Mr. Thomas Wolfgang Howard, Dipl.-Ing.

Abstract:

In consideration of the growing production of stainless steel, which averaged at about 6 %/year between 2012 and 2021 and reached 58.3 Mio t/year in 2021, the accumulation of the corresponding residues such as dust increases as well [1]. During the production of stainless steel via electrical arc furnace (EAF), which is the most commonly applied production route, approximately between 10-20 kg of dust accrue per ton of steel [2]. This dust contains a quite significant amount of Cr and Ni. If no recycling of those dusts is carried out, these metals are lost for further operations and potentially interact in a harmful way with the environment in case of present leachable components, such as Cr^VI+. This would lead to economic loss as well as ecological harm. All of todays in industry applied processes to recover valuable metals from such dusts are of pyrometallurgical nature, which are carbon based and quite energy intensive. 

Hydrometallurgical operations for the recovery of metals from Cr-Ni-rich AOD and EAFD have been examined by Aromaa et all. and Stefanova et all. but with the goal to selectively extract Zn [3, 4]. Therefore, after thoroughly charactering the dust including X‑Ray diffraction (XRD), scanning electron microscope-energy dispersive X‑ray (SEM‑EDX), Elemental analysis with inductively coupled plasma‑optical emission spectroscopy (ICP‑OES) and thermogravimetric analysis (TGA), five different acids (hydrochloric, sulphuric, nitric, vinegar and citric acid) were investigated in their potential to leach Cr and Ni. Out of the five acids used, hydrochloric acid was the most promising candidate to conduct a parameter variation with. In the conducted follow-up experiments, a clear trend of increased extraction rates can be observed for higher temperatures and longer leaching times. To counter specific problems observed in previous experiments, a double walled reaction vessel including a lid with four openings for stirrer, reflux condenser, acid addition and pH‑electrode was used. Through applying these methods, the FeCr₂O₄ and NiFe₂O₄ spinel phases, which contain the metals of interest, were able to be leached in a satisfactory amount. The paper summarizes the results of the characterization in conjunction with the obtained extraction rates and conclusions on the mineralogical phases and their leachabilities under different conditions.