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2009:065
M A S T E R ' S T H E S I S
Improvement of the DesulphurisationProcess by Slag Composition Control
in the Ladle Furnace
Stephen Famurewa Mayowa
Luleå University of Technology
Master Thesis, Continuation Courses Minerals and Metallurgical Engineering
Department of Chemical Engineering and GeosciencesDivision of Process Metallurgy
2009:065 - ISSN: 1653-0187 - ISRN: LTU-PB-EX--09/065--SE
IMPROVEMENT OF THE DESULPHURISATION PROCESS
BY SLAG COMPOSITION CONTROL
IN THE LADLE FURNACE
Famurewa Mayowa Stephen
Supervisors Professor Bo Björkman(LTU) Sven-Olof Ericsson(OVAKO)
Luleå University of Technology
Master Thesis in Minerals and Metallurgical Engineering
Department of Chemical Engineering and Geosciences
Division of Process Metallurgy
ii
ABSTRACT
The cleanliness of steel with respect to non-metallic inclusions and the precise alloy
compositions in the steel products have always been of great concern in steel making
technology. The development of steel making process to meet the compositional requirements
for specific mechanical properties such as ductility, toughness, fatigue and machinability
requires dynamic and continuous investigations.
The refining of molten steel in the ladle furnace to meet the required compositional range
requires the optimisation of the process parameters. For sulphur removal control, parameters
such as argon gas flow rate through the porous plugs, inductive stirring effect, vacuum
pressure of the tank degasser, amount & composition of the top slag should be optimised. In
this thesis project an investigation was carried out on the factors that influence the top slag
composition before vacuum treatment and also to optimise the top slag composition for
precise sulphur removal. 12 heats were followed during the project; slag samples, steel
samples, temperature and oxygen activities were taken at eight different process stages at
Ovako steel mill. A relatively large variation was observed for all the oxide components of
the slag phase before vacuum treatment in all the heats followed. A PLS analysis made shows
that topslag composition before degassing is influenced by the amount of slag former added,
oxygen potential at tapping, the yield of Al and Si deoxidants into the steel at tapping. The
model has a poor predictability because some important parameters such as ladle glaze
condition, amount of EAF slag tapped and refractory wear could not be measured.
An alternative solution of extra slag practice was suggested instead of modelling the
composition and mass of carry over slag left after slag removal. The extra slag practice
involves the addition of lime during tapping so as to aid the removal of all the slag before
ladle refining and thus optimisation of the new synthetic slag for precise sulphur removal
could be easily achieved.
Finally the investigation of the desulphurisation process shows that degassing time, argon gas
flow rate through the porous plugs are as well important as the slag mass and composition in
order to achieve a precise sulphur removal.
iii
ACKNOWLEDGEMENT
I am eternally grateful to my creator and my saviour whose mercy and love has been without
limit in my life. To him who gave this opportunity, His mercy endures forever
I want to appreciate Swedish institute (S.I), who has granted me the scholarship to study in
LTU. My profound gratitude goes to my supervisor at Ovako Steel AB, Sven-Olof Ericsson
for accepting me to carry out this research work under his supervision and also for sharing his
rich experience with me during the course of the work. I appreciate my supervisor in LTU
Professor Bo Björkman for his contribution in this project work and his pedagogic style of
knowledge transfer in the classroom. I also like to appreciate the technical support of Jan-Eric
Andersson, Robert Eriksson, Patrik Undvall, Sölve Hagman, Lars-Erik Borgström, Ove
Grelsson, Rolf Nilsson and all the team members working at the EAF and Ladle furnace at
Ovako Steel AB Hofors. I also appreciate the moral support of the members of Pingst Kyrkan,
Hofors during my stay and all my friends in Luleå.
This will be incomplete if i don’t appreciate my dearly beloved Abiola, who has been a good
companion for me. My parents Mr and Mrs I. B Famurewa you are part of what I am today.
This could not have been, without the support of my wonderful brothers, Sunday and Festus.
I am grateful unto you all.
Famurewa Mayowa Stephen
July 2009, Hofors
iv
TABLE OF CONTENTS
ABSTRACT .............................................................................................................................. ii
ACKNOWLEDGEMENT ...................................................................................................... iii
TABLE OF CONTENTS ........................................................................................................ iv
1.0 INTRODUCTION .............................................................................................................. 1
1.1 Background ...................................................................................................................... 1
1.2 Historical Background of Ovako ...................................................................................... 2
1.3 Process Description at Ovako Steel AB Hofors ............................................................... 3
1.4 Effects of Sulphur on Steel ............................................................................................... 4
1.5 Aim of the Project ............................................................................................................ 4
2.0 LITERATUTRE REVIEW ............................................................................................... 6
2.1 General Steelmaking ........................................................................................................ 6
2.1.1 Electric Arc Furnace .................................................................................................. 6
2.1.2 Ladle Furnace Refining ............................................................................................. 7
2.2 Refining Processes ........................................................................................................... 8
2.2.1 Deoxidation ............................................................................................................... 8
2.2.2 Alloying ..................................................................................................................... 8
2.2.3 Stirring ....................................................................................................................... 9
2.3 Desulphurisation ............................................................................................................... 9
2.3.1 Thermodynamic Theory ............................................................................................ 9
2.3.2 Slag Properties ......................................................................................................... 12
2.3.2.1 Composition ......................................................................................................... 12
2.3.2.2 Sulphide capacity ................................................................................................. 15
2.3.2.3 Oxides Activities .................................................................................................. 16
2.3.2.4 Sulphur Distribution Ratio ................................................................................... 17
2.3.2.5 Temperature ......................................................................................................... 18
2.3.3 Kinetic Theory ......................................................................................................... 18
2.3.3.1 Argon Gas flow rate ............................................................................................. 19
v
2.3.3.2 Viscosity ............................................................................................................... 21
2.4 Dilution Slag .................................................................................................................. 21
3.0 MATERIAL AND METHOD ......................................................................................... 23
3.1 Material .......................................................................................................................... 23
3.2 Method ........................................................................................................................... 23
3.2.1 Experimental Procedure .......................................................................................... 23
3.2.2Analysis Procedures and Techniques: ...................................................................... 25
4.0 RESULTS AND DISCUSSIONS .................................................................................... 27
4.1 Synthetic Slag Composition ........................................................................................... 27
4.2 Top slag compositional changes .................................................................................... 27
4.3 Mass Balance .................................................................................................................. 30
4.4 Regression Analysis for the Top Slag Variation ............................................................ 31
4.5 Oxygen Activities ........................................................................................................... 34
4.6 Equilibrium sulphur Distribution ................................................................................... 36
4.7 Regression Analysis for the Desulphurisation Process .................................................. 38
4.8 Equilibrium Condition during Vacuum Treatment ........................................................ 41
4.9 Equilibrium Sulphur Content in the Bulk steel .............................................................. 43
4.10 Optimisation of the top slag composition .................................................................... 44
5.0 CONCLUSION AND RECOMMENDATIONS ........................................................... 46
5.1 Conclusion ...................................................................................................................... 46
5.2 Recommendations .......................................................................................................... 47
REFERENCES ....................................................................................................................... 48
APPENDICES ........................................................................................................................ 50
1
1.0 INTRODUCTION
1.1 Background
Steel and its products are undoubtedly the pillar and anchor of material developments through
the ages. It is a substantial part of material science and a key material in product development
in modern technological advancement. It is the base material for over 2500 different grades of
products (1). The potential ability to modify its structures, crystal arrangements, chemical
compositions and several other material properties leads to its wide areas of present use and
continuous possibility of future developments(1).
The world production of crude steel as reported by world steel association is to a great extent
more than any other metal product, this also proves its wide versatility in material
consumption. Its world productions in million metric tons are 1251, 1251 and 1329 in the year
2006, 2007 and 2008 respectively (2).
Figure 1 shows the production of steel in the world in 2008. There was a decrease in the crude
steel produced in the world as well as in Europe and Sweden in 2008 compared to 2007. The
production of steel in Sweden has been between 5.2 and 6 million metric tons in the past 6
years with minimum of 5.2 in 2008 (2).
Figure 1: World Steel production (2)
2
The production of steel could be classified into two, based on the raw materials; ore based and
scrap based raw materials. Steel from iron ore (hematite or magnetite) are mainly produced in
integrated steel mills while steel from scrap based materials are produced in EAF operated
mills. Steel products could also be classified into three based on the composition of alloy
additives; low alloy steel, medium alloy and high alloy steel (1).
1.2 Historical Background of Ovako
Ovako is a leading European long special steel producer whose production covers low alloy
steels and carbon steels in the form of bar, wire, rod, tubes and rings. The primary operation
areas include, heavy vehicle, automotive and engineering industries. It has 15 production sites
in Sweden Finland, Italy, France and Netherlands with several sales companies in Europe and
the USA with a total annual production of about 2million tones of steel of "right quality" (3).
The origin of Ovako could be traced to strong Nordic steel production technology and the
forerunners to the company were founded for over 300 years ago. Present day Ovako was
established in 2005 by a merger of 3 re-known steel companies, Ovako Steel, Fundia Steel
and Imatra Steel. Due to strategic and technical reasons, the new company decided to
continue its operation in a specific steel product (3).
Ovako consists of four product divisions namely; Bar, Wire, Bright Bar and Tube&Ring.
Figure 2 below shows the four product divisions with their respective production sites.
Figure 2: The Group Structure of Ovako showing the
products and their production sites (3)
3
1.3 Process Description at Ovako Steel AB Hofors
At Ovako Steel two scrap baskets with a total weight of about 110ton are charged into the
oval bottom tapped (OBT) electric arc furnace (EAF). The electrical melting with graphite
electrode and combustion from the oxy-fuel burners proceed after the first scrap charge with
1.9ton of slag former (lime) addition. The second scrap charge into the furnace is followed by
the addition of 1.6ton slag former (lime or dolomite) and then by carbon and oxygen injection
for slag foaming. Dust and off gas produced during the melt down are collected by off gas
evacuation system. Sampling is carried out during the melting to check the temperature in the
furnace and also elemental compositional of the molten scrap. The desired phosphorous
refining and heat condition is achieved after about 48 minutes of power-on. The steel is
tapped into the ladle where it is deoxidized with aluminum and silicon (FeSi). Sample of the
steel is taken after tapping and deoxidation in advance for further refining. The ladle is
transported further by crane to the ASEA-SKF unit and the ladle glaze from the previous heat,
tap hole sand, EAF slag and part of deoxidation products (Al2O3 and SiO2) which have floated
to the top of the steel and other impurities are removed at the mechanical deslagging process.
The steel in the ladle is then transported further in a ladle wagon to the heating unit where it is
heated using electric energy through three graphite electrodes. Alloying is done through
lumpy alloys and wire feeder. Also slag formers are added. The ladle proceeds to the vacuum
degassing where desulpurisation is done as well as gas and inclusion removal. A schematic
description of the entire steel making process at Ovako Steel AB is shown in Figure 3.
Figure 3: Steel making Process at Ovako Steel AB
Scrap Charging
Melting
Slag Removal
Ladle Furnace
Vacuum
Ingot Teeming Stripping
Ingot
To Rolling Mill
4
The steel temperature is finally adjusted to casting temperature and the composition is
checked to be in agreement with the aimed composition. The 100ton refined molten steel is
finally teemed into 24 ingots each of 4.2ton weight using up-hill teeming. The ingots are
stripped and then transported to the pit furnaces for heat treatment prior to rolling or forging.
The final products after processing in the rolling mill and tube& ring mill are in the form of
bar, tube and ring.
The production of steel grades used for the manufacturing of ball bearing requires very low
oxygen content in other to reduce the possibility of formation of non metallic oxide inclusions
such as Al2O3, and etc, which have deleterious effect on the final products (fatigue life, crack
initiation point) (4). A lot of research work has been done on the reduction of total oxygen
content of steel; a successful result achieved was a further reduction from about 20ppm to
5ppm. This success led to the increase in the effect of sulphur in the steel, these effects
become somewhat more intense than earlier noticed (4).
1.4 Effects of Sulphur on Steel
Sulphur has a positive effect on steel when good machinability is desired of the steel product.
In some other steel products sulphur content is refined to its minimum due to its negative
effect on the mechanical properties. The following effects of sulphur become more significant
when the oxygen content is successfully reduced.
i. Formation of undesirable sulphides which promotes granular weaknesses and cracks
in steel during solidification.
ii. It lowers the melting point and intergranular strength and cohesion of steel
iii. Sulphur contributes to the brittleness of steel and when it exists in sulphide phase it
acts as stress raiser in steel products. (4,5)
1.5 Aim of the Project
The above mentioned effects of sulphur are highly undesirable in the production of some
special steel products, for example ball bearing steel grades; since the operating condition of
such steel grades requires high fatigue strength and other similar mechanical properties. The
present state of production in Ovako at the commencement of this project was able to meet
5
the low sulphur requirement ranges of the different steel grades but with a low level of
accuracy. These involve extra sulphur addition when the sulphur removal is too high or
further refining when the removal is too low.
