Ionic Liquid-based metal recovery from aqueous industrial process and waste streams
Due to the COVID-19 crisis the PhD defence of Enas Othman will take place (partly) online.
The PhD defence can be followed by a live stream.
Enas Othman is a PhD student in the research group Sustainable Process Technology (SPT). Her supervisor is prof.dr. S.R.A. Kersten from the Faculty of Science and Technology (S&T).
One of the EU’s strategies to combat climate change is the transition toward low-carbon energy technologies such as the transition to electric vehicles (EV’s). The accelerated introduction of EVs increases the demand for lithium-ion batteries (LIBs), where Cobalt (Co) supply is expected to be the bottleneck in their production. Co is one of the most critical raw materials on earth due to its rare existence and its near-monopolistic supply structure. Therefore, there is a growing incentive, if not critical need. for recycling Co from different waste sources such as industrial wastewater streams and spent LIB’s leachate. Although the process of Co retrieval plays an important role in replenishing Co stockpiles, it also contributes to reducing the environmental impact of industrial Co use and reducing the reliance on Co mining, during which the miners are exposed to highly hazardous conditions.
Liquid-liquid extraction (LLX) is one of the most widely applied hydrometallurgical techniques for metal separation and recovery. LLX is defined as the migration of a solute from the feed into a solvent, followed by back-extraction to retrieve the end product and reuse the solvent. It allows efficient processing of large volumes of aqueous solution in a continuous mode with relatively low capital and operational costs. At the beginning of the 21st century, a new type of solvents, known as ionic liquids (ILs), found their way to scientists. These liquids, defined as salts which is in the liquid state at ambient temperatures or below 100 oC, have interesting properties such as a negligible vapor pressure, high thermal stability and a wide liquidus temperature range. ILs are often described as designer solvents and green alternative solvents because of their ability to fine-tune their structure-function relationship and negligible vapor pressure, respectively. Compared to conventional volatile organic compounds (VOCs), ILs are also considered to be a safer alternative due to their low flammability. However, a serious issue related to the application of some ILs is their solubility in aqueous media, which might lead to a pollution threat for the aquatic environment. Although, this is not true for all ILs, the application of each individual IL should be thoroughly checked for this aspect, even more so if one or more of the IL components is considered to be rather toxic.
In this PhD thesis, several aspects of process upscaling of an IL-based LLX system are considered. The goal is to investigate the application of an IL-based LLX process for the extraction and recovery of different transition metals (notably Co) from different process streams. In order to achieve this, fundamental research, experiments, modelling and a preliminary economic analysis are all included. It is decided to use tetraoctylphosphonium oleate [P8888][Oleate] as the solvent due to its unique selectivity towards transition metals. This makes it a good candidate for the extraction of Co from industrial wastewater and spent synthetic LIB leachate. First, the LLX process is investigated at lab-scale in Chapter 2 to recover Co from a chloride-based aqueous medium. Extraction-regeneration processes are followed for five consecutive cycles using [P8888][Oleate] as the solvent and aqueous Na2CO3 solutions for regeneration. The main criteria to evaluate the feasibility of any potential application of ILs are the following: the extraction efficiency, the recovery and the loss of the IL and the production of commercial valuable end-product.
In Chapter 3, the IL [P8888][Oleate] is applied to selectively extract and recover valuable metals (Co, Ni, Mn and Li) from synthetic leachates resembling those from spent lithium-ion battery cathodes, under different conditions. The parameters investigated for this selective separation and recovery process include the extraction pH, contact time, composition of the regeneration solutions and number of required extraction/ regeneration stages within one cycle. Additionally, an economic potential analysis (EP0) is performed to evaluate the commercial feasibility of the proposed process.
In Chapter 4, we address a number of questions regarding IL-based metal extraction at the more fundamental level. These questions relate to the metal affinity, extraction mechanism and the effect of presence of inorganic anions and cations other than Co in the aqueous phase. The selection of the extraction mechanism eventually considered as most adequate is based on: (1) model fitting of the extraction isotherms as obtained at different temperatures, (2) evaluating different thermodynamic parameters, including equilibrium constant, Gibbs energy, enthalpy, and entropy of the complexation reaction and (3) analysing the physical properties of the LLX system such as the change in viscosity and electric conductivity.
In Chapter 5, a LLX process is performed using a single droplet extraction column. The aim of these experiments is, first, to investigate the mass transfer of Co from water to IL droplets in more details, secondly, to identify the mass transfer limitation, and finally, to estimate the mass transfer and kinetic parameters. This investigation evaluates various mass transfer models, with(out) a chemical reaction using a statistical cross-validation method (CV5). The outcome of the CV5 analysis, includes the effect of column length, droplet diameter, droplet rising velocity (and thus contact time) and the Co concentration in the continuous and dispersed phases on Co uptake.
Chapter 6 discusses, the extraction of Co by [P8888][Oleate] using a laboratory scale KARR® reciprocating plate extraction column. The influence of different operating variables on the performance of the column are evaluated; these variables are the frequency of agitation and the flow rates of the solvent and the aqueous phase. Moreover, experiments are performed to study the effect of the solvent-to-feed ratio on the hydrodynamic performance of the column and the ratio at which flooding is likely to occur. Additionally, a mathematical model using literature correlations for mass transfer, holdup and droplet diameter is developed. The results from this model are compared to the experimental data obtained in this study as well as those reported in literature.
Finally, it is concluded that the IL [P8888][Oleate] shows a good potential for the extraction and recovery of valuable transition metals due to its selectivity towards the targeted metals. However, the main drawback when performing the LLX process in a laboratory scale extraction column (KARR®) is the need for the addition of a substantial amount of diluent to avoid flooding, which narrows the window of operation due to the limited IL/feed ratio and accordingly lowers the extraction efficiency.
