Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
93
OPTIMALIZATION SLAG COMPOSITION IN LADLE FURNACE CONSIDERING
TO EFFECTIVE STEEL DESULFURIZATION
Buľko B.1, Kijac J.1, Domovec M.2
1
Department of Ferrous Metallurgy and Foundry, Faculty of Metallurgy, Technical University
Košice, Slovak republic
2
Železiarne Podbrezová, a.s., Slovak republic
OPTIMALIZÁCIA ZLOŽENIA PANVOVEJ TROSKY VZHĽADOM
K EFEKTÍVNEMU ODSÍRENIU OCELE
Buľko B.1, Kijac J.1, Domovec M.2
1
Katedra metalurgie železa a zlievarenstva, Hutnícka fakulta, Technická univerzita Košice,
Slovenská republika
2
Železiarne Podbrezová, a.s., Slovenská republika
Abstrakt
Svetový trend výroby ocele smeruje k ekonomickej výrobe ocele s neustále rastúcimi
nárokmi na kvalitu produkovanej ocele, čo spôsobuje zvýšenie podielu výroby mikrolegovaných
ocelí na úkor výroby ocelí bežných akostí [1]. Zároveň vychádzajú do popredia ekologické
aspekty spojené s výrobou ocele. Tieto náročné požiadavky je možné plniť len za pomoci
správne fungujúceho troskového systému v ktoromkoľvek agregáte. Voľbou vhodného
troskového režimu je možné nielen zvýšiť čistotu ocele, znížiť náklady spojené s jej výrobou,
ale i znížiť množstvo trosky vznikajúcej na 1 tonu vyrobenej ocele. Modifikáciou zloženia
trosiek je možné meniť aj ich fyzikálne vlastnosti, vplývajúce najmä na priebeh reakcií na
rozhraní troska - tekutý kov, opotrebenie výmurovky, ale aj na možnosti ich ďalšieho
spracovania za účelom zníženia množstva haldovaných trosiek [2]. Odsírenie ocele v panvovej
peci závisí na teplote, aktivite kyslíka v troske a oceli, ale najmä na chemickom zložení
a fyzikálnych vlastnostiach trosky. Nevyhnutnou požiadavkou pre efektívne odsírenie ocele je
minimálny obsah ľahko redukovateľných oxidov v troske. Existuje veľa vzťahov popisujúcich
odsírovacie schopnosti trosky medzi ktorými existujú aj funkčné závislosti. V tomto článku sú
prezentované grafické závislosti medzi jednotlivými parametrami pre približne 229 tavieb
vybranej ocele. Na základe uvedeného sú zostavené parametre pre optimálne pracujúci troskový
systém v panve. Jeden z významných faktorov je obsah MnO v troske. Napriek vyššiemu
rozptylu získaných parametrov je rozdeľovací koeficient síry (Ls) jedným z hlavných
ukazovateľov ktorý úzko súvisí s optickou bazicitou trosky.
Abstract
The world trend of steelmaking forwards to economical steel production with
increasing demands on quality of produced steel. This causes increasing production of
microalloyed steel at the expense of production of conventional steel [1]. At the same time
outgoing in forefronts ecological aspects related with steelmaking. These serious demands can
be possible fill only with the aid of properly functioning slag system in the every metallurgical
device. Choice convenient slag system it can be possible not only increase purity of steel, lower
costs arising with its production, but reduce amount of slag on 1 ton made steel [2].
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Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
Desulphurization of steel at ladle furnace depends on temperature, amount of oxygen and
sulphur in the steel, but mainly on chemical composition and physical properties of slag.
Necessary requirement for effective desulphurization is also minimum amount of easy reducible
oxides in the slag. There are many correlations for expression of slag desulphurization
capability, where their functional dependency on each other can be found. This paper present
graphical correlation between individual parameters using 3-D surface graphs from set of
approximately 229 heats and based on these, the optimal parameters for slag desulphurization
capability are expressed. One of the most significant parameters is content of MnO in slag.
Despite the higher scatter of obtained values, the distribution coefficient of sulphur (Ls) is the
one of wide range of parameters where exist the close dependence on optical basicity.