However the focus of this project work is to improve the desulphurisation process during
vacuum degassing at the ladle furnace, by slag composition control. It is focussed on
increasing the level of accuracy of the process to meet the desired sulphur content of the steel
product and also to shorten the degassing period. This involves an extensive study of the
thermodynamics of the process and kinetics. The ladle refining of different steel grades and
different slag practice were followed daily. Slag properties (especially composition), steel
compositions, temperature and some other factors were analysed for their sulphur removal
potential using some empirical models and later compared with actual measurements in the
plants. The compositional variation of the slag formed after deoxidation was studied with
respect to the dissolved oxygen content of the steel at tapping. The mass of slag remaining
after deslagging and its influence on the final slag composition were also investigated.
Optimum synthetic top slag practice with improved sulphide capacity, for accurate
desulphurization for different steel grades was to be estimated and the effects of the different
kinetic parameters were to be investigated.
6
2.0 LITERATUTRE REVIEW
2.1 General Steelmaking
2.1.1 Electric Arc Furnace
The Electric Arc Furnace is a Steel making technology which is employed for about twenty-
five percent of the world steel production (5). External high current electric arc heating with a
better thermal control than the basic oxygen Process is used to melt steel scrap and converts it
into liquid steel. The cycle of operation for the production of steel in the EAF involves;
charging of scrap (direct reduced iron is included in some charges), melting down, refining,
sampling (composition and temperature) and tapping. The scarp charged into the furnace
could be home scrap (scraps within the steel mill), process scrap (scraps from the
manufacturing of steel products) or obsolete (scraps from the end of life of used equipments),
and the choice depends on type of steel products (1). All the mentioned scrap types are used in
Ovako steel production process. Metallurgically, preheating the scrap is beneficial, as it
reduces the energy requirement for melting the scrap which further reduces tap to tap time and
the overall productivity. It also decreases the hydrogen contents in the steel as dry charge are
fed into the furnace but the extent of preheating is limited to avoid evolution of undesired
dioxin. The furnace is mainly eccentric bottom tapped vessel (though oval bottom tapped
vessel, equipped with spout also exist), made of heavy steel plates with a dish-shaped
refractory hearth and three vertical graphite electrodes extending downward from a dome-
shaped removable roof. The furnace could be tilted backward for slag removal and forward
for about 10-18º for tapping. The furnace is also often equipped with oxy fuel burners for
energy efficiency reason. Fluxing agents (lime and dolomite) are added as slag formers to
remove impurities. Oxygen and carbon are also injected into the furnace for slag foaming.
Slag Foaming
This is a common praxis in the EAF. Carbon or coke is injected into the furnace to increase
the melt down efficiency by supplying additional energy from combustion with injected
oxygen and also to cause carbon boil which promotes stirring to achieve a good slag/metal
mixing. Another important function of this praxis is to cause a foaming of the slag provided
7
the viscosity of the slag is not too low. The slag foam decreases the energy loss, decreases
refractory wear and protect the water cold panel at the top of the vessel (1, 7).
The injection of oxygen performs some refining operation in the EAF, especially phosphorous
removal although manganese, silicon, chromium and iron are also oxidized. The oxygen
content of molten steel is often extrapolated using the carbon content in an online production
process. In theory dissolved oxygen and carbon content of steel will react to form carbon
monoxide until equilibrium is reached
C + O = CO(g) ∆Gº = -18319 - 41.369T -------------------------(1)
% C X % O = 0.0025 X CO pressure---------------------------------------- (2)
Reaction 1 will reach equilibrium when the relationship in equation 2 is attained (5).
2.1.2 Ladle Furnace Refining
The secondary stage of steelmaking process is done in an open topped cylindrical container
lined with refractory called ladle. The primary step is done either in the converter or EAF and
crude steel is produced (7). The unit metallurgical processes in the ladle include; Electrical
heating, deslagging, wire feeding, stirring with gas or electromagnetic fields and vacuum
treatment (5, 7).
The units of operation mentioned above enables the following refining and adjustment
operations;
i. Deoxidation
ii. Alloying
iii. Stirring to achieve temperature and composition homogeneity and improved refining
iv. Desulphurisation
v. Degassing to remove hydrogen, nitrogen and other gaseous inclusions
vi. Removal and modification of inclusion
vii. Adjustment of temperature to optimum range before casting (7)
.
8
2.2 Refining Processes
2.2.1 Deoxidation
The oxygen content of the steel tapped into the ladle after melting in the EAF is often high, as
oxygen is injected into the EAF for refining, slag foaming, and other process control
measures. Final steel products require a very low content of oxygen and also further refining
and alloying are most desirable at the minimum oxygen content, for this reasons there is a
need to kill or deoxidize the crude molten steel. The addition of strong deoxidizers such as
aluminum and ferrosilicon is done during tapping, they could either be placed in the preheated
ladle before tapping or run into the tapping stream so as to utilize the mixing effect of the
tapping stream to achieve thorough deoxidation (5,8). The reaction is shown in equation (3)
and (4), and the oxides nucleate to diffuse to the ladle wall or absorbed into EAF slag.
2Al + 3O = Al2O3(slag) ∆Gº = -1205115+386.714T ------------------------------------(3)
Si + 2O = SiO2 (slag) ∆Gº = -580541+220.655T ------------------------------------- (4)
The deoxidation reactions shown in equations (3) and (4) are exothermic and thus the
temperature of the liquid steel is increased, however the steel also loses heat by radiation from
the top surface, heating of ladle lining and by flux through the lining and shell (5). The rate of
heat loss is reduced in most ladle operation by preheating the ladle before tapping.
2.2.2 Alloying
The adjustment of the final composition of the molten metal is done at the heating and wire
feeding position of ladle station. A wire feeder runs wire of alloying elements at controlled
speed into the steel (5,7). Most of the alloying elements are lumpy ferroalloy since they are
cheaper to produce and available in different grades to suit the final steel compositional
requirements. The addition of alloying elements results into temperature drop of the molten
steel and the calculation to meet the final composition is often done by computer
programmes.
9
2.2.3 Stirring
Generally, ladle furnace technology is equipped with either one or both of the two stirring
facilities; electromagnetic accessory for inductive stirring and permeable refractory block at
the bottom of the ladle called porous plugs for gas stirring. These two stirring means are
important for good metal/slag interaction to achieve an effective ladle refining. It enhances
homogenous temperature and composition of the steel. It also aids continuous slag metal
reaction with the aim of sulphur, hydrogen, nitrogen and inclusion removal (6).
2.3 Desulphurisation
Desulphurisation is an essential practice in the production of clean steel products such as
bearing steels with high fatigue strength which function under high impact operational
requirement. Ovako Steel AB specializes in the production of bearing steel products,
desulphurisation becomes an important subject to be continuously investigated for highly
clean products which can withstand market competition and satisfy customer's demand.
Based on the production route and the type of steel product, desulphurisation could be done at
different points in the steelmaking process and with different reagents; however it is mainly
carried out in a reducing conditions when the oxygen activity is low(8). At Ovako Steel AB,
desulphurisation is done during the vacuum degassing in the ladle furnace using lime
saturated multicomponent slag.
The parameters which influence the desulphurisation process are either thermodynamic or
kinetic parameters; the main parameters are discussed later in the report while the theories
related to this project work are explained below.
2.3.1 Thermodynamic Theory
When studying the thermodynamics of slag, metal and gas interactions in the ladle refining
with consideration to sulphur removal, the reactions below are important.
[ ] ( ) [ ] ( )( ) ( ) ( ) ( )
[ ] ( ) [ ] ( ) )7(2121
)6(2121
)5(
22
22
−−−−−−−−−−−−−−−−−−−−−−−−−+=++
−−−−−−−−−−−−−−−−−−−−−−−−−+=+
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+=+−−
−−
gg
gg
SOOS
SOOS
SOOS
The equilibrium constant for the reaction in equation (6) is expressed as;
10
( ))8(
%
2
2
2
2
2
2
2
2
6
6
−−−−−−−−−−−−−−−−−−−−−−⋅⋅
=
⋅=
−
−
−
−
S
O
O
slagS
S
O
O
S
P
P
a
SfK
P
P
a
aK
Also sulphide capacity can be written as
( )2
2
2
2
%
)9(6
S
OslagS
S
Os
P
PSC
f
aKC
⋅=
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−⋅
=−
−
Where [S] and [O] are dissolved sulphur and oxygen in the steel respectively while (S2-) and
(O2-) are sulphide and oxide (with free oxygen ion) in the slag respectively. aS2- and aO2- are the
activities of sulphide and oxide in the slag. K6 is the equilibrium constant for gas-slag reaction
in equation (6) (4, 8, 9).
Since oxides activities and the partial pressure of gaseous phases are not readily available as
process parameters, sulphide capacity is often expressed in terms of temperature and
composition as process control tool (5, 13').
Sosinky and Sommerville derived an expression to correlate optical basicity with sulphide
capacity at temperature range between 1400 andd 1700˚C (4, 14).
)11(.........
...........
)10(2.256.435464022690
332211
333222111−−−−
+++
+Λ+Λ+Λ=Λ
−−−−−−−−−−−−−−−−−+Λ+
Λ−=
∑∑
nXnXnX
nXnXnXicityOpticalBaswhere
TCLog
ththth
S
X is the mole fraction of the oxides in the slag system, n is the number of oxygen atom in a
molecule of each oxide and Λth represents the optical basicity of each oxide (14).
Young et al showed that equation (11) only applied to range where Λ = 0.8, and therefore
reported correlations for ranges with Λ < 0.8,
( ) ( ) )12(%%2223.01171082.2384.42913.13 322
12 −−−−−−Λ−Λ+−= −OAlSiOTCLog s
11
Andersson et al (4) studied the distribution of sulphur and the extent of sulphur removal using
equation (7)
[ ]
( )[ ]
)13(375.1935
%
%
375.1935
,
%
)(%
7
7
2
2
−−−−−−−−+++−
=
=
+−
=
⋅⋅=⋅=
OSSS
S
SS
O
O
S
S
O
aLogfLogCLogT
LLog
S
SL
TKLogAlso
Cf
a
S
S
P
P
a
aK
To calculate the activity coefficient of sulphur in the metal, Wagner's expression can be used
[ ] )14(.% −−−−−−−−−−−−−−−−⋅=∑ iefLogj
iS
aO and aS are the activities of oxygen and sulphur in the molten steel respectively. j
Se , is the
interaction parameter between sulphur and other elements j in the steel. K7 is the equilibrium
constant for the gas-metal reaction in equation (7).
The activity of oxygen in the steel could be calculated assuming equilibrium between the
dissolved oxygen and aluminum in the steel and alumina in the slag or alumina inclusion in
the steel bulk.
[ ] [ ]3215
32
32exp
)15()/(387.012053
OAl
OAl
aa
a
RT
GK
molKJTGOAlOAl
=
∆−
=
−−−−−−−−+−=°∆=+
Where aAl2O3 is the activity of alumina in the slag or as inclusion, and aO and aAl are the
activities of oxygen and aluminum respectively. The activity of Al can be calculated using
equations (14) and (16) while the alumina activity can be calculated using Ohto and and Suito
empirical expression in equation (17). aAl2O3 is taken to be 1, if alumina inclusion is
considered in the equilibrium. K17 is the equilibrium constant for the deoxidation reaction
shown in equation (15) (4, 15).
[ ]( ) ( )[ ] ( ) )17(560.1%033.0
%
%167.0%275.0
)16(%
32
232
−−−−−++−
=
−−−−−−−−−−−−−−−−−−−−−−−−−−⋅=
OAlSiO
MgOCaOaLog
Alfa
OAl
AlAl
The oxide composition is in weight percent and the expression has been proven to be valid for
temperature ranges close to 1600ºC (4). Also the expression is suggested for CaO;10-
12
60wt%,SiO2; 10-50wt%, Al2O3;0-50wt% and MgO: 0-30wt% which is a fairly good range for
the slag studied in this project work
2.3.2 Slag Properties
The process of desulphurisation depends to a great extent on the properties of the slag phase.
The ability to extensively describe the thermodynamic and thermophysical properties of the
notable phases as a function of the composition and temperature of the slag is a strong control
tool for desulphurisation (16). It has also been established that high amount of slag is
favourable for sulphur removal (17).
