Selective extraction of silver from jarosite residues produced in the zinc hydrometallurgical process using thiourea under acidic conditions: Kinetic analysis and leaching optimization

In this work, an industrial hazardous jarosite residue produced in the zinc hydrometallurgical process containing silver was used to evaluate the feasibility of using thiourea (Tu) for silver leaching under acidic conditions in the presence of oxalate (C₂O₄²⁻ ion denoted as Ox²⁻). The shrinking core kinetic model (SCKM) was applied to evaluate the effect of variables on the reaction rate. The chemical reaction of H₃O⁺ with the jarosite surface was the controlling stage. The results of batch experiments revealed that silver extraction is influenced by various factors in the descending order: pH > T (temperature) > [Tu] (thiourea concentration) > [Ox²⁻] (oxalate concentration). The extraction efficiency of Ag was 98 % at high levels of both T and [H₂SO₄], although a higher [Tu] was required. High Tu consumption was associated with the stability of Tu due to the presence of Fe³⁺ and Cu²⁺ ions leached along with Ag⁺, which affect the redox potential catalyzing Tu oxidation. Temperature is the most effective variable with a reaction rate constant of k_exp = 1.1 min⁻¹ at 60 °C, but it causes a decrease in silver extraction to 92.5 %. The kinetic parameters were obtained using the SCKM; furthermore, the experimental results were statistically validated through a surface response experimental design.