Coke Mineral Transformations in the Experimental Blast Furnace

Energy & Fuels 2008, 22, 3407–3419

3407

Coke Mineral Transformations in the Experimental Blast Furnace Kelli Kazuberns,*,† Sushil Gupta,† Mihaela Grigore,‡ David French,‡ Richard Sakurovs,§ Mats Hallin,| Bo Lindblom,| and Veena Sahajwalla† CooperatiVe Research Centre for Coal in Sustainable DeVelopment (CCSD), School of Materials Science and Engineering, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia, CSIRO Energy Technology, Lucas Heights, Bangor 2234, Australia, CSIRO Energy Technology, Newcastle 2300, Australia, and LKAB R&D Metallurgy, Lulea, Sweden ReceiVed April 29, 2008. ReVised Manuscript ReceiVed June 20, 2008

Blast furnace efficiency may be improved by optimizing coke reactivity. Some but not all forms of mineral matter in the coke modify its reactivity, but changes in mineral matter that occur within coke while in the blast furnace have not been fully quantified. To determine changes in mineral matter forms in the blast furnace, coke samples from a dissection study in the LKAB experimental blast furnace (EBF) were characterized using SEM/EDS analysis, EPMA (microprobe), and low-temperature ashing/quantitative XRD analysis. Variations in alkali concentration, particularly potassium, dominated the compositional changes. At high concentrations of potassium, the mineral matter was largely potassium-bearing but even more potassium was diffused throughout the coke and not associated with mineral matter. There was little difference in potassium concentration between the core and surface of the coke pieces, suggesting that potassium diffused rapidly through the whole coke. Iron, calcium, silicon, and aluminum concentrations were relatively constant in comparison, although the mineralogy of all elements changed significantly with changing temperature.

Introduction One of the paths to reducing the environmental footprint of blast furnace ironmaking is to minimize the energy consumption of the process. Japanese studies1,2 have shown that, by using a highly reactive coke, the temperature of iron oxide reduction in the furnace can be lowered from 1000 to 800-900 °C, which can improve reduction efficiency and thus lower energy consumption.3 Generally, highly reactive cokes tend to break down more readily than less reactive cokes. At lower temperatures, the degradation behavior is influenced by coke reactivity, which is dependent upon other coke properties: porosity, carbon structure, and constituent minerals.4 Of these three parameters, the influence of mineral matter is least understood. The influence of coke mineral matter has been * To whom correspondence should be addressed. Telephone: 61-2-93856957. Fax: 61-2-9385-6565. E-mail: [email protected]. (K.K.); [email protected] (S.G.). † The University of New South Wales. ‡ CSIRO Energy Technology, Lucas Heights. § CSIRO Energy Technology, Newcastle. | LKAB R&D Metallurgy. (1) Yagi, J. Summary of the project research on a super high efficiency ironmaking process. Science and Technology of Innovative Ironmaking for Aiming at Energy Half Consumption, Tokyo, Japan, Nov 27-28, 2003; Ishii, K., Ed.; pp 251-257. (2) Naito, M.; Nakano, M. Improvement of blast furnace reaction efficiency by controlling temperature of thermal reserve zone. Science and Technology of Innovative Ironmaking for Aiming at Energy Half Consumption, Tokyo, Japan, November 27-28, 2003; Ishii, K., Ed.; pp 105-108. (3) Grigore, M.; Sakurovs, R.; French, D.; Sahajwalla, V. Influence of mineral matter on coke reactivity with carbon dioxide. ISIJ Int. 2006, 46 (4), 503–512. (4) Sato, H.; Patrick, J. W.; Walker, A. Effect of coal properties and porous structure on tensile strength of metallurgical coke. Fuel 1998, 77 (11), 1203–1208.

