Critical factors in the development of blast furnace burden materials - LKAB's Vision

Lawrence Hooey1 Development of blast furnace burden materials is an important area for optimising the economy of the blast furnace process as the raw material costs are the major cost in the production of steel. Identifying critical factors for this optimisation require close co-operation between the blast furnace engineers and the raw material suppliers. Improvements in laboratory test values of completely new pellets compared to values of existing products are not sufficient to develop blast furnace materials. At LKAB investment in the experimental blast furnace has proven to be a key tool in the development of pellets prior to full-scale testing and is providing valuable knowledge. Close co-operation with customers provides the necessary direction for burden development and deepens LKAB's knowledge of the critical factors to be considered. Keywords: blast furnace, ferrous burden, pellets 2nd International meeting on Ironmaking, Vitória, Brazil, September 12-15, 2004, pp 803-815 1Senior Research Engineer, LKAB, Research and Development, Varvsgatan 45, S-97128, Luleå, Sweden. Lawrence.Hooey@LKAB.com

1. Introduction LKAB's mining operations are located in northern Sweden, above the arctic circle. The mines are located in Kiruna and Malmberget. Two pelletising plants are located in Kiruna and one in Svappavaara (grate-kiln processes); and two pellet plants are located in Malmberget of which one is a straight-grate and the other LKAB's own steel-belt process similar to a straight grate. Research facilities are concentrated in Malmberget at the Agglomeration Research Centre and in Luleå at the Experimental Blast Furnace. The ores are mainly high-grade magnetite and easily concentrated to over 70% Fe. Such pure concentrates allow the modification of pellet chemistry based solely on additives. LKAB's iron ore production amounted to 21.5 Mtonnes in 2003 which consisted of 15.3 Mtonnes BF- and DR-pellets, and the rest sintering fines. For the blast furnace, LKAB produces 2 types of blast furnace pellets - a recently developed acid pellet, the KPBA pellet, with 2.5% SiO2, designed for use with sinter; and the Olivine pellets (KPBO and MPBO) for use either in mixed burdens or in 100% pellet operations. The development process for new blast furnace pellets such as the KPBA required the use of the experimental blast furnace (EBF) commission in 1997 and located in Luleå.1 Due to the extremely complex nature of reduction and melting in the blast furnace laboratory scale tests were not sufficient to optimise the pellet properties and the EBF provides the required testing prior to full-scale testing in commercial furnaces. Figure 1 shows LKAB's pellet development route. The focus of this paper is on the critical factors affecting a commercial furnace that must be considered in this development path.

Pellet testingin full scale BETTER

PRODUCT

Pellet productionlab scale

Pellet testingin pilot scale

LKAB’sEBF

The customer’s furnace

Figure 1. LKAB's development route for blast furnace burden testing and pellet development

2. The laboratory versus the blast furnace Research tests undertaken in the laboratory can never fully simulate blast furnace conditions. They can assist in evaluating specific phenomena taking into consideration the difference between the blast furnace and the laboratory test conditions. With the laboratory results an explanation of blast furnace behaviour is sought. Quality control of materials via various tests is a somewhat different matter. Although in most cases the tests aim to simulate blast furnace conditions, caution must be observed when comparing completely different materials, as opposed to the very useful function of controlling daily production of the same product. 2.1 Pellet standardised laboratory tests The number of laboratory tests for blast furnace pellets is extensive with the ISO standard tests listed in Table 1. Many other standards and tests unique to individual operations make this list very long. LKAB works with customers to ensure that standard quality parameters are developed, but this work is considered separately from product development or blast furnace burden mixture development. Table 1. ISO standard tests used for blast furnace pellets Test Measurement values Purpose ISO 4700 /crushing strength daN Pellet cold strength ISO 3271 /tumble strength Fractions +6.3mm and -0.5mm Tendency for abrasion ISO 13930 / low temperature reduction disintegration

Fractions +6.3, -3.15 and -0.5mm Tendency for low temperature degradation

ISO 4698 /free-swelling Volume% increase Tendency for swelling ISO 4695 / Reducibility Reduction rate at 40% reduced Reducibility ISO 7992 /Reduction under load Reduction rate at 40% reduced

