The recycling of rare earth elements and base metals from recycling slag powders of nickel metal hydride (NiMH) batteries by ionic liquid leaching was studied. Leaching tests were conducted by using the ionic liquid 1-methylimidazolium hydrogen sulfate (HmimHSO(4)) selected after some preliminary leaching tests on slag powders. The HinimHSO(4) concentration of aqueous ionic liquid solution was 30% (v/v) in all tests. The slag powders were ground and sieved to obtain two different particle-size fractions namely 63-90 pm and 90-125 pm in order to evaluate the leaching behavior and the kinetics. The most important metals present in the initial slag powders are iron (35.70% w/w), aluminum (9.95% w/w), manganese (3.45% w/w) and rare earth elements (REEs) such as lanthanum, cerium, neodymium and yttrium (totally 22.50%w/w). In the leaching experiments three parameters were studied: temperature, particle size and time. The average dissolution ratios of 63-90 pm particle size slag powder were 100% for iron, manganese and yttrium after 2 h at a temperature of 80 degrees C. However, the maximum leaching efficiencies were about 15% for lanthanide and cerium and 20% for neodymium under the same conditions. As an explanation for the low recovery rates, a phase analysis with QEMSCAN (Quantitative Evaluation of Materials by SCANning electron microscopy) indicated decomposition in the initial REE phases but a precipitation as sulfates. The kinetic study of Fe and Mn dissolutions showed that the leaching process follows the shrinking core model with an unchanged particle size controlling diffusion through the product layer. (C) 2016 Elsevier B.V. All rights reserved.