Kinetics of reduction of lead chloride and oxychloride with sodium carbonate

Abstract

This study investigates the kinetic behaviour of the reduction reaction between lead chloride (PbCl₂) and lead oxychloride (PbO·PbCl₂) with sodium carbonate in the presence of carbon in a reducing atmosphere. The relevance of this research stems from the need to develop effective technologies for processing lead-containing secondary production dusts and to optimise the composition of sodium-containing fluxes. Thermodynamic evaluation of the reactions using the Outotec HSC Chemistry software package demonstrated that, within the temperature range of 500-1000°C, both reactions exhibit negative ΔG values and high equilibrium constants, indicating their thermodynamic feasibility. Kinetic studies were carried out using thermogravimetric analysis under isothermal conditions in the temperature range of 800-1100°C for the PbCl₂–Na₂CO₃–C system and 500-800°C for the PbO·PbCl₂–Na₂CO₃–C system. It was established that the reductive interaction proceeds via a multistage mechanism with a change in the rate-controlling step as the degree of conversion increases. For the PbO·PbCl₂–Na₂CO₃–C system, the initial stage of the process is satisfactorily described by the first-order reaction equation, indicating the predominance of chemical control; the apparent activation energy at this stage is 33.5 kJ/mol. As the degree of conversion increases, the activation energy rises to 62-80 kJ/mol, which indicates a transition to a regime complicated by diffusion and structural limitations. The PbO·PbCl₂–Na₂CO₃–C system is characterized by higher reactivity and rapid attainment of significant conversion degrees. At moderate conversion levels, the process proceeds predominantly under chemical control (E_a ≈ 43-44 kJ/mol); however, at α ≥ 55%, an increase in activation energy up to 74 kJ/mol is observed, indicating an increasing influence of mass transfer. It is shown that the presence of oxygen in the structure of lead oxychloride significantly affects the kinetic characteristics of the reductive process in sodium-containing flux systems. The obtained results can be used in optimizing technological schemes for processing lead-containing dusts in order to enhance metal recovery and improve the energy efficiency of the process.