Injection of BF flue dust into the BF - a full-scale test at BF No. 3 in Luleå Björn Jansson, Lena Sundqvist Ökvist. Abstract

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1 Injection of BF flue dust into the BF - a full-scale test at BF No. 3 in Luleå Björn Jansson, Lena Sundqvist Ökvist SSAB Tunnplåt AB, SE LULEÅ, Sweden Phone bjorn-l.jansson@ssab.com lena.sundqvist@ssab.com Abstract Co-injection of BF flue dust and pulverized coal (PC) has previously been tested in the LKAB experimental blast furnace (EBF). Based on the test results, full-scale tests in BF No.3 SSAB Luleå Works were decided. Two different methods for addition of BF flue dust to PC have been tested. The results are successful from the point of view of process and valuable amounts of C and Fe contained in the BF flue dust are efficiently used. The BF flue dust is very abrasive and evaluation of the best method for continuous injection and necessary equipment modifications is in progress.

2 Introduction In the summer of 2000, the blast furnace operation at SSAB Tunnplåt AB in Luleå changed from two-furnace operation into one-furnace operation, when BF No. 2 was rebuilt and enlarged. The new furnace, BF No. 3, produces close to 2.3 Mtonnes of hot metal a year compared to 2.0 Mtonnes with BF Nos. 1 and 2. Some general data of BF No. 3 can be seen in table 1. The BF-gas cleaning system was changed from a dust catcher and a wet scrubber into a cyclone and a wet scrubber. One of the reasons for changing Table 1 Design data of BF No. 3 BF No. 3 Year of construction 2000 Design SSAB/Kvaerner Start of campaign Aug 2000 Hearth diameter, m 11.4 Working volume, m Total volume, m No of tuyeres 32 No of tapholes 2 Runners Replaceable troughs Top pressure, kpa 150 Charging equipment Belt / BLT central feed Daily output, tonnes 6700 into a cyclone was to improve the removal of dry BF flue dust. Close to 80 % of the dust is estimated to be removed as dry BF flue dust in the new system, compared to approximately 55 % in the old system. Due to the fact that the blast furnace operates with 100 % pellets and there is no sintering plant in operation, BF flue dust together with other in-plant fines are recycled to the BF by making a cold-bonded briquette. The different materials are mixed with water and cement, compacted, cured and finally stored for 3 to 4 weeks to reach sufficient strength. The briquettes are then charged into the furnace together with pellets, limestone, manganese slag and nut coke. BF flue dust has a negative influence on the cold strength of the briquettes and large amounts of cement have to be used to reach an acceptable quality level. If BF flue dust could be removed from the briquette, several advantages would be obtained. The quality improves, the amount of cement/binder decreases and the amount of briquette fines becomes lower. The production capacity in the briquette plant is limited, and so, if BF flue dust would be removed, other materials can substitute the BF flue dust in the briquette, thereby increasing the total amount of material recycled. An alternative way to take care of the BF flue dust is injection into the tuyeres of the blast furnace. It has been shown that iron oxide, injected together with coal powder, is reduced within a few milliseconds and that it is likely that the oxygen liberated from the iron oxide can participate in the combustion of the coal [1][2][3]. This implies that all reactions regarding the iron oxide part of the flue dust can be completed in the raceway. It has also been shown [1][3] that a better reduction degree can be obtained, if the iron oxide is mixed with a low-volatile coal compared to a high volatile coal depending on different ignition and combustion behaviour between the two coal types. To get a better understanding of the effects of BF flue dust injection, a trial was made at the LKAB Experimental Blast Furnace (EBF) [4]. Based on that trial, it was concluded that injection of BF flue dust decreased the consumption of reducing agents and decreased the Si content of the hot metal. Because of these encouraging results, it was decided to continue with full-scale tests at BF No. 3 to see, if the same results could be achieved. 2

