HOT METAL QUALITY : ALWAYS A HOT TOPIC T. Bakker1 W.M. Husslage2 M.A. Reuter2 P. den Exter 3 A.G.S. Steeghs3 1
2
Corus Research, Development & Technology Delft University of Technology, Faculty of Applied Earth Sciences 3 Corus Strip Products IJmuiden PO Box 10.000, 1970 CA IJmuiden, The Netherlands
[email protected]
Key Words: Blast Furnace Ironmaking, Hot Metal Quality, Silicon, Sulphur
INTRODUCTION The Corus IJmuiden site in the Netherlands is one of the four integrated steelmaking routes within the Corus Group. IJmuiden annual output is 6.5 million ton steel, predominantly for strip and packaging applications. The core of the ironmaking facilities are the two blast furnaces #6 and #7 (Table I). Typical production levels of these furnaces are 7000 and 10,000 thm/day respectively, with a total annual output of 5.85 million ton hot metal. Both furnaces are operated at low coke rates (300 kg/ton) and consequently high levels of PCI injection (>200 kg/thm). Ferrous feedstock for the blast furnaces is a mixture of 50% olivine pellets and 50% ultra high basicity sinter, both produced on-site. Both furnaces are operated at a low slag volume just above 200 kg/ton hot metal. Table I : Characteristics of Corus IJmuiden blast furnaces IJmuiden BF #6 IJmuiden BF #7 Working volume (m3) 2328 3790 Hearth diameter (m) 11.0 13.8 Charging system PW Bell/MA Specific productivity (t/m3.24h) 3.1 2.8
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Table II : Typical hot metal composition and targets % Targets Si 0.40 0.40 ; 100% >0.25 0.30 50% SiO2) was partly abstracted into the intra-particle coke porosity, whereas other slags (lower in SiO2) remained as a distinct phase in the inter-particle voids. 5. The holdups of the fluids within the bed could not be predicted based on suitable dimensionless equations given in literature. Neither could a suitable dimensionless equation be derived for the experiments. 6. Residence time of fluids in the bed ranges from a few seconds to 30 minutes, depending on the type of fluid and the mode of feed to the bed. The results of this work can aid in understanding the type of surface across which transfer of sulphur from metal to slag takes place. Initial experiments of simultaneous flow of slag and metal (containing sulphur) through a packed coke bed indicated that both fluids interact in the bed and thus favor transfer of sulphur. Equilibrium limit – Apart from the observations on quenched furnaces and other samples, two other indications for the equilibrium thesis can be put forward. Firstly, others[Steiler] demonstrated that slag and metal reach equilibrium with respect to sulphur. Secondly, it appears that (S)/[S] for furnaces is a site specific aspect, i.e. furnaces on the same site tend to have similar sulphur distribution ratios. This indicates that obviously slag chemistry is a dominant factor, rather than the operational performance of a furnace.