investigated by a number of authors,3,5–9 but these studies concentrated on coals, cokes with mineral additives,7,9 individual forms of mineral matter, and their effects on coke reactivity. Although some qualitative studies have been made,10 the changes in mineral matter that occur within coke in the blast furnace have not been fully quantified. This study partially redresses the situation by investigating the mineral matter transformations that occur in coke in a specially designed blast furnace. Characterization and mineral distribution of midradial experimental blast furnace (EBF) coke samples are presented. This is part of a wider study of coke reactions within the blast furnace. Experimental Section EBF Samples. The current study is based on coke samples from a dissection study conducted in 2002 in the LKAB EBF situated in Luleå, Sweden (campaign 10). The fuel and coke rates were of the order of 500 and 350 kg/thm, respectively, while the hot blast temperature was around 1200 °C. The EBF has a working volume of 8.2 m3 and a diameter of 1.2 m at the tuyere and is 6 m high (5) Mahoney, M.; Rogers, H.; Andriopoulos, N.; Gupta, R. Understanding Mineral Matter in Australian Coking Coals and PCI Coals; C9059; ACARP: Newcastle, Australia, 2002. (6) Hermann, W. Coke reactivity and coke strength. Part 1: Summary and outlook. Coke Making Int. 2002, 14 (1), 18–31. (7) Price, J.; Iliffe, M.; Khan,M.; Gransdan, J. Minerals in coal and high temperature properties of coke. 53rd Ironmaking Conference, Chicago, IL, Mar 24-27, 1994; pp 79-87. (8) Hilding, T. Evolution of Coke Properties while decsending through a Blast Furnace. Master’s degree, Lulea University of Technology, Lulea, Sweden, 2005. (9) Kerkkonen, O.; Mattila, E.; Heiniemi, R. The correlation between reactivity and ash minerology of coke. 55th Ironmaking Conference, Iron and Steel Society, Pittsburgh, PA, Mar 24-27, 1996; pp 275-281. (10) Kerkkonen, O. Influence of ash reactions on feed coke degradation in the blast furnace. Coke Making Int. 1997, 9 (2), 34–41.

10.1021/ef800295d CCC: $40.75  2008 American Chemical Society Published on Web 08/02/2008

3408 Energy & Fuels, Vol. 22, No. 5, 2008

Kazuberns et al. Table 1. Proximate and Ash Analysis of Coke Samples percent (db) moisture ash mineral matter volatilty fixed carbon SiO2 (%) Al2O3 (%) TiO2 (%) Fe2O3 (%) K2O (%) Na2O (%) CaO (%) MgO (%) SO3 (%) other (%)

feed coke

KL20

KL30

KL35

0.5 11.5 14.6 0.5 88 60.30 27.76 1.39 5.08 1.49 0.88 1.29 0.93 0.27 0.61

0.7 15.7 19.4

0.6 17.4 36.2

0.7 18.5 33.5

84.1 42.87 19.38 0.96 2.81 16.57 7.03 0.84 0.62 8.33 0.58

82.2 35.94 21.68 1.10 4.12 20.08 6.52 1.81 0.83 7.03 0.89

81.3 37.63 15.57 0.81 3.61 26.91 7.22 0.94 0.60 5.88 0.85

Figure 1. EBF schematic showing approximate depths and temperatures of samples.