2.3.2.1 Composition
The different phases in a multi-component slag system play significant roles in ladle refining
with focus on desulphurisation.
CaO + [S] = CaS + [O] ------------------------------------------------ (18)
The general equation for desulphurisation is written in equation (18) above, one of the
important conditions to enhance the reaction is the activity of CaO. An optimum slag
composition should be saturated with CaO, in other words the activity of CaO in the slag
should be close to unity to facilitate the exchange of the dissolved sulphur in steel with
oxygen ion (6,18). The ternary phase diagrams of CaO-SiO2-Al2O3 can be used to establish the
optimum slag composition at a particular temperature. Figure 2 shows an isothermal section
of CaO-SiO2-Al2O3 slag system, the highlighted portion shows the homogeneous liquid region
at temperature 1600ºC, while the remaining sections show undesired solid regions.
Any composition within the homogeneous liquid region with an activity of CaO close to unity
is good for desulphurisation process. For the system considered in Figure 4, liquidus line 'ab'
represents a perfect unity of CaO activity.
13
Figure 4: Isothermal section of the phase diagram of the system, CaO-SiO2-Al2O3 at 1600°C. The shaded area indicates the anticipated
homogeneous liquid region at 1600°C. C3S=3CaO⋅SiO2, C2S=2CaO⋅SiO2,
CA=CaO⋅Al2O3, CA2=CaO⋅2Al2O3, CA6=CaO⋅6Al2O3, A3S2=3Al2O3⋅SiO2. (18)
Figure 5, 6 and 7 show the activities of CaO, SiO2 and Al2O3 respectively at different points in
the homogeneous liquid region of the slag system at the 1600C. An optimum slag
composition for good desulphurisation can be carefully chosen by exploring these diagrams to
have the highest possibilities of CaO activity, basicity and efficient fluidity. It should be noted
that composition affects viscosity of the slag, if the CaO of the multicomponent slag system is
higher than 60% it has a negative effect on the sulphide capacity of the slag as it becomes
heterogeneous and more viscous, thus the kinetic will be negatively influenced (6).
Figure 5: Activities of CaO in CaO-SiO2-Al2O3 multicomponent system. Standard State of pure AlO1.5
with the relations 2AlO1.5= Al2O3 and
(aAlO1.5)2
= aAl2O3 at 1600° C. (14)
1600ºC
14
Figure 6: Activities of SiO2 in CaO-SiO2-Al2O3 multicomponent system
at 1600°C (14)
Figure 7: Activities of Al2O3 in CaO-SiO2-Al2O3 multicomponent system . Standard State of pure AlO1.5
with the relations
2AlO1.5= Al2O3 and (aAlO1.5)2
= aAl2O3 at 1600°C). (14)
15
2.3.2.2 Sulphide capacity
An important property of slags which plays a vital role in the investigation and control of
desulphurization process is sulphide capacity. It is the potential ability of a completely
homogeneous molten slag to remove sulphur during slag metal interaction (4, 19). This potential
ability is used to estimate the amount of sulphur that a slag of a given composition will retain
under a specified condition of oxygen and sulphur pressures (19). It is often used to establish
the sulphur distribution ratio between slag and metal at equilibrium. Its ability to compare the
desulphurization characteristics of different slags has led to the creation of several models for
its measurement (6, 10, 17, 16, 14). Figure 8 shows the sulphide capacities of a CaO-Al2O3-SiO2
system at different compositions. For an optimal slag composition aimed to achieve an
effective desulphurisation, compositions close to line ab (-Log Cs = 1) should be ensured.
Compositions close to line ab also have CaO activity close to unity and high basicity which
are important for desulphurisation (14).
Figure 8: Isothermal section of the system, CaO-SiO2-Al2O3 at 1650°C
showing the Log of Sulphide capacity with composition in mass % (14)
a
b
16
Basicity is defined in its simplest form as the ratio of %CaO / %SiO2, it is known that
desulphurization is improved with slags of higher basicity. Basic slags have high content of
basic oxides which are network breakers with ability to release its oxygen ion (O2-) in
exchange for the dissolved sulphur in steel. Significant correlations have been made between
sulphide capacity and basicity. Figure 9 shows three different sulphide capacity models;
Sosinky & Sommerville, Young et al and KTH models. The first two models were calculated
from optical basicity while the third is a model developed in the division of process
Metallurgy in KTH (4). The three models in figure 9 show that sulphide capacity is improved
with increased basicity.
Figure 9: Sulphide Capacity values as functions of Basicity (4)
In this project work optical basicity, a measure of the electron donor power of slag will be
used in the estimation of the sulphide capacity, because its model have been proven to have a
fair agreement with empirical data and the parameters can be accessed.
2.3.2.3 Oxides Activities
The activities of oxides in the molten slag and alloying elements in the molten metal as well
as the temperature of the process determine the equilibrium oxygen potential in the system.
The measurements of the activities of oxides in the slag and dissolved oxygen in the steel are
important for control of desulphurization process (10). Oxides activities in the slag affect the
equilibrium activity of oxygen in the steel and also the basicity of the slag. Figure 10 shows
that at high basicity, the activity of alumina is low and the oxygen activity will also be low
provided the Al content of the steel is high at this condition. This is a necessary requirement
17
for desulphurization. Also Turkdogan E. established that low SiO2 content of the slag is
favorable for improved sulphur removal, due to its equilibrium with Al and Si content of the
steel. (20).
Figure 10: Alumina activities for typical ladle furnace slag using the
KTH model and Ohta and Suito Equation (16)
.
2.3.2.4 Sulphur Distribution Ratio
It is an estimation of the sulphur reduction in a desulphurization process. It is the ratio of the
sulphur content in the slag and metal phase at the end of vacuum treatment. A good
estimation of this parameter indicates a good control of the process. It is a function of
temperature, sulphide capacity of the slag, oxygen activity and sulphur content of the molten
steel (16). An optimized slag which can be used to control desulphurisation can be obtained by
exploring the ternary diagram shown in figure 11
Figure 11; Isothermal section of the system, CaO- SiO2- Al2O3-MgO
with 5%MgO at 1600°C showing the sulphur distribution between
Metal and Slag at equilibrium (14)
18
2.3.2.5 Temperature
An essential parameter in desulphurization is the temperature at which the process is carried
out. It influences the viscosity (favourable kinetic condition) and sulphide capacity of the slag
and also sulphur distribution in the metal and slag. Most models that have been developed to
evaluate the sulphide capacity of slag were mainly functions of temperature and composition
(10,11,12,17). Figure 12 shows that sulphide capacity is improved at higher temperature. The
calculation was done at constant MgO and SiO2 contents of the slag and also at constant %Al
and %C content of the steel(11). It is also reported that desulphurisation is slower at the later
period of vacuum degassing due to reduced sulphur content of the steel and temperature drop
during the process which is unfavorable for the sulphide capacity (4).
Figure 12: Sulphide capacity as a function of temperature
and Al2O3 in the topslag(11)
.
2.3.3 Kinetic Theory
The transfer of sulphur atoms from the metal phase to the slag phase and the transfer of
oxygen ions from the slag phase to the metal phase during sulphur refining process is
controlled by mass transfer through diffusion (12, 19). Fick’s law of diffusion could be applied
to the process as below,
19
[ ] [ ] [ ]
[ ] [ ] [ ] ( )19%%%
%%%
−−−−−−−−−−−−−−−−−−−−
=∂
∂
−−=
∂⋅
∂
∂
Φ∂−=
emmt
m
SSM
AK
t
S
SiSD
tA
SV
xDJ
ρδ
Where J- Diffusion flux D- Diffusion constant
x∂
Φ∂- Concentration gradient
[ ]−mS% Initial Concentration of Sulphur in the melt %wt
[ ]eS% - Concentration of Sulphur in the slag/Metal interface at equilibrium %wt
δ - Boundary Layer Kt- Total mass transfer coefficient M- Mass of steel A - Slag-Metal interface area V- Volume of steel
mρ - Density of steel
[ ]t
S
∂
∂ %- Sulphur removal rate and [%S] is the instantaneous sulphur concentration in steel
As earlier mentioned sulphur removal depends on the stirring rate and viscosity, both
properties affect the slag metal interface area and also mass transfer coefficients of the
process. It should be noted that these constant δ and Kt are difficult to measure in a real
process and the modeled values are specific for particular stirring conditions (12). The
conventional assumption of a flat and horizontal slag metal interface area 'A' has been proven
to be an under estimation as the slag is dispersed in the steel and the interaction area is more
than supposed (12).
2.3.3.1 Argon Gas flow rate
The manipulation of the inductive and gas stirring during the Ladle refining is a very
important factor in the desulphurization process control. An investigation of the influence of
argon gas flow rate on desulphurization during vacuum treatment is shown in Figure 13. With
respect to desulphurization, the optimum condition for vacuum treatment in the figure is at the
argon gas flow rate of 1.8m3Ar/min, thus a better desulphurisation is achieved optimum
slag/metal mixing (6).
20
Figure 13: Steel Desulphurisation during Vacuum treatment at various Ar stirring rates (13)
The position of the inductive stirrer and the rate of flow of argon gas through the porous plugs
were studied by Hallberg et al (17) in the creation of a process model for sulphur refining at
Ovako Steel AB. It could be deduced from figure 14 that argon gas flow rate through the
porous plug has an influence on the fluid flow in the ladle, a low flow rate at the first porous
plug which is closer to the inductive stirrer and high flow rate at the second porous plug
which is at the opposite side is necessary for a good desulphurisation (17).
Figure 14: Influence of different argon gas flow rates on sulphur removal for
combined gas and inductive stirring (17)
21
Most of the top slag is concentrated above the second porous slag where they are hit by the
gas plumes from the plug and thus a greater contact area between slag and steel is created.
This is substantiated by the authors, using computational fluid flow dynamic predictions. It
should be noted that the flow rates through the porous plugs are not fixed throughout a heat at
Ovako Steel AB, they are changed by the operators based on reactions observed in the camera
view.
Some previous projects done at Ovako Steel AB have also investigated the effects of argon
gas stirring before and after vacuum treatment on desulphurisation (6). The control of argon
gas flow rate was difficult at the heating station of ladle due to poor flow of gas through the
porous plug. The only trial that was done was unsuccessful with a poor sulphur removal and
poor vacuum treatment.
Another trial with inductive stirring and calm argon gas stirring at the final heating unit
process after the vacuum treatment shows no influence on the final sulphur content of the
steel though it had a positive impact on inclusion removal (6).
2.3.3.2 Viscosity
It is a thermo-physical property that influences the kinetics of the ladle metallurgy (16). The
viscosity of both the steel and slag affects the mass transfer during the ladle refining. At low
viscosity of the slag, mass transfer rate of sulphur is improved due to easy dispersion in the
steel and the slag/metal interfacial area is increased (13). A low melting point CaO rich slag
can be synthesized by adding a correct proportion of Al2O3, and the viscosity is adjusted in
some steel plants by the addition of CaF2. (6,13). Viscosity values for steels are reasonably well
established at steel making conditions but the viscosity of slags are not, they are rather
extrapolation of temperature and composition in a multicomponent slag system (16).
2.4 Dilution Slag
A major quest in this research is to identify the source, composition and mass of the slag
which remains after mechanical slag removal in steel making process line at Ovako. This
poses a problem to the optimization of the slag mass and composition for a precise sulphur
22
removal. A preliminary study of the process shows the following possible sources of the
dilution slag;
1. EAF Slag: Hot heel is a common praxis at Ovako steel, about 110tons of steel scrap is
charged into the EAF and less than 105tons is tapped into the Ladle leaving some steel behind
in the furnace. Despite the hot heel practice, it is unavoidable to have a small mass of slag
entrained in the tapped steel.
2. Tap hole sand: After each tapping, the tap hole is blinded with Olivine sand. It is of course
certain that this sand will be lost into the steel during tapping, as the hole is opened before
steel falls into the ladle. The tap hole sand is rich in MgO and SiO2 and its quantity in the
tapped stream depends on the age of the tap hole, this also considered as a probable source of
dilution slag during ladle refining.
3. Ladle glaze: Ladle glaze is formed when draining the Ladle into the mould, as top slag
comes in contact with refractory (21). As teeming proceeds the temperature of the system drops
and fluidity of the slag reduces, thereby forming non metallic particles after reacting with
refractory, this particles hang on the wall of the ladle as glaze and are flushed off when liquid
steel is poured into the Ladle during subsequent heat. The condition, composition and the
amount of the glaze depends on the steel type produced at a particular heat.
