The mechanism of cobalt extraction by a tri-alkyl-amine hydrochloride (TAAHCl) was studied. Based on extraction solution chemistry, a mathematical expression was derived to identify metal transfer mechanisms. Both experimental results and theoretical consideration identified COCl42- anions as the most favorable species for ion exchange with chloride anions in protonated TAAHCl. A new emulsion liquid membrane (ELM) system using TAAHCl as carriers was tested for the separation of cobalt from a nickel hydrometallurgical waste effluent. The effect of complex agent addition, carrier concentration, and acidity of the internal and external aqueous phases on cobalt extraction efficiency was investigated. Under optimum conditions, the enrichment of cobalt with a recovery rate of 99.8% was obtained by a one-stage process. The treated effluent contained mainly nickel, which is suitable for further nickel recovery, while the cobalt in the separated internal phase can be recovered as a value added by-product.