Key words: steel desulfurization, ladle furnace, sulphur distribution coefficient
Introduction
Correct and optimal managing of slag mode in ladle furnace allows effective
production of steel with required chemical composition and properties. During steel
desulphurizing in ladle furnace is very significant influence of MnO in slag on desulphurizing
capability of slag [3]. Analysis was based on parameters from electric steel plant where steel
produced by electric arc furnace was in next step utilized in ladle furnace and finally casted on
continuous casting machine.
Experimental methods and used materials
For interpretation of MnO influence on slag desulfurization capabilities was chosen
steel with basic chemical composition shown in Table 1.
Table 1 Basic composition of investigated steel
Element
C
Mn
Si
Min. content [%]
0.17
1.15
0.15
Max. content [%]
0.2
1.3
0.35
P
S
0.025
0.02
Al
0.02
0.03
Slag was investigated by many parameters and their mutual relations.
Based on molecular theory is basicity expressed by ratio of basic to acid oxides B1 or
B2 (1) [4]:
B1 =
(CaO )
(SiO 2 )
or
B2 =
(CaO ) + (MgO )
(SiO 2 ) + ( Al 2 O3 )
(1)
Other criterion of examination of slag is sulphidical factor SF, also known as
Mannesmann’s coefficient (2):
SF =
(CaO ) : ( Al O )
(SiO2 ) 2 3
(2)
In polycomponent system is optical basicity OB specified by formula [5] (3):
n
OB = ∑ λi xi
i =1
(3)
Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
Where:
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xi is ionic fraction of element “i“
λi is optical basicity of given oxide
For complex and effective evaluation properties of slag, can be used also other criteria
as (4), (5), and (6) [6]:
Summary of easily reducible oxides SO:
SO= (FeO)+(Fe2O3)+ (MnO)+(Cr2O3)+ (V2O5)+(P2O5)
(4)
Desulphurizing potential DP of slag based on sulphur distribution coefficient:
∑ (S ) *
DP =
S
1
3
Al
(5)
2
Calcium – aluminate ratio CA:
CA =
(CaO )
( Al 2O3 )
(6)
For practical calculations can be used sulphidical capacity Cs depending on
temperature and optical basicity of slag:
log C s =
22690 − 54640 * OB
+ 43,6 * OB − 25,2
T
(7)
A very useful criterion is desulphurisation level R:
R=
S start − S end
S start
(8)
For valuation refining possibilities of slag is useful to use most of shown here
criteria, because between many of them exist functional relations. For given parameters also
exist interval of optimal values, where refining capabilities of slag is on high level.
Results and their analysis
In Table 2 is shown average chemical composition of slag from 229 analysed charges
and slag parameters based on mentioned criteria. In parenthesis is shown average optimal
parameters of selected 16 charges with the best desulphurizing.
Table 2 Statistical analysis samples of slag from ladle furnace for examined steel grade
Value
Element of slag
Average content [%]
Slag parameter
[-]
CaO
59.64 (60.19)
0.61(0.77)
R
Al2O3
20.19 (20)
0.83 (0.83)
OB
MgO
8.23 (8.45)
2.97 (3.03)
CA
MnO
0.68 (0.33)
26.18 (37.48)
Ls
FeO
0.81 (0.70)
1.53 (1.09) [%]
SO
SiO2
10.38 (9.99)
5.99 (6.17)
B1
(S) slag
0.31 (0.35)
2.23 (2.30)
B2
2.88 (4.32)
DP
0.06 (0.07)
Cs
0.297 (0.31)
SF
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Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
The highest slag sulphidical capacity cs was gained at higher temperatures of steel and
higher value of basicity B2 as shown in Figure 1.
Area with highest
sulphidical capacity
Fig.1 Relation of slag sulphidical capacity on steel temperature and slag basicity B2
In the Figure 2. and Figure 3. is shown relation of sulphur distribution coefficient on
summary of easily reducible oxides and on MnO content in slag.
