RECYCLING OF STEELMAKING SLAG FROM ELECTRIC ARC FURNACE

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1 25/13 Archives of Foundry, Year 2004, Volume 4, 13 Archiwum Odlewnictwa, Rok 2004, Rocznik 4, Nr 13 PAN Katowice PL ISSN RECYCLING OF STEELMAKING SLAG FROM ELECTRIC ARC FURNACE Ľ MIHOK 1, 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.

2 166 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 O 3 ; 12.16% SiO 2 ; 35.87% CaO; 3.72% MgO; 3.46% Al 2 O 3 ; 6.82% MnO; 0.760% P 2 O 5 ; 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.

3 167 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 O 5 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 Al 2 O 3, SiO 2, P 2 O 5 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

4 168 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 Al 2 O 3 SiO 2 P 2 O 5 S CaO MnO FeO A B , C D E F G H Table 2. Tabela 2. List of analyzed heats, part two Wytopy przeprowadzone w drugiej części prób Heat no Demetallized slag addition, kg Charged lime decrease, kg I J K L M N 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 J K L M N 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.

5 169 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 % 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 [2] Ľ. Mihok, D. Fedičová: Recycling of demetallized steelmaking slag into charge of basic oxygen converter. Metalurgija 39, 2000, pp [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 [4] K. Seilerová: Influencing of slag regime in EAF LF system. Dissertation Theses, Faculty of Metallurgy, TU, Košice 2004.

6 170 STRESZCZENIE RECYKLING ŻUŻLI Z PROCESU WYTAPIANIA STALI W ELEKTRYCZNYCH PIECACH ŁUKOWYCH 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.