In this work, the extraction of silver from the refractory type Gumusk
oy-Pasa (Turkey) ores by thiourea leaching is studied. Optimum leachin
g 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-degr
ees-C. The oxidizing period is about 24 h with the application of 10(5
) Pa oxygen pressure, and about 2 h with the application of 10(6) Pa o
xygen pressure. In the thiourea leaching step, 2 h of leaching at 45-d
egrees-C is found sufficient. The reaction rates of preliminary oxidat
ion and the thiourea leaching steps can be described with a ''shrinkin
g core'' model; the activation energies are calculated according to th
e 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 throu
gh the reacted layer surrounding the unreacted core to the solution.