Development of fluxed blast furnace pellets with application

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METEC Congress 2003, 3rd Internat. Conference on Science and Technology of Ironmaking, Düsseldorf, June 16-20, 2003, pp 256-261
Development of fluxed blast furnace
pellets with application of coatings
1
Lawrence Hooey
1
Mats Hallin
2
Kalevi Raipala
1)
2)
LKAB R&D Metallurgy, Box 952 SE-971 28
Luleå, Sweden
Rautaruukki Corporate R&D, Fundia Koverhar,
Lappohja, FIN-10820, Finland
SUMMARY
High iron content fluxed pellets have been tested in
pilot and full-scale trials. The reduction behaviour of
the pellets in both pilot-scale and commercial blast
furnaces was acceptable. The furnaces' behaviour
with the experimental pellets was acceptable and had
the potential to improve the blast furnace operations.
However, there was an unexpected result in the fullscale trials: the hot metal carbon content was lower
and sulphur distribution poorer than when using
regular olivine pellets. This behaviour appears to be
associated with high temperature clustering and
meltdown properties of the fluxed pellets that have
been observed in dissections of the experimental
blast furnace. Further testing in the experimental
furnace showed that application of either quartzite or
olivine coating at 3.6 kg/t pellet restored hot metal
quality and shows potential for improving blast
furnace stability and reducing problems associated
with alkali circulation.
1. INTRODUCTION
In 1997 LKAB commissioned the Experimental Blast
Furnace (EBF) in Luleå, Sweden. The role of this
1.2m hearth diameter pilot blast furnace was to
provide an intermediate step between laboratory and
full-scale testing of experimental pellets. The furnace
is a complete blast furnace operation that produces
circa 35 tonnes/day of hot metal and has been
described previously.1,2 The EBF has been run in 11
campaigns of about 8 weeks each. After extensive
testing in the EBF, a new grade of acid pellets for use
with sinter, the KPBA pellets, were introduced in
2001.3
In addition to pellets designed for use with sinter, a
new grade of fluxed pellets designed for the Nordic
blast furnaces of SSAB and Fundia Wire are being
developed. The furnaces are currently burdened with
LKAB's olivine pellets (MPBO and KPBO).
In order to modify the properties of the pellets to suit
the furnace operation and long term development
plans, the pellet chemistry and reduction behaviour
must be considered. Nordic blast furnaces have a
number of features that have acted as the driving
force for pellet development. Among the main
features are desire for very low slag volumes (150
kg/tHM or even lower), high productivity (circa 3
t/d/m3 w.v.) and high rates of injectants and oxygen.
With these objectives in mind, the suitable pellet
properties are listed in Table 1.
The factors listed in Table 1 are the main
considerations, of course provided mechanical
strength, pelletising properties, particle size and so
on, are all satisfactory. If the pellets can be produced,
the factors in Table 1 are all affected by the choice of
additives.
Tab. 1. Summary of desired pellet behaviour and
furnace operation.
Pellet
Properties
Pellet
Chemistry
Furnace Operation
Suitable for flux injection and
100% pellet operation
Slag rate reduction
High
Reduction
strength, Low
swelling
Reducibility
High
temperature
properties
Meltdown and
slag formation
High oxygen injection (low top
gas temp.)
High productivity, stable
operation
Low residence times
Low cohesive zone, stable
burden descent
Dripping behaviour of slag and
iron meltdown must be good
for very low slag volumes
2. DEVELOPMENT OF FLUXED PELLETS WITH
COATINGS
With the objectives listed in Table 1, development of
the fluxed pellets followed the scheme in Figure 1.
Various pellet types were tested in laboratory and the
EBF before being introduced to industrial furnaces.
The EBF is quenched at the end of each campaign in
order to evaluate the material behaviour in the
furnace. Nitrogen is introduced in the top and
removed via the tuyeres to prevent oxidation and a
heat-front from moving up the furnace. The furnace is
then excavated layer by layer with extensive
sampling and evaluation of materials.
In campaign 4 excavation it was noted that the pellets
had very high reduction strength - that is the pellet
shape was maintained quite low in the furnace and
there was no indication of swelling or cracking of
pellets. A detailed comparison with other pellets is
beyond the scope of this paper.
