25/13 Archives of Foundry, Year 2004, Volume 4, № 13 Archiwum Odlewnictwa,

Rok 2004, Rocznik 4, Nr 13 PAN – Katowice PL ISSN 1642-5308

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.; Lubomir.Mihok@tuke.sk.

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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% Fe2O3; 12.16% SiO2; 35.87% CaO; 3.72% MgO;

3.46% Al2O3; 6.82% MnO; 0.760% P2O5; 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. Demetallized slag in slag yard.

Rys. 1. Składowisko żużla pozbawionego części metalowych.

Concerning steel composition, main attention was paid to phosphorus and

sulphur contents, as the demetallized slag had 0.760% P2O5 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 Al2O3, SiO2, P2O5 and lower contents of FeO. Additions of

demetallized slag didn´t increase the phosphorus and sulphur 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 sulphur 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

no. MgO Al2O3 SiO2 P2O5 S CaO MnO FeO

A 3.412 2.985 12.91 0.9547 0.1658 29.98 7.715 33.35

B 2.813 2.026 11.67 0,7719 0.1710 29.44 7.664 35.37

C 2.195 1.897 10.63 0.6481 0.1817 29.97 7.919 36.89

D 2.110 1.612 9.63 0.5217 0.1678 25.47 7.882 42.50

E 3.045 0.961 6.33 0.3803 0.2194 21.62 5.196 50.10

F 1.990 1.286 7.40 0.4758 0.1993 25.95 6.913 44.31

G 2.483 1.085 7.00 0.4129 0.1788 22.01 5.916 49.07

H 1.953 1.576 9.55 0.5306 0.1648 25.24 7.844 42.42

Table 2. List of analyzed heats, part two Tabela 2. Wytopy przeprowadzone w drugiej części prób

Heat no Demetallized slag

addition, kg Charged lime decrease, kg

I 200 100

J 200 100

K 400 200

L 400 200

M 800 400

N 800 400

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 P S

I 0.010 0.026

J 0.009 0.044

K 0.005 0.032

L 0.005 0.034

M 0.005 0.031

N 0.005 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 1.16265

P content in steel, wt % 0.006965

S content in steel, wt % 0.035663

With demetallized slag additions

Basicity of slag 1.3295

P content in steel, wt % 0.004642

S content in steel, wt % 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 steel-

making 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 deme-

tallized 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 żużli 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.