(401d) Hydrodynamics of Two-Phase Ionic Liquid Solvent Systems in Countercurrent Chromatography

In industry, current approaches for liquid-liquid extraction separations often involve the use of organic solvents which have low flash points and are inherently unsafe. The long residence times, and the large volumes in the mixer-settler or pulsed column extraction units that are most commonly used, further increase the high hazard ratings for the process. A promising intensified separation technology is high-performance countercurrent chromatography (CCC). It is a form of liquid-liquid chromatography that takes places along a continuous length of tubing, which is wound around a drum (called a bobbin). Within the CCC column, i.e. the coiled tubing, one of the liquid phases is held stationary by centrifugal forces generated as a result of rotating the bobbin in a planetary motion, while the second (mobile) phase is then continuously pumped through it in such a way that there is good retention of the stationary phase. Rotation of the column results in a variable centrifugal field. This creates a series of simultaneous mixing and settling zones along the length of tubing, promoting solute transfer between the phases, and therefore, separation of species with different partition coefficients. Due to the high-speed rotation of the column, a large number of stages can be generated in a single machine. For example, a sample injected into the mobile phase can experience 800 repetitive mixing and settling steps in a minute when the motor rotor speed is 800 rpm, thus promoting highly efficient separations.

In this work, the application of ionic liquid biphasic solvent systems in a custom-designed CCC device has been investigated for the potential separation and recovery of valuable and intrinsically important metals, such as uranium, thorium, lanthanides and noble metals. Due to their dual nature and unique physiochemical properties, ionic liquids have gained increasing attention in recent years as potentially suitable candidates for replacement of volatile organic solvents in liquid-liquid extractions. Some of their most favourable features include their negligible vapour pressure, chemical and thermal stability, nonflammability and their adjustable miscibility and polarity. The unique combination of the two technologies, ionic liquids and CCC, represents a new and exciting philosophy to liquid-liquid separations. It overcomes the problems associated with the extractive process related to the use of organic solvents (flammability, volatility) and to the performance of the equipment (large inventories, long residence times) and will result in safer, more environmentally friendly approaches.

The viscosity of ionic liquids is often higher than that of common molecular solvents and can pose significant problems for traditional CCC devices, which are mostly low pressure. As a result, to date, their application to CCC has been very limited and greater insight into the hydyrodynamics that govern or control the extraction process is required. To investigate the hydrodynamic behaviour of the ionic liquid biphasic solvent system, visual observations have been carried out on a transparent 2-D spiral CCC column at different angles around the central axis of rotation using a high-speed camera. The images acquired have revealed the phase distribution and the mixing and settling areas within the column undergoing planetary motion, as well as the interfacial characteristics that can promote mass transfer (such as waves). A necessary condition to separate solutes using the CCC technique is that the column retains some of the stationary phase. Therefore, the influence of several operational parameters on the volume fraction of stationary phase retained was also studied including mobile phase flow rate, mobile phase pumping direction (Head --> Tail or Tail --> Head), rotational speed and direction of rotation. The settling and mixing characteristics of the two-phase mixture, and phase retention within the coils have been related to the densities, viscosities and interfacial tension of the ionic liquid solvent systems employed.

Acknowledgements

The project was funded by the UK Engineering and Physical Science Research Council (EPSRC) (Project title: Application of ionic liquid-liquid chromatography (ILLC) to extraction of metals), in collaboration with QUILL Research Centre at The Queen's University of Belfast, Northern Ireland, U.K.