Recovery of cobalt powder and tungsten carbide from cemented carbide scrap - Part II: Recovery of submicron cobalt powder from the leach solution

In this study, it is aimed to treat cemented carbide scrap (hard metal WC-Co) by nitric acidic leaching, followed by the precipitating of cobalt hydroxide, and finally obtaining a submicron size pure cobalt metal powder by thermal breakdown, and the hydrogen reduction of this hydroxide in a horizontal tube furnace in one process step. A comparison of different precipitating reagents and optimization of the precipitation parameters are also conducted, along with a search for the possible existence of an environment-friendly and economic process. According to the results obtained, it is possible to dissolve cobalt with high efficiencies (min. 91.5 %) via HNO₃ leaching of cemented carbide scrap. The optimum 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 > 98 % in the form of hydroxide by addition of an alkaline reagent such as NaOH. Cobalt hydroxide is transformed into a pure cobalt powder (99.7 %) by a thermal breakdown process (>300°C, 0.5 h, with argon) and by a reduction with hydrogen (800°C) under controlled conditions in 1.5 h to yield a submicron cobalt powder. The metallic Co-powder obtained has a purity of 99.7 % Co and BET specific surface area of 0.37 m²/g.