SILVER EXTRACTION FROM A REFRACTORY TYPE ORE BY THIOUREA LEACHING

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.