Pressure drop at 80% reduced Bed shrinkage at 80% reduced

Reducibility Softening/melting behaviour

What is notably missing from these laboratory test are behaviours that cannot measured including:

- Effect of alkali and sulphur on test values - Values for material that is removed prior to testing due to size

range used in the standardised tests - Interaction with other burden components - Effects of moisture, transport and storage on pellet properties - Effects of blast conditions, injectants on different burden

mixtures 2.2 Pellet quality development The purpose of the quality control tests parameters fall into two categories: 1) Quality control for current products used in the blast furnace 2) Measurement of values for pellets in pot tests that have not been tested in the blast furnace and therefore no BF process evaluations exist

An example of the goal as quality control is the quality development of the KPBA pellets. Figure 2 shows the development of the LTD values since the introduction of the pellet in 2000. Small adjustments are made in the pelletising process and composition to ensure a stable product within specified quality control parameters. In this case the LTD has been raised to over 90, with target values >90% +6,3mm.

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ISO13930 +6.3mm Figure 2. Development of KPBA LTD quality since introduction in 2000. For the development of new pellets or burden mixtures, the subject of this paper, there are basic characteristics that must be met:

- pellets must be strong enough to survive transport and handling without undue generation of fines

- Pellet must not show swelling properties or a tendency for disintegration - Interaction with other burden components must be suitable - Reducibility must be adequate

The specific limits for example for cold compression for different pellet types are different. For fluxed type pellets containing limestone flux, this limit is most certainly higher due to how the slag builds and binds in the pellets and the necessary cold strength is a function of the pelletising process not a requirement for the blast furnace. For examples, Figure 3 shows distinct differences in bonding between LKAB fluxed and olivine pellets. Examples of different compression strengths for different pellets can be seen in Table 2, with examples from LKAB and from IOC.2 Just within this list there is a difference of over 60 kg/pellet for different products. It would no doubt be

catastrophic to produce the fluxed or the limestone pellets with a cold strength of the olivine or acid pellets, if the pelletising process could be run at all. Alternately, producing an acid pellet with a crushing strength of 270 or 280 kg/pellet would require great changes in the pelletising process that would affect other quality values. Hence the usefulness of this value for quality control but not comparison of completely different products.

LKAB Fluxed Pellet LKAB Olivine Pellet Figure 3. Slag formation in a fluxed pellet showing hematite with silicate slag formation; and in an olivine pellet showing partly reacted relic olivine, magnesioferrite, and hematite. Width of photos circa 220 micron. Table 2. Comparison of normal ISO 4700 values for various pellets that have been used commercially. Pellet type daN (kg/pellet) LKAB Fluxed (full-scale experimental) CaO/SiO2=1 285 LKAB MPBO (standard Olivine pellet - Malmberget) 220 LKAB KPBO (standard Olivine pellet - Kiruna) 230 LKAB KPBA (standard Quartzite pellet) 250 IOC Fluxed Pellet (CaO/SiO2=0.91) 239 IOC Standard Limestone pellet (CaO/SiO2 = 0.2) 276 IOC Acid Pellets (CaO/SiO2=0.09) 235 For development purposes, an understanding of what values should be expected in the quality control tests is important. If these deviate strongly from expected values then this should be a red flag for the pellet in the development process. Thus, for the development of NEW PELLETS and UNTESTED BURDEN MIXTURES I am defining two cases: False positive: a pellet showing superior properties in the laboratory but unsuitable properties in the blast furnace False negative: a pellet showing unsuitable properties in the laboratory but superior properties in the blast furnace An example of a false negative is illustrated in the following example. Within LKAB we measure the ITH value which is a test used to determine high temperature

