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Mineralogy and reactivity of cokes in a working blast furnace
FUPROC-03694; No of Pages 8 Fuel Processing Technology xxx (2013) xxx–xxx
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Mineralogy and reactivity of cokes in a working blast furnace Sushil Gupta a,⁎, ZhuoZhu Ye a, 1, Byong-chul Kim a, c, 1, Olavi Kerkkonen b, Riku Kanniala b, Veena Sahajwalla a, 1 a b c
Centre for Sustainable Materials Research and Technology, School of Materials Science & Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Rautaruukki Oyj, Raahe, Finland Hyundai Steel Company, Dangjin, Chungnam, Korea
a r t i c l e
i n f o
Article history: Received 31 August 2012 Received in revised form 5 February 2013 Accepted 11 February 2013 Available online xxxx Keywords: Blast furnace Tuyere-drilling Coke Mineralogy Reactivity
a b s t r a c t Coke samples from tuyere level of a blast furnace were obtained through tuyere drilling. Mineral matter of tuyere level cokes was quantified using SIROQUANT and examined using scanning electron microscope. The apparent CO2 reaction rates were measured using a fixed bed reactor. About 50% of the total inorganic matter of tuyere cokes was found to occur as amorphous or glassy phase. At most of the tuyere level locations, Quartz and mullite contents of the feed coke decreased significantly or disappeared. Silicon carbide and gupeiite were found to be the most notable and common silicon and iron bearing minerals of cokes particularly in the raceway region while gehlenite and spinel were the typical calcium and magnesium bearing minerals respectively. Tuyere level cokes also indicated the presence of significantly high potassium levels as well as the graphite formation. The apparent reaction rate of tuyere level cokes is shown to increase up to ten times of the feed coke reaction rate, and is strongly related to the total amount of potassium species. The study highlights the strong influence of the raceway temperature, the alkali loading and the hot metal presence on the modification of the mineralogy and reactivity of tuyere level cokes with implications on coke selection criterion. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Coke is essential and the most expensive material for blast furnace process for ironmaking. Due to growing cost and increasing environmental concerns, efficient utilization of coke has become one of the top priorities of the iron producers. Coke performs many roles in a blast furnace (BF); thermal role to provide heat energy; chemical role to reduce iron oxide ore, and mechanical role to provide suitable permeability for upward flowing gases. High strength coke is desired for maintaining stability of blast furnace operation. Often, supplementary fuels such as coal, oil or natural gas are injected through tuyeres to replace some percentage of coke. At high injection rates, the thickness of coke layer in the cohesive zone decreases as well as the total residence time in the blast furnace increases. At high fuel injection rate, the mechanical strength as well as high temperature coke strength after reaction (CSR) become more critical, as less coke is available to perform both mechanical and chemical roles. Therefore, high strength coke is always desired under the belief that it will resist coke fine generation and hence accumulation in lower zone of a blast furnace.
⁎ Corresponding author. Tel.: +61 2 9385 4433. E-mail address: [email protected] (S. Gupta). 1 Tel.: +61 2 9385 4433.
Effect of coal organic matter such as the rank and the maceral types on coke strength and reactivity has been extensively studied [1–3]. In recent years, there has been growing interest to examine the role of inorganic matter of cokes on the reactivity and hence on coke strength in high temperature zones of a blast furnace. Coke reactivity is influenced by the minerals present in original coke as well as their modifications that occur in severe reacting conditions including with recirculating alkalis in a blast furnace. The presence of iron, the alkalis and the alkaline earths bearing phases generally increase the gasification reactivity of carbonaceous materials including coke [4–13]. Recirculating potassium present inside a blast furnace has been reported to enhance the solution loss reactivity of blast furnace cokes [14–16]. Due to the complexity and the severity of high temperature reaction environment, it is often inconvenient to get coke samples from a working blast furnace. In past, a number of studies have been reported about blast furnace coke properties [17–30]. However, limited studies have considered inorganic elements in their true mineral forms rather than as oxides. Therefore, understanding the role of coal minerals on coke mineralogy and their transformations at tuyere-level is an exciting area of interest. Quantitative mineralogy of tuyere cokes is required to further improve the understanding on the implications of coke mineral transformations on blast furnace operations. This paper summarizes and quantifies the different types of minerals which form within cokes as they reach tuyere-level of a working blast furnace and their association with the reactivity.
0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.02.009
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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S. Gupta et al. / Fuel Processing Technology xxx (2013) xxx–xxx
2.2. SIROQUANT analysis Coke minerals were quantified using carbon-free mineral sample which was prepared using radio-frequency low-temperature oxygen plasma ashing. Philips PW1050 goniometer located at the CSIRO, Sydney was used for mineral analysis. The XRD spectra of powder ash samples were obtained using CuKα radiation at 40 kV and 35 mA by scanning over a 2Theta range from 2–90° with a step interval of 0.04° 2θ and 5 s count time per step. SIROQUANT™, a personal computer quantitative X-ray diffraction analysis software, was used to process the XRD spectra to quantify the mineral phases [31]. 2.3. Fixed bed reactor
Fig. 1. Tuyere drilling equipment used to extract the coke samples.
2. Materials and methods 2.1. Tuyere drilling In this study, tuyere-level coke samples were obtained from a medium sized blast furnace (#1) of 8 m hearth diameter of Rautaruukki Oyj Steel Works at Raahe, Finland. The effective volume of the blast furnace was 1086 m3 and was equipped with 21 tuyeres. A specially designed mobile tuyere drilling equipment was employed to extract the coke samples from one of the tuyeres (Fig. 1). A steel probe was pushed into the furnace during a stoppage through an opened tuyere and quenched after withdrawing. A typical high strength coke (CSR ~ 66) was used during the drilling campaign. Coke was introduced into the burden using two bells with a two chamber charging system. Preheated, heavy distillation residue oil was injected through tuyeres at the rate of 102 kg/thm while coke rate was 400 kg/thm. Further detail of operating conditions and related data can be found elsewhere [17]. Fig. 2 shows the steel probe after withdrawing from the furnace illustrating a wide size range of coke particles. The core material was about 60 kg and divided into ten sections of 25 cm each [17]. Metal and slag particles were sticking to several coke particles particularly in the deadman region. Core material was manually sieved into a number of size fractions. Coke lumps from the largest sieved group (+ 19 mm) were selected to represent the cokes arriving at tuyerelevel. Clean pieces of cokes were carefully selected to minimise the slag or metal contamination. This paper includes the analysis of the coke lumps (+19 mm) only which are referred as the tuyere cokes for convenience throughout the discussion.
