• Vollprecht, Daniel
• Öfner, Wolfgang
• Frisch, Gero
• Sedlazeck, Klaus Philipp
• Mischitz, Robert
• Schopf, Simone
• Schlömann, Michael
• Müller, Peter
• Pomberger, Roland

Abstract

Industrial waste waters often contain dissolved critical metals, which became the focus of recycling over the last years, but are not recovered yet in most industries leading to a loss of these metals. Our research project focuses on an integral system for the recycling of critical metals from industrial waste waters. The fixation of the metals from the waste water is based on fluidized bed reactors (Ferrodecont process), filled with zero valent iron (ZVI). This process was originally invented in a previous research project for the remediation of contaminated sites, i.e., the reduction of hexavalent chromium. The metal-containing water is pumped from the bottom into the reactor which induces a fluidization of the bed and the reaction between the ZVI and the metals yields to reduction-induced precipitation and/or adsorption of the critical metals.The following step after the fixation of the critical metals is solid-liquid separation. To obtain a high quality separation, the effectiveness of magnetic matrix separators and mechanic separation tech-niques (e.g., filtration techniques, gravity driven separation such as decanter and centrifuges) were evaluated. Hence, the goal was a minimization of residual solids in the liquid phase and a low water content in the emerging solid phase. First results revealed that clogging due to the µm-sized particles prevents a proper usage of matrix separators and filtration. However, decanters and centrifuges ob-tained satisfying results. Centrifuges were used in a laboratory scale so far, nevertheless, an upscaled dewatering test series was conducted with a decanter yielding solid contents up to 42 wt% com-pared to solid contents of the input suspension ranging from 0.8 to 1.5 wt%.Chemical analyses of the separated sludge indicated an enrichment of the critical metals in the sludge of partly more than 10 wt%, but electron microprobe analyses could not reveal how the met-als are fixed exactly within the Fe-(oxy-)hydroxide sludge. Anyhow, the Fe-(oxy-)hydroxides and the critical metals have to be separated from each other in order to enable an economically feasible recycling process. This enrichment can be achieved by either applying extracting methods (e.g. selec-tive leaching) or passive enrichment (e.g., microbial induced dissolution of iron through reduction). Selective leaching is performed by using ionic liquids, different acids and bases as well as oxidation and reducing agents. Alternatively, galvanic and solvent extraction are performed. In addition to that, a thermal sample treatment is tested in order to divide critical metals and iron into different oxide phases by dehydroxylation.So far, selective extraction of Mo and W yielded 8 and 6 times higher concentrations compared to the original waste water. Resulting leachates are presently tested for solvent extraction. Alternative-ly, we could show on the lab scale that the majority of Mo and W was leached by using ionic liquids or deep eutectic solvents, whereas Fe remained almost completely in the sludge (Fe(aq) < 1 ppm). The effectiveness of microbially induced dissolution is currently under investigation.

Topics

• impedance spectroscopy
• chromium
• precipitation
• leaching
• iron
• liquid phase
• solvent extraction
• selective leaching