THE CHOICE OF PELLETS IN A MIXED BLAST FURNACE BURDENAND HOW IT EFFECTS PROCESS CONDITIONS

Nicklas Eklund and Anna DahlstedtLKAB Research and DevelopmentBox 952, S-971 28 Luleå, Sweden

Phone: +46 920-38000 Fax: +46 920-19505firstname.lastname@lkab.com

ABSTRACT

LKAB olivine blast furnace pellets (KPBO, MPBO) were developed in the early 1980’s andhave proven outstanding properties, particularly in 100% pellet operation on low slag volumesand high production rate, as demonstrated by SSAB Tunnplåt AB. This has to a large extentbeen attributed to the high temperature properties of olivine pellets. The olivine addition ren-ders the pellet a high melting temperature and a narrow melting interval. When used in mixed(sinter-pellet-lump ore) burdens this has not always been regarded as an advantage, as it is theproperties of the mix that needs to be optimised, rather than that of an individual component.Therefore, the first objective, when the LKAB Experimental Blast Furnace was ready for ope-ration in 1998, was to develop and test a pellet specially designed to give good blast furnaceprocess conditions when mixed with basic sinters. The idea was to create a pellet with hightemperature properties matching sinter and with an acid slag phase that mixes well with thebasic sinter slag. Naturally reducibility and pellet strength during reduction had to be main-tained at good levels. As a result of this work, KPBA was launched in 2001. Examples fromthe development work including trials in the LKAB Experimental Blast Furnace are presentedand the differences in process conditions between sinter operation with KPBO and KPBAtype pellets will be discussed from results in laboratory testing.

1 BACKGROUND

The goal for LKAB is to increase pellet production and aim for the position as the leadingsupplier of blast furnace pellets, in terms of product quality and pellet performance1. The stra-tegy is to ensure that the use of LKAB pellets and access to research facilities creates addedvalue in the customer process, compared to the use of sinter or pellets from any competitor. Acornerstone in the research and product development effort is the LKAB Experimental BlastFurnace. It has been in operation, in Luleå since 1998.

The decision to build the experimental blast furnace was based on previous experience frompellet development work, clearly showing that the step from laboratory testing to a trial in afull-scale blast furnace is big and risky. Although a number of laboratory test routines haveevolved over the years, with static or dynamic test conditions testing burden characteristics,none of these can be used to predict blast furnace process behaviour with a new burden mate-rial. Proper evaluation of pellet performance from full-scale trials is also rather difficult andwhen trials can be conducted is governed by a number of factors, other than the needs fromthe product development work. After an initial campaign to try out the furnace, eight testcampaigns have been run in the experimental blast furnace, each lasting five to ten weeks.

1.1 LKAB Blast Furnace pellets

An increasing portion of LKAB’s production constitutes of pellets and olivine blast furnacepellets (KPBO and MPBO) has become LKAB’s major product. They were first developedtwenty years ago and world class blast furnace efficiency has been reached by SSAB on 100%olivine pellet operation2. This is partly explained by the high iron content of the olivine pelletpermitting low slag rates. LKAB olivine pellets also have a favourable high temperature beha-viour, with a high melting temperature and a narrow melting interval. This is believed toresult in a narrow cohesive zone, in turn resulting in a comparatively small pressure drop overthat part of the furnace, which allows for high production blast furnace rate. The olivineaddition is what renders olivine pellets their high smelting temperature3.

Most LKAB pellet customers do not operate their blast furnaces on 100% LKAB pellets andKPBO mixed with other burden materials has sometimes resulted in a blast furnace operationdescribed as a bit sensitive to disturbances in operation. A possible explanation can be linkedto the high melting temperature, resulting in a late release of the pellet slag. In 1997, the focusof the LKAB product development work on blast furnace pellets was altered, from furtherimprovement of a pellet for 100% pellet blast furnace operation, to the creation of a pelletespecially designed to give good blast furnace process conditions when mixed with basic sin-ters. At the same time the experimental blast furnace was available for pellet testing and it hasbeen invaluable in the development process. The result from the work is the new pellet grade,named KPBA and launched in 2001. In this presentation some differences between KPBOand KPBA experienced in the experimental blast furnace will be discussed.

