1

Olivine reactions during reduction in blast furnace pellets Elin Ryösä1,2

1Uppsala University, Villavägen 16, SE-752 36 Uppsala

2LKAB, Technology and Business Development, Box 952, SE-971 28 Luleå

Abstract

The use of olivine mineral as an additive in LKAB’s blast furnace pellets has given the pellets great beneficial qualities in the blast furnace operation. This report aims to map out the olivine reactions on a micro scale as they descend in the blast furnace. The pellets have undergone reactions in an oxidating environment in the pelletising plant and the forsterites have developed a complex set of coronas before entering the blast furnace. The samples used in this study come from three different levels in LKAB’s experimental blast furnace (EBF). Methods used are light optic microscopy, SEM equipped with EDS and WDS, and micro-Raman spectroscopy. Results show that the first and second corona from the oxidation will merge into one phase consisting of secondarily formed olivine with inclusions of glass. The iron content in the olivine rims will increase with increased reduction. Magnesioferrite solid solution will decrease with increasing reduction to magnetite. Magnesium solid solution will stabilize wüstite at increasing reduction degree and temperature. When reduction is commenced FeO in olivine is unstable and reduced out of olivine forming metallic iron droplets. The liberated quartz will react with potassium, further increasing the reduction and lowering melting points in the lower shaft where slag melting occurs. The olivine is consumed in the lower shaft at the wall position and in the cohesive zone in the centre position of the EBF, leaving a glass phase with varying composition.

1. INTRODUCTION Olivine is used as an additive to LKAB’s iron ore blast furnace pellets. They have added great beneficial qualities to the blast furnace operation. Olivine is a solid solution between forsterite (Mg2SiO4) and fayalite (Fe2SiO4). The olivines used in LKAB’s pellets were mined in Åheim, Norway with the initial formula of forsteritic composition – (Mg1.9Fe0.1)SiO4. Additives besides forsterite are quartz and limestone, and bentonite is used as a binder. These ingredients are rolled and heated in an oxidising atmosphere together with magnetite to the finished pellets product in LKAB’s pelletising plant, having reached temperatures of 1350°C. The magnetite is oxidised to hematite. The forsterite undergoes a series of reactions during the oxidation in the pelletising plant due to Fe2+ in olivine being oxidised forming quartz and hematite as previously described by Niiniskorpi (2001). A core of the forsterite remains with the primary composition in the finished pellets

product, except for thin inclusions of hematite determined through optical microscopy. It will also develop a complex set of coronas. The first corona consists of a two phase lamellar structure, each phase too small to be examined individually with the optical microscopy or SEM used in the previous study. They are believed to be magnetite or magnesioferrite and a silica phase. Grains of magnesioferrite are embedded in an amorphous silica rich phase in the second corona. Between the coronas a layer is usually observed containing Ca-Fe-Mg-silicate of pyroxenitic composition. Magnesioferrite commonly occurs in hematite close to the olivine grains. In this study micro-Raman spectroscopy is applied to the oxidised pellets to determine the phases that appear in this complex system. However, the main aim of this article is to describe the further reactions of forsterite and its coronas in the reducing environment of the blast furnace. The pellets used in this study are collected from LKAB’s experimental blast furnace (EBF) divided

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into the upper and lower shaft, and the cohesive zone. There are horizontal variations in the material observed in previous studies with e.g. higher alkali deposition at the wall position (Eliasson et al, 2007). It has further been observed that the reduction degree is higher at the wall position even in the upper shaft (Ryösä et al, 2008). However, the reduction degree of the burden is generally related to the main gas flow in the blast furnace. Therefore, rather than describing the pellets and its olivines based on their horizontal position they will be described based on the degree of reduction of the surrounding iron oxides.

2 METHODS 2.1 Samples In this paper oxidised pellets from LKAB’s pelletising plant and reduced pellets from the EBF are studied. The amount of pellets used for examination for each campaign is given in Table 1. The locations of the probes are

described in Figure 1. The probed samples are cooled within minutes whereas the excavated pellets take hours to cool. The effect of the different quench times have been carefully examined in another study (Ryösä et al, to be submitted) and the results of that study are taken into consideration in this project, e.g. some phases that were clearly results of a slow quench time have been excluded. Table 1. Amount of pellets studied in light optic microscopy and amount of pellets chosen for further analysis in SEM. Campaigns 14 and 16 – only probing performed. Campaign 15 – both probing and excavation performed. Microscopy SEM

Oxidised pellets 21 9

Campaign 14 (upper and lower shaft) 93 41

Campaign 15 (upper and lower shaft) 111 47

Campaign 16 (cohesive zone) 21 7

2.2 Instrumentation The pellet samples were cut, polished and examined with reflected light optic

Figure 1. (a) The location of charging level, probes and tuyeres in the experimental blast furnace. (b) The position of the wall and centre in cross section of the blast furnace shaft. (c) Zones within the pellets.

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microscopy, microprobe analysis performed with a Cameca SX50 equipped with wavelength dispersive spectrometers (WDS) and scanning electron microscopy (SEM) a Philips XL30 equipped with energy dispersive spectrometers (EDS). The WDS is calibrated against standards and give absolute values whereas the EDS analysis is calibrated against Cu. Further, the micro-Raman spectroscopy was utilised. The micro-Raman spectrometer was equipped with a Kaiser’s HoloSpec imaging spectrometer equipped with thermoelectrically cooled charge-coupled device detector (Andor). It uses an argon ion laser as a light source with the wavelength 415 nm (green light) and maximal effect of 50 mW. Spectra were collected from -220 to 2300 cm-1 and the measurement time was about five minutes. The spot size of the micro-Raman instrument is not smaller than for the SEM (~5 µm) but will yield further structural information if there are several phases present on a small surface.

