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INVESTIGATION OF THE ALKALI UPTAKE AND SLAG FORMATION REACTIONS
IN THE LKAB EXPERIMENTAL BLAST FURNACE
Bo Lindblom, Elin Rutqvist and Mikael Pettersson LKAB, Box 952,
SE-971 28 Luleå, Sweden [email protected]
Abstract The alkali uptake and slag formation mechanisms for two
different pellets in combination with various slag formers were
studied in this work. The studied materials were in basket samples
charged to the LKAB Experimental Blast furnace (EBF) and taken out
in the cohesive zone after quenching. The samples consisted of LKAB
regular olivine pellets, with and without kaolinite coating, and
slag formers (BOF-slag, quartzite and limestone). From scanning
electron microscope analysis, it was found that the highest amounts
of alkali were found together with aluminium oxide. Coated pellets
form slag areas with kaliophilite. Non-coated pellets form slag
areas with kaliophilite/Na,K-metasilicate due to lower Al2O3
content. BOF slag and quartzite contributes more to the slag
formation than limestone. Slag that is formed close to BOF slag
shows increased calcium oxide content, which leads to a lower
alkali content. Quartzite forms acid slag with a higher alkali
uptake. Alkali is also found on the surface of the slag former as a
result of condensation of alkali gas in the blast furnace. The
formation of solid phases was simulated using the thermochemical
software FactSage™. The operational data from the EBF shows a more
stable process with a higher gas utilisation and a lower tendency
of slips during the period with kaolinite coated pellets compared
to operation with uncoated pellet. Introduction The circulation of
alkali in the blast furnace process is a problem that has been
known for a long time. Scaffolds, instability and increased dusting
are effects related to alkali. During the last years tests with
oxide coated blast furnace pellets has been performed to decrease
the negative effects. The coating has shown to increase the
stability of the process and in some cases decrease the dusting1,
2) . The detailed reason for this stabilising effect is not known
in detail. The aim of the present work is to increase the
understanding of these mechanisms. The LKAB Experimental Blast
Furnace (EBF) A simplified layout of the EBF is shown in Figure 1.
It has a working volume of 8.2 m3 and a hearth diameter of 1.2 m.
There are three tuyeres placed at 120 degree intervals. As great
effort has been made to keep heat loss at a minimum, insulating
refractories were chosen. Only the bosh area and the tuyeres are
water-cooled. The blast is normally preheated to 1200°C in two
pebble heaters. The furnace is equipped with in-burden probes at
three different levels, for sampling of solid materials as well as
gas temperature and composition during operation.
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Figure 1. The LKAB Experimental Blast Furnace. The EBF was at
the trial equipped with a bell less top for the burden distribution
control. One mechanical stock rod and one radar are used to monitor
the burden descent and control the charging of the furnace. The
furnace has one tap hole, which is opened with a drill and closed
with a mud gun. The hot metal and slag are tapped into a ladle
without separation. To make dissection and repair easy, the hearth
is detachable and can be separated from the furnace3, 4).
Excavation The blast furnace was operated continuously in a
campaign of eight weeks. It was the 12th consecutive trial
campaign. The tap-to-tap time was 60 minutes and normal tapping
duration 5 - 15 minutes. The burden before and during the trial
consisted of LKAB regular olivine pellets, with kaolinite coating,
and slag formers (BOF slag, quartzite and limestone). At the end of
the campaign, the furnace was quenched. To stop chemical reactions
as fast as possible, nitrogen was added from the top as the blast
was stopped. As the cooling gas is added from the top, a heat wave
moving upwards is avoided and therefore changes in the material
during cooling is kept to a minimum. The furnace was cooled for
three weeks before dissection of the furnace started. Sample
preparation At the end of the campaign, stainless steel-mesh
baskets, filled with test material were inserted at the top of the
furnace. The baskets were added in a timed interval to get a good
spread throughout the furnace before quenching. Every basket
inserted into the EBF was divided into three separate sections.