1.2 The LKAB Experimental Blast Furnace

The LKAB Experimental Blast Furnace has been described in several publications4,5 and willonly be briefly outlined here. The layout is shown in Figure 1. It is fully owned by LKAB andoperated in co-operation with MEFOS. SSAB supplies media, some raw materials and carryout analyses, etc

Figure 1 Layout of the LKAB Experimental Blast Furnace Plant

The experimental blast furnace has a working volume of 8.2 m3 and a diameter of 1.2 m attuyere level. From tuyere level to stock line the height is 6 m. Three tuyeres are placed with120 degrees separation. The furnace is equipped with systems for injecting pulverised coal, oiland slag formers. It is typically operated with 1 bar top pressure.

The raw materials system consists of four bins for pellets or sinter, one bin for coke and foursmall bins for slag formers. Each material is weighed separately according to the recipe. Untilnow a bell type charging system has been used, with moveable armours. Great effort has beentaken to keep heat losses at a minimum. Therefore insulating refractories were chosen andonly the bosh and tuyeres are water-cooled. The blast is normally preheated to 1200°C inpebble heaters. The furnace has one tap hole that is opened with a drill. After each tap, the taphole is closed with a mud gun. The top gas is transported to a dust catcher. The gas is furthercleaned in a venturi scrubber and a wet electrostatic precipitator. Finally, the top gas is flaredin a torch.

Process data are logged every second and transferred at regular intervals to another databasewhere process data calculations are carried out. Data in this database is used for reports, trendcharts and mass- and heat balance calculations. Chemical analyses for raw materials, hotmetal and slag are also stored in this database.

Burden probes are installed at different levels. The probes are equipped with two differentheads. One is used to collect material samples from the furnace. The other is used to collectfurnace gas for analysis and to measure the temperature. To make dissection and repair easythe hearth is detachable and can be separated from the furnace in one to two hours.

As to now the experience is that the LKAB Experimental Blast Furnace is a very sensitivetool for detecting differences in properties for different pellets6,7. The response time is muchshorter for the experimental furnace compared to a commercial furnace. Evaluation of dataand comparisons to full-scale operation has been described8.

2 EXPERIMENTAL WORK

The development and testing of KPBA included extensive work in the laboratory as well as inthe LKAB Experimental Blast Furnace and also in full-scale trials. Pellets were tested on theirown and in mixtures. Experiences from work on pellet-sinter mixes in the experimental blastfurnace will be presented and also some results from high temperature testing and micro stu-dies needed to discuss the results.

2.1 Material

Table 1 lists some typical data for the materials treated in this study. In laboratory testing a10-12.5 mm fraction of the materials have been used. The choice of sinter fractions for trialsin the LKAB Experimental Blast Furnace has been based on practical considerations andvaries between tests.

2.2 The LKAB Experimental Blast Furnace

KPBA type pellets have been charged to the LKAB Experimental Blast Furnace for severalperiods, sometimes as the only iron bearing material and at other times as part of a mixed bur-den. Operation has generally been described as very stable. Mixtures of sinter and KPBA orKPBO have been tested in two experimental blast furnace campaigns.

Table 1 Typical data for materials used in the studiesKPBO KPBA Pellet A Sinter A Sinter B/C Sinter D

Chemical compositionFe tot (%) 66.3 66.9 65.9 60.5 59.5 57.5FeO (%) 0.4 0.4 - 11.0 10.5 5.0MgO (%) 1.55 0.5 0.4 1.65 1.35 0.4SiO2 (%) 2.15 2.35 2.7 4.0 4.4 5.0b2 (CaO/SiO2) 0.22 0.23 0.6 1.65 1.85 2.2Crushing Strength – ISO 4700(daN) 250 290 220Low temperature disintegration (550°C) – ISO 13930>6.3 mm (%) 85 90 77<0.5 mm (%) 5 2 13Reduction Under Load (1050°C) – ISO 7992R40 (%/min) 1.2 1.1 1.4∆p (mmWG) 20 25 3

In the fourth campaign KPBA type pellets and KPBO were tested together with Sinter A.Pellets and sinter were charged to the furnace in equal amounts. Trial periods are listed inTable 2.

In campaign 6 KPBA type pellets were tested mixed with a different sinter and also with thissinter and another pellet. As the sinter came in two different size fractions, the 6-20 mm frac-tion is “Sinter B” while “Sinter C” is the >20 mm fraction. Test periods within the trial arealso listed in Table 2.