3 Characterisation of BF flue dust and coal The amount of dry BF flue dust produced each year is between 30 and 40 ktonnes, which equals to 13 to 18 kg/thm. BF flue dust consists of fine particles from the BF burden materials, mainly carbon from the coke and iron oxide from pellets. Normally, the carbon content of the BF flue dust from BF No. 3 is higher than the iron content. A typical chemical analysis can be seen in Table 2. Table 2 Chemical analysis of BF flue dust and coal. All values are stated in percentage by weight. Fe CaO SiO 2 Al 2 O 3 MgO Na 2 O K 2 O S Zn C Ash LOI Volatiles BF flue dust PC n.a , The screen analysis in Table 3 shows that the BF flue dust is coarser than the PC. A fractional analysis, Table 4, shows that most of the carbon is in particles larger than 75 microns and iron in particles smaller than 250 microns. Thermal analyses are run using a Netzch STA 409 instrument for simultaneous Thermo Gravimetric (TG), Differential Thermal Analysis (DTA), and Mass Spectrometric (MS) measurements. Samples, roughly 70 mg in mass contained in an alumina crucible, are heated from room temperature to 1200 C, at a heating rate of 10 C/min in air atmosphere around the sample at a constant flow rate of 200 ml/min. Ar gas with a constant flow of 100 ml/min is supplied to the chamber. The samples are ground and mixed in a mortar. Table 3 BF flue dust and coal particle size distribution Opening µm BF flue dust % Passing Opening µm P Coal % Passing Table 4 Fraction analyses of BF flue dust, all values in percentage by weight. < µm µm µm µm µm Wt in fraction >250 µm ,0 23, Fe CaO SiO Al 2 O MgO C S Zn As can be seen in the left diagram of Fig. 1, moisture is released during the first part of the test and chemical bound water in the temperature interval of C, when testing coal. Intensity H2O, CO2, H2 3E-09 2E-09 1E-09 0E+00 H 2 O Hydrocarbons H 2 CO Temperature, C 4E-10 3E-10 2E-10 1E-10 0E+00 Intensity hydrocarbons Intensity H2O, CO2, CO 1E-08 8E-09 6E-09 4E-09 2E-09 0E+00 H 2 O H 2 CO 2 1E-09 8E-10 6E-10 4E-10 2E-10 0E Temperature, C Fig. 1 Composition of gases generated during TG/DTA tests made on samples of pulverized coal to the left and BF flue dust to the right (hydrocarbons = C x H y). O 2 CO Intensity O2, H2 3

4 Both are registered as endothermic reactions on the DTA analysis. A CO 2 evolution starts at 250 C. In the temperature interval of ~ ºC, volatile matters are released and CO 2 is generated. An endothermic peak at ~550 C coincides with the generation of H 2. At ~600 C, the generation of CO 2 and H 2 O is increased coinciding with a slightly decreased O 2 intensity. From 750 C until the end of the test, there will be a CO 2 evolution. The rate of weight loss is quite similar during the whole test. The total weight loss for the test is 76 %. During thermal testing, BF flue dust shows similar behaviour as in other studies[7]. The main results of the gas analyses obtained at the test with BF flue dust are summarised in the right diagram of Fig. 1. The O 2 content of the gas is lower during this test compared with that one during the test with coal and CO is generated. Hydrocarbons and chemically bound are not detected and there is no weight loss below 500 C. Decomposition of carbonates occurs in the temperature range of ºC coinciding with an endothermic peak and a CO 2 evolution. The CO 2 generated reacts further with C producing CO that can be used for reduction of e.g. iron oxides resulting in a second peak of CO 2 coinciding with a narrow endothermic peak that is detected approximately at 800 C. When the temperature reaches 850 C, the CO generation starts to increase and at ~ 950 C a large endothermic peak coinciding with CO 2 is detected. At approximately 1150 C, the total weight loss is 44 % and the sample increases in weight, when the temperature is further increased, as a result of an exothermic reaction. Injection of BF flue dust Fig. 2 Coal preparation and coal injection plant Coal preparation and injection system The coal preparation and injection system is a BMH-system. A schematic view of it can be seen in Fig. 2. Raw coal is transported from the raw coal yard to the injection plant by truck. The coal is emptied into a raw coal hopper and transported by a vertical conveyor to a raw coal silo of 600 m 3. Coal is charged from the silo into the ball mill by a chain conveyor. The size of the coal particles is adjusted by changing the setting of the windscreen in the mill. The coal powder is then stored in a 800 m 3 silo before it is injected into the blast furnace. Nitrogen 4