reduction strength. Pellets are tumbled in an I-drum 200 times and screened after undergoing the ISO4695 reducibility test. Table 3 shows values for two types of pellets used in commercial blast furnaces. MPBO olivine pellets are the standard product from the straight-grate plant and Pellet A was an experimental pellet produced in the smaller steel-belt plant using different additives. Table 3. Reducibility and high temperature reduction strength measured as screen fractions after ISO4695 and tumbling 200 times in an I-drum and screening. MPBO Pellet A Reducibility ISO4695 R40 %/min 0.55 0.50 LKAB test ITH +6.3mm 68 45 LKAB test ITH -0.5mm 23 45 Standard MPBO pellets are used extensively in Sweden and Finland showing very good operation of the furnaces. Pellet A in the commercial furnace showed that the pellets were also suitable for the blast furnace with no evidence of disintegration. Assuming the laboratory behaviour of the pellets is characteristic of the pellets deviations from the values would represent a change in pellet behaviour. This means that if the laboratory test were accepted as relevant, Pellet A would not have been charged to the commercial furnace - even though it is a suitable pellet and may become a commercial product in the future. Extensive testing of Pellet A in the EBF revealed no process behaviour indicating disintegration and this was confirmed by extensive material probing of the EBF shaft. The EBF provided the necessary bridge between the laboratory tests that suggested a problem with reduction strength, and the reality that the pellets are suitable for a full-scale furnace. 3. Development criteria for blast furnace pellets In order to develop burden materials, specifically pellets, and overall burden design for blast furnaces the following are key criteria for the overall development process and application:

- Pelletising characteristics such that supply and quality are secure and stable

- Pellet characteristics in transport and reduction are suitable - Pellets must have a benefit over and above the pellets or material in

present use (otherwise why develop a new pellet or change the mixture?) 3.1 Critical parameters in the blast furnace related to ferrous burden Blast furnaces have undergone substantial development in the past decades. Optimisation of charging, control, cooling systems, injection technology and so on have done much to reduce reductant rates and increase furnace productivity and improve hot metal quality.

Each blast furnace operation has conditions that must be assessed on an individual basis. Many of the following factors are related and the overall combination must be considered. Slag chemistry In the first consideration, chemistry of slag is critical. Targets for desulphurisation, slag volume, slag applications such as road construction and cement must be met. Of course, burden trims can be used as well to meet these criteria and the economics of using a trim or additional pellet additives must be considered. Blast conditions There are many different blast parameters and injectants used. Differences in coal, oil or natural gas injection, oxygen enrichment, blast moisture and blast temperature impact on furnace behaviour and behaviour of the reduction of burden materials. Productivity demands alter the desired blast volumes and oxygen enrichment. Top pressure increases blast pressure and increases gas residence times. Burden chemistry The circulation of alkali and sulphur loading are critical parameters for blast furnaces. The balance between alkali output and desulphurisation capacity of slag is a constant consideration for blast furnace engineers. Each operation has its own strategy. Burden interaction Most blast furnaces operate with a mix of pellets, sinter and/or lump ore and burden trims such as limestone. Examples of interaction of the components in the cohesive zone have been described in the First International Meeting.3 Economic optimisation The variable costs for production of hot metal lie in the costs of ferrous burden and the cost of reducing agents, especially coke. For example, the balance between coke costs, productivity losses and the use of lump ores. Strategies for furnace operation with high lump ore rates are desirable when possible, however high productivity furnaces cannot overcome the deleterious effects of lump ores on furnace behaviour. 3.2 Development considerations for blast furnace materials The most important behavioural considerations for ferrous burden are listed in Table 4 along with LKAB's approach to development of new pellets and burden mixtures. Since its commissioning in 1997, the EBF has been compared to several full-scale operations and found to be highly suitable for evaluating blast furnace behaviour and the quality of ferrous burdens. The EBF is undergoing continuous improvements in equipment, control strategies and so on, enhancing its effectiveness as a tool in blast furnace research.

Table 4. Important considerations in ferrous burden material development and testing methods at LKAB. Target area for development Effectiveness of

standard laboratory tests

LKAB Evaluation Techniques

Effectiveness of EBF testing

Fines generation Reduction strength Medium to low EBF shaft probing Very good EBF testing and full-scale tests for stability

Excellent

Descent stability Effect of alkali/sulphur on swelling and degradation and fines generation

Low - sulphur and alkali not included in tests

Participation in RFCS projects - sampling of scaffolds

Excellent

Evaluation of slag chemistry from EBF

Fair but improving - Sulphur loads lower than commercial furnaces when using oil injection with high quality Swedish diesel oil.

