Scandinavian Journal of Metallurgy 2001; 30: 225–231 Copyright C Munksgaard 2001Printed in Denmark. All rights reserved

SCANDINAVIANJOURNAL OF METALLURGY

ISSN 0371-0459

Physical modelling of hot metal flow in a blastfurnace hearth

Matti Juhani Luomala, Olli Juhani Mattila and Jouko Juhani HärkkiLaboratory of Process Metallurgy, PO Box 4300, FIN-90014 University of Oulu

Physical modelling was applied in order to gain information aboutthe flow behavior in the blast furnace hearth. For that purpose,residence times of tracer injections were determined based onthe electric conductivity measurements and flow paths were visu-alised by using KMnO4 (s). Especially the influence of a floatingcoke bed and blocked center of the deadman on flow character-istics in the blast furnace hearth were investigated using a ring-shaped distributor and penetrated taphole, simulating sinteredmud mass. When a coke-free space is formed under the cokebed, the iron discharge route forms a V-shaped cross-sectionalarea in which iron flows completely inside the coke bed. The

In order to produce pig-iron competitively, a longcampaign life of a blast furnace is required. Generally,the blast furnace hearth is considered to be the mostcritical area for the length of a campaign because thehearth is the most difficult region to repair. For re-lining the hearth, the furnace needs to be blown outand as a consequence, production losses as well ascapital investments are remarkably high. Hearth lin-ing usually consists of different kinds of carbon-basedmaterials and a so-called ceramic cup. In certain con-ditions, flow of molten iron causes severe erosion ofhearth lining. The flow behaviour of iron also plays akey role in terms of a proper drainage of hearth andcontrol of the taps [1]. The amount of residual slagaffects overall function and productivity of the blastfurnace.

In spite of wide-ranging measurement and monitor-ing facilities of the blast furnace, flow conditions inthe hearth cannot be directly determined. Therefore,different models – either physical, numerical or evenelectrical – describing the actual process are needed.Although mathematical modelling has become morecommon, physical modelling is still used for its clar-ity. It is sensible to study some variants, e.g., differentshape or porosity of the deadman or different hearthgeometries, with numerical simulation.

Fukutake & Okabe [2, 3] studied the effect of the

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share of iron entering the V-shaped area can be increased eitherby increasing the drain rate or by decreasing the height of thecoke-free layer. Inactive center of deadman causes iron to flownear the hearth wall which leads to severe erosion of hearth.

Key words: blast furnace, hearth, hot metal flow, physicalmodelling, residence time, hearth wear.

c Munksgaard, 2001

Accepted for publication 11 September 2000

rate of tapping, the viscosity of slag and the numberof tapping operations on drainage of viscous liquidsand developed an empirical relationship between theamount of residual slag and the above-mentionedparameters. Tanzil, Pinczewski and co-workers inAustralia extended the modelling work, e.g. byundertaking the experiments with 2 immiscibleliquids [4–6]. Ohno et al. [7] carried out excellent re-search which included numerical calculations, modelexperiments and measurements on operating blastfurnaces in order to clarify the flow of iron and slagin the hearth [7]. Standish & Campbell [8] investigatedthe residence-time distributions of liquid in a cold-scale model of a blast furnace hearth with and with-out coke-free layers at the base and coke-free guttersnear the wall. It was shown in their studies that thepresence of different packing regions in the hearth al-ters the residence-time distribution. Gudenau et al. [9]studied the influence of the immersion depth of thedeadman on the flow pattern and local velocities [9].Roy et al. [10] and Chatterjee et al. [11] used a 2-di-mensional slice model to study the effect of differentoperating variables, e.g., slag viscosity, casting rate,initial slag height on residual slag volume. RecentlyBachhofen et al. [12] simulated hearth wear in a physi-cal model and the results were found to be in goodagreement with those measured in operating blast

Luomala et al.

furnace. The influence of different parameters on theflow conditions in the hearth and on hearth drainagewere additionally examined by mathematical simula-tion.