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Pellet production in lab scale
Pellet testing in lab scale
Pellet production in pilot scale
Pellet testing in pilot scale
EBF
Pellet production in full scale
Pellet testing in full scale
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PRODUCT
Fig. 1: LKAB's Pellet development path
One observation that was noted, however, was a
tendency for pellets to form clusters. The
phenomenon appears to be a solid-state sintering of
iron during reduction. Figures 2a and b shows
clusters forming in the middle shaft of the furnace in
the campaign 4 excavation. The clustering, however,
did not affect the furnace stability and was not
reflected in the furnace operation.
It is well established that alkalis can have negative
impact on reduction behaviour in the blast furnace such as swelling4. It was decided to test coating of
the pellets to prevent sticking, and especially in the
case of the fluxed pellets, to see if alkali absorption
could be improved.
Fig. 2a: Clustered pellets forming in layer 12 of
excavation of campaign 4 with proto-type MPB1
pellets. The height above tuyeres was circa 2.4 m.
Degree of metallisation was circa 36%at mid-radius.
Fig 2b: Close-up of clustered pellets in layer 15 (3
pellet layers below Fig. 2a. The height above tuyere
level was 2.0 m. Degree of metallisation was 71% at
mid-radius.
For these general reasons, in EBF campaigns 7 and
8 fluxed pellets were tested with applications of
olivine, quartzite and dolomite sprayed in slurry form
onto the pellets. The coating amount was chosen at
3.6 kg coating material/tonne pellets plus 0.4 kg
bentonite to improve the binding of the coating.
3. Laboratory Evaluation of Fluxed Pellets
After various experimental pellet types had been
tested, a type called 'MPB1' fluxed pellet emerged as
having properties and chemistry that were the most
suitable for possible replacement of MPBO pellets in
Nordic blast furnaces.
The MPB1 fluxed pellet composition and metallurgical
results are compared to MPBO in Tables 2 and 3.
The chemistry of the MPB1 is suitable because it
allows removal of some of the limestone charged to
the furnace. This in turn decreases the required
thermal energy and coke rate. The metallurgical
properties as measured in the laboratory are as good
as or superior to MPBO with higher compression
strength, LTB, higher softening temperature and
lower pressure drop.
Tab. 2: Chemistry of MPBO and MPB1 Experimental
pellets
MPBO
MPB1
Olivine Pellets
Experimental
Fluxed Pellets
Fe
66.8
66.5
CaO
0.29
1.65
MgO
1.46
0.41
SiO2
2.15
1.70
Al2O3
0.41
0.38
CaO/SiO2
0.13
0.97
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Tab. 3: Metallurgical properties of MBPO and
MPB1 pellets
MPBO MPB1
Cold Strength (daN)
213
289
ISO 13930 LTB (%+6.3)
74
80
ISO4695 R40 (%/min)
0.54
1.1
TRT7992 dP
(mmWg)
18
2.4
High temperature
1263
1310
softening temperature* oC
32
17
Swelling (LKAB test)**
*
Rautaruukki's softening and melting test
**
LKAB's swelling test using higher temperature
o
(1000 C) that yields higher values than the ISO4698
o
test (900 C)
4. EBF TESTING OF FLUXED PELLETS
The final MPB1 testing was made in the EBF in
campaigns 7 and 8. Tables 4,5 and 6 show
summaries of operating data.
As the furnace operating periods are typically 2-3
days long the furnace fuel rate cannot necessarily be
optimised. Furnace stability is considered the most
relevant for comparing pellets. The standard
deviations in ETA CO, burden descent rate and
burden resistance index are used to evaluate the
stability. Production statistics such as fuel rate and
productivity can be compared for longer experimental
times or if extreme behavioural differences are
present. Table 4 and 5 show that the stability of the
MPB1 is the same or better than MPBO. Table 6
shows that there were no significant differences in
fuel rate or production in pilot scale.
Coated MPB1-type pellets were also tested in
campaigns 7 and 8. In campaign 7 the goal was to
establish if the coating technique and to see if the
coatings remained on the pellets after coating,
transportation, screening and charging to the furnace.
This preliminary testing proved successful, but the
periods were too short to achieve a very reliable
comparison. Comparison of flue dust generation in
Figure 3 showed that coating material was not being
removed via top gas. Chemical assays also showed
that no appreciable coating was lost in transport and
handling.