This made it possible to study pellets in contact with different
slag formers at the same depth in the furnace. Three baskets
containing olivine pellet and three
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containing kaolinite coated olivine pellet where tied to each
other to minimise the differences in temperature and depth in the
furnace within the baskets during the decent. After quenching, the
EBF was excavated and the baskets were collected. The temperature
in the area where the baskets were found has been appreciated to
set to about 1200°C. The content of the baskets were highly
sintered due to the depth at where they were found. To stabilise
the surfaces in contact with each other, the samples were moulded
in a 2-component plastic. This made it possible to divide each
section into smaller parts and to find areas without separating
interesting contacts. After finding contact areas between pellets
and slag formers, the parts were once again moulded in plastic,
using normal 40mm cups suitable for analysis in scanning electron
microscope (SEM). Before analysing the samples in the SEM, standard
preparations were conducted such as coating the top of the sample
with graphite. Analysis methods Sulphur was analysed with a Leco
analyser. All other elements were analysed with X-ray fluorescence.
A scanning electron microscope, Philips XL30 TMP(W) with integrated
EDX, was used in the morphological studies. The chemical analysis
and the morphological studies were made at the LKAB laboratories.
Thermodynamic calculations For the thermodynamic calculations
FactSageTM thermochemical software and database systems has been
used5). FactSage is a fusion of two thermochemical softwares,
Fact-win (formerly F*A*C*T) and ChemSage (formerly SOLGASMIX). It
can be used both for stoichiometric reactions and complex
multiphase multicomponent equilibrium. FactSage consists of
databases and calculation modules that enables access and
manipulation of pure substances and solution databases. The program
permits calculations of chemical equilibrium state of a system that
is defined with regards to temperature, pressure (or volume) and
total amounts and/or equilibrium activities of any phase
constituent in the system. Thermodynamic data was extracted from
the FACT 5.3 database. The database holds data for pure substances,
non-ideal solid solutions and oxide liquids. Multicomponent solid
solution models were used for spinel, pyroxenes, melilite, olivine,
etc. The modified quasichemical model, provided in the FACT 5.3
database, was used for the oxide liquid (named slag in this paper).
Results Operation data Prior to the test period with kaolinite
coated olivine pellet, a reference period with regular olivine
pellet were conducted. Regular olivine pellets were charged in the
EBF for 72 hours to establish normal circumstances. Eta-CO
increased during the kaolinite period and displayed a more stable
trend compared to the reference period. The decent rate also
stabilised during the kaolinite period, a decreased amount of slips
were registered. The volume and velocity of the top gas during the
test period indicated a more stable gas flow and a slightly
decreased gas velocity. SEM point analysis In this work, the
composition of the oxide material in selected areas in pellets, or
between pellets and slag formers, have been determined using
SEM-analysis. Data have been transformed to
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normalised molar percent of the oxides. In the following
chapters the oxide areas are named “slag”. In order to state if the
analysed oxide areas have been in liquid (slag) or solid state
(crystalline phases) in the blast furnace, some of the point
analysis compositions have been used as input to the FactSage
calculations (see section Results - FactSage calculations).
Lime
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7
Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Limestone Pellet
Figure 2: Lime and kaolinite coated pellet
Figure 2 shows a piece of lime, in the left lower corner, close
to a “slag” area in the outer layer of a pellet. The analysis shows
high concentration of CaO in the lime with increased SiO2 content
closer to the pellet. The “slag” area in the pellet shows a low
content of CaO and does not seem to be effected by the lime. The
darker grey “slag” areas in the pellet, point 4, 5 and 7, are most
probably olivine, with a characteristic rim of metallic iron. The
lighter grey areas, point 3 and 6, may be kaliophilite
(KAlSiO4).
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Figure 3: Covering on a kaolinite coated pellet
Figure 3 shows a covering on the same pellet as in figure 2.
Four different “slag” compositions were found. Darker grey areas
consist of high content of MgO with periclase (MgO) and
monticellite (MgCaSiO4) compositions, see point 3, 5 and 6. Lighter
grey areas are lime and calcium silicates, see point 1, 2, 4 and 7.
The covering could be a result of lime that has been affected by
olivine.
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0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7
Point number
Moe
-l%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Figure 4: Kaolinite coated pellet in close contact with
lime-like covering
The “slag” areas in figure 4 are located in the outer layer of a
pellet, in close contact with the lime-like covering from figure 3.
