Recovery of diamond and cobalt powders from polycrystalline drawing die blanks via ultrasound assisted leaching process—part 2: Kinetics and mechanisms

The leaching of industrial polycrystalline diamond (PCD) blanks in aqua regia at atmospheric pressure between 60^◦C and 80^◦C was performed using an ultrasound to improve the rate of cobalt removal in order to be able to reuse very expensive polycrystalline diamond. Because cobalt (20 wt.%) is used as a solvent catalyst in the production of PCD, its recovery is very important. The cleaned PCD are returned to the production process. Kinetic models were used in the study of cobalt dissolution from polycrystalline diamond blanks by measuring the declining ferromagnetic properties over time. For a better understanding of this leaching process, thermochemical aspects are included in this work. The lowest free Gibbs energy value was obtained with a low solid/liquid ratio and the full use of an ultrasound. A transition from a reaction-controlled to a diffusion-controlled shrinking core model was observed for PCD with a thickness greater than 2.8–3.4 mm. Intermittent ultrasound doubles the reaction rate constant, and the full use of ultrasound provides a 1.5-fold further increase. The obtained maximum activation energy between 60^◦C and 80^◦C is 20 kJ/mol, for a leaching of diamond blank with grain size of 5 µm.