Process intensification for valuable metals leaching from spent NiMH batteries

Acid leaching is an essential unit in hydrometallurgy, yet it is time consuming and requires concentrated acids. Process intensification is examined for the leaching of valuable and rare earth elements (REEs), such as Ni, Co, La, Nd, and Ce, from spent nickel metal hydride (NiMH) batteries using subcritical water extraction (SWE) and microwave-assisted extraction (MAE). Results of SWE revealed that HCl performed better than H2SO4 and HNO3. Leaching efficiency increased as reaction temperature increased from 100 to 150 degrees C, and it increased with HCl concentration. SWE was very effective using 0.5 N HCl, solid concentration of 20 g/L, initial pressure of 9.8 bar, 100 degrees C for 5 min, and leaching efficiency of 99.85% for Ni, 91.72% for Co, 96.26% for La, 85.75% for Nd, and 84.37% for Ce was found, and the total energy consumption was 2,640 kJ. Leaching efficiency of MAE using 0.5 N HCl decreased slightly when heating rate increased from 30 to 70 degrees C/min, and it increased as reaction time increased from 5 to 30 min. Compared with SWE and conventional leaching, MAE was the most energy-efficient, and total of 874.1 kJ was required to reach 85.93% for Ni, 77.60% for Co, 64.04% for La, 75.01% for Nd, and 57.17% for Ce, respectively under experimental conditions of 1 N of HCl, solid concentration of 20 g/L, 100 degrees C for 5 min. Novel acid leaching methods showed promising potential for metals recovery from e-waste.