Closer to the surface kaliophilite/magnesium oxide “slag” is found
(point 1, 3 and 4) and further in to the pellet the “slag” appears
more olivine-like, see point 6 and 7. Two points, one with
relatively high vanadium oxide content, point 5, and one with
perovskite (CaTiO3) content, point 2, are also found.
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeOMnO
Figure 5: “Slag” area in centre of kaolinite-coated pellet
The “slag” area in figure 5 is located in the centre of the
kaolinite-coated pellet in figure 4. The point analysis shows five
different “slag” areas; CaTiO3 (point 1), periclase MgO (point 2
and 4), a MgO-V2O5–SiO2 “slag” (point 3 and 6), a kaliophilite-like
“slag” with high FeO content (point 5) and pyroxene MgSiO3 (point
7).
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0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 9 10
MgOAl2O3
CaOTiO2V2O5
Point number
Mol
e-%
Na2O
SiO2K2O
FeO
Pellet Limestone
Figure 6: Kaolinite coated pellet and lime Figure 6 shows the
contact between a pellet and a piece of lime. The analysis shows
high concentration of CaO in the lime with increased SiO2 content
closer to the pellet, see point 8, 9 and 10. Inside the pellet a
kaliophilite-like “slag” is found (point 3, 4, 5 and 6) and the two
points closer to the centre of the pellet are probably
monticellite, a calcium rich olivine.
Sample 5-3-2 Mg Al
Si Ca Fe Figure 7: Mapping over a kaolinite coated pellet and
lime. Figure 7 shows a mapping over a contact between a coated
pellet and a piece of lime Calcium is mostly present inside the
lime. The “slag” area closest to the pellet consists of calcium
silicate.
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BOF-slag
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 9 10 11Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeOMnO
Fe
Figure 8: Center of BOF-slag
Figure 8 shows the centre in a piece of BOF-slag with two
different “slag” compositions. Areas that appear white are metallic
iron, see point 1, 4 and 7. Darker grey areas are periclase, see
point 2, 3, 6, and 9 with a content of manganese, giving a
approximate composition of (Mg0,88Fe0,06Mn0,06)O. Lighter grey
areas are mainly consisting of calcium silicate with some vanadium
oxide content, see point 5, 7 and 10. Here are also some Al2O3,
TiO2 and K2O/Na2O present but in quite low concentrations. All
points in this analysis display relatively low concentrations of
alkali.
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 9 10Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeOMnO
Fe
Figure 9: BOF-slag and a non-coated pellet
The “slag” area in figure 9 is located in the outer layer of a
regular olivine pellet, in close contact with a piece of BOF-slag.
The different “slag” combinations that appear in this area are very
much like the ones found inside the BOF-slag in figure 9. White
areas are once again metallic iron as in point 1. Darker grey areas
are calcium oxide-rich compounds, see points 6, 7 and 8 and light
grey areas are magnesium oxide rich as point 5 and 10. Compared to
the BOF-slag in figure 8, periclase has an increased iron content,
giving a approximate composition of (Mg0,74Fe0,17Mn0,09)O. Silica
concentrations are higher in the calcium silicate “slag” and have a
higher magnesium concentration, compared to the composition in the
centre of BOF-slag seen in figure 9.
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0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 9 10 11Point number
Mol%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeOMnO
Figure 10: Kaolinite coated pellet and BOF-slag
Figure 10 shows the contact between a kaolinite-coated pellet
and BOF-slag. As in the earlier sample with BOF-slag in contact
with a non-coated pellet, see figure 9, it is possible to see that
vanadium oxide once again follows calcium silicate and that
manganese oxide follows magnesium oxide. The combinations of
elements are very similar to the types found in earlier samples.
The main difference that can be found is that magnesium no longer
is present in calcium silicate. A slightly higher aluminium
concentration can also be found in this sample. Light grey areas
are high in magnesium content as seen in points 2, 4, 6, 8 and 10.
Dark grey areas have a low magnesium concentration, see points 1,
3, 5, 7,9 and 11, but are rich in CaO and SiO2. As in previous
samples containing BOF slag, only low concentrations of alkali are
found. An approximate composition for the periclase type compound
is in this case, (Mg0,51Fe0,44Mn0,05)O.