Table 2 List of sinter-pellet trials in campaigns 4 and 6

Burden Ratio Trial length (h)Campaign 4Period A KPBA/Sinter A 50/50 86Period C+D KPBO/Sinter A 50/50 100

(48+52)Campaign 6Period 1 KPBO / Sinter C 50/50 41Period 2 KPBO / Pellet A / Sinter B 23/23/54 41Period 3 KPBA / Pellet A / Sinter B 23/23/54 72Period 4 KPBA / Pellet A / Sinter B / Sinter C 35/14/18/33 24Period 5 KPBA / Sinter C 50/50 24Period 6 KPBA / Sinter A 60/40 49Period 7 KPBA / Pellet A / Sinter B 30/21/49 23

2.3 Description of High Temperature Tests

The aim of high temperature testing is to learn something about the behaviour of a burdenmaterial or a mixed burden in the high temperature zone of the blast furnace, where burdenmaterials soften and melt and slag formation starts. A number of different laboratory testshave been developed, but there is no general agreement on how to perform such tests. It iscommon to try to decide the pressure drop over a pellet bed, to monitor bed height and todecide when liquid slag and molten iron is formed. Results are sensitive to the choice of testconditions and a comparison between materials depends on what strategy is adopted on howto make “equal reduction conditions”. As reducibility differs between burden materials thesame reduction treatment will result in varying reduction degree and vice versa. In both casesmaterial properties may be effected. Matters are further complicated by the need to test bur-den mixtures instead of individual materials.

The work to develop the KPBA pellet included laboratory testing of high temperature proper-ties. Some of it was carried out in the LKAB metallurgical laboratory and is described here.Tests were also carried out by SGA (Studien-Gesellschaft für Eisenerz-Aufbereitung), accor-ding to the REAS-test. Only a selection of results can be covered here and a summary of whatwill be included can be found in Table 3.

Table 3 Investigations discussed in the report

KPBO KPBA Pellet B Sinter A Sinter B/C Sinter DHigh temperature REAS testing at SGA100 % X XHigh temperature testing by LKAB (65% pre-reduction)100% X X X X XMixed with Sinter A 50% 50% 50%Mixed with Sinter B 50% 50% 50%Mixed with Sinter D 50% 50% 50%

Descriptions of the REAS-test can be found in literature 9,10. It is a test where reduction andheating to about 1550°C is done in a sequence. Reduction is monitored from gas analysis ofinlet and outlet gases. Pre-reduction to 900°C is carried out according to a blast furnace simu-lating gas- and temperature program. It is ended at a fixed time or when the degree of reduc-tion reaches a predetermined level. The pressure drop, the bed height and the mass of drippingmaterial are monitored and used for evaluation.

High temperature laboratory testing at LKAB is performed in a graphite crucible and nitrogenatmosphere on pre-reduced samples. The pre-reduction is carried out in a blast furnace simu-lating test regime where temperature and gas composition (N2, CO, CO2 and H2) is continuou-sly altered. The sample weight is recorded. For the high temperature testing, samples are pre-reduced, most commonly, to 65%. During the high temperature test the temperature is increa-sed from 900 to 1550°C, in an electrical high frequency furnace at a selected rate. A load isapplied to the sample at 1100°C and any changes in bed height monitored. The mass of meltdripping out of the sample crucible is measured. Tests were performed on pure pellet andsinter samples as well as on mixtures.

2.4 Material samples from the experimental blast furnace

When a campaign in the experimental blast furnace is ended there is an opportunity to quenchthe material inside, from operating conditions. This has been done seven times so far. To endall chemical reactions as quickly as possible nitrogen is added from the furnace top. The nitro-gen blow starts before the blast is switched of and as soon as the gas flow from the tuyeresends the furnace is flushed with nitrogen, exiting the furnace through the tuyeres. A ratherhigh nitrogen flow is kept for some hours and the temperature is brought down quite rapidly.The off gas temperature is below 1000°C after less than an hour and below 600°C withineight hours of shut down. As the cooling is done from the top, there is no further heating ofthe burden at any position in the furnace, above the operating temperature. The furnace is leftto cool for about two weeks before the dig-out starts.

