Recycling of nickel metal hydride (NiMH) batteries; Characterization and recovery of nickel, AB5 alloy and cobalt
Nickel metal hydride (NiMH) batteries are used today for applications that can assist in the adaptation toward carbon-neutral energy sources (i.e. hybrid vehicles and smart grids). Recovery of metals such as nickel, cobalt and rare earth elements (REEs) from discarded NiMH batteries is important for economic and/or technological reasons. Recirculation of valuable materials back into society can be achieved by the production of pure concentrates of elements from waste, or by regenerating and reusing materials.
Recycling of NiMH batteries can be done through pyro- and hydrometallurgical methods, which produces pure and valuable metal fractions. The current work focuses on the separation of three different types of material from NiMH waste: (1) metallic nickel powder, (2) reusable hydrogen storage alloy (i.e. AB5) and (3) cobalt-enriched surface compounds.
Bipolar NiMH batteries were investigated in this work, which are different in design compared to previous investigations on recycling of batteries. In these batteries the conductive nickel network is made up of nickel powder, instead of conventional nickel foam grids.
Previous investigations suggest that spent electrode materials are not completely degraded. Reusing parts of the discarded NiMH battery has however not yet been commercially realized. For the current work spent NiMH batteries were characterized separately. Corrosion products form on the anode particles during battery cycling, and in this work rare earth hydroxides (i.e. REE(OH)3) were identified. The hydrogen storage alloy can potentially be reused if corrosion products are removed.
In the current work it was re-confirmed that nickel, even in powder form, can be recovered by dissolution of the waste matrix using hydrochloric acid. Cyanex 923 was briefly considered for treating the leachate solution. The extraction of REEs and aluminum from the leachate was affected by the amount of acid present, which was attributed to preferential extraction of acid. Controlling the acid in the dissolution step is therefore important in order to further treat leachate solutions.
The relatively novel recycling approach of recovering AB5 alloy using carboxylic (malonic, maleic, acetic and citric) acid was considered. An interesting result was that REE(OH)3 could be removed using acetic and citric acid, without dissolving much of the remaining spent AB5 alloy. The practical uses were otherwise limited due to similar dissolution rates of the AB5 alloy and other waste matrix.
A novel recycling approach of using ascorbic acid was suggested and tested. Separation of the cobalt layers from the cathode hydroxide particles is possible because these cobalt layers have different oxidation states (higher than 2) compared to the waste matrix. The ascorbic acid treatment does not dissolve the anode material. Ascorbic acid can therefore potentially be used as a pre-step to chemically separate the anode material from the cathode material in a mixed electrode waste flow.
hydrogen storage alloy