Apparent CO2 reaction rate of tuyere cokes was measured using a fixed bed reactor as detailed elsewhere [17]. Measurements were carried out under optimized condition to avoid any effect of particle size and flow rates so that reaction can occur uniformly throughout coke particle. The coke specimens were crushed to a narrow size range of approximately 0.6–1 mm to minimise the effect of particle size variation. Crushed samples were dried overnight at a temperature of 378 K. About 1.2 g of the specimen was supported on a sintered glass frit inside the quartz tube placed in an electrically heated furnace. A mass-flow controller was used to maintain a gas flow rate of 0.75 L per minute through the sample bed from top to bottom. A thermocouple was used to monitor the sample bed temperature. Carbon dioxide was passed through an oxygen trap to remove traces of oxygen and moisture before reaction. The CO concentration of the exhaust gas was continuously monitored by an infrared analyzer via computer to calculate the apparent reaction rates. A high resolution field emission scanning electron microscope (FESEM) (Hitachi 4500II) fitted with an Oxford ISIS energy dispersive X-ray analyzer (EDS) was used to examine the presence of inorganic phases. Specimens were prepared by embedding coke pieces in resin followed by polishing and carbon coating. 3. Results and discussions Majority of inorganic matter of coke comprises four elements silicon, aluminium, iron and calcium with small amounts of alkalis, alkaline earths as well as sulphur and phosphorus. Often these elements are represented as oxides as shown in Table 1. Table 2 provides the percentage of various minerals present in the low temperature ash specimens of feed coke and the coke samples from ten locations across the tuyere entrance. On the basis of major elemental abundance, the minerals discussed in this paper are divided into four groups. 3.1. Si and Fe bearing minerals The reaction environment such as the temperature and the gas composition of a blast furnace varies from tuyere entrance to the deadman. On the basis of reaction zones, ten sections of core are combined to represent three tuyere-level zones. During drilling, core material was approximately 50% compacted. Therefore, a total 2.5 m long material in the probe represents coke samples from the tuyere entrance to the centre of the blast furnace (>4 m). The locations from 25 cm–100 cm
Fig. 2. The steel probe showing an overview of core material.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Table 1 Proximate and oxide analysis of cokes. DC1
DC2
DC3
DC4
DC5
DC6
DC7
DC8
DC9
DC10
Proximate analysis (%, adb) Moisture 0.50 Ash yield 11.2 Volatile matter 1.10 Fixed carbon 87.2
Feed coke
0.10 10.8 0.30 88.8
0.10 13.0 0.20 86.7
0.20 10.7 0.10 89.0
0.10 12.3 0.00 87.6
0.20 12.6 0.10 87.1
0.20 13.2 0.10 86.5
0.10 21.2 0.00 78.7
0.10 25.7 0.00 74.2
0.10 27.7 0.00 72.2
0.20 37.5 0.00 62.3
Elements as oxides (wt.%) SiO2 6.50 Al2O3 3.05 TiO2 0.17 Fe2O3 0.67 CaO 0.30 MgO 0.09 K2O 0.14 Na2O 0.05 SO3 0.11 P2O5 0.07 Mn3O4 0.01 V2O5 0.01 SiO2/Al2O3 2.13
4.56 3.21 0.20 1.31 0.39 0.48 0.27 0.13 0.13 0.03 0.04 0.01 1.42
4.38 3.08 0.20 3.70 0.97 0.19 0.11 0.07 0.20 0.03 0.03 0.01 1.42
2.49 2.85 0.18 1.61 0.75 1.40 0.43 0.17 0.49 0.05 0.19 0.02 0.88
4.00 3.07 0.18 1.83 0.58 1.03 0.63 0.33 0.40 0.04 0.15 0.02 1.30
4.60 3.10 0.19 1.43 0.62 0.94 0.75 0.35 0.39 0.04 0.14 0.01 1.48
4.87 3.41 0.17 1.25 0.58 0.84 1.04 0.39 0.42 0.04 0.14 0.01 1.43
5.09 2.79 0.21 7.22 2.11 1.18 0.99 0.36 0.95 0.05 0.19 0.03 1.83
4.94 2.17 0.21 11.51 2.79 1.11 1.22 0.52 0.90 0.06 0.18 0.06 2.27
4.95 2.24 0.22 13.89 2.07 0.81 1.57 0.78 0.89 0.05 0.16 0.05 2.21
6.31 2.60 0.34 18.37 4.09 1.38 1.99 1.03 0.99 0.07 0.21 0.09 2.43
represent the raceway region while the locations from 100 cm to 200 cm indicate the birdsnest and the locations beyond 200 cm represent the deadman zone of blast furnace. Different tuyere regions are distinguished by grey shades in the figures of this paper.
Fig. 3 shows the variation of coke ash parameters at various locations from the tuyere entrance. In the raceway region the ash yield of tuyere cokes does not change significantly (Fig. 3a). However, the ash yield increases progressively beyond the birdsnest zone locations.