A furnace excavation includes observation and sampling of the burden components and greateffort is spent on documentation. Samples are taken from predetermined positions in the blastfurnace and tested for strength, chemical composition and/or prepared for microanalysis.

Textures of a large number of reduced pellet samples have been analysed at Sintef (Norway)and some results have been reported11. The experimental blast furnace has been quenchedtwice on olivine pellets and once on a KPBA type pellet. Some differences in melting charac-teristics can be observed.

3 RESULTS

3.1 Experimental Blast Furnace

Some operational results from the trials are gathered in Table 4.

Table 4 Process parameters in pellet-sinter trials in the LKAB Experimental BF.

Campaign Campaign 4 Campaign 6

Period A C D 1 2 3 4 5 6 7

Blast parametersBlast flow 1420 1620 1620 1765Blas temp. 1155 1170 1170 1175Blast moisture 28 29 29 24 24 24 24 23 26 26Blast O2 23,7 23,7 23,7 22,2 22,9 22,8 22,8 22,8 22,6 22,6Oil 59 61 61 46 52 53 54 54 53 57RecipeCoke 463 465 468 469 473 457 468 467 463 465Tot red. 522 526 529 515 525 510 521 520 516 522Slag rate 142 156 154 174 210 203 185 181 168 180Tap analysis (hot metal)HM Temp 1433 1421 1423 1451 1439 1451 1437 1439 1431 1434C (%) 4,4 4,4 4,4 4,4 4,4 4,4 4,4 4,4 4,4 4,4Si (%) 1,5 0,9 1,1 1,7 1,6 1,7 1,9 1,7 1,6 1,6Process parametersç CO

i) mean 47,4 45,2 44,9 45,0 45,8 47,2 46,0 45,5 46,0 46,6ç CO std dev 2,6 2,7 3,0 0,7 2,8 0,7 0,5 0,5 1,3 0,9BRI ii) mean 7,2 7,0 7,0 6,3 6,6 6,6 6,5 6,4 6,4 6,1BRI std dev 1,1 0,5 0,3 0,2 0,3 0,6 0,2 0,1 0,2 0,1Burden DescentRate std dev iii)

0,6 1,3 1,0 0,6 0,9 0,7 0,6 0,6 0,6 0,5

i) CO2/(CO+CO2) - From top gas analysisii) Burden Resistance Index – The pressure drop compensated for bosh gas flowiii) Burden Descent Rate (cm/min) – From stock rod measurements

What was most striking in campaign 4 was the much more stable operation during period A,when KPBA type pellets were mixed with sinter, compared to conditions during periods Cand D, when olivine pellets were used. With olivine pellets there were periodic changes (6-8h) in operating performance. The difference in stability does not show in Table 4 but is evi-dent in Figure 2. In the diagram gas utilisation (ç CO), burden resistance index (BRI) and bur-den descent rate are plotted (defined as explained in Table 4). The time scale should be inter-preted as “mddhh” and it is 6 hours between each mark on the time axis.

A fact that can also be extracted form the table is the difference in blast volume. Alterationswere done in an unsuccessful attempt to stabilise operation. Of course it influences all quan-titative comparisons between the test periods.

The reason for the unstable operation on sinter and olivine pellet mixture is believed to beproblems in the melting and slag formation process in the melting zone. The relationship bet-ween burden resistance and hot metal silicon is poor 8 for the Sinter-KPBO mixture and indi-cates instabilities in the melting zone. When KPBA is charged there is a correlation. Possiblemechanisms and effect on melting zone geometry have also been discussed by Hooey et al 8

and will be further dealt with below.

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Figure 2 Process data from mixed burden trial in campaign 4

Results from campaign 6 are also presented in Table 4. In this trial the process stability didnot differ depending on pellet grade as much as it did in campaign 4. The trial is divided intoseveral different periods, when varying burden mixtures were charged to the furnace, as outli-ned in Table 2.

The most critical periods to evaluate for comparison between pellet types, when mixed withthis sinter, should be periods 1 and 5 when the respective pellet grades were used in 50%mixtures with sinter C (>20 mm). It can be seen from the tabulated data that differences inprocess parameters are very small between these two periods. It is also worth noticing thatthere is a slight difference in oil injection rate and blast oxygen that results in a difference inproduction rate.

