Recycling of spent NiMH batteries

Abstract

The utilization of existing metallurgical infrastructure and integration of secondary process streams into primary metals production can provide advantages over separate recycling plants. This paper focuses on the integration of a pregnant leach solution (PLS) into a nickel production plant that contains Ni, Co, Zn, Mn, Fe, Al and Cd ions, derived from a NiMH recycling stream. The solution composition highlights the challenge related to spent battery investigations, as although toxic Cd is not present in NiMH battery chemistries, it ends up into the battery waste collection and prevents direct integration of the battery waste into the primary process. The aim of the study is to remove Zn, Fe, Al, Mn and Cd ions from the PLS, so that the Ni and Co remaining in solution could be integrated into the nickel production process. A two-step solvent extraction process using Di-2-ethylheyxl phosphoric acid (D2EHPA) was developed. In the first step, Zn, Al and Fe were removed at pH 1.5, whereas in the second step Cd and Mn were removed at pH 2.3. Different process parameters (pH, O/A ratio, temperature, extractant concentration, kinetics) were optimized at each step of the solvent extraction. The purified and fully integrable PLS was found to contain Ni ~28 g/L and Co ~3.7 g/L.