Silver leaching from pyrargyrite oxidation by ozone in acid media

• Pyrargyrite is a silver antimony sulfide refractory to cyanidation.
• The silver leaching of pyrargyrite is possible with ozone in acid solutions.
• Ozone and acid concentrations are the variables affecting the most the silver leaching.
• Extreme experimental conditions permit a silver dissolution up to 80%.

Pyrargyrite (Ag3SbS3) is one of the most important silver ores because of its abundance and occurrence in mineral deposits; it is refractory to cyanide, and therefore, silver recovery in standard cyanidation processes for precious metals extraction is low. The present work proposes the oxidative leaching of pyrargyrite with ozone in acid solutions as an alternative option to treat pyrargyrite concentrates.The stoichiometry of the pyrargyrite-ozone reaction and the effect of the stirring speed, ozone concentration in gas, particle size and sulfuric acid concentration on silver leaching were investigated. All investigated variables affected silver dissolution; however, increased ozone and acid concentrations resulted in a higher dissolution rate. A factorial experimental design demonstrated that the ozone concentration had the greatest effect on silver leaching. The dissolved Ag/Sb molar ratio and the O3 consumed/Ag3SbS3 leached molar ratio were observed to be approximately 3 and 14, respectively, suggesting that the reaction occurs according to the following stoichiometry:Ag3SbS3+14.5O3+0.5H2O+2H+=3Ag2++SbO3−+3HSO4−+14.5O2Ag3SbS3+14.5O3+0.5H2O+2H+=3Ag2++SbO3−+3HSO4−+14.5O2A silver dissolution of 80% was obtained when one gram of pyrargyrite (fines, d80 = 13 μm) in 800 mL of 0.18 M sulfuric acid solution was oxidized with 1.2 L/min oxygen containing 0.079 g O3/L at 25 °C and a stirring speed of 800 rpm.Pyrargyrite dissolution exhibits a linear behavior when the concentrations of the reagents are not limited. However, when conditions are extreme (very fine particle size and high acid and ozone concentrations), the silver and antimony dissolution rates decrease, and the characteristics of the remaining solid change, suggesting the presence of a solid byproduct that apparently passivates the dissolution reaction.