There are only few 3-dimensional physical hearthmodels reported in the literature and none of themhave presented a proper distribution of iron drippinginto the hearth. In this paper, an attempt has beenmade in the direction of proper liquid distribution onthe cross-section of the model. In addition, in the pres-ent work, the taphole penetrates into the packed bed,simulating the effect of sintered mud mass. In thisstudy, the share of iron which flows solely in the cokebed, when coke bed floats in the iron bath, is defined.The influence of different operating parameters on therenewal of deadman coke and the wear of refractorylining are discussed.

Additionally, the influence of inactivated center ofdeadman on flow paths in the bottom of the hearthhas been examined.

Experimental conditions

It is essential that dynamic similarity is obtained be-tween the flows in the physical model and in the ac-tual process. In the present work, this is assured byconsidering 3 dimensionless numbers. The dimen-sionless numbers in eqs. (1), (2) and (3) are given bythe following symbols. The Froude number is modi-fied by Niu et al. [13], Galilei and Reynolds numbersby Fukutake et al. [14]:

Remod Ω|lu dpj

(1ªe)m, (1)

Frmod Ωm2

dpjg(1ªe)2 , (2)

Ga Ω|2

l gd3pj3

m2l (1ªe)3 , (3)

where u, dp, j, g, e, |, m are superficial velocity, par-ticle diameter, sphericity of particle, gravitational ac-celeration, voidage of bed, density and viscosity ofliquid, respectively.

Table 1 shows a comparison of the dimensionlessnumbers defined by eqs. (1)–(3) between the physicalmodel and those estimated in the blast furnace. Table2 shows the major variables influencing the flow con-ditions in the hearth.

The magnitude of the Reynolds number is close tounity, which indicates that the effect of inertial and

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Table 1. Comparison of dimensionless numbers between the actualblast furnace process and the physical model

Blast furnace [–] Physical model [–]

Reynolds number 4.59 0.608–2.434*Froude number 4.89 ¡ 10ª7 1.563 ¡ 10ª6–2.501 ¡ 10ª5*Galilei number 1.97 ¡ 108 9.47 ¡ 105

* Corresponds to drain rates 0.004–0.016 m3/min, respectively.

Table 2. Major variables influencing the flow conditions in the hearth

Blast furnace Physical model

hearth diameter (m) 8.0 0.8taphole diameter (mm) 40 10diameter of the coke (mm) 30 3.4sphericity of coke 0.5 0.85porosity of bed 0.4 0.37density of iron (kg/m3) 6800 1000viscosity of iron (kg/ms) 6.0 ¡ 10ª3 1.0 ¡ 10ª3

Tapping rate (m3/min) 0.485 0.004–0.016Superficial velocity (m/s) 1.61 ¡ 10ª4 (1.326 – 5.305) ¡ 10ª4

viscous forces are quite similar. Because the Froudenumber represents the relationship between inertialand gravitational forces and the Galilei number therelationship between inertial and gravitational forcesto viscous forces, it is evident that the gravitationalforce governs the flow behaviour in the hearth. Al-though values of, e.g., the Froude number in themodel are not quite the same magnitude as in theblast furnace, it is evident that the gravitational forceis also dominant in the water model.

Apparatus and experimental procedure

Fig. 1 shows the experimental apparatus and auxiliaryinstruments. A single SAN-particle is transparent, butthe packed bed of SAN-particles used in the modelwas not transparent, so the defining of flow paths di-rectly was not possible by, e.g., a digital videocamera.Instead of optical sensors, residence times were deter-mined based on electric conductivity measurements.Therefore, 40 ‘‘fixed’’ electric resistance sensors andone sensor at the tap hole were placed in the modelas shown in Fig. 2. In addition, one movable sensorwas attached to varying tracer injection point. The in-fluence of sensors on flow characteristics was as-sumed to be negligible. Measuring sensors werelocated only on the other half of the model, becausethe flows on the another half were assumed to besimilar according to geometrical similarity.

Water was fed into the model through flow meterand distributor. A distributor simulated the real mass

Hot metal flow in a blast furnace hearth

Fig. 1. Experimental apparatus.