12
In campaign 8, the coated MPB1 pellets were tested
for longer periods (circa 2-3 days) in the EBF. Table 5
shows the basic results of stability were again
comparable to MPB1 or MPBO.
Tab. 4: Summay of furnace operation in Campaign 7
testing of MPB1 Pellets
-----ETA CO---- ---PV Bosh --- Descent
Time Average STD Average STD
STD
h
MPBO
27
45,6
1,0
5,9
0,2
1,2
MPB1
76
46,1
0,9
6,3
0,9
0,7
2
2
1.7
Pvbosh =(Pblast -Ptop )/Vbosh
where P is in atm
3
2
absolute; Vbosh = bosh gas volume Nm /s/m hearth
area
BDR= burden descent rate, cm/min
Tab. 5: Summary of furnace operation in Campaign 8
testing of MPB1 pellets, and MPB1 pellets with
coatings of quartz (MPB1-Quartz) and olivine (MPB1Olivine).
-----ETA CO---- ---PV Bosh --- BDR
Time Average STD Average STD STD
h
MPBO
60
47.6
1.0
6.0
0.3
0.55
MPB1
42
47.4
1.1
7.3
0.6
0.52
MPB1- 67
46.9
0.9
7.2
0.7
0.35
Quartz
MPB1- 76
47.5
1.4
6.6
0.4
0.48
Olivine
Tab. 6: Summary of furnace operation in Campaigns
7 and 8.
Prod.
Blast
Blast
Coke
Coal
3
t/h
Nm /h
O2
rate
rate
%
kg/tHM kg/tHM
Cam.7
MPBO 1.32
1721
22.5
441
90
MPB1
1.32
1725
22.6
442
98
Cam.8
MPBO
MPB1
MPB1Quartz
MPB1Olivine
1.55
1.56
1.54
1737
1738
1744
24.7
24.7
24.7
403
400
400
127
123
127
1.57
1744
24.7
396
124
kg flue dust/thm
10
8
5. FULL-SCALE TESTING OF FLUXED PELLETS
6
4
2
0
MPB1
MPB1 +
Dolomite
MPB1 +
Olivine
MPB1 +
Quartzite
MPBO
Fig. 3: Flue dust generation measured in campaign 7
for coated and uncoated pellets.
The MPB1 pellets were tested in two industrial
furnaces described in Table 7. The operations
differed slightly with Fundia furnace operating with oil
injection and SSAB Oxelösund operating with coal
injection. Both furnaces continued to use other pellets
in the burden during the one to two week trial periods.
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The results of the trials showed little change in the
fuel rate and production rate statistics.
Tab. 7: Blast furnaces used for full-scale testing
Fundia
SSAB
Oxelösund #2
567
760
Working volume m3
3
2.9
2.5
Productivity t/m /d
Injectants kg/tHM
Oil c. 90
Coal 95
26.5
24
O2 in blast %
Slag rate kg/tHM
c. 160
c. 155
Tab. 8: Summary of BF production and fuel rate at
SSAB Oxelösund
Time Burden
Prod. Fuel rate
days
Rate Kg/thm
t/d
Ref.
18
70% MPBO
1920 473
30% KPBO
Test
8
c. 60% MPB1 1920 472
40% KPBO
furnaces would have to operate either with higher
alkali circulating loads or with higher sulphur content
hot metal. Either way, the behaviour was undesirable.
The reason the oxygen potential increased occurred
cannot be determined directly from the trial data.
However, it was thought that the clustering behaviour
seen in the excavation might have an impact on the
full-scale furnaces that was not visible in the EBF. In
EBF campaign 8, which was running in parallel to the
full-scale trial, was evaluated in more detail for the
hot metal-slag quality relationships.
4,8
4,4
[C]
4,0
MPBO
3,6
MPB1
3,2
0,0
Tab. 9: Summary of BF production and fuel rate at
Fundia Wire, Koverhar.
Time Burden
Prod. Fuel rate
days
Rate Kg/thm
t/d
Ref.
14
80% MPBO
1553 466
20% other
Test
8
60% MPB1
1548 466
20% MPBO
20% other
However, the behaviour of the silicon, carbon,
sulphur and potassium in the slag and hot metal
showed unexpected but very consistent trends in
both the industrial furnaces. Looking at the
relationships between silicon and carbon, both
furnaces showed a drop in carbon content for
equivalent hot metal silicon content (Fig. 4 and 5).
