Removal of critical metals from waste water by zero-valent iron

Critical metals, defined as metals with growing economic importance that might be susceptible to future scarcity, are present in industrial waste waters. However, they are mostly not recovered in standard chemical-physical waste water treatment as they often have no negative impact on the environment. In this study, an adaption of the well-known technologies using zero-valent iron (ZVI) which have recently been improved by using a fluidized bed reactor (Ferrodecont process) to recover these metals from synthetic aqueous solutions and real industrial waste water samples is presented. Batch experiments with synthetic aqueous solutions (V =1 L, various pH depending on used compounds) showed that more than 99% of many dissolved critical metals (c(0) = 50 mg/L, e.g. tantalum (Ta), neodymium (Nd), gallium (Ga), indium (In)) could be removed yielding residual concentrations of mostly < 50 mu g/L and metal concentrates (about 4 g) with approximately 1 wt% critical metals bound to an iron hydroxide matrix. A solid to liquid weight ratio of 1:1 was found to be ideal for metal removal, whereas the optimum pH was dependent on the intended critical metal to be removed due to their specific speciation. Batch experiments with six industrial waste waters generally confirmed the results from experiments with synthetic solutions, as again about 99% of In, copper (Cu), molybdenum (Mo), tungsten (W) and chromium (Cr) could be removed from some samples. Pilot-scale tests in a fluidized bed reactor (FBR) mostly confirmed the results of batch experiments, as more than 90% of Cr, vanadium (V), In, cobalt (Co) and nickel (Ni) were removed. X-ray diffraction (XRD), Raman spectroscopy and electron microprobe analyses (EMPA) of metal concentrates showed that critical metals are mainly bound to lepidocrocite (gamma-FeOOH) due to the specific operational conditions and to specific mineral phases which are characteristic for a particular industrial waste water such as an In-Mo-S compound. (C) 2018 Elsevier Ltd. All rights reserved.