Experimental Designs towards an Understanding of Process Phenomena in Steel Making

Ragnhild E. Aune and Seshadri Seetharaman Division of Materials Process Science,

Royal Institute of Technology, SE-100 44 Stockholm, Sweden Tel.:+46-8-790 83 55 Fax: +46-8-790 09 39 E-mail: raman@kth.se

Key words: experimentation, thermal diffusivities, x-ray imaging, coke, graphitization, reduction, foaming, mould powder

INTRODUCTION With the increasing demands on product qualities due to the high technological applications, metallurgical processes, above all, with respect to steel production, need a sound understanding and optimization based on process phenomena. Many of the processes occur under dynamic conditions with the properties of the system undergoing continuous changes. The depletion of reactants due to ongoing reactions or the retardation of the transport of the products causes changes in local physical properties, which, in turn, can lead to further complications. Since these changes and their consequences can not be estimated by suitable models with our existing knowledge, experimental studies and visualization have become very important. Appropriate experimental design, at the laboratory scale, in order to understand process phenomena is thus an important area in high temperature steel research. Unfortunately, due to funding constraints, importance of experimentation has been set aside and today, there are very few laboratories in the world that are suitably equipped for these kind of studies. The present paper highlights some of the experimental studies carried out in the laboratory of Materials Process Science, Royal Institute of Technology, Stockholm, Sweden. The paper deals with studies in coke making, iron making, steel making and continuous casting areas.

STUDIES ON COKE PROPERTIES 1

Coke is an important ingredient in blast furnace iron making. The chemical as well as physical functions of coke in blast furnace operation are well-known. Coke is an amorphous form of carbon produced by the coking process in coke ovens. The amorphosity enhances the reactivity of coke with respect to direct as well as indirect reductions. Coke reactivity, i.e. degree of coke gasification by CO2 , is one of the most important properties among the high temperature properties of coke. If coke reacts excessively with CO2, it will become more porous and be broken into smaller pieces, resulting in a poor flow of gase and molten products. As a consequence, the reactivity should be kept as low as possible although, at the same time, the reactivity should be high enough to guarantee satisfactory reduction and carbonization of the hot metal. Acquisition of the optimum reactivity is very important for the blast furnace operation. The reactivity is considered to be dependent on the degree of graphitization, the porosity as well as by the ash content. The degree of graphitization will control the chemical reaction rate. It is generally known that cokes with a higher degree of graphitization show lower reactivity. Coke reactivity would increase with an increase in the porosity. Porosity and pore size control the gas diffusion rate. Ash may also affect the reactivity since alkalis contained in ash catalyze the gasification reaction. As the ash content increases, coke reactivity may increase. As a measure of the reactivity, CRI (Coke Reactivity Index) is widely used all over the world. CRI is obtained by measuring the weight loss (%wt) of coke after reaction with CO2 under controlled condition, and is a rigorous but time consuming measurement. In the present laboratory, attempts have been made to monitor thermal diffusivity as a measure of the structural changes occurring in coke. The impact of thermal diffusivity on the above-mentioned properties of coke is presented in Table I.

Table I The relationship between the reactivity and the thermal diffusivity through the degree of graphitization, the porosity as well as the ash content.

Degree of

Property Graphitizationdecrease

Porosity increase

Ash content increase

Reactivity Increase Increase Increase

Thermal diffusivity Decrease Decrease Decrease

It was in this regard aimed at elucidating the sensitivity of thermal diffusivity to these parameters. Coke samples were taken from five different levels, i.e. I - V, of a pilot blast furnace situated in Luleå, Sweden as shown in Figure 1.

Tuyere lev

I

V

M R C

Figure 1 Schematic diagram of a blast furnace. The po The distances (mm) from the top to the levels I-V are abosamples were taken from the center (C), the place just besideof level I is, for instance, denoted by I-C. Pellets having 10 coke lumps for the thermal diffusivity measurement. For somthat of the first pellet, was taken from the identical coke lumthe direction dependence of the thermal diffusivity. The thermal diffusivity measurements were carried out over purified argon by the laser flash apparatus (Model TC-7000(1):

2/1

2

tL1388.0 ⋅

=α

where L is the thickness of the sample and t1/2 is the time reqtemperature. Measurements were started at room temperatcooling cycle until about 873 K.