Table 2 Mineral phases of low temperature ash specimens of cokes (wt.%). Mineral phase
Mineral yield Quartz Mullite Sanidine Silicon carbide (cubic) Silicon carbide-H Pyrrhotite Hematite Magnetite Gupeiite Xifengite Wuestite Fersilicite Iron Gehlenite Anorthite Merwinite Monticellite Diopside Oldhamite Bassanitea Enstatite ferroan Spinel Hexahydrite Epsomite Periclase Syngenite Jarosite Picromerite Leonite Aphthitalite Anatase Rutile Alumina Corundum Osbornite Graphite Amorphous a
Formula
Mineral phases (wt.%)
Coke codes →
Feedd
DC11
DC21
DC31
DC41
DC51
DC61
DC71
DC81
DC91
DC101
SiO2 Al6Si2O13 K[AlSi3O8] SiC SiC Fe1−xS Fe2O3 Fe3O4 Fe3Si Fe5Si3 FeO FeSi Fe Ca2Al2SiO7 Ca·[Al2Si2O8] Ca3·Mg[SiO4]2 CaMg[SiO4] CaMg[Si2O6] CaS 2CaSO4(H2O) Mg silicate-mix MgAl2O4 MgSO4·6H2O MgSO4·7H2O4·7H2O MgO K2·CaSO4·H2O KFe3(SO4)2(OH)6 K2Mg(SO4)2·6H2O K2Mg(SO4)2·4H2O (K,Na)3·Na(SO4)2 TiO2 TiO2 Al2O3 Al2O3 TiN C
11.2 18.1 16.5 – – – 2.3 0.0 1.3 – – – – 1.0 – – – – – – 3.2 – – – – – – 1.8 – – – 0.6 0.4 – – – – 51.6
10.6 0.6 0.9 – 4.8 1.5 – – – 3.9 – 1.7 – 0.3 0.3 0.5 – – 0.9 – 2.6 0.9 4.0 0.2 – – – 1.1 – – – – – 5.6 1.3 0.3 – 67.6
18.6 4.3 0.1 – 5.4 – – – – 11.3 1.5 1.1 – 1.5 9.7 – – – – 2.6 7.5
26.0 0.9 0.6 – 2.0 – – – – 7.2 0.4 0.8 – 3.0 14.4 – 5.4 – – 2.7 6.2
13.6 1.2 1.2 1.0 1.6 – – – – 6.1 – 0.8 – 1.1 – – – – – 0.2
2.5 – – – – 0.5 – – – – – 7.9 – – 5.8 38.3
6.3 – – – – 0.7 – – – – – 1.7 – 1.1
12.1 3.1 – – 8.6
13.5 1.5 1.0 – 1.8 – – – – 3.9 – 0.4 – 0.3 – – – – – 0.9 1.4 1.4 8.0 0.9 – – 1.4 0.1 6.3 – 0.5 – – – 2.1 0.3
13.5 1.5 0.3 – 1.9 – – – – 4.3 – 0.6 – 0.7 – – – – 1.1 0.8 – 1.6 8.3 – – – 6.5 0.4 4.8 – – – – 0.3 1.7 0.5 0.9 63.9
27.7 0.5 – – 0.2 – – – – 2.2 – – – 2.3 10.6 – 2.7 – – 2.5 1.9 1.7 3.5 – 0.5 – 2.5 – 3.0 –
33.3 0.3 – – – – – – – 1.3 – – – 2.5 11.4 – 10.4 1.2 – 1.4 0.2 – 2.3 – – 1.2 5.4 – – 1.0 2.4 – – – – 0.8 7.4 51.8
23.1 0.2 0.9 – – – – – – 0.7 0.1 – 0.5 1.2 2.2 – – – – – 0.8 – 1.4 0.6 –
46.0 0.4 0.7 – – – – – – – – – – 3.3 11.9 – 20.0 – – – 1.4 – 1.0 – 1.2 1.5 – – – – 10.2 – – – – 0.6 4.1 42.9
46.6
3.3 – – – 0.5 – 1.3 7.6 50.3
67.7
– – – – 0.7 65.3
4.8 – 1.4 1.8 12.4 – – – – 0.5 71.7
Sulphate phases including bassanite and jarosite are formed during low temperature ashing.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 5. Variation of potassium (oxide basis) content of cokes with their locations in the probe from tuyere entrance.
Fig. 3. Variation of the ash yield (a), silica to alumina ratio (b) and iron oxide content (c) of tuyere cokes with the distance of their locations in the probe from the tuyere entrance.
High ash yield in the deadman region can be attributed to several factors such as the high degree of gasification due to prolonged stay, the adsorption of recirculating alkalis as well as the penetration of few slag and metal droplets in some cases. Unusual changes in the ash chemistry of coke samples from any adjacent 25 cm sections within the same tuyere zone can provide an indication of the presence of slag or hot metal presence in a given coke specimen. Silica to alumina ratio of cokes (~ 1.5) at most of the tuyere locations is significantly
less compared to that of feed coke (2.09) (Fig. 3b). This shows a substantial loss of coke silica as it reaches to the tuyere regions. Total iron oxide content of coke increases significantly in the deadman region (Fig. 3c). Fig. 4 shows microscopic images of typical tuyere cokes from three tuyere zones. Fig. 4b clearly shows the hot metal droplets sticking on the tuyere cokes as bright spots. It was difficult to completely remove the penetrated molten iron from cokes in all the samples leading to high iron levels in the deadman regions (Fig. 3c). Potassium content of tuyere coke increases significantly compared to that of the feed coke (Fig. 5). This is attributed to the adsorption of recirculating potassium in the blast furnace. Potassium content of deadman cokes is more than fifteen times of that present in the feed coke. The amount of potassium intake by cokes at tuyere level is also found to be inversely proportional to the temperature of the tuyere zone as well as the degree of coke graphitization [17]. Fig. 6a and b compares the cross-sections of the feed coke and a typical raceway coke respectively. The raceway cokes are distinguished by relatively enlarged pores with inorganic species often occurring along the pore boundaries. The pores were more apparent in the cokes from relatively higher temperature zones such as the raceway zone, which is most likely related to higher intensity of in-situ reduction of coke minerals. SiC is observed mainly in the raceway region cokes and is almost absent in the birdsnest and deadman cokes (Fig. 7a). In the raceway zone, high amount of SiC of tuyere cokes (Fig. 7a) is also accompanied by low amount of residual SiO2 polymorphs (Fig. 7b) as well as low mullite content (Fig. 7c). The presence of SiC in the raceway coke highlights the strong influence of the tuyere temperatures on the amount
Fig. 4. Steel probe images illustrating cokes present in the raceway, birdsnest and deadman sections.
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Fig. 6. SEM images of the cross-sections of the feed coke and typical raceway coke.
Fig. 7. (a) Variation of SiC (b) quartz and (c) mullite content of the cokes with their locations in the probe from tuyere entrance.