Figure 2. Oxidised pellet sample – micro-Raman spectra. a) Forsterite core with inclusions of hematite and cristobalite. Further spectra of first corona

(lamellar) around forsterite core. b) Second corona with magnesioferrite and cristobalite.

3 RESULTS 3.1 Oxidised pellets The olivines in oxidised pellets have previously been described in depth by Niiniskorpi, (2004). However, some phases are too small for analysis with SEM which was utilised in his study. The inclusions in the forsterite core, previously assumed to be hematite, and the two phase lamellar corona (first corona) and surrounding silicate phase and iron oxide (second corona) were subjected to micro-Raman analysis in this study. The resulting spectra are presented in Figure 2 and EDS chemical analyses are given in Table 2. Table 2. Oxidised pellets - EDS analysis (normalised wt-%) of selected phases. The second corona analysis was focused on the silicate phase. All iron as FeO. MgO SiO2 FeO

Forsterite core 46.7 46.8 6.6

Inclusion in forsterite core 40.4 43.6 16.1

First corona 33.0 44.1 22.9

Second corona 1.4 88.9 9.7

Magnesioferrite 17.3 0 82.7

In Table 3 the Raman modes of the measurements are compared with previously measured modes for hematite, magnetite, magnesioferrite, orthoenstatite, forsterite and vitreous silica (Chopelas, 1991; Gasparov et al, 2005; Geissberger and Galeener, 1983; Lin, 2003; McMillan et al, 1983; McMillan, 1984; Shebanova and Lazor, 2003; Shim and Duffy, 2002; Wang et al, 2002 and Winell et al, 2006). Some of the possible phases in this study have Raman modes that overlap in the literature, but through studying the most intense modes the method can be used to identify the phases. Forsterite core: Forsterite is clearly visible in the forsterite core from the strong modes at 826 and 859 cm-1 which are related to the SiO4-tetrahedra internal stretching modes (Chopelas, 1991). The inclusions in the forsterite core are hematite shown by the modes at 294, 414 and 614 cm-1 which are the most intense in this analysis and in the literature. The last mode at 1327 cm-1 is

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possibly microcrystalline graphite (Winell et al, 2006). The modes at 414, 504 and 614 cm-1 can be assigned to both hematite and vitreous silica. The envelope at 1050-1150 cm-1 is the antisymmetric vibration of the SiO4-tetrahedra in vitreous silica indicative of phyllosilicate structure – Si2O5

2- (Virgo et al, 1980). First corona: In the first corona the characteristic narrow forsterite double peaks are clearly visible in the 800-900 cm-1 region. The modes at 215, 297, 335, 482, 549 and 696 cm-1 corresponds to magnesioferrite in the literature. The peak at 696 cm-1 has no overlap with the other possible phases. In the literature magnetite has many modes that agree with the modes

found in this studied phase; however it has the strongest peak at 668 cm-1 and is therefore excluded. The envelope at 1015-1035 cm-1 is displaced compared to that found in the forsterite core and is more in line with orthopyroxene. Many of orthopyroxene’s peaks in literature overlap with magnesioferrite, especially the strongest peaks at 664-686 cm-1 which explains its wideness in the recorded spectrum. Second corona: The strong band at 706/712 cm-1 is displaced compared to the first corona but fits with reported modes of magnesioferrite. The envelope at 1050-1200 cm-1 is assigned to vitreous silica. The peak at 410/414 cm-1 is the low frequency mode of vitreous silica.

Table 3. Oxidised pellets – micro-Raman modes measured for forsterite core with inclusions (For core), first corona (1st cor) and second corona (2nd cor). Compared against modes for Fe2O3, Fe3O4, MgFe2O4, MgSiO3, Mg2SiO4 and vitreous silica (Vitr. SiO2) in the literature. Only values between 200-1400 cm-1 are given. The dash between data indicate a range of modes and slash gives specific modes found. The most intensive modes are marked with a star. Cf Figure 2 and Table 2.

For core 1st cor 2nd cor Fe2O3 Fe3O4 MgFe2O4 MgSiO3 Mg2SiO4 Vitr. SiO2

215 215 217 *203/217

227 231 229 *224 226 226

248 250 250 243 238 242

*294 *290

297 297 *299

305 305 306 302 304

335 333/337 336 333 *343 329/339

*414 *416 *410/414 *408 402/422 422 *435

504 482 486 496 450-490 459/486 *490

549 557 *538 523/554 524/552 545

614 614 605/614 *609 570 624 608 605

664 672 659 *668 *664

*696 *706/712 706 *687/715 *686

*826 *825/828 *824 791

*859 *857/859 852 *856

884 886 881

921 923 927 920

965 966 1011 965 1000-1050

(1050-1150) (1015-1035) (1050-1200) 1032 1067/1199

*1327 1333 *1334/1375 *1316 1314

Figure 3. Schematic representation for the optical determination of the reduction degree of pellets from the EBF.

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3.2 Reduced pellets The reduction degree of the pellets is relative from optical examination ranging from 0% – all hematite – to 100% all metallic iron, see Figure 3. These reduction percentages are not absolute and shall not be compared to chemically determined reduction degrees. The olivines will be described based on the reduction degree of the pellets zone is, i.e. based on the predominant surrounding iron oxides. The olivine mineral phase which has the primary forsteritic composition will be referred to as the forsterite core in this study. It is not always clear whether the silicate phases originating from the second corona are crystalline or amorphous. Hence, these silicate phases will be referred to as glasses. The micro-Raman spectroscopy measure-ments will give some information on the long-range order in point analyses where the glass phase is dominating.

