recycling of steelmaking slag from electric arc furnace

recycling of steelmaking slag from electric arc furnace

Archives of Foundry,
Year 2004, Volume 4, № 13
Archiwum Odlewnictwa,
Rok 2004, Rocznik 4, Nr 13
PAN – Katowice PL ISSN 1642-5308
25/13
RECYCLING OF STEELMAKING SLAG
FROM ELECTRIC ARC FURNACE
Ľ MIHOK1 , K.SEILEROVÁ, D.BARICOVÁ
Technical University, Faculty of Metallurgy, Letná 9, Košice, Slovakia
SUMMARY
The paper presents results of research of demetallized EAF slag recycling into
charge of electric arc furnace. Effects of recycling on steelmaking process parameters,
on composition of steel and slag are discussed. Economic and environmental benefits
of proposed technology are presented.
Key words: electric arc furnace; demetallized slag; recycling.
1. INTRODUCTION
In modern apprehension metallurgical smelting slag is not considered as a waste,
but as valuable secondary raw material that can be utilized both in metallurgical
processes and in other industrial applications. While whole amount of blast furnace slag
is utilized in civil engineering, road building and cement industry applications, full
utilization of steelmaking slag forms urgent problem for researchers in metallurgy and
foundry industry. Owing to slag – free tapping, steelmaking slag always contains
a portion of solid metal – raw steel. Utilization of this metallic part in scrap charge is
normal practice. Remaining demetallized slag is utilized mostly in road building
industry, but benefits of demetallized slag recycling into steelmaking furnace are
studied in present time.
Refining process in oxygen converter produces converter slag as a by – product,
that has relatively stabile properties and composition. Different situation is related to
slag produced in process of electric arc furnace steelmaking. Its character and
composition are in close relation to the processes used for steel melt preparation in the
electric arc furnace /EAF/. The use of two – regime smelting with oxidation and
reduction periods result in production of two types of slag. Modern tendencies in EAF
1
prof. ing. D.Sc.; [email protected].
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steelmaking define it as the refining – remelting process followed by the molten steel
treatment in ladle furnace. This process produces slag with only small variations in
composition and properties.
The aim of the work presented in this contribution was to study the possibilities
of EAF slag recycling into the refining – remelting process in EAF and to recognize all
possible positives and negatives of the recycling. Though electric arc furnaces used in
foundries producing steel castings are of relatively small production capacities and
amounts of produced slag are not too large, dumping of steelmaking slag is the worst
alternative.
2. PRESENT STATE OF EAF SLAG TREATMENT
After tapping of steel from EAF the slag is poured into the slag ladle and
transported to the slag yard. After solidification and cooling the slag is crushed in
multi-staged crushing process (from coarse to fine crushing). Each crushing stage is
followed by electromagnetic removal of metallic portion. The metallic material is
returned into EAF charge. It is necessary to note the pieces of iron metal contain
different portions of slag not removed by crushing.
Demetallized slag, named “artificial stone”, Fig.1, is sold mainly to road building
companies. By this way only part of the demetallized slag is utilized, most of it is
dumped on specialized dumping site. Typical analysis of the demetallized slag from
EAF (in wt%): 19.11% FeO; 15.87% Fe 2 O3 ; 12.16% SiO2 ; 35.87% CaO; 3.72% MgO;
3.46% Al2 O3 ; 6.82% MnO; 0.760% P2 O5 ; 0.149% S. The structure of slag consists of
di- and tricalcium silicates together with phosphides, oxidic RO – phase and calcium
ferrites together with calcium sulphide.
In contrast to oxygen converter slag the slag from EAF has very little contents of
free CaO. No problems with spontaneous decomposition of slag pieces, caused by lime
hydratation, are observed.
3. RESEARCH OF DEMETALLIZED SLAG RECYCLING
The trials with demetallized EAF slag recycling were evoked by successful
research on oxygen converter slag recycling when recycling of up to 3 t of demetallized
slag into charge of 170 t converter had beneficial effects on parameters of steel refining,
that followed from early slag formation in the process. Recycling of slag into oxygen
converter was established as a daily practice (1, 2).
The trials with recycling of demetallized EAF slag (3, 4) were performed in two
parts. In the first part 200 kg and 300 kg of demetallized slag were added to EAF
charge. EAF smelting capacity was 60 t of liquid steel. The fraction 0 to 8 mm of
demetallized slag was used. Eight heats were analyzed: heats A, B were produced
without demetallized slag additions; heats C, D, E were produced with 200 kg of
demetallized slag additions; heats F, G, H with 300 kg of demetallized slag additions.
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Chemical and microscopic analysis of steel and slag samples taken during tapping, were
performed.
Fig. 1.
Rys. 1.
Demetallized slag in slag yard.
Składowisko żużla pozbawionego części metalowych.
Concerning steel composition, main attention was paid to phosphorus and
sulphur contents, as the demetallized slag had 0.760% P 2 O5 and 0.149% S. No increase
of phosphorus and sulphur contents was found. Similarly, no influence of demetallized
slag additions on contents and composition of non – metallic inclusions in steel was
found.
Composition of the slags taken in the first part of the trials is in Table 1. It can be
seen from the table, that reference heats produced without demetallized slag additions
have higher contents of Al2 O3 , SiO2 , P2 O5 and lower contents of FeO. Additions of
demetallized slag didn´t increase the phosphorus and sulph ur contents nor in steel nor in
slag. Structural composition of slag was the same as the one found in EAF heats slags
produced without demetallized slag additions.
