Metal extraction and Recovery with Electrochemical Methods

The recycling of batteries is currently based on pyro- or hydrometallurgical processes that are energy and chemical-intensive. Therefore, novel strategies using electrochemical methods are investigated. By in situ generation of leaching reagents the need for chemicals can be minimized and the handling of dangerous substances like hydrogen peroxide is avoidable. The separation of extracted metals requires several complex steps that need additional chemicals for solvent extraction, stripping and precipitation. To overcome these downsides, electrochemical methods for leaching and recovery of valuable metals are a promising alternative.
Electrochemical leaching
Instead of the conventional leaching with sulphuric acid with added hydrogen peroxide the proposed method uses peroxydisulfate electrochemically generated in situ by oxidation of sulphuric acid. This reduces risks and costs related to the transport and handling hydrogen peroxide and other reagents. The electrochemically generated persulfate increases the reactivity substantially as is demonstrated by studying the leaching of lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NCM) and industrial black mass. Conditions were to find a sulphuric acid concentration of 5 M and a moderate temperature of 65 °C as best-suited parameters. The durations for full conversion of black mass are shortened substantially, and the extraction rates are increased significantly for LCO and NCM by a factor of two.
The electrochemical leaching was investigated as in-cell and ex-cell leaching process and the latter with leaching in a separate reactor was found to be more suitable for efficient metal extraction.
Electrochemical recovery
The electrochemical recovery and separation of the metals was investigated as next step. Separate electrowinning of cobalt and nickel is a challenging task due to their similar redox potentials. The selectivity of metal plating was optimized under potentiostatic conditions, at -1.0 V (vs. Ag/AgCl) a high selectivity for cobalt of over 90 % from a 1:1 mixture of Co/Ni sulphate was achieved. More negative potentials lead to substantially higher nickel content in the plated material. The solution pH is a relevant factor as well, yet only a minor influence on the Co/Ni ratio was observed in the range from pH 3 – 6.5. However, low pH is unfavourable due a substantial decrease in current efficiency. A strong temperature effect was observed showing the highest cobalt content of 95 % at 5 °C that decreases to 80 % at 60 °C (Fig. 1b). Unfortunately, low temperature reduces the current density below 10 mA/cm², thus, electrolyte optimization is necessary for sufficient conductivity to run the process.
Additionally, the use of organic additive to form metal complexes was investigated to alter the redox potentials. Depending on the used additives, over 98 % selectivity could be achieved.
In summary, we successfully improved leaching of battery materials with electrochemical means and are currently showing new potential pathways for nickel and cobalt recovery and separation. Combining both steps in one electrochemical cell promises to be an efficient and sustainable process.