Journal of Chromatography A, 1220 (2012) 21–25 Contents lists available at SciVerse ScienceDirect Journal of Chromatography A jou rn al h om epage: www.elsevier.com/locat e/chroma Modelling and optimisation of preparative chromatographic purification of europium Frida Ojala a , Mark Max-Hansen a , Dejene Kifle b , Niklas Borg a , Bernt Nilsson a,∗ a Department of Chemical Engineering, Centre for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden b Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway a r t i c l e i n f o Article history: Received 18 July 2011 Received in revised form 7 November 2011 Accepted 17 November 2011 Available online 23 November 2011 Keywords: Ion-exchange chromatography Rare earth elements Europium Calibration Optimisation Kinetic dispersive model a b s t r a c t A model commonly used to describe the separation of biomolecules was used to simulate the harsh environment when eluting neodymium, samarium, europium and gadolinium with a hot acid. After calibration, the model was used to optimise the preparative separation of europium, as this is the most valuable of the four elements. A kinetic dispersive model with a Langmuir mobile phase modulator isotherm was used to describe the process. The equilibration constant, the stoichiometric coefficient and the column capacity for the components were calibrated. The model fitted the experimental observations well. Optimisation was achieved using a differential evolution method. As the two objective functions used in optimising the process, productivity and yield, are competing objectives, the result was not a single set point but a Pareto front. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Rare-earth elements are currently used in many electronic devices due to their specific properties, and the demand for these elements in pure fractions is increasing. The source of rare-earth elements is minerals consisting of mixtures of several rare earths, and it is thus necessary to separate them. The price of rare-earth ele- ments increases with the demand on purity [1], and it is therefore of economic interest to purify the elements to a high level, pro- vided a cost-effective separation process is available. However, it is not easy to separate these elements as they have similar chemical properties [2]. Commercial separation is usually carried out using liquid–liquid extraction, while small-scale separation is often per- formed by means of preparative ion-exchange chromatography [2]. Small-scale separation is utilised when the demands on purity are high, and the elements of interest are of high value. The subject of this study was the separation of the elements neodymium (Nd), samarium (Sm), europium (Eu) and gadolinium (Gd) by preparative ion-exchange chromatography. Europium is the most valuable of the four elements [1], and this was the tar- get component for purification. Cerium was used in the overloaded experiments, as it was believed to have similar properties to the other elements but is cheaper; making it more suitable when large ∗ Corresponding author. Tel.: +46 46 222 8088; fax: +46 46 222 4526. E-mail address: bernt.nilsson@chemeng.lth.se (B. Nilsson). quantities are required. To minimise the cost of purifying the ele- ments while ensuring the desired level of purity, it is essential to optimise the separation process. Computer simulation was used to shorten the optimisation time and reduce the costs associated with extensive experimental studies. Ion-exchange chromatography is a well-established separation technique [3,4], utilising the variation in the electrostatic interac- tion between the stationary phase and the substances to achieve separation. Model-based optimisation of batch-wise separation using liquid chromatography has been applied to most kinds of chromatography processes, for example, hydrophobic interaction chromatography [5], reversed-phase chromatography [6] and ion- exchange chromatography [7]. The components involved in the above-mentioned processes are biomolecules, whereas in the case presented here the components are small ions, and elution is per- formed using a hot acid. Although the separation of europium by ion exchange using an acid is a known process [8,9], optimisation by means of modelling has not been widely studied. It is therefore of interest to investigate whether the models used, which were deigned to reproduce the separation of biomolecules, can describe the harsh environment in which a hot, strong acid is used to elute small metal ions. The main objectives of this work were the calibration and validation of the model. The experimental system was then opti- mised using the model, with europium as the target component. A kinetic-dispersive column model was used to model the sepa- ration. Calibration was initiated by visual adjustment, after which 0021-9673/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2011.11.028