Characterization and Dissolution of HEV NiMH Batteries

Licentiate thesis, 2010

The recycling of batteries is an important issue in a sustainable society where materials recycling of complex products is a necessity. Without an efficient recycling there will be an unnecessary loss of both materials and energy due to inefficient or non-existent recycling. This project concerns the recycling of nickel metal hydride (NiMH) hybrid electric vehicle (HEV) batteries and aims to outline the potentials and possibilities inherent in this waste stream. The main valuable metals in the batteries are nickel, cobalt and lanthanoids. The active cathode material is a triprecipitate of mainly nickel with zinc and cobalt in hydroxide form with yttrium and magnesium added, pasted onto a nickel metal foam-mesh support. The active anode material is a hydrogen absorbing alloy containing approximately 54% nickel and 31% lanthanoids and smaller amounts of manganese, cobalt, aluminium and yttrium. The recovery of these metals should be performed using a method which minimizes the use of energy and chemicals. The work presented in this thesis is a characterization of the batteries and a dissolution study of individual electrode materials as a first step in a hydrometallurgical recycling process. A nickel removal and purification process has also been investigated, the so called Mond process, by which nickel reacts with carbon monoxide and forms nickel tetracarbonyl. The nickel tetracarbonyl both forms and is highly volatile at a low temperature, unlike other relevant carbonyl compounds. This means that the volatilized nickel can easily be separated from the rest of the material and thereby purified. This gives both pure nickel and an interesting residue to work with. The positive electrode active material, nickel hydroxide, can be dissolved without dissolving the electrode backing material (nickel metal). A rapid dissolution of the active material on the positive electrode can be achieved at low temperature and relatively low acid concentration (pH 0.5-1.5), while leaving the major part of the nickel metal undissolved. This is possible with all the acids tested, nitric, sulphuric and hydrochloric acid. Therefore the most suitable acid depends on the choice of process following the dissolution.