I

I I

II

IV

el

sitions of where the coke samples were taken have been marked out.

ut 3960, 5290, 5700, 6290 and 6980, respectively. At each level, coke the wall (R) and in between (M). The coke sample taken from the center

mm in diameter and around 2.5 mm in thickness were machined from the e samples, the second pellet, the normal axis of which is perpendicular to

p and was subjected to the thermal diffusivity measurement to investigate

a temperature range, between room temperature and 1623 K, in a flow of H/MELT Ulvac-Riko). The thermal diffusivity, α , is obtained from Eq.

(1)

uired for the temperature of the rear surface to reach half of its maximum ure and were carried out during the heating cycle, and then during the

X-ray diffraction measurements were carried out to determine the average crystallite size of graphite along the structural c-axis, Lc, and that in the structural ab plane, La. These crystallite sizes are generally used as a measure of the degree of graphitization. The ash content (mass%) of the coke samples was determined by measuring the weight change of a sample before and after burning the sample at 1173 K for 12 h. The measurements were compared with those obtained for pure graphite samples. Such calibration studies showed that the thermal diffusivity increases with the degree of graphitization. It was established that the apparent thermal diffusivity, observed during the cooling cycle, is larger than that observed during the heating cycle. The difference may be due to the graphitization of the samples during the high temperature measurements assuming that the sintering of coke is negligible by observing the average weight and thickness decreases as –3 and –0.2 %, respectively. In Figure 2 the temperature dependences of the apparent thermal diffusivities for coke samples taken from the levels I-V are presented. It can be clearly seen that the coke samples taken from the deeper level of the blast furnace show larger thermal diffusivities. It should be pointed out that the coke samples in the deeper level have been exposed to the higher temperatures. There is the possibility that the coke samples in the deeper level might have sintered more due to that, and is less porous resulting in higher thermal diffusivity.

500 1000 15001

2

3

4

5 I-CI-MI-R

Ther

mal

diff

usiv

ity /

10-6

m2 s-1

500 1000 15001

2

3

4

5 II-CII-MII-R

500 1000 15001

2

3

4

5 III-CIII-MIII-R

500 1000 15001

2

3

4

5 IV-CIV-MIV-R

500 1000 15001

2

3

4

5 V-CV-MV-R

Temperature / K

Figure 2 Temperature dependences of the apparent thermal diffusivities for coke samples taken from levels I-V. It was also established that the measured porosity exhibits no difference among different levels, and the level dependence of the thermal diffusivity could not be explained from the viewpoint of the porosity. The coke in the deeper level might also be more graphitized (crystallized) yielding to higher thermal diffusivity. Figure 3 shows the crystallite sizes Lc and La versus level of the blast furnace. It can be seen from the figure that the value of Lc is larger as the level is deeper although the value of La is constant within the scatter regardless of the level. This is in conformity with the thermal diffusivity measurement results. Experiments were also designed to study the rate of graphitization at any given temperature for a given coke sample. The experiments were designed in this case in the high temperature X-Ray Diffraction unit, Philips x-pert system available within the group. The samples were scanned over peak (002) in the scanning range (20-30)o in steps of 0.02o. The isothermal measurements were carried out at 700, 800, 900, 1000, 1100 and 1200oC. The results showed very clearly that the graphitisation starts at as low a temperature as 700oC. The measurements also indicated that the graphitization of coke is instantaneous and there is no change with time. Thus, the transformation energy is very small and the thermal diffusivity of coke is, in fact, a measure of the temperature history of the coke sample.

1

2

3

4

I II III IV VLevel of the blast furnace

L c /

nm

CMR

3

4

5

6

7

8CMR

Level of the blast furnaceI II III IV V

L a /

nm

Figure 3 Crystallite sizes Lc and La versus level of the blast furnace.

