Enhanced hydrochloric acid leaching of cobalt

Dissolution rates of cobalt in hydrochloric acid with and without the presence of thiourea have been studied by means of weight loss and rotating disc (with solution analysis) methods. In the absence of thiourea an apparent activation energy value of 67.7 ± 3.4 kJ. mol⁻¹ was determined from weight loss measurements and 70.3 ± 7.5 kJ. mol⁻¹ using rotating disc techniques. These high values are inconsistent with a diffusion controlled process and this was supported by the lack of dependency of the dissolution rate on the rotational speed. A reaction controlled process is indicated.In 8.4 mol. dm⁻³ hydrochloric acid the thiourea concentration producing optimum dissolution rates was found to be ∼ 1 × 10⁻³ mol. dm⁻³. Above 60°C rates of leaching were enhanced by some 700–800%. Tests using radiochemical labelling of the sulphur and carbon in the thiourea molecule confirmed previous proposals that H₂S is produced as a by-product of the cathodic depolarisation reaction involving the thiourea and is adsorbed on the cobalt surface in the molecular state forming activated anodic sites which enhanced anodic dissolution.