4. Deoxidation Products: To enhance further refining after melting of scrap in the EAF,
reduction of oxygen contents of the steel is important. The oxygen contents which is often
between 100 and 1000ppm before tapping depending on the extent of refining and the heat
condition in the EAF, is reduced by the addition of aluminium metal and Ferrosilicon alloy.
The reaction products, Al2O3 and SiO2 indigenous inclusions are major sources of inclusion in
steel making and as well increase the amount of the EAF slag as they are absorbed after
nucleation and separation from the steel bulk.
With consideration to all the theories of sulphur removal as well as thermodynamic and
kinetic properties discussed earlier, this project work will advance sulphur removal process at
Ovako steel AB Hofors using slag composition control. It will investigate the slag mass and
composition during different process stages and attempt to optimize the slag with the aim of
controlling sulphur refining process.
23
3.0 MATERIAL AND METHOD
3.1 Material
The raw materials used at Ovako Steel AB are steel scraps of different grades, including
home, process and obsolete scraps. The selection of the scrap materials is based on the size
and grade of the scrap and also on the cleanliness or type of the steel to be produced.
The additives used in the process includes anthracite, coal, oxygen, slag formers (Lime,
dolomite Alumina and pure Alumina), deoxidants (Aluminium metal and Ferrosilicon) and
other alloys. The chemical composition of the slag formers used at the ladle furnace refining
is given in table 1.
Table 1: Chemical Composition of slag formers
SiO2 MnO S TiO2 CaO Al2O3 MgO FeO
Lime 2,91 0,17 0,065 0,050 92,30 0,93 0,80 0,430
Alumina 2,78 0,08 0,017 0,198 17,58 63,78 12,13 0,679
Pure Alumina 2,22 0,02 0,006 0,020 4,22 91,90 0,45 Measurement is in wt-%
3.2 Method
3.2.1 Experimental Procedure
A number of heats were followed to observe the compositional changes in the steel and slag at
different process stages, right from melt down in the EAF to the end of casting. The aim was
to observe the consequential effects on desulphurisation. Different steel grades were followed
to investigate the variation in the studied parameter. The studied parameters include
temperature, slag composition, steel composition, oxygen activity, mass of additives and also
consideration was taken of the vacuum pressure and argon gas flow rate at degassing.
24
Eight sampling points were observed at different subunits in the integrated process line for
thorough follow up of the equilibrium conditions and changes in the system. The schematic
diagram of the sampling points is shown in Figure 15.
Figure 15: Sampling points for plant trials
B - Bulk steel sample, S - Slag Sample, O - Oxygen activity, T - Temperature
1. B1, S1, O1, T1: Steel sample, Slag sample, Oxygen activity and Temperature in the
EAF just before tapping respectively
2. B2, S2 : Steel sample and Slag sample at the end of tapping;
3. S3: Slag sample before slag removal
4. B4, O4, T4 : Steel sample, Oxygen activity and Temperature at arrival at the ASEA-
SKF Ladle furnace station before alloying
5. B5, S5, O5, T5 : Steel sample, Slag sample, Oxygen activity and Temperature before
degassing
6. B6, S6, O6, T6: Steel sample, Slag sample, Oxygen activity and Temperature after
degassing.
7. B7, S7, O7, T7: Steel sample, Slag sample, Oxygen activity and Temperature after
extra alloying (if there is any).
8. B8 : Steel sample during casting.
25
3.2.2Analysis Procedures and Techniques:
Temperature: The temperature of the molten steel was measured in the EAF with the aid of
Robot and at the Ladle furnace using the automatic sampling lance. In some occasions for the
purpose of this thesis work the temperature was measured using the electro nite Celox R7
oxygen activity measuring equipment.
Chemical Composition Steel Samples: Automatic lances, similar to the temperature lances
said earlier were used to take steel samples at each point mentioned above, the samples were
sent to the operation laboratory were immediate analysis was made. The samples were
analysed by optical emission spectroscopy (Bausch & Lomb, ARL OES 4460) for the
concentrations of Al, Cr, Mn, Si, P, Mo, V, Ca, Ti, Mg and other minor elements. The relative
analysis accuracy of these elements depends on their concentrations. Sulphur and carbon in
the samples were analysed with LECO CS-444 using the melting and combustion method.
Chemical Composition of Slag Samples: The slag samples were taken at each designated
process stage using the slag spoon, the samples were saved for further analysis. The samples
were prepared before analysis; they were ground into powder in a ring mill, sieved to collect
fine particles less than 10µm which are void of metallic iron. Chemical reagents were added
according to standard and then thoroughly mixed together. The prepared samples were then
heated in a laboratory furnace for about 8 minutes before they were arranged in the PAN
analytical Axios equipment to analyse the oxide composition using the wavelength dispersive
X-ray fluorescence technique. The concentration in weight percent of CaO, Al2O3, SiO2,
MgO, MnO, TiO2, Cr2O3 and other oxides in minor concentration were measured. A portion
of the samples taken after grinding to fine size was also analysed for sulphur and carbon
content. This analysis was done in LECO-CS 200 equipment using the melting and
combustion method.
26
Oxygen content
The dissolved oxygen content in the steel bulk is a key parameter in refining during steel
making process. In the project work the activity of oxygen in the steel melt was measured
using the Celox sensor designed by Heraeus Electro-Nite. The sensor contains ZrO2 elctrolyte
with a molybdenum wire in Cr/Cr2O3 as a reference electrode while the bulk steel is the
second electrode. An electromotive force (e.m.f) is built up when different oxygen activities
are sensed by the two electrodes, the value of which is displayed on the screen of the
equipment. The thermocouple attached to the sensor measures the temperature of the system.
A calculation is generated automatically using the measured temperature and e.m.f to obtain
the oxygen activity which is then displayed on the screen of the equipment.
27
4.0 RESULTS AND DISCUSSIONS
4.1 Synthetic Slag Composition
The synthetic compositions of the mixture of the different slag formers used at the steel mill
during the trial sampling are given in Table 2. Synthetic slag 1 and 2 are mixtures of 65%
Lime / 35%Alumina and 60%Lime / 40% Alumina respectively. The mass of the slag varies
between 800 and 1000kg based on the degree of sulphur removal for the different steel
grades. Synthetic slag 3 contains 73% Lime and 27% Pure Alumina, it has a higher mass than
the previous slags (1300kg).
Table 2: The Initial composition of three different synthetic slags
Synthetic slag 1: 65%Lime&35% Alumina, Synthetic slag 2: 60%Lime&40%Alumina, Synthetic slag 3: 73%Lime&27% Pure Alumina (Measurement is in wt-%)
4.2 Top slag compositional changes
The analysis of the compositional variation of topslag before degassing for the 12 heats
followed during the thesis work is given below. The usual slag practice for ladle refining is
either synthetic slag blend 1 or 3(table 2), the compositional change of the top slag after
heating and before vacuum treatment for the 12 heats is shown in figure 16. The change is
quite high for some oxides while it is low for others. For SiO2, MnO, MgO, FeO and Al2O3,
the average top slag composition before vacuum treatment for all the heats is higher than the
synthetic slag blends added, while CaO is lower. It could be seen from table 3 that the range
of wt%CaO is between 49.8 %- 64.5%, and wt%Al2O3 is between 21.90% - 31,9% with
relative deviation of 4.01% and 2,47% respectively. The range of SiO2 is between 5.10 - 10%
while MnO is 0.09 - 3.14%.
Composition CaO Al2O3 SiO2 MgO S TiO2 FeO MnO
Synthetic slag 1 66,23 22,79 2,77 4,48 0,05 0,110 0,59 0,14
Synthetic slag 2 62,50 25,92 2,75 5,00 0,04 0,12 0,62 0,13
Synthetic slag 3 68,52 25,49 2,18 0,71 0,05 0,04 0,31 0,13
28
65
60
55
50
33
30
27
24
21
10,0
7,5
5,0
4
3
2
1
0
4
3
2
1
0
8
6
4
2
CaO % Al2O3 % SiO2 %
MnO % FeO % MgO %
Figure 16: Box Plot-Investigating the variation in the top slag composition before Degassing where represents mid spread of the data (50% of the heats), and represent
the mean and median of the data set.
Table 3: The composition of top slag before degassing for composition
Charge %SiO2 %MnO %CaO %Al2O3 %MgO %FeO ao(ppm) Temp °C
1 6,80 0,24 57,30 26,00 8,20 1,66 2,8150 1610
2 10,00 3,14 49,80 27,40 7,50 2,21 4,1400 1596
3 5,60 0,18 64,50 28,40 2,50 0,61 2,4140 1624,7
4 6,90 0,84 62,00 26,60 3,20 1,24 - -
5 6,28 3,49 59,01 25,40 6,02 3,46 5,1205 1639
6 7,50 0,77 55,10 26,70 8,00 0,81 3,6064 1610
7 5,10 0,23 60,40 24,10 6,80 2,60 2,5326 1606
8 6,20 0,40 56,70 31,90 4,90 1,00 - 1597
9 5,90 0,31 63,50 28,00 3,30 0,73 3,0263 1625,9
10 9,20 1,20 56,60 21,90 6,70 3,90 3,4036 1618,7
11 7,20 0,13 59,00 25,50 5,70 0,70 2,0369 1577,9
12 6,40 0,09 56,50 27,90 8,10 0,33 3,1259 1608,6
Mean 6,92 0,92 58,37 26,65 5,91 1,60
Std dev 1,43 1,17 4,01 2,47 2,03 1,18
Rel dev. 20,62 127,51 6,87 9,26 34,30 73,76
The composition is measured in wt-%
The higher extreme values for the CaO in the box plots (though not considered as outliers for
the distribution), are connected to high proportion of lime in the synthetic slag blend and also
high mass of the synthetic slag (this is a common practice for high clean steel which requires
extreme sulphur removal). The lower extreme values are peculiar for heats with high oxygen
activities ao at tapping; which implies a low yield of Al and Si at tapping and also higher
potential to retain some deoxidation products as inclusion in the steel bulk or on the ladle wall
29
until ladle refining stage when they are removed to the top slag. For %Al2O3, the outlier is an
uncommon blend of slag former (68% Lime and 32% pure Alumina) which contains very
high %Al2O3 . On the contrary MnO, FeO and SiO2 have high relative deviation with wide
spread of the values and most of the values close to one extreme end. This is an indication that
their compositions in the topslag are not controlled by the variation in the slag former blend.
The upper extreme values for these oxides contents are results of high ao at tapping, Al/O/
Al2O3 equilibrium before vacuum degassing and also the quantity of carry over slag remaining
after the mechanical slag removal. The lower extreme values, especially for SiO2, depict special
heats with high purity requirements; they are refined with synthetic slag former of high mass
and low initial SiO2. The SiO2 content of the slag of such heats is reduced further during
vacuum treatment as the interaction between slag and steel degassing became improved. This
reduction (equation 21) is aided by high %Al and low ao of such heats, and it in turn favours
good desulphurisation.
The trend for the variation of MgO from 2.5 to 8.2% is a little bit compounding, as it depends
on the amount of EAF slag remaining after slag removal, age of the ladle in use, lime saturation
of the top slag and it also depends on the slag former blend. If pure Alumina is used, then the
MgO content of the top slag before vacuum treatment will be very low due to the low content of
MgO in it. It can be clearly seen from figure 17 that the solubility of MgO refractory into the
topslag varies with CaO content of the top slag; lime saturated topslag has a low solubility
potential of the refractory and vice versa. It was observed that the ladle age is also an
influencing factor for the MgO content of the topslag before vacuum treatment. Newly lined
ladles have greater potential to wear than old ladles though they give better thermal resistance
and heat conservation than the old ladles.
1
3
5
7
9
45 50 55 60 65
% CaO
%M
gO Before Degassing
After Degassing
Figure 17: MgO pick up from the refractory into the top slag
30
4.3 Mass Balance
A mass balance was done to investigate the probable mass of dilution slag which was
transferred to the top slag after slag removal. In the calculation, the mass of the CaO content
of the synthetic slag was assumed to be constant throughout the ladle refining, this could be
substantiated, as the calcium content of the steel is so low and thus has a very low
thermodynamic potential to react. Also the CaO content of the dilution slag can be ignored
because of its small mass compared to its content in the synthetic slag.
The changes in the wt%CaO content of the slag was thus used to calculate the extra slag
which floats to the top slag and the new top slag mass. The mass balance procedure for a heat
could be followed in appendix X.
Furthermore, an attempt was made to calculate the elemental mass balance between the steel
and slag for each of the heats just before vacuum treatment during the ladle refining. The
results shows that extra slag was often added to the steel during heating and inductive stirring.
