Liquid-liquid extraction is a process for separating chemical components in a liquid phase. In this process, the target component is transferred from the liquid phase into a second liquid phase, the extraction agent. The basic principle here is the different solubility of the valuable substance in the two phases. As a rule, the two phases are an aqueous phase and an organic phase. Liquid-liquid extraction is used in the chemical, pharmaceutical, perfumery and food industries.

Liquid-liquid extraction is often used for wastewater treatment. A well-known example is the removal of phenol. This is used as a starting material for the production of plastics, but also for the production of acetylsalicylic acid (active ingredient of Aspirin®). Phenol has a strong toxic effect both locally and systemically, which makes removal from process waters indispensable. Liquid-liquid extraction can effectively remove phenol in extraction columns, for example, by using methyl isobutyl ketone (MIBK). In this process, the organic phase (MIBK) is introduced at the bottom of the column and the aqueous phase (containing phenol) is introduced at the top of the column. As it passes through the column, the aqueous phase releases the phenol and the organic phase absorbs it.

The fundamentals of physical extraction are well understood. The focus of research is therefore on hydrodynamic aspects, which allow the prediction of the droplet size distribution and the disperse phase fraction in the column. Related to this, interfacial phenomena are not yet fully understood, which complicates the design of extraction columns. For example, the Marangoni effect accelerates mass transfer from the aqueous to the organic phase. Eruptions at the phase interface influence the ascent direction and flows along the phase interface influence the coalescence behavior of the droplets.

Research area

The extraction methods mentioned so far refer to physical extraction, in which the valuable or harmful substance is soluble in both phases. Metal ions are generally not soluble in the organic phase. To enable extraction nevertheless, we resort to liquid ion exchangers. These complex the metal ion and thus make it soluble in the organic phase. This type of extraction is accordingly called reactive extraction.

In the context of the energy transition and electromobility, urgent questions of battery recycling arise. For several years, the Institute of Process Engineering has been working on the separation of the black mass contained in lithium-ion batteries. This consists mainly of oxides of the metals lithium, cobalt, manganese and nickel. The subject of this research is the resource-efficient separation of these metals. To this end, we are investigating the use of extraction columns and membrane extraction for feasibility studies. Membrane extraction is a special process that is characterized by a high phase interface, i.e. a high yield in a small space. In addition, the extraction is nondisperse, so there is no need for downstream phase separation.

For the design and also the scale-up of plants, we use and develop computer-aided computational fluid dynamics (CFD) models with the aid of droplet population balance models (PBM).

For research we have various apparatuses as well as measuring techniques at our disposal. These include a Kühni DN32 column, a Kühne DN100 column, mixer-separator cascades, pulsed columns and a membrane extraction plant. In the area of measurement technology, we have an ICP-OES, as well as high-speed cameras and corresponding measurement technology for determining the droplet size.