of SiC formation. It may be noted that local gas composition could also influence the amount of SiC formation. Fig. 8 compares the variation of different iron bearing phases present in the tuyere cokes at different locations. Wuestite is absent at most of the birdsnest and deadman locations (Fig. 8a). In the raceway region, metallic iron showed an opposite trend with the distance from tuyere entrance (Fig. 8c) as shown by wuestite. Some of iron occurs as silicides which decrease with increasing distance of the locations of cokes from tuyere side (Fig. 8b). The presence of large size of some ferrosilicon phases suggests that the interaction of molten iron and silica phases of coke could have significant contribution. The ferro-silicon phases are also known form due to in-situ reduction within coke [13]. Therefore, the formation of ferro-silicon alloys in tuyere cokes will be influenced by the interaction of hot metal with coke minerals as well as in-situ reduction of original coke minerals. Fig. 9 illustrates typical Si bearing phases in some of the raceway cokes. Fig. 9a shows the presence of elemental Si along pores which may be attributed to in-situ reduction of quartz as silicides and silicon carbide has been reported to form due to carbothermal reduction of silicon mineral with pyrite in chars at more than 1673 K [32]. Fig. 9b illustrates the formation ferro-silicon (most likely gupeiite) formed due to molten iron interactions. Fig. 9c illustrates the dispersion of fine ferro-silicon grains in the carbon matrix and consequences on the modifications of carbon matrix of coke. Further studies are continuing to clarify the implications of ferro-silicon formation on tuyere coke. Table 2 also shows the presence of graphite crystals in tuyere cokes. Total percentage of graphite in tuyere coke did not clearly relate with the temperatures of various tuyere zones which could be related to
differences in the presence of catalysing species for graphitisation such as hot iron at different locations [32,33]. 3.2. K and Na bearing minerals Tuyere cokes showed increased levels of potassium and sodium compared to their concentration in the feed coke (Table 1). However, the potassium content of tuyere cokes increased significantly due to recirculating alkalis (Fig. 5, Table 1). Potassium species are mainly formed in the tuyere cokes outside the raceway region, and increase along the deadman zones (Fig. 10). Total amount of potassium expressed as oxide basis did not relate to the total percentage of the identified potassium minerals. This can be attributed to the fact that a part of total potassium occurred as non-crystalline silicate phases. Some of the potassium and sodium are observed as sulphate in low temperature ash samples and are not related to the actual mineralogy of tuyere cokes (Table 2). Sulphate minerals such as jarosite or bassanite are often formed as a consequence of the interaction of potassium or sodium species with residual sulphur in tuyere cokes. In tuyere cokes, potassium could occur as intercalated graphite phase or dispersed between graphene layers [9,18]. The XRD peaks of intercalated potassium compounds were not seen in the tested tuyere coke. Raceway cokes from relatively higher temperatures had a narrow interspacing of carbon layers, and also showed lower amount of potassium. Potassium retention in tuyere cokes was inversely related to both temperature as well as the extent of graphitization. Therefore, on the basis of our
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 10. Variation of K bearing phases of cokes with their locations in the probe from tuyere entrance.
tuyere cokes such as in the deadman is most likely attributed to slag contamination. Majority of new calcium bearing phases of the tuyere cokes were absent in the low temperature ash specimen of feed coke [17]. Gehlenite is observed to be the most common calcium bearing phase in tuyere cokes (Table 2). Fig. 12a illustrates the formation of calcium and magnesium silicate such that grey spots show the calcium silicate phase. Some of the calcium in tuyere cokes is also stabilised as oldhamite (Table 2), which is believed to display high thermal stability. The presence of bassanite in tuyere cokes is the consequence of low temperature ashing. Spinel is the most common magnesium bearing mineral of the tuyere cokes (Table 2). On the basis of the EDS composition, a typical spinel phase of tuyere coke is identified in Fig. 12b. 3.4. Apparent CO2 reactivity and mineralogy
Fig. 8. Variation of wuestite (a), gupeiite (Fe3Si) and (c) metallic iron content of cokes with their locations in the probe from tuyere entrance.
observations and previous reported studies [9,18], potassium in tuyere cokes was most likely dispersed between carbon planes which transform to sulphate during low temperature ashing. 3.3. Ca and Mg bearing minerals In general the percentage of calcium bearing minerals of coke do not increase significantly except at few locations in the deadman region (Fig. 11). The presence of high calcium phases in some of the
The apparent reaction rates of tuyere coke increase with distance from the tuyere entrance (Fig. 13). The reaction rate of the deadman coke is the highest and about more than ten times the apparent reaction rate of the feed coke. The variation of reaction rate of cokes across different tuyere zones is related to the amount of recirculating potassium adsorbed in cokes which is also inversely interrelated to the temperatures of the different tuyere zones. The apparent reaction rates of cokes did not give a clear correlation with total amount of all iron bearing phases. However, the reaction rate of tuyere cokes with high amounts of metallic iron and wuestite is high (Fig. 14a). Fig. 14b shows that the tuyere coke reactivity increases with the total amount of potassium bearing species. Fig. 14b further shows that the reaction rate of tuyere cokes does not change significantly after the K bearing minerals in coke become about 1.5%. This means, any further increment of potassium levels, such
Fig. 9. (a) Illustration of in-situ reduction of silica, (b) gupeiite (Fe3Si) type and (c) dispersion of ferro-silicon alloys in carbon matrix of tuyere cokes.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 11. Variation of Ca bearing minerals of cokes with their locations in the probe from tuyere entrance.
as that occurs in the birdsnest and deadman zones, may not further increase the reaction rate of tuyere cokes. The apparent reaction rate of tuyere coke can also be influenced by other factors such as the differences in the ordering of carbon structure of cokes [17]. However, within the raceway region, high reaction rate of cokes cannot be attributed to their increased carbon structure ordering alone. On the other hand, a relatively small increment of the potassium level seems to have greater effect on reaction rate. This shows that the catalytic effect of recirculating potassium has a strong effect on the tuyere coke reaction rate. 4. Conclusions A batch of tuyere coke samples of a working blast furnace was examined to quantify mineral phases and reactivity. The following conclusions were made. 1. The ash yield of tuyere cokes increases from the tuyere entrance to the deadman zone. The study shows that more than 50% of total inorganic matter of cokes at tuyere levels occurs as amorphous or glassy phase. Quartz and mullite, the predominant minerals of the feed coke, were found to significantly decrease or completely disappear at most of the tuyere-level locations. SiC and gupeiite were found to be the most notable new Si bearing minerals in the tuyere cokes, and were most abundant in the raceway regions.
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Fig. 13. Apparent reaction rates of tuyere cokes vs their locations in the probe from tuyere entrance.
2. A number of new calcium bearing new phases were formed in the tuyere cokes. However, gehlenite and spinel were found to be the most common calcium and magnesium bearing minerals of the tuyere cokes respectively 3. The presence of recirculating potassium was found to be the most distinctive feature of tuyere cokes particularly outside the raceway. The study confirmed the presence of graphite formation in cokes at tuyere levels. 4. The apparent CO2 reactivity of tuyere cokes was found to be significantly higher compared to the feed coke reactivity and increases with distance from tuyere entrance. The apparent reaction rate is found to increase with increasing amounts of potassium bearing phases. The study highlights the strong influence of blast furnace conditions such as the raceway temperatures, alkali loading and molten iron on the mineralogy and reactivity of cokes at tuyere levels of a working blast furnace.
Acknowledgement Authors acknowledge the financial support from the Australian Coal Association Research Program as well as the technical support from Rautaruukki Oyj, Finland. We also express sincere thanks for continuous input and support from Dr Dave Osborne, Mr Sid McGuire and Mr Stephen Brant. We also express our sincere thanks to Dr David French for the SIROQUANT measurements.
Fig. 12. (a) Illustration of calcium silicates and (b) spinel phase presence in the raceway cokes.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 14. (a) The apparent reaction rates of tuyere cokes vs potassium bearing minerals and (b) combined metallic iron and wuestite content of tuyere cokes.
