Assessing acid performance in NiMH battery recycling: A comparative study of inorganic and organic acids

The growing demand for critical metals has intensified the need for efficient recycling routes for spent NiMH batteries. This study presents a comparative assessment of inorganic (sulfuric acid – H 2 SO 4 and phosphoric acid – H 3 PO 4 ) and organic acids (citric acid – C 6 H 8 O 7 and aminoacetic acid – glycine – C 2 H 5 NO 2 ) for the leaching of the black mass obtained after the physical processing of NiMH battery modules. The objective was to identify the most suitable leaching agent considering metal recovery efficiency, reaction parameters, and reagent cost. The novelty of this work lies in the systematic comparison between organic and inorganic acids, including the first-time reported use of glycine for this purpose. Experiments with H 2 SO 4 achieved leaching efficiencies of 100% for rare earth elements (REEs), 97.7% for Ni, and 99.2% for Co at 2.0 mol/L, 25 °C, solid-liquid ratio 1/7.5 (g of solid material per L of acid) for 60 min. The use of H 3 PO 4 enabled the selective leaching of Ni and Co, while REEs precipitated as insoluble phosphates. Leaching with organic acids proved feasible but demanded the highest temperature (90 °C) and reaction time (120 min) than inorganics to promote protonation and ensure metal solubilization. C 6 H 8 O 7 had leaching efficiencies of 92.7% for Ni and 96.7% for Co, while C 2 H 5 NO 2 achieved leaching percentages above 98% in acidic conditions (pH 2.0) and under the lowest solid-liquid ratio (1/20). A preliminary cost analysis indicated that H 2 SO 4 provides the most cost-effective leaching among the evaluated acids. • 8.6 kg Ni, 2.0 kg REEs, and 0.5 kg Co can be recovered from a 37.2 kg NiMH pack • H 2 SO 4 showed the best balance between leaching efficiency, mild conditions, and cost • H 3 PO 4 selectively leached Ni and Co, while REEs precipitated as insoluble phosphates • Organic acids are greener options but require harsher conditions, more time, and cost