(S)/[S]
From a physico-chemical perspective, the equilibrium sulphur distribution between slag and metal is the result of four parameters, i.e. the activity of sulphur in the hot 80 metal, the activity of sulphur in the slag, the prevailing 70 oxygen activity and temperature. Young et al. [Young, Cripps Clark] 60 summarized this as follows : 1/2 (S)/[S] = K.Cs.fs/pO2 50 40 30
IJmuiden furnaces
20 10 0 1.20
1.22
1.24
1.26
1.28
1.30
1.32
1.34
1.36
1.38
1.40
(MCaO+1/2MMgO) (MSiO2+1/3MAl2O3)
Figure 7 : Sulphur distribution of European furnaces as a function of basicity (EBFC data 1998) 5
K represents the reaction constant for the reaction between sulphur dissolved in the metal and gaseous S2, and is relatively strongly dependant on temperature. Cs is the sulphur capacity of the slag and is dependant on slag composition and to lesser extent on temperature. fs represents the activity of sulphur in the metal. It is dependant on hot metal composition, carbon in particular. Because carbon saturation is strongly dependant on temperature fs is also slightly dependant on temperature. pO2 is the prevailing oxygen activity and is most difficult to assess. It can be indirectly derived from the [Mn]/(MnO) ratio in metal and slag but also related to the blast pressure via the Boudouard reaction[Steiler]. Apart from control by blast pressure it is believed that also the position and shape of the cohesive zone can have an effect on the oxygen activity in the hearth[Steiler]. The relative contribution of controllable parameters, i.e. temperature, C s (via slag chemistry factor [MCaO+1/2MMgO]/[MSiO2+1/3MAl2O3][refs]) and potentially pO2, is shown in Figure 6. The effect on (S)/[S], relative to a reference condition (T=1500°C; slag factor = 1.3; aO=2.10-6; (S)/[S]=40) is expressed as a factor, indicating the change in (S)/[S]. This figure demonstrates that oxygen activity has the most significant effect, followed by temperature (mainly via its effect on K) and slag composition. However, from a comparison of data of different European furnaces only a reasonable correspondence between the slag composition factor and the observed (S)/[S] was found (Figure 7). This observation further supports the hypothesis that desulphurization is governed by equilibrium considerations, rather than transfer restrictions. Furthermore, it suggests that slag composition is the most dominant factor. Operational experience – An investigation into day-to-day operational performance, to be discussed subsequently, demonstrated that equilibrium with respect to sulphur between slag and metal is also attained on a short term. Short-term variation in sulphur levels on a torpedo-to-torpedo basis could be explained by variation in slag chemistry during a cast. An explanation will be given for the operational observation that hot metal sulphur tends to be high at the beginning of a cast (in the absence of slag) and to decrease towards the end. The latter observation initially was an argument against attainment of equilibrium, because it suggests that the presence of slag during tapping is crucial to maximally desulphurize.
S in metal
The [S] trend over 2 years, shown in Figure 8, indicates that the typical level varies between 0.025 and 0.04%. The long term variation in sulphur trends can be related to variations in the parameters in the sulphur mass balance, i.e. sulphur input, slag volume and slag basicity. However, Figure 8 and 9 demonstrate that [S] also significantly varies on a short term. Figure 9 shows the distribution of the difference 0.10 IJmuiden 6 0.09 between either cast average or torpedo 0.08 analysis and the daily average sulphur 0.07 0.06 1.8 value. Short term variations in the order of 0.05 0.005-0.010% [S] are common. The 0.04 K f(T) 1.6 0.03 distribution is symmetrical, i.e. high 0.02 1.4 0.01 C f(Bas) sulphur casts are balanced by casts with 0.00 low sulphur. The behavior of both furnaces C f(T) 1.2 0.10 IJmuiden 7 0.09 is very similar. f f(T) 0.08 1.0 s
s
s
S in metal
0.07 0.8
0.6
0.06 0.05
pO21/2 f(aO)
0.04 0.03 0.02
0.4 1400 0.011425
0.00 1.1 04/01/98 1.0E-006
1450
1475
1500
1525
1550
1600 T
1575
(MCaO+1/2M MgO)
07/01/98 1.2 1.5E-006
10/01/981.3 01/01/99 2.0E-006
04/01/99 1.4 2.5E-006
07/01/99
Bas = 01/01/00 04/01/00 10/01/99 1.5 (MSiO2+1/3MAl2O3
07/01/00
3.0E-006 aO
Figure impact of different parameters Figure 68: :Relative Variation in hot metal sulphur : dailyonvalues and individual torpedo analyses; sulphur distribution (see text)IJmuiden BF#6 and BF#7 6