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Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Mineralogy and reactivity of cokes in a working blast furnace Sushil Gupta a,⁎, ZhuoZhu Ye a, 1, Byong-chul Kim a, c, 1, Olavi Kerkkonen b, Riku Kanniala b, Veena Sahajwalla a, 1 a b c
Centre for Sustainable Materials Research and Technology, School of Materials Science & Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Rautaruukki Oyj, Raahe, Finland Hyundai Steel Company, Dangjin, Chungnam, Korea
a r t i c l e
i n f o
Article history: Received 31 August 2012 Received in revised form 5 February 2013 Accepted 11 February 2013 Available online xxxx Keywords: Blast furnace Tuyere-drilling Coke Mineralogy Reactivity
a b s t r a c t Coke samples from tuyere level of a blast furnace were obtained through tuyere drilling. Mineral matter of tuyere level cokes was quantified using SIROQUANT and examined using scanning electron microscope. The apparent CO2 reaction rates were measured using a fixed bed reactor. About 50% of the total inorganic matter of tuyere cokes was found to occur as amorphous or glassy phase. At most of the tuyere level locations, Quartz and mullite contents of the feed coke decreased significantly or disappeared. Silicon carbide and gupeiite were found to be the most notable and common silicon and iron bearing minerals of cokes particularly in the raceway region while gehlenite and spinel were the typical calcium and magnesium bearing minerals respectively. Tuyere level cokes also indicated the presence of significantly high potassium levels as well as the graphite formation. The apparent reaction rate of tuyere level cokes is shown to increase up to ten times of the feed coke reaction rate, and is strongly related to the total amount of potassium species. The study highlights the strong influence of the raceway temperature, the alkali loading and the hot metal presence on the modification of the mineralogy and reactivity of tuyere level cokes with implications on coke selection criterion. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Coke is essential and the most expensive material for blast furnace process for ironmaking. Due to growing cost and increasing environmental concerns, efficient utilization of coke has become one of the top priorities of the iron producers. Coke performs many roles in a blast furnace (BF); thermal role to provide heat energy; chemical role to reduce iron oxide ore, and mechanical role to provide suitable permeability for upward flowing gases. High strength coke is desired for maintaining stability of blast furnace operation. Often, supplementary fuels such as coal, oil or natural gas are injected through tuyeres to replace some percentage of coke. At high injection rates, the thickness of coke layer in the cohesive zone decreases as well as the total residence time in the blast furnace increases. At high fuel injection rate, the mechanical strength as well as high temperature coke strength after reaction (CSR) become more critical, as less coke is available to perform both mechanical and chemical roles. Therefore, high strength coke is always desired under the belief that it will resist coke fine generation and hence accumulation in lower zone of a blast furnace.
⁎ Corresponding author. Tel.: +61 2 9385 4433. E-mail address: [email protected] (S. Gupta). 1 Tel.: +61 2 9385 4433.
Effect of coal organic matter such as the rank and the maceral types on coke strength and reactivity has been extensively studied [1–3]. In recent years, there has been growing interest to examine the role of inorganic matter of cokes on the reactivity and hence on coke strength in high temperature zones of a blast furnace. Coke reactivity is influenced by the minerals present in original coke as well as their modifications that occur in severe reacting conditions including with recirculating alkalis in a blast furnace. The presence of iron, the alkalis and the alkaline earths bearing phases generally increase the gasification reactivity of carbonaceous materials including coke [4–13]. Recirculating potassium present inside a blast furnace has been reported to enhance the solution loss reactivity of blast furnace cokes [14–16]. Due to the complexity and the severity of high temperature reaction environment, it is often inconvenient to get coke samples from a working blast furnace. In past, a number of studies have been reported about blast furnace coke properties [17–30]. However, limited studies have considered inorganic elements in their true mineral forms rather than as oxides. Therefore, understanding the role of coal minerals on coke mineralogy and their transformations at tuyere-level is an exciting area of interest. Quantitative mineralogy of tuyere cokes is required to further improve the understanding on the implications of coke mineral transformations on blast furnace operations. This paper summarizes and quantifies the different types of minerals which form within cokes as they reach tuyere-level of a working blast furnace and their association with the reactivity.
0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.02.009
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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2.2. SIROQUANT analysis Coke minerals were quantified using carbon-free mineral sample which was prepared using radio-frequency low-temperature oxygen plasma ashing. Philips PW1050 goniometer located at the CSIRO, Sydney was used for mineral analysis. The XRD spectra of powder ash samples were obtained using CuKα radiation at 40 kV and 35 mA by scanning over a 2Theta range from 2–90° with a step interval of 0.04° 2θ and 5 s count time per step. SIROQUANT™, a personal computer quantitative X-ray diffraction analysis software, was used to process the XRD spectra to quantify the mineral phases [31]. 2.3. Fixed bed reactor
Fig. 1. Tuyere drilling equipment used to extract the coke samples.
2. Materials and methods 2.1. Tuyere drilling In this study, tuyere-level coke samples were obtained from a medium sized blast furnace (#1) of 8 m hearth diameter of Rautaruukki Oyj Steel Works at Raahe, Finland. The effective volume of the blast furnace was 1086 m3 and was equipped with 21 tuyeres. A specially designed mobile tuyere drilling equipment was employed to extract the coke samples from one of the tuyeres (Fig. 1). A steel probe was pushed into the furnace during a stoppage through an opened tuyere and quenched after withdrawing. A typical high strength coke (CSR ~ 66) was used during the drilling campaign. Coke was introduced into the burden using two bells with a two chamber charging system. Preheated, heavy distillation residue oil was injected through tuyeres at the rate of 102 kg/thm while coke rate was 400 kg/thm. Further detail of operating conditions and related data can be found elsewhere [17]. Fig. 2 shows the steel probe after withdrawing from the furnace illustrating a wide size range of coke particles. The core material was about 60 kg and divided into ten sections of 25 cm each [17]. Metal and slag particles were sticking to several coke particles particularly in the deadman region. Core material was manually sieved into a number of size fractions. Coke lumps from the largest sieved group (+ 19 mm) were selected to represent the cokes arriving at tuyerelevel. Clean pieces of cokes were carefully selected to minimise the slag or metal contamination. This paper includes the analysis of the coke lumps (+19 mm) only which are referred as the tuyere cokes for convenience throughout the discussion.