5 is used as conveying gas from the injection vessels to the blast furnace. Approximately 1000 nm 3 /h of nitrogen is used. Injectants, kg/thm Injectants feb 07-feb 08-feb BF flue dust % of injectants 20% 09-feb 10-feb 11-feb 12-feb 13-feb 14-feb 15-feb Fig. 3 Daily averages of injected material 15% 10% 5% 0% BF flue dust Mixing BF flue dust with coal powder Two different methods have been used to mix the BF flue dust with the coal. The first method was to mix BF flue dust and raw coal at the coal yard by making a three-layer bed - two layers of coal and one layer of BF flue dust. The mix was handled as a normal coal, i.e. it was transported by truck to the coal injection plant. This method was very simple to perform, but had several disadvantages. First of all the and percentage of BF flue dust included. mixture was not homogeneous. The amount of BF flue dust in the different mixtures made varied between 5 % and 20 %, and as can be seen in Fig. 3 the daily average percentage of BF flue dust in the injected material varied between 4 % and 16 % as a result. However, the short-term variation was probably much greater. Secondly, unground coal has different properties compared to BF flue dust and BF flue dust also consists of several materials with different properties. As mentioned earlier, the grinding equipment acts as a windscreen and oversized particles are circulated, until the correct size distribution has been reached. This resulted that some materials were overground and that the filling level in the mill increased. This was noted, when the pressure drop over the mill and filter increased and the vibrations in the mill, measured at the power drive, increased to high levels. The grinding capacity was also reduced, because the mill was fed with a material that did not have to be ground. To improve the situation regarding the control of the addition of BF flue dust to the coal powder, an old coal injection system was taken into operation again. BF flue dust was transported from the dust cyclone to the former fine coal bin. The old injection plant was rebuilt to permit blowing of BF flue dust to the fine coal bin in the new coal injection plant. The amount of BF flue dust blown was adjusted to the amount of coal ground. The mixture could then be blown to BF No. 3 and distributed to the 32 tuyeres without having to take any other measures. By using this method, the mechanical problems involved with coal preparation could be avoided. Instead, problems with wear in the pipes from the coal distributor to the blast furnace occurred. It could also be noticed that the wear in the injection lances increased. Results Process conditions Besides the difficulties to keep a constant flow of BF flue dust, because of uneven mixing during the 1 st campaign, and wear of the pipes during the 2 nd campaign, additional BF related disturbances also occurred during the campaigns. During the 1 st test campaign there were problems with the charging equipment, decreased blast temperature, because one hot stove was temporary out of operation and three BF stoppages that resulted in increased variations in process data. During the 2 nd campaign the availability of the BF was close to 100%. 5

6 Table 5 Average ( x ) and variation (σ ) of some process parameters for each reference and test period. 1 st Campaign 2 nd Campaign Ref Test Ref 1a Test 1 Ref 1b Ref 2 Test 2 Ref 3 Test 3 x Blast flow knm3/h x Blast temperature C σ, Blast temperature x O 2 enrichment % x Blast moisture g/nm x Blast velocity m/s x Blast pressure(gauge) kpa x Top pressure(gauge) kpa x Gas efficiency % σ, Gas efficiency x Total heat loss MW MW Heat losses tuyeres, MW apr 11-apr 13-apr 15-apr BF flue dust % of injectants 10% 17-apr 19-apr 21-apr 23-apr 25-apr 27-apr 8% 6% 4% 2% 0% Fig. 4. Tuyere heat losses during the test periods 2 and 3 of 2 nd campaign. % Flue dust Gas distribution Occasions with a low burden level followed by a changed gas distribution were registered during both test campaigns. During the 1 st campaign, the central gas flow was significantly decreased, but in the 2 nd test campaign the effect was opposite. During the second and third test period of the 2 nd test campaign, the heat losses at the tuyeres decrease, Fig. 4, when the BF flue dust is injected. For the same period, the total heat loss varies independently, if BF flue dust is injected or not. Table 6 Hot metal composition and variation during the 1 st and 2 nd test campaigns. Time in the below table corresponds to the time used for evaluation excluding the periods of change. Time, x C σ C x Si σ Si x S σs x Temp. σ Temp. Slag B2 B4 (S)/[S] h w% w% w% C C kg/thm 1 st test Ref campaign Test Ref 1a Test Ref 1b nd test campaign Ref Test Ref Test Hot metal and slag The composition of hot metal and slag can be found in Table 6. The variation of the chemical composition of hot metal and slag increases during the test compared with the reference period during the 1 st test campaign. 6