The typical performance in a three day period is shown in Figure 10 for IJmuiden BF6. This figure refers to a period of a week undisturbed production, followed by two furnace stops to accommodate changing hot metal demand levels of the steel plant. The behavior on August 21st is typical for undisturbed furnace operation. The production level typically was 7500 ton/day (3.2 t/m3/24h) achieved at a blast rate of 3700 Nm3/min and 33% O2-enrichement in the blast. The coke rate was 310 kg/thm with corresponding PCI rate of 210 kg/thm. The first plot gives an overview of the cast practice. Both IJmuiden blast furnaces have three tap holes. Two of the tap holes are in operation, of which one is tapping at any moment. A tap hole is opened by drilling. Casts are sequentially ordered with no inter-tap period. The typical cast length for BF #6 is in the order of 2-3 hours. A standard procedure is to open the second tap hole if no slag is cast within 60 minutes after tap
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IJmuiden 6 8
Cast average
6
4
2
Torpedo-based
0 10
IJmuiden 7 8
6
4
2
0 -0.05 -0.04 -0.03 -0.02 -0.01 0.00
0.01
0.02
0.03
0.04
0.05
S-Sdaily average (%)
Figure 9 : Variation in short term [S] (either torpedo or cast average) versus daily average value (2 year period) 7
Al2O3
9 17 16
60
15 08/21/00
08/22/00
08/23/00
08/24/00
50
(S)/[S]
00:00:00 00:00:00 Figure 1000:00:00 : Overview of three-day period 00:00:00 21-23/8/00 IJmuiden BF#6 hole opening. Two of such incidents arise for example during casts #9261 and #9273 in Figure 10. Under 40 normal tapping conditions slag is observed within 30 minutes. The torpedo car logistics are also indicated in the cast plot. Prior to transfer to the steel plant, a hot metal sample is taken from a torpedo car to be subsequently analyzed. Slag is sampled at the end of a cast. The 30 slag sample is taken from the slag runner, just prior to closing of the tap hole. Both hot metal and slag analyses are indicated in Figure 10. The cast temperature is 20 measured typically three times during a cast by a thermocouple at the skimmer, of which only the highest temperature is recorded. 10 and ‘jumpy’ in comparison to the silicon trend, i.e. The behavior of sulphur seems to be more irregular subsequent analyses are not following a continuous trend upwards or downwards, as silicon does. Torpedo cars with high sulphur can be directly followed by low sulphur analyses and vice versa. There does not seem to be a 0 1.20 1.25 1.30 1.35 1.40 link between the silicon and sulphur trend. (M +1/2M ) CaO
In addition to the normal sampling/analysis scheme extra slag and hot metal samples have been collected in the morning shifts of 21st-23rd August. Hot metal was sampled at the skimmer, rather than from the torpedo car. Slag sampling (when present) was according to the normal procedure, i.e. at the slag dam. Typical sampling frequency was 10-15 minutes. The resulting analyses are indicated in Figure 10 as well. As the sampling moment of the slag and metal sample was identical the additional samples could aid in better understanding the relation and interaction between metal and slag.
MgO
(MSiO2+1/3MAl2O3)
Figure 11 : Relation between slag basicity and (S)/[S] of The main conclusion from the frequent samples taken during frequent sampling campaign. sampling campaign is that the sulphur distribution between slag and metal is predominantly more or less at chemical equilibrium. In Figure 10 the molar ratio expressed as (MCaO+1/2MMgO)/(MSiO2+ 1/3MAl2O3), calculated from the slag analysis, is indicated as well. In Figure 11 this ratio is plotted against the sulphur distribution (S)/[S], showing a clear relationship. A similar relation is found for basicity. The introduction of acid burden prior to a stop also has a strong influence on (S)/[S] and [S]. Another remarkable conclusion from the frequent sampling campaign was that the trend in slag composition during a cast always appears to be similar. All casts sampled showed a trend where the basicity of the slag increases towards the end of the cast. As Figure 10 shows, this is caused both by an increase in CaO and a decrease in SiO2 in the slag during a cast. Typical increases in basicity of 0.05 (e.g. from 1.45 1.50) are common. Note that this behavior occurred even after the interruption in cast #9261. In this situation the tap hole changeover was delayed, the South tap hole was reopened. Experience is that after a short closing period the same tap hole can cast for a prolonged period. Note also, that although slag was cast before the interruption it takes over an hour to cast slag again (it was even decided to shortly open the North tap hole). The second part of the cast shows a similar development in slag composition as the first part.