The optimum inductive stirring power of about 1000W together with the new top slag
composition often enhanced the lifting up and absorption of the inclusions (from deoxidation
products attached to the ladle wall), ladle glaze and also EAF slag that clustered on the ladle
walls, into the top slag.
In the mass balance, mass input was not equal to mass output due to the influence of the carry
over slag. The elemental mass difference in the steel and top slag could be said to be a result
of the carry over slag whose composition is unknown. Table 4 shows the mass difference of
some selected elements, before vacuum treatment. It could be seen that excess Al, Si as well
as Mg appear into the system in all the heats in the mass balance made just before the vacuum
treatment. (With the exception of heat 3 which could be due to measurement error). This
support earlier observation on the large deviations of SiO2 and MgO contents of the topslag
before degassing, since their initial mass in the slag formers used during refining are small.
For Mn, the additional mass though very small has a meaningful influence on the top slag
composition before degassing. The higher values of Mn in Table 4 are peculiar to heats with
high manganese contents, this could be taken as an error in measurement of Mn alloy, and
these values are less than 5% of the total mass of the alloy added at such heat.
31
Table 4: The mass balance between slag& steel of selected elements before vacuum treatment
Heats Si Mn S Ti Ca Al Mg
Extra
Slag
1 15,12 6,39 -0,08 0,51 0,02 26,32 27,81 146
2 30,13 39,56 2,22 0,69 -0,01 4,90 25,88 256
3 23,68 6,72 -1,40 -0,05 0,02 8,27 14,21 74
4 32,52 -38,94 1,65 -0,17 0,01 7,04 20,90 130
6 15,90 -50,47 0,46 0,01 11,53 25,24 165
7 6,32 4,63 -2,89 0,71 0,13 14,91 82
8 5,09 1,78 0,71 0,06 61,31 8,58 164
9 11,79 -3,15 -1,66 0,17 0,00 8,63 20,75 93
10 22,10 -16,09 -0,32 0,81 0,02 -3,02 16,37 137
11 16,29 3,98 -0,40 0,46 0,06 18,35 9,26 97
12 14,71 4,92 2,77 0,67 0,04 26,83 26,56 149 *Heat 5 was not recorded because no sample was taken at sample point 4, and thus mass balance was incomplete
*All calculations are in Kg
The mass balance done in this work, only gives an idea of an upward flow of oxides from the
steel to the topslag as well as the reduction of oxides into the steel. It is difficult to calculate a
perfect mass balance at some points because the reactions proceed rapidly until equilibrium is
achieved. The assumption of fixed mass of CaO in the slag before degassing also influences
the result of the mass balance, the values are not extremely precise but the trend is good for
analysis.
4.4 Regression Analysis for the Top Slag Variation
A partial least square (PLS) regression was done to further investigate and probably model the
factors influencing the variation in the composition of the top slag before degassing. To
increase the accuracy of the PLS analysis 12 extra heats from previous experiments carried
out during ladle refining in Ovako were added to the 12 heats investigated in this thesis work.
The PLS modelling was done using SIMCA-P +10. The SiO2 and CaO composition of the top
slag for the 24 heats were set as the dependent variables while all other variables were set as
predictors. The predictors include Al yield, Si yield at deoxidation, mass of Si and Al metal
added at deoxidation, calculated mass of carry over slag, temperature at tapping and carbon
content at tapping (oxygen potential). The predictors also include the SiO2 and CaO content
of the removed slag and mass of slag former added.
An overview of the model obtained is shown in figure 18 and 19; from figure 18 it could be
seen that heats with low %C at tapping, lower yield of deoxidants at tapping and also higher
amount of extra slag have a tendency for an increasing SiO2 content of the top slag and a
decreasing CaO content. On the contrary, the figure also shows that heats with either high
slag former addition or low high %C at tapping have tendency for an increasing CaO content
and decreasing SiO2 content of the top slag before degassing.
32
A plot of the influence of the independent variables on the heats is shown in figure 19 (score
plot), the heats with high slag former are grouped together in a direction of low SiO2 contents
(high CaO content) while the heats with low %C at tapping (high ao) tends in the opposite
direction favouring high SiO2 content (low CaO content) of the top slag.
-0,50
-0,40
-0,30
-0,20
-0,10
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
-0,50 -0,40 -0,30 -0,20 -0,10 0,00 0,10 0,20 0,30 0,40 0,50 0,60
w*c
[2]
w*c[1]
M1 (PLS)w*c[Comp. 1]/w*c[Comp. 2]
X
Y
C
Slag forme
CaO
SiO2
Al YieldSi Yield
Al(kg)
Si(Kg)
Extra slag
Temp
SiO2_degas
CaO_degas
Figure 18: Loading scatter plot of the investigated parameters
-4
-3
-2
-1
0
1
2
3
4
-4 -3 -2 -1 0 1 2 3 4
t[2]
t[1]
M1 (PLS)t[Comp. 1]/t[Comp. 2]
N3589-803Q
N3601-803Q
N3602-255G
N3614-803F
N3620-803Q
N3622-803F
N3634-803QN3677-803Q
N5857-824BN5858-256G
N5902-804Q
N5944-277Q
N6040-123A
N6052-280TN6066-803F
N6067-803Q
N6145-143A
N6174-803F
N6175-826B
N6192-803F
N6204-825B
N6206-803F
N6208-803J
N6236-824B
Figure 19: Score Scatter plot of the observed heats
33
The contribution of each of the independent variables to the model for the prediction of the
SiO2 and CaO content of the top slag is shown in figure 20. The most important parameters
influencing the top slag composition before degassing as shown by the VIP plot below are;
amount of the synthetic slag former added, amount of carry over slag, yield of the deoxidants
into the steel and the carbon content at tapping (oxygen potential).
0
1
2
Ex
tra
sla
g
Sla
g form
e
Si Y
ield
Al Y
ield
Al(kg) C
SiO
2
Ca
O
Si(K
g)
Tem
p
VIP
[2]
Var ID (Primary)
M1(PLS), VIP[Comp. 2]
Figure 20: VIP plots of the predictors in the model
In summary the model created shows a fairly good explanation of 0.64 for the variation of the
dependent variable (CaO and SiO2) in all the observed heats, but it has a poor predictability of
0.34. This implies that the model is unreliable to predict the composition of the top slag
before degassing for any arbitrary heat besides the heats observed. The reason for this is that,
some other important variable parameters were not involved in the PLS regression, this
parameters include the description of the ladle glaze, amount of EAF slag tapped, mass of the
EAF slag removed and mass of refractory wear. Similar regression was done for the
prediction of Al2O3; the model was also mostly described by the same important parameters
as observed for CaO and SiO2.
34
4.5 Oxygen Activities
An important thermodynamic parameter which influences the desulphurisation process apart
from slag composition is the oxygen potential of the slag-steel system (equation 13). A
control of the desulphurisation process is impossible if the oxygen activity is not known. An
attempt was made to create a model and also modify the existing ones in order to estimate the
ao at any given point during the ladle refining.
Three models were established for online prediction of ao and they were also compared with
the measured values. The first model assumed equilibrium between Al and O dissolved in the
steel melt and Al2O3 in the slag (4). Ohta and Suito expression in equation (17) was used to
calculate the Al2O3 activities in the slag while Wagner's expression in equation (16) was used
to calculate the Al activities in the steel. The values of the interaction coefficients used were
taken from T. A Engh (8).
The second model assumed equilibrium between Al and O dissolved in the steel and Al2O3
inclusion in the steel bulk, for this model the activity of Al2O3 inclusion was assumed to be 1
since at this stage of refining ( before degassing) it is valid to consider that the inclusion is
Al2O3 saturated (15).
In the third case a model was created using several existing online process data (measured ao,
temperature and %Al in the steel) to predict the ao at a particular temperature and within an Al
range.
Appendix XI shows graphical relationship between ao and %Al in the melt at different
temperature. The formula obtained in the model fit in appendix XI was used in the third
model calculation. Figure 21 shows a comparism of the three models with the measured ao
values. Model 2 and 3 gave fairly close values compared with measured values while model 1
shows a high discrepancy.
35
0,0E+00
1,0E-04
2,0E-04
3,0E-04
4,0E-04
5,0E-04
0 2 4 6 8 10 12 14
Heats
ao
(pp
m) Ohta&suito
Al2O3 =1
Data
Measured ao
Figure 21: Measured and calculated oxygen activities before degassing
It could be seen from figures 21 and 22 that model 2 which assume equilibrium between
Al/O/Al2O3inclusions has a quite close pattern with the measured ao values and also has a high
degree of explanation for the deviation of its values from measured value. It has a good
agreement with measured values.
y = 0,1363x - 1E-05
R2 = 0,4968
y = 0,7739x + 2E-06
R2 = 0,8197
y = 0,4968x + 0,0001
R2 = 0,6223
0,0E+00
1,0E-04
2,0E-04
3,0E-04
4,0E-04
5,0E-04
1,00E-04 2,00E-04 3,00E-04 4,00E-04 5,00E-04 6,00E-04
Measured ao
Ca
lcu
late
d a
o
Ohta&suito
Al2O3 =1
Data
Figure 22: Refitting Oxygen activities Models
36
A refit of these models was done and the modified models gave better ao prediction as seen in
figure 23 below.
0,00E+00
1,00E-04
2,00E-04
3,00E-04
4,00E-04
5,00E-04
6,00E-04
0 2 4 6 8 10 12 14
Heat
ao
(p
pm
) Ohta&suito
Al2O3=1
Data
Measured ao
Figure 23: Modified oxygen activities Models
For online utilisation of ao values, it could be inferred from the investigation above that model
2 gives a fairly accurate result with better predictability.
4.6 Equilibrium sulphur Distribution
The slag-metal sulphur distribution ratio Ls after desulphurisation, as earlier mentioned, is
another important parameter in the modelling of sulphur removal in steel making. Ls was
calculated using equation (13) where fs was obtained with Wagner's expression in equation
(16) and ao was calculated using Al/O/Al2O3inclusion equilibrium. Sulphide capacity was
calculated using Young et al expression, written in equation (12) but in few cases when
optical basicity is very close to 0.8, Sosinky & Sommerville expression was used due to better
correlation (14).
Figure 24 shows the values of measured and calculated sulphur distribution ratio after vacuum
treatment. The calculated Ls gives the probable sulphur distribution at equilibrium, while
measured Ls gives the actual distribution at the prevailing kinetic conditions. A perfectly
37
linear correlation could not been observed from figure 24 because equilibrium was not
reached after vacuum treatment for few heats. This could be explained by kinetic reasons of
either short time or poor interaction between the reacting phases. Calculated Ls is the sulphur
partition value at optimum desulphurisation condition, i.e. assuming favourable kinetic
conditions such as low vacuum pressure, optimised argon gas purging and good viscosity.
The optimum vacuum pressure (<5 torr) was always attained during the plant trials but an
investigation of the gas flow for all the heats shows that argon gas flow rate has a major
influence on the success of the desulphurisation.
0
200
400
600
800
1000
1200
0 200 400 600 800 1000 1200 1400
Calculated Ls
Me
as
ure
d L
s
Figure 24: Distribution of sulphur between slag and steel after
Vacuum treatment against calculated Ls
The heats in figure 24 with lower values of measured Ls compared to calculated Ls have poor
gas stirring. For most of these heats the average flow rate of argon gas in either/both porous
plugs was lower than 45ltr/min as opposed to about 80ltr/min for others and 100ltr/min
suggested by Hallberg et al (17) . Another reason could be that the sulphur composition of the
slag after desulphurisation is inhomogeneous, this influences the measured Ls; this is
supported by previous investigation done by Andersson M. et al (4).
38
4.7 Regression Analysis for the Desulphurisation Process
A multivariate data analysis was carried out to investigate the important parameters during the
process of desulphurisation and their influences. This will enhance the optimisation of top
slag composition for precise sulphur removal with consideration to other factors. Similar to
the PLS analysis done earlier for the variation in top slag composition before degassing, 12
extra heats from previous research on ladle refining done at Ovako were added to the 12 heats
investigated in this thesis, to increase the accuracy of the prediction of the modelling. In the
PLS modelling of the desulphurisation process, the dependent variable was set to be the
degree of desulphurisation (DD) (definition can be seen in equation 20), while the predictors
are; average argon gas flow rate in porous plug 1 and 2 (Ar1, Ar2), mass of slag former,
temperature (temp), time, oxygen activity (ao), basicity (B1), CaO/Al2O3 ratio, aluminium
content of the steel [%Al], coefficient of sulphur activity in the steel (fs) and slag composition.
The influence of the cross interaction between CaO and Al2O3 was also included in the model.
