Jul 22, 2021 The WC-Co hard metal sludge powder was rinsed using deionized water to remove cutting oil and impurities, and then mechanochemical ball-milled for 2 h in various concentrations (0.5–2 M) of aqueous H 2 SO 4 (95%; Junsei), using a planetary ball mill (Pulverisette 5; Fritsch GmbH) at 300 rpm and a ball:powder ratio of 5:1. The ball-milling process used 6-mm diameter cemented carbide balls