REDUCTION OF IRON ORE PELLETS 1

An experimental design for the investigation of the mechanism of reduction in the blast furnace shaft, and the role of coke in the same by dynamic x-ray photography was carried out in the present laboratory. By this method, the reduction of a single pellet in elevated temperatures, and a reducing atmosphere can be monitored in real time. Thus the shape change of hematite to metallic iron can be observed, as well as the formation of oxide slags. A laboratory furnace was programmed to follow a given temperature profile. A controlled gas flow apparatus regulated both the inert and reducing gases. Pellets were reduced under a variety of conditions to examine the mechanisms of reduction, and to improve the simulation of the blast furnace with the experimental equipment. Dynamic x-ray photography allowed real time observation of the pellet during its reduction. An x-ray source and an x-ray detector were placed on either side of the furnace, quartz windows with a diameter of 40mm were cut into the furnace to allow penetration of the x-rays. A PC recorded and displayed the x-ray images as they were taken to allow live observation of the reduction. For each reduction, an iron ore pellet (KPBA/KPBO, provided by LKAB, Sweden) was placed in an inert alumina crucible. A reducing gas flow mixture of argon, carbon monoxide, and carbon dioxide flowed over the pellet from above. A limited number of reductions on each temperature profile were done in the presence of carbon (graphite) to simulate the presence of coke in the blast furnace. The graphite was either graphite powder surrounding the pellet or a graphite plate on which the pellet was placed. Reduction temperatures ranged from 700 to 1500°C. The reduction studies were carried out under isothermal as well as non-isothermal conditions.

While most experiments had a CO:CO2 gas flow ratio of 13:1 or 19:1, some reductions were run with less reducing atmospheres, i.e. more CO2. One experiment had a temperature ramp of 3°C/minute as opposed to the standard 5°C/minute. A few attempts were also made to simulate the pressure in the blast furnace by reductions of pellets baked in graphite powder and subjected to a load from above.

After reduction, the pellets were examined for further information on the elemental mass transfer during the reduction process. Observable physical changes to the pellet's appearance and mass loss during the reduction were recorded. Vertical cross sections were taken of many pellets, and Scanning Electron Microscopy (SEM) was used to obtain an elemental analyzes of these cross sections.

In Figure 4 the results of the isothermal reduction experiments at 1350oC, with a gas mixture of CO and CO2 in the ratio 13: 1, is presented. The entire pellet melted and the molten phase wetted the sides of the crucible up to the height of the pellet and higher. These pellets lost approximately 10% of their mass during the reduction, corresponding to a loss of one third of the oxygen originally in the pellet. SEM analysis of the pellets reduced at 1350°C, with CO:CO2 ratio of 1:1 to 4:1, indicated that the pellets had been reduced to wüstite (FeO) but no iron had formed as yet. The effect of graphite on the reduction process was also studied by reducing pellets on a graphite plate substrate or embedded in a loose graphite powder bed. Both provided interesting descriptions of the reduction process and the elemental mass transfer involved.

Figure 4 Before (left) and after (right) isothermal reduction at 1350°C with 13CO:CO2. The x-ray images of the pellets baked in a graphite bed show that the pellets became significantly darker during the reduction, indicating the presence of metallic iron. Additionally they lost 33 % of their original mass. Unlike the temperature ramped pellets without graphite, the mass loss in these pellets also resulted in a significant loss of volume which started at about 1450°C, accelerating at 1505°C, and ending when the pellet melted at 1500-1550°C (the highest melting point observed in the present study). Interestingly, the molten phase of this material did not get stuck to the alumina crucible substrate, but rather maintained an acute surface interface angle. Pellets baked in graphite and subjected to loads of 30-70 g demonstrated much the same behavior. Pellets reduced on a graphite substrate were also found to reduce fully to metallic iron. These pellets, however, revealed a preferential formation of the darker iron phase on the surface in contact with the carbon substrate. With a CO:CO2 ratio of 3:1, a phase separation was observed at 1425°C as the pellet appears to become semi-molten at this temperature forming a dark phase in contact with the substrate and a lighter phase above in contact with the gas flows from above. At 1485°C, the lighter phase became a gaseous "bubble" which enlarged to 150% its original volume. (a) (b) (c) (d)

Figure 5 Temperature ramp reduction on a graphite substrate at 3CO:CO2. (a) at 800°C before reduction, (b) at 1425°C after 123 min., (c) at 1485°C after 135 min. and (d) at 1489°C after 136.5 min.

