Silver Leaching from Jarosite by Thiourea in Acid Media: effect of Copper and Sulfur on the Stability of the Thiourea-Silver System

摘要

The jarosite residue has been produced during many years in the hydrometallurgical zinc plants to remove iron and sulfate from the zinc liquor. The jarosite is a stable species that may contain some elements in its structure, e.g., silver, that would be interesting to recover. As jarosite is refractory to the standard cyanidation method for silver recovery, some pretreatment for decomposition in alkaline or acid medium at elevated temperature is required to recover the silver contained. The thiourea leaching in acid media is proposed as an alternative method to cyanidation, for the recovery of silver from jarosite residues. The stability of thiourea was evaluated in synthetic solutions by titration with iodate. It was demonstrated that the presence of copper and sulfur has a detrimental effect on the thiourea stability and consequently on the silver leaching. The release of copper from the jarosite to the solution by the thiourea leaching increases the redox potential of the solution causing the thiourea oxidation and thus hindering the silver leaching The thiourea leaching tests of an industrial Mexican jarosite showed that the silver contained in the jarosite can be completely leached (10,000 ppm thiourea, pH = 1, 90°C and 2 h) if the thiourea concentration in the system is sufficient. During the leaching, copper is also released to the solution; it increases the redox potential and can cause the thiourea oxidation. When the jarosite/solution ratio is decreased from 100 g/L to 5 g/L, the silver leaching increases from 76% to 100%. This fact can be associated with the thiourea concentration in solution, that increases from 4% to 71% due to the diminution of the copper released into the solution.