from the stock line to the tuyeres.11,12 The coke was prepared at the SSAB Luleå coking plant by blending of 67% low-medium volatile Australian coals, high volatile US coals, and a small percentage of pet coke. The CRI for the feed coke was recorded as 19, and CSR was 72. The ferrous burden used for testing was LKAB pellets.8 After completion of the campaign, the EBF was quenched by purging nitrogen from the top of the furnace continuously for 10 days. Top quenching also restricted the upward movement of heat flux and retarded any subsequent reactions of the burden constituents.11–13 Approximately 20-35 coke pieces were collected from several vertical and radial locations of the EBF after quenching. Physical locations of the EBF cokes are illustrated in Figure 1. The temperatures of coke layers based on separate thermal probe measurements are also shown. On the basis of the thermal profile coke, KL01 represents the stock line coke, sample KL20 represents the thermal reserve zone coke, and sample KL30 represents cohesive zone coke. On the basis of the characteristics of the samples at excavation, sample KL35 is used to represent the extreme lower end of the cohesive zone. Characterization of Coke Samples. Coke samples were characterized visually and by proximate analysis, ultimate analysis (Table 1), and SEM/EPMA. It was noted that, when comparing a selection of the feed material to samples retrieved from the hightemperature region of the furnace, there is very little evidence of physical breakdown of the sample and no evidence of obvious reactions at the surface. The coke samples were examined by a scanning electron microscope (Philips XL 30) and energy dispersive x-ray analysis (EDS). Coke pieces were mounted in an epoxy slow-setting resin in plastic molds and polished to give a sample cross-section. SEM micrographs were taken of each sample at different locations to observe if the core and the surface of any sample appeared to be different (Figure 2). In each coke sample, the elemental composition of mineral phases at several spots was also analyzed with EDS analysis, with particular attention being paid to mineral transformations in the coke. (11) Hilding, T.; Gupta, S.; Sahajwalla, V.; Bjo¨rkman, B.; Wikstro¨m, J.-O. Degradation behaviour of a high CSR coke in an experimental blast furnace: Effect of carbon structure and alkali reactions. ISIJ Int. 2005, 45 (7), 1041. (12) Dahlstedt, A.; Hallin, M.; Tottie, M. LKAB’S experimental blast furnace for evaluation of iron ore products. SCANMET 1, Luleå, Sweden, 1999; pp 235-245. (13) Hilding, T.; Gupta, S.; Sahajwalla, V.; Bjorkman, B.; Wikstrom, J. Degradation behaviour of a high CSR coke in an experimental blast furnace. Effect of Carbon Structure and Alkali Reactions; Kazuberns, K., Ed.; Sydney, Australia, 2004.

Figure 2. SEM micrographs at different positions on coke samples.

A Cameca SX50 electron probe microanalyser (EPMA) was used to map elements across the same coke cross-sections (but not the same analysis points) as were examined with the SEM. In this case, testing was conducted at every 20-25 µm across the entire sample to ascertain if there were variations in elemental composition from the rim to the core of a coke piece during its descent down the blast furnace. The mineral phases present in the feed and EBF cokes were identified using X-ray diffraction analysis. The carbon was removed from the mineral matter using radio frequency oxygen-plasma ashing at low temperature (120 °C), low-temperature ashing (LTA),14 to minimize alteration of the mineral species. The XRD analysis of the cokes was carried out on a Philips PW1050 goniometer at CSIRO, Lucas Heights. The cokes were subjected to low-temperature oxygen-plasma ashing using an LFE 4-chamber asher as specified in Australian Standard 1038, Part 22. The mass percentage of LTA, mineral matter content, was determined in each case. X-ray diffraction analysis was carried out using Cu KR radiation with generator settings of 40 kV and 35 mA. The 2θ range from 2 to 60° was scanned in steps of 0.04° 2θ using a count time of 5 s/step. Quantitative analyses of mineral phases in each LTA (14) Gluskoter, H. J. Electronic low temperature ashing of bituminous coal. Fuel 1965, 44, 285–291.

Coke Mineral Transformations in the EBF

Energy & Fuels, Vol. 22, No. 5, 2008 3409

Figure 3. Bulk composition of elements in coke ash on descent through the EBF. Table 2. Potassium Mineral Forms Identified by Quantitative XRD for Coke Samples