Apparent CO2 reaction rate of tuyere cokes was measured using a fixed bed reactor as detailed elsewhere [17]. Measurements were carried out under optimized condition to avoid any effect of particle size and flow rates so that reaction can occur uniformly throughout coke particle. The coke specimens were crushed to a narrow size range of approximately 0.6–1 mm to minimise the effect of particle size variation. Crushed samples were dried overnight at a temperature of 378 K. About 1.2 g of the specimen was supported on a sintered glass frit inside the quartz tube placed in an electrically heated furnace. A mass-flow controller was used to maintain a gas flow rate of 0.75 L per minute through the sample bed from top to bottom. A thermocouple was used to monitor the sample bed temperature. Carbon dioxide was passed through an oxygen trap to remove traces of oxygen and moisture before reaction. The CO concentration of the exhaust gas was continuously monitored by an infrared analyzer via computer to calculate the apparent reaction rates. A high resolution field emission scanning electron microscope (FESEM) (Hitachi 4500II) fitted with an Oxford ISIS energy dispersive X-ray analyzer (EDS) was used to examine the presence of inorganic phases. Specimens were prepared by embedding coke pieces in resin followed by polishing and carbon coating. 3. Results and discussions Majority of inorganic matter of coke comprises four elements silicon, aluminium, iron and calcium with small amounts of alkalis, alkaline earths as well as sulphur and phosphorus. Often these elements are represented as oxides as shown in Table 1. Table 2 provides the percentage of various minerals present in the low temperature ash specimens of feed coke and the coke samples from ten locations across the tuyere entrance. On the basis of major elemental abundance, the minerals discussed in this paper are divided into four groups. 3.1. Si and Fe bearing minerals The reaction environment such as the temperature and the gas composition of a blast furnace varies from tuyere entrance to the deadman. On the basis of reaction zones, ten sections of core are combined to represent three tuyere-level zones. During drilling, core material was approximately 50% compacted. Therefore, a total 2.5 m long material in the probe represents coke samples from the tuyere entrance to the centre of the blast furnace (>4 m). The locations from 25 cm–100 cm
Fig. 2. The steel probe showing an overview of core material.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
S. Gupta et al. / Fuel Processing Technology xxx (2013) xxx–xxx
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Table 1 Proximate and oxide analysis of cokes. DC1
DC2
DC3
DC4
DC5
DC6
DC7
DC8
DC9
DC10
Proximate analysis (%, adb) Moisture 0.50 Ash yield 11.2 Volatile matter 1.10 Fixed carbon 87.2
Feed coke
0.10 10.8 0.30 88.8
0.10 13.0 0.20 86.7
0.20 10.7 0.10 89.0
0.10 12.3 0.00 87.6
0.20 12.6 0.10 87.1
0.20 13.2 0.10 86.5
0.10 21.2 0.00 78.7
0.10 25.7 0.00 74.2
0.10 27.7 0.00 72.2
0.20 37.5 0.00 62.3
Elements as oxides (wt.%) SiO2 6.50 Al2O3 3.05 TiO2 0.17 Fe2O3 0.67 CaO 0.30 MgO 0.09 K2O 0.14 Na2O 0.05 SO3 0.11 P2O5 0.07 Mn3O4 0.01 V2O5 0.01 SiO2/Al2O3 2.13
4.56 3.21 0.20 1.31 0.39 0.48 0.27 0.13 0.13 0.03 0.04 0.01 1.42
4.38 3.08 0.20 3.70 0.97 0.19 0.11 0.07 0.20 0.03 0.03 0.01 1.42
2.49 2.85 0.18 1.61 0.75 1.40 0.43 0.17 0.49 0.05 0.19 0.02 0.88
4.00 3.07 0.18 1.83 0.58 1.03 0.63 0.33 0.40 0.04 0.15 0.02 1.30
4.60 3.10 0.19 1.43 0.62 0.94 0.75 0.35 0.39 0.04 0.14 0.01 1.48
4.87 3.41 0.17 1.25 0.58 0.84 1.04 0.39 0.42 0.04 0.14 0.01 1.43
5.09 2.79 0.21 7.22 2.11 1.18 0.99 0.36 0.95 0.05 0.19 0.03 1.83
4.94 2.17 0.21 11.51 2.79 1.11 1.22 0.52 0.90 0.06 0.18 0.06 2.27
4.95 2.24 0.22 13.89 2.07 0.81 1.57 0.78 0.89 0.05 0.16 0.05 2.21
6.31 2.60 0.34 18.37 4.09 1.38 1.99 1.03 0.99 0.07 0.21 0.09 2.43
represent the raceway region while the locations from 100 cm to 200 cm indicate the birdsnest and the locations beyond 200 cm represent the deadman zone of blast furnace. Different tuyere regions are distinguished by grey shades in the figures of this paper.
Fig. 3 shows the variation of coke ash parameters at various locations from the tuyere entrance. In the raceway region the ash yield of tuyere cokes does not change significantly (Fig. 3a). However, the ash yield increases progressively beyond the birdsnest zone locations.
Table 2 Mineral phases of low temperature ash specimens of cokes (wt.%). Mineral phase
Mineral yield Quartz Mullite Sanidine Silicon carbide (cubic) Silicon carbide-H Pyrrhotite Hematite Magnetite Gupeiite Xifengite Wuestite Fersilicite Iron Gehlenite Anorthite Merwinite Monticellite Diopside Oldhamite Bassanitea Enstatite ferroan Spinel Hexahydrite Epsomite Periclase Syngenite Jarosite Picromerite Leonite Aphthitalite Anatase Rutile Alumina Corundum Osbornite Graphite Amorphous a
Formula
Mineral phases (wt.%)
Coke codes →
Feedd
DC11
DC21
DC31
DC41
DC51
DC61
DC71
DC81
DC91
DC101
SiO2 Al6Si2O13 K[AlSi3O8] SiC SiC Fe1−xS Fe2O3 Fe3O4 Fe3Si Fe5Si3 FeO FeSi Fe Ca2Al2SiO7 Ca·[Al2Si2O8] Ca3·Mg[SiO4]2 CaMg[SiO4] CaMg[Si2O6] CaS 2CaSO4(H2O) Mg silicate-mix MgAl2O4 MgSO4·6H2O MgSO4·7H2O4·7H2O MgO K2·CaSO4·H2O KFe3(SO4)2(OH)6 K2Mg(SO4)2·6H2O K2Mg(SO4)2·4H2O (K,Na)3·Na(SO4)2 TiO2 TiO2 Al2O3 Al2O3 TiN C
11.2 18.1 16.5 – – – 2.3 0.0 1.3 – – – – 1.0 – – – – – – 3.2 – – – – – – 1.8 – – – 0.6 0.4 – – – – 51.6
10.6 0.6 0.9 – 4.8 1.5 – – – 3.9 – 1.7 – 0.3 0.3 0.5 – – 0.9 – 2.6 0.9 4.0 0.2 – – – 1.1 – – – – – 5.6 1.3 0.3 – 67.6
18.6 4.3 0.1 – 5.4 – – – – 11.3 1.5 1.1 – 1.5 9.7 – – – – 2.6 7.5
26.0 0.9 0.6 – 2.0 – – – – 7.2 0.4 0.8 – 3.0 14.4 – 5.4 – – 2.7 6.2
13.6 1.2 1.2 1.0 1.6 – – – – 6.1 – 0.8 – 1.1 – – – – – 0.2
2.5 – – – – 0.5 – – – – – 7.9 – – 5.8 38.3
6.3 – – – – 0.7 – – – – – 1.7 – 1.1
12.1 3.1 – – 8.6
13.5 1.5 1.0 – 1.8 – – – – 3.9 – 0.4 – 0.3 – – – – – 0.9 1.4 1.4 8.0 0.9 – – 1.4 0.1 6.3 – 0.5 – – – 2.1 0.3
13.5 1.5 0.3 – 1.9 – – – – 4.3 – 0.6 – 0.7 – – – – 1.1 0.8 – 1.6 8.3 – – – 6.5 0.4 4.8 – – – – 0.3 1.7 0.5 0.9 63.9
27.7 0.5 – – 0.2 – – – – 2.2 – – – 2.3 10.6 – 2.7 – – 2.5 1.9 1.7 3.5 – 0.5 – 2.5 – 3.0 –
33.3 0.3 – – – – – – – 1.3 – – – 2.5 11.4 – 10.4 1.2 – 1.4 0.2 – 2.3 – – 1.2 5.4 – – 1.0 2.4 – – – – 0.8 7.4 51.8
23.1 0.2 0.9 – – – – – – 0.7 0.1 – 0.5 1.2 2.2 – – – – – 0.8 – 1.4 0.6 –
46.0 0.4 0.7 – – – – – – – – – – 3.3 11.9 – 20.0 – – – 1.4 – 1.0 – 1.2 1.5 – – – – 10.2 – – – – 0.6 4.1 42.9
46.6
3.3 – – – 0.5 – 1.3 7.6 50.3
67.7
– – – – 0.7 65.3
4.8 – 1.4 1.8 12.4 – – – – 0.5 71.7
Sulphate phases including bassanite and jarosite are formed during low temperature ashing.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 5. Variation of potassium (oxide basis) content of cokes with their locations in the probe from tuyere entrance.