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Figure 11: “Slag” area (dark) in centre of kaolinite-coated
pellet
Figure 11 shows a “slag” area in the centre of the same pellet.
The iron oxide in this area is not fully reduced and light grey
areas are wüstite. Two types of “slag” areas are present. Lighter
grey areas are monticellite or diopside (point 1,3,4 and 7) and
darker grey areas consist of an alkali rich phase, probably
kaliophilite (point 2,5 and 6).
Pellet BOF-slag
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Sample 5-3-4 Mg Al
Si Ca Fe Figure 12: Mapping over contact between pellet and
BOF-slag. Figure 12 shows a mapping over a contact between a pellet
and a piece of BOF-slag. Magnesium is mostly present inside the BOF
slag. No detectable concentrations of alkali can be found in this
area. As in earlier point analysis two main “slag” compositions can
be found, calcium silicate and periclase.
Quartzite
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 43 44 45 46 47 48 49 50Point number
Mol%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Figure 13: Quartzite and olivine pellet
Figure 13 shows the contact between quartzite and a pellet.
Point 1 is closest to the pellet and point 50 is in the quartz. The
first points closest to the pellet are very similar. All the points
have high concentration of silica and alkali. Al2O3 is also present
to some extent. The concentration of alumina tends to increase
during the steps away from the pellets and towards the centre of
the slag former. This can be seen in points 43 to 48 compared to
points 1 to 8, as well as in the mapping analysis in figure 14. In
these points, the Al2O3 concentration is higher than the
concentration of alkali. Concentration of sodium decreases as the
distance from the pellet increases.
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Sample 4-3-8 Na Al
Si K Fe Figure 14: Mapping over quartzite and olivine pellet
Figure 14 shows a mapping over a piece of quartzite and an olivine
pellet. The highest concentrations of alkali are to be found at the
surface of the slag former. Probably due to condensation and
sticking of material. Inside the quartzite, two “slag” areas are
found. The darker areas consist of SiO2, K2O and a higher
concentration of Al2O3. The lighter areas consist of almost pure
SiO2.
Pellet-Pellet contact
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 9 10 11Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Figure 15: “Slag” area between two kaolinite-coated pellets
Figure 15 shows a “slag” area in contact with two
kaolinite-coated pellets. The point analysis shows that the lighter
grey areas consist of K2O together with Al2O3 and SiO2 with a
composition close to kaliophilite, KAlSiO4, see point 1, 2, 4, 6, 8
and 9. The darker grey areas consist of three different
“slag”-compositions, almost pure periclase MgO, forsterite Mg2SiO4,
and diopside Mg0.6Ca0.4SiO3, see point 3, 5, 7,10 and 11.
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0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
1 2 3 4 5 6 7 8 9 10Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Pellet 1 Pellet 2
Figure 16 Contact area between two kaolinite coated pellets
Figure 16 shows the contact area between two kaolinite-coated
pellets close to a piece of BOF-slag. Grey areas are “slag” and
white areas are metallic iron. The point analysis shows that the
“slag” composition can be divided into four different types. One
type of “slag” consists of K2O and minor amounts of Na2O together
with Al2O3, SiO2 and MgO with a composition close to
kaliophilite/MgO, see point 1, 3, 6, 9 and 10. The other “slags”
are periclase, a diopside-like “slag” and an iron containing
perovskite (Ca,Fe)TiO3, see point 2, 4, 5 and 7.
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
1 2 3 4 5 6 7 8 9Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Pellet 1 Pellet 2
Figure 17: Contact area between two kaolinite-coated pellets.
Magnification 278X
Figure 17 shows another contact area between the same two
kaolinite-coated pellets, see figure 16. The point analysis
consists of alkali, alumina, silica and magnesia, a
kaliophilite/MgO-like “slag”. The alkali rich “slag” area at this
contact area contains more magnesia compared to figure 16. “Slag”
areas with potassium and sodium metasilicate ((K,Na)2SiO3)
composition are also found, see point 1 and 7.