Further experiments with pellets embedded in graphite powder showed that the reduction reaction was nearly topo-chemical. Thus, these studies reveal the intricate mechanism of the reduction of iron ore pellets and the impact of factors like the rate of descending of the pellets in the blast furnace, the impact of CO:CO2 ratios, the contact with solid carbon, the carburization reaction as well as the slag formation. Efforts are presently underway to follow the reduction reaction simultaneously by x-ray imaging as well as by thermal analysis by incorporating a thermo balance in the present equipment.

BUBBLE FORMATION 2

In a number of steelmaking processes, gas is injected to the steel bath for inducing various reactions, for example, oxygen injection in converter processes or argon purging in ladled treatments. Gas is also generated due to the chemical reactions occurring in the steel bath. Bubbles can be formed by nucleation in supersaturated solutions or superheated liquids. In most of the modeling of the processes, the injected gas is normally accounted for, but rarely the gas generated due to chemical reaction. It is important to identify this phenomenon and investigate it in case the process needs to be optimized. Further, the first step in obtaining foam is the formation of bubbles. The nucleation of bubbles is aided by a heterogeneous site, e.g. an impurity or a wall, by reducing the interfacial energy associated to forming a new interface between the gas phase and the liquid phase. In the present laboratory, experiments were designed in order to study this phenomenon by water model experiments. The experiments were aimed at bubble formation due to chemical reaction between two liquid phases. The reaction chosen for the studies was the reaction between oleic acid and an aqueous solution of sodium bicarbonate, producing carbon dioxide. The reaction can be represented as follows: CH3(CH2)7CH=CH(CH2)7COOH (l) + NaHCO3 (aq) → CH3(CH2)7CH=CH(CH2)7COO - ⋅ Na+ + CO2 (g) + H2O (2) The produced CO2 would form the bubbles. The formation and the transport of the bubbles were monitored by suitably positioned CCD cameras. A schematic diagram of the experimental set-up is presented in Figure 6. The sample cells, containing the aqueous bicarbonate solution, were positioned on a glass top sample table. Oleic acid was added slowly and cautiously to the sample cell, making sure that emulsification was prevented.

Figure 6 The experimental set up for bubble formation studies. The higher density sodium bicarbonate solution formed the bottom phase while the top phase consisted of oleic acid (specific gravity 0.89). The interface between the liquids was monitored by CCD cameras positioned at the side as well as the cell bottom as shown in Figure 6. Since the bubbles could form anywhere over the interface, the side view cameras were placed slightly above the interface to cover the full depth of the interface. The camera was placed on a stand allowing it to turn sideways and, as a result, the entire width of the sample cell could be covered. The bubble formation process was recorded on video tapes for analysis.

Gas bubbles were noticed to form underneath the interface, in the lower aqueous phase as shown in Figure 7. The bubbles migrated upwards against the gravitation, due to buoyancy, but were retained at the interface between the aqueous and the oleic acid phase by a

thin film of bicarbonate solution. When buoyancy exceeded the interfacial forces the bubbles detached from the interface surrounded by an aqueous film and rapidly ascended to the surface of the oleic acid layer. At the surface, the bubbles were held up shortly (typically, less than 2 s) before the bubbles burst and the gas merged into the surrounding atmosphere. The water, that had surrounded the bubble, formed lenses at the oleic acid surface.

Figure 7 Gas bubble formed under the interface between the aqueous phase and the oleic acid a) t = 0s, b) t = 12s, c) t = 17s. The camera is positioned diagonally below the interface. The white surface seen in the figure is the interface.

At the start of the experiments, the newly formed bubbles existed as single bubbles. These bubbles, however, frequently gathered into groups with populations ranging from 2 up to 25 bubbles. At the time of detachment, the bubbles were predominantly present in such clusters. Within a cluster, bubbles were often of comparable size. However, situations with one or a few larger bubbles surrounded by several small bubbles were not uncommon. The number of bubbles present in the clusters fluctuated with time, and as a result a general trend could not be established between the population size and time or between population size and increasing bicarbonate solution.

