Modern civilization is indivisibly connected to technology that is run by electric energy. The introduction of Nickel based batteries (e.g., nickel-metal hydride, Ni-MH) and later the breakthrough of Lithium-Ion batteries (Li-Ion) in the late 90s by Sony gave rise to portable devices like smartphones and laptops. As a result of the technological improvements of our globalized world, the demand for portable energy storages has increased significantly over the last decades. However, with a finite lifespan of these portable energy storage systems, a large number of spent batteries will consequently enter the waste stream and need to be handled. Due to limited resource availability, geopolitical tensions, and poor working conditions in some parts of the world, the recycling of spent batteries is becoming increasingly important.[1] Within the hydrometallurgical recycling route, liquid-liquid extraction (more commonly known as solvent extraction (SX)) offers a promising method to recover critical metals (e.g, Li, Co, Ni, Mn, Cu) due to its high selectivity, high efficiency, and low energy consumption. While conventional aromatic diluents are widely used in solvent extraction due to their favorable polarity and phase behavior, many of them, such as Solvesso 150, exhibit significant drawbacks, including toxicity and poor biodegradability. In terms of workplace safety, the associated exposure risks and fire risks due to the high volatility of many aromatic diluents can be considered a significant issue for the environment as well as for workplace safety. Besides aromatic diluents, many aliphatic compounds also present similar problems. Although HVO100 is already used as a green and sustainable alternative for classic aliphatic diluents in solvent extraction, the need for more sustainable aromatic compounds still remains.[2, 3]
In our research approach, we investigated the two compounds 1,4-di-tert.-pentylbenzene and 1-methyl-4-(tert.-pentyl)benzene as suitable and sustainable improved diluent alternatives and elucidate their properties and application in solvent extraction processes. The experimental design included well-known extractants like the acidic extractants Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid), DEHPA (di-(2-ethylhexyl)phosphoric acid), as well as the quaternary ammonium salt extractant Aliquat 336, and a new sustainable diketone extractant. The compounds 1,4-di-tert.-pentylbenzene and 1-methyl-4-(tert.-pentyl)benzene were compared against the common diluents tert.-butylbenzene (TBB), Ethylbenzene, Solvesso 150 ND as well as the aliphatic green diluent HVO100. An artificial aqueous solution containing typical battery metals such as aluminum (Al), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), lithium (Li), and manganese (Mn) was used as the aqueous feed. The organic phases were composed as a 30% (v/v) compositions of extractant in diluent. The experiments were performed using a 1:1 ratio of organic to aqueous phase through a variety of different experimental conditions. Using Inductively coupled plasma-optical emission spectroscopy as the analytical method, distribution values (D) and separation factors (α) were calculated and compared. Initial results indicate that the new compounds are comparable to those already in use and have the potential to substitute more harmful aromatic compounds as diluents in the solvent extraction process.[4, 5]
Currently ongoing is our approach to evaluate the potential for industrial application. We intend to repeat the experiments using real industrial waste from end-of-life Ni-MH batteries. Moreover, we will investigate, measure, and provide additional important physical data, such as dielectric constant (εr), density (ρ), solubility of water in diluent and solubility of diluent in water, as well as viscosity (η), which are either missing or not yet reported in the literature for these compounds.