The MnO/Mn relationship, a good indicator of the
oxygen potential in the hearth, showed that the
oxygen potential increased when MPB1 pellets were
used (Tab. 10).
Tab. 10: Relationships showing (MnO)/[Mn] ratios
indicating higher oxygen potential in the high MPB1
periods.
Reference
High MPBI Period
Period
Fundia Wire
2.39
2.69
SSAB Oxel #2
1.62
1.98
The increase in oxygen potential is clearly reflected in
the poorer desulphurisation relative to alkali output.
Figures 6 and 7 show that the distribution of sulphur
between slag and metal became poorer for
equivalent alkali output - which means that the
0,2
0,4
MPBO(>80%)
[Si]
0,6
0,8
1,0
MPB1 (>60%)
Fig. 4: Fundia Wire results showing a drop in hot
metal carbon content for a given silicon content.
5,0
4,6
[C]
4,2
3,8
0,3
0,5
0,7
[Si]
MPBO 70%, KPBO 30%
0,9
MPB1 70%, KPBO 30%
Fig. 5: SSAB Oxelösund results showing a similar
drop in hot metal carbon content for a given silicon
content.
100
75
(S)/[S]
MPBO
50
25
MPB1
0
0,2
0,4
0,6
0,8
K2O Content of Slag
MPBO (>80%)
1,0
MPB1 (>60%)
Fig. 6: Fundia Wire results showing poorer
desulphurisation for a given alkali output.
1,2
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50
MPBO + KPBO
80
40
(S)/[S] 30
MPB1-Quartz
60
MPB1-Olivine
MPBO
(S)/[S]
20
40
10
MPB1 + KPBO
20
0
0,2
0,4
0,6
0,8
Slag K2O Content
MPBO 70%, KPBO 30%
MPB1
0
1,0
0,1
MPB1 70%, KPBO 30%
MPBO
Fig. 7: SSAB Oxelösund results showing poorer
desulphurisation for a given alkali output
Log (Slag K2O,%)
The coating of the MPB1 type pellets appears to
successfully alleviate the problem of hot metal
quality, with consistently higher carbon content and
better sulphur distribution and alkali output (Fig. 9).
The alkali output was seen to be better for equivalent
optical basicity for coated-MPB1 pellets compared to
uncoated MPB1 pellets (Fig. 10).
4,2
MPB1 MPBO
MPBO
MPB1
MPBO-Quartz
MPB1-Olivine
Fig. 8: Results of EBF Campaign 8 showing higher
carbon content versus silicon for coated MPB1 pellets
compared to both MPB1 and MPBO pellets.
MPB1
MPB1-Olivine
-0,6
MPB1-Quartz
0,66
0,67
0,68
0,69
MPB1-Quartz
MPB1-Olivine
Clustering of pellets, combined with high meltdown
temperatures could delay carburisation of the iron
that is essential for the lowering of oxygen potential in
the iron and slag.
[C] 4,4
1,75
-0,5
Fig. 10. Results of EBF Campaign 8 showing higher
alkali output for a given optical basicity of slag for
coated MPB1 pellets compared to uncoated MPB1
pellets
4,6
1,5
-0,4
MPB1
MPB1-Olivine MPB1-Quartz
[Si]
-0,3
Optical Basicity
The particular success of coating of MBP1 pellets
compared to uncoated MPB1 or MPBO appears
linked to two phenomena noted in studies of
materials removed from probe samples and from the
excavations:
- Clustering
- Alkali circulation
1,25
MPB1-Olivine
-0,2
-0,7
0,65
7. DISCUSSION
1
0,7
-0,1
The results from EBF campaign 8 are consistent with
the behaviour observed in the full-scale tests. Figure
8 shows the relationship of hot metal C and Si for the
pellets in campaign 8. The MPB1 pellets clearly show
lower carbon contents for equivalent hot metal silicon
contents.
4
MPB1
0,5
K2O wt%
MPB1-Quartz
Fig. 9. Results of EBF Campaign 8 showing higher
carbon content versus silicon for coated MPB1 pellets
compared to both MPB1 and MPBO pellets.