7 w% Si Ref 1 Test Ref 2 Linear Ref 1 LinearTest Linear Ref During the 2 nd test campaign, the chemical compositions are in general quite stable with low variations during all reference and test periods. The S content of the hot metal and the S distribution ratio ((S)/[S]) are slightly reduced during the test periods compared to the corresponding reference periods Temperature, C Fig. 5 Si contents of hot metal during reference and test periods of the 2 nd test campaign. The Si content of the hot metal was significantly decreased during the previous BF flue dust injection tests made in the experimental BF [5]. As can be seen from Fig. 5, there is a decrease of approximately 0.02 % in Si content at the first test during the 2 nd test campaign. The same evaluation of the hot metal composition has been made for the other two test periods during this campaign. The second test gives similar behaviour for C and Si during the reference period and test period. The last test period of 2 nd campaign indicates higher C and Si contents during the test compared with the reference periods. Table 7 Consumption of reducing agents during the 1 st and 2 nd test campaign 1 st Campaign 2 nd Campaign Ref Test Ref 1a Test 1 Ref 1b Ref 2 Test 2 Ref 3 Test 3 Coke rate kg/thm PC kg/thm BF flue dust kg/thm Injected PC equivalents kg/thm Total kg/thm Reducing agents Table 7 shows the averages of PCR, BF flue dust rate and coke rate for each period of reference and test. The injected BF flue dust is included as PC equivalents, based on the C content, in the total amount of reducing agents. The consumption of coal and coke are in general unchanged or lower, when BF flue dust is injected compared to if only coal is used, except for the test 1 period of the 2 nd test campaign, when the consumption of reducing agents is decreased during the reference period after the test (ref 1b). During this test period, the blast moisture was significantly higher compared to the reference period, because steam was added to the blast. When the total consumptions of reducing agents are considered, similar relationships can be seen except for the 1 st campaign. During the 1 st campaign, the total amount of reducing agent increased by 4 kg/thm, when BF dust was added to the coal. Wear One problem that occurred during the campaigns was wear in pipes, especially from the coal distributor to the blast furnace, and the lances. The wear was much more pronounced during the 2 nd campaign and the reason is probably that the grinding in the 1 st campaign reduced the particle size and made the surface of the coke particles less abrasive. 7

8 a b outer c inner c d outer inner Fig. 6. Cross section of a lance and different stages of wear, from bottom to top cross section, wear on inner lance, severe damage close to the lance tip and a damaged blowpipe. Fig. 6 a shows the cross section of a lance with the space between the inner and outer lance for air cooling. Fig. 6 b and Fig. 6 c show the wear of the lances and Fig. 6 d shows the most severe occasion of wear, when a lance failure caused the breakdown of a blowpipe. Wear in the inner lance causes a hole, which makes it possible for coal and BF flue dust to enter the space between the inner and outer lance, Fig. 6b. The consequence is wear in the outer lance and a reduced cooling capability. When the cooling is reduced, a more severe damage can occur. In this case, the damage is close to the tip of the lance, Fig 6 c. If the outer lance fails, the flow of coal can be directed toward the blowpipe, finally causing a breakdown, Fig. 6d. Discussion Full-scale injection trials with BF flue dust have been made earlier, for example at the Donetsk Steel Plant BF 1 in Ukraine [6]. Compared to the trials made at the Donetsk Steel Plant, which did not show any problems with grinding or wear in pipes and lances, the trials at SSAB have shown that the abrasiveness of the BF flue dust is a problem that has to be taken into account. There are of course several differences between the two plants and the materials injected that can explain this. The BF flue dust at SSAB contains for example more carbon, which implies that there is more coke in the dust. The fact that wear of the pipes did not occur during the 1 st test campaign, when the BF flue dust was ground, indicates that grinding of the material both decreases the particle size and reduces the abrasiveness of the particles. One thing that was experienced at both plants was that the process and the hot metal quality were not changed significantly in any negative way. The C included in the BF dust seems to be efficiently used. The total consumptions of reducing agents are unchanged or decrease in most of the tests, when BF dust is added to the 8

