Silver extraction from a refractory type ore by thiourea leaching

In this work, the extraction of silver from the refractory type Gümüşköy-Pasa (Turkey) ores by thiourea leaching is studied. Optimum leaching conditions and kinetics are investigated. In order to extract 98% of Ag, ores should be oxidized in a H₂SO₄ solution at pH = 1 and 80°C. The oxidizing period is about 24 h with the application of 10⁵ Pa oxygen pressure, and about 2 h with the application of 10⁶ Pa oxygen pressure. In the thiourea leaching step, 2 h of leaching at 45°C is found sufficient. The reaction rates of preliminary oxidation and the thiourea leaching steps can be described with a "shrinking core" model; the activation energies are calculated according to the Arrhenius equation. Activation energies are 21 kJ/mol and 27 kJ/mol for the first and second steps, respectively. The reaction controlling step in the preliminary oxidation is the diffusion of sulphate ions; in the thiourea leaching is the diffusion of Ag-thiourea complex through the reacted layer surrounding the unreacted core to the solution.