Innovative Valorization of Secondary Raw Materials

In order to answer the ever-growing demand for more sustainable metal resources, a number of new and distinct approaches for the treatment of different secondary raw materials were investigated in detail. These included recovery of metals from electric arc furnace dust (EAFD), recovery of tellurium from Doré slag, and recovery of noble metals from base metal sulfate solutions. In the first method, the effect of 27 different lixiviants on the dissolution of electric arc furnace dust (about 12 Mt produced globally every year) was explored. From these results, citric acid was selected as the most suitable medium to produce three base metal fractions—Fe-, Pb- and Zn-rich streams—from the EAFD containing 33.2 wt-% Zn, 17.9 wt-% Fe and 1.6 wt-% Pb. The complete process developed consisted of several metallurgical unit processes including an initial alkaline roasting stage with NaOH at 450 °C, followed by selective leaching with 0.8 M citric acid for 120 min. The residue formed after leaching was shown to be a chemically suitable raw material for reuse in the EAF, whereas the related pregnant leach solution (PLS) could be further purified to produce a Pb-rich residue, and a Zn-rich electrolyte appropriate for use in state-of-the-art metallurgical plants. A new, innovative process flowchart for industrial application was proposed as a result of these findings. The recovery of tellurium from Doré slag produced in a TROF (Tilting, Rotating Oxy Fuel) furnace was also demonstrated via a combined conventional hydrometallurgical and innovative electrochemical route. The Doré slag was first leached (30% aqua regia) to produce a multimetal solution that contained 421 ppm of Te. From the experiments undertaken it was determined that electrowinning (EW) is the preferable Te recovery method at concentrations above 300 ppm, whereas below this threshold value, an innovative method based on electrodeposition redox replacement (EDRR) was demonstrated to be more effective. EDRR was also investigated for Ag recovery from synthetic zinc sulfate solutions (Zn 60 g/L) where the content of Ag varied between 1 ppb to 250 ppm. The investigations show that an exceptionally high enrichment ratio of Ag (9.86) from solution to the electrode could be achieved. Furthermore, although H2 evolution was also shown to affect overall energy efficiency, EDRR was shown to outperform conventional EW. In addition, Pt—present as a trace amount (~1 ppb) in complex multimetal Ni-rich (>140 g/L) industrial process solution—was also successfully recovered by the EDRR method on a novel electrode comprised of pyrolysed carbon, PyC. In this case, the results indicated that high purity (90 wt-% Pt) and extraordinary enrichment ratio (1011) were detected on the electrode surface compared to the industrial process solution. Moreover, it was also found that other precious metals like Pd and Ag could be simultaneously enriched on the PyC electrode surface.