Influence of Na⁺, K⁺, Mn²⁺, Fe²⁺and Zn²⁺ions on the electrodeposition of Ni-Co alloys: Implications for the recycling of Ni-MH batteries

Abstract

Abstract.A hydrometallurgical recycling procedure for the recovery of a mixed rare earths sulfate and an electrodeposited Ni-Co alloy has been described. The latter step was found to be complex, due to the presence of several ions in the battery electrode materials. Electrochemical evaluation of the influence of the ions on the Ni-Co alloy deposition was carried out by cyclic voltammetry test. It was found that ions such as K + , Fe 2+ and Mn 2+ improved the current efficiency for the Ni-Co deposition process on a copper surface. On the other hand, Na + and Zn 2+ ions exhibited a deleterious behaviour, minimizing the values of the reduction current. The results were used to suggest the inclusion of additional steps in the process flow diagram of the recycling operation, in order to eliminate deleterious ions from the electroplating solution. IntroductionNi-MH batteries are mainly composed of nickel (in oxidized and metallic form), cobalt, alkaline electrolyte substances and rare earths metal alloys [1][2][3][4][5]. Although it is difficult to get exact numbers for the usage of Ni-MH batteries in Latin America, it has been reported that this kind of batteries account for 2.7% (by weight) of the total household spent batteries, as collected in communal recycling procedures in Venezuela [6]. This number can be regarded as representative, for most countries within the region. New legislation changes issued in the Latin American countries have encouraged the application of valorisation strategies, not only to prevent environmental damage, but also to recover metal values from this kind of electronic wastes. A feasible solution for this local problem could be the adoption of small-scale hydrometallurgical facilities, in order to process these wastes without the need of exporting them to other countries, most of which are usually located in Europe and East Asia [7]. Nickel, cobalt and rare earths are strategic materials, which are worth to be recovered from depleted Ni-MH batteries; nevertheless, the hydrometallurgical recycling of Ni-MH batteries is a challenging issue, due to its internal chemical complexity [1][2][3][4][5][6][7][8]. In a previous paper [7], a small-scale hydrometallurgical processing route was devised, comprising a chemical stage and an electrochemical stage. During the chemical stage, the active electrode materials of the Ni-MH batteries were leached using aqueous H 2 SO 4 . The obtained pregnant solution was then submitted to pH adjustment in a settling reactor, in which the rare earths were recovered as an insoluble complex sulphate. The remnant solution, containing nickel, cobalt and some other ions is then electrolyzed, in order to recover a Ni-Co alloy [7]. This latter stage is not yet well understood, because of the interaction of the several ions present in the solution.