The sizes of the bubbles were determined at the time of bubble detachment from the oleic acid-aqueous phase. It was noticed that the average diameters of the bubbles found in a cluster somewhat decreased with the cluster population, i.e. the number of bubbles present in the cluster. Thus, to see variations in bubble size, the bubble sizes of unclustered, or single, bubbles were studied. The bubble size was seen to increase for approximately 5 min., after which it fluctuated around a fairly constant value. The final bubble diameters fluctuated between 1.2 and 1.8 mm, seemingly independent of the bicarbonate concentration.

A preliminary estimation of the rate of reaction in the present experiments was carried out. In this estimation, it was assumed that the effect of gas diffusion into the bulk of the aqueous phase on the reaction rate is negligible. In other words, the reaction rate was approximated as the detachment rate of the bubbles. With this approximation, the reaction rate seems to be controlled by the chemical reaction for 0.8 - 1.0 M solution. The detachment rates show an apparent increase with increasing sodium bicarbonate solution. However, the results fluctuate in the lower concentrations.

SLAG FOAMING 3, 4

Slag foaming is a common feature in steel-making processes. It is found in most converters and is prevalent in the blast furnace, during pre-treatment of hot metal and in bath smelting processes. Foaming of slags is also common in the Electric Arc Furnace (EAF). Attempts to rationalize the foaming behavior have been made by introducing the foaming index, which correlates the foam height H with the superficial gas velocity U:

UH

=Σ (3)

Lahiri and Seetharaman5 showed that, for a number of slags, the foaming index for foam of uniform bubbles sizes can be expressed as:

bd

C

ρ

µΣ = (4)

where µ is the surface tension, ρ the density and the bubble diameter. C is determined by the gas fraction in the foamy slag, the bubble shape and the ratio of bulk to surface viscosity of the slag. Lahiri and Seetharaman

bd5 further pointed out that the foaming index

is independent of the gas flow rate when the gas fraction of foam is constant, a condition that could be reasonably valid at steady state. The above studies were made for foaming under steady state where both the gas flow rate and the chemical composition of the slag were constant. In production processes, however, gas generation rate is not constant. Oxidation and reduction reactions of the oxides changes the slag composition continuously. Lining and fluxes dissolve into the slag, altering further the slag composition as well as the slag volume with time. The change in slag composition during high alloy steel making is even more complex due to the presence of oxides like Cr2O3 and V2O5 in the slag phase as the metal cations of these oxides can be found in different oxidation states. Besides the change in slag composition, the gas generation rate depends on the rates of the chemical reactions producing gas, which vary significantly with the progress of the process. The foaming process was followed by the means of an x-ray imaging equipment connected to a high temperature furnace (max temperature 1650°C). Approximately 50 g of pre-melted slag samples were placed in a graphite crucible with alumina lined sides (inner diameter = 40 mm). The slag behavior was recorded on video tapes connected to a CCD camera in the x-ray equipment. The foam height was determined optically through scale inserted in the image. The video recordings were later used for evaluation of the foam behavior. Six different synthetically produced slags with typical compositions found in tool and stainless steel making processes were investigated, se Table II. As can be seen from the table the surface active components Cr2O3, Fe2O3 and V2O5 were varied.

Table II Initial composition of the six different synthetic slags investigated.

Slag no. B %Al2O3 %Fe2O3 %MgO %MnO %Cr2O3 %V2O51 1.5 7 5 5 2 5 0 2 1.5 7 5 5 2 10 0 3 1.5 7 5 5 2 15 0 4 1.5 7 10 5 2 10 0 5 1.5 7 5 5 2 5 2 6 1.5 7 5 5 2 5 5

Figure 8 shows the variation of gas fraction in the samples with progress of the reaction. For clarity, only two slags are shown, i.e. Slag 2 and 5. The behavior is, however, similar in the case of the other slags.

0 10 20 30 40 50 60 700,35

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0,75

0,80

0,85 Slag 2 Slag 5

Gas

frac

tion,

ε

Time (min)

Figure 8 Gas fraction, ε, during progress of reaction for Slag 2 and 5.