Periods 2 and 3 can also be compared. Gas utilisation (ç CO) does differ, both the mean valueand the standard deviation. The gas utilisation is higher and more stable for the KPBA period.Burden resistance is at the same level although it varies more in the KPBA period. Burdendescent rate, on the other hands varies more in the KPBO period. Evaluations of the correla-tion between top gas temperature, burden resistance and hot metal quality shows that the rela-tionship is poor in the KPBO period 8. This indicates unstable melting rate or direct reductionin the lower part of the furnace.

An attempt to evaluate and compare a large selection of all the process data monitored andstored was done, using Multivariate Data Analysis (MVDA) to model data from the testperiods in campaign 6. MVDA provides an opportunity to model large data sets thus trans-

forming the information to a format allowing for presentation in a comprehensive graphicalformat. The software package Simca 9.0 was used for the analysis. Process data are stored atleast for minute averages and those data has been used to calculate hourly averages, in turnused for the multivariate data analysis.

Figure 3 shows results from a model on data from the trial in campaign 6. Several groups ofdata can be identified, corresponding to one or more of the trial periods. The fact that data aregrouped in this way shows that the changes that were made, in burden mixture in this case,leads to changes in operating parameters that are greater than any other process variations.

There are four groups of data that can be distinguished from Figure 3. Data points from period1 are found in the upper right corner of the plot and those of period 2 further to the left. Period3 is found towards the lower left quadrant and periods 4-7 in the lower right corner. When thedifferences in burden materials are taken into consideration it is evident that most of the datapoints in the upper part of the diagram corresponds to periods when KPBO was charged andthose in the lower part to periods when KPBA was used. The choice of sinter size fraction isalso important to the operation. All data points to the left correspond to charging of fine sinterand those to the right to charging of the coarser sinter grade.

To gain some information on what it is that distinguishes data from the various test periodsfrom one another the “load plot” in Figure 4 should be studied. Here is illustrated in what waysome important process parameters contribute in the model. Explanations of the parametersare found in Table 5.

Table 5 Explanation of the variables in the load plot

Parameter Description Parameter DescriptionW1 (CP) Cross probe temperatures

“outer wall”η CO Gas utilisation

W2 (CP) CP temp. “inner wall” P5 Pressure drop, lower part of furnaceI (CP) CP temp. “intermediate” BRI Burden resistanceC1 (CP) CP temp. “very centre” BP Blast pressureC2 (CP) CP temp. “ next to centre” BDR Burden descent rateCx (CP) Centre index (Cross Probe) RAFT Flame temperatureWx (CP) Wall index (Cross Probe) HMT Hot metal temp.TGT av Average top gas temp. Si Silicon content of HMTGT d Delta top gas temp. C Carbon content of HMTGV Top gas velocity C/HMT C/HMTFGV Furnace gas velocity SB Specific blastO2 % Oxygen enrichment S Sulphur content of HMOil Oil injection BM Blast moistureHLF Heat losses to furnace cooling Prod.rate Production rateHLT Heat losses to tuyere cooling DRR Degree of direct reduction

In the load plot groups of parameters and orientations can usually be found and it is often pos-sible to relate them to the aspects generally accepted to be important to blast furnace opera-tion. Such orientations have been indicated in Figure 4. In this case several parameters descri-bing gas distribution are grouped along the horizontal axis (identical to component 1 in themodel). Also gas utilisation and burden resistance are oriented in this direction. A componentrelating to productivity can be identified in the vertical direction. Parameters describing hotmetal heat content can also be grouped and the direction is marked on the load plot.

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Figure 3 Score plot (mixed sinter-pellet burden trial in campaign 6)

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Figure 4 Load plot (corresponding to the score plot in Figure 3

When the load plot is studied together with the score plot some interesting observations canbe made. The gas distribution parameters in the left-right direction of the load plot has had alarge impact on the model in this specific case and from the score plot it is clear that this isrelated to the two size fractions of sinter that was used. Operation on the finer sinter (periods2 and 3) gives higher burden resistance (BRI, BP, P5), a more centre working furnace (C1, C2and Cx, from cross probe measurements) and also a better gas utilisation (η CO). The coarsersinter, on the other hand, can be related to a more wall working furnace, higher top gas tempe-rature and also higher direct reduction and gas velocities in the furnace.