9 PC. When comparing the reference and test period of the 1 st campaign, the stoppages and the significantly lower blast temperature during the test period have to be considered. During the 2 nd campaign, the total consumption of reducing agents of test 1 is similar to that of the reference period before the test, but higher than the reference period after the test. To avoid too high flame temperatures, steam was used to increase the blast moisture, when the injection was out of operation. According to literature, the used amount of steam corresponds to an increase in approximately 8 and 3 kg/thm of reducing agents for the test 1 and reference 1a periods, respectively. Volatiles are released from the PC, followed by cracking and combustion, but not from the BF flue dust. The combustion causes a decreased O 2 content in the off gas. Endothermic peaks at 800 C and at C, coinciding with generation of CO and CO 2, probably correspond to the reduction of iron oxides and the Boudoard reaction. However, when the majority of the sample consist of PC the behaviour of coal dominates, but the addition of BF flue dust will reduce the percentage of VM. According to the literature, the hematite is reduced or thermally decomposed at the conditions in the raceway [6][9][10]. The released O 2 contributes to the combustion of coal. The reduction of iron oxides in BF flue dust will probably decrease the flame temperature, if it is co-injected with coal. The reduction of Si in hot metal that was noticed in the test at the EBF was not found in the full-scale test at SSAB. One explanation can be a high SiO(g) generation in the EBF, because the raceway occupies a large area of the total cross section compared to BF No. 3. The influence of iron oxide on flame temperature and partial oxygen pressure [9] will then be more pronounced in the EBF operation. Another explanation can be the initial Si content of hot metal that is much higher in the EBF compared with in BF No. 3. When injecting hematite or FeO containing materials the FeO content of the raceway slag will increase. FeO can take part in the desiliconisation reaction of hot metal according to the reaction Si +2(FeO) SiO 2 +2Fe. The reduction rate is increased, when the Si content of hot metal increases. FeO is also consumed by char and coke fines in a direct reduction reaction. The amount of char increases normally with an increased PCR and the PCR is considerably higher at BF No. 3 compared with the EBF [9][10]. Injection of BF flue dust will increase the amount of tuyere slag. The slag basicity B2 will increase from 0.02 to 0.11 and the MnO content will increase as well. According to the literature, hematite is indirectly reduced to wustite in the raceway [4][9]. An increased FeO content will have a positive effect on the melting point and viscosity of the tuyere slag formed. An estimation of the melting point, according to a phase diagram of the system Al 2 O 3 -CaO-MgO-SiO 2, indicates a melting point of over 1700 C of the tuyere slag formed during the reference period and that it decreases more than 100 C during the test period [8]. If the increased content of FeO is taken into account, the decrease of melting point will be much more. The effect on the tuyere slag properties might have a direct effect on the S behaviour in the raceway region and indirectly by improving the permeability at the end of raceway. The decreased heat losses at the tuyeres may be explained by an increased gas permeability at the end of raceway caused by improved tuyere slag properties and consumption of char and coke fines by FeO. The decreased content of VM in a mixture of coal and BF flue dust probably causing slightly delayed ignition might also have some effects. 9

10 Conclusion The full-scale trials in Luleå have indicated that it is possible to inject BF flue dust without deteriorating the process or the hot metal quality and that the C and Fe in the BF flue dust is well utilised. Injection of BF flue dust can be a solution to increase the amount of materials recycled in an operation that lacks sintering facilities. The future work will be focused on solving the problem of wear in pipes and lances. Acknowledgements The laboratory staff and the colleagues at SSAB are greatly acknowledged for their support. BDX is also acknowledged for their guidance and service concerning transportation and handling of BF flue dust. The authors wish to thank the MIMER research foundation for the cooperation in characterisation and laboratory work. References [1] A. Babich, H.W. Gudenau, D. Senk, A. Formoso, J.L. Menendez, V. Kochura : [2] Experimental modelling and measurements in the raceway when injecting auxiliary substances. International BF lower zone symposium, Wollongong 2002 [3] H.W. Gudenau, K. Stoesser, H. Denecke, V. Schemann: Environmental aspects and recycling of filter dusts by direct injection or use of agglomerates in shaft furnaces, ISIJ International vol 40 (2000) no3, pp [4] Wirtsch, H.W. Gudenau, H. Denecke, S Wippermann: Iron ore and iron containing dust injection into the blast furnace, 1998 ICSTI/Ironmaking conference proceedings, pp [5] P. Sikström, L. Sundqvist: Recycling of flue dust into the blast furnace, TMS Conference, Luleå, Sweden (2002) [6] H.W. Gudenau, A. Babich, H. Denecke, S. Yaroshevskii, V. Kochura: Einblasen von Gichtstaub mit kohlenstaub in den hochofen, Stahl und eisen119 (1999) Nr 12, pp [7] S. A. Mikhail, A-M. Turcotte. Thermal reduction of steel-making secondary materials I. Basic-oxygen-furnace dust. Elsevier Science B.V. Thermochemia Acta p (1998) [8] Slag Atlas 2nd Edition, Verein Deutscher Eisenhüttenleute (VDEh), Verlag Stahleisen GmbH, (1995), , , 381. [9] Kosei Kushima, Masaaki Naito, Kiyoshi Shibata, Hiroji Sato, Hitoshi Yoshida, Morimasa Ichida. Iron ore injection into the blast furnace raceway, 47th Ironmaking Conference, Toronto, Canada, Apr. 1988, Iron and Steel Society, Inc., pp , 1988 [10] C. Yamagata, Y. Kajiwara, S. Suyama, T.Miyake, Desiliconisation Reaction of Pig Iron with High FeO containing Blast Furnace Slag under Prezzurised and coke coexisting conditions. ISIJ International, Vol. 30, 1990, No. 9, pp