Fig. 3. Variation of the ash yield (a), silica to alumina ratio (b) and iron oxide content (c) of tuyere cokes with the distance of their locations in the probe from the tuyere entrance.
High ash yield in the deadman region can be attributed to several factors such as the high degree of gasification due to prolonged stay, the adsorption of recirculating alkalis as well as the penetration of few slag and metal droplets in some cases. Unusual changes in the ash chemistry of coke samples from any adjacent 25 cm sections within the same tuyere zone can provide an indication of the presence of slag or hot metal presence in a given coke specimen. Silica to alumina ratio of cokes (~ 1.5) at most of the tuyere locations is significantly
less compared to that of feed coke (2.09) (Fig. 3b). This shows a substantial loss of coke silica as it reaches to the tuyere regions. Total iron oxide content of coke increases significantly in the deadman region (Fig. 3c). Fig. 4 shows microscopic images of typical tuyere cokes from three tuyere zones. Fig. 4b clearly shows the hot metal droplets sticking on the tuyere cokes as bright spots. It was difficult to completely remove the penetrated molten iron from cokes in all the samples leading to high iron levels in the deadman regions (Fig. 3c). Potassium content of tuyere coke increases significantly compared to that of the feed coke (Fig. 5). This is attributed to the adsorption of recirculating potassium in the blast furnace. Potassium content of deadman cokes is more than fifteen times of that present in the feed coke. The amount of potassium intake by cokes at tuyere level is also found to be inversely proportional to the temperature of the tuyere zone as well as the degree of coke graphitization [17]. Fig. 6a and b compares the cross-sections of the feed coke and a typical raceway coke respectively. The raceway cokes are distinguished by relatively enlarged pores with inorganic species often occurring along the pore boundaries. The pores were more apparent in the cokes from relatively higher temperature zones such as the raceway zone, which is most likely related to higher intensity of in-situ reduction of coke minerals. SiC is observed mainly in the raceway region cokes and is almost absent in the birdsnest and deadman cokes (Fig. 7a). In the raceway zone, high amount of SiC of tuyere cokes (Fig. 7a) is also accompanied by low amount of residual SiO2 polymorphs (Fig. 7b) as well as low mullite content (Fig. 7c). The presence of SiC in the raceway coke highlights the strong influence of the tuyere temperatures on the amount
Fig. 4. Steel probe images illustrating cokes present in the raceway, birdsnest and deadman sections.
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Fig. 6. SEM images of the cross-sections of the feed coke and typical raceway coke.
Fig. 7. (a) Variation of SiC (b) quartz and (c) mullite content of the cokes with their locations in the probe from tuyere entrance.
of SiC formation. It may be noted that local gas composition could also influence the amount of SiC formation. Fig. 8 compares the variation of different iron bearing phases present in the tuyere cokes at different locations. Wuestite is absent at most of the birdsnest and deadman locations (Fig. 8a). In the raceway region, metallic iron showed an opposite trend with the distance from tuyere entrance (Fig. 8c) as shown by wuestite. Some of iron occurs as silicides which decrease with increasing distance of the locations of cokes from tuyere side (Fig. 8b). The presence of large size of some ferrosilicon phases suggests that the interaction of molten iron and silica phases of coke could have significant contribution. The ferro-silicon phases are also known form due to in-situ reduction within coke [13]. Therefore, the formation of ferro-silicon alloys in tuyere cokes will be influenced by the interaction of hot metal with coke minerals as well as in-situ reduction of original coke minerals. Fig. 9 illustrates typical Si bearing phases in some of the raceway cokes. Fig. 9a shows the presence of elemental Si along pores which may be attributed to in-situ reduction of quartz as silicides and silicon carbide has been reported to form due to carbothermal reduction of silicon mineral with pyrite in chars at more than 1673 K [32]. Fig. 9b illustrates the formation ferro-silicon (most likely gupeiite) formed due to molten iron interactions. Fig. 9c illustrates the dispersion of fine ferro-silicon grains in the carbon matrix and consequences on the modifications of carbon matrix of coke. Further studies are continuing to clarify the implications of ferro-silicon formation on tuyere coke. Table 2 also shows the presence of graphite crystals in tuyere cokes. Total percentage of graphite in tuyere coke did not clearly relate with the temperatures of various tuyere zones which could be related to
differences in the presence of catalysing species for graphitisation such as hot iron at different locations [32,33]. 3.2. K and Na bearing minerals Tuyere cokes showed increased levels of potassium and sodium compared to their concentration in the feed coke (Table 1). However, the potassium content of tuyere cokes increased significantly due to recirculating alkalis (Fig. 5, Table 1). Potassium species are mainly formed in the tuyere cokes outside the raceway region, and increase along the deadman zones (Fig. 10). Total amount of potassium expressed as oxide basis did not relate to the total percentage of the identified potassium minerals. This can be attributed to the fact that a part of total potassium occurred as non-crystalline silicate phases. Some of the potassium and sodium are observed as sulphate in low temperature ash samples and are not related to the actual mineralogy of tuyere cokes (Table 2). Sulphate minerals such as jarosite or bassanite are often formed as a consequence of the interaction of potassium or sodium species with residual sulphur in tuyere cokes. In tuyere cokes, potassium could occur as intercalated graphite phase or dispersed between graphene layers [9,18]. The XRD peaks of intercalated potassium compounds were not seen in the tested tuyere coke. Raceway cokes from relatively higher temperatures had a narrow interspacing of carbon layers, and also showed lower amount of potassium. Potassium retention in tuyere cokes was inversely related to both temperature as well as the extent of graphitization. Therefore, on the basis of our
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 10. Variation of K bearing phases of cokes with their locations in the probe from tuyere entrance.