3.2.1 Upper shaft

The temperature measured in the upper shaft is ~800-900°C in the centre and ~700°C at the wall position of the EBF. Morphology of olivine phases

Hematite (0-40% reduction): The olivines found in hematite have not changed in morphology from the olivines found in oxidised pellets. An example of these olivines is seen in Figure 4 a where the core of the grain has inclusions of hematite and vitreous silica and the first corona consist of magnesioferrite, olivine and orthoenstatite in a lamellar structure and is surrounded by the second corona containing vitreous silica (glass) and magnesioferrite grains. Magnetite and wüstite (20-60% reduction): When the hematite is reduced to magnetite and wüstite the lamellae in the first corona are merging into a more

Figure 4. Upper shaft of EBF – backscatter images of olivine phases: (a) Hematite: Forsterite core with hematite and vitreous silica inclusions surrounded by first and second corona. (b) Magnetite/wüstite: Forsterite core with magnetite and K2O-4SiO2 inclusions surrounded by first olivine rim and second corona. (c) Magnetite/wüstite: Forsterite core with the first olivine rim and second corona merging. (d) Wüstite/metallic iron: Forsterite core and second olivine rim with inclusions of glass.

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homogenous phase, hereafter referred as the first olivine rim. However, this step is gradual; the first olivine rim have traces of the lamellae and a gradient in the backscatter image from darker closest to the core to lighter further away from the core, see Figure 4 b. The glass phase in the second corona is still enclosing the first olivine rim, wetting completely against the surrounding iron phase. Wüstite and metallic iron flakes (60-80% reduction): As the wüstite structure matures – often with the introduction of metallic iron flakes – the first olivine rim and the glass phase in the second corona merge into one thick rim, hereafter referred to as the second olivine rim. The mean thickness of the different rims around forsterite cores found in hematite, magnetite, wüstite and wüstite/metallic iron are given in Table 4. Table 4. Upper shaft - Mean thickness (µm) of the first olivine rim, the glass in the second corona and the second olivine rim found in hematite, magnetite, wüstite and wüstite/iron; 59 olivines were measured. Rim type Hem Magn Wüst Wüst/iron

1st olivine rim 3.2 2.9 2.4 -

2nd corona (glass) 2.9 2.7 1.9 -

2nd olivine rim - - - 5.8

Total 6.1 5.6 4.3 5.8

The merging of the first olivine rim and the glass in the second corona are visible in Figure 4 c. This merging is also obvious in the total thickness data of the first olivine rim and glass in the second corona in Table 4, as it matches the total thickness for the second olivine rim. Some glass remains as smaller inclusions (spots) within the second olivine rim, see Figure 4 d. The inclusions inside the forsterite core are still visible at this point.

Chemistry of olivine and glass phases

First corona and olivine rims: The composition of the first corona, the first and second olivine rim will change from Mg-rich to Fe-rich as reduction proceeds to the wüstite/metallic iron conditions at this level in the EBF, see Figure 5.

Figure 5. Upper shaft of EBF - chemical analysis (EDS and WDS in wt-%) of first corona, first and second olivine rims in the system CaO-MgO-FeO found in different iron oxide surroundings, see legend. Other oxides are omitted for clarity. Glass phases: Chemical mapping of a forsterite core with the microprobe shown in Figure 6 reveal that potassium and calcium levels are elevated in the glass phase in the second corona. In spots where the potassium levels are higher the amount of calcium is low. In the first olivine rim the potassium and calcium levels are lower than in the glass phase. It is obvious that potassium is elevated in some of the inclusions inside the forsterite core as well together with elevated amounts of iron and silica. The silica is also higher in the glass in the second corona compared to the forsterite core and first olivine rim. The glass phases found in the second corona and as inclusions in the second olivine rim vary in composition and it is possible to classify them in four glass groups presented in Figure 7. When grouped into the different iron surroundings the glasses were found in – and into the different corona and olivine rims they are connected with – there are not much variation within each glass group. Considering the number of analyses in Figure 7 Glass 1 is the most common outside the first corona in hematite and magnetite. Micro-Raman measurements at 40-60%

reduction

Micro-Raman analysis of the inclusions in the forsterite core, the first olivine rim and

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Figure 6. Upper shaft of EBF at 40-60% reduction: Chemical mapping (WDS) of forsterite core and the surrounding phases. The forsterite boundaries are clearly seen in the magnesium mapping, cf Figure 4 b.

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Figure 7. Upper shaft of EBF: classification of glass phases found in the second corona or as inclusions in the second olivine rim. The values are means of a number of analysis (see number in parenthesis) and given in wt-%. Method of analysis is WDS and EDS. The first letter in the legend denote the predominant surrounding iron phase; H = hematite, H/M = hematite/magnetite, M/W = magnetite/wüstite, W/I = wüstite/iron.

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the glass in the second corona are presented in Figure 8 and the chemical analysis by WDS in Table 5. The observed modes are compared with literature data in Table 6 (Brawer and White, 1975; Chopelas, 1991; Gasparov et al, 2005; Lin, 2003 and Shebanova and Lazor, 2003).

Figure 8. Pellets from upper shaft of EBF at 40-60% reduction – micro-Raman spectra, cf Table 5 and 6. a) Forsterite core with inclusions and first olivine rim. b) Glass in the second corona. Forsterite core: The modes in the region 800-1000 cm-1 are assigned to forsterite according to literature. Magnetite is visible in the modes at 546 and 673 cm-1. In the close-up between 1050-1200 cm-1 region in Figure 8 a there are weak modes at 1102 and 1162 cm-1. These are assigned to K2O-4SiO2 glass. The presence of potassium inclusion in the chemical mapping of the forsterite

core inclusions supports this. First olivine rim: The same modes as in the forsterite core are found in the first olivine rim, i.e. forsterite, magnetite and K2O-4SiO2. Additional weak modes at 1011 cm-1 indicate the presence of orthopyroxene. Many of orthopyroxene’s peaks are found in the spectra for first olivine rim, especially the strongest bands at 664 and 686 cm-1 where the first one is overlapping and the last one is buried in the magnetite peak. Second corona: The glass phases sometimes still show the characteristic olivine modes at 821 and 852 cm-1. The peaks are slightly dislocated and broader compared to the forsterite core and first olivine rim meaning the disorder has increased. There is also magnetite present at 664/668 cm-1. The modes for orthopyroxene and K2O-4SiO2 glass are also present in the glass but the modes are broad. 3.2.2 Lower shaft