The degree of desulphurisation DD is well modelled by the x-variables due to its high degree
of explanation (0.899) and also a good predictability of (0.772).
[ ] [ ][ ]
)20(100%
%%−−−−−−−−−−−−−−−−−−−−−
−= X
S
SSDD
i
fi
Where [%S]i is the Initial sulphur content of steel before degassing while [%S]f is the final sulphur content of steel after degassing.
An overview of the model is shown in figure 25 and figure 26. From the loading plots of the
predictor in figure 25, it could be seen that the tendency of having a very high DD is favoured
by high mass of slag former, longer time of degassing good argon gas flow through the
porous plug, %Al in the steel and the cross interaction between CaO&Al2O3. An increasing
basicity is also very important to obtain good desulphurisation but it requires high slag mass
to achieve an extreme desulphurisation. The SiO2 content of the top slag has a negative
influence on DD. The score plot of the observed heats in figure 26 shows an outlier heat, the
DD predicted by the model from the x-variable parameters of this heat does not correlate with
measured DD, and this is as a result of unusually low Al2O3 and very high CaO content in the
top slag and also very poor argon gas flow rate. The figure also shows the grouping of all
heats into extreme sulphur removal and low sulphur removal, this is explained by the
x- variable contributions in each heat.
39
-0,40
-0,30
-0,20
-0,10
0,00
0,10
0,20
0,30
0,40
-0,10 0,00 0,10 0,20 0,30 0,40
w*c
[2]
w*c[1]
M1 (PLS)w*c[Comp. 1]/w*c[Comp. 2]
X
Y
Cross
ao
Temp
SiO2
CaOAl2O3
Ar 1
Ar 2
fs
Time
%Al
Slag forme
B1
CaO/Al2O3
DD
CaO*Al2O3
Figure 25: Loading scatter plot of the investigated parameters
-3
-2
-1
0
1
2
3
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
t[2
]
t[1]
M1 (PLS)t[Comp. 1]/t[Comp. 2]
N3589-803Q
N3601-803QN3602-255G
N3614-803F
N3620-803Q
N3622-803F
N3634-803QN3677-803Q
N5857-824B
N5858-256G
N5902-804Q
N5944-277Q
N6040-123A
N6052-280T
N6066-803F
N6067-803Q
N6145-143A
N6174-803FN6175-826BN6192-803F
N6204-825B
N6206-803F
N6208-803J
N6236-824B
Figure 26: Score Scatter plot of the observed heats
The contribution of each of the independent variables to the model for the prediction of the
DD is shown in figure 27. The most important parameters influencing the DD as shown by
40
the VIP plot of the model are; time, temperature, mass of slag former, %Al, basicity, slag
composition, oxygen activity and argon gas in the second porous plug.
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1,60
1,80
2,00
Tim
e
Tem
p
Sla
g form
e
%A
l
Ar
2
Ar
1
CaO
*Al2
O3
B1
SiO
2
ao
Al2
O3
CaO fs
CaO
/Al2
O3
VIP
[2]
M1 (PLS)VIP[Comp. 2]
Figure 27: VIP plots of the predictors in the model
An observation from this PLS regression analysis worth mentioning is the influence and
importance of the ao in the model. Oxygen activities ao is dependent on temperature, so it is
difficult for PLS model to show the precise influence of ao on DD. It is expected that high ao
should have a negative influence on DD but this could not be clearly seen on the model
overview, since the degassing for all the heats were not carried out at the same temperature.
Heats with longer degassing time usually commenced at higher temperature for the reason of
favourable kinetics during the length of degassing, this however made it difficult to see the
expected influence of ao on DD in this simple model.
41
4.8 Equilibrium Condition during Vacuum Treatment
A study of the possible thermodynamic equilibrium which has influence on the
desulphurisation process was examined. The desulphurisation reaction during the vacuum
treatment shown earlier in equation (18) is influenced by reoxidation reactions and also the
equilibrium between Al/Si content of the bulk steel and the SiO2 content of the slag. Higher
SiO2 content of the top slag before degassing (due to the dilution of the synthetic slag by carry
over slag) contributes to the oxygen potential of the system especially when %Al in the steel
is low. This condition is not favourable for good sulphur removal. If the temperature is high
(which implies high ao), and %Al is low, the silica content in the slag might increase during
the vacuum treatment.
The reduction reaction is shown in equation (21), the forward reaction is favoured if the Si
content of the steel is low and the Al content is high and with a rigorous argon gas stirring (20).
(SiO2) + 4/3[Al] = [Si] + 2/3(Al2O3) ----------------------------------------- (21)
For the 12 heats investigated, the influence of the above mentioned equilibrium condition on
sulphur removal is shown in figure 28. The smooth curve on the graph is the equilibrium
relationship for Al reduction of SiO2 from lime saturated calcium aluminate slag at
temperature close to 1600˚C, as explained by E. T. Turkdogan (20).
The points on figure 28 explain the drift of the system towards equilibrium during degassing.
Each heats have two points on the curve, the point with higher Al represent the condition
before degassing when equilibrium has not been achieved while the corresponding point with
lower Al depicts equilibrium condition after degassing (though it was suspected that few heats
did not attain equilibrium with the kinetic conditions that prevailed during the process).
42
0
10
20
30
40
50
60
70
80
0,000 0,020 0,040 0,060 0,080 0,100 0,120 0,140 0,160
[%Al]
SiO
2/S
i
Equil Lit 1 2 3 45 6 7 8 910 11 12
Figure 28: Aluminium reduction of SiO2 from Lime saturated molten slag during degassing
From figure 28 it could be noted that all the heats are moving towards the smooth equilibrium
curve as the vacuum treatment proceeds, some final points are yet far away from the curve
either due to lower reaction temperature compared to that of the equilibrium curve or
equilibrium has not yet been achieved before the process was stopped.
Heats with high %Al before degassing have low ao and have higher potential for the reduction
of the slag and consequently giving a favourable condition for desulphurisation to proceed.
Heats with low %Al have the tendency for increasing SiO2 content of the slag and increasing
oxygen potential of the system which is unfavourable for sulphur removal.
The initial ao activity before degassing is also another important parameter which influences
the equilibrium drift in the plot. Heats with high ao before degassing have the tendency of
increasing SiO2 of the slag and thereby limiting the extent of the desulphurisation even if the
slag is lime saturated.
43
4.9 Equilibrium Sulphur Content in the Bulk steel
A mass balance was made in terms of sulphur content of the system at equilibrium, in order to
project the sulphur content of the steel after vacuum treatment. The mass difference of
sulphur in the steel before and after degassing was equated to the mass difference of sulphur
in the slag.
[ ] [ ] ( ) ( )
[ ] ( ) [ ] ( )22%%
%
%%%%
−−−−−−−−−−−−−−−−−−−−−−+⋅
+=
−=−
SsSl
iSisf
islfslfsis
MLM
SMSMS
SMSMSMSM
Ms - Mass of steel Bulk Msl - Mass of slag [%S]i -Initial Sulphur content of steel before degassing [%S]f - Final Sulphur content of steel after degassing (%S)i - Initial Sulphur content of Slag before degassing (%S)f - Final Sulphur content of slag after degassing Ls - Sulphur Partition It should be noted that the compositional change in the top slag was considered in the
calculation of Ls since mass change is of no effect on it. To simplify the calculation of the
equilibrium sulphur, average extra slag mass of 200kg was added to the top slag for all heats
since a range of about 150-250kg was estimated in the conditional mass balance done earlier.
Figure 29 shows that few heats have higher measured [%S]f values after degassing compared
to calculated values, similar reasons hold for the observed variation in the measured and
calculated values of Ls explained earlier.
44
0,000
0,005
0,010
0,015
0,020
0,025
0,000 0,005 0,010 0,015 0,020 0,025
Calculated [%S]f
Mea
su
red
[%
S]
f
Figure 29: Calculated and measured equilibrium sulphur in the steel
The heats with values far away above the equilibrium line are the same heats with wide
variation in Ls, equilibrium was not achieved after vacuum treatment, perhaps due to poor
argon gas stirring or time factor.
It should be noted that some of these deviations observed in the calculated parameters could
be due to the model used in calculating the sulphide capacity of the slags, other models such
as 'IRSID' and 'Thermoslag' could have been used as well but their parameters were not
accessible.
4.10 Optimisation of the top slag composition
The regression analysis made for the top slag composition before degassing and other
empirical process investigation shows a very low degree of the prediction of the carry over
slag composition. This is as a result of few important parameters which could not be
measured; this however made the optimisation of the top slag for precise sulphur removal
more challenging. An alternative measure to achieve this result is 'extra slag practice'. The
amount of slag carry over could be reduced to an insignificant amount by improving the
ability of the first slag from the EAF to absorb inclusion and entrapped glaze. A short
inductive stirring before slag removal is also necessary to aid the separation of these
entrapped oxides into the floating slag.
45
The extra slag practice involves the addition of lime (200-400kg), during tapping to increase
the activity of CaO and thus improve the ability of the EAF slag to absorb the entrapped
oxides in the steel. A short inductive stirring is necessary to remove the ladle glaze and other
oxides, and also enhance the upward transport to the slag at the top of the vessel. This practice
will enhance the accurate estimation of slag former with right mass and sulphide capacity for
a precise sulphur removal since dilution of the new synthetic slag would have been reduced to
an insignificant level.
46
5.0 CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
The main factors influencing the top slag composition before vacuum treatment include,
oxygen content of steel at tapping, extent of deoxidation at tapping, ladle glaze and amount of
EAF slag left after slag removal. The variation in the top slag composition and the
desulphurisation process for the heats investigated, could be summarised below
• All heats have at least 100% increase in the SiO2 content of the top slag before
degassing, including heats with low oxygen potential at tapping, where the yield of
silicon from FeSi into the steel is high. This is an influence of the unknown mass of
slag and oxides remaining on the ladle wall or entrapped in the bulk steel after EAF
slag removal. It was also noted that heats with high oxygen potential before tapping
and low mass of synthetic slag former, have an tendency for increased SiO2 content in
the top slag before degassing
• MnO and FeO in the top slag before vacuum treatment are also traced to slag or oxides
entrapped in the steel or on ladle walls. These two oxides appear to be low when low
oxygen activity is achieved after deoxidation at tapping or when the %Al content of
the steel is very high. They are easily reduced by Al.
• MgO obviously comes from two major sources; carry over slag and refractory erosion.
Heats for high clean steel, have very low MgO and high CaO in the synthetic slag
blend, this however result in low MgO content of the topslag before degassing as slag
with high CaO has low tendency for the dissolution of the refractory.
• The most important factors influencing top slag composition before degassing are
amount of slag former added, mass of carry-over slag, oxygen potential and the yield
of the deoxidants at tapping.
• The PLS model created has a very unreliable predictability for the top slag
composition due to lack of other important parameters. This necessitated an alternative
solution of extra slag practice to improve the absorption of oxides in the steel bulk.
• A number of heats has lower sulphur partition ratio after desulphurisation compared to
the calculated values due to poor argon gas flow, this is also substantiated in the
regression analysis made.
47
• The second regression analysis made shows that the sulphur removal can be optimised
by controlling the following parameters; mass& composition of the top slag, vacuum
treatment time, % Al, oxygen activity in the steel before degassing and argon gas flow
rate.
• To achieve a precise sulphur removal with an optimised top slag composition, the
argon gas flow and inductive stirring effect during the vacuum treatment must be
optimised as well.
5.2 Recommendations
The following praxes highlighted below are recommended based on the investigation made in
the research thesis and the prevailing process practices at the steel plant.
• It is strongly recommended that further investigation should be carried out on
deoxidation at tapping as the process seems to be inconsistent. In some instances, low
ao could not be achieved due to Al/O/Al2O3 equilibrium. It is suggested that a steel
sample should be taken shortly before tapping after which there will be no further
oxygen blowing, to really know the oxygen level in the steel bulk and avoid additional
estimation of extra aluminium metal mass by operators. Consistent and accurate
amount of FeSi should be added during tapping, and also the material handling of the
deoxidants should be improved.
• It could be a better practice to add extra sulphur into the system only after vacuum
treatment, for heats with high final sulphur requirement, to reduce loss of sulphur into
the slag during vacuum treatment.
• The improvement of the argon gas stirring for optimum utilisation of the sulphide
capacity of the slags should be investigated. This will decrease the degassing time and
also contribute to the expected result for precise sulphur removal using slag
composition control.
• Further research should be carried out on the mass of lime to be added during tapping
to improve the ability of the EAF slag to absorb entrapped oxides in the steel bulk.
