Recovery of cobalt powder and tungsten carbide from cemented carbide scrap - Part I: Kinetics of cobalt acid leaching

The aim of this study is to investigate the effectivity of nitric acid leaching and the leaching kinetics of cobalt from cemented carbide scrap. The studied variables were reaction temperature, time, nitric acid concentration and stirring speed. The best leaching conditions were found to be 25°C, 2 h, 0.5 M HNO ₃, stirring speed of 900 rpm (corresponds to Reynolds number of 25 000) and 1/10 solid/liquid ratio. A cobalt leaching efficiency of 91.5 % was reached under these conditions. An positive effect of higher temperatures is not apparent and cobalt can be extracted at room temperature. At higher temperature, the rate of cobalt leaching decreases. The leaching rate is also decreasing with increasing HNO ₃ concentration. It is found that the stirring speed has the most important influence on the leaching rate. The leaching process follows the kinetic models 1 - 2/3 x - (1 - x) ^2/3 = kt and the apparent activation energy is determined to be 13.77 kJ/mol at 25 to 70°C. It is concluded that the leaching process of cobalt from cemented carbide scrap in nitric acid solution is controlled by diffusion.