Extraction of zinc from scrap steel using zinc-bromine battery technology
Steelmaking will likely not reach its carbon neutrality by 2050 without technical innovations. The steel industry struggles with scrap recycling because of zinc content within scrap needing to be of a suitable level (16 wt%) to be economically viable. Current steel recycling methods (pyrometallurgy and hydrometallurgy) are energy intensive or only pass the problem of recycling onto a separate waste stream. This research investigates the extraction and recovery of zinc from scrap steel using zinc-bromine battery (ZBB) technology. Zinc-bromine flow batteries are a promising technology for energy storage due to their low cost, with coulombic efficiencies of 95 % and energy efficiencies of 75 %. In this research, a static membrane-free ZBB was developed, characterised and optimised for zinc recovery from steel. The electrochemical performance of the cell was characterised using charge-discharge profiles, OCP measurements were also used to characterise the cell. The topography and surface elemental composition of electrode materials were characterised ex-situ by scanning-electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The ZBB cell was firstly developed, characterised and optimised using conventional ZBB materials in order to optimise the design and operation of the cell prior to the incorporation of steel substrates. The cell was designed with an open top, this allowed steel substrates to be dipped into the electrolyte solution without the need to disassemble the cell. The highest performance of the cell was determined to be a 2 M ZnBr2 electrolyte solution with no supporting electrolyte, with a 6 mm thick hydrophobic carbon felt cathode and a 4.65 mm thick hydrophilic carbon felt anode. The area of the electrodes was 7.07 cm2. The charging current and discharging current was determined to be ±100 mA (14.14 mAcm-2). This was achieved by changing controllable variables and directly comparing results against one another. The work demonstrated the feasibility of a lab-scale ZBB that would allow for zinc recovery from steel substrates, this research formed the basis for all experiments involving steel going forward. Galvanized steel as a substituted anode was also investigated. The use of galvanized steel shows the importance of the cut-off voltage upon discharging: if less than 0.5 V, the cell co-extracted iron into the electrolyte solution, affecting cell durability and the purity of zinc recovered. Optimising the cell for galvanized steel was then investigated. The study revealed that electrochemical methods can be used to alleviate issues that are caused by galvanized steel, mainly iron contamination. The cells optimum conditions for galvanized steel included a cut-off voltage of 0.50 V to alleviate iron contamination. The optimum current was determined to be 100 mA, between 5-50 mA the cells coulombic efficiency was poor (23-40 %) in comparison to that of 100 mA, applying a current above 100 mA led to degraded performance, with zinc remaining on the surface of the streel regardless of cut-off voltage. Measurements conducted with real scrap steel show a negative impact on the cell. The unknown composition of the scrap led to utilising the battery in a new way, using the OCP of ZBBs for each individual piece of scrap to determine whether zinc was present or not. EDS analysis was also less helpful than previous experiments due to the surface being covered in unknown elemental compounds. The electrochemical stability of the cell was compromised, and ex-situ characterisation failed to determine the exact composition on the surface of an individual piece of scrap steel. The research indicates that the cell is technically feasible to aid the steel industry in the recovery of zinc from scrap metal. The quick dipping of steel into the electrolyte solution determined whether zinc was present on the steel substrate or not. The electrical power yielded during discharge could be harnessed for on-site industrial processes. Further research should be conducted into ensuring the cell is not compromised over multiple cycles, through change of material or redesigning of the cell. Research should also be conducted on cleaning methods for scrap steel that would not compromise the zinc on the steel surface to benefit the longevity of the cell.
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