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0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
7 10 11 13 16 22 28 34 36 42 48
Point number
Mol
e-%
Na2OMgOAl2O3SiO2K2OCaOTiO2V2O5FeO
Pellet 1 Pellet 2
Figure 17: Contact area between two non-coated pellets.
Figure 17 shows the contact area between two non-coated pellets.
An iron rim has been formed at the pellet surface and the “slag”
areas are distributed within the iron structure. Points containing
pure iron have been omitted. Two main “slag” compositions were
identified: an alkaline-containing “slag” with a composition of
kaliophilite/Na,K-metasilicate, see point 7, 10, 16, 28, 34, 36, 42
and 48 The second “slag” has a low alkali content and is a CaO-MgO
silicate with an average composition near Mg0.5Ca0.5SiO3, see point
11 and 22.
FactSage calculations The thermochemical software FactSage™ has
been used to simulate the formation of solid phases of the “slag”
areas analysed with SEM. Tables 2 to 7 below show examples of
results from the simulations both at a low temperature of 400 °C
and at a high temperature, 1150 °C which is close to the
temperature in the blast furnace at the level where the baskets
were found. Table 2 shows the result of a alkali rich “slag” area
with low MgO content found at the contact of two kaolinite-coated
pellets, see figure 15 pt 1. The solid phases are kaliophilite,
nepheline, spinel and K-beta aluminate at 400 °C. When the
temperature increases to 1150 °C kaliophilite has started to melt,
nepheline has melted completely and K-beta aluminate has formed
“slag” and corundum. About 20 wt.-% “slag” is formed. Table 2:
Pellet contact between two kaolinite-coated pellets, low MgO
content. Kaliophilite Tlow = 400
oC Thigh = 1150 oC
Input amount [mole] Main comp. Chem. Formula Part comp. mole
Main comp. Chem. Formula Part comp. moleNa2O 2 Kaliophilite KAlSiO4
36 Kaliophilite KAlSiO4 29MgO 2 Nepheline NaAlSiO4 4 Slag 21Al2O3
35 Spinel 3 SiO 2 10SiO2 40 MgAl 2O 4 2 K 2O 5K2O 20 FeAl 2O 4 1 Al
2O 3 3CaO 0 K-beta alumina K2Al12O19 2 Na 2O 2FeO 1 Spinel 3
MgAl 2O 4 1FeAl 2O 4 1
Tliq = 400 oC Corundum Al2O3 12
Two simulations were also made from “slag” areas with high
alkali and magnesia content, one at the contact between two coated
pellets, figure 14 pt 4-6, and one at the surface of a non coated
pellet, figure 15 pt 8. Table 3 shows the result from the contact
between two kaolinite coated pellets. The dominating phases are
kaliophilite, monoxide and K-aluminate at 400 °C. FactSage
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was not able to calculate “slag” phases instead pure liquids
were chosen. At 1150 °C sodium and potassium metasilicate were in
liquid phase. About 17 wt.-% “slag” is formed. Table 3: Pellet
contact between two kaolinite-coated pellets, high MgO content.