The gas fraction changed during progress of reaction in all cases. Initially the gas fractions had a value of about 0.6 - 0.85, but were found to decrease with time. Gas bubbles could be held up at the surface before exiting to the surrounding atmosphere thereby forming a layer of high gas-fraction foam, a situation sometimes referred to as two-phase foam. In contrast, when the gas bubbles moved into the atmosphere without holding up at the surface, the foam was of dispersion-type. In industrial processes, both these types may be found. The distinction is not always obvious. During the present measurements the foam was initially only of the dispersion-phase type, but once the gas generation rate increased the slag turned into a two-phase foam. In other words, the samples formed foam of both high and low gas fraction over the denser slag layer, se Figure 9.

(a) (b) Figure 9 X-ray images of two slag melts.The arrows point out the foamy layer and the crucible bottom and the dashed circles indicate

bubbles. (a) A true foam layer at the top. (b) The foamy layer is of a dispersed phase type. The slags containing V2O5 consisted at all times of foam of the dispersion type. The gas escape rate was found to be independent of the viscosity of the slags, contrary to the foaming index concept. Traditional theories for foaming do not seem to be valid for slag foaming under dynamic conditions. In this case the foam displays a fluctuating behavior, which the presently available models are not able to take into account. The concept of foaming index does not seem to be applicable either, resulting in the need for alternative theories. From the x-ray images, the rate of foam height change, dHfoam/dt, was estimated and matched with the difference between the gas generation and escape rates, Ug-Ue, the latter being a function of the rate of the reaction. The proportionality relationship between these two functions is shown in Figure 10.

-6

-4

-2

0

2

4

6

-0,0001 -0,00005 0 0,00005

Ug-Ue (min)

dHfo

am/d

t (m

m/m

in)

Figure 10 The relationship between mismatch in gas flows and the change in foam height for a slag.

As can be seen from Figure 10, the chemical reaction rates and the impact of the same on mass transport have to be considered in process simulations when modeling foaming phenomena.

MELTING OF MOULD POWDERS DURING CONTINUOUS CASTING 6

The heating up and melting of a mould flux during continuous casting were simulated in special designed laboratory equipment. The composition of the mould powders investigated is tabulated in Table III.

Table III Composition of the two different mold flux powders investigated

Flux %SiO2 %CaO %MgO %Al2O3 %TiO2 %Fe2O3 %MnO %Na2O %K2O %F %CTotal Loss on ignition CaO/SiO2A 28.8 36.5 1.3 6.5 0.3 0.8 3.3 7.2 0.1 5.9 2.4 10.9 1.27 B 32.7 28.8 1.77 4.7 0.11 1.24 < 0.1 11.3 0.31 9.4 6.6 14.1 0.88

During the experiment the liquid steel was maintained in an alumina container. On top of the liquid steel, mould flux was added to a total depth of 110 - 120 mm. The start of the experiment was marked by the first addition of the mould flux powder. The mould flux powder was added continuously during the experiment in order to maintain a fixed bed height. X-ray equipment was employed to monitor the melting rate at predefined intervals of 20 or 30 s, depending upon the melting behavior of the powder, by following the positions of Pt-markers carefully placed on top of the powder bed to visualize the bed movement on the x-ray images, see Figure 11. The lowering of the positions of the Pt-markers indicated the sinking rates of the powder beds. This method enabled to record the actual sinking rate of the central part of the flux bed, thus eliminating influence by wall effects such as sticking. The x-ray images also revealed to some extent the differences in material density, thus enabling the identification of different layers forming in the bed structure; powder, sintered, mushy and liquid.

Flux BFlux A

Liquid

Mushy

Sintered

Powder

Liquid

Powder Platinummarkers

Gas generation

1 cm

Flux BFlux A

Liquid

Mushy

Sintered

Powder

Liquid

Powder Platinummarkers

Gas generation

1 cm1 cm

Figure 11 X-ray image showing the cross section of the mould flux bed, for flux A and B respectively, during melting trials in laboratory equipment.