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(S)/[S]
(S)/[S]
Influence of slag over metal ratio – The observation that slag and metal are at equilibrium contradicted the operational ‘feeling’ that in the absence of slag in the initial stages of a cast [S] tends to be higher than in later stages of the cast. A closer analysis showed that the observation is IJmuiden 6 100 correct, but that the underlying 90 80 mechanism is more complex than the 70 insufficient opportunity for slag/metal 60 50 contact to desulphurize in the absence 40 30 of slag. 20 Figure 12 gives the relation 10 34.72 + 0.07307*x 0 between the degree of slag cast over 0 10 20 30 40 50 60 70 80 90 100 slag over metal time metal and the corresponding sulphur distribution (S)/[S]. It can be observed IJmuiden 7 100 that a high slag over metal ratio is no 90 guarantee for an improved sulphur 80 70 distribution. At slag over metal ratios 60 50 >95% sulphur distribution (S)/[S] 40 ranges from 10-80. Nor does a cast 30 20 with relatively short periods of 10 37.79 + 0.08151*x 0 simultaneous slag/metal tapping lead to 0 10 20 30 40 50 60 70 80 90 100 slag over metal time low values for (S)/[S]. In other words, the relation between presence or Figure 12 : Sulphur distribution as a function of percentage of time absence of slag and attainment of that slag was cast simultaneously with metal equilibrium is not very strong, if not absent. A clear example is also shown in Figure 10 60 during cast #9259, where slag was absent during most 14 of the cast, but where hot metal sulphur levels are still 50 12 relatively low. 40 overgangsmengers (2-taps)
hele mengers
vol zonder slakdekking
zonder slakdekking
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It is believed that two other mechanisms can account for the ‘feeling’ that higher sulphur levels tend to occur in initial part of a cast. First of all the observation that basicity tends to increase towards the end of a cast. Consequently, chemical conditions for desulphurization become more favorable during a cast. Secondly, Figure 10 demonstrates that the relative flow of slag to metal increases towards the end of a cast. As a result, even at constant (S)/[S] the sulphur level in the metal will be lower simply because of mass balance considerations.
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30
6 20 4 10
0 0.00
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[S]
IJmuiden 6
400 100
80
begin zonder slak
begin met slak
vol met slak
vol met slak
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To validate the observation of higher sulphur levels at the beginning of a cast the hot metal sulphur levels as a 60 200 function the type of analysis were evaluated. Hot metal 40 analyses refer to sampling of torpedo cars just after 100 being filled completely, i.e. prior to transfer to the steel 20 plant. A distinction can be made between four types of 0 0 0.00 0.02 0.04 0.06 0.08 0.10 0.00 0.02 0.04 0.06 0.08 0.10 torpedo cars, i.e. exclusively cast in the absence of slag, [S] [S] cast partly in the absence and partly in the presence of slag, entirely cast in the presence of slag and overlap Figure 13 : [S] distribution of sulphur for different types of torpedo cars (see text) 9
torpedo cars, being cast first in the presence of slag and subsequently completed during the next cast mostly in the absence of slag. The distribution of sulphur levels for the four types of torpedo cars is shown in Figure Error! Reference source not found. The averages and standard deviations are summarized in Table IV. This figure and table confirm that nor the presence nor the absence of slag guarantees either high or low sulphur levels in the metal. The higher average sulphur levels of torpedo cars cast entirely in the absence of slag is most likely related to the less advantageous conditions for desulphurization during the initial parts of a cast, i.e. from slag chemistry perspective and slag to metal flow rate. Table IV : IJmuiden BF#6 and BF#7 : Influence of presence of slag on [S] [S]avg (%) SD (%) IJ6 IJ7 IJ6 IJ7 2-tap torpedo, start=slag, end no slag 0.0363 0.0339 0.0101 0.0102 1-tap torpedo, start =no slag, end = no slag 0.0405 0.0413 0.0126 0.0162 1-tap torpedo, start =no slag, end = slag 0.0342 0.0319 0.099 0.0103 1-tap torpedo, start = slag, end = slag 0.0317 0.0303 0.0090 0.0090
