Leaching kinetics of tellurium, selenium and silver from copper anode slime by sulfuric acid leaching in the presence of manganese(IV) oxide and graphite

Leaching of constituent metals from copper anode slime/CAS (22.23% Se, 1.53% Te, 9.66% Ag) by sulfuric acid in the presence of manganese(IV) oxide (MnO₂) and graphite has been investigated at a range conditions by varying stirring speed (200–600 rpm), H₂SO₄ concentration (0.5–3.0 M), MnO2/CAS and graphite/CAS mass ratio (0–1/1) and temperature (25–90 °C). The metal leaching was facilitated by the galvanic interaction with MnO₂, while graphite play role as a catalyst. Under the most suitable conditions (500 rpm, 2.0 M H₂SO₄, 0.8/0.8/1 MnO₂/graphite/CAS mass ratio and 90 °C temperature), the extent of leaching of Se, Te and Ag from the CAS, together with Mn from MnO₂ after 6 h was 90.8%, 81.9%, 80.7% and 84.1%, respectively, as compared to 76.8% Te, 69.4% Se, 67.2% Ag and 55.9% Mn after leaching of CAS with MnO₂ without graphite under the similar conditions, and against still lower leaching in the absence of MnO₂ (32.6% Te and negligible Se and Ag). The leaching results (CAS+MnO₂+graphite) of tellurium and silver at temperature 25–50 °C well fitted to the mixed and surface chemical reaction models, respectively, and changed to follow the diffusion and mixed control models in the temperature range 60–90 °C with the respective apparent activation energies of 17.8 and 12.2 kJ/mol. That of selenium was controlled by the surface chemical reaction with the estimated activation energy of 27.6 kJ/mol in the temperature range 25–90 °C. And manganese leaching followed the mixed-control model (activation energy of 56.7 kJ/mol) at the all tested temperature.