Abstract:The formation of SO3 in lead smelting flue gas causes low temperature corrosion in smelting equipment and pipelines. It also initiates furnace wall corrosion by forming alkali-metal iron trisulfate. This study delves into the underlying causes of SO3 generation during lead smelting processes. During the pyrometallurgical smelting of sulfide ores, a large amount of sulfur-oxide gases (SO?) is produced, mainly SO2 with small amounts of SO3. When the temperature is below the acid dew point temperature, SO3 reacts with moisture to form acid mist, corrosive micro droplets, or surface adhering liquid films, which have destructive impacts on metal surfaces. Mitigation measures include keeping the coldend temperatures above the acid dew point or using acid resistant materials. To deal with the hazards of SO3 while retaining SO2 for sulfuric acid production, targeted SO3 removal needs to strike a balance between the risks of secondary pollution and the challenges in managing desulfurization byproducts. This research establishes a theoretical framework for inhibiting SO3 formation by clarifying its generation mechanisms.The thermodynamic equilibrium of the gas-phase system in lead smelting flue ducts was studied. The lead smelting flue gas was simulated by using the FactSage software, with a focus on the influence mechanism of key factors such as temperature and gas-phase composition on the equilibrium components in the lead smelting flue gas and the homogeneous oxidation of SO3. Under homogeneous conditions, the maximum amount of SO3 is generated at 500 °C. Under the conditions where the temperature was fixed at 300 °C, 400 °C, 800 °C and 1 200 °C respectively, the influence of the initial O2 content on the content of each substance at the final equilibrium of the reaction was studied. In the medium temperature range of 500—800 °C, reducing the content of O2 has a significant effect on controlling the content of SO3 in the flue gas.Meanwhile, a solid-phase catalytic experimental platform including a gas distribution system, a reaction system and an absorption and detection system was constructed to study the impact of solid oxides in lead ash on the conversion of SO2 to SO3. The results show that the change trend in the homogeneous experiment results is consistent with that in the thermodynamic simulation research results. From the heterogeneous catalytic experiment of SO2 after adding lead ash, it can be clearly seen that the effect of lead ash on the conversion of SO2 to SO3 is quite obvious. Secondly, through component analysis of lead ash, it can be obtained that the main oxides in lead ash are Fe2O3, ZnO, CdO and PbO. The lead dust in lead smelting flue ducts promotes the conversion of SO2 to SO3 in the flue gas. The oxides Fe2O3 and PbO in lead ash play a catalytic oxidation role in the formation of SO3. In this experiment, in the presence of lead ash, the conversion rate of SO3 exceeds 4.5% within the temperature range of 600—800 °C. The heterogeneous catalytic effect of lead ash is significantly higher than that of gas-phase oxidation. Under the independent experimental conditions of Fe2O3, when the temperature is 800 °C, the conversion of SO3 reaches a maximum value of 15.45%. Under the independent experimental conditions of PbO, when the temperature is 800 °C, the conversion of SO3 reached a maximum value of 4.23%.The SO2 molecules are adsorbed on the surface of the catalyst, which are lead ash, Fe2O3, ZnO, CdO and PbO in the experiment. They react with the adsorbed O2 to form adsorbed SO3, and then desorb from the catalyst surface and enter the gas phase. During this process, the properties of the active sites of the catalyst (such as the structure of the active center, electronic properties, etc.) have a great influence on the reaction rate. Furthermore, the reason why the oxide Fe2O3 plays a significant role is similar to that of V2O3 as a catalyst. In the initial stage of the reaction, the lattice oxygen of Fe2O3 in the oxide oxidizes SO2 to SO3, and itself is reduced. The catalytic effects of different oxides manifest through two distinct mechanisms: lattice oxygen-mediated oxidation and surface adsorption-based activation. Then, the oxygen in the gas phase replenishes the lattice oxygen lost on the catalyst surface, so that the catalyst can be regenerated and continuously catalyze the oxidation reaction of SO2 to SO3. However, ZnO, CdO and PbO cannot provide lattice oxygen and only exist as active sites for the catalytic oxidation of SO2. Therefore, reducing Fe2O3 in lead ash can inhibit the formation of SO3 in flue gas.The formation behavior of SO3in lead smelting flue gas is dual-regulated by thermodynamic equilibrium and reaction kinetics, with significant differences between homogeneous and heterogeneous reaction mechanisms. In industrial applications, the design of flue gas purification systems should integrate thermodynamic suppression strategies (high-temperature decomposition) and kinetic regulation approaches (catalyst passivation) to minimize SO3 emissions.