Fig. 2. Location of sensors in the model.

flows in the tuyere zone of a blast furnace as meas-ured by Mitsufuji et al. [15]. By the use of a horizontalprobe at a tuyere level, they observed that the liquidflow concentrates into a narrow area in front of theraceway region. Furthermore, Nakajima et al. [16] es-timated that almost 70% of the total particulate liquidsflow down through the narrow area. The diameter ofthe perforated ring-shaped distributor tube used inthe experiments was 0.62 m. Experiments were con-ducted in stationary state, i.e., the liquid level did notchange during the tests. Water was drained outthrough the pipe (diameter 10 mm) which penetrated13 cm into the coke bed.

When the stationary state was obtained, 20 ml ofsaturated NaCl-solution was injected in the desiredlocation, which caused a change in the electrical con-ductivity of water. The conductivity in every sensorwas measured as a function of time and the datalogger

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was used in data acquisition. Afterwards, it was poss-ible to determine residence times between injectionpoint and taphole or between any 2 points. To visual-ize the flow paths, KMnO4 (s) was placed near thewall or bottom.

Results

Drain rateThe drain rate has significant influence on residencetimes as shown in Fig. 3. The bigger the drain rate,the shorter residence time, i.e., the bigger the streamvelocity in the hearth. After a dimensionless distanceof 0.4, the difference becomes clearer because streamvelocities are much higher near the taphole.

Fig. 4 shows the same data in slightly differentform. When tracer is injected in the middle of centerline (DøΩ0.5), the residence time seems to be linear as

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a function of flow rate. As the injection point drawsaway from the taphole, the curvature of the residencetime curve increases.

Floating coke bedSometimes, especially in smaller blast furnaces, thecoke bed floats in the iron bath and coke-free space isformed under the coke bed, which may contribute tosevere erosion of hearth refractory [17]. Fig. 5 showsthe relationship between residence time and dimen-sionless distance from taphole wall according to the

Fig. 3. The effect of drain rate on residence time.

Fig. 4. Residence time as a function of drain rate at different injectionpoints.

Fig. 5. Residence time as a function of the height of coke-free space.

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height of the coke-free space. The reason that there isa peak in the residence time curve can be explainedas follows. Before the position showing the peak,water flows toward the taphole through the packedbed where the stream velocity is slow, but after theposition showing the peak, it descends nearly verti-cally through the packed bed, enters the coke-freespace and flows toward taphole.

By inspection (Fig. 5), the position of the peak is notalways the same. Fig. 6 shows the position of thepeak as a function of the height of coke-free space.Considering the accuracy of the experimental pro-cedure, the relationship was linear in the measuredrange of the floating height of the deadman. The big-ger the height of coke-free space, the closer the posi-tion of the peak in the residence time curve, i.e., thesmaller part of iron flows solely through the coke bed.

The effect of the flow rate on the position of peakwas also investigated. By inspection of Fig. 7, the posi-

Fig. 6. Position of maximum (peak) in residence time curve as a func-tion of the height of coke-free space.

Fig. 7. Position of peak as a function of drain rate.

Hot metal flow in a blast furnace hearth

Fig. 8. Determined iron discharge route when coke bed floats. Thepatterned area indicates the area in which iron flows completely insidethe coke bed.

tion of the peak moves away from the taphole withincreasing flow rate. A similar finding was also madeby Standish & Campbell [8]. They suggested that thismay affect hearth heat transfer, skull formation andrefractory wear.

To estimate the amount of iron which flows onlythrough the coke bed i.e. not through the coke-freelayer, further experiments were done. As shown inFig. 8, 4 points representing peaks in the residencetime curve at different angles from the taphole weredetermined in order to obtain a cross-sectional areafor iron flow that occurs only in the coke bed. Theheight of the coke-free layer was 3 cm and the drainrate was 12 l/min. According to Fig. 8, about 45% ofmolten iron flows through coke bed.

Blocked center of deadmanThe deadman may become inactive for many reasons,e.g., low coke strength, low production rate, too longmaintenance stop, water leakage into the furnace, im-proper burden distribution, hearth construction,hearth erosion, etc. If the deadman becomes blocked,it will cause bad hot metal quality, irregular burdendescending, abnormal hearth erosion etc. [18]. In thephysical model, liquid flow through the center of thedeadman was prevented with cylindrical block (DΩ0,2 m). Fig. 9 shows clearly the change in residencetime compared to the normal situation where the po-rosity of the coke bed is homogeneous. Because thecenter is totally impermeable, it takes a longer time toreach the taphole when injection is made beyond theblock. Fig. 10 shows comparison between flow paths

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Fig. 9. The influence of blocking the center of the deadman on residencetime.