The temperature measured in the lower shaft is ~900-1100°C in the centre and ~750-800°C at the wall position of the EBF. Morphology and chemistry of olivine and

glass phases

Wüstite/metallic iron (80% reduction): The olivines in the wüstite core of pellets retains the second olivine rim that wets completely against the wüstite and metallic iron, see Figure 9 a. There is generally a distinct gradient in the rim that becomes more Fe-rich further away from the forsterite core. Spots of glass phases are visible inside and surrounding the second olivine rim and are also visible outside of the rim in the pores of the wüstite and iron. The inclusions in the forsterite core are still visible. In pellets with a thick metallic iron border surrounding the wüstite core there are generally no olivines

Table 5. Upper shaft at 40-60% reduction - WDS analysis (normalised wt-%) of selected phases. Na2O MgO Al2O3 SiO2 K2O CaO MnO FeO

Forsterite core 0 53.28 0 41.10 0 0 0.13 5.48

Inclusion in forsterite 0.09 41.97 0.06 38.50 0 0.05 0.07 19.10

1st olivine rim 0.49 38.00 0 40.71 0.84 0.34 0.09 19.54

2nd corona (glass) 3.67 18.83 0.42 55.99 4.66 6.74 0 9.69

Magnesioferrite 0 9.21 0 0 0 0 0.19 90.60

Mg-wüstite 0 1.33 0.33 0 0 0 0.08 98.25

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left inside the core. Figure 9 b shows such an area with mainly magnesiowüstite and metallic iron present along with a glass phase. Chemical analyses of the mentioned phases are given in Table 7. Metallic iron (80-100% reduction): In some pellets that have a specific iron structure with large features there are no olivines left, only the glass phase com-parable observations in some wüstite cores. However, in pellets with olivine free core – both wüstite and iron cores – the shell usually contain olivines. The forsterites in metallic iron have a generally thinner second olivine rim compared to the olivines found in wüstite, see Figure 9 c. The glass can

appear as inclusions in the second olivine rim and as patches outside the rim. The composition of the second olivine rim in the lower shaft changes greatly when wüstite is reduced to metallic iron, see Figure 10. In wüstite the second olivine rim is more iron rich than in metallic iron surroundings. Some of the olivines integrate Ca into the rim for both wüstite and metallic iron. At the wall position of the EBF a certain shape of olivine is sometimes found as presented in Figure 9 d. The forsterite core is usually in direct contact with a thick cracked glass rim. There is no olivine rim remaining – the only exception is a second olivine rim

Table 6. Upper shaft at 40-60% reduction – micro-Raman modes for forsterite core with inclusions (For core), first olivine rim (1st ol rim) and glass in second corona (Glass). Compared against modes for Fe3O4, MgSiO3, Mg2SiO4 and K2O-4SiO2 in the literature. Only values between 200-1400 cm-1 are given. The dash between data indicate a range of modes and slash gives specific modes found. The most intensive modes are marked with a star. Cf Figure 8 and Table 5.

For core 1st ol rim Glass Fe3O4 MgSiO3 Mg2SiO4 K2O-4SiO2

225 223-249 223/225 226 238 226

242

307 303-339 306-335 306 302 304

336 *343 329/339

432 421 402/422 422

527 450-490 524

*546 542 540 *538 552 545 *520

570 608 595

*673 *669 *664/668 *668 *664

706 *686 780

*825 *825 *821 *824

*857 *855 *852 852 *856

882 882 881

921 921 927 920

966 965 965

1011 1016 1011

1102 1102 1102 1032 *1100

1162 1162 1162 1160

Table 7. Lower shaft at 80-100% – WDS analysis (normalised wt-%) of forsterite cores with second olivine rim and Mg-wüstite (Figure 9 a and c), cracked glass rim (Figure 9 d) or no olivine rim at all (Figure 9 e). Analysis of inclusion in forsterite core with second olivine rim, and periclase found in the core of the forsterite with no rim. The glass and magnesiowüstite in the olivine free core (ol free core) of pellets are also presented (Figure 9 b). Na2O MgO Al2O3 SiO2 K2O CaO TiO2 V2O5 Cr2O3 MnO FeO

Forsterite core (2nd oliv rim) 0 54.02 0 40.70 0 0 0 0 0.03 0.08 5.10

Inclusion in forsterite core 0.04 49.30 0 40.93 0.19 0 0 0 0.03 0.06 9.44

Mg-wüstite 0 2.74 0.36 0 0 0.11 0.60 - 0 0.12 95.57

Forsterite core (cracked rim) 0.05 52.42 0 40.21 0 0 0 - 0.04 0.09 7.17

Cracked glass rim 1.76 23.23 0 27.71 43.38 0.22 0.11 - 0 0.27 3.32

Forsterite core (no oliv rim) 0 54.20 0.08 41.54 0 0.77 0.22 0.75 0.05 0.39 1.97

Periclase in forsterite core 0 85.77 0.09 0.22 0 0 0 0.72 0.04 0.74 12.37

Glass; ol free core 1.88 5.16 9.39 34.10 22.82 3.47 1.16 0.99 0.28 0.59 20.15

Magnesiowüstite; ol free core 0 6.75 0.37 0 0 0 0.21 0.55 0.04 0.40 91.68

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Figure 9. Lower shaft of EBF- backscatter images of olivine phases, cf Table 7 for chemical analysis: a) Wüstite/metallic iron: Forsterite core and second olivine rim. b) Wüstite/metallic iron: Glass and magnesiowüstite in core of pellets with no olivines. c) Metallic iron: Forsterite core with second olivine rim and glass as spots on the outside of the rim. d) Metallic iron: Forsterite core with thick cracked glass rim from wall position in the EBF. e) Metallic iron: Forsterite with metallic iron and periclase inside the core.

