Stainless steel production generates significant quantities of flue dusts that are considered as hazardous waste and are normally treated with pyrometallurgical methods in separate treatment plants. These dusts also contain valuable metals but the extraction and recovery of the valuables is difficult due their complex composition. Especially zinc content of the dust causes troubles in treating the dusts. In this thesis a hydrometallurgical method based on NaOH leaching was investigated as considered to be suitable for on-site treatment. The aim was to selectively remove zinc so that the remaining (Ni, Cr, Fe) oxide material could be recycled back to melting. The studied dust samples included argon-oxygen decarburization (AOD1 and AOD2) converter dusts and electric arc furnace (EAF1 and EAF2) dusts from the two production lines of Outokumpu Stainless Tornio Works. The effect of five different leaching factors (NaOH concentration, temperature, liquid-to-solid ratio, oxygen/nitrogen gas bubbling and agitation) on zinc dissolution was investigated using fractional factorial designs of the experiments. NaOH concentration, temperature and agitation affected zinc extraction predominantly and the greatest zinc extraction was achieved with high level of those three factors. The maximum amount of extracted zinc was: 80 % from AOD1, 50 % from AOD2, 60 % for EAF1 and 30 % for EAF2. Difference in zinc extraction between the dusts arose from the mineralogical differences. In all dusts zinc occurred both as easily soluble zinc oxide and as poorly- soluble ferrite form. This study showed that alkaline leaching of zinc from stainless steel AOD and EAF dusts using NaOH solutions is selective and practically no iron, chromium and nickel were dissolved. However, the dissolution of zinc is limited due to the presence of zinc ferrites. The leaching process can work in zinc removal if zinc ferrites are decomposed prior to leaching, for example by roasting.
|Publication status||Published - 2015|
|MoE publication type||G3 Licentiate thesis|
- NaOH leaching
- Flue dust
- Stainless steel