The difference between KPBO and KPBA operation is not as clearly elucidated in this model.From Table 4 it is clear that gas utilisation (η CO) is better during period 3 (KPBA) comparedto period 2 (KPBO) and that can explain why the data from period 3 are found to the left ofthose from period 2 in the score plot. The top gas temperature (TGT) is lower in period 3 andthe hot metal temperature higher and both would work to move data to the left. Productivity isslightly better in period 3 compared to period 2 and may explain the difference in the verticaldirection.

3.2 High Temperature Laboratory Testing

Results from high temperature testing on KPBA and KPBO in the LKAB high temperaturetest are presented in Figure 5. It can be seen that KPBA both softens and drips out of the cru-cible at lower temperatures (by about 100°C) compared to KPBO. The temperature intervalfor dripping is also larger for KPBA than it is for KPBO. The difference in composition bet-ween KPBA and KPBO is basically 1% MgO, which of course is a substantial difference incomposition of the slag formers in the pellet.

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Figure 5 Results from high temperature testing of KPBA and KPBO

It is evident that the small amount of MgO not only increases the melting temperature of theslag phase of the reduced material but also has great impact on the melting of the metalliciron, as that is what the majority of the sample consists of. As the melting temperature of the

metallic iron is governed by the carbon content it is reasonable to conclude that also thecarbon pick-up rate depends on the pellet slag.

As the high temperature test is carried out in inert atmosphere, in a graphite crucible, transportof carbon in the condensed phases is more important compared to conditions in the blast fur-nace, where carbon oxides are present in the gas phase. Therefore the formation of an initialliquid phase may be more crucial to carburisation of the metallic iron. It can be observed thatthe melting of KPBA starts when a small amount (of slag phase) drips out of the crucible.

Tabell 6 shows results from REAS testing carried out at SGA on KPBO and KPBA. It con-firms the difference in melting temperature, also with a reducing gas atmosphere. In this test itis observed that a fayalitic type slag drips out prior to the metallic iron phase, from both pelletgrades.

Tabell 6 Results from REAS-test on 100% pellet samples.

KPBA KPBOReduction behaviourR40 (%/min) 0,45 0,42Time to reach 65% reduction (min) 160 159Softening behaviourTE (Temp. at 50% shrinkage) (°C) 1260 1290Temp at max pressure drop (°C) 1300 1340Melting and dripping behaviourTSS (Temp of primary slag) (°C) 1305 1350TBF (Temp of melting iron) (°C) 1365 1420

Table 7 Results from LKAB high temperature tests

Temperatureat 30%

contraction(°C)

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contraction(°C)

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dripping(°C)

Temperatureat the end of

dripping(°C)

Temperatureinterval ofdripping

(°C)Only pelletsKPBO 1170 1325 1490 1510 20KPBA 1140 1260 1380 1440 60Only SinterSinter A 1255 1410 1545 Very little

dripped mtrlSinter B 1525 Very little

dripped mtrlSinter D 1180 1360 - Not meltedSinter and KPBOSinter A 1215 1365 1520 1545 25Sinter B 1420 1550 80Sinter D 1190 1367 1540 Not completely

melted>10

Sinter and KPBASinter A 1220 1350 1480 1500 20Sinter B 1370 1500 130Sinter D 1135 1290 1470 / 1505 1510 / 1520 40 / 15

Results from trials in the LKAB high temperature test with sinter-pellet mixtures are listed inTable 7. The results extracted from experimental data like those plotted in Figure 5 are thetemperatures corresponding to 30% and 50% bed contraction, respectively and the tempera-

tures when the first molten material drips out of the sample crucible. Also the temperatureswhen the last melt drips and bed contraction ends are listed.

The tabulated data shows that mixes with Sinter A melts in a narrow temperature interval,with Sinter B in a rather wide temperature range and mixtures containing Sinter D in anintermediate range and that this is not changed by the choice of pellets. What can also be seenis that the mixtures with KPBA melts at lower temperatures than those containing KPBO.

