Thermodynamic and kinetic analysis of jarosite Pb–Ag sludge thermal decomposition for hydrometallurgical utilization of valuable elements

Jarosite is a specific type of hazardous waste determined by the origin of the primary raw materials of the hydrometallurgical process of zinc production. It represents a very demanding raw material for further treatment and recover of valuable metals due to their complex phase structures. The aim of this study was to collect relevant data for the designing the process of thermal decomposition of jarosite Pb–Ag sludge desired structure, which includes the transformation of Fe(II) and Fe(III) sulfates to insoluble hematite, retention of Cu, Zn and In in the form of soluble sulfates, with the possibility of their subsequent valorization by hydrometallurgical methods. Thermal decomposition of the industrial sample of jarosite Pb–Ag sludge is investigated under different temperatures (670–750 °C) and atmospheres (air and nitrogen) by thermodynamic, thermogravimetric and kinetic analysis. Physicochemical characterization of samples was done by ICP-MS, AAS, IMS, XRD, SEM techniques. The examined sample is ammonium jarosite type with the dominant phases NH 4 Fe 3 (SO 4 ) 2 (OH) 6 × H 2 O (44.00%) and ZnFe 2 O 4 (15.25%). Experimental results, physicochemical characterization and thermodynamic analysis confirmed that the maximum content of hematite (Fe 2 O 3 ) and metal sulfates (Cu, Pb and Zn), as required compounds for selective metal extraction, without the presence of unreacted ammonium jarosite, was obtained at 730 °C. A kinetic study obtained by thermogravimetric analysis showed that the thermal decomposition of jarosite Pb–Ag sludge takes place in two phases. The activation energy, calculated using the Kissinger–Akahira–Sunose iso-conversion method, was 235.4 kJ mol −1 and 208.8 kJ mol −1 for the decomposition of jarosite in air and nitrogen, respectively.