Acid decomposition and silver leaching with thiourea and oxalate from an industrial jarosite sample

Abstract Jarosites are a waste product of most zinc processing plants, since these compounds are formed as a measure to eliminate iron and other impurities from zinc solutions in acidic media. However, they often occlude valuable elements, such as silver, in their structure. In the present study, an alternative is presented for the recovery of silver that is present in this type of waste. The effect of pH, temperature, oxalate concentration and solid/liquid ratio (S/L) on thiourea stability and the selective recovery of silver by acid decomposition and reductive leaching of jarosite were evaluated. The results show that increasing the temperature of the system favors the decomposition of the jarosite and the release of silver to the solution, obtaining recoveries of 86 and 100% silver at 50 and 60 °C, respectively, with 0.13 M thiourea, 0.2 M sodium oxalate, pH 2 and a S/L ratio of 50 g/L. At higher S/L ratios (500 g/L), an increase in the oxalate concentration to 1.2 M was necessary to achieve the decomposition of the jarosite, liberating both the iron and the occluded silver. Additionally, increasing the oxalate concentration is necessary to act as a complexing agent for the large amount of ferric and cupric ions released by the jarosite decomposition.