Fig. 10. Comparison between flow paths with and without a blockedcenter of the deadman.

at the bottom of the model with and without a block-ed center drawn on the basis of visual inspection.

Discussion

The flow behaviour of molten iron in a blast furnacehearth is strongly affected by drain rate and prop-erties of the deadman. The influence of floating cokebed and the blocked center of the deadman wereespecially investigated in this research.

In the model experiments, the share of iron whichflows only through the coke bed is about 45% and itdepends on the drain rate and the height of the coke-free layer. Ohno et al. [7] presented comparable resultsin the form of equal travelling time distribution. Theshape of the patterned area in Fig. 8 is, however,clearly different from that calculated by Ohno et al.[7]. The difference can be explained by different distri-butions of iron dropping into the hearth and different

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taphole penetration depths. In the present investiga-tion, the iron was distributed properly and the dimen-sionless taphole penetration depth was 0.1625, whilstin the Ohno calculations, the distribution was as-sumed to be uniform and fluid was drained at thewall.

The greater the share of iron that flows only in thecoke bed, the better carbunization of pig iron is. Theshare can be increased either by increasing drain rateor by decreasing the height of the coke-free layerwhich can be deduced from the information in Fig. 7and Fig. 6, respectively. In the former case, carbuniz-ation does not necessarily improve because the resi-dence time of pig iron shortens. Additionally, theabove-mentioned actions do not agree with the ac-tions generally proposed to extend the hearth life-time[19].

Reduced carbonization of iron by coke may be de-creased to such an extent that the renewal rate ofdeadman coke slows down significantly. When the ce-ramic cup is worn out, carbon bricks are exposed tounsaturated hot metal which dissolves the carbonlining.

The inactive center of the deadman causes iron toflow near the hearth wall which is clearly seen in Fig.10. The above-mentioned phenomenon may lead tosevere erosion of hearth refractory, especially if theflow velocity of iron is detrimentally high. It is alsonotable that due to the long taphole, iron is suckedthrough the coke bed to the taphole before it reachesthe corner of the hearth bottom under the taphole.

The experiments were carried out using only oneliquid phase, i.e., the influence of the slag phase wastotally ignored. However, the results should be practi-cable because 2 liquid phases have more effect on thebehaviour of liquid-liquid interface, and less on theflow behaviour, e.g., at the bottom. Saturated NaCl-solution as a tracer is not the best choice because it isheavier than water. During the visual inspection, theinfluence of the wall effect on flow paths was as-sumed to be insignificant.

Conclusions

When coke-free space is formed under the coke bed,the iron discharge route forms a V-shaped cross-sec-tional area in which iron flows completely inside thecoke bed. The share of iron entering the V-shaped areacan be increased either by increasing the drain rate orby decreasing the height of the coke-free layer. Thisimproves the carbunization of pig iron but increasesthe wear of hearth lining.

The inactive center of the deadman causes iron to

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flow near the hearth wall, which leads to severe ero-sion of hearth refractory, especially if the flow velocityof iron is detrimentally high.

With a longer taphole, the wear of the corner of thehearth bottom under the taphole can be reduced.

List of symbols

dp particle size (mm)Fr Froude numberD diameter (m)Dø dimensionless distance from tapholeg gravitational acceleration (m/s2)Ga Galilei numberR radius of the hearthRe Reynolds numberu superficial liquid velocity based on empty

column (m/s)e fractional voidage of bedm viscosity (Pa¡s)| density (kg/m3)j sphericity of packing particle

Subscriptsl liquidmod modified

Acknowledgements

The authors wish to express their gratitude to the Na-tional Technology Agency of Finland (TEKES) and theAcademy of Finland for the financial support. TheRautaruukki Group and technical support from theWorkshop of the Department of Process Engineeringare also gratefully acknowledged.

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