied by a volume increase and a further loss of the coke structure integrity. The alkalization mechanism of coke minerals by potassium vapor was clearly confirmed by partially alkalized coke mineral as shown in Fig. 9(b). The gray phases were mainly original silicon oxides, while the surrounding gray white phases were an alkalized phase. Cracks were clearly observed at the interface between the primary phases and alkalized phases. The catalyzed gasification of coke carbon by alkalized coke minerals has been proposed as another important mechanism of coke degradation.

charged materials. Further research is required to construct a novel mathematical model coupling the results of this experimental method with the traditional recycling model of alkalis in a blast furnace. The reactivity and behavior of coke fines generated by K penetration in an industrial BF should also be further studied.

Acknowledgements This work was financially supported by the Open Foundation of the State Key Laboratory of Advanced Metallurgy (41603007), the National Natural Science Foundation of China and Baosteel Group Co., LTD of Shanghai for the Key Joint Project (U1260202), and the National Science Foundation for Young Scientists of China (51304014). References

4. Conclusions An experimental approach has been developed to investigate the influence of potassium and sodium vapors on the structure mineral matter and high temperature behavior of blast furnace cokes. Key findings of this study are: (1) Coke textures were observed to peel off layer by layer after alkalization by potassium vapor; macro fissures were observed for K/Coke ratios above 3/100. Coke samples alkalized by sodium vapor did not show such peeling behavior. (2) In a significant contrast to original coke minerals, a number of potassium-bearing and sodium-bearing phases were detected with SEM/EDS after the alkalization by potassium and sodium vapors. XRD investigation confirmed the formation of kalsilite or potassium alumina silicate (KAlSiO4) and sodium alumina silicates (Na6Al4Si4O17). The formation of intercalation compounds could not be confirmed in the alkalized coke samples due to the chemically instability of intercalation compounds. (3) The catalytic effect of sodium and potassium-bearing minerals on coke gasification appeared to be quite similar; the degradation of coke strength by sodium vapor was found to be somewhat stronger than that by potassium. Even so, potassium showed a stronger damage capability to coke properties due to the coke fines generated in high amount by K penetration. (4) The interaction of alkali vapors with micro graphite crystals of coke led to the formation of intercalation compounds which can cause an expansion of the coke carbon matrix and promote the formation of cracks. The interaction of alkali vapors with coke minerals led to the formation of kalsilite, potassium or sodium aluminum silicates which caused a lattice expansion, subsequent cracks and also catalyzed the gasification of coke carbon. (5) The severe degradation of coke quality caused by alkali vapors was attributed to their strong influence on both coke carbon matrix and coke minerals, as well as their catalytic effect on carbon gasification reaction. (6) The resistance capability of coke to alkalis attack can be evaluated with the method proposed in this study, which provides a guidance for the selection of appropriate coke for a blast furnace. With the obtained alkalis/coke ratio at which coke quality was decreased rapidly, the highest limitation of alkalis in an operation blast furnace can be estimated using the recycling model of alkalis in a BF, which provides a guidance for the control of alkalis content in the

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