Table 3. Sodium Mineral Forms Identified by Quantitative XRD for Coke Samples

percent mineral matter potassium mineral forms in ash jarosite (K,H3O)Fe3(SO4)2(OH)6 kalsilite KAlSiO4 nepheline Na6K1.2Al7.2Si8.8O32 potassium nitrate KNO3 potassium sodium sulfate K3Na(SO4)2 wt % K in crystalline form wt % K in amorphous

feed

KL20

KL30

percent mineral matter

1.1

6.9 93.1

sodium mineral forms in ash

KL35

3.7

2.1

2.5

1.7

1.5

5.1

1.8

2.4

3.3

10.8

4.9

10.4

40.1 59.9

19.5 80.5

26.7 73.3

albite Na(AlSi3O8) anorthite (Ca,Na)(Si,Al)4O8 diopside-jadeite see belowa nepheline Na6K1.2Al7.2Si8.8O32 potassium sodium sulfate K3Na(SO4)2 wt % Na in crystalline form wt % Na in amorphous a

were made using Siroquant “6” interpretation software15 based on the Rietveld XRD analysis technique.16

Results and Discussion Bulk Chemistry and Elemental Distribution. The variation in ash chemistry with depth/temperature is shown in Figure 3 based on data from Table 1 Both potassium and sodium concentrations increase with an increasing temperature, potassium more so than sodium, from minimal amounts within the feed coke sample. This is consistent with previous studies; alkali levels in the coke increase because of recirculating alkalis in gaseous form within the blast furnace

feed

KL20

KL30

KL35

0.5 0.8 1.8

12.1 87.9

1.7

1.5

5.1

10.8

4.9

10.4

96.6

10.0 90.0

21.9 78.1

(Ca0.55Na0.30Fe0.07Mg0.06)(Mg0.59Fe0.08Ti0.01Al0.32)(Si2O6).

at temperatures above 700 °C.17,18 Potassium is known to increase the reactivity of coke with carbon dioxide and may (15) Taylor, J. C. Computer programs for standardless quantitative analysis of minerals using the full powder diffraction profile. Powder Diffr. 1991, 6 (1), 2–9. (16) Ward, C. R. Quantitative mineralogical analysis of coal and associated materials by X-ray diffractometry. Annual International Pittsburgh Coal Conference, 2001; pp 309-326. (17) Forsberg, S. Chemical and physical effects of alkali on blast furnace coke. 1st International Cokemaking Congress, Verlag Gruckauf GmbH, Essen, Germany, 1987; pp C6 1-19. (18) Helleisen, M.; Nicolle, R.; Steiler, J. M.; Jusseau, M.; Meltzheim, C.; Thirion, C. Characterization of the behaviour of coke in the blast furnace by dead man coke samples. 1st International Cokemaking Congress, Verlag Gluckauf GmbH, Essen, Germany, Sept 13-18, 1987; pp C2 1-19.


Kazuberns et al.

Figure 4. Potassium distribution of EBF samples shown by (a) microprobe and (b) EDS/SEM. Table 4. Iron Mineral Forms Identified by Quantitative XRD for Coke Samples

Table 5. Calcium Mineral Forms Identified by Quantitative XRD for Coke Samples

percent mineral matter iron mineral forms in ash coquimbite Fe1.55Al0.45(SO4)3(H2O)9 fayalite (Fe0.94Mg0.06)2SiO4 diopside-jadeite see belowa hematite Fe2O3 hercynite FeAl2O4 iron Fe iron silicon Fe0.905Si0.095 jarosite (K,H3O)Fe3(SO4)2(OH)6 magnetite Fe3O4 pyrrhotite Fe1-xS wuestite FeO wt % Fe in crystalline form wt % Fe in amorphous a

feed

KL20

KL30

percent mineral matter KL35

0.6 0.4 1.8 2.8 0.1 0.7

0.2

0.2

0.2 0.1

1.1 5.7 0.1

0.2

0.2

0.1

100 0

25.5 74.5

calcium mineral forms in ash

0.1

akermanite Ca2MgSi2O7 anhydrite CaSO4 anorthite (Ca,Na)(Si,Al)4O8 bassanite CaSO4 · 0.5H2O calcite CaCO3 diopside-jadeite see belowa gehlenite Ca2Al2SiO7 fluorapatite Ca5(PO4)3F oldhamite CaS wt % Ca in crystalline form wt % Ca in amorphous a