70
R = -0,4820
60
50
40
Ls
30
20
10
0
0
1
2
3
4
5
6
SO [%]
Fig.2 Relation of sulphur distribution coefficient on
summary of easily reducible oxides SO
Fig.3 Relation of sulphur distribution coefficient
on MnO content in slag
Based on shown figures and calculated relations was formatted the order of individual
factors effect on sulphur distribution coefficient Ls, Table 3. Most important is influence of MnO
in slag with value R2=32.41%, which confirms Figure 4.
One of the problems with desulphurising of steel comes from electric arc furnace in
the ladle furnace is higher spread of incoming sulphur, as shown in Figure 4. This spread is
caused by unstable sulphur content in steel scrap and it is necessary to eliminate it with higher
amount of slag in ladle furnace, aimed to specified sulphur content in final product [7].
Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
97
Table 3 The order of individual factors effect on sulphur distribution coefficient Ls
Order
Factor
R2 [%]
1
MnO
32.41
2
Al2O3
7.15
3
P2O5
5.24
4
TiO2
4.71
5
SiO2
3.4
6
Cr2O3
2.79
7
FeO
2.47
8
Temperature
2.15
9
MgO
0.09
Fig.4 Histogram of incoming sulphur content in the ladle furnace
The first step to solve this problem is selection of steel scrap according to lowest
spread of sulphur content. In the mass production of steel it is probably most difficult practically
realizable step in terms of technological and economical aspects.
The next important step is providing primary steel desulphurizing in the electric arc
furnace, what is determined by technology and slag mode. In as much as today prevalent
technology of modern electric arc furnace with only oxidation cycle is not effectively of steel
desulphurizing on sufficient level according to oxygen activity in steel.
Required sulphur content in high deoxygenated steel is reached by ladle furnace slag.
In respect of wider spread of incoming sulphur content and unstable penetration of electric arc
furnace slag into the ladle slag is necessary to calculate with some anomalies from optimal slag
chemical composition for given steel, however it is possible to adjust slag chemical composition
near to optimal values. Significant influence for steel desulphurization has content of easily
reducible oxides in slag.
The one of easily reducible oxides is MnO, which has interesting influence on sulphur
distribution coefficient. MnO makes itself felt as indicator of steel and slag oxygenation [8].
Also between MnO and FeO content in slag exists dependence. In the Figure 5. is
shown distribution of manganese between steel and slag as a function of FeO content in slag.
Distribution coefficient of manganese is increasing with FeO content in slag. When increasing
content of FeO in slag, declines manganese content in the steel.
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Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
Fig.5 Distribution of Mn between slag and steel as the
function of FeO content in the steel
Fig.6 Distribution of Mn between slag and steel as
the function of the sulphur distribution
coefficient
Function of manganese distribution between slag and steel can be presented as a
significant controller of steel oxygenation. This can be shown as a function between MnO/Mn
and sulphur distribution coefficient in the Figure 6. High influence of manganese distribution is
in the ratio (MnO)/[Mn]<1.
Conclusion
Based on shown data and utilizing statistic methods was formatted optimal values of
slag parameters for given steel, according to the best desulphurization in the ladle furnace at
Table 4. For optimal steel desulphurizing is very important to control the content of the easily
reducible oxides in slag, mainly content of MnO which must be less than 0.8%. Also was found
optimal Calcium-aluminate ratio 3:1 and Optical basicity of slag in range 0.83 – 0.86. It can be
very complicated to respect this parameters in practical steelmaking, but it is possible to
approach them.
Table 4 The optimal values of desulphurizing slag for given steel
Parameter
Basicity B1
Basicity B2
Sulphidical factor SF
Calcium–aluminate ratio CA
Summary of easily reducible oxides SO
Sulphur distribution coefficient Ls
Sulphidical capacity Cs
MnO content
Desulphuration potential DP
Desulphurization level R
Optical basicity OB
Value
6.17
2.3
0.31
3.03
< 2%
38
0.07
<0.8%
4.32
0.77
0.83 – 0.86
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Acta Metallurgica Slovaca, 15, 2009, 2 (93 - 99)
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