TRIALS WITH MIXED BURDEN IN THE LKAB EXPERIMENTAL BLAST FURNACE

Nicklas Eklund

LKAB Research and Development

Box 952 SE – 971 28 Luleå

nicklas.eklund@lkab.com

Abstract Experimental fluxed pellets were produced and tested in the LKAB Experimental Blast Furnace (EBF). The fluxed pellets were mixed with sinter and lump ore. As a reference KPBA acid pellets were tested with the same mixture. Evaluation of EBF operation showed the KPBA pellets to be superior to the experimental fluxed pellets with respect to reductant rate and the balance between alkali output via slag and desulphurisation. Both the behaviour in the furnace and examination of materials sampled with the EBF shaft probes indicate that the softening and melting behaviour of the mixture of fluxed pellets and basic sinter was not as suitable as the acid pellets in the mixture.

Introduction At the end of 1997 the LKAB Experimental Blast Furnace (EBF) was blown-in. The EBF is an important tool for the blast furnace process and material testing and it is of great importance in the research and development work at LKAB. With the EBF it is possible to overcome the vast step between laboratory scale testing and the operation of commercial blast furnaces. The EBF is used successfully not only for developing new blast furnace products it is also an excellent tool for testing different process conditions. During campaign 11 of the EBF a mixed burden trial was conducted. Two periods within the comprehensive trial period was compared. In both periods pellet, sinter and lump ore was used in the same burden ratio. In the first period the commercial pellet KPBA was used in the burden mixture. LKAB launced the KPBA in 2001. It was developed to be a complement to higly basic sinter and work well in mixed blast furnace burden. In the second period a basic experimental pellet was used. Difference between the two pellet types was seen by monitoring process data. When including the basic experimental pellet in the ferrous burden higher coke rate, lower hot metal temperature, lower gas utilisation and variation in furnace pressure drop compared to the KPBA was seen. 1. The LKAB Experimental Blast Furnace The LKAB Experimental Blast Furnace was built and in comissioned at the end of 1997. It is fully owned by LKAB and is situated at Mefos premises in Luleå, Sweden. It is an important tool for LKAB when developing iron ore pellets. The LKAB experimental blast furnace has been described in several publications[1, 2, 3] and will only be briefly introduced here. A summary of plant and operation characteristics is described. General specifications and working parameters are shown in table 1 and the plant layout is shown in figure 1.

Figure 1. Experimental blast furnace plant layout.

Table 1. EBF characteristics Working volume 8,2 m3

Hearth diameter 1,2 m Working height 5,9 m

3 Tuyeres 54 mm diameter Top pressure Up to 1,5 bar

Charging Bell top/MA Injection Coal, oil, natural gas,

slag formers Blast Up to 2000 Nm3/h

Blast heating Pebble heaters Max. blast temp. 1250 oC Tapping volume c. 1,5 t/tap

Tap time 5-10 min Tapping interval c. 60 min Reducing agents 510-540 kg/tHM

Burden material are screened at +6 mm for pellets, 6-50 mm for sinter, coke 15-30 mm and fluxes 10-20 mm. The burden material is stored in large bins outside the furnace which is situated in door. Before entering the plant a sample of the iron ore, coke or slag former are taken for further analysis (chemical or physical). Iron ore can be stored in one of four, coke is stored in one larger bin and slag formers is stored in four small bins. A skip transport material to a receiving hopper. Either a bell or a rotary Top Charger is used to charge the burden material. While using the bell system there is a moveable armour for distribution control. Pebbles heaters provides up to 1250 oC in blast temperature. To minimise heat losses insulating refractories were chosen. The hot blast enters the furnace through one of three tuyeres. The tuyeres are separated by 120 degrees and has a diameter of 54 mm resulting in a blast velocity of 150 m/s at normal blast volume. The experimental blast furnace is equipped with one taphole. Hot metal and slag is taped into a ladle. Normal tap-to-tap time is one hour. At each tapping hot metal and slag are sampled and sent to SSAB for analysis. Typical response time is c. 20 min. Top gas is transported through the uptakes and down comers to a dust catcher. The gas is further cleaned in a venturi scrubber and a wet electrostatic precipitator. Finally the top gas is flared in a torch. During operation it is possible to collect material or gas and temperature by in-burden probes, figure 2. Horisontal probes can be positioned at five different furnace heights. One inclined probe collects material from the bosh area. One vertical temperature measerment gives the temperature profile over the blast furnace height. Figure 2. In-burden probes at the EBF.

