Recovery of submicron cobalt-powder by acidic leaching of cemented carbide scrap

This study investigates on the treatment of cemented carbide scrap (hardmetal WC-Co) by nitric acid leaching, followed by the precipitation of cobalt hydroxide to finally obtaining a submicron size pure cobalt metal powder by thermal breakdown and the reduction of this hydroxide in a horizontal tube furnace using hydrogen gas. According to the results obtained, it is possible to dissolve cobalt with high efficiencies (min. 91.5%) via HNO₃ leaching from cemented carbide scrap. Effective leaching conditions were found to be: <90 μm, 25 °C, 2 h, 0.5 M HNO₃, 900 rpm and 1/10 solid/liquid ratio. Moreover, it has been found that cobalt could be precipitated with an efficiency of > 97 % in the form of hydroxide by addition of an alkaline reagent like NaOH. Cobalt hydroxide was transformed into a cobalt powder (99.7 %) by a simple thermal breakdown process (<300°C, 0.5 h, with Argon) and by a reduction with hydrogen under controlled conditions in 1.5 h to yield a sub-micron-structure. Weight loss ratios in the reduction process vary between 5.9 and 9.7 % at the end of a 2.0 hour calcination and reduction step. The thermal breakdown and reduction kinetics of the cobalt hydroxide, amount of impurities it contains, and its morphology have significant effects on the properties of cobalt-powder. The metallic Co-powder obtained has a purity of 99.7 % Co and specific surface area of 0.37 m² / g. The characterization studies of the WC leach residue aiming for the re-utilization possibilities in the Hard Metal Industry (or Powder Metallurgy), revealed that it has a density of 11.9 g/cm³, and a BET value of 1.08 m ²/g.