3.3 Samples from the experimental blast furnace

The texture of numerous samples from the experimental blast furnace have been studied andcharacterised, primarily pellets produced from LKAB concentrate and sampled from the fur-nace at dissections or during operation. The KPBA-type pellets differ from other materials ashighly reduced samples are often found to be hollow. This suggests that a low melting fayali-tic slag forms and melts before the metallic iron is melted. Figure 6 shows an example. Thesepellets were sampled during a furnace dissection.

Figure 6 KPBA type pellets from the cohesive zone of the experimental blast furnace.

4 DISCUSSION

Trials in the LKAB Experimental Blast Furnace have shown that operating parameters changewhen burden mixtures of sinter and KPBO or KPBA, respectively, are charged to the furnace.There are indications, from two trials with different sinter grades, that process stability is bet-ter when KPBA is chosen instead of KPBO. In one of the trials, problems with the sinter andKPBO mixture was obvious. In the other trial the instabilities were not as clear and mostlyrelated to hot metal quality. Unstable conditions in the melting zone are believed to cause theproblems. There have been indications of similar phenomena in full-scale operation. Resultsfrom high temperature laboratory testing can be shown to support that theory.

LKAB olivine pellets, KPBO, has an open porous structure were the original ore particleshave sintered together to some extent. Most of the bonding is from sintering of hematitegrains rather than slag “gluing” particles together. A thin film of silicate slag covers the grainsurfaces of the hematite structure. The overall amount of slag is quite small and it originatesfrom the gangue, from the bentonite added as binder and from the added olivine. Magnesiumfrom the olivine is partly transported to the iron oxide and forms magnesioferrite. Olivine isnot completely reacted in the pelletising process. During reduction of KPBO pellets, magne-sium can be dissolved into the silicate, as the temperature increases, and thus increasing themelting temperature of the slag phase. Magnesia also increases the melting temperature ofwustite (or magnesiowustite) and this effect may also be important. Melting of the slag there-fore takes place at a very high temperature. In the LKAB high temperature test all materialdrips out of the sample crucible at the same time, indicating that the slag only melts when themetallic iron does. In the REAS test some slag dripped out of the crucible before the metalliciron melted. From the samples we have from the experimental blast furnace it is not evidentwhether slag melts out of the pellets before the metal melts or not. That would need to befurther investigated.

The structure of oxidised KPBA pellets is not very different from olivine pellets. The amountof silicate slag phase is somewhat higher but that is not easily observable. A small amount ofolivine is added in KPBA, so some olivine particles are present and also some magnesiofer-rite. However, the magnesium content is obviously to low to prevent the silicate slag frommelting and therefore a fayalitic slag phase melts out from the pellets. This was observed inthe REAS test. Also in the LKAB high temperature test, a portion of the material melts outsome time before the rest does. A hollow centre is common in KPBA samples from the expe-rimental blast furnace and is interpreted as a result of fayalitic slag phase having melted out ofthe comparatively less reduced centre and leaving the pellet.

How the melting characteristics of the pellet influences the overall melting of a mixed burden,can of course vary greatly, depending on the properties of other burden components. The roleof slag formers and other materials have not been mentioned here but are important althoughthis discussion focuses on pellets and sinter. As sinters often have a relatively high meltingtemperature and also a rather high basicity (compared to the blast furnace slag at least) thepresence of a low melting acid slag may have an important role in fluxing the sinter slag. Thedifference in melting temperature between the pellets may not seem so great, “only” 45°C to100°C in the two tests conducted. The temperature itself may not be the only important factorbut the composition of the primary slag could have great impact. If KPBO does in fact meltalmost homogeneously that could be of disadvantage if it causes local “flooding”. The matterwill be further studied.

5 CONCLUSIONS

The work in the LKAB Experimental Blast Furnace when the new pellet grade, KPBA, wasdeveloped contributed greatly to the successful result. A pellet grade that had proven to pro-mote stable blast furnace operation under various operating conditions could be selected forfull-scale testing, and is now available to LKAB pellet customers.

A comparison from blast furnace operation on mixed sinter-pellet burdens, using the newKPBA pellet and the regular, KPBO, olivine pellet has been presented. Results indicatedifferences in process stability, located to the melting zone of the furnace. Laboratory hightemperature testing shows that KPBA melts at lower temperature, compared to KPBO. Thatmay be the reason for the differences in burden behaviour and furnace operation. In the twomixtures that were studied, KPBA seemed to be the pellet choice.

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