Balance between alkali output and desulphurisation

Behaviour of minerals in burden in lower shaft/cohesive zone

Nil

Evaluation of commercial furnaces 100% Effective

EBF testing for stability

Excellent

Excavation of EBF Excellent

Interaction of burden components

Softening, melting, carburisation Nil Some non-standard advanced softening/melting tests useful

Evaluation of commercial furnaces 100% Effective

Reducibility of burden materials

Applies only to lump ores and possibly sinter or pellets with extreme acid composition*

Low to Nil Evaluation of EBF probe samples, basket samples from EBF excavations

Fair to Excellent - burden probes not necessarily big enough to take large particles of lump ore

Testing of dust suppression techniques in laboratory and full-scale

Not applicable

Top gas temperature limitations Testing of injection effects

Control of moisture in transport (dusting suppression, environmental considerations)

Low to Nil

Testing of modified blast conditions in EBF

Good - undergoing development

*Various pellets over a wide composition range have been tested. Lump ores, acid sinter or very high silica pellets may exhibit problems with reducibility.

Fines generation The ISO13930 test and the ITH tests are used and, as discussed in above, low values for test pellets must be taken as possible indicators of problems in the blast furnace. However, degradation and reduction strength of ferrous burden has been found to be highly dependent on alkali circulation and trace elements in the gas phase4 as well as on hydrogen content of the gas.2 Lump ores exhibit the most extreme degradation. Alkali circulation also increases particulate fines especially in the lower shaft. High concentrations of potassium compounds have been found in the <0.5mm material from the lower shaft and inclined probings such as the probing shown in Figure 4.

Inclined upper c. - 4.3 mlower c. -5 m

Upper shaft -1 m

Lower shaft -3.45 m

Tuyere -6 m

wt% Upper shaft Lower shaft Inclined SiO2 8,37 10,88 16,48

TiO2 0,14 0,11 0,07

Al2O3 1,55 3,81 5,60

MnO 0,23 0,19 0,27MgO 4,10 3,29 4,67CaO 5,13 9,35 9,47K2O 0,29 18,89 28,45

Na2O 0,35 1,48 4,82

P2O5 0,09 0,10 0,12

V2O5 0,05 0,10 0,04

Fe 52,56 21,48 6,26C 9,00 16,18 10,70

Figure 4. The experimental blast furnace probe locations and an example of the composition of <0.5mm material typically screened from the samples. Scaffolding Scaffolding phenomena continues to be studied. It is well known that alkali circulation and zinc circulation have strong impacts. Strategies include burden distribution control and control of alkali input and circulation via input if silicious materials, but this affects desulphurisation capacity of slag. Optimisation of the control of alkali circulation effects is not yet optimised. Balance between alkali removal and desulphurisation Every blast furnace has its own requirements regarding the sulphur content of hot metal versus the need to reduce alkali circulation. Some burdens are more sensitive to alkalis due to scaffolding tendencies and fines generation as described above. Furnace construction and cooling systems, charge distribution and other factors also have a large impact that must be considered.

An example of alkali output and desulphurisation capacity is shown in Figure 5. It was determined not to commercialise the MPB1 type experimental pellet (CaO/SiO2 = 1) due to the less favourable balance in alkali out and desulphurisation compared to commercial MPBO pellets.

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Figure 5. Differences in desulphurisation and alkali output via slag for two burdens charged to a commercial furnace.5 MPBO, KPBO are commercial olivine pellets. MPB1 is an experimental fluxed pellet with CaO/SiO2 = 1) Interaction of burden components The importance of interaction of burden components in the cohesive and melting zones of the blast furnace was described in the previous paper presented at this meeting.3 The details of the article will not be repeated in detail here. Summarising the previous experience, there exists the possibility of formation of slags in the melting zone that refreeze and cause problems with carburisation and hot metal quality or with gas distribution. 'Matching' of the measured softening or meltdown temperatures in laboratory tests - meaning that each burden component should melt at the same temperature - is not necessarily useful and even ill-advised. The well-known practice of mixing acid pellets with sinter, which have greatly differing meltdown temperatures, has proven very effective in the blast furnace illustrating clearly this point. Reducibility of materials In LKAB experience, pellet reducibility as measured in standardised tests has not shown to directly correlate with blast furnace performance. Figure 6 shows the reduction extent of 8 different pellet types charged as basket samples in EBF campaign 5 showing no difference in the reduction degrees of different pellets.6 Optimisation of laboratory reducibility is therefore not considered a critical factor for the very wide range of pellets tested.