As can be seen from Figure 11, mould flux A shows a totally different bed structure than flux B. The cross section in this case, consists only of the loosely packed powder layer located directly on top of the liquid slag layer. Only a very thin sintered and/or semi-liquid layer could be detected. On the other hand, mould flux B is built up by a multi-layer structure with the loosely packed powder

layer on top. This layer represents the major part of the total bed height. Below the powder layer, the sintered layer is present. This can be detected due to its slightly higher density observed in the x-ray images. It was also to some extent possible to identify the sintered layer due to the decrease in vertical movement of the Pt-markers as they crossed the interface between the loosely packed and sintered layers. In Figure 12, this point is marked by arrows and is detected due to the decreased sinking rate relative other Pt-markers still positioned in the powder layer, i.e. the distance between the marker curves starts to narrow. Underneath the sintered layer, flux B shows a semi-liquid layer. It is interesting to note the presence of metallic droplets in this two phase layer. Observations during the experiments reveal that the metal drops are released from the bulk metal and transported through the liquid slag.

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 00

1

2

3

4

F lu x B - M a r k e r 1 F lu x B - M a r k e r 2 F lu x B - M a r k e r 3 F lu x B - L iq u id p o o l

Dis

tanc

e fro

m li

quid

ste

el s

urfa

ce [c

m]

T im e [ s ]

0

1

2

3

4

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

P o in t w h e r e m a r k e r e n t e r s t h e s in t e r e d la y e r o f t h e f lu x b e d .

F lu x A - M a r k e r 2 F lu x A - M a r k e r 3 F lu x A - L iq u id p o o l

Figure 12 Plot showing the tracking of Pt-tracers added to the mould flux in melting trials for the flux A (upper graph) and B (lower graph).

Experimental observations showed that during the initial period of time after the addition of the mould flux on top of the free liquid iron surface, the gas evolution was strong. The gas at this initial stage is believed to, due to the rapid raise in temperature, consist of a mixture of several gas components detected earlier in DSC measurements carried out parallelly. The agitation due to this was so vigorous that powder fluidization was observed. 15 - 45 s after the first powder addition had taken place, the gas evolution decreased rapidly and only local gas channeling could be observed. This corresponds approximately to the period of time necessary to form a liquid slag layer and the semi-steady-state to be established.

SUMMARY The present paper demonstrates the importance of proper experimental studies to understand different process phenomena. A number of experimental designs directed towards the understanding of different process phenomena in iron and steel making is presented, i.e.: • the graphitization of coke studied by thermal diffusivity measurements as well as x-ray diffraction in static and dynamic modes, • the reduction sequences of iron ore pellets monitored by x-ray image analysis, • the bubble formation due to two-phase reactions studied by suitably designed water-model experiments, • the foaming phenomena under dynamic conditions monitored by the x-ray technique after suitable experimental arrangement, and • the melting of mould fluxes investigated by novel designing of the experiment.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the contributions of Dr. M. Hayashi, Dr. A. Kapilashrami, Dr. M. Görnerup, Miss S. Barron and Mr. R. Abas. The present paper is based on their experimental studies.

REFERENCES 1. M. Hayashi, P. Fredriksson, A. Jakobsson and S. Seetharaman, "Coke–Its Properties and its Role in Blast Furnace Process", The

International Workshop on Science &Technology of Innovative Ironmaking for Aiming at Energy Half Consumption, Nov. 27-28, 2003, Tokyo, Japan.

2. A. Kapilashrami, A. K. Lahiri and S. Seetharaman, "Bubble Formation Through Reaction at Liquid-Liquid Interfaces", Steel

Research, 2005 (In press). 3. A. Kapilashrami, M. Görnerup, A. K. Lahiri and S. Seetharaman, "Slag Foaming under Dynamic Conditions", Metall. Mater.

Trans., Oct. 2004 (Sent for publication). 4. A. Kapilashrami, M. Görnerup, A. K. Lahiri and S. Seetharaman, "The Fluctuations in Slag Foam under Dynamic Conditions",

Metall. Mater. Trans., 2005 (Accepted for publication). 5. A. K. Lahiri and S. seetharaman, "Foaming Behaviour of Slags", Metall. Mater. Trans. B, vol. 33B, 2002, pp. 499-502. 6. M. Görnerup, M. Hayashi, C.-Å. Däcker and S. Seetharaman, "Mould Fluxes in Continuous Casting of Steel – Characterisation

and Performance Tuning", The 7th International Conference on Molten Slags, Fluxes and Salts, Jan 25-28, 2004, Cape Town, South Africa.