Kinetics study of platinum and base metals precipitation in gas–liquid chloride system

The use of dissolved sulphur from gaseous phase was tested for its ability to precipitate platinum ions in chloride system with alterations in the parameters such as metal ion concentration, hydrochloric acid concentration, pressure and temperature. The precipitation process was analysed on the basis of mass transfer coefficient, diffusivity models and also by absorption kinetic models. The maximum physical and reactive absorption capacity for sulphur dioxide was found to be 0.015 and 0.018 mol/L, respectively, at 298.15 K and 1.125 bars in a short contact time of 10 min. Based on the sulphur dioxide solubility data in gas–liquid system for both physical and reactive absorption conditions, the introduction of sulphur atoms in the chloride system was achieved. The mass transfer coefficients obtained for all metal ions were in the range 0.10–0.26 min^-1, while the diffusivity of SO2 at different temperatures and pressures was found to be in the range (2.36–3.43) * 10^-9 and (1.36–2.22) * 10^-9 m^2s^-1, respectively. Thermodynamic parameters such as the changes in Gibbs free energy, enthalpy and entropy were also evaluated. The results indicated that the precipitation of platinum and base metals in gas–liquid (G–L) chloride system was an endothermic process.