tuyere cokes such as in the deadman is most likely attributed to slag contamination. Majority of new calcium bearing phases of the tuyere cokes were absent in the low temperature ash specimen of feed coke [17]. Gehlenite is observed to be the most common calcium bearing phase in tuyere cokes (Table 2). Fig. 12a illustrates the formation of calcium and magnesium silicate such that grey spots show the calcium silicate phase. Some of the calcium in tuyere cokes is also stabilised as oldhamite (Table 2), which is believed to display high thermal stability. The presence of bassanite in tuyere cokes is the consequence of low temperature ashing. Spinel is the most common magnesium bearing mineral of the tuyere cokes (Table 2). On the basis of the EDS composition, a typical spinel phase of tuyere coke is identified in Fig. 12b. 3.4. Apparent CO2 reactivity and mineralogy
Fig. 8. Variation of wuestite (a), gupeiite (Fe3Si) and (c) metallic iron content of cokes with their locations in the probe from tuyere entrance.
observations and previous reported studies [9,18], potassium in tuyere cokes was most likely dispersed between carbon planes which transform to sulphate during low temperature ashing. 3.3. Ca and Mg bearing minerals In general the percentage of calcium bearing minerals of coke do not increase significantly except at few locations in the deadman region (Fig. 11). The presence of high calcium phases in some of the
The apparent reaction rates of tuyere coke increase with distance from the tuyere entrance (Fig. 13). The reaction rate of the deadman coke is the highest and about more than ten times the apparent reaction rate of the feed coke. The variation of reaction rate of cokes across different tuyere zones is related to the amount of recirculating potassium adsorbed in cokes which is also inversely interrelated to the temperatures of the different tuyere zones. The apparent reaction rates of cokes did not give a clear correlation with total amount of all iron bearing phases. However, the reaction rate of tuyere cokes with high amounts of metallic iron and wuestite is high (Fig. 14a). Fig. 14b shows that the tuyere coke reactivity increases with the total amount of potassium bearing species. Fig. 14b further shows that the reaction rate of tuyere cokes does not change significantly after the K bearing minerals in coke become about 1.5%. This means, any further increment of potassium levels, such
Fig. 9. (a) Illustration of in-situ reduction of silica, (b) gupeiite (Fe3Si) type and (c) dispersion of ferro-silicon alloys in carbon matrix of tuyere cokes.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
S. Gupta et al. / Fuel Processing Technology xxx (2013) xxx–xxx
Fig. 11. Variation of Ca bearing minerals of cokes with their locations in the probe from tuyere entrance.
as that occurs in the birdsnest and deadman zones, may not further increase the reaction rate of tuyere cokes. The apparent reaction rate of tuyere coke can also be influenced by other factors such as the differences in the ordering of carbon structure of cokes [17]. However, within the raceway region, high reaction rate of cokes cannot be attributed to their increased carbon structure ordering alone. On the other hand, a relatively small increment of the potassium level seems to have greater effect on reaction rate. This shows that the catalytic effect of recirculating potassium has a strong effect on the tuyere coke reaction rate. 4. Conclusions A batch of tuyere coke samples of a working blast furnace was examined to quantify mineral phases and reactivity. The following conclusions were made. 1. The ash yield of tuyere cokes increases from the tuyere entrance to the deadman zone. The study shows that more than 50% of total inorganic matter of cokes at tuyere levels occurs as amorphous or glassy phase. Quartz and mullite, the predominant minerals of the feed coke, were found to significantly decrease or completely disappear at most of the tuyere-level locations. SiC and gupeiite were found to be the most notable new Si bearing minerals in the tuyere cokes, and were most abundant in the raceway regions.
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Fig. 13. Apparent reaction rates of tuyere cokes vs their locations in the probe from tuyere entrance.
2. A number of new calcium bearing new phases were formed in the tuyere cokes. However, gehlenite and spinel were found to be the most common calcium and magnesium bearing minerals of the tuyere cokes respectively 3. The presence of recirculating potassium was found to be the most distinctive feature of tuyere cokes particularly outside the raceway. The study confirmed the presence of graphite formation in cokes at tuyere levels. 4. The apparent CO2 reactivity of tuyere cokes was found to be significantly higher compared to the feed coke reactivity and increases with distance from tuyere entrance. The apparent reaction rate is found to increase with increasing amounts of potassium bearing phases. The study highlights the strong influence of blast furnace conditions such as the raceway temperatures, alkali loading and molten iron on the mineralogy and reactivity of cokes at tuyere levels of a working blast furnace.
Acknowledgement Authors acknowledge the financial support from the Australian Coal Association Research Program as well as the technical support from Rautaruukki Oyj, Finland. We also express sincere thanks for continuous input and support from Dr Dave Osborne, Mr Sid McGuire and Mr Stephen Brant. We also express our sincere thanks to Dr David French for the SIROQUANT measurements.
Fig. 12. (a) Illustration of calcium silicates and (b) spinel phase presence in the raceway cokes.
Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009
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Fig. 14. (a) The apparent reaction rates of tuyere cokes vs potassium bearing minerals and (b) combined metallic iron and wuestite content of tuyere cokes.
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Please cite this article as: S. Gupta, et al., Mineralogy and reactivity of cokes in a working blast furnace, Fuel Processing Technology (2013), http:// dx.doi.org/10.1016/j.fuproc.2013.02.009