The EBF is equipped with an ABB control system for the continuous monitoring. The data in the ABB system is then transported to an OracleTM database for evaluation using spreadsheets and statistics software. The database is also connected to “Ironmaster”, a data retrieval, analysis and display software for monitoring on a short-term basis and examining data from the database. Chemical analysis of charged and tapped materials are also put into the database. As to now the experience is that the EBF is a very sensitive tool for detecting differences in properties for different pellets[4, 5]. The response time is much shorter for the experimental furnace compared to a commercial furnace. Evaluation of data and comparisons to full-scale operation has been described[2]. 2. Experimental Developing blast furnace pellets is an extensive work. Several types of pellets are produced in small amounts for laboratory testing. The most appropiate pellet, in terms of chemical analysis and metallurgical properties, is produced in full-scale and shipped to the experimental blast furnace. 2.1 Material During campaign 11 a mixed bureden was tested in the experimental blast furnace. One experimental pellet, pellet F, were compared to a reference pellet, KPBA. The KPBA was designed to work well in mixed burdens. Both pellet types were mixed with different types of iron bearing materials, sinter and lump ore. Table 2 shows information of the burden material. Table 2. Chemical analysis of tested material Name Type of iron ore Fe wt% CaO/SiO2 MgO KPBA Acid 66,8 0,23 0,57 Pellet F Fluxed 65,8 1,05 0,56 Sinter 59,8 1,65 1,47 Lump Ore 67,5 0,03 0,12

2.2 Trial periods in the EBF Intention of the test in the experimental blast furnace was to verify the behaviour of a fluxed pellet mixed together with sinter and lump ore . Properties and blast furnace performance of the fluxed pellet is compared to a reference pellet, KPBA. This acid pellet is originally made to work well in mixed burdens. As can be seen from table 3, period 1 and period 2 are identically except for the pellet difference. Evaluation made in this paper is focusing on these two periods. The length of each period is restricted to stable operation, i.e. influence of furnace stop, extra charge of coke; blast changes etc should not affect chosen periods. With these criteria the operation time for period 1 is 45 hours and for period 2 it is 33 hours, table 3.

Table 3. Comparison of trial periods in the EBF Period description Period 1 Period 2 Duration (h) 45 33 Burden ratio (%) KPBA 38 Pellet F 38 Sinter 52 52 Lump Ore 10 10

2.3 Description of softening-melting test at LKAB[6] High temperature laboratory testing at LKAB is performed in a graphite crucible and nitrogen atmosphere on pre-reduced samples. The pre-reduction is carried out in a blast furnace simulating test regime where temperature and gas composition (N2, CO, CO2 and H2) is continuously altered. The sample weight is recorded. For the high temperature testing, samples are pre-reduced to a reduction degree of 65%. During the high temperature test the temperature is increased from 900 to 1550 oC, in an electrical high frequency furnace at a selected rate. A load is applied to the sample at 1100 oC and any changes in bed height monitored. The mass of melt dripping out of the sample crucible is measured. Tests were performed on pellet and sinter mixtures. 3. Results 3.1 Characteristics of burden material in laboratory testing Table 4 illustrate results from laboratory testing. As can be seen in table 4 there are differences between the two pellets tested in the experimental blast furnace. Calcium is added as CaCO3 in the pelletisizing process which will dissociate into CaO in a narrow temperature interval. Calcium oxide is very reactive and will react with the surroundings, mainly iron oxide. At temperatures above 1200 oC calcium ferrites melt and develop a slag containing silicate bonds. These bonds are a complement to the hematite bonds[7]. Higher cold compression strength (CCS)-values of the pellet F is an indication of more silicate slag bonding in the pellet. The melt formation in the pellet core starts at lower temperature. This liquid phase leads to structural and volume changes of the pellet. The core shrinks, compared to the more rigid shell, when magnetite particles draws together. Strain occurs which may lead to cracks[7]. During reduction these strains are released and will make the pellet to break. Thus, pellet F has lower ISO13939-values. There is a great difference in pressure drop in the reduction under load test, ISO7992. Due to its high basicity of pellet F it will not be as much deformed as KPBA at test temperature. Table 4.Characteristics of material tested in the experimental blast furnace ISO13930 ISO4695 ISO7992 ISO4700 +6,3 mm +3,15

mm -0,5 mm

R40 %/min ITH +6,3 mm

ITH –0,5 mm

DP mmVp

R40 %/min

CCS avg. daN

KPBA 85,6 95,8 2,6 0,58 70 25,1 23 1,14 260 Pellet F 66,2 75,6 20,1 1,05 72,6 18,9 1,9 1,2 292 Sinter 1,35 74 10,4 Lump 0,79 42,1 30,2

3.2 Results from the EBF A summary of period 1 and 2 is given in table 5 and table 6. To faciliate evaluation of period 1 and period 2 blast furnace parameters as blast temperature, top pressure, oxygen enrichment etc was kept as constant as possible. Table 5. Operation parameters Period 1 Period 2 Operation time (h)

45 33

Blast temp. (oC)

1247 1247

Blast pressure (bar)

1,32 1,37

σblast pressure 0,023 0,021 Top pressure (bar)

1,0 1,0

σtop pressure 0,02 0,02

Table 6. Fuel rates during operation Period 1 Period 2 Coke rate (kg/tHM)

415 421

Oil rate (kg/tHM) 113,8 113 Red.rate (kg/tHM)

528,8 534

EtaCO 44,87 43,38 σetaCO 1,12 1,18 Slag rate (kg/tHM)

235,5 236

During operation in the experimental blast furnace it was seen that higher fuel rate in period 2 did not increase the heat level in the blast furnace, figure 3. Operation time for period 1 was 45 h and 33 h for period 2.