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of monticellite, a Ca-containing olivine. In some pellets the olivines have lost their second olivine rim but do not have the thick slag rim as shown in Figure 9 e. The forsterite core contains less iron than the primary forsterite and it is common to find grains of metallic iron and periclase inside the core, see Table 7. Glass phases: All glass classes found in the upper shaft are also present in the lower shaft except for Glass 1. However, there are more complex glasses in the lower shaft and an additional three classes can be made and are presented in Figure 11. The glass phases found in connection to olivines with no rim are always lower in FeO representing a higher reduction degree than glasses found in metallic iron in contact with a second olivine rim. The amount of alkali in the glasses increases with increasing reduction degree as evident in all glass classes. Further it is obvious that the amount of analyses of the alkali containing glasses

increases with increasing reduction degree, i.e. 62% of the glasses analysed in wüstite/metallic iron in contact with a second olivine rim are alkali containing glasses, the corresponding numbers of glass analyses in metallic iron with or without the second olivine rim are 91% and 99% respectively.

Figure 11. Lower shaft of EBF – chemical analysis (EDS and WDS in wt-%) of the second olivine rims found in wüstite and/or iron surroundings placed in the ternary system CaO-MgO-FeO.

Table 8. Lower shaft at 80-100% reduction – WDS analysis (normalised wt-%) of forsterite core1, second olivine rim and glass found outside the rim. Another forsterite core2 is analysed with a cracked slag rim where p1 is closest to the forsterite core and p4 the furthest away, cf Figure 12 and Table 9.

Na O MgO Al O SiO K O CaO TiO MnO FeO

Figure 10. Lower shaft of EBF: classification of glass phases found outside the forsterite core; as inclusions in the second olivine rim (2nd rim) or as partial glass outside rim if present. The values are means of a number of analysis (see numbers in parenthesis) and given in wt-%. Method of analysis is WDS and EDS. The first letter in the legend denote the predominant surrounding iron phase; W/I = wüstite/iron and I = iron.

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Micro-Raman measurements at 80-100%

reduction Micro-Raman analysis of the inclusions in the forsterite core, the second olivine rim and the glass in contact with the second olivine rim together with the forsterite core in contact with the cracked glass rim are presented in Figure 12 and the chemical analysis by WDS in Table 8. The modes are compared to literature in Table 9 (Chopelas, 1991 and Groppo, 2006). Forsterite core (second olivine rim): Most of the modes fit with the forsterite in literature except for 591, 681, 1117, 1190 and 1261 cm-1. These bands have been searched for in literature on possible phases with a similar composition to that in Table 8 but no conclusive match is found. A comparison with epoxy resin gives a good match. Since the samples were cast in epoxy resin it is a possibility of this contamination in the measurement. The inclusions in the forsterite core are transformed into olivine.

Figure 12. Pellets from lower shaft of EBF at 80-100% reduction – micro-Raman spectra. (a) Forsterite core with inclusions, second olivine rim and glass, situated in metallic iron surroundings. (b) Forsterite core and cracked slag rim in metallic iron.

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Second olivine rim and glass: The modes for olivine are clearly visible in these phases; the peaks are dislocated to the left compared to the forsterite core as it gets richer in iron. The peaks become less intense as well meaning the disorder in the long-range order increases. Unidentified peaks also appear here and are assigned to the epoxy resin. Forsterite core with cracked glass rim: The olivine modes are observed in both the forsterite core and in the cracked glass rim. The rim experiences a shift to the left with increasing iron and potassium content and

the peaks become less intense showing disorder in the lattice. Traces of the epoxy resin are also visible in these measurements suggesting the inclusions in the forsterite core are transformed to olivine. 3.2.3 Cohesive zone

The temperature in the cohesive zone is measured around ~1100-1200°C. There are pellets present at this level with a wüstite core, but as described above they do not contain any olivines, i.e. the olivines described in this section are only found in

Table 9. Lower shaft at 80-100% reduction – micro-Raman modes for forsterite core with inclusions (For core1), with second olivine rim (2nd rim) and glass found outside the rim. Modes also measured for forsterite core (For core2) with a cracked glass rim (Crkd glass). Compared against Mg2SiO4, Fe2SiO4 and Epoxy resin from literature. Only values between 200-1400 cm-1 are given. The dash between data indicate a range of modes and slash gives specific modes found. The most intensive modes are marked with a star. Cf Figure 12 and Table 8.

For core1 2nd rim Glass For core2 Crkd glass Mg2SiO4 Fe2SiO4 Epoxy resin

232 226 224

310 303-310 242 239

304 284

389-392 329/339 362 391

440 416 422

591 596 594 597 545 507 572

611 646 608 *640

681 678 670 698 670

*827 *825-827 *825 *827 *825-827 *824 *814 *822

*860 *858-860 *858 *858 *841-858 *856 *841

885 882-885 885 885 869 881

896 900

923 918-923 921 920-923 920 932 917

967 955-959 *986 964 961-964 965

*1018 1015

1117 1120 1117 *1112-1120 1114

1190 1176 1190 1190 1185

1261 1260 1261 1261

*1293

Table 10. Cohesive zone at 80-100% reduction – WDS analysis (normalised wt-%) of forsterite core, inclusion in core and the forsterite part of the third olivine rim, cf Figure 13 a. Further analysis of forsterite spot and its rim embedded in glass, cf Figure 13 b. The glass analyses are given as means: Glass 8 = alkali-MgO-SiO2 (8 analyses), Glass 9 = alkali-Al2O3-CaO-MgO-TiO2-SiO2 (23 analyses) and Glass 10 = alkali-Al2O3-CaO-MgO-FeO-TiO2-SiO2 (6 analyses). Analysis of glass and wüstite in pellets core with no olivines are also presented. Na2O MgO Al2O3 SiO2 K2O CaO TiO2 V2O5 Cr2O3 MnO FeO