6. EFFECT OF COATING OF FLUXED PELLETS
4,8
0,3
2
The clustering of pellets in the blast furnace process
has not received attention. Due to the stability of the
descent of MPB1 pellets, scaffolding and clustering
do not appear directly related. The reduction and
meltdown conditions of the blast furnace are very
complex with large amounts of circulating potassium,
sulphur compounds, zinc, interaction with other
burden components, as well as temperatures beyond
the melting point of iron. The minerals applied to the
pellet surface are likely not the same materials on the
surface at the start of clustering. The interactions
between reducing gas, reduction and melting
behaviour at the pellet surface, effect of coating
minerals on sulphur and alkali distribution in the
furnace and other factors must be considered.
Some alkali behaviours have been studied in the
EBF. Generally, alkalis are stable in silicates and but
are unstable or unreactive with basic materials.
However, the form of the material must be
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considered. For example, coarse quartzite does not
appear highly reactive, with only a surface reaction
taking place, as shown in the example in Figure 11.
In the case of olivine pellets or acid lump ore,
potassium appears to be reacting to form K2O-SiO2FeO slags. For alkali control, silicates (olivine or
quartzite) coatings are likely to be effective as
indicated in the campaign 8 results in Figure 10. The
coating particle sizes are less than 100 micron,
thereby giving a very high surface area for reaction.
*1*2*3
1) Coating MPB1 pellets with 3.6 kg olivine or
quartzite improved the desulphurisation and
carburisation of the hot metal, and appeared to
improve the furnace stability and hot metal
quality.
2) Coated-MPB1 pellets may be a suitable
replacement for MPBO pellets.
3) The behaviours of the EBF and the full-scale
furnaces were very similar. The EBF is providing
reliable evaluations of pellet quality for full-scale
furnaces
9. FUTURE WORK
Full-scale testing of coated-MPBO pellets is
underway at the time of writing.
ACKNOWLEDGEMENTS
1 mm
Relic silica
86% SiO2, 11% K2O, 2% Na2O
33% CaO, 33% SiO2, 19% Al2O3, 5% MgO, 2% K2O,
2% Na2O, 2% S, 1% FeO, 1% MnO.
Fig. 11: Reaction of alkali with coarse quartzite in a
probe sample from the start of the cohesive zone of
the EBF. On the right is quartzite additive, on the left
is a piece of basic sinter.
These preliminary tests of coating of MPB1 pellets
were followed by testing of coated MPBO pellets, with
successful results.5
8. CONCLUSIONS
From both pilot -scale and full-scale testing of MPB1
pellets, the following conclusions can be made:
1) MPB1 fluxed pellets gives similar production, fuel
rate and general blast furnace operation as
MPBO pellets.
2) Replacing MPBO pellets with MPB1 pellets
results in different slag formation behaviour which
affected the desulphurisation and alkali behaviour
in the blast furnaces.
3) Replacing MPBO pellets with MPB1 pellets
resulted in a lower carbon content hot metal for a
given hot metal silicon level.
The behaviours 2-3 were impossible to predict in
laboratory scale, but were detected in experimental
blast furnace trials of 2-3 days for each test material.
From the results of the coating of MPB1 pellets and
testing in the EBF the following can be concluded:
We wish to thank SSAB Oxelösund and Fundia
Koverhar for their encouragement and for permission
to publish this work.
REFERENCES
1. Sterneland, J.; Hallin, M.: “The Use of an
Experimental Blast Furnace for Raw Material
th
Evaluation and Process Simulation“, 6 JapanNordic Countries Joint Symposium, Nagoya,
Japan, November 2000.
2. Dahlstedt, A.; Hallin, M.; Tottie, M.: “LKAB's
Experimental Blast Furnace for Evaluation of Iron
Ore Products“, Proceedings of Scanmet 1, Luleå,
Sweden, 1999.
3. Hooey, L.; Sterneland, J.; Hallin, M.: “Evaluation
of Operational Data from the LKAB Experimental
th
Blast Furnace”, 60 Ironmaking Conference
Proceedings, March 2001.
4. George, D.W.R.; Peart, J.A.: “The Influence of
Alkalis on Blast Furnace Performance“, Alkalis in
Blast Furnaces, McMaster Symposium on Iron
and Steelmaking, Hamilton, 1973.
5. Sterneland, J.; and Jönsson, P.G.: “The Use of
Coated Pellets in Optimising the Blast Furnace
Operation“,ISIJ, 43 (2003), Nr. 1, p.26-35.