Selective Dissolution of Electrode Materials from NiMH Batteries

Paper in proceeding, 2010

Nickel-metal hydride (NiMH) batteries are one of the most common battery types used in hybrid electric vehicles (hev). The electrodes of these batteries contain large amounts of nickel, cobalt and rare earth metals. Recovery of these metals is economically and environmentally beneficial as it mines an easily accessible source of raw material. Recovery should be performed using a method which minimizes the use of energy and chemicals. One way to perform the recovery is to selectively dissolve electrode materials during hydrochemical processing. In the case of NiMH batteries, the positive electrode active material, nickel hydroxide, can be dissolved without dissolving the electrode backing material (usually nickel metal). To rapidly dissolve all of the nickel hydroxide, and as little as possible of the nickel metal, becomes an optimization problem: where is the suitable point at which to stop dissolving material in combination with the choice of chemical surrounding Experiments were performed with a temperature controlled by pH-stat, which maintains a constant pH during dissolution. This work shows that rapid dissolution of the active material on the positive electrode can be achieved at a low temperature and a relatively low acid concentration (pH 0.5 –1.5) whilst leaving the major part of the nickel metal undissolved. This is possible with all three tested minerals: acids nitric, sulphuric and hydrochloric. The speed of dissolution is mostly affected by the pH and only slightly affected by the type of acid. Therefore the most suitable acid depends on the choice of process following the dissolution.