Kaliophilite/K-aluminate Tlow = 400
oC Thigh = 1150 oC
Input amount [mole] Main comp. Chem. Formula Part comp. mole
Main comp. Chem. Formula Part comp. moleNa2O 4 Kaliophilite KAlSiO4
27 Kaliophilite KAlSiO4 27MgO 13 Monoxide MgO 13 Monoxide MgO
13Al2O3 20 K- aluminate KAlO2 13 K- aluminate KAlO2 13SiO2 36
K-metasilicate K2SiO3 5 Slag 9K2O 25 Na-metasilicate Na2SiO3 4 K 2
SiO 3 5CaO 0 Na 2 SiO 3 4FeO 1
Tliq = 975 oC
Table 4 shows the result from simulations of a “slag” area at
the surface of a non-coated pellet. Compared to the coated pellets
in the above simulation the alumina content is lower and there by
the K-aluminate is not found, instead the dominating phases are
potassium and sodium metasilicate. Also in this simulation FactSage
was not able to calculate “slag” phases instead pure liquids were
chosen. At 1150 °C sodium and potassium metasilicate were in liquid
phase. The higher amount of metasilicates resulted in an increased
amount of liquid phases (61 wt.-%) compared to the coated pellets,
see table 2 and 3. Table 4: Surface of regular olivine pellet
without coating. Kaliophilite/K-metasilicate Tlow = 400
oC Thigh = 1150 oC
Input amount [mole] Main comp. Chem. Formula Part comp. mole
Main comp. Chem. Formula Part comp. moleNa2O 9 K-metasilicate
K2SiO3 12 Slag 25MgO 13 Na-metasilicate Na2SiO3 9 K 2 SiO 3 11Al2O3
4 Kaliophilite KAlSiO4 8 Na 2 SiO 3 9SiO2 44 Olivine Mg2SiO4 7 K 2
Si 2 O 5 5K2O 20 K - disilicate K2Si2O5 4 Kaliophilite KAlSiO4 8CaO
0 Olivine 6FeO 9 Mg 2 SiO 4 4
MgFeSiO 4 2Fe 2 SiO 4 1
Monoxide 10MgO 6
Tliq = 975 oC FeO 3
Alkali and silica rich “slag” is also found at the contact
between coated pellet and quartzite, see table 5 and figure 10 pt
43. Because of the high silica content the dominating minerals at
low temperature are leucite (feldspathoid), and K-feldspar. At
1150°C about 19 wt.-% is “slag”, which is more than the “slag”
formed at the contact between two coated pellets, see table 2 and 3
Table 5: Kaolinite coated pellet and Quartzite. K-feldspar/Leucite
Tlow = 500
oC Thigh = 1150 oC
Input amount [mole] Main comp. Chem. Formula Part comp. mole
Main comp. Chem. Formula Part comp. moleNa2O 1 Leucite KAlSi2O6 22
Leucite KAlSi2O6 24MgO 1 K-feldspar KAlSi3O8 5 Slag 19Al2O3 17
Andalusite Al2SiO5 2 SiO 2 15SiO2 67 Albite NaAlSi3O8 2 K 2O 1K2O
13 Al 2 O 3 2CaO 0 Na 2O 1FeO 0
Tliq = 950 oC
Table 6 and 7 show the result of simulations of olivine-like
“slag” areas with high and low CaO content at the centre of a
pellet, see figure 16 pt 7 and figure 17 pt 1. With low CaO content
the dominating mineral is olivine and only a small amount
feldpathoids and kaliophilite at 400 °C.
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Olivine mineral is unaffected when temperature increases to 1150
°C and about 19 wt.-% is “slag”. Table 6: Centre-“slag” with low
calcium oxide content. Olivine/Pyroxene Tlow = 400
oC Thigh = 1150 oC
Input amount [mole] Main comp. Chem. Formula Part comp. mole
Main comp. Chem. Formula Part comp. moleNa2O 2 Olivine 25 Olivine
25MgO 50 Mg 2 SiO 4 24 Mg 2 SiO 4 24Al2O3 4 MgFeSiO 4 1 MgFeSiO 4
1SiO2 37 Nephelite NaAlSiO4 3 Slagg 16K2O 2 Leucite KAlSi2O6 2 SiO
2 8CaO 1 Kaliophilite KAlSiO4 2 Na 2O 2FeO 1 Al 2 O 3 2
CaO 1K 2 O 1
Tliq = 700 oC Leucite KAlSi2O6 2
Table 7 shows the result of the calcium oxide rich olivine-like
“slag”. At low temperature the dominating mineral is clinopyroxene
(the solid solution augite). At 1150 °C clinopyroxene has started
to melt and about 32 wt.-% is “slag”. Table 7: Centre-“slag” with
high calcium oxide content. Olivine/Pyroxene Tlow = 400
oC Thigh = 1150 oC
Input amount [mole] Main comp. Chem. Formula Part comp. mole
Main comp. Chem. Formula Part comp. moleNa2O 1 C-Pyr 11 Slagg 29MgO
14 CaMgSi 2 O 6 8 FeO 5Al2O3 1 CaFeSi 2 O 6 2 SiO 2 14SiO2 42
Melilite 7 CaO 7K2O 0 Ca 2 FeSi 2 O 7 6 C-Pyr 8CaO 28 Olivin 3
CaMgSi 2 O 6 5FeO 12 Fe 2 SiO 4 1 CaFeSi 2 O 6 2
MgFeSiO 4 1 Melilite 5FeMgSiO 4 1 Ca 2 FeSi 2 O 7 2
Na2Ca3Si6O16 1 Ca 2MgSi 2 O 7 3Wollastonite 3
CaSiO 3 2Olivine 3
Tliq = 900 oC CaFeSiO 4 1
Discussion The alkali uptake and slag formation reactions in
different pellets are highly dependent on the various slag formers.