7.0 93.0

9.5 90.5


result in coke fracture because of swelling of the coke grains by intercalated potassium. While an increase in reactivity may be advantageous (reducing the blast furnace operating temper-

feed

KL20

KL30

0.1

KL35 0.4

0.2

0.2

0.8 0.9

0.2

0.4

0.1

1.8 0.4 0.6 0.2 100 0

15.9 84.1

4.6 95.4

45.1 54.9


ature), the duty of coke is to also act as burden support within the furnace, and hence, the coke fracture needs to be minimized.19 The resulting plot of potassium across the feed and three EBF samples (Figure 4) confirms the much greater amounts of (19) Harris, D. J.; Young, D. J. Potassium effects in solution loss reaction of metallurgical coke. Ironmaking Steelmaking 1989, 16 (6), 399–405.



Figure 5. Sodium distribution of EBF samples shown by (a) microprobe and (b) EDS/SEM.

Figure 6. Plot of potassium versus aluminum and silicon from microprobe data, showing correlations between elements.


Figure 7. Plot of sodium versus aluminum and silicon from microprobe data, showing correlations between elements.

Figure 8. Aluminum distribution of EBF samples shown by (a) microprobe and (b) EDS/SEM.

Kazuberns et al.



Figure 9. Silicon distribution of EBF samples shown by (a) microprobe and (b) EDS/SEM. Table 6. Aluminium Mineral Forms Identified by Quantitative XRD for Coke Samples aluminum mineral forms in ash albite Na(AlSi3O8) anorthite (Ca,Na)(Si,Al)4O8 coquimbite Fe1.55Al0.45(SO4)3(H2O)9 diopside-jadeite see belowa hercynite FeAl2O4 gehlenite Ca2Al2SiO7 kalsilite KAlSiO4 nepheline Na6K1.2Al7.2Si8.8O32 mullite Al6Si2O13 wt % Al in crystalline form wt % Al in amorphous a

Table 7. Silicon Mineral Forms Identified by Quantitative XRD for Coke Samples percent mineral matter

feed

KL20

KL30

KL35

silicon mineral forms in ash

0.5 0.8 0.6 1.8 0.1 0.4 3.7

2.1

2.5

1.7

1.5

5.1

19.4

2.3

0.9

1.0

51.7 48.3

11.8 88.2

5.7 94.3

33.6 66.4


potassium in the cokes from the lower blast furnace levels than in the feed sample. From Figure 4, it is observed that there are occasional points with very high levels of potassium; these are associated with mineral matter. The remainder of the potassium is uniformly distributed across the specimen, with no evidence of surface enrichment, and does not appear to be associated with mineral matter. This potassium is most likely intercalated in the coke carbon. Occasional high level spikes of potassium

akermanite Ca2MgSi2O7 albite Na(AlSi3O8) anorthite (Ca,Na)(Si,Al)4O8 cristobalite SiO2 diopside-jadeite see belowa fayalite (Fe0.94Mg0.06)2SiO4 gehlenite Ca2Al2SiO7 iron silicon Fe0.905Si0.095 kalsilite KAlSiO4 mullite Al6Si2O13 nepheline Na6K1.2Al7.2Si8.8O32 quartz SiO2 wt % Si in crystalline form wt % Si in amorphous a

feed

KL20

KL30

0.1

KL35 0.4

0.5 0.8 0.4

0.2

0.1

1.8 0.4 0.4 0.1

19.4

3.7

2.1

2.5

2.3

0.9

1.0

1.7

1.5

5.1

8.5 91.5

13.5 86.5

18.4

1.8

42.0 58.0

11.8 88.2


observed are primarily associated with high aluminum, silicon, or both, seen in the maps (Figures 4, 8, and 9).


Kazuberns et al.

