SCIENCE: Leaching properties of electric arc furnace dust prior/following alkaline extraction

Article in Journal of Environmental Science and Health Part A (2007) 42, 323-329 by Visnja Orescanin, Ruder Boskovic and others, Institute, Laboratory for Radioecology, Bijenicka cesta, Zagreb, Croatia.

Extracts

This study was carried out to determine the appropriate treatment of electric arc furnace (EAF) dust prior to permanent disposal. The total heavy metal content as well as heavy metal leaching from EAF dust was investigated in five composite samples obtained from three Croatian and Slovenian steelworks. ... The results of the study indicate that permanent disposal of EAF dust require the following procedure: alkaline digestion (followed by leachate purification and alkaline zinc electrolyses), chromate reduction (if necessary), solidification of leaching residue and its testing using the leaching analyses. ...

Electric arc furnace (EAF) dust is a major by-product hazardous waste generated by the secondary steelmaking industry. It is a complex, fine-grained, high-density material. The composition of EAF dust is directly associated with the chemistry of the metallic charges used in process. Phase analysis showed that EAF dust was composed mostly of metal oxides, silicates and sulfates.[1,2] According to its chemical composition and leaching properties, the material was classified as hazardous waste which requires special treatment prior to permanent disposal. Generally, there are two main ways of the handling of this sort of waste material. The first is oriented on the conversion of EAF dust into a non-hazardous waste by the solidification/stabilization procedure, which results in reduced heavy metal leaching. For that purpose, zeolitised ashes as well as the original coal fly ash were used.[3,4] Additionally, leachability/toxicity of EAF dust was also decreased by embedding it into the mixture for the production of concrete[5] and Portland cement.[6] The secondway of the handling of EAF dust is oriented toward the recycling of its valuable constituents like zinc and lead. For that purpose, various technologies were developed. They include magnetic and mechanical separation of EAF dust particles,[7] alkaline leaching withNaOH[8,9] and alkaline leaching with NaOH followed by fusion leaching residues with caustic soda,[10,11] microwave assisted caustic leaching[12] and high temperature metal recovery.[13] Acid leaching method gave also promising results.[14] ...

Results and discussion

Major components of electric arc furnace (EAF) dust (Table 1) were iron (ranging from 210.1 to 397.5 g/kg), zinc (ranging from 57.7 to 218.0 g/kg), manganese (ranging from 25.3 to 63.9 g/kg), calcium (ranging from 27.00 to 33.00 mg/kg) and lead (ranging from 5.4 to 16.4 g/kg). From an inspection of these values it is evident that all EAF dust samples highly differ among each other in elemental concentrations as well as in pH values. Generally, samples from Slovenian steelworks (SL-1 and SL-2) had almost two times lower iron concentrations, significantly higher pH values and higher content of zinc, lead, nickel, total chromium and chromium (VI) compared to Croatian steelworks (SK- 1, SK-2 and ST). Concentrations of Ni and Cr (VI) were negligible in the samples SK-1, SK-2 and ST. Significant differences in the composition of all five EAF dust samples are connected with the composition of steel scrap material used in steel production, the type and proportion of nonmetal additive, the type and proportion of ferroalloys and technological parameters as well as techniques and dynamic of slag separation. ...

Conclusion

Electric arc furnace (EAF) dust in its original form presents in the same time a hazardous waste material as well as a valuable secondary raw material for the production of zinc. Due to its origin (a by-product of steel production from the steel scrap) this material significantly varied in its composition.

High concentrations of zinc and lead were determined in the TCLP extracts of initial EAF dust samples, having maximum values 204 mg/L and 42.2 mg/L respectively. Zn values were more than 15-fold lower and Pb values more than 200-fold lower following alkaline digestion of EAF dust in comparison with the TCLP extracts of initial EAF dust. As regards the DIN 38414-S4 extracts of either initial or alkaline digested dust, only Cr( VI) exceeded permissible level, and its reduction to Cr(III) using FeSO4·7H2O is required prior to permanent disposal.NaOH-extractable zinc ranged from 50.3% to 73.2%. Removal efficiency largely depended on the zincite/franklinite ratio.

It could be concluded that EAF dust should be treated prior to permanent disposal according to the following procedure: alkaline digestion (followed by leachate purification and alkaline zinc electrolyses), chromate reduction (if necessary), solidification of leaching residue and its testing using appropriate leaching procedures (TCLP, SPLP, DIN 38414-S4).