In the second part of the trials bigger additions of demetallized slag into EAF
charge were examined. In addition to it, quantity of charged lime was proportionaly
decreased. List of heats analyzed in the second part is in Table 2. Also in this part the
samples of steel and slag, taken during tapping, were analyzed.
Table 3 shows phosphorus and s ulphur contents in steel from all six analyzed
heats. No increase in the contents was recorded, even in the heats with maximum
decrease of charged lime quantity. It was obvious lime was compensated by basic
constituents in added demetallized slag. Utilization of part of demetallized slag by its
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recycling into EAF charge and savings in lime consumptions are the most valuable
results of investigated technology.
Table 1. Chemical composition of slag samples, %
Tabela 1. Skład chemiczny próbek żużla, %
Heat
MgO
Al2 O3
SiO2
P2 O5
no.
A
B
C
D
E
F
G
H
Table 2.
Tabela 2.
3.412
2.813
2.195
2.110
3.045
1.990
2.483
1.953
2.985
2.026
1.897
1.612
0.961
1.286
1.085
1.576
12.91
11.67
10.63
9.63
6.33
7.40
7.00
9.55
0.9547
0,7719
0.6481
0.5217
0.3803
0.4758
0.4129
0.5306
S
0.1658
0.1710
0.1817
0.1678
0.2194
0.1993
0.1788
0.1648
CaO
29.98
29.44
29.97
25.47
21.62
25.95
22.01
25.24
MnO
7.715
7.664
7.919
7.882
5.196
6.913
5.916
7.844
FeO
33.35
35.37
36.89
42.50
50.10
44.31
49.07
42.42
List of analyzed heats, part two
Wytopy przeprowadzone w drugiej części prób
Heat no
I
J
K
L
M
N
Demetallized slag
addition, kg
200
200
400
400
800
800
Charged lime decrease, kg
100
100
200
200
400
400
Table 3. Phosphorus and sulphur contents (wt %) in heats, part two
Tabela 3. Zawartość fosforu i siarki (% wag.) w wytopach przeprowadzonych w części drugiej
prób.
Heat no
I
J
K
L
M
N
P
0.010
0.009
0.005
0.005
0.005
0.005
S
0.026
0.044
0.032
0.034
0.031
0.039
To confirm the results of experiments, statistic evaluation of 378 EAF heats was
performed, 116 of them were produced without demetallized slag additions, 262 with
demetallized slag additions. Basicity of slag, contents of phosphorus and sulphur in
steel were evaluated. The results of statistic evaluation are in Table 4.
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As can be seen from the table, additions of demetallized slag increased basicity
values, the sulphur contents were not influenced, the phosphorus contents decreased.
Table 4. Statistic evaluation of 378 EAF heats
Tabela 4. Statystyczna ocena 378 wytopów w elektrycznym piecu łukowym
Mean value
Without demetallized slag additions
Basicity of slag
P content in steel, wt %
S content in steel, wt %
With demetallized slag additions
Basicity of slag
P content in steel, wt %
S content in steel, wt %
1.16265
0.006965
0.035663
1.3295
0.004642
0.036666
4. CONCLUS IONS
The aim of presented work was investigation of possibilities for recycling of
electric arc furnace demetallized slag into electric arc furnace charge. The results are as
follow:
1. Industrial experiments with recycling of demetallized EAF slag into EAF charge
showed the additions of demetallized slag had no negative effects on smelting
process parameters, on composition and cleanness of produced steel.
2. Additions of demetallized slag enable to save part of charged lime.
3. Decrease of charged lime amounts in ratio 1 (lime) : 2 (demetallized slag) has no
negative effects on de-phosphorization process in EAF.
4. On the basis of experiments and statistic evaluation recycling of 800 to 1000 kg
of demetallized slag with corresponding decrease of 400 to 500 kg of lime is
recommended for EAF with 60 t capacity.
5. Important environmental and economic effects result from recommended
technology.
REFERENCES
[1] Ľ. Mihok, D. Fedičová, J. Hric: Increase of recycling ratio of demetallized steelmaking slag by addition to oxygen converter charge. Acta Metallurgica Slovaca 5,
Special Issue 3/1999, pp. 214 – 220.
[2] Ľ. Mihok, D. Fedičová: Recycling of demetallized steelmaking slag into charge
of basic oxygen converter. Metalurgija 39, 2000, pp. 93 – 99.
[3] K. Seilerová, Ľ. Mihok, D. Baricová, M. Domovec, K. Balco: Recycling of demetallized slag from electric arc furnace. Hutnícke listy 4-5, 2000, pp. 30 – 33.
[4] K. Seilerová: Influencing of slag regime in EAF – LF system. Dissertation Theses,
Faculty of Metallurgy, TU, Košice 2004.
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RECYKLING ŻUŻLI Z PROCESU WYTAPIANIA STALI
W ELEKTRYCZNYCH PIECACH ŁUKOWYCH
STRESZCZENIE
W artykule przedstawiono wyniki badań nad recyklingiem żu żli stalowniczych
w elektrycznym piecu łukowym. Przedyskutowano wpływ tego procesu na parametry
wytapianej stali, jej skład i ilość powstającego żużla. Omówiono korzyści ekonomiczne
i środowiskowe tego procesu.
Recenzent: prof. dr hab. Mariusz Holtzer.