Use of mixed-metals isotherm and log–log McCabe Thiele's diagram in solvent extraction—A case study

Nickel and cobalt are invariably associated with other transition element impurities like copper, zinc, iron, etc. It is essential to remove these impurities in order to attain the high standards of purity required for specific applications. Solvent extraction is a well-tested route for this purpose. The present paper describes the application of two novel concepts, viz. mixed-metals isotherm and log–log McCabe Thiele's (MT) diagram in solvent extraction. Mixed-metals isotherms can be applied to a system in which two metal ions compete for a single extractant. If these two metal ions are required to be extracted as a group, mixed-metals isotherms can precisely predict their behavior. Log–log MT diagrams are useful in determining the requirement of the number of stages in counter current extraction, especially when the feed and raffinate concentrations differ by several orders of magnitude. The two concepts have been successfully applied to develop a process for the separation of high purity cobalt and nickel from ocean nodules leach liquor.