An Investigation on the Effect of Viscosity on Mass Transfer Coefficient Using Cyanex 272 Solvent Extractant during Cobalt-Nickel Extraction and Separation

Abstract

The separation of cobalt from nickel in aqueous solutions remains a major challenge in hydrometallurgy due to their chemical similarities. Solvent extraction (SX) using organophosphorus acids, particularly Cyanex 272, offers a promising approach for achieving selective separation with high yields and purity. This study investigates the effect of organic phase viscosity on mass transfer coefficients during SX of cobalt (Co) and nickel (Ni), with the objective of identifying operating conditions that maximize diffusivity and extraction efficiency. Viscosity experiments were conducted using Cyanex 272 in kerosene as diluent and iso-octanol as modifier, with a saponification ratio of 70% at 25?C in an agitated water bath, thermostatically controlled and mass transfer coefficient experiments were carried out in a Drop Swarms apparatus of 0.10 m diameter and 0.60 m height, equipped with a multi-channel precise syringe pump and a computerized video imaging system to help monitor and capture the movement of drops in the extraction column. Results show that viscosity decreases significantly with the addition of diluents and modifiers, leading to higher mass transfer coefficients and improved solute diffusion. At 15% v/v Cyanex 272, viscosity was reduced to below 10 cP, ensuring optimal diffusivity. Mass transfer experiments confirmed preferential extraction of cobalt over nickel, explained by coordination chemistry: cobalt forms tetrahedral, hydrophobic complexes that retain less water, while nickel forms octahedral, hydrophilic complexes that retain more water molecules. The novelty of this research lies in its systematic linkage between viscosity control and mass transfer efficiency, providing a clear operating window for SX optimization. By demonstrating how diluents and modifiers can be strategically employed to reduce viscosity and enhance cobalt selectivity, the study offers practical guidance for industrial hydrometallurgical operations. These findings underscore the importance of Cyanex 272 as a cost-effective and efficient extractant for cobalt-nickel separation, with direct implications for improving recovery yields and process sustainability. This research provides both fundamental insight into solvent extraction mechanisms and practical guidance for hydrometallurgical operations, contributing to more efficient and sustainable production of battery-grade cobalt and nickel.