Forsterite core 0.18 50.67 0 40.98 0 0 0.04 0.07 0 0.11 7.96

Inclusion in core 0.35 46.17 0 39.82 0.17 0.09 0 0 0 0.11 13.24

3rd olivine rim 0.19 51.77 0.21 41.03 0.82 1.31 0.76 0.31 0 0.68 2.92

Forsterite spots 0 51.70 0 41.06 0.02 0.35 0 0.31 0 0.83 5.70

Forsterite spot rim 0.07 52.18 0.10 42.00 0.32 0.34 0.06 0.30 0.05 0.98 3.59

Glass 8 4.91 19.77 2.19 49.52 13.84 3.60 1.43 0.38 0 - 3.60

Glass 9 4.65 10.06 7.07 44.84 13.07 10.47 5.92 0.42 0 - 2.83

Glass 10 5.35 7.07 6.49 33.42 13.05 7.85 4.65 0.83 0.06 - 20.80

Glass in olivine free core 3.44 11.73 5.10 43.42 12.86 8.47 2.72 0.97 0.05 0.37 10.87

Wüstite in olivine free core 0 10.55 0.37 0 0 0 0.70 1.71 0.37 0.22 85.97

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the metallic shells of pellets. In pellets from the wall position of EBF there are no olivines observed in any zone within the pellets. Morphology and chemistry of olivine and

glass phases

Metallic iron (80-100% reduction): The morphology of some forsterite cores has not changed much from previous levels except the olivine rim. They are comparable to the olivines with no rim found in the lower shaft, but they have metallic iron droplets in the rim which in the backscatter image lay in a darker phase than the forsterite core. This is called the third olivine rim and is presented in Figure 13 a. The darker phase is analysed as forsterite with less iron than the primary forsterite, see Table 10. The composition of the magnesiowüstite and glass found in the core of some pellets is also presented. Some of the olivines have been further reacted until the forsterite core structure with inclusions is no longer visible and broken up into spots, see Figure 13 a and b. These smaller units of forsterite have small inclusions of metallic iron and a rim with a darker backscatter image (low in iron) than the core of the forsterite spots. They are embedded in a glass phase and the chemical analyses of these phases are given in Table 10. Glass phases: The glass phases can be

divided into an additional three classes (8, 9 and 10) given in Table 10. The main difference from the higher levels in the EBF is that the majority of the glasses contain notable amount of titanium and vanadium.

Figure 14. Figure 14. Pellets from cohesive zone of EBF at 80-100% reduction – micro-Raman spectra of forsterite core with inclusions in connection to third olivine rim. Further spectra for the glass phase outside the rim are presented, cf Table 10 and 11 for chemical analysis and Raman modes. Micro-Raman measurements at 80-100%

reduction

The forsterite core with inclusions and a third olivine rim surrounded by a glass phase were studied with micro-Raman spectro-scopy and the spectra are presented in Figure 14. The chemical analyses of the phases are

Figure 13. Cohesive zone at 80-100% reduction – backscatter images: (a) Forsterite core with inclusions, third olivine rim and surrounded by a glass phase. To the left is a partly decomposed olivine embedded in a thick glass phase. (b) Glass phase probably derived from olivine.

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given in Table 10 and the Raman modes in Table 11 where they are compared against modes found in literature (Chopelas, 1991). Forsterite core: All the modes in Table 11 are in agreement with the modes for forsterite according to literature. Table 11. Cohesive zone at 80-100% reduction – micro-Raman modes for forsterite core with inclusions (For core) in contact with third olivine rim. The glass found outside the rim is also presented. Compared against Mg2SiO4 and Fe2SiO4 from literature. Only values between 200-1400 cm-1 are given. The dash between data indicate a range of modes and slash gives specific modes found. The most intensive modes are marked with a star. Cf Figure 14 and Table 10.

For core Glass Mg2SiO4 Fe2SiO4

226/242 239

307 304 284

329/339 362

440 422

587 545 507

609 609 608

*825 *825-826 *824 *814

*857 *856-859 *856 *841

886 882-884 881 900

923 920 932

965 966-970 965

Glass: The modes recorded for the glass phase all correspond to olivine, however

they are slightly dislocated compared to the forsterite core. The background increases with distance from the forsterite core and the peaks loose intensity meaning the disorder in the long-range order of the lattice increases but the orthosilicate structure is somewhat retained. The chemical analysis show high alkali content which gives a more open silicate structure and thus resulting in the high background in the spectra.

4 DISCUSSION From the chemical analyses of phases presented in this study there are clear trends for the time related diffusion of magnesium and iron with increasing temperature and reduction in the blast furnace burden. These trends have been plotted in Figure 15 as the means of chemical analysis of FeO in all olivine rims and glasses, and MgO in the iron oxides divided into the degree of reduction of the pellets. 4.1 Upper shaft 4.1.1 Olivine rim and glass phases

The Fe3+ in magnesioferrite in the first and second corona from the oxidised pellets

Figure 15. The mean values of chemical analysis (EDS and WDS in wt-%) of FeO in olivine rim and glass and MgO in iron oxides plotted with increasing reduction degree. Amount of analyses are: olivine rim = 385, glass = 594, magnesioferrite and Mg-wüstite found near olivine = 164 and Mg-wüstite in olivine free pellets core = 7.