Figure 10. Iron distribution of EBF samples shown by (a) microprobe and (b) EDS/SEM. Table 8. Sulfur Mineral Forms Identified by Quantitative XRD for Coke Samples

Table 9. Other Mineral Forms Captured by Siroquant for Coke Samples

percent mineral matter sulfur mineral forms in ash anhydrite CaSO4 bassanite CaSO4 · 0.5H2O coquimbite Fe1.55Al0.45(SO4)3(H2O)9 jarosite (K,H3O)Fe3(SO4)2(OH)6 oldhamite CaS potassium sodium sulfate K3Na(SO4)2 pyrrhotite Fe1-xS wt % S in crystalline form wt % S in amorphous

feed

0.9

KL20

percent mineral matter

KL30

KL35

0.2

0.2

0.2 0.6

1.1 0.2

0.1 100 0

10.8

4.9

0.2

0.1

65.8 34.2

35.4 64.6

10.4

90.5 9.5

Sodium follows similar trends to potassium, as shown in Figure 5. Figure 6 shows the association of potassium with aluminum and silicon from the WDS microprobe step scan data collected across the cross-sections examined. In the feed coke, most of the aluminosilicate minerals present do not contain potassium. In KL20, it is clear that there are a number of regions in this compositional diagram that are favored: there is a considerable amount of potassium that is not associated with aluminosilicates, and most of the points are close to the line joining nepheline and potassium. In KL30, nearly all of the aluminosilicates have

percent of other mineral forms in ash magnesite MgCO3 periclase MgO anatase TiO2 rutile TiO2

feed

KL20

KL30

KL35

0.2 0.3 0.4

0.4

0.4

associated potassium. This indicates that all of the coke mineral matter has been in contact with potassium. As for the KL20 sample, much of the potassium is not associated with aluminosilicates. At KL35, the potassium level associated with aluminosilicates has increased still further. The situation for sodium is similar. As with potassium, there are many aluminosilicate minerals in the feed coke that do not contain sodium. In the EBF samples, sodium appears to have been in contact with most of the mineral matter. However, in comparison to potassium, there is a considerable amount of mineral matter that has little associated sodium in the blast furnace samples and there are fewer points where there is sodium in the absence of aluminum and silicon. This suggests that sodium and potassium interact with coke minerals in a similar but not identical fashion. At the higher temperatures, the circulating free alkalis are made up primarily of potassium; the sodium proportion is decreasing. The bulk chemistry of aluminum within the coke ash is seen



Figure 11. Calcium distribution of EBF samples shown by (a) microprobe and (b) EDS/SEM.

to remain fairly constant throughout the descent through the blast furnace, although it appears to increase around 1500 °C. Figure 8 shows the resulting microprobe plots for aluminum for all samples, as well as corresponding SEM/EDS maps for aluminum. The ash chemistry also shows an increase in the amount of aluminum, calcium, and magnesium at 1500 °C; however, the microprobe and SEM/EDS results show that the nature of distribution has not changed, suggesting capture of a distinct mineral form involving all three elements at 1500 °C. It is more likely that these elements do not change significantly during descent below the cohesive zone. Silicon shows a reverse of the pattern seen for aluminum in the ash chemistry plots (Figure 3), decreasing slightly at 1500 °C. On a microscale, the correlation between the silicon and aluminum distribution microprobe plots suggests that the aluminum in the coke is generally associated with silicon but as different phases as the bulk chemistry changes (Figures 6 and 7). The bulk chemistry of iron decreased initially with increasing temperature but returned by 1500 °C to about the same levels seen in the feed coke sample. The microprobe results for the iron distribution showed no significant changes in the overall amount of iron between the feed and KL35 sample, and what iron was apparent within the sample occurred in discrete mineral forms, suggesting that the variation noted in the bulk chemistry for aluminum, silicon, and iron may have been due to sampling, as shown in Figure 10. Calcium distribution, as shown in Figure 11, has more isolated mineral forms and no significant variations with temperature, similar to the microprobe results for iron.

The initial concentration of sulfur in the feed coke ash was low (

Coke Mineral Transformations in the Experimental Blast Furnace

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