HOT METAL SILICON PHENOMENOLOGY DURING PRODUCTION LEVEL VARIATIONS Stable process conditions - In periods of stable blast and process conditions the standard deviation in hot metal silicon analyses of individual torpedo cars is small. A typical standard deviation of 0.065% at an average [Si] of 0.40% is achieved. Silicon is controlled by adjusting PCI rate to the amount of oxygen blown per unit time and the silicon analyses of previous casts. Figure X demonstrates that the trend in silicon torpedo-after-torpedo is relatively smooth and not ‘jumpy’. Trends tend to extend across a changeover of tap hole, indicating that hot metal is more or less homogeneous in the furnace hearth. Flex practice – The so-called flex practice has been introduced to accommodate blast furnace production level to changing hot metal demand from the steel plant. Blast rate is reduced to 70-90% of the target blast rate level to reduce furnace output. This practice occasionally, but not always, leads to increased silicon levels in the hot metal. Figure X shows two contrasting situations where both a strong and no reaction on [Si] was observed.
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blast rate
S i
5000 4000 3000 2000 1000 0 1.0 0.8 0.6 0.4 0.2 0.0
Figure 15 : Furnace blow-in after stop. Notice the initial high silicon levels in the hot metal
Furnace blow-in after stop – Hot metal silicon tends to develop according to a specific trend during blow-in after a furnace stop. As shown in Figure 15, initially high levels of silicon (>0.70%) are observed. Subsequently, silicon rapidly drops (within an hour) to levels below the target level (typically 0.20%). Then [Si] gradually stabilises typically in 6 hours after blow in on the target silicon level.
blast rate
Mechanisms involved – To improve control of silicon under unstable process conditions understanding of the underlying mechanisms is a prerequisite. Hot metal silicon is the result of the complex interaction between tuyeres conditions (pblast; Tflame), operating point of the process (gas distribution, actual reductant demand of the lower furnace), the position of the cohesive zone and slag chemistry. E.g. the increase in [Si] under flex practice conditions might be related to an improved overall gas utilization by an improved gas distribution at reduced blast rate, leading to a lower reductant demand in the lower furnace. Consequently, the cohesive zone will move upward. The increased height over which silicon transfer from gas to metal can take place ultimately leads to higher levels of silicon in the metal. On the other hand, several hypotheses might explain the high silicon levels during the initial stages of blow-in after a stop. The relatively low blast pressure during blast rate increase might explain these high [Si] levels. Lower blast pressures are believed to promote the transfer of silicon from coal/coke ash to the gas phase and thus the amount of silicon transferred to the metal. Another hypothesis 5000 4000 3000 2000 1000 0
S i
no reaction
reaction
1.0 0.8 0.6 0.4 0.2 0.0
Figure 14 : Flex practice situation. Contrary to the first flex situation the second flex leads to increased silicon level in hot metal. assumes that in the initial stages after a stop relatively large amounts of gas are required for a ton hot metal. The higher gas/metal ratio leads to relatively increased exposure of metal droplets to gaseous silicon and thus higher levels of silicon in the final metal. A third hypothesis links the addition of acid burden to the burden prior to a stop to increased levels of silicon in the hot metal.
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During the frequent sampling campaign (depicted in Figure X) also metal and slag has been sampled just after a furnace blow-in (cast #9273). The results give clues on the mechanisms involved. In this situation the furnace has been stopped at relatively short notice and had thus been prepared by charging acid burden relatively shortly before going off-blast (1.5 hours). No additional coke had been charged, but 25 kg above the coal set point has been injected prior to going off blast (1.5 hours) and during blow-in (3 hours), to compensate for heat losses during the stop. The silicon analyses show a rapid fall from >0.7% to