Figure 3. Hot metal temperature and coke rate during period 1 and 2.

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Period 1 present a more stable blast furnace operation, figure 4. In period 2 nitrogen and the direct reduction rate, based on oxygen balance, showed an unstable behaviour.

Figure 4. Nitrogen and DRR during period 1 and 2. With higher fuel rate in period 2, +6kg/tHM, it is expected to have lower burden resistance index (BRI) due to relatively higher coke rate. Also the heat level in the EBF should be higher compared to period 1. However the EBF operation does not show higher heat level, figure 5. This means that the KPBA/sinter mixture works in a more favourable way compared to period 2.

Figure 5. Blast furnace pressure drop linked to the heat level. Figure 6 shows the sulphur ratio and alkali behaviour. There is 100% alkali output via slag at approximately 0,55 %K2O. Operation in period 1 with KPBA shows better alkali output via slag even though this period had a higher basicity, table 7. This can be seen in the sulphur distribution and potassium behaviour in figure 6.

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(%)

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3,55

3,6

3,65

3,7

3,75

3,8

3,85

3,9

3,95

4

0 0,5 1 1,5 2 2,5

%Si

BR

I

Period 1 Period 2

Figure 6. Alkali and sulphur content in the slag. Table 7. Average slag composition during period 1 and 2 Period 1 Period 2 SiO2 36,5 38,6 CaO 36,1 34,4 MgO 12,6 12,3 Al2O3 15,3 14,7 K2O 0,40 0,40 TiO2 0,56 0,79 MnO 0,15 0,24 V2O5 0,02 0,03 Basicity, B2=CaO/SiO2 0,99 0,90

There is instability in the slag oxidation state during period 2 as shown in figure 7 by the variation in vanadium and manganese.

Figure 7. Variation in slag during period 1 and period 2.

0

0,05

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Period 1 Period 2

3.2.1 Consideration of charged limestone A calculation has been made on the gas utilisation. Due to the difference in charged amount of limestone, table 8 shows the effect on gas utilisation without limestone. Various amount of limestone were charged due to the difference in the pellet feed. Period 1 used 40 kg/tHM of limestone whereas period 2 used 20 kg/tHM of limestone. Table 8. Initial conditions Period 1 Period 2 Top gas volume, Nm3/tHM 1789,62 1804,42 CO in top gas, Nm3/tHM 439,71 449,30 CO2 in top gas, Nm3/tHM 357,85 344,28 Original etaCO 44,87 43,38 CO2 from limestone, Nm3/tHM

8,32 4,01

Assuming 100% of limestone calcinates in the lower temperature zone in period 1 and period 2, table 9. Even though all CO2 generated by the limestone is deducted from the two periods, the gas utilisation in period 1 is higher. Table 9. Modified gas utilisation by deducting CO2 from limestone Period 1 Period 2 CO2 in top gas, Nm3/tHM, deducting CO2 from limestone

349,53 340,27

EtaCO 44,29 43,10 3.3 Results from meltdown test Two mixtures of burden material was used in the meltdown test. In both cases the burden ratio was 50% sinter and 50% pellet. Results, figure 8, show that the mixture of sinter/KPBA melt at a lower temperature, approximately fifty degrees, compared to the other mixture, sinter/pellet F. For both mixtures about 3% of the total material weight forms a primary melt for a short temperature interval before the rest of the material melts out.

Figure 8. Results from meltdown test with pellet/sinter-mixtures.

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1101 1147 1194 1241 1287 1334 1381 1427 1474 1520

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Primary melt

Also, interrupted meltdown test were performed to try to investigate the constituents of the primary melt for both pellet types. Both laboratory reduced and ferrous burden sample from the experimental blast furnace were used. Pellets and sinter from the lower shaft probe were introduced to the meltdown test. Table 10 shows the chemical analysis, in wt%, from the primary melt in the interrupted test. The primary melt of sinter/KPBA from the experimental blast furnace and from the lab-reduced mixture the melt did not contain any visible slag. From the sinter/pellet F mixture the primary melt consisted only of slag, table 10. Table 10. Chemical analysis of the first melt from the meltdown test Period 2

