Design optimisation study of solvent extraction: chemical reaction, mass transfer and mixer-settler hydrodynamics

It is a well-known fact that a typical engineering design problem usually deals with more than one design criterion. If each design criterion is stated as an objective function to be optimised, then the engineering design problem becomes a multicriterion optimisation problem, requiring the simultaneous optimisation of more than one objective function. In this paper, it is shown how the design of solvent extraction flow-sheets can be stated as a multicriterion optimisation problem, using the positive weighted sum approach. This is used not only to obtain parametric optimisation (i.e., the best operating conditions: agitation speed, residence time and phase flow ratio) but also to help in structural optimisation (i.e., to synthesise the best process flow-sheet: number of stages, flow structure and phase recycle ratio). We demonstrate this over a case study, namely, the selective separation of two chemically akin and hard to separate metals, zinc and cadmium, commonly found together in the leaching liquor of complex ores. With this case study, it is shown that the design solutions are richer and more wide-ranging when put together from the vantage point of multicriterion optimization, whereas they become narrow-minded and/or biased if the starting point is a single criterion point of view. Three other conclusions of less general validity were also obtained: (i) the opposite effects of feed phase flow-rates on recovery and purity; (ii) the high sensitivity of short optimum residence times to variations in agitation speed; (iii) the ability of counter-flow associations of a variable number of mixer–settler units to accommodate changes in metal purity and overall recovery in response to drivers in market prices and environmental policies. D 2004 Elsevier B.V. All rights reserved.