48
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Capacity Concept on High basicity Ladle Slags Used in Bearing Steel Production. ISIJ
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Composition by application of a sulphide capacity Model; Iron and steel making
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Ladle Refining Operations; ISIJ International, Vol 41 (2001),No. 11,pp1289-1302.
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Thermo-calc, applied for removal of sulphur during the ladle treatment. Licentiate
thesis 1997, KTH.
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during Lalde treatment, Iron and Steelmaking, Vol. 34, No 4, pp312-324, 2007.
49
16. Margareta Andersson, Malin Hallberg, L. Jonsson, Pär Jönsson, Slag-Metal Reactions
during ladle treatment with focus on desulphurisation; Iron and steel making 2002
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Hydrogen Refining Using Process Models in Production.
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CaO-SiO2-Al2O3-MgO-FexO slags at 1723K; Steel Research International 76(2005)
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Proceedings May 2004.
50
APPENDICES
Appendix I: Slag composition of all heats
Heat SiO2 % MnO % S % TiO2 % CaO % Al2O3 % MgO % FeO % ao(ppm) Temp˚C
Sample in the EAF before Tapping
1 136 1694
2 N/A 1730
3 632 1713,2
4 159 1703,3
5 12,50 7,01 0,11 0,34 34,70 4,48 9,38 22,27 928 1692,6
6
7
8
9 11,8 0,6 0,10 0,54 44,1 5,6 10,3 17,6
10 14,3 5,1 0,10 0,68 49,0 5,7 4,8 13,6
11 14,9 6,2 0,11 0,51 45,9 6,4 10,8 8,7
12 18,4 5,5 0,08 0,46 36,7 6,3 15,8 12,6
Sample after Tapping
1
2
3 14,40 5,38 0,076 0,51 35,90 12,20 13,40 15,80
4 20,00 3,09 0,053 0,20 19,20 27,60 20,60 7,56
5 11,18 4,36 0,12 0,18 29,36 16,71 14,37 18,09
6 20,2 2,4 0,06 0,18 23,2 23,7 23,9 5,5
7 17,8 0,44 0,51 0,11 27,3 25,4 25,7 3,5
8
9 19,3 3,7 0,09 0,33 30,9 20 22,1 10,1
10 14,7 5,0 0,10 0,60 48,2 6,0 4,9 12,6
11 16,7 5,2 0,11 0,49 41,7 7,2 12,2 11,1
12 22,6 2,8 0,16 0,27 26,1 14,3 27,0 5,8
Sample Before Slag Removal
1 21,20 1,24 0,18 0,11 28,60 24,80 22,80 2,13
2 17,60 3,28 0,08 0,19 28,20 23,90 15,30 9,62
3 15,10 5,08 0,09 0,52 35,80 17,70 11,80 12,23
4 16,70 2,51 0,07 0,17 22,90 33,80 16,60 6,47
5 10,57 3,61 0,07 0,14 29,44 30,38 13,88 11,93
6 17,90 1,20 0,21 0,22 22,60 29,50 23,70 5,20
7 15,20 0,55 0,54 0,10 31,70 25,90 22,10 3,90
8 15,90 0,60 0,84 0,14 30,50 21,60 28,20 2,50
9 23,00 2,50 0,04 0,19 23,60 22,00 24,30 4,30
10 18,50 2,30 0,16 0,29 40,60 20,90 12,90 4,30
11 18,00 1,70 0,25 0,28 38,20 20,30 15,20 5,10
12 19,70 0,70 0,48 0,12 25,40 23,70 26,90 2,30
Continue on next page
51
Heat SiO2 % MnO % S % TiO2 % CaO % Al2O3 % MgO % FeO % ao Temp
Sample Before Degassing
1 6,80 0,24 0,28 0,15 57,30 26,00 8,20 1,66 2,8150 1610
2 10,00 3,14 0,37 0,17 49,80 27,40 7,50 2,21 4,1400 1596
3 5,60 0,18 0,23 0,07 64,50 28,40 2,50 0,61 2,4140 1625
4 6,90 0,84 0,21 0,07 62,00 26,60 3,20 1,24 - -
5 6,28 3,49 0,43 0,13 59,01 25,40 6,02 3,46 5,1205 1639
6 7,50 0,77 0,52 0,14 55,10 26,70 8,00 0,81 3.6064 1610
7 5,10 0,23 0,35 0,14 60,40 24,10 6,80 2,60 2,5326 1606
8 6,20 0,40 0,53 0,07 56,70 31,90 4,90 1,00 - 1597
9 5,90 0,31 0,15 0,11 63,50 28,00 3,30 0,73 3,0263 1626
10 9,20 1,20 0,19 0,13 56,60 21,90 6,70 3,90 3,4036 1619
11 7,20 0,13 0,31 0,11 59,00 25,50 5,70 0,70 2,0369 1578
12 6,40 0,09 0,86 0,14 56,50 27,90 8,10 0,33 3,1259 1609
Sample After Degassing
1 8,00 0,04 1,30 0,17 56,10 28,60 7,20 0,27 1,4850 1535
2 12,00 0,43 1,10 0,19 49,90 29,30 8,60 0,67 1,7140 1564
3 2,10 0,03 0,77 0,12 58,70 35,30 3,60 1,05 0,8280 1521,5
4 5,00 0,10 0,77 0,10 55,80 34,00 6,10 0,47 0,9190 1546,4
5 7,72 0,32 1,94 0,21 52,16 29,78 8,46 0,56 4,6400 1597,9
6 8,70 0,27 2,40 0,23 52,50 28,60 9,40 0,40 3,5099 1577
7 5,90 0,05 1,30 0,19 59,50 27,30 7,40 0,76 1,4550 1541
8 6,40 0,20 1,30 0,15 52,10 32,20 7,10 3,70 1,3340 1540
9 5,80 0,13 0,75 0,16 60,60 30,20 3,70 1,10 1,0370 1515,6
10 8,70 0,09 1,20 0,17 56,40 28,30 6,10 0,53 2,2865 1545,6
11 7,60 0,03 0,91 0,19 59,00 27,60 4,70 0,41 1,0609 1512,4
12 7,50 0,05 2,10 0,21 58,30 26,70 6,20 0,38 0,6765 1537,6
Sample at completion of Refining
1
2
3
4
5 7,50 0,77 0,52 0,14 55,10 26,70 8,00 0,81
6 9,20 1,20 0,19 0,13 56,60 21,90 6,70 3,90 3,4036 1619
7 7,20 0,13 0,31 0,11 59,00 25,50 5,70 0,70
8
9 8,00 0,04 1,30 0,17 56,10 28,60 7,20 0,27
10 12,00 0,43 1,10 0,19 49,90 29,30 8,60 0,67 1,7140 1564
11 5,00 0,10 0,77 0,10 55,80 34,00 6,10 0,47 0,9190 1546
12 7,72 0,32 1,94 0,21 52,16 29,78 8,46 0,56 4,6400 1598
52
Appendix II: Steel samples for all Heats
Heat %C %Si %Mn %S %Cr %Ni %Al %Ti %Ca %Mg
Sample ein the EAF Before Tapping
1 0,24 0,02 0,25 0,024 0,41 0,23 0,162 0,0004 0,00008 0,00003
2 0,08 0,01 0,14 0,027 0,26 0,26 0,215 0,0002 0,00017 0,00011
3 0,06 0,01 0,19 0,012 0,18 0,10 0,236 0,0002 0,00005 0,00003
4 0,09 0,01 0,23 0,011 0,27 0,32 0,175 0,0001 0,00045 0,00006
5 0,04 0,02 0,19 0,014 0,44 0,18 0,245 0,0003 0,00013 0,00005
6 0,14 0,02 0,16 0,016 0,21 0,09 0,180 0,0003 0,00031 0,00004
7 0,29 0,02 0,20 0,020 0,24 0,10 0,257 0,0005 0,00005 0,00003
8 0,76 0,03 0,31 0,014 0,45 0,12 0,002 0,0001 0,00292 0,00037
9 0,14 0,01 0,25 0,011 0,24 0,07 0,198 0,0003 0,00012 0,00003
10 0,13 0,01 0,17 0,012 0,35 0,13 0,277 0,0004 0,00339 0,00025
11 0,28 0,02 0,24 0,017 0,37 0,17 0,186 0,0005 0,00018 0,00003
12 0,20 0,01 0,24 0,022 0,42 0,13 0,052 0,0001 0,00049 0,00010
Sample After Tapping
1 0,24 0,18 0,30 0,026 0,62 0,22 0,027 0,0004 0,00019 0,00006
2 0,03 0,08 0,19 0,028 0,36 0,27 0,004 0,0003 0,00024 0,00005
3 0,04 0,24 0,18 0,014 0,19 0,10 0,044 0,0002 0,00016 0,00003
4 0,04 0,18 0,22 0,014 0,29 0,32 0,030 0,0003 0,00021 0,00004
5 0,04 0,10 0,19 0,016 0,43 0,18 0,015 0,0003 0,00019 0,00003
6 0,08 0,20 0,20 0,017 0,32 0,10 0,033 0,0002 0,00023 0,00009
7 0,28 0,25 0,27 0,019 0,34 0,10 0,056 0,0006 0,00019 0,00006
8 0,60 0,27 0,37 0,014 0,57 0,13 0,065 0,0005 0,00019 0,00012
9 0,11 0,20 0,26 0,012 0,26 0,07 0,063 0,0003 0,00016 0,00006
10 0,08 0,25 0,21 0,013 0,44 0,13 0,045 0,0004 0,00017 0,00004
11 0,37 0,26 0,29 0,013 0,49 0,16 0,062 0,0006 0,00023 0,00011
12 0,20 0,27 0,28 0,024 0,52 0,13 0,050 0,0004 0,00017 0,00005
Sample Before Alloying at Ladle Furnace station
1 0,23 0,18 0,30 0,028 0,62 0,22 0,024 0,0003 0,00018 0,00005
2 0,04 0,08 0,18 0,029 0,36 0,27 0,004 0,0003 0,00025 0,00009
3 0,04 0,21 0,19 0,016 0,2 0,10 0,019 0,0002 0,00018 0,00003
4 0,05 0,18 0,23 0,014 0,29 0,32 0,014 0,0003 0,00020 0,00003
5 0 0 0 0 0 0,00 0 0 0 0
6 0,07 0,20 0,20 0,017 0,33 0,11 0,026 0,0002 0,00019 0,00003
7 0,28 0,27 0,27 0,022 0,36 0,10 0,105 0,0006 0,00012 0,00006
8 0,61 0,29 0,38 0,016 0,59 0,13 0,068 0,0005 0,00016 0,00009
9 0,11 0,20 0,26 0,014 0,27 0,08 0,050 0,0003 0,00017 0,00003
10 0,09 0,25 0,23 0,015 0,48 0,13 0,033 0,0003 0,00018 0,00003
11 0,38 0,28 0,30 0,016 0,52 0,17 0,064 0,0006 0,00016 0,00003
12 0,19 0,27 0,28 0,023 0,54 0,13 0,042 0,0004 0,00016 0,00004 Continue on the next page
53
Appendix III: Interaction Coefficients of activities of dissolved elements in steel (17)
j C Si Mn P S Cr Mo Al
ejs 0,110 0,0630 -0,026 0,290 -0,028 -0,011 0,003 0,035
ejAl 0,091 0,0056 0,000 0,000 0,030 0,000 0,000 0,045
j Cu V Sn Ti W Ca Co
ejs -0,0008 -0,016 -0,004 -0,072 0,01 0,000 0,003
ejAl 0,0000 0,000 0,000 0,000 0,00 -0,047 0,000
Heat %C %Si %Mn %S %Cr %Ni %Al %Ti %Ca %Mg
Sample Before Degassing
1 0,36 0,29 0,30 0,025 1,7 0,22 0,056 0,0005 0,00020 0,00017
2 0,17 0,27 0,62 0,036 1,14 3,55 0,034 0,0005 0,00023 0,00009
3 0,09 0,22 0,22 0,012 1,45 0,10 0,144 0,0003 0,00020 0,00011
4 0,11 0,20 1,18 0,013 2,03 0,48 0,081 0,0004 0,00021 0,00009
5 0,12 0,12 0,85 0,037 0,51 0,18 0,051 0,0004 0,00020 0,00005
6 0,14 0,20 1,14 0,040 1,02 0,15 0,045 0,0005 0,00020 0,00011
7 0,42 0,27 0,32 0,016 1,42 0,10 0,057 0,0011 0,00025 0,00020
8 0,76 0,34 0,65 0,013 1,68 0,13 0,057 0,0013 0,00022 0,00024
9 0,15 0,20 0,28 0,011 1,46 0,08 0,079 0,0003 0,00017 0,00013
10 0,28 0,25 0,65 0,013 1,7 0,13 0,053 0,0009 0,00019 0,00013
11 0,50 0,28 0,30 0,013 1,4 0,17 0,057 0,001 0,00022 0,00016
12 0,34 0,29 0,28 0,017 1,72 0,14 0,033 0,0008 0,00020 0,00023
Sample after Degassing
1 0,91 0,31 0,31 0,013 1,67 0,22 0,044 0,001 0,00023 0,00031
2 0,22 0,25 0,70 0,023 1,25 3,66 0,033 0,0005 0,00020 0,00018
3 0,92 0,25 0,22 0,001 1,44 0,10 0,081 0,0009 0,00017 0,00029