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Figure 6. Reduction extent of different pellet types charged in baskets to the EBF.6. Legend: 'Burden Acid 1' indicates the base material that was in the furnace, and other markings are adjacent basket samples. It should be noted that for lump ore reducibility is a consideration. Lump ores have been found to affect reductant rate depending either on lump ore quality or process conditions. Table 5 lists conditions found for two different blast furnace operations tested in the EBF. In the case of Test 1, there was a significant increase in reductant rate compared with 100% pellet operation, and this was consistent with the full-scale operation. In Test 2, there was no increase in reductant rate however there were significant problems with hanging and slipping, and this was also consistent compared to the full-scale operation. Table 5. Results of two EBF tests with lump ores in the ferrous burden. The operations are compared to similar operation with 100% pellets. The results are quite similar to corresponding comparisons in commercial operations.

Test 1 Test 2Burden Pellets 76%

Lump ore 24% (one main type)

LKAB pellets 35%sinter 40%Lump ores* 25% *(2 main types mixed)

Coke rate +0.9 kg/% lump ore No changeProdution rate -6-7% No change

(excluding hang time)Stability Moderately worse Extremely poor -

hanging disrupting productionFlue dust Increase +11 kg/thm No changeOyxgen enrichment 1.5% 2.5%Injection type Coal 100 kg/thm Oil 61 kg/thmLump ore LOI Circa 5% < 1% in type one;

c 2% in type two.

Top gas temperatures and injectants Blast conditions and the effect of high or low levels of oxygen enrichment, coal injection with additives, high injection levels of oil or natural gas etc. can alter the behaviour of materials under reduction and other factors in blast furnace behaviour. In some cases when the furnace uses extensive amounts of oxygen top gas temperatures may be depressed to below 100oC and condensation, heat level or output of Zn can be problematic. Thus moisture charged with pellets should be minimised. In one instance the extra reductants required amounts to 0.5kg coke/kg charged moisture/tonne hot metal.7 Development of methods to lower the amount of water necessary to control dusting during transport under these circumstances can be critical. In other cases, where no oxygen is available, top gas temperature may be unnecessarily high and merely increase fuel consumption. While not affected necessarily by ferrous burden quality, knowledge of how blast parameters affect the energy balance in the furnace is important for optimisation of the furnace. This is working from the opposite direction - development of furnace practice to suit materials under certain conditions. Research is ongoing to test the effect of blast conditions and injectants on blast furnace behaviour. 4. Summary There are two distinct development areas for blast furnace ferrous burden: 1) Quality development of current products 2) Development of new products In the case of quality development, LKAB emphases improving product stability, as well as improving quality values as measured in the laboratory and as seen in operation of blast furnaces burdened with LKAB pellets. We strive for close co-operation with our customers to ensure the pellets meet the blast furnace demands. For development of new pellets, a need in the customer's blast furnace process is identified and pellets are developed and tested in the Experimental Blast Furnace. The first priority is that the EBF results are improved over reference burden using conditions as close to the customer's operation as possible. This indicates potential for optimisation and little risk in full-scale testing. References 1 L. Hooey, J. Sterneland and M. Hallin, 60th Ironmaking Conference, ISS, March 25-28, Baltimore, USA. pp. 197-208. 2 W.K. Lu and G. Chung, 62nd Ironmaking and Steelmaking Conference, ISS, April 28-30, 2003, Indianapolis, USA. pp.435-445. 3 L. Hooey and M. Hallin, 1st International Meeting on Ironmaking, ABM, September 24-26, 2001, Belo Horizonte, Brazil.

4 K. Grebe and K.P. Stricker, 2nd ICSTI and 57th Ironmaking Conference, ISS, March 22-25, 1998, Toronto, Canada. pp. 245-256. 5 L. Hooey, M. Hallin and K. Raipala, METEC Congress 2003 3rd Interational Conference on Science and Technology of Ironmaking, Düsseldorf, June 16-20, 2003, pp. 256-261. 6 J. Sterneland and M. Andersson, 61st Ironmaking and Steelmaking Conference, ISS, March 10-13, 2002, Nashville, USA. pp. 53-64. 7 K. Raipala , Personal communication, Rautaruukki Oyj Koverhar Operation, September 2003.