Browsing by Author "Aromaa, Jari, Dr., Aalto University, Finland"
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
- Application of High-Speed Silver Electrorefining
School of Chemical Technology | Doctoral dissertation (article-based)(2021) Aji, Arif TirtoThe main focus of the current study was on the investigation of high current density (HCD) operation of silver electrorefining through empirical modelling of the process. The first part of modelling consists of the phenomena on the anode surface i.e. silver dissolution, passivation due to gold, and copper dissolution. Second part of modelling was optimization of the process by modelling of the electrolyte properties i.e. conductivity, density, viscosity and electrolyte circulation system. All of the modelling was conducted based on the experimental results of laboratory-scale measurements in synthetic electrolyte, under conditions similar to industrial operation. Kinetic modelling of silver dissolution in the current study suggests that the application of HCD is technically feasible. Based on the experimental results, the main cause for silver anode passivation is the gold content in the anode, since gold did not dissolve in the dilute nitric acid solution used in electrolysis. Meanwhile, copper present in the anode dissolved during the process and accumulated in the silver electrolyte. Though copper increased the conductivity of the electrolyte, the high copper content also resulted in copper contamination of the silver cathode. By modelling the kinetics of the dissolution of anode metals, limitations for copper and gold content could be established for optimum HCD operation. Electrolyte has two main roles in the electrorefining process; it is the medium for current transfer as well as the inventory and supplier of silver ions. Accordingly, optimization of the electrolyte composition allows the minimization of energy consumption while maintaining the purity of the deposit. In the current work, the optimal electrolyte conditions and circulation system were established as a function of a HCD operation. The optimal anode and electrolyte conditions in HCD silver electrorefining were found to be at max. Au of 6-8% in the anode and 100-150 g/dm3 [Ag+], 50-75 g/dm3 [Cu2+], 5-7 g/dm3 [HNO3] in the electrolyte. - Innovative Valorization of Secondary Raw Materials
School of Chemical Technology | Doctoral dissertation (article-based)(2020) Halli, PetteriIn 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.