The contact areas between lime and pellets seem to have the lowest
occurrence of alkali. The lime does not react much with the primary
“slag” in the pellets. Instead a rim of calcium- and magnesium
silicates were present in the contact areas. The BOF-slag seems to
be more reactive with primary “slag” than lime. “Slag” areas in
regular olivine pellets in contact with BOF-slag contain calcium
silicates and enhanced concentrations of MnO and FeO in periclase,
which also has been observed in earlier studies6). Areas with
BOF-slag in contact with kaolinite coated pellets are similar, but
they have a slightly higher aluminium oxide content. The contact
areas between BOF-slag and pellets seem to have a low occurrence of
alkali, both with- or without kaolinite coating present. Quartzite
on the other hand forms an acid area with a higher alkali uptake.
On the quartzite surface, high alkali concentrations were observed,
probably from gaseous alkali that has condensed on the surface.
Inside the quartzite Al2O3 was found together with K2O. The
thermodynamic calculations show that Leucite, KAlSi2O6 , is the
dominating alkaline containing mineral at both 400 °C and 1150
°C.
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In “slag” areas between two kaolinite coated pellets alkali is
mainly in the form of kaliophilite, KAlSiO4. The thermodynamic
calculations show that kaliophilite is the dominating alkaline
containing mineral at both 400 °C and 1150 °C. In “slag” areas
between non-coated pellets, alkali is mainly in the form of
kaliophilite/Na,K-metasilicate, due to the lower Al2O3 content.
Thermodynamic calculations show that in a olivine type “slag” with
low CaO content, olivine is dominating at both 400 °C and 1150 °C.
In a olivine type “slag” with higher CaO content, clino-pyroxene
dominates at 400 °C. Due to its lower melting point the amount of
“slag” is higher at 1150 °C. Acknowledgements The authors would
like to thank Mr Harry Palo at LKAB for the assistance in sample
preparation and SEM-analysis work, Mr Per-Ola Eriksson, Mr Timo
Räty, Mr Anders Keskitalo and Kjell-Ove Sjöholm for their
assistance during the blast furnace campaign and excavation work.
References 1) Sterneland Jerker, Jönsson Pär G, The use of coated
pellets in optimising the blast furnace
operation. ISIJ International, Vol 43, 2003 No.1 pp26-35. 2)
Kristina Zarins., Mineralogical evolution of kaolinite coated blast
furnace pellets,
examensarbete Uppsala universitet, 2003 3) Jerker Sterneland och
Mats Hallin., The use of an experimental blast furnace for raw
material
evaluation and process simulation., 6th Japan-Nordic Countries
Joint symposium in Nagoya, Japan 29-30 november 2000.
4) Anna Dahlstedt och Niklas Eklund., The choice of pellets in a
mixed blast furnace burden
and how it effects process conditions., The 14th Conference on
Hungarian pig iron and steel making, Balatonsèplak, 12-13 September
2002.
5) FactSage Thermochemical Software and databases. Bale C.W.,
Chartrand P.,Degterov S.A.,
Ben Mahfoud R., Melançon J., Pelton A.D., Eriksson G., Hack K.,
Petersen S. Calphad, Vol. 26, No. 2, pp. 189-228,2002. Published by
Elsevier Science Ltd.
6) Lena Sundqvist Ökvist, Co-injection of Basic Fluxes or BF
Flue Dust with PC into BF
charged with 100 % pellets, Doctoral thesis, Luleå University of
Technology, (2004), ISSN: 1402-1544
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