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becomes unstable in the blast furnace’s reducing gas. In the second corona it will be reduced to Fe2+ and expel magnesium to react with the vitreous silica as described in Equations 1 and 2. The number of analyses of Glass 1 in Figure 7 clearly shows that the silica rich phase is consumed early. Potassium gas can also work as a reducing agent and form K2O·4SiO2 according to Equation 3. 3MgFe2O4 + 1.5SiO2 + COg → 2Fe3O4 + 1.5Mg2SiO4 + CO2,g [1] 3MgFe2O4 + 3SiO2 + COg → 2Fe3O4 + 3MgSiO3 + CO2,g [2] 3MgFe2O4 + 7SiO2 + 2Kg → 2Fe3O4 + 3MgSiO3 + [K2O·4SiO2] [3] In the first corona the magnesium from magnesioferrite will enter the orthopyroxene and form olivine according to Equation 4. 3MgFe2O4 + 3MgSiO3 + COg → 2Fe3O4 + 3Mg2SiO4 + CO2,g [4] Some pyroxene remains in the first olivine rim as evident from micro-Raman spectra, cf Figure 8 and Table 6. The magnetite lamellae are clearly visible in the first olivine rim and will remain until further reduction, cf Figure 4 b. In this reaction step between 20% to 40-60% reduction the iron is stable in the first olivine rim and glass as displayed in Figure 15. The glasses in the second corona in contact with the first olivine rim have mainly a pyroxenitic composition like glass classes 2, 3 and 4 in Figure 7. This glass will react with the first olivine rim phases and form the second olivine rim containing olivine that is more iron rich than the primary forsterite. Any calcium or alkali that was in the glass will be left as glass inclusions in the second olivine rim. Equation 5 shows the second corona (Glass 2) reacting with the first olivine rim phases where Fe3+ in magnetite is completely reduced. The composition for

the second corona is calculated from Glass 2 in Figure 7. Fe3O4 + 6Mg2SiO4 + 3(Mg0.8Fe0.2)SiO3 + COg → 9(Mg1.6Fe0.4)SiO4 + CO2,g [5] 4.1.2 Iron oxides

The reason for the initially high amount of magnesium in magnesioferrite (almost pure MgFe2O4, cf Figure 15) is because it is trapped spatially by hematite that does not allow magnesium into its lattice. With the reduction of all hematite to magnetite it allows magnesium to diffuse further close to the olivine. With further reduction to wüstite the magnesium dissolve into wüstite. These reactions are shown in Equation 6 and 7. 6Fe2O3 + MgFe2O4 + 2COg → 5(Mg0.2Fe0.8)Fe2O4 + 2CO2,g [6] (Mg0.2Fe0.8)Fe2O4 + COg → 3(Mg0.07Fe0.93)O + CO2,g [7] 4.2 Lower shaft and cohesive zone When the wüstite core forms in the lower shaft pellets the reduction gas diffusion into the pellets is slowed down by the thick metallic iron border that is formed around the core. This allows the wüstite to survive down into the cohesive zone. When the temperature increases the reaction rate among olivines and wüstite increases as well, cf Figure 15 from 80% reduction in the lower shaft to 80-100% reduction in the lower shaft and cohesive zone. This reaction is described in Equation 8 based on the means of chemical analysis in Figure 15. (Mg1.9Fe0.1)SiO4 + (Mg0.07Fe0.93)O → (Mg1.77Fe0.23)SiO4 + (Mg0.2Fe0.8)O [8] The increasing amount of magnesium into the wüstite stabilises the wüstite reduction to iron further with the formation of magnesiowüstite (Harkki, 1980). Magnesium can keep diffusing out of the olivine making it more iron rich as long as the magnesiowüstite is present. Increasing iron will depress the melting point of olivine and

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absorbing potassium will further decrease it and form potassium and aluminium rich melts. The olivine will react with silica and aluminium, and potassium from the blast furnace gas and form the glass in Table 7, shown schematically in Equation [9]. (Mg1.77Fe0.23)SiO4 + SiO2 + Al2O3 + K2,g → [K2O·0.3Al2O3·0.5MgO·FeO·2SiO2] [9] The reason for the presence of olivines in the shell of pellets but no olivine in the core is simply due to the fast reduction to metallic iron. The metallic iron does not form an alloy with magnesium metal at these temperatures. The Fe2+ is reduced out of the second olivine rim in the metallic shell of pellets in the lower shaft. The second olivine rim will loose iron as evident in Figure 15 going from 80% reduction in the lower shaft to 80-100% reduction in both lower shaft and cohesive zone. Eventually metallic iron droplets will form as the second olivine rim becomes the third olivine rim. Iron free forsterite and silica will also form according to Equation 10. 5(Mg1.6Fe0.4)SiO4 + 2COg → 4Mg2SiO4 + 2Fe0s + SiO2 + 2CO2,g [10]

Figure 16. Upper stability of olivine with different iron content in olivine for three selected temperatures. The stability of iron in magnesium and iron containing olivine under different reduction conditions can be calculated out of the relation in Equation 11 in accordance with Equation 12 where [Fe2SiO4] is the solid

solution component in olivine (XFe) assuming ideal solution. From experi-mentally determined ∆G0 values (Jacob et al, 1989) the stability of olivine at different temperatures and oxidation degree is plotted in Figure 16. Above the respective curve iron containing olivine is stable, below iron and silica is stable. Temperature rather than composition is the critical parameter.

2

ln0

O

Fe

p

XTRG ⋅⋅=∆ [11]

2Fe0s + O2,g + SiO2 → [Fe2SiO4] [12] The free silica from Equation 10 attracts potassium that is deposited from the furnace gas, furthering the reduction of iron. Eventually a glass with e.g. the composition of Glass 8 in Table 10 will form out of the consumed forsterite core shown schematically in Equation 13. (Mg1.9Fe0.1)SiO4 + SiO2 + Kg → Fe

0s +

[K2O·2MgO·4SiO2] [13]

Figure 17. Potassium content in glasses versus aluminium from upper and lower shaft, and cohesive zone in the EBF. The early formation of rimless forsterites at the wall position in the lower shaft suggest the high alkali deposition help consume these olivines faster as no olivines were found at the wall position in the cohesive zone. The role of alkali in slag formation at the wall position in the lower shaft has previously been discussed (Eliasson et al, 2007). A further consequence of potassium

19

incorporation in glasses is an increasing amount of Al3+ (Figure 17) substituting Si4+ to preserve charge balance in glasses.