Probeno. 11142 Lab-reduced

Sinter/Pellet F SiO2 22,51 19,66 Al2O3 4,3 3,38 TiO2 1,17 1,33 MnO 0,56 0,87 MgO 5,44 5,34 CaO 29,22 27,08 Na2O 0,57 0,05 K2O 3,64 0,07 V2O5 0,73 0,82 P2O5 0,16 0,19 FeO 31,7 41,20

4. Discussion It can be concluded that the period 2, indicates following characteristics: • Higher coke rate, approximately 6 kg/tHM • Lower hot metal temperature • Lower gas utilisation • Unstable direct reduction and nitrogen content in the top gas • Variation in the pressure drop over the blast furnace Blast furnace operation at SSAB Luleå, 1994, with self-fluxed pellet, 100% pellet burden, indicated for irregularities[8]. The pressure drop of burden increased and fluctuated in a wide range and the heat flux at the lower part of the shaft increased. It was considered[9], theroretically, that an excessive basic slag lead to slag formation problem. During the evaluation work of the EBF trial an attempt was made to see if the irregularities in the second period were due to slag formation problem. The first melt collected from the interrupted meltdown test is a basic slag, table 10, with an FeO content ranging from approxiamtely 32 % to 41%. Alkali content of sample from the lower shaft probe is higher since this material has been exposed to some alkali recirculation. When the sinter/pellet F mixture reach the cohesive zone area it is expected to start softening and eventually melt. First melt, table 10, will enter part of the coke slit. The FeO content decreases and the residual slag composition alters. Since an high FeO content will lower the melting point of slag the residual slag increases its melting point.

In figure 9 and figure 10 MgO has been added to CaO. As the FeO content in figure 9 and figure 10 is decreased from 30% to 15% the melting point is increased from approximately 1350oC to 1650oC. Also, viscosity will increase with an increased melting point.

Figure 9. Phase diagram with 30% FeO[10].

Figure 10. Phase diagram with 15% FeO[10]. An increased melting point will make the slag to solidify in the coke slit. Thus, part of the coke slit will not be in use for distribution of bosh gases and the FeO content of the slag will not be reduced entirely. Thus, some FeO will proceed below the softening and melting zone into the dripping zone. Further reduction will take place. As the reduction of FeO is endothermic the heat level is decreased. To maintain the heat level more fuel is needed. In period 2, sinter/pellet F, disturbances is seen, section 3, and the coke rate is 6 kg/tHM higher compared to the reference production KPBA.

Acknowledgements The author would like to thank Lawrence Hooey, Mats Hallin, Guangqing Zuo and Bo Lindblom for fruitful discussions. A special thank to Hans Nordström, Johan Bucht and Jan-Erik Hägg in Malmberget for making laboratory testing and chemical analysis. Finally I would like to thank the probe sampling team during campaign 11, under the supervison of Per-Ola Eriksson. References [1] M. Hallin: “Evaluation of Ferrous Burden Properties in an Experimental Blast

Furnace After Quencing and Dissection”, International BF Lower Zone Symposium, Wollongong, (2002), 22-1.

[2] L. Hooey, J. Sterneland, M. Hallin: “Evaluation of Operational Data from the LKAB Experimental Blast Furnace”, Iron & Steel Society’s 60th Ironmaking Conference, (2001), 197.

[3] J. Sterneland, M. Hallin: “The Use Of An Experimental Blast Furnace For Raw Material Evaluation and Process Simulation”, Proc. 6th Japan-Nordic Countries Joint Symposium, (2000), 9.

[4] L. Sundqvist-Öqvist, A. Dahlstedt, M. Hallin: “The Effect on Blast Furnace Process of Changed Pellet Size as a Result of Segregation in Raw Material Handling”, Iron & Steel Society’s 60th Ironmaking Conference, (2001), 167.

[5] L. Hooey, J. Sterneland, M. Hallin: “Evaluation of High Temperature Properties of Blast Furnace Burden”, 1st Internat. Meeting on Ironmaking, Belo Horizonte, Brazil, (2001).

[6] A. Dahlstedt, N. Eklund: “The Choice of Pellets In A Mixed Blast Furnace Burden and How It Effects Process Conditions”, Proc. 14th Conference on Hungarian Pig Iron and Steel Making, (2002).

[7] V. Niiniskorpi: “Phases and Microsturctures in LKAB’s Olivine- and Dolomite-Fluxed Pellets”, Ironmaking conference proceedings, (2001), 767.

[8] L. Hooey, B. Sundelin: “Evaluation of High Temperature Properties of High Iron content Fluxed pellets”, ICSTI/Ironmaking Conf. Proc., (1998), 1609.

[9] J. Ma: “Injection of Flux Into the Blast Furnace via Tuyeres For Optimising Slag Formation” ISIJ International, 39 (1999), 697.

[10] Slag Atlas (Ed. Verein Deutscher Eisenhüttenleute), 2nd edition, Düsseldorf, 1995, chapter 3.