Development of fluxed blast furnace pellets with application
Transcription
Development of fluxed blast furnace pellets with application
Welcome Publications 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. Welcome Publications 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 FINAL 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 Welcome Publications 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. Welcome Publications 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 Welcome Publications 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 Welcome Publications 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.