5 CONCLUSIONS The olivines in the pellets have a reaction history from an oxidising atmosphere. The initial phases are: - forsterite core – inclusions of hematite and vitreous silica - first corona – fine lamellae of magnesioferrite, olivine and ortho- pyroxene - second corona – magnesioferrite and vitreous silica When the hematite is reduced to magnetite and wüstite in the upper shaft of the EBF the olivine phases have reacted and contain: - forsterite core – inclusions of magnetite and K2O·4SiO2 - first olivine rim – magnetite, olivine, orthopyroxene and K2O·4SiO2 glass - second corona – magnetite, olivine, orthopyroxene, K2O·4SiO2 glass, several glass phases With further reduction to wüstite and metallic iron in the upper and lower shaft the first olivine rim and the second corona merge and form the second olivine rim; containing olivine and inclusions of glass. Inclusions in the forsterite core will react and form iron rich olivine. In the wüstite core of pellets in the lower shaft and cohesive zone the olivines are completely consumed as magnesium diffusion from the olivine phases into wüstite continues. In the metallic shells of pellets from the same level the olivines survive further down into the EBF due to lacking magnesium diffusion into neighbouring metallic iron. Instead these olivines are subjected to great reduction; reducing the FeO component into metallic iron droplets forming the third olivine rim. Silica is released and attracts potassium which furthers the reduction and leaves a

glass with the main composition K2O·2MgO·4SiO2 or variations to this composition. In conclusion it can be said that the olivine reactions are first governed by reduction up to the wüstite phase. In the wüstite phase magnesium and iron diffusion is controlled by the increasing temperature. When wüstite is reduced to metallic iron the reduction will once again control the olivine reactions. Potassium plays an important role in the olivine reactions throughout the EBF.

ACKNOWLEDGEMENTS

Acknowledgements to prof em Hans Annersten and Dr Bo Lindblom for valuable discussions and suggestions. Thanks to Elin Rutqvist for help with the samples and discussions. Thanks to Hans Harryson for help with microprobe analysis. Grateful acknowledgement to the LKAB’s workers at the EBF for probe and excavation samples.

REFERENCES Chopelas, A., (1991). Single crystal Raman spectra of forsterite, fayalite, and monticellite. American Mineralogist, 76, pp. 1101-1109. Eliasson, E., Hooey, P.L., Annersten, H., and Lindblom, B., (2007). Formation of potassium slag in olivine fluxed blast furnace pellets. Ironmaking and Steelmaking, 34, 5, pp. 422-430. Gasparov, L.V., Arenas, D., Choi, K-Y., Güntherodt, G., Berger, H., Forro, L., Margaritondo, G., Struzhkin, V.V., and Hemley, R., (2005). Magnetite: Raman study of the high-pressure and low-temperature effects. Journal of Applied Physics, 97, pp. 10A922-1-3 Groppo, C., Rinaudo, C., Cairo, S., Gastaldi, D., and Compagnoni, R., (2006). Micro-Raman spectroscopy for a quick and reliable identification of serpentine minerals from ultramafics. European Journal of Mineralogy, 18, pp. 319-329. Harrki, J., (1980). On the hydrogen reduction of magnesiowüstite. Report TKK-V-B 11-13, Helsinki University of Technology, Finland. Jacob, K.T., Kale, G.M., and Iyengar, G.N.K., (1989). Chemical potentials of oxygen for fayalite- quartz-iron and fayalite-quartz-magnetite equilibria. Metallurgical Transactions B, 20B, 679-685. Lin, C-C., (2003). Pressure-induced metastable phase transition in orthoenstatite (MgSiO3) at room temperature: a Raman spectroscopic study. Juornal of Solid State Chemistry, 174, pp. 403- 411.

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Niiniskorpi, V., (2001). Phases and microstructures in LKAB’s olivine- and dolomite-fluxed pellets. Proceedings 60th Ironmaking Conference 2001, Baltimore, pp. 767-780. Ryösä, E., Wikström, J., Lindblom, B., Rutqvist, E., Benedictus, A., and Butcher, A., (2008). Investigation of minerals and iron oxide alterations in olivine pellets excavated from the LKAB experimental blast furnace. Proceedings Ninth International Congress for Applied Mineralogy 2008, Brisbane, QLD, pp. 545-555. Ryösä, E., Lindblom, B., and Annersten, A. To be submitted. Effect of quench rate on phases in olivine fluxed blast furnace pellets. Shebanova, O.N., and Lazor, P., (2003). Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum. Journal of Solid State Chemistry, 174, pp. 424- 430. Shim, S-H., and Duffy, T.S., (2002). Raman spectroscopy of Fe2O3 to 62 GPa. American Mineralogist, 87, pp. 318-326. Virgo, D., Mysen, B.O., and Kushiro, I., (1980). Anionic constitution of 1-atmosphere silicate melts: implications for the structure of igneous melts. Science, New Series, 208, 4450, pp. 1371- 1373. Wang, Z., Lazor, P., Saxena, S.K., and O’Neill, H. St. C., (2002). High pressure Raman spectroscopy of ferrite MgFe2O4. Materials Research Bulletin, 37, pp. 1589-1602. Winell, S., Annersten, H., and Prakapenka, V., (2006). The high-pressure phase transformation and breakdown of MgFe2O4. American Mineralogist, 91, pp. 560-567.