Cobalt and Nickel are important metals, with their alloys being used to produce, amongst other things, strong magnets and high-strength, corrosion-resistant machine components. They are often found together in, for example,... [ view full abstract ]
Cobalt and Nickel are important metals, with their alloys being used to produce, amongst other things, strong magnets and high-strength, corrosion-resistant machine components. They are often found together in, for example, nickel-cobalt laterite deposits. Separation of the two metals is challenging due to their physical and chemical similarities, though high purities are desirable for many applications. Current separation techniques generally rely on liquid-liquid extraction methods, though these require water-immiscible organic solvents which are often flammable, volatile or toxic.
Recent developments in ionic liquids mean that they are increasingly being seen as viable alternatives to organic solvents. Their low volatilities and the ability to tune their properties give rise to the potential for highly efficient metal extraction processes [1]. Experiments have shown that trihexyltetradecylphosponium (TTDP) chloride is able to separate cobalt and nickel from an aqueous mixture with a high degree of selectivity to obtain high purity cobalt (>99.8 %) and nickel (>99.5 %) [2]. This work aims to examine this separation in more detail by firstly examining the thermodynamics and kinetics of the separation using thermodynamic and molecular models, then using these results to produce flowsheet models which can be used in process engineering.
Molecular dynamics simulations were performed in order to examine the dynamics of the binary phase system, including estimating viscosities, and investigating the mechanism of the transfer of the cobalt from the aqueous phase to the ionic liquid phase. These simulations also give useful insights into the compositions of the two phases at equilibrium as reagents flow through the phase boundary. These simulations required the parameterisation of new Co2+ and Ni2+ models based on the OPLS forcefield. The result was that we were able to predict both thermodynamic and transport properties to a good degree of accuracy.
Thermodynamic models were used to inform the flowsheet modelling. A SAFT-g-Mie model was constructed which included descriptions of the electrolytes. After parameterisation with respect to experimental densities and experimental heat capacities, this model is able to predict densities and activities of the aqueous / TTDP system over a range of conditions and compositions. COSMO-RS was also used to provide further insight in to the thermodynamics of the extraction process.
The theoretical data were then used to aid in the production a flowsheet for the process. Flowsheet simulations are widely used in the chemical industry to design plants and, given key input data such as mass and energy balances, equilibrium relationships and rate correlations, important predictions can be made including phase compositions and optimised operating conditions.
References:
- A. Stojanovic, B.K. Keppler, “Ionic liquids as extracting agents for heavy metals”, Sep. Sci. and Tech. 47 (2012) 189-203
- S. Wellens, R. Goovaerts, C. Moller, J. Luyten, B. Thijs, K. Binnemans, “A continuous ionic liquid extraction process for the separation of cobalt from nickel” Green Chem. 15 (2013) 3160-3164
Carbon capture and other industrial applications , Challenges and advances in fluid phase equilibria