4 0,13 0,22 1,21 0,001 2,11 0,48 0,020 0,0007 0,00027 0,00027
5 0,17 0,11 0,87 0,016 0,52 0,18 0,026 0,0005 0,00024 0,00012
6 0,15 0,19 1,15 0,013 1,03 0,14 0,022 0,0006 0,00029 0,00020
7 0,96 0,28 0,33 0,011 1,42 0,10 0,045 0,0014 0,00022 0,00025
8 0,98 0,34 0,65 0,007 1,67 0,13 0,041 0,0014 0,00023 0,00038
9 0,90 0,22 0,29 0,004 1,47 0,08 0,052 0,0007 0,00021 0,00018
10 0,92 0,26 0,67 0,005 1,71 0,13 0,032 0,0013 0,00024 0,00029
11 0,97 0,29 0,30 0,010 1,4 0,17 0,048 0,0014 0,00020 0,00019
12 0,93 0,30 0,29 0,007 1,7 0,14 0,025 0,0013 0,00025 0,00029
Final sample
1 0,94 0,30 0,31 0,010 1,68 0,22 0,037 0,0009 0,00025 0,00031
2 0,22 0,25 0,70 0,022 1,25 3,65 0,029 0,0005 0,00020 0,00019
3 0,93 0,31 0,22 0,001 1,44 0,10 0,076 0,0009 0,00119 0,00045
4 0,15 0,25 1,20 0,001 2,15 0,48 0,028 0,0005 0,00255 0,00047
5 0,20 0,11 0,87 0,020 0,52 0,18 0,027 0,0005 0,00019 0,00011
6 0,15 0,20 1,17 0,022 1,03 0,15 0,030 0,0006 0,00023 0,00019
7 0,97 0,27 0,32 0,010 1,41 0,10 0,043 0,0013 0,00024 0,00019
8 0,99 0,34 0,65 0,006 1,67 0,13 0,041 0,0014 0,00022 0,00033
9 0,95 0,21 0,31 0,007 1,47 0,08 0,044 0,0006 0,00020 0,00018
10 0,94 0,26 0,67 0,006 1,71 0,13 0,029 0,0013 0,00025 0,00032
11 0,97 0,28 0,29 0,008 1,39 0,17 0,042 0,001 0,00023 0,00017
12 0,93 0,31 0,30 0,006 1,7 0,14 0,034 0,0013 0,00027 0,00033
54
Appendix IV: Variation of SiO2 content of slag
0
5
10
15
20
25
0 1 2 3 4 5 6 7 8Process Stage
%S
iO2
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Heat 12
1-Sample in EAF before Tapping
2-Sample at tapping
3-Sample before slag removal
4-Synthetic slag
5-Sample before degasing
6-Sample after Degassing
7-Sample at completion of refining
Appendix V: Variation of MnO content of slag
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8Process Stage
%M
nO
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Heat 12
1-Sample in EAF before Tapping
2-Sample at tapping
3-Sample before slag removal
4-Synthetic slag
5-Sample before degasing
6-Sample after Degassing
7-Sample at completion of refining
55
Appendix VI: Variation of CaO content of slag
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5 6 7 8
Process Stage
%C
aO
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Heat 12
1-Sample in EAF before Tapping
2-Sample at tapping
3-Sample before slag removal
4-Synthetic slag
5-Sample before degasing
6-Sample after Degassing
7-Sample at completion of refining
Appendix VII: Variation of Al2O3 content of slag
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8Process Stage
%A
l 2O
3
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Heat 12
1-Sample in EAF before Tapping
2-Sample at tapping
3-Sample before slag removal
4-Synthetic slag
5-Sample before degasing
6-Sample after Degassing
7-Sample at completion of refining
56
Appendix VIII: Variation of MgO content of slag
0
5
10
15
20
25
30
0 1 2 3 4 5 6 7 8
Process Stage
%M
gO
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Heat 12
1-Sample in EAF before Tapping
2-Sample at tapping
3-Sample before slag removal
4-Synthetic slag
5-Sample before degasing
6-Sample after Degassing
7-Sample at completion of refining
Appendix IX: Variation of FeO content of slag
0
5
10
15
20
25
0 1 2 3 4 5 6 7 8Process Stage
%F
eO
Heat 1 Heat 2 Heat 3 Heat 4 Heat 5 Heat 6
Heat 7 Heat 8 Heat 9 Heat 10 Heat 11 Heat 12
1-Sample in EAF before Tapping
2-Sample at tapping
3-Sample before slag removal
4-Synthetic slag
5-Sample before degasing
6-Sample after Degassing
7-Sample at completion of refining
57
Appendix X: Mass Balance of Heat 12
Heating and Alloying Si Mn S Ti Ca Al Mg
Analysis before alloying [%] 0,25 0,23 0,02 0,0003 0,0002 0,03 0,00004
Alloying [kg] 14,42 457,30 0,10 0,0000 0,0000 36,74 0,00000
Sample before degassing [%] 0,25 0,65 0,01 0,0009 0,0002 0,05 0,00013
Mass difference ('amt after' - 'Amt Before' - 'Alloy addition') [kg] -7,67 -23,90 -1,73 0,6184 0,0150 -15,51 0,09262
SiO2 MnO S TiO2 CaO Al2O3 MgO FeO
Synthetic slag [%] 2,77 0,14 0,05 0,1127 66,34 22,70 4,46 0,66
Weight of synthetic slag [kg] 22,05 1,10 0,36 0,8961 527,42 180,49 35,45 5,25
Slaganalysis before degassing[%] 9,20 1,20 0,19 0,1300 56,60 21,90 6,70 3,90
Weight before degassing [kg] 85,73 11,18 1,77 1,2114 527,42 204,07 62,43 36,34
Mass difference in slag [kg] 63,67 10,08 1,41 0,3153 0,00 23,59 26,99 31,10
Equivalent element in slag [kg] 29,77 7,81 1,41 0,1890 0,00 12,49 16,27 24,18
Elemental Sum in slag&steel [Kg] 22,10 -16,09 -0,32 0,8074 0,02 -3,02 16,37
Mass of Top slag = Mass of CaO in sythetic slag / % Cao before degassing = 931.84Kg
Extra slag = Mass of Top slag before degassing - Mass of synthetic slag = 136,84kg
Degassing Si Mn S Ti Ca Al Mg
Steelanalysis before degassing[%] 0,25 0,65 0,01 0,0009 0,0002 0,05 0,00013
Extra alloying [kg] 0,00 0,00 4,37 0,0000 0,0000 0,00 0,14800
Steel analysis after degassing [%] 0,26 0,67 0,01 0,0013 0,0002 0,03 0,00029
Mass difference ('amt after' - 'Amt Before' - 'Alloy addition') [kg] 14,56 31,66 -12,42 0,4288 0,0549 -20,82 0,01964
SiO2 MnO S TiO2 CaO Al2O3 MgO FeO
Slaganalysis before degassing [%] 9,20 1,20 0,19 0,1300 56,60 21,90 6,70 3,90
Weight before degassing [kg] 85,73 11,18 1,77 1,2114 527,42 204,07 62,43 36,34
Extra slag addition [kg] 0,00 0,00 0,00 0,0000 0,00 0,00 0,00 0,00
Slag analysis after degassing [%] 8,70 0,09 1,20 0,1700 56,40 28,30 6,10 0,53
Weight after degassing [kg] 81,36 0,84 11,22 1,5897 527,42 264,65 57,04 4,96
Mass difference in slag [kg] -4,37 -10,34 9,45 0,3784 0,00 60,57 -5,39 -31,39
Equivalent element in slag [kg] -2,04 -8,01 9,45 0,2268 0,00 32,07 -3,25 -24,40
Elemental Sum in slag&steel 12,52 23,65 -2,97 0,6556 0,05 11,25 -3,23
Mass of Top slag = Mass of CaO before degassing / % Cao after degassing = 935,14Kg
Extraslag=Mass of Topslag after degassing - Mass of Topslag before degassing=3,30kg
Final sample Si Mn S Ti Ca Al Mg
Steel analysis after degassing [%] 0,26 0,67 0,005 0,0013 0,0002 0,032 0,00029
Extra alloying [kg] 0,00 0,00 0,09 0,00 0,00 0,00 0,00
Steel analysis during casting [%] 0,26 0,67 0,006 0,0013 0,0003 0,029 0,00032
Mass diff (Final mass - Initial mass[kg]) 0,00 0,00 0,94 0,0000 0,0103 -3,10 0,03102
58
Appendix XI: Model for predicting ao activity for online process using existing process data
0,1
1
10
100
1000
1500 1550 1600 1650 1700 1750
Temp (C)
ao
(p
pm
)
0,002-0,004%Al 0,008-0,019 0,020-0,030 0,031-0,049 0,050-0,069 >0,069
.
Appendix XII: Comparison of Different Models of estimating ao
Calculated Measured
Heats Ohta&suito Al2O3 =1 Data ao
1 2,67E-05 2,32E-04 2,75E-04 2,82E-04
2 6,85E-05 2,94E-04 2,44E-04 4,14E-04
3 3,16E-06 1,64E-04 2,84E-04 2,41E-04
4 1,86E-05 1,28E-04 1,59E-04 1,59E-04
5 3,84E-05 4,05E-04 3,95E-04 5,12E-04
6 4,33E-05 3,00E-04 2,89E-04 3,61E-04
7 1,17E-05 2,16E-04 2,61E-04 2,53E-04 8 2,07E-05 1,90E-04 2,34E-04 3,22E-04
9 1,56E-05 2,29E-04 2,91E-04 3,03E-04
10 4,06E-05 2,58E-04 2,93E-04 3,40E-04
11 1,65E-05 1,46E-04 1,84E-04 2,04E-04
12 3,64E-05 3,26E-04 2,85E-04 3,13E-04
59
Appendix XIII: Modified oxygen activities
Calculated Measured
Heats Ohta&suito Al2O3=1 Data ao
1 2,69E-04 2,97E-04 3,52E-04 2,82E-04
2 5,76E-04 3,78E-04 2,89E-04 4,14E-04
3 9,65E-05 2,36E-04 3,70E-04 2,41E-04
4 2,10E-04 2,52E-04 1,19E-04 1,59E-04
5 3,39E-04 5,21E-04 5,94E-04 5,12E-04
6 3,55E-04 3,85E-04 3,80E-04 3,61E-04
7 1,59E-04 2,76E-04 3,25E-04 2,53E-04 8 2,25E-04 2,43E-04 2,69E-04 3,22E-04
9 1,88E-04 2,94E-04 3,85E-04 3,03E-04
10 3,71E-04 3,30E-04 3,88E-04 3,40E-04
11 1,95E-04 1,86E-04 1,70E-04 2,04E-04
12 3,40E-04 4,18E-04 3,73E-04 3,13E-04
Appendix xiii: Calculated and Measured Equilibrium Sulphur distribution
Calculated Ls Measured
Heats Ohta&suito Al2O3=1 Meas. ao Ls
1 4498 517 426 130
2 671 156 111 48
3 71815 1161 1434 970
4 10917 1030 885 750
5 3890 348 275 121
6 1818 263 218 185
7 14948 810 690 118 8 3424 373 220 186
9 25155 1711 1297 188
10 3207 505 383 240
11 5893 666 478 91
12 2901 324 338 300
60
Appendix xiv: Equilibrium Sulphur content in steel
Calculataed Measured
Heat Ohta&suito Al2O3=1 Meas
ao [%S]f
1 0,005 0,005 0,005 0,010
2 0,020 0,018 0,019 0,023
3 0,001 0,001 0,001 0,001
4 0,001 0,001 0,001 0,001
5 0,009 0,012 0,012 0,016
6 0,014 0,015 0,014 0,013
7 0,002 0,003 0,003 0,010 8 0,005 0,005 0,006 0,006
9 0,001 0,001 0,001 0,004
10 0,004 0,003 0,004 0,005
11 0,003